I am considering putting a twin turbo on my engine... I dont really understand the way it affects the motor...I was wondering what would be a good compression ratio to begin with...would 10:1 be too high? Does it really matter? If it does raise the comp too high would i not be able to use pump gas? That is really everything I want to know...because I read that it does raise the compression...but I wonder why is it that other cars run high *** boost(like 14-18 psi) and still run on pump gas...I appreciate the info...

 

10:1 is to high for you

yes it really matters

it "raises" compression NO pump gas with 10:1

because thos other engines where built for turbocharging

 

A turbocharger is forced induction, which means that it forces more are into the motor than a naturally aspirated motor (no turbo or supercharger) , by increasing the amount of air that enters into the motor, you increase the compression ratio. the more boost (lbs of pressure forced into a motor) you run the higher your compression ratio will be. I would reccomend a 8 to 1 compression ratio.

The reason for a low compression ratio is detonation, the higher your compression ratio the more likely to have detonation. by lowering your intake charge tempature, you can force more air into a motor without detonation.

pump gas- gas is rated on octane, which is the tempature at which it completely burns. the higher the octane rating, the higher the burn point
so with a lower octane fuel you have a higher risk of detonation.

To give you idea of whats possible ive seen supras run 45 psi of boost with racing gas and a COLD intake charge. so 14-18 is easily possible

 

To elaborate on what bandit said;

its not actually the boost (14-15 psi or whatever) that increases the compression ratio. Its the added air.

In essence, if you have 10:1 comp, then the air/fuel that enters the combustion chamber will be compressed 10 times its original volume. Add twice or three times the air, and compress it the same amount, and the compression will go up. Even though the piston still travels the same distance, it now has more "stuff" to compress, thus a higher compression ratio.

 

Thank you for the info...It goes appreciated... would a cam selection matter? would it have a good ar bad effect if I get a big cam?

 

Another thing...what heads would you suggest...it is going on a 305...so its hard to find heads with larger chambers and 1.94/1.5 valves...

 

Ok im not a big turbo expert, but somethings i will suggest, just what ive observed, while being on here.

First off, turbo is not just something you throw on in an afternoon, and makes the car super quick. Its expensive, and has to be carefully selected to suit your need.single turbo maybe, but not a twin turbo, its pain staking to get all that plumbing right.

Secondly, if you have the cash to do a twin turbo setup (cuz with the questions youre asking, you definately dont have the knowledge to do it yourself), why would you do it on a 305? you may have the skill to do the welding, i dont know, but ill give you the benefit of the doubt, atleast you are trying to learn.

My point is, you cant just stick on some turbos over night and expect miracles, the cam, the heads, the pistons and the bottom end of an engine has to stand up to twin turbos. If you have the $2000-3000 to do up a twin turbo setup, spend the extra $1500 on a 350 core, get it machined, and spend a bit more on the proper pistons, cam, rods, etc....

 

Yes I know...I just wanna do a 305...because I want to...why does everyone say the same thing?...just answer my questions please...If you are gonna question what I wanna do...please...do not respond...just wanted to say that...

Now...would it lower my compression if I used pistons for 6" rods on a 5.7 rod? or should I just get thicker gaskets? because I am set on buying trickflow heads, but the ones with the 76cc cha,bers have 2.02/1.6 valves...so I am gonna buy the 56cc ones and try to do what I asked...or make some sort of spacer to increse the size of the combustion chamber...

 

We question what you want to do, because clearly you don't know much about what you're doing. We could tell you what would "work", and just follow along blindly, and you end up with some piece of crap that *works*, but doesn't do very much. Pistons go up and down but it knocks like crazy and only makes 4psi of boost.

Why is it the people who come up with the craziest schemes get the most defensive when someone questions their choices??? If you weren't asking for advice on your choices then why bother posting in the first place???

No, you can't use a piston meant for a different rod, good thinking, but it won't work.
No, you don't use thicker gaskets, they kill your quench and blow out easier.

You use bigger dished pistons (yes, you will need to replace the stock ones) and bigger chamber heads. 76cc chambers might be too big on a 305, 64cc would probably work nice. What's wrong with 2.02/1.6 valves? And don't tell me they're going to hit the block, or else i'll really know you need to do some homework.

 

I get a little frustrated because everyone says the same thing...I know a little bit of what to do...I dont know very much about turbos...I know that it wont be done over night... and I am going to get 10cc dish pistons...I can build the motor and do all the plumbing for the turbos and stuff, so I can build it...I just want to know what would work BEST for my particular motor...which is a 305...so please dont misunderstand me I dont want to sound like I know everything when I really dont, I just want to build something no one really wants to build...
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Oh and I also intend to heavily modify the bottom end...forged crank, hyper pistons, aluminiun rods...the works...

 

not to pick on you, but the reason no body does a 305 is because you will spend just as much money on a 305 as a 350. but not make as much power.
now this is what i would do, build on the 305(leave it alone), get your set up figured out, and see how fast you can go. then build a 350 right, with a forged bottom end, correct compression,and good valvetrain. if you just have to build a 305, then good luck.

btw what is the year and compression of your 305?
a carbed lg4 has an 8.6:1 compression ratio, which could be left alone and built on

also dont do hyperutectics pistons, a set of forged aluminum speed pros (although more expensive) will hold up better last longer and take more boost

just my

 

It is an 88 roller block...the block is just bare...the compression on those was 9.3 I believe...

 

9.3 and boost is a good combination. If you want to turbo your 305, then go for it - I boosted my stock V6 with good results on cast pistons and crank, and am now building a v6 (N/A) that will rape stock TPI's, then the turbo is going back on.

Can't express enough that tuning is the key. I have seen 30psi on cast pistons in a 10sec k-car (yes I said k-car), again with a good tune (obviously).

Junkcltr just finished a turbo 305 TPI in his pickup and had good results...

https://www.thirdgen.org/forums/powe...turbo-tpi.html

As already mentioned, take your time and do some research before throwing parts at the car.

Here is a good place to start...

http://www.turbobygarrett.com/turbob...ch_center.html

 

Thanks for the info...My goal is to make a good road race car...I am into drags and stuff but I like taking corners and stuF like that...that is what I do...Anyway thanks...

 

OK, lets get this clear. Noone here has picked on you or ignored you, you’re getting frustrated for people telling you like it is.

I’m going to be very blunt about this, not to pick on you but because this is brutally clear to those of us that do understand what you want to do and know how to do it. The problem isn’t “I dont know very much about turbos...,” based on the questions you’re asking the problem is that you know next to nothing about engines, how they go together, what the advantages and disadvantages of parts are. No one would choose hypereutectic pistons for this, it’s doubtful that most people would look seriously at aluminum rods… and these are things that you mention to show us that you know what you’re doing.

My suggestion to you is to either pay someone to do this right for you, or spend a few years learning about engines and how they work. Maybe blow up a couple of naturally aspirated engines and rebuild them in the process, so you know how things work together and what works for what. Parts combinations are much more obvious and forgiving for NA engines and bad parts combos for NA engines tend to run badly and you have some chance of swapping parts and getting something that works. With boost a bad combination tends to go boom and you have nothing useful left. It’s a harder way to learn.

As far as the 305… boost a 305, there’s nothing wrong with that. There’s a few people that have done that on this board, I’m considering doing that if a leakdown test shows that it’s healthy, I don’t have a clue why that is even debated.

Compression/boost… alright everyone, lets get this straight. Boost DOES NOT increase compression. Compression is just a calculated number based on the ratio of the total volume of the cylinder/chamber divided by the volume left with the piston at the top of the bore. And that’s static and dynamic compression, dynamic compression just compares numbers while the valves are closed and compressing is actually happening. Compression, especially dynamic compression gives you some idea of detonation sensitivity and increasing air density (boost) increases detonation sensitivity. Put A, B and C together in this paragraph and you have the answer to the cam timing question.

 

What a straight forward guy...you are right though...I do know next to nothinmg about engines, like I siad...I can omly put it together...that is why I ask...no worries though...
what a straight forward guy...

 

Im gonna piggyback on your thread here. Doing turbo sbc is kind of a dream of mine that I've put off until when Im a bit older and can afford to do it. I understand boosting a motor built to be naturally aspirated is generally out of the question due to compression ratios.

But I was thinking, would you be able to get around doing all the work to put together a new bottom end if you ran it on E85? The octane is a lot higher... seems like it might be an interesting work around to me since it wouldn't detonate nearly as easily. Obviously, you'd have to tune the thing anyway, so just tuning it for E85 wouldnt be that big of a deal. That doesn't account for ring gaps though. It's my understanding that motors built for forced induction generally have much larger ring gaps. But I see people turbo previously N/A motors all the time so maybe it's more of a "do it if you can, but if you cant do with what you got" kinda thing...? Or maybe that's why all those motors dont last all that long?

 

I dont know of anywhere to buy e85 around here...plus I really dont know if it will work...dont know much about anything like that really...

 

Dynamic compression is the most important thing as you said. With the 305ci I have been playing with it is very sensitive around 3200-3600 RPM. Exactly where the stock cam makes peak efficiency. I had to pull a lot of timing in that area to keep it from rattling. If it had a bigger cam it would be less sensitive and bring the sensitive RPM higher which would also make it less sensitive due to the higher RPM. A better combo. overall and also make more HP.

 

E85 has pros and cons. For a low budget race setup it is excellent. For a daily driver it is aweful. My opinion is to build a setup for pump gas. If you want more out of it at the track or on the street once in a while then bump up the timing and run E85.

 

Heh, sounds like you know _exactly_ where your peak torque (cylinder pressure, most sensitivity to detonation) is. The thing is that unless you’re running a lot of gear you probably don’t want to move that up any.

It’s interesting because it doesn’t entirely react like you would expect of something that is mostly alcohol. You’d expect it to burn cool and be much less detonation sensitive, and for the most part it is, but I’ve seen a bunch of people that start burning plugs and other parts when they lean on it hard, and find that they actually have to run cooler plugs with it then with gasoline, which doesn’t entirely make intuitive sense.