Need some help... No power after 4800 rpms
09-17-2008, 05:55 PM
#1
Staging Lane
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Need some help... No power after 4800 rpms Took the car to the dyno today and runs great below 4800 rpms. Above 4800 and the tuner said it feels like power drops and let out.
Logs show AFR's 11.0 - 11.5. Making 12 psi, then drops to 10 psi. Setup is as follows:
403 LS2
Patriot truck heads
8.2:1
TC78 0.96 a/r
rear mount
stock manifolds
2.5 to 3" y-pipe
3" charge pipe (mostly flattened out for clearance)
Any ideas on what's happening? Tuning was done on a load bearing dyno. Tried it without a load and went all the way through to 6000 rpms, but power dropped from about 550 @ 4800 to 400 whp @ 6000.
Open to ideas... backpressure? flow choked out because of flattened pipe? bad luck, lol?
Logs show AFR's 11.0 - 11.5. Making 12 psi, then drops to 10 psi. Setup is as follows:
403 LS2
Patriot truck heads
8.2:1
TC78 0.96 a/r
rear mount
stock manifolds
2.5 to 3" y-pipe
3" charge pipe (mostly flattened out for clearance)
Any ideas on what's happening? Tuning was done on a load bearing dyno. Tried it without a load and went all the way through to 6000 rpms, but power dropped from about 550 @ 4800 to 400 whp @ 6000.
Open to ideas... backpressure? flow choked out because of flattened pipe? bad luck, lol?
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06-11-2009, 06:11 PM
#12
Staging Lane
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Whoa, finally an update...
Took the car to another tuner and dyno. Found out my base fuel pressure was too high and it dropped a few psi right around 4500 rpms and the AFRs were going LEAN... Also, the timing was sky high and the plugs were gapped around 38.
Retuned the car with the lower fuel pressure and tighter plug gap (32). Now, it's good until 5800 rpms. Checked backpressure and it was at 33 psi max and 11 psi on the intake side. AFRs go rich at 5800, no matter what we do to the fuel trims.
I understand that the backpressure ratio is 3:1 and it should be more like 2:1. Will exhaust pressure rise with increased boost? I was thinking of raising the boost to 14 or 15 psi to lower the backpressure ratio... Any thoughts? Will it work to see if backpressure is the issue? If not, I think I will try new valve springs next...
Took the car to another tuner and dyno. Found out my base fuel pressure was too high and it dropped a few psi right around 4500 rpms and the AFRs were going LEAN... Also, the timing was sky high and the plugs were gapped around 38.
Retuned the car with the lower fuel pressure and tighter plug gap (32). Now, it's good until 5800 rpms. Checked backpressure and it was at 33 psi max and 11 psi on the intake side. AFRs go rich at 5800, no matter what we do to the fuel trims.
I understand that the backpressure ratio is 3:1 and it should be more like 2:1. Will exhaust pressure rise with increased boost? I was thinking of raising the boost to 14 or 15 psi to lower the backpressure ratio... Any thoughts? Will it work to see if backpressure is the issue? If not, I think I will try new valve springs next...
06-25-2009, 06:43 PM
#15
back pressure is too high, you nee more exhaust flow from the turbine.
You can wrap the exhaust before the turbo or have it coated and that will help, you can then put a larger turbine housing on it.
Realistically the T78 is just too small to feed a 403 as a rear mount and make the power you are looking for. You need a 88mm sized turbo and large turbine setup.
You can wrap the exhaust before the turbo or have it coated and that will help, you can then put a larger turbine housing on it.
Realistically the T78 is just too small to feed a 403 as a rear mount and make the power you are looking for. You need a 88mm sized turbo and large turbine setup.
06-29-2009, 11:46 AM
#16
Staging Lane
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Thanks for the reply Zombie... Exhaust is wrapped before the turbo already.
Swapped the valvesprings and the sudden drop in power went away. Power peaks low at about 5k. I think I'm done for now. Tuner recommended a more aggressive cam if I didn't want to redo the turbo right now.
Swapped the valvesprings and the sudden drop in power went away. Power peaks low at about 5k. I think I'm done for now. Tuner recommended a more aggressive cam if I didn't want to redo the turbo right now.
06-29-2009, 01:16 PM
#17
Thanks for the reply Zombie... Exhaust is wrapped before the turbo already.
Swapped the valvesprings and the sudden drop in power went away. Power peaks low at about 5k. I think I'm done for now. Tuner recommended a more aggressive cam if I didn't want to redo the turbo right now.
Swapped the valvesprings and the sudden drop in power went away. Power peaks low at about 5k. I think I'm done for now. Tuner recommended a more aggressive cam if I didn't want to redo the turbo right now.
06-29-2009, 09:11 PM
#18
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im having the same problem with mine! at about 4200 everything drops way off.. boost and power. i was told it was cause my turbo was too small.. i have a cheap shitty Chinese t61 with a .68 a/r on a stock ls1.. but other people say it should be ok.. i just dont know.. im saving up to buy a turbo but i would hate to buy 1 then still have the same problem