LT-4 Mods

[SJ LT-4]

I recently had some work done to my car. My plan was to have the heads ported, put in a LT-4 hot cam, port the intake manifold, and add a 58 mm throttle body. I already have a Borla exhaust system, Hooker short headers from Lingenfelter, forced air system through the font license plate, K&N air filter, Stainless pipe (where the cats once resided), centerforce clutch and 373 rear end.

Once the engine was opened up, I was called to the shop to look at the engine. The bearings were worn both in the crank shaft area and the cam area. This was a manufacturing defect. The engine only has 45000 miles on it. It was too late to take it to the dealer, sooo... I had the the engine machined true, odered new bearings, had the cylinders honed, put in new rings, and a new timing chain, new plugs & wires, while I was at it.

When finished, I was surprised at the dyno readings. My HP at the rear wheels was only 305, which nets out to be around 385 at the engine. The Torque at the engine is about 398. I was told I would have about 420 HP at the engine (Lingenfelter told me the same). The mechanic couldn't explain it. He said he performed the same work on an LT-1 and got about 420-425 Hp. He said the only difference was that the LT-1 had long headers vs. the shorties that I have. The other part is that the dyno ran my numbers assuming that I had a stock 342 rear end, instead of the 373 that I actually have. I don't know how much HP difference that would make, but I plan on getting the dyno readings done again, once I break in the engine. I appreciate any comments.

 

[Shriker]

Generally , stock LT-4's should be in the 290-300 rwhp range. With all the stuff you had listed , your car should be putting down at least 335+. The 3.73 gears will show less hp on the dyno (maybe 3-5 rwhp) but not that much less. The other mods you have listed should result in over 40 rwhp , so even if your baseline stock car made 290, it should now show 330.

I have seen a few LT-4's with long tube headers/catback, Hot Cam, lid/filter, and a good tune make 345-350 rwhp (with stock 3.45 gears). You may want to get a good custom tune, that can make quite a difference. Also , I am sure the car needs to break in a bit as well.

Was the dyno a dynojet? Were the numbers corrected? Did they do before/after pulls ? How have other stockers dynoed on that particular dyno , like other LT-4's or LS-1 Fbods and Vettes?

 

[Edmond]

Don't forget about that ram air. You just can't measure how much of a benefit that ram air system will be on a dyno unless he has a fan blowing air into it. Am I right?

 

[bob eshleman]

lt4-mods

Hi all is well except fot the 58mm throttle body. The lt4 5.7 350 cu.in isn't large enough for it.The 58mm needs 1000cfm. The 52mm is designed for the 750 range to match the intake.The58mm has max air flow @3/4 throttle/ very low velocity @ lower rpms. ohoracing.com has a complete site on mods.good luck bob.

 

[Springer's LT4]

Hi all is well except fot the 58mm throttle body

;stupid


I agree, 58 mm TB is hurting you more then helping. Even a 52 mm on that motor is a bit much. You'd need to be turning much higher RPM's or have a stroker before you'd gain anything from a larger TB.

My LT4 with just a Hot Cam and some basic intake porting puts down 337 at the rear wheels.

 

[No Go]

I would say the 58 mm TB is not helping you, but would not say it is hurting the combination.

It ain't a carburetor, just an air valve-velocity isn't a factor with EFI in this case.

Heck, Holley sells a single plane EFI system for 250 HP engines and it comes with a 1000 CFM TB!

Only thing a TB can do is restrict.

I would worry about the local porting job and/or the quality of the rebuid. LT4 heads are not LT1 heads so maybe something was done that wasn't helpful to the flows. Or maybe you now have too high compression if they decked the block or heads (already at 10.8 stock) knock sensor maybe pulling timing out.

Custom programing/dataloging is required with the changes you have made and to get full potential.

Good Luck!

 

[bob eshleman]

no go

HI what is your currant status with your project?It is not likely that any reptable rebuilder has done any damage but you never know. Go ahead ask about comp ratio and timming specs.How extensive was the port work and how about the intake?There is a lot in the heads and intake if cnc work is done properly.I expect you have already set you TPS and verified proper voltage.I still suspect the 58mm TB. How can one expect proper calibration if you have max air flow at 3/4 throttle?Please send any outher info that can be of help to me by my email and perhaps we can work it out together. thanks bob

 

[sothpaw]

springer, what porting?

Did you mean ported cyl. heads and hot cam? Or are you speaking about a ported intake manifold?

If you have a specific performance part you're happy with, I'd love to get a reference.

 

[Springer's LT4]

I've got hot cam with a ported intake, cylinder heads are stock.

 

[UB2 SLOW]

I would say the 58 mm TB is not helping you, but would not say it is hurting the combination.

I disagree with you. I started off with a 58mm and had to go to a 52mm. The car was really sluggish at the lower rpms with the 58mm and the car dynoed about 2hp less all the away across the power curve until about 5700rpms. After 5700 It made about 3-4hp more than the 52mm. The high end gain was not enough to compensate for the low loss. The tq curve also had the same results.

 

[CKA_Racing]

Re: lt4-mods

Hi all is well except fot the 58mm throttle body. The lt4 5.7 350 cu.in isn't large enough for it.The 58mm needs 1000cfm. The 52mm is designed for the 750 range to match the intake.The58mm has max air flow @3/4 throttle/ very low velocity @ lower rpms. ohoracing.com has a complete site on mods.good luck bob.

Wasting your time with a 58mm unless you plan to punch it out to 383 or 396 or above later.
Not saying it's bad at this point since you have it already.