DirtyDaveW

On my way home just now, I was wondering about this. You guys are going to think/know I'm stupid on this topic but I wondered, "If my T76 is at 18 psi and I had a 88mm at 18psi, why would the 88 be any better?". Now I know from the ET/MPH of the 88 guys that a 88 equipped car can clobber a similar 76 equipped car, but....why? If the 18 psi is the same? Is it the recovery ? Similar to the difference of a small paint type air compressor versus one of those refridgerator sized compressors? It might take a 120v cheap air compressor about 30 seconds of running to build back up to 90psi but the 240v monsters might take 5 seconds if that.
Just curious.

Kevin Blown 95 TA

Maybe the air isn't as hot with the larger compressor so it helps in that regard.

96capricemgr

I don't really know anything about turbos and I suspect there are cases where your observation does NOT pan out but I do know different impellers have different efficiency curves(maps) and I would bet the answer lay somewhere in there.

As Kevin said boost heats air, but the other side of it is to make boost there is exhaust backpressure which could play into things as well.

A friend runs a speedshop and he is getting into turbo builds, I have just observed a little when I am in his neck of the woods. I have never actually worked on a turbo car just like to understand how things work.

Kevin Blown 95 TA

Yeah, you have to look at the compressor maps and see where your airflow is in relation to impeller RPM and boost and see where you fall in the efficiency zones. If you take a small blower and run it out of it's (adiabatic) efficiency zone it can maybe still make as much boost as the larger blower, but it will heat up the air and with the air being less dense, you're not really getting more air, and then you have to cool off the air to stay out of detonation so it shrinks back down. So somehow compressing more air and forcing it into the engine without heating it up too much is the key. You need to stay out of the surge zone, too, or else it can be hard on the blower, so you can't just pick any blower configuration and have it work. I'm no turbo expert as you can see, but a lot of what is true for for belt driven blowers is true for exhaust turbine driven blowers.

engineermike

Little of what those guys said plus. . .

For starters, the 76's efficiency maps best match a much smaller motor than yours. Sure, it'll make great power on a 355 or 383, but it'll be more efficient and make more peak power on 200 cid. The 88 compressor is a much better fit. For example, a 350 cid engine making 800 hp at 18 psi will be running the T76 at about 60% efficiency (efficiency drops like a rock when you approach choke flow - or sonic velocity in the compressor inlet which flow can not exceed). A 200 cid engine flowing the same amount of air will put the compressor at a much happier 70%+ efficiency. A T88 on the 350 at the same flow rate would be in the 78% range.

So what does efficiency do for you? Probably more than you think.

1st, the thing that everyone knows, is that poor efficiency increases charge temperature. High temps are bad for power, as everyone knows, but you can offset this some by intercooling.

2nd, poor compressor efficiency means it takes more power from the turbine to drive it, for the same boost and air flow. More power from the turbine means more turbine pressure drop required. This, of course, means more exhaust pressure on the engine, more pumping losses, and less power.

Finally, the turbine wheel and snail on the T88 is much larger than the T76. This translates into higher mass flows at lower pressures to achieve the same turbine power output to the compressor. As mentioned above, lower pressure driving the turbine leads to lower pumping losses and more power.

Mike

DirtyDaveW

Perfect! Just the kind of info I was hoping to get! Thanks,
Dave

DirtyDaveW

Thanks Kevin.
____When I first contemplated getting a turbo for this car I had a 383 solid roller. Unfortunately, the guy that machined and assembled it did a half *** job (though he's highly respected/well known and has many superstar motors to his credit). After that motor came apart at 7,000 miles (waited 2 yrs for it and a few thousand dollars, sigh..) I started looking seriously into the turbo build. I read a lot of posts here and elsewhere but my head was spinning regarding efficiencies and compressor maps. So, based on several positive posts, I contacted Jose at ForcedInductions and gave him my specs and goals. He recommended the TC76, which I purchased right after that, from him.

____I started discussing the build with a friend in Houston(used to moderate the LS1tech.com board) and he, like Injuneermike, said he felt the 383 was too big for a T76 so I traded the damaged 383 and some cash for a Jeep for my daughter and bought a stock bore 93' LT1 block, all the forged internals and four bolt main caps. Seeking to keep the displacement of the engine smaller to help the T76 stay in a better efficiency range.

____I'm torn now after reading InjuneerMikes post. It appears the 88mm would be a better choice but the thing is, I'm already at/beyond the limits of traction with my current setup. My car is primarily a street car with hopes of getting to the track once a month. I'm going to step up to some ET streets this year. The Nitto DR's let me have 'some' traction on the streets but they're hopeless at the track, once I get to the higher boost levels. I'll have to see what the economy will bring this year. If it's good, then my inner-child will probably demand the 88, lol....

96capricemgr

I can guess who built your engine and the "superstar motors" is just all part of the myth that let him rip people like you off.

If you actually look at what he put out, the level of effort and expense put in and then compare it to other setups it is not impressive. People drooled over his stuff when a belt drive timing/distributor converted heads/ intake pure unlimited budget lite weight racecar on E85 runs SLOWER than another guy does in a heavier car with ported OEM LT1 heads and intake with an opti and $30 timing set. Nobody wants to hear about a car going fast on OEM stuff though because it doesn't fit into benchracing "knowledge".

engineermike

First of all, it is true that smaller motors will make more max power with the T76. People tend to blow this way out of proportion, though. I doesn't even mean that a smaller motor will make more power in your setup. For instance, I did exactly what you did. . . started with a 383 then downsized to a 360 to match the turbo better. I actually lost power and gained lag. I went right back with a 388 and liked it much better. You see, if you limit your boost level for reliability or fuel reasons, you'll actually make more power with a larger motor. It's when you start max'ing out the turbo that it makes more power with less cid. For example, at 23 psi boost, a 383 might make 950 hp and a 350 make 1050 hp, both limited by compressor choke. However, at 12 psi the power is more related to the NA flow of the engine, so a 383 might make 700 hp and a 350 make 650 hp.

I ran 9.90 at 146 mph in a full weight+ 95 Z28 using a 388 and a T76GTS

T76 is a lot of power in a very small package, plus it spools up fast. The T88 has much larger wheels in it and will spool up much slower. Also, I've been told that the T88 has an old-style Garrett straight blade turbine wheel that spools up even slower still. And finally, the T88 is a much larger "medium frame" that may not physically fit in your location, plus it requires a larger downpipe.

DirtyDaveW

Hi Dwayne, You and I had discussions in the past on cams, etc but I think you're under the impression I'm talking about Bret. If so, I'm not But perhaps I pm'd you to tell you whom had built the 383 solid roller? Just wanted to be clear, I'm not talking about Bret. All he's done for me was recommend my current cam.

Thanks guys

DirtyDaveW

Aha.. thanks for the clarification. Lots of useful information here.

RealQuick

Dave, if the current power level isn't enough, my recommendation would be to step up to a T76GTS if you don't want to modify the turbo kit. The kit was designed around that turbo anyway, so it will fit. Only change is the compressor outlet is 3" instead of 2.5" (simple silicone coupler change). The T76GTS will make ~100rwhp more than the TC76.

You still haven't maxxed your current setup out yet, so I'd be interested to see where that is.

Jon

DirtyDaveW

Thanks for the tip Jon. Right now, I'm redoing my crossover to get some more ground clearance and relocate the O2 sensors. After the routine temps start staying in the 50's I'm going to install the ported TFS heads and ported intake.