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Just realize the text content disappeared somehow, and I am not able to edit anymore.
Car just went through 3.8 rebuild with high lift cam, long header runners, no CAT but considerable quiet tail section, COP and Ecumater EMU CLASSIC. Wondering how can I smooth out the torque dip between the 3100 to 4200? The dip of the torque is at 300NM and it was my peak torque before the rebuild. The car behave like choking when I gave big or full throttle at low RPM below 2000 with above 2nd to 4th gear but it behave normal when just gave it small or reasonable throttle and choking disappear with gradual accelerate. We found the car behave less choking when we adjusted it leaner AFR at lower rpm. When car drive with high RPM and full throttle it behave like a rocket and none of this happen. So I think it is reflecting the dyno chart. So what can we do to smoothen out the curve? thanks
Aside from mapping- have you checked that the resonance valve is operating correctly?
thanks Pete,
Yes we confirmed the resonance valve is operating normally.
Tuner mentioned my headers is too long according to the setup.
So could a shorter and bigger diameter headers help to smooth the dip?
thanks
Was air:fuel recorded during the dyno run? That big dip could be from a very very rich mix. The stock ecu tune is overly rich in that same range.
Relative to the familiar, and mild, 993SS cam, yours looks very aggressive on the lift. I’m guessing the duration numbers are at zero lift, so can’t compare to Dougherty’s profiles- https://www.drcamshafts.com/911profiles.htm
It would make sense for the torque peak to move up in the rpm with itb and a very hot cam, but “choking” sounds like too much fuel.
Also- 378fwhp is pretty strong when Ed Pink’s 4.0 is doing 390hp. But maybe a very aggressive street-unfriendly cam can make those numbers?
Agree with nick, first thing I’d check is AFR. Given how high your peak Hp and Tq are you almost certainly have larger injectors than stock. Tuner may have focused more on getting the top end right (the new bits) and left the low end close to stock, which would be overfueled with bigger injectors (and/or higher pressure). Might lose a little resolution with bigger injectors too (small throttle change causing big fuel flow change).
So plotting AFR in that graph will answer a lot of questions on the mix and fueling through the load range. That has to be sorted first, and shown ideally with timing. I personally wouldn't yet focus on headers.
So... that cam seems super aggressive (granted I'm not an expert here...) . A popular street cam is the DC993SS which uses 248/234 duration, but it's not too agressive.
I built a hot street engine (granted a 2.8L that revs to 8500) and we used a DC 70 profile which is 274/264 which comes on torque around 3500-4000.
Anyway, I think you need to really take a look at the AFR fueling timing FIRST, and have that fully optimized. Then you can get a realistic assessment of what the cam is doing.
I don't have the AFR plotted.
But tuner tried many adjustment this is what he can best get out of the it.
He mentioned there at 3-4 rpm range there is isn't enough air going in and we make it leaner at the range then the startle became less.
Could it be scavenging issue that cause the air not been drawn enough?
The car drive like rocket to me at hight rpm specially the resonance valve is open.
Attached is my exhaust system with car deleted.
Tuner told me my piping is too small and the Y piece joining left and right side resonators sit above the big center resonator before going to larger resonator at the bottom.
Piping after the collector is 2.5 inch, could it be too small?
And the exhaust has droning seems like air bouncing back and go.
So I planning to remove the tail section and try to add two resonators only at each side after the collector and then to the rear center join left and right bank exhaust and then out at right tail with 3 inch piping.
Dunno about the drone or pipe sizing, exhaust is a bit of a black art IMO. Just wanted to chime and compliment the welder. Nice work on the headers - that’s a sweet looking bundle of snakes you have there. 😀
12-04-2023, 10:02 PM
