SBC TBI build question

[turbows6]

I'm rebuilding the engine in my 1987 3/4 ton suburban. It has a 350 TBI engine, and I was wondering if anyone on here has messed with TBI before. I would like to put a flat top piston and run .040" piston to head clearance to take advantage of the quench in the closed chamber head. This will raise the compression from 8.3:1 to 10:1. I am a little concerned that may be a little high for an iron headed engine that will be used for towing occasionally. I have considered running a "computer compatible" camshaft that 206*/212* that's around 10* more to bleed a tad of the compression off, and lowering the thermostat from 195 to 180, and only running premium gas when towing.

Do you think this is too much compression?

 

[K5Cruiser]

I had an 89 Suburban 3/4 ton 4x4 with a TBI 383. The compression ratio in it was somewhere between 9.5 to 9.7 with iron heads. I always ran premium and didn't have any problems. 9.5 is about the highest you would want to go. I think 10 would be pushing it unless you wanted to splurge and get some aluminum heads.

I would suggest you talk to the guys at turbocity, I believe the guy who is knowledgeable about the TBI stuff is Tom, but can't remember for sure. There website is turbocity dot com.

 

[mikestertwo]

10:1 is a little high. I would keep it in the low 9's. I did a similar thing to my 3/4 ton pickup. I used a cam close to the one you are considering but I converted to TPI. I also changed the heads to heads with 1.94" intake valves. The engine makes a lot more torque. I tow a single car trailer with no problems using regular gas.

 

[turbows6]

I just ask myself this question, Is it worth running more piston to head clearance to lower the compression only to loose the advantage of the closed chamber heads? I have read that much over .040" clearance and you are effectively back to an open chamber configuration which is more prone to knock and it will loose torque and fuel economy. It's a catch 22 in my opinion because you lower compression = less knock, but open chambers = more knock. There are also constraints on cam selection due to the factory speed density setup being sensitive to changes that effect vaccum. From the little research I've done you pretty much have to burn your own chips if you want a good one. (We have it damn good here in the turbo Buick community when it comes to chips) It's a 2 wheel drive, so it's a little lighter than a 4x4. I think if you were running 9.5:1 in a 4x4 I could probably get away with it.

I always have headers and free flowing exhaust to fall back on also.
On a side note it is easy to work on one of those things! I didn't even have to get jack stands out to pull the engine lol.

Maybe I should buy an alcohol kit for it :tongue:

 

[mikestertwo]

There are also constraints on cam selection due to the factory speed density setup being sensitive to changes that effect vaccum. From the little research I've done you pretty much have to burn your own chips if you want a good one. (We have it damn good here in the turbo Buick community when it comes to chips) It's a 2 wheel drive, so it's a little lighter than a 4x4. I think if you were running 9.5:1 in a 4x4 I could probably get away with it.

That's why I converted to TPI with a MAF. It's not sensitive to cam selection and is easy to tune. I use TunerCat to burn my own chip. IMHO I would go with a better flowing head (TBI heads don't flow very well) and keep the compression down in the low 9's

 

[turbows6]

I don't really care if the heads flow good numbers honestly. Under 3000 rpm they are actually good torque heads, but they practically nose over after 4000 . I just don't have the time and interest to start burning chips for it. I just want it to be reliable and get the best fuel mileage possible and make a little more torque and power than stock. It was just time for the motor to come out. The last time I drove it far was a day trip to Iowa to pick up a car. Round trip was around 550 miles. It burned a CASE of oil on the trip

 

[TNTBuick]

I don't have experience with TBI engines, so this may not be applicable to your application, but for a sbc n/a engine ideal quench is about .040. With a .040 gasket, deck to piston clearance would be close to .0. I have a n/a 383 ready to assemble and had instructed the machine shop to cut the deck to .0 but they, probably wisely, left .002. I think detonation problems start at about .048 to .050 quench.

Ideal dynamic compression ratio for pump gas is probably between 8.3 and 8.5. A static compression ratio of 10.0 may be high for the short cam you are considering, depending on the rest of your combination. United Engine and Machine Co., manufacturers of Keith Black pistons, has a calculator at their website, kb-silvolite.com. where you can "what-if" and find the best combination. HTH. Good luck with your rebuild.

 

[RmvBfrFlght]

. . I just don't have the time and interest to start burning chips for it.

In this case leave the engine as stock as possible. Same cam, heads, thermostat (!!!), compression the way GM built it. Otherwise the tune will be a compromise.

As for your question about compression ratio, those heads are most likely swirl ports. Check for a ramp behind the intake valve in the bowl area. Swirl port heads look to have half the bowl blocked off.

If they are swirl ports the compression can be bumped a little. About 9.25 is OK. The reason is that the chambers burn very rapidly. That reduces the ignition lead requirement dramatically. Which in turn reduces the chance of detonation, there isn't time for it to form.

And with swirls quench isn't as important at it usually is. The ramp takes care of setting up the chamber turbulence.

RemoveBeforeFlight