I know the more experienced and technical folks probably get frustrated when a lot of the same questions are repeated - so I always try to search and read threads (in this case, I searched and also read some of the threads listed in the Turbo Sticky).

My issue is more from lack of understanding and of course, the factors that come in with different builds and opinions.

So to get some reference specs out of the way - forged internals, 4 bolt mains, HSR intake, and rings originally had some gap for nitrous. 195cc AFR heads. Cam card - http://www.compcams.com/Company/CC/c...csid=1111&sb=0
HP curve starts to flatten and torque starts to fall at 4750 RPM.
Builder assures me the engine will safely stand up to 600 HP/TQ all day. Exhaust is pretty free flowing.
(last Dyno, which was with the carb, had the power at 371HP/381TQ).

Originally when my engine was built, the shop gapped the rings a little looser because I was planning on running nitrous. Since then, I've had people convince me that a turbo was the better way to go, especially since I had some extra gap on my rings already.

Now the tricky part is - my cam does a really nice job for me.

Since I have a pretty good initial power climb, I was going to get a bigger turbo that would take a little longer to spool up, so it'd be picking up just a bit before my curve started to flatten/drop.

The guy I was talking to (who did a beautiful job on modding his Grand National and turning it into a beast) said that it would be a waste of the turbocharger potential, and "would launch me to the moon" if my wastegate failed to keep the boost under control (as well as blow up my engine).

Is my approach dumb? I know it may not be "optimal" as far as getting the most out of the turbo, but I get really good power down low already.

Sorry if this post is a little disjointed, working a lot of long hours. If there's any other information anyone would like, please let me know - and thank you all for the tons of great information (and sorry again for my sometimes lacking ability to absorb it).

 

That cam should work pretty good for a turbo with the 113ls and 109ic. Im abit confused by your post about what your really asking

 

If i understand it right you want to keep your combo as is and buy a turbo but kida wanted it to spool later 4000's rpm range to follow your tq curve?

Not a bad idea except it doesnt really work that way. Once you add turbo to exhaust the engine sees it as a huge restriction until it starts spinning fast enough to pass the ehaust gas and make boost. So your power curve is somewhat killed "off boost" until turbo starts coming alive to overcome the restriction.

So i would size the compressor side of things to the hp you want to make. Then select turbine size and housing that will give you spool starting in the lower mid 3000's range so your full boost by peak torque rpms 4200-4500-ish. I'd place the stall speed around 3600-3800
A good electronic controller will control boost climb rate if you want a more linear power curve

 

Completely the wrong way to look at it. Your engine is not going to constantly run above 6000-RPM, and if you focus your turbo spool to come in just as your torque curve flattens and/or drops, just after 5000-RPM, you will have a small window of boost to utilize and that will defeat the whole purpose. You will want your boost to come in when your stall comes in, or if your running a stick, you will want it to come in when your cam comes alive. Boost will enhance your torque curve, substantially, and if you line up against someone who does it the right way he will leave you in the dust by the time your boost comes in. Don't think for a second that just because boost comes in later you will pull harder up top, because that will depend on your max RPM and cam specs, which again will be wasted because you won't be turning that high...

Assuming what you quoted above was FWHP, your not even in the elevens. Take a ride in an eleven second car under boost, not naturally aspirated, but boosted. It throws the concept of "peak" power right out of the window, and THAT, is good power down low....

 

Thank you both for the replies - I'm glad you both were able to understand my late night mumblings, thank you!

I've got a manual transmission, so my stall speed is controlled by my left foot.

I've tried reading a few compressor maps, and used the Garrett Turbo calculator, and here are a few of the turbos it suggested (that I was told would launch me to the moon):

GT4202R
GTX4202R (only different is a new billet wheel?)
4708r
GTX4294R

Orr89 - so you think my idea could work, I just need to make sure my turbo is spec'd out properly?

Do any of those turbos sound like a good 'starting point'? Or is the Garrett application giving me some wonky suggestions because what I'm trying to do is unorthodox?

Street Lethal -- thanks for you insight as well! I was typing up my post when you were typing up yours. Okay, so it seems like the better way is to go ahead and replace the cam.

I'll start digging around the Turbo threads to see what cam has worked well for other guys with hydraulic rollers, and bounce off questions in my thread here, help keep my questions contained to one place.

Thanks again!

 

So I played around with Comp Cams cam selector today (which I know is pretty basic), one of it's options is picking if you're using a forced induction setup... and it kicked out my cam as an option! lol

The other one it suggested (as a "Great fit" versus mine which was a "good fit" is this one:

http://www.compcams.com/Company/CC/c...?csid=177&sb=2

So now I'm working on educating myself on cam specs.

The new cam it suggestions also has "9.5 to 1 compression" in the description -- is that saying that I should make sure my compression is not higher than that? (currently it's 10:1).

Why would the cam be concerned about compression (excluding the increased cylinder pressure from the forced induction)?

 

I am kinda on Street's side that you dont want the turbo coming in too late.
I would have it come in slightly below peak trq, right where a converter would be or in your case right where the cam starts coming alive which should be upper 3000's rpm.

It will enhance trq curve alot so have turbo follow the cam curve to a point, it just has to be spooled and making boost before you hit peak trq.

And i wouldnt necessarily change cam. Theres nothing wrong with yours except it does need good valvesprings when you turn it 6000 rpm and then add boost. Need some good pressure, probably 150-160 lbs seat, 400-420 open like afr 8019 springs

Comps cam selector sucks by the way. That xfi292 is a giant cam. It wont run well at all in your motor. Great fit?! Shame on comp

 

Thanks! Good news is, I now have 8019 springs in there (you and others had helped me out in another thread sometime back - https://www.thirdgen.org/forums/tech...estions-w.html ).

So sounds like at this point, I just need to work with a turbo supplier to get me the right sized turbo, and all the little things that go with it!

I see a lot of folks talk about the different "sides" to the turbo, but I can't seem to quite follow how they arrive at the conclusions, like if they say something should be changed to increase or decrease the A/R.

Is there a set of questions or plug-and-chug equations I can use to figure out which turbo/specs would kick in around 3000 RPM, and give me the power range I'm looking for?

 

You can size the compressor easily if there are maps for them. Equations work well.

Turbine is harder. Not many manufacturers have maps and then you need pressure ratios which you dont really know yet but can kinda guess at

So turbine side is kinda a guess based on experiences and backpressure readings

 

Working on reading compressor maps -- I found a page that has been pretty helpful with walking through the math:

http://www.enginelogics.com/read-a-t...ompressor-map/

 

When working through some of the math here, should I be using the estimated BSFC for with the turbo, or when it's naturally aspirated? (for N/A I've been using .43, and from what I've read, for turbocharged engines, it's around .55-.60, so I was using .55).

This factor seems to change pressure ratio enough to change which turbos I should look at, so just want to make sure I'm using the right figures.

 

Use the turbo stuff .55-.60 ish should suffice

 

Thanks! Okay, so I'm nearing the end of things here! Hopefully last question. That article says "f you are trying to choose between 2 turbos, pick the one with the better efficiency where most of your driving is done."

I'm taking this to literally mean "If most of the time you're cruising at 3000 RPM, that's where your focus should be for getting right in the 'bullseye' of the compressor map".

When I do that, the higher RPM points wind up pretty far to the upper-right on the map for most turbos, and well outside the maximum efficiency.

The exception to this is the Garrett T80, RPM 3750 through 5625 are all in the highest efficiency, but that's 72% (compared to 78% of other turbos).

The next best map looks like the GT4708. At 3750, it's at 77%, then up through 5625 RPM it's all sitting at the max efficiency of 79%

So, unless I've really goofed up the math someplace, does this sound right/make sense?

My corrected airflow (pounds/min) works out to be:
21.1 @ 3000 RPM
42.3 @ 3750
69.7 @ 4500
66.0 @ 4750 (I think I did the math right, but don't know why the lb/min went down)
80.4 @ 5625 (annnd it jumps up - maybe because of possible intake charge temp changes and V.E.?)

 

what boost PSI did you use to calculate flow. If it wasn't the same it's plausible that's part of your issue calculating. The fact that flow goes down at 4750 and up again after that indicates you've got something weird with your VE (thereby throwing off your calculations) or that you've got an error of some kind.

I think you're somewhat mixing up the idea of a belt driven centrifugal supercharger compared to a turbocharger. The reason I say that is the higher RPM you go with a centrifugal the more boost it tends to make, so you end up with a torque curve that some what artificially "extends" the RPM range.

Honestly, this may sound weird but something in your combo doesn't add up. AFR 195s are a significant amount of head, that's not a horridly small cam and you've got decent compression and you're making somewhere in the neighborhood of 350hp at the flywheel. You should be making a significant amount more power than that IMO. Have you done any work with your timing yet? Any signs of ignition issues or valve float? What about cam timing? Are you running an exhaust system that may be causing you issues? I would skip the turbo until you figure out if you've got a decent tune.

 

I took his power figure to be rear wheel hp so its in line with xfi280/195 head combos

Anyway need to size where you will do most of your boost driving, not general driving. At cruise at 3000, you better NOT be in boost lol else that is one tiny turbo

My car for example. I can run anywhere from 6-7 psi to 25 psi. Thats 400's whp all the way to 1000 whp. Huge different in optimal turbo for those ranges. So most of my driving will be 14-15 psi on street. Comfortable 700's whp. So i would check size on that parameter.
Now my end goal was well over 1000 hp so i need a turbo that will support that. So i check that range as well.

 

Thanks for the information everyone! Yes my power is what I've got at the wheels.

Draconic, thanks for pointing out something I overlooked -- the expected boost being different! At 3000 RPM, the expected boost is only around 10 PSI, for everything over 4500 RPM, it's all at 20 PSI. That definitely explains some of the other variations.

Orr, thanks for the clarification! So I really want to make sure those 'sweet spots' are up where I'm at or near WOT, so the 4000+ area?

 

Yeah estimate where you want turbo to start spooling and the lowest rpm you expect to see full boost

My setup may see boost start at 3600, and full boost by 4000 to 4500 depending on amount. If lower like 12-15 psi it may be all in by 4000-4200. If 24 it takes alittle more time to build so may be in by 4500-5000 as converter stall changes slightly with more torque from more boost. I hold that boost to 7000.

You want to do the calcs at both points, the shift point and starting boost threshold point

 

Thanks again for all your help! It definitely looks like the GT4708 is going to be the way to go for me, not only does it line up well with the efficiencies (assuming I did it all right), but it seems it'll be able to offer plenty of power growth in the future.

After 4k RPM, the PSI is stating 20.1. At that point, what sort of other things should I take into consideration?

My current compression is 10:1, how do I figure what I should drop it to? (if at all).

Thanks!

 

Pump gas i'd say atleast 9:1. Race or e85 may be fine where you are at

What power you goin for again? 20 psi is alot

 

Thanks Orr,
I though 20psi was high, which is why I wanted to ask - I thought maybe I goofed up some math someplace, or maybe the math on one of the pages I was reading/using is making assumptions about some factors, which is causing the PSI to be higher than it would be in practice.

I have no issues with running premium, though most places around here that's 91 octane, sometimes luck out with 92, 93 is rare.

For the moment, I'm looking for 600 at the crank in it's current incarnation; that's what the engine was built to safely handle (with use of nitrous, which is hard on engines, so it could probably take more of a beating, but I don't want to push my luck yet).