LT1/396 just noses over at 5K

grumpyvette

Administrator
Staff member
396 LT4 evaluation
Ok so I just purchased this car. Its a 96 LT4 that has been built completely, my main concerns are weather all of these parts are a good match, I'm no engine builder! I've been having some issues since day one and bought the car knowing it had gremlins. I've been thru just about the entire ignition system and I'm about to tackle the fuel system, but before I do I would like to know if the injectors I have are appropriate for my situation. The car runs amazing until about 4.5k rpm's where it just falls completely on its face, if I grab another gear, we are off again until 4.5k where we start the whole thing over again. If i ease through the rpms it goes just fine thru 6k (as far as I was willing to take it). It only happens under heavy acceleration. The car is displaying no codes and has good fuel pressure. Could it be injectors, I have the receipts from them being installed in 07 previous owner had every receipt from forever ago. Also I'm curious about whether or not a "tune" to the computer could be causing this problem.The engine now has Accel 150130 30lb injectors installed can anyone tell me if these are an acceptable unit for what I have? If I were to replace them what would be your suggestion?

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Every part on the car, including the tires appears to be new, although every receipt is dated around 07 and 08. I don't think the car ever worked properly and as a result has sat parked for an extended period of time.

Someone with more knowledge than me please chime in here.
Thank You.





As always testing and dealing in prove-able facts helps,

youll want to pull trouble codes,
and verify ALL the sensors are working correctly
guessing seldom works
.
IF the MAT sensor, or heat sensors ,or any other sensors not tested and proven to be functing correctly its suspect.

http://garage.grumpysperformance.co...lay-switch-locations-and-info.728/#post-43477


http://garage.grumpysperformance.com/index.php?threads/c4-c5-corvette-trouble-codes.2697/

http://garage.grumpysperformance.co...ard-starting-tpi-crossfire-or-lt1-vette.1401/

http://garage.grumpysperformance.co...p-after-running-out-of-fuel.11309/#post-51444

obviously I'm not there to test but it seems like a likely suspect is that the injectors size limitations are limiting fuel flow at near peak rpms
looking at the engine posted specs Id think 470-500 hp was a reasonable expectation for fly wheel hp.

a few quick calculations suggest a 38 lb-42 lb flow rated injector set might be a better match, obviously a bit of testing with a fuel/air ratio scan tool might be well worth doing to prove or dis-prove the need but generally if your trying to get near 500 hp you will need a predictable fuel flow and that generally required a larger than 30 lb injector size.
running injectors at over 80% pulse duration generally causes problems
ID also point out that a restrictive exhaust system mounted behind the headers or a partially clogged catalytic converter or muffler would also cause similar results as either will restrict flow as the volume of exhaust gasses increases with rpms , youll also want to verify the valve spring load rates and clearances, if something is close to binding or the valve springs are not maintaining the desighned load rates youll loose valve train stability at lower than intened rpm levels.

With a Hydraulic Roller Camshaft: 130-140 lbs Seat Pressure/300- 355 lbs open , the common load range on valve spring pressure.
TESTING BEFORE GUESSING HELPS
Id strongly suggest you put the car on a dyno to confirm, & verify the exhaust back pressure in both headers collectors, verify your fuel pressure stays at 42 psi, stock fuel pumps can produce the intended 42 psi at rpms below about 5K rather easily but frequently can't keep up with demand at higher rpms, you may need to upgrade the fuel pump and maybe the line size and fuel filter and get a print-out showing your fuel/air ratio every 500rpm, on both steady state and under hard & rapid acceleration.
ID make darn sure they also test for and print out the plenum vacuum, exhaust back pressure, fuel pressure , air temperature, coolant temperature and ignition advance curve on the dyno run, the fuel pumps rated flow ,means very little its the pressure and volume reaching the injectors under load at high rpms that needs to be verifiied
Fuel-Pump-Flow-Requirements.jpg

http://garage.grumpysperformance.com/index.php?threads/replacing-a-c-4-fuel-pump.33/
http://members.shaw.ca/corvette86/FuelSystemDiagnosis.pdf
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http://www.thecalculatorsite.com/conversions/liquidvolume/liters-to-gallons-(us).php
A common upgrade that frequently works on C4 corvettes but its marginal on a serious race engine in my experience, especially if used with stock fuel lines and filter.
http://www.summitracing.com/parts/vpn-gca758-2/overview/
heres a pump I find works well with AN#8 lines and filters
http://www.summitracing.com/parts/aei-11169


EXAMPLES

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http://garage.grumpysperformance.com/index.php?threads/calculate-fuel-injector-size.1200/

http://www.summitracing.com/parts/acc-150840/overview/

http://garage.grumpysperformance.com/index.php?threads/is-backpressure-hurting-your-combo.495/

RELATED
https://www.cantonracingproducts.com/Trackside/PDF/street_thunder.pdf

http://www.grandsportregistry.com/dyno.htm


IT RARELY HURTS TO BUY A FEW REFERENCE MATERIALS OR ASK QUESTIONS

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the basics of engine performance stay rather consistent

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set the float levels and verify the fuel pressure consistent at about 5 psi entering the carb inlet port.
your fuel/air ratio should be in these ranges
Idle- up too about 2500 rpm try for 14.7:1-15:1 f/a ratio
from about 2500 rpm- too about 4500 rpm try to smoothly and predictably transition the fuel/air ratio mix richer to about 13.5:1
from about 4500 rpm- too about 6500 rpm and higher ,try to smoothly and predictably transition the fuel/air ratio mix richer to about 12.5:1
this is only a starting point on the tune but it generally gets you in the ball park and tends to reduce the chances of the engine reaching detonation conditions.
the ignition advance curve needs to be checked, the chart below is a very good starting point to work from

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set the plug gaps at about .045, make sure the valves are adjusted correctly

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useful related info

http://garage.grumpysperformance.com/index.php?threads/c4-c5-corvette-trouble-codes.2697/

http://garage.grumpysperformance.co...ard-starting-tpi-crossfire-or-lt1-vette.1401/

http://garage.grumpysperformance.com/index.php?threads/replacing-a-c-4-fuel-pump.33/

http://garage.grumpysperformance.co...-system-trouble-shooting-flow-chart-info.596/

http://garage.grumpysperformance.com/index.php?threads/valve-train-clearances-and-problems.528/

http://garage.grumpysperformance.co...e-springs-and-setting-up-the-valve-train.181/

http://garage.grumpysperformance.co...-springs-get-compressed-when-installed.11356/
 
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