I've been wanting to do this for years, and I'm finally ready! Here is my project... Its a 1988 GMC Jimmy (fullsize). It has a tbi 383 with vortec heads. The engine has an Eagle stroker kit in it with Keith Black pistons, zero decked block, a 203/210 roller cam with .480 lift, ported and polished vortec heads, an RPM Air Gap intake with a homemade adapter to run 2 tbi units. It's about 10:1 and runs great on 87 octane, maybe 325 hp. I run it to about 5800 rpm. Tuning is done with EBL with the twin tbi mod. I've been running this engine in some form or another for about 15 years, and in its current iteration for maybe 4 or 5 years.

My questions are:

What header setup? Ebay turbo headers, shorties or block huggers turned upside down, custom fabricated, or something else? I'm medium skilled on metal fabrication, but I would be really slow trying to fab a set of headers... but I can if it's best.

What turbo? I know, probably the most common question... I was originally thinking ebay twins, but I decided I'd rather run a single brand name turbo (single for cost savings). At 10:1, I've got plenty of punch off the line, so I think I'd rather err on the large side, but then again, I have no practical knowledge.

Intercooler? I've read that carb/tbi turbo systems cool the intake charge quite a bit with the fuel. Can I get away with no intercooler for my first iteration? Is that foolish?

Camshaft? At the least, I'm going to pull the motor to put a new set of gaskets in it. I've considered putting in a different cam while it's on a stand. Any suggestions on this? When I put the heads on, my machinist used a valve spring setup that he said is good to about .525 lift.

Ring gap? My rings were gapped for NA, but have a lot of miles on them, and there's no way I'm going to be running a ton of boost at my compression. Can I leave them be, or is that foolish to not re-gap them?

Thanks in advance for any advice yall can give me! Feel free to answer the questions I neglected to ask too, lol!

 

Desired hp will determine everything, from turbo size to whether intercooler will be needed.

 

You could just run a set of cast manifolds on it, good heat retention, tons of outlet options for the SBC. Could also find a set of quality forward facing turbo manifolds. There is a sticky on the top of this page regarding cheap 304 SS.

S475 would be a good fit on this engine, good to at least 850hp, will be responsive enough and still work well at lower boost numbers. You can geta more specialized recommendation with more detail.

Intercooler will depend a bit on boost, they are always good but with a carb you should be good to at least 8psi.

A little bigger cam would be nice, but that is a fairly heavy vehicle so its good to stay modest. You didn't mention trans or gearing. The turbo is going to broaden your usable top end a little bit, but matching the cam is a good plan.

What is your ring gap at now? You really want 0.006" or so per inch of bore on the top ring, a little more on the second. Turbo engines make a bunch of heat, better to have a little excess here. If this is a fresh engine built for NA it may be as tight as 0.004" and that is too tight.

You will have to say goodbye to 87 octane, you might get away with 89 on low boost and an intercooler, but you will very likely have to run premium.

 

Thanks for the replies!
bbi, I'm hoping for 500+ hp. I figured 10 psi is the most I could get away with on pump gas (obviously don't really know though), and the math says I would end up with a little over 500 hp.

Ben, I'll check out the sticky on stainless, and look at what's available for a reasonable cost. As for trans and gearing, I'm running a 700r4, 4.10 gears, and 35s. I've still got plenty of grunt with this setup, but wouldn't want to sacrifice any of it. I was thinking possibly slightly more duration and lift with a tighter LSA to preserve the bottom end. We'll see... I dug up my piston instructions, and it says for street NA, the ring gap factor should be .0065 which is what my machinist said he set it up as, and as I said, I've got about 15 years on it (50k miles?). I've already prepared myself to run premium gas. That'll be nice at 10-12 mpg right now, lol.

Does 10 psi and 500+ sound doable? Intercooler at that level? Thanks!

 

Ok, 325hp @ 5,252rpms = 83% ve. 10psi out of the turbo with 1psi pressure drop through the air filter =1.73 pressure ratio (pr). 1.73pr compressed at 75% efficiency gives us 150°f out of the turbo. I normally figure 70% efficiency for an intercooler, since you'll also be evaporating the gas, I'll up the total cooling efficiency to 80%, giving air intake temp at the ports of just 78°F (on a 60° day). That gives us a density ratio of 1.53. 325x1.53=497.5hp, 52lbs/min. So you need a turbo capable of moving 52lbs/min at 1.73pr, a Garrett gtx4202r 76mm will do that at 76% efficiency, now for turbine side.

Assuming 1500°f egts, exhaust corrected flow with drive: boost ratio 1:1 is 58lbs/min. Turbine expansion ratio required to power comp at 1.73pr is 1.35. So turbine side needs to flow 58lbs/min at 1.35 expansion ratio.

Looking at gt4202r turbo, turbine side is way too small. Gt47 turbine fits perfect. In this case you can use a Garrett gt47 off a caterpillar c15 with a 1.42 housing, good thing about these is they are cheap, I bought an almost new one for $150, and they are heavy built and durable. An added bonus is it will be wastegated in case you need it.

Sizing the turbo this way will be a "laggy" turbo, letting the engine make the brunt of the power down low. Once around 3,000rpms turbo will start to come online and follow the power curve of the motor, while keeping drive:boost ratio 1:1.