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Old 01-28-2008, 04:02 PM   #1
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Ideal Compression for Turbo

Hey everyone, what would be the most ideal compression ratio to have on a motor seeing about 10-15 pounds of boost?
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Old 01-29-2008, 06:48 PM   #2
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Re: Ideal Compression for Turbo

Not that I'm an expert, but I recently saw Gale Banks (expert) say that about 9:1 is ideal...

Anyone else feel free to chime in...

Raf
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Old 01-29-2008, 08:59 PM   #3
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Re: Ideal Compression for Turbo

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Which is better, high boost and low compression or low boost and high compression?
You are better off with a low compression/high boost combination. The combination of supercharger boost coupled with your static or geometric compression ratio provides a resultant Effective Compression Ratio. There is a complex formula for determining the Effective Compression Ratio. However as an example if you are running an 8.75:1 static compression ratio and 6 pounds of boost you will have an Effective Compression Ratio of 12.32:1. Under normal circumstances with a Centrifugal supercharger you can usually run an Effective Compression Ratio of from 12.50:1 to 13.10:1. This is obviously on the realms of science, therefore, if you have questions please do not hesitate to call RSE's Tech Line 0870 474 0153 for assistance.
How much compression ratio can you run?
Again, this can vary from engine to engine but a good rule of thumb with a RSE Supercharger is you can run about nine pounds of boost on a 9:1 engine. If you have more compression ratio then you need to run less boost. If you have less compression ratio then you can run more boosts Click the image to open in full size. In ideal circumstances you would like to have as much compression ratio and as much boost as you can run without detonation.
That's from http://www.rse.co.uk/rsesupercharger.htm

So 9.0:1 sounds about right for my goals, or lower yet?
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Old 01-29-2008, 09:23 PM   #4
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Re: Ideal Compression for Turbo

I'm figuring about 10 pounds on the street maybe a little less, then like 15 at the track, and methanol injection could be used to fight detonation??
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Old 01-30-2008, 02:31 PM   #5
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Re: Ideal Compression for Turbo

anywhere between 8.5-9.0:1 for a street car on pump gas should be good. Meth will help out alot at the track. If you had more of a race car you could get away with higher CR if you plan to run race gas. Also Aluminum heads will allow you to run more compression and timing with less detonation. As well as intercooling.
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Old 01-30-2008, 07:22 PM   #6
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Re: Ideal Compression for Turbo

Think a T-61 with a .63 a/r is enough to support "at least" 450 horse at around 10 pounds through an intercooler?
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Old 01-30-2008, 08:07 PM   #7
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Re: Ideal Compression for Turbo

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Think a T-61 with a .63 a/r is enough to support "at least" 450 horse at around 10 pounds through an intercooler?
depends on engine size. with a turbo that small, on anything like a 350 you're going to want a bigger A/R housing. the .63 will spool up wicked fast with a 350, however if you wanted to keep RPM's down to like 5500 it would work out for you as you'd probably see all 10lbs by 3000 rpms
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Old 01-30-2008, 08:19 PM   #8
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Re: Ideal Compression for Turbo

You're going to get a lot of oppinions on this. Some guys like higher c/r and less boost to give it some grunt when out of boost. Others like low c/r with more boost to give it mild manners for easy cruising and lots of boost to turn up the chaos when you step on it. Like the article said more boost = lower c/r; less boost = higher c/r.
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Old 01-30-2008, 09:07 PM   #9
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Re: Ideal Compression for Turbo

A lower compression boosted motor will get better mpg while cruising compared to a higher compression less boost setup correct? Since there's no boost while cruising anyways the lower compression motor should be more economical?
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Old 01-30-2008, 09:42 PM   #10
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Re: Ideal Compression for Turbo

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A lower compression boosted motor will get better mpg while cruising compared to a higher compression less boost setup correct? Since there's no boost while cruising anyways the lower compression motor should be more economical?
Yes. Plus if using pump gas, less cr and more boost will make more power than more cr and less boost.
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Old 01-30-2008, 09:55 PM   #11
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Re: Ideal Compression for Turbo

Alrighty then, the motor will be on pump gas only so I guess that settles that. As for the rotating assembly.... I don't wanna go cheap with it, because I want it to last for a long time. Would it be wise to go with a forged crank, rods and pistons? If I go with forged pistons, is there a way to get rid of the annoying rattling during cold start ups?? (Like using a block warmer in colder weather) Or, would i be better off with hypers or casts since it's mainly a street machine and only some strip?
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Old 01-30-2008, 10:18 PM   #12
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Re: Ideal Compression for Turbo

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Originally Posted by blue82_z28 View Post
Alrighty then, the motor will be on pump gas only so I guess that settles that. As for the rotating assembly.... I don't wanna go cheap with it, because I want it to last for a long time. Would it be wise to go with a forged crank, rods and pistons? If I go with forged pistons, is there a way to get rid of the annoying rattling during cold start ups?? (Like using a block warmer in colder weather) Or, would i be better off with hypers or casts since it's mainly a street machine and only some strip?

i've never heard any rattling during start up with forged pistons, so i dont know what you're talking about. As far as a rotating assembly. you can do some speed pro forged pistons, some forged i-beams and a cast steel crank for the amount of power you're looking for. Thats a super budget rotating assembly what you can get anywhere for around $500.
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Old 01-30-2008, 10:20 PM   #13
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Re: Ideal Compression for Turbo

Oh, alright... I guess I was misinformed about the noise then. On a side note, how much boost are you running, if you don't mind me asking?

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Old 01-30-2008, 11:56 PM   #14
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Re: Ideal Compression for Turbo

if you go i beam rods, get the scat rods with 7/16" rod bolts. they will hold power
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Old 01-31-2008, 08:17 PM   #15
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Re: Ideal Compression for Turbo

What's the going rate for a forged crank?? I found an Eagle crank between 5-600 good for 1000+ horsepower, would that help insure longevity even if I decide not to push that much power?
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Old 01-31-2008, 09:53 PM   #16
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Re: Ideal Compression for Turbo

No, higher compression less boost will get better MPG. Think about it - higher CR is more efficient, burns the mixture better, etc. You get more power from it (when not in boost). It is more responsive, etc.

That being said, i'd far rather have a 8-8.5:1 engine with 14psi of boost than a 10:1 with 6psi of boost. Just throw in a larger stall speed if and let it rev up. But then again i'd only install a turbo if I wanted a lot more power - hence why i'd want the high boost low CR combo.

Crank and rods are typically rated for HP, but that's kind of a nonsensical way to measure it. RPM is the main thing that will damage a crank/rods. The cylinder pressure at that RPM is a drop in the bucket compared to it.

7000RPM NA making 450 HP will destroy a stock rotating assembly due to the HP. 5000RPM making 600HP with boost will run fine. Just my of course.
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Old 01-31-2008, 10:19 PM   #17
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Re: Ideal Compression for Turbo

Can someone point me in the right direction for a good crank that's not overly expensive and can handle at least 700 horsepower? I'll probably be making a tad bit over 600 I would assume when at the track, but a respectable 500 or so on the street.
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Old 01-31-2008, 11:12 PM   #18
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Re: Ideal Compression for Turbo

What are we even talking about here, a 350?? or?
1/2pc RMS?

If you're replacing the crank, may as well go to a 383....
Are you set on the T61 there?

Good selection here;
http://www.ohiocrank.com/sbcCranks.html
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Old 01-31-2008, 11:55 PM   #19
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Re: Ideal Compression for Turbo

Yes, a 350 .30 over.
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Old 02-01-2008, 12:34 AM   #20
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Re: Ideal Compression for Turbo

Plenty out there- scat, eagle, ect. Just go to summit or jegs and start shopping.
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Old 02-03-2008, 01:30 PM   #21
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Re: Ideal Compression for Turbo

an engine with low cr and high boost will spool faster that is the reason for low cr on a boosted engine. but if you are running a small turbo with low boost and i high cr it still won't spool fast because that turbo is designed to fill say 1.5L of displacement at 20lbs of boost, so there fore if you are filling 2.8L at 10lbs then it is doing relatively the same job so cr will still hinder it's ability to spool. that example would be a twin turbo setup. i do not know the exact reason but it is proven on a 300zx forum btw 300zx turbos are perfect for a tt 355 running 1/2 the boost
use http://www.turbofast.com.au/javacalc.html
it will help you for all of your turbo calculation needs
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Old 02-03-2008, 03:48 PM   #22
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Re: Ideal Compression for Turbo

What?

All indications and research has shown me that a higher SCR turbo engine combo will have a lower boost threshold (spool sooner) than a low SCR turbo engine combo. The higher SCR combo helps accelerate the exhaust gases just a bit quicker down the exhaust than a lower SCR, due to a higher amount of squeeze (higher SCR).

A higher SCR, will also have better off boost manners, as in quicker throttle responce, more effciant burn of the charge, etc. This low SCR high boost combination is backwards thinking from the '60s before EFI and better engine technology was around.
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Old 02-03-2008, 04:06 PM   #23
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Re: Ideal Compression for Turbo

i am implying the use of the same turbos in an engine that had had the pistons changed with the same dynamic cr spooled later with higher compression, and i would like to see where you got your information please.

Basically i'm saying that you would have to use a relatively small turbo with high scr and low boost to produce a low boost threshold.
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Old 02-03-2008, 05:32 PM   #24
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Re: Ideal Compression for Turbo

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This low SCR high boost combination is backwards thinking from the '60s before EFI and better engine technology was around.
'60s?? Uh huh. Everything else being equal, a lower CR higher boost engine running at the ragged edge of detonation will make LOADS more power than a high CR low boost engine running at the edge of detonation. It's no surprise that you can run more boost if you have lower CR in the beginning.

It's a matter of personal choice. If you want wicked throttle response just go to a bigger CID engine or a roots blower. Use the lightweight flywheel and all that other jazz.
If you want max power, just up the stall speed of the converter so you're in your powerband faster (no bog/lag) and put the CR in the basement and crank up the boost. Some cars are just fine with 10:1 CR and 6psi of boost (not an old school carbed setup, but some newish FI type of things). Yet that same car with 8.5:1 and 16psi of boost would be a hell of a lot faster.

The throttle response isn't *completely* sluggish.

Side note - What's the CR of the Buick Grand National? Something like 8.0:1 ? They seemed to do pretty well with that EFI injected setup.
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Old 02-03-2008, 08:17 PM   #25
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Re: Ideal Compression for Turbo

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i am implying the use of the same turbos in an engine that had had the pistons changed with the same dynamic cr spooled later with higher compression, and i would like to see where you got your information please.

Basically i'm saying that you would have to use a relatively small turbo with high scr and low boost to produce a low boost threshold.
Uh huh, and the difference in dynamic comprsseion ratio would be?........ That's right they would be the same for the same power, since it's the dynamic compression thats more directly related to power output. I had a chart somewhere that showed effective dynamic compression ratio for differening amounts of boost pressure on different static SCRs. The only thing it lacked was to account for actual head flow, but to do that would make any conversaion of effective dynamic compression ratio null and void.

My information comes from many places, most notably those that are making insane amounts of power in 'street cars', the 1500+ HP cars, that run, along with many other sources including, but not limted to every book that I could get my hands on about turbocharging (some included supercharging), such as turbochargers by Hugh MacInnes, Maximum Boost by Corky Bell, Hot Rod, Car Craft, Popular Hot Rodding, Turbo and High Tech Performance, and many other sources, some include the interweebnet, but I take most of that with a grain of salt unless I either know the person, or the person has a strong rep to have correct informantion. This is all inaddition to personal experiance, building or at least assiting on quite a few turbo builds, most street driven, many daily drivers.
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Old 02-03-2008, 08:26 PM   #26
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Re: Ideal Compression for Turbo

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'60s?? Uh huh. Everything else being equal, a lower CR higher boost engine running at the ragged edge of detonation will make LOADS more power than a high CR low boost engine running at the edge of detonation. It's no surprise that you can run more boost if you have lower CR in the beginning.
Again, they will make the same power for the same dynamic compression, this has been proven over and over again, and some information I have seen actually indicates that the higher SCR/lower boost combo will make more power down low, due to the sooner onset of boost threshold. This seems to increase the area under the power curve, in that the peak HP is not increased, but the ramp to peak power is steeper with the higher SCR.

Quote:
It's a matter of personal choice. If you want wicked throttle response just go to a bigger CID engine or a roots blower. Use the lightweight flywheel and all that other jazz.
If you want max power, just up the stall speed of the converter so you're in your powerband faster (no bog/lag) and put the CR in the basement and crank up the boost. Some cars are just fine with 10:1 CR and 6psi of boost (not an old school carbed setup, but some newish FI type of things). Yet that same car with 8.5:1 and 16psi of boost would be a hell of a lot faster.
Again, EVERYTHING I have read says otherwise. The higher SCR/lower boost combination will generally edge out a lower SCR/hugh boost combination, due to that lower boost combo having a slight bit more area under the curve.

Quote:
The throttle response isn't *completely* sluggish.

Side note - What's the CR of the Buick Grand National? Something like 8.0:1 ? They seemed to do pretty well with that EFI injected setup.
It seems to be sluggish enough for OEM manufactures to take notice of the "sluggish responce" that the buyers were complaining about with turbo cars and upped the SCR, and sometimes lowering the boost over older offerings.

the Grand National was using low SCR, due to what it was based on, the "hot air" set-ups in the turbo Regals of the late '70s and early '80s, where low SCR was needed to combat pre-det.

look at more current offerings, many OEM turbo set-ups run close to or over 10:1 SCR with mild boost and the buyers are eating it up, because the throttle responce is there AND it will pass emmisions by makeing a complete combustion at lower RPM, where there will be little to no boost created, and the most used part of the RPM band, Idle and cruise RPM, most of the time between 1800 and 2500 RPM, depending on the car and gearing.

The guys that are making huge power are usually running 10:1+ SCR, and high boost.
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Old 02-03-2008, 09:15 PM   #27
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Re: Ideal Compression for Turbo

Well I guess we can agree to disagree then? No point in arguing over the internet, it just makes me feel stupid.

I haven't read as much as you say you've read about turbocharging, but what little I have read tells me that lower CR and higher boost ends up with the same "dynamic compression ratio" (I put that in quotes because I think it's almost a fictitious concept, and not easy to measure and put an accurate number on), but loads more power. If it didn't matter the source of the compression (talking dynamic here) we'd just setup the car to run 14:1 SCR and skip the turbo. Unfortunately the turbo gives more power. Generally speaking you can't use much boost if you already have 10:1 SCR unless we're talking about race fuel, aluminum heads, sophisticated FI, highly efficient intercooler, and boost controls etc. The more 'tricks' you use the further you can push the detonation threshold. If you start lower you can simply use more boost. Personally i'd rather have more of a cushion.

Quote:
The higher SCR/lower boost combination will generally edge out a lower SCR/hugh boost combination, due to that lower boost combo having a slight bit more area under the curve.
I'll have to do some more research, you've piqued my interest.
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Old 02-03-2008, 09:44 PM   #28
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Re: Ideal Compression for Turbo

I'm not in on this argument, but a higher c/r engine will always have better throttle response than a lower c/r engine, boosted or not. And how is dynamic c/r a fictitious concept?
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Old 02-03-2008, 09:49 PM   #29
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Re: Ideal Compression for Turbo

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Well I guess we can agree to disagree then? No point in arguing over the internet, it just makes me feel stupid.

I haven't read as much as you say you've read about turbocharging, but what little I have read tells me that lower CR and higher boost ends up with the same "dynamic compression ratio" (I put that in quotes because I think it's almost a fictitious concept, and not easy to measure and put an accurate number on), but loads more power. If it didn't matter the source of the compression (talking dynamic here) we'd just setup the car to run 14:1 SCR and skip the turbo. Unfortunately the turbo gives more power. Generally speaking you can't use much boost if you already have 10:1 SCR unless we're talking about race fuel, aluminum heads, sophisticated FI, highly efficient intercooler, and boost controls etc. The more 'tricks' you use the further you can push the detonation threshold. If you start lower you can simply use more boost. Personally i'd rather have more of a cushion.


I'll have to do some more research, you've piqued my interest.
I have to say I disagree with the idea that we'd just go straight to 14:1 and skip the turbo (I'm not being argumentative here, just furthering the discussion), since the purpose of any power adder, ok lets leave out nitrous for now, since that can be it's own discussion, but the purpose of a turbo or super charger is to force more air in than the engine can take naturally. That means that even with a 14:1 SCR you are still only taking in the same volume of air as the engine has displacement. It is because of this forced induction that make engine capable of making more power than the displacement in theory allows. If I had 14:1 I'd still run plenty of boost.

BTW, you should start looking at things like the Hot Rod Pump Gas Drags, there are quite a few cars making quite a bit of HP on 93 octane pump gas. The top cars are over 1300 HP, as they have to weigh 3300 lbs and the top contenders are running mid 8s. Most are running turbos, some turbos and nitrous, some add water in there as well. the only three things allowed in the PGDs is the supplied 93 octane fuel (Rocket Brand), nitrous, and water. No octane boosters, no alcohol, no propane, etc. There have been a few nitrous only contenders with big cubic inch, but I prefer the whistle of a turbo.

I do agree however that dynamic compression ratio seems to be a bit subjective, though I did read about how to actually measure dynamic compression, and even then it seemed to have holes that would allow for inaccuracies.
I thought I had a calculation for it, but just checked the book I thought it would be in, and it's not there, might be in a book stased away in the basement.
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Old 02-03-2008, 10:11 PM   #30
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Re: Ideal Compression for Turbo

Yes, I think we can all agree that higher CR will give more throttle response - no debate there.

And yea, dynamic is just kind of loosely defined thats what I meant by "fictitious". If we were to take dynamic compression as the final word about detonation then we'd be far off base. ie. we can't say "ok 10:1 dynamic is the limit for 93 octane, period". Dynamic CR is simply static CR with cam timing. It doesn't take into account ambient (or post-intercooler) temperature for example. There's a pile of other factors that come into play with detonation avoidance that dynamic compression doesn't see. I think dynamic compression is a valuable number, but it's just one bit of information needed to see if your engine will detonate. (iron/AL heads, cooling sytem -reverse or normal flow, chamber style, quench distance, temperature, spark plug style, etc etc etc). Also what number do YOU use for dynamic? Valve closing, or timing at .050" or timing at .050"+15 or....?

Hot rod pump gas drags - That's EXACTLY my cup of tea right there. I'm going to look into that, thank you. I fully intend on (one day, near future) have a 900-1000HP small block with turbochargers and running pump fuel. Pump or propane, whatever the case may be.

I just think if we are limited to, lets say xyz effective compression (DCR+boost), and we use

8:1 SC + 14psi
vs
10:1 + 7psi (just throwing out numbers here)

The loss in SCR is only say 8% (thereabouts right?) but the gain in boost would give lets say 30% power (again, thereabouts). So if we're talking about a non daily driver, where you want max power between 2500-6000RPM lets say, and we've got the turbos making boost above 3000RPM - I think the lower CR engine would make most power. You'll hit 6psi at lets say 4000RPM on both engines, so the first engines powerband will continue to skyrocket, where the second one will be only rising slightly (only based on RPM, not additional boost).
I think the higher boost engine would make it down the 1320 a bit faster, but would be a bit more doggy street light to streetlight. I think now we've boiled it down to a matter of personal opinon - yes?
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Old 02-03-2008, 10:38 PM   #31
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Re: Ideal Compression for Turbo

Quote:
Originally Posted by Sonix View Post
Yes, I think we can all agree that higher CR will give more throttle response - no debate there.

And yea, dynamic is just kind of loosely defined thats what I meant by "fictitious". If we were to take dynamic compression as the final word about detonation then we'd be far off base. ie. we can't say "ok 10:1 dynamic is the limit for 93 octane, period". Dynamic CR is simply static CR with cam timing. It doesn't take into account ambient (or post-intercooler) temperature for example. There's a pile of other factors that come into play with detonation avoidance that dynamic compression doesn't see. I think dynamic compression is a valuable number, but it's just one bit of information needed to see if your engine will detonate. (iron/AL heads, cooling sytem -reverse or normal flow, chamber style, quench distance, temperature, spark plug style, etc etc etc). Also what number do YOU use for dynamic? Valve closing, or timing at .050" or timing at .050"+15 or....?

Hot rod pump gas drags - That's EXACTLY my cup of tea right there. I'm going to look into that, thank you. I fully intend on (one day, near future) have a 900-1000HP small block with turbochargers and running pump fuel. Pump or propane, whatever the case may be.

I just think if we are limited to, lets say xyz effective compression (DCR+boost), and we use

8:1 SC + 14psi
vs
10:1 + 7psi (just throwing out numbers here)

The loss in SCR is only say 8% (thereabouts right?) but the gain in boost would give lets say 30% power (again, thereabouts). So if we're talking about a non daily driver, where you want max power between 2500-6000RPM lets say, and we've got the turbos making boost above 3000RPM - I think the lower CR engine would make most power. You'll hit 6psi at lets say 4000RPM on both engines, so the first engines powerband will continue to skyrocket, where the second one will be only rising slightly (only based on RPM, not additional boost).
I think the higher boost engine would make it down the 1320 a bit faster, but would be a bit more doggy street light to streetlight. I think now we've boiled it down to a matter of personal opinon - yes?
A little bit of opinion yes, but neither will in theory make more power than the other, given that both have the same effective dynamic compression ratio, they should both make the same power, I said that in a previous post, just the higher SCR combo, with have much better street manners and have less laggy responce.
the problem with your numbers though is that they are arbetrary. I don't recall how to effectivly calculate the loss in compression, it's not just straight math, as that would be 20% less, there is a calculation for it though, and the effcetive loss is less than that.

Read through some books, especially Maximum Boost by Corky Bell, it backs up what I've been saying, and is a book written more for later model cars, where as Turbochargers by Hugh MacInnes is older and is more theoretical based, also very technical.
If you read through many magazines that deal with real life street cars most of the time they also indicate the same. The ONLY advantage to a low SCR combo is the tuning window is more forgiving, and gives you a wider margin of error. I will trade this ANY day for better throttle responce.
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Old 02-03-2008, 11:40 PM   #32
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Re: Ideal Compression for Turbo

I use intake closing point w/rod length to deturmine dynamic c/r. Using dynamic c/r by itself is a bit arbitrary because you need to assume ideal running conditions for it to truly be accurate, but it's a much better guage of preign/detonation limits than mechanical c/r alone... on n/a engines. When out of boost dynamic c/r is still a good guage of detonation resistance, but it doesn't tell you much about boosted operation because the engine is filling +100%. When in boost mechanical c/r & boost psi is the best guess at running c/r (assuming no valve overlap losses).

Intake closing point has a huge effect on the actual c/r of a n/a engine. Case in point- the factory 10/1 engines of the late 60's. They were 10/1 mechanically, but only 8/1 dynamically. The late intake closing point allowed the engine to backpump air into the intake at low rpm making it safe on pump gas. As rpm rises the losses become less and less and the increased chamber turbulence decreases chances of detonation permitting the increase in pressure.
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Old 02-03-2008, 11:43 PM   #33
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Re: Ideal Compression for Turbo

Yea, I have Corky Bells book and it's GREAT! Very technical, and I love that. I've read it a few times now, it takes more than one read to absorb it all eh?
I'll look for the one by Hugh MacInnes, since Corkys book was a bit thin on blow through carb setups. (ie. more for later models cars, yes).

Loss/gain from compression is typically quoted as 4% per point. I think that's a very rough rule of thumb though.

Alright, fair enough, we have different goals then.
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Old 02-04-2008, 01:15 AM   #34
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Re: Ideal Compression for Turbo

i have not read much since you posted this
Quote:
Quote:
Originally Posted by mxcrazed
i am implying the use of the same turbos in an engine that had had the pistons changed with the same dynamic cr spooled later with higher compression, and i would like to see where you got your information please.

Basically i'm saying that you would have to use a relatively small turbo with high scr and low boost to produce a low boost threshold.


Uh huh, and the difference in dynamic comprsseion ratio would be?........ That's right they would be the same for the same power, since it's the dynamic compression thats more directly related to power output. I had a chart somewhere that showed effective dynamic compression ratio for differening amounts of boost pressure on different static SCRs. The only thing it lacked was to account for actual head flow, but to do that would make any conversaion of effective dynamic compression ratio null and void.


but none the less i did not say that the engine would produce less power dcr and ve are closely related there for they would produce similar power but you have only read books and i said that "using the same turbos" which is a very important part of my argument for lack of a better term. if you merely change the pistons to raise the scr the turbos will spool later FACT from first hand dyno testing posted on a 300zx forum.
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Old 02-09-2008, 03:16 AM   #35
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Re: Ideal Compression for Turbo

What if we approach this debate from the quantity of oxygen molecules available to combine with fuel to produce power?

More oxygen molecules means we can add more fuel, and therefore deliver more power. This is the basic principle common to all power adders (supercharger, turbo, nitrous).

As the piston approaches TDC on the compression stroke, cylinder pressure increases. Increasing pressure increases temperature. At a certain temperature, the fuel/air mixture will ignite without a spark plug (think diesel). This would be pre-ignition. Detonation is different than pre-ignition and can happen long before the temperature required for pre-ignition due to a variety of factors.

To simplify this mental exercise, let's call the pre-ignition temperature the limiting factor even though detonation usually happens first. Think unlimited octane fuel.

Take two engines, same displacement, one engine has 8:1 CR and the other 10:1 CR. Let's say they both have the same cam that closes the intake valve exactly and instantly at BDC. This means there is no difference between SCR and DCR. Which engine has the larger combustion chamber? The 8:1 CR engine.

If we start with a perfectly balanced mixture of fuel and air (at the same starting temperature and pressure for both engines) then fill each combustion chamber to the pressure that produces the temperature just below the pre-ignition temperature. Which combustion chamber has more oxygen molecules available for combustion? The larger combustion chamber of the 8:1 engine.

The 8:1 engine can produce more BTU's per cycle, has more heat available to be converted to motion, generates more cylinder pressure and force on the connecting rod, and therefore can produce more power than the 10:1 engine.

If we run these two engines in this artificial world we've created, the 8:1 can take more psi of boost since it is not compressing the charge as much as the 10:1 engine.

The 8:1 engine makes more power under max boost than the 10:1 engine, while the 10:1 engine makes more power off-boost.

It seems to me, the real trick is to choose an SCR and a cam that can produce the optimal balance of off and on boost power for a given application (different vehicles used in different ways will result in different optimal combinations).

There are a lot of things to consider with the cam beyond the point at which it closes the intake valve and produces the dynamic compression ratio. Overlap is a bad thing when you have exhaust back pressure in the case of a turbo. Less of a problem with a belt driven supercharged application with an unrestrictive or possibly tuned exhaust. Too much overlap in a supercharged application and you will push part of the fresh charge out the tailpipe without being burned in the cylinder. Bad for mileage, and some loss of power since it took crank power to compress the wasted portion of the charge.

All power adders create more exhaust. What can the cam do to help this? More exhaust lift? Or increased duration in the form of opening a bit sooner to blow down and reduce pumping losses, but not so soon that the savings are less than the loss on the reduced power stroke.

The SCR/cam question is exactly what I ask in the following post. It's a tough question, probably why I'm not getting a lot of answers. I'm sure the fact that the post is really long, detailed, and loaded with a bunch of questions on a bunch of different fronts doesn't help either.

http://www.thirdgen.org/techboard/po...ld-vortec.html

Enter at your own risk....

Last edited by 87 Burb; 02-09-2008 at 04:10 AM.
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Old 02-09-2008, 02:09 PM   #36
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Re: Ideal Compression for Turbo

Theoretically a boosted engine with a larger combustion chamber has a -slight- edge on one with a smaller chamber, but realistically it's such a small difference in total volume that I doubt it would make a major difference. If you boost both to the edge of preignition neither is going to make significantly more power because they're both reaching the same cylinder pressure, except that the larger chamber'd one would have slightly more air. And then you need to take into consideration the extra heat and compression losses generated by pumping this extra air into the cylinder. Just different means to the same end.
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