Tuning issues on S/C 1996 RT/10

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I have a 1996 RT/10 with a Paxton s/c system (modified, see general build spec below), running about 11psi.

I was having the car tuned on a dyno yesterday and things did not seem to be right. The car was making good torque to 3,600rpm (~600ftlb wheel) but then it was dropping off. Meaning that peak wheel power (at ~5,500rpm) was 'only' around 520bhp. I'd have thought that with such a s/c set-up, torque should continue to raise beyond 3,600rpm, or at least flatten (i.e. rather than drop off)? I was certainly looking for a peak wheel power around 100bhp more than I got!

The boost goes up pretty linearly with revs. But it seems that the charger is becoming less efficient (in terms of volume air flow rate) beyond 3,600rpm. So, general initial question - am I correct in thinking that, given a s/c running 11psi, the torque is dropping off too early in the rev range and that, consequently, hp is down on what it should be? And, if so, what the most likely causes might be? I have some ideas but wanted to get more of a range of opinions. Thanks in advance.

Build:
Paxton Novi 2000 s/c kit, running 11psi max boost
Bellanger headers and Corsa exhaust
T&D roller rockers (1.7, I think)
70mm throttle bodies
Uprated charge cooler
AEM Infinity EM
690cc/min injectors running at 80psi (top mount and fuel rail, pumps and swirl pot in trunk, new braided hoses throughout)
Valaya Racing 'stage two' head work, including race valves
708 cam

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NoLiftShift said:

Something is definitely off. My old Paxton viper made 580whp(11psi) with just the split-sec box for the tune. I forgot the torque number but it was very close if not more then the HP. The only other parts I had on the car at the time was corsa exhaust. Everything else was stock. I then changed out the pulley to the 14psi one and added a water/meth injector and made 735whp. This was with the fuel system and split-sec ECU that came with the Paxton kit. I then bumped up the psi, upgraded the fuel system, Motec ECU, ported heads/intake, and put down 990whp 901wtq.

With the build you have you should be putting down a lot more with no falloff in torque. I can look around and see if i can find the old dyno graph with the base 11psi pulley so you can compare.

Thanks for the information. Yes, your 11psi dyno graph would be very useful, thanks. My car was run on a 'DynoLog' rolling road, which do generally produce lower figures than other dynos.

[ViperPict]

Thanks for that.

Is that wheel or crank hp? In which case, not so very different from car I think (still not got the dyno graphs yet to be sure). I can't see the full curves so difficult to see at what point your torque is peaking. Does the video also obscure the curves?

Thanks again.

[ViperPict]

OK, below are my two dyno plots.

The first is as the car was as it came into the shop, the second as it left after having the cats removed, the air filter cleaned and the tune optimised.

So final wheel power is ~560rwhp @ ~5,600rpm and torque 616lbft @~4,100rpm. That is up ~100rwhp and 75lbft, respectively.

Note that it was run on a DynoLog system that I'm told generally give a lower number than others (e.g. Mustang) - hopefully no can of worms opened here...

The boost dropped off around 5,600rpm (slipping supercharger belt) - extrapolating the slope of torque curve prior to that would suggest ~520ftlb at 5,900rpm or ~580rwhp.

So, my question is, does this seem reasonable for a car (with the set-up described above) running 11psi? Or should I be looking for any other issues that are maybe hindering output? The tuner thought that it was running well but that it didn’t rev out very well (but that could be due to charger belt slip or the intake/ exhaust may still be slightly restrictive). And advice greatly appreciated!

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NoLiftShift said:

Looks good! Actually right on par with my old car. The more I think about it, maybe I was running around peak efficiency for the setup on the motor at that time. I now definitely remember complaining about the belt slip and drop in power at the top. Sorry I made it sound like you were way off the power numbers. The numbers are so close to my old one. Belt slip is always going to be there with that type of belt unless you go to a cog type. The dip in power at the top end can be a combo of the belt slip, exhaust and the heavy stock flywheel. I upgraded to an aluminum one and noticed a big improvement. This big ass motor is never going to spin the way you want it too. I also ran with no cats and just Corsa's. Your Fuel system is up to the task as your AFR is right on the money. You need to slap on the smaller pulley (14psi if I remember) and you are going to make "at least" 700. It's really that big of a jump in power. You're on the right path to make some big power for the road. Have fun!!

And yes DynoLog or Mustang dyno's as called in the states, generally give a lower number than others.

Edited by NoLiftShift on Wednesday 9th November 14:51

Edited by NoLiftShift on Wednesday 9th November 14:59

Thanks very much for the feedback. A bit of reassurance that everything is working OK. Elsewhere I have been advised to do leakdown and compression tests. Might see if that reveals anything.