Reason for power dropping off

[razza]

Hi all,
I had the first go at the dragstrip on the weekend and did pretty terribly. The Evo Zero 3.909 ratio gearing might be partly to blame, probably as much as my shitty driving, but another issue is my 7500 RPM limit. My tuner set it there as the car stopped making power about that point. Full boost is at 5250 RPM and with the rev limit where it is it gives the car a very narrow powerband. So with the mod list below can anyone see anywhere which might be causing power to drop. One item that has crossed my mind is the intercooler. Its a 21x9x2.5" chinese style. I had the inlet and outlet changed from 2.25" to 2.5". Could this cause a drop in power up top when the turbo is really starting to pump.

Mod list is below as well as dyno graph. Any advice greatly appreciated. Cheers!

9.0:1 forged pistons
Aftermarket rods
ARP head studs
Cometic head gasket
ID1000 injectors
300lph fuel pump
Fuelab fuel filter and adjustable fuel pressure regulator
Forced Performance GT35R with 0.68 exhaust housing @ 28psi
JMF Race Inlet Manifold
Larger intercooler w/ 2.5" intercooler piping & Tial BOV
Degree'd Kelford 272 cams with dual valve springs
3" Exhaust
Standalong ECU

You must be logged in to view this image or video.

 

[gsxeclipse102]

5200 rpm full boost seems pretty bad. Have you checked for boost leaks? 7500 is way too low for a gt35 , smim, and kelfords. You should be around 8500-9000

 

[razza]

Shouldnt be any boost leaks as it was checked as part of the pre-dyno tune checks. But something I will double check regardless. Thanks for your post.

 

[TalonSpeed]

.68ar housing. If you looking to make power 8k-9k rpm, u need a larger AR housing. Try adjust cam gears to dial in LESS overlap.

 

[razza]

Thanks for your reply. I actually got the same response on another forum that I asked the same question. So obviously these boltup exhaust housings are somewhat restrictive. But I did find this - https://www.instagram.com/p/2jJYy2k58J/ The dyno chart shows the car making power upto 9500RPM. Though I realise it isnt climbing past 7000RPM it isnt dropping off like mine does. Any thoughts? Not trying to argue with you, just want to be 100% before I make any changes.

 

[TalonSpeed]

If that car has the same bolt on housing that you do, and the same compressor, and the same cams. Then the difference in power is down to the tune, cam timing, and any mods to the head. Intake and exhaust manifold play a part, but cam timing is what you should adjust. I run my Kelfords straight up 0 0 on a 2.3 Bep 55AR HX40. My car doesnt pull much past 7k. It sucks. Backpressure hot exhaust bleeds back into the combustion chamber during cam overlap. Its okay as long as backpressure doesnt greatly exceed intake pressure, but running a larger compressor on a small housing = backpressure that can be twice as high as intake pressure. Best way around it is to remove overlap and hope that the dull in midrange is negated by the quick spooling nature of a small housing. I plan on pulling as much as 8 degrees of overlap out. This will smooth the idle out, dull the midrange, but drastically increase the top end power.

Read Quote

The same car taht wouldn't run past 7200, now with 4* advanced exh cam and 4* retarded intake cam was spooling a little slower but once it was on it was like a locomotive, she was pullin and nothin was stoppin' her.. The car now rockets past the entire 8k RPM range and buries the stoock tack to the 9K RPM limiter i have if i don't shift early in 1st and second gears when they start to "freewheel" the LSD and both front tires the revs are insanely high. I'm shifting at right around 8k in 1-3rd gears, and have no need to get near 8k in fourth as long as i'm DSM brakes But before with the cams both overlaped, i could spin the tires and even with no traction the engine hovered around 7200-7500 RPM untill the car caught traction or caught up, now it will rev to the moon if i let it

From this thread:
http://www.dsmtuners.com/threads/cam-gear-tuning-for-dummies-understanding-what-is-happening.375887/