Dr.G

iTrader: (1)

I take the car in this afternoon to get dynoed to see how much (if any) HP was gained by going to a true dual exhaust setup. To save time in tuning the car right, my tuner suggests we just go ahead and also put the new smaller pulley (2.87) on the supercharger, a drop down from my 3.12. Now the last time I had the car dynoed with the 3.12 pulley and only HKS mufflers I put down 341rwhp and 299lb/torq at only a little under 5lbs of boost.

So now, more free flowing exhaust, smaller pulley, I hit a tad over 7.5lbs of boost and the car puts down, ready for this....344HP and 314lb/torq!!! WTF?!! the air/fuel ratio was consistent with my last run, steady on 12. For the 2nd run, a little adjusting to the SAFC2 and the car drops about 15 HP and air/fuel line drops to 11. Each following run we would get an oscillating line on the air/fuel graph from as high as 13 to as low as 11 and no higher gain in HP than 345! My tuner was baffled as was I, how could I increase 3lbs. in boost and only gain 3 maybe 4 HP?? The ONLY changes to my car from my last dyno has been the exhaust, addition of a BOV, and now the smaller pulley- that's it! Can anyone offer any explanation.

chuckb

same dyno?

DaveGS4

iTrader: (2)

Dr G, I'd make sure to PM TTurbpimp on this one to get him to respond. When he had the setup I have on my GS on his car, he did a LOT of dyno tuning to arrive at the 3" pulley he had custom made. I seem to recall him telling me that the smaller pulley didn't or wouldn't do anything because the Vortech SC that is used was maxed out and adding more boost didn't yield more power.

BA_GS400

iTrader: (1)

yep, that's a tough one man.

How did the lower part of the curve look when comparing the two runs?

chuckb

did they take too much timing out to compensate for boost. seem like with same timing curve it should make more hp.

jtanoyo1

Can you scan your dyno slip and post it here for us to see? I wanna take a look at it and try to see what I can gather. That is a bit strange.

jcm3

Definately check the timing and possible belt slippage from the s/c pulley.

Pearlpower

iTrader: (2)

I believe this s/c unit is capable of more boost that the 2.87 is providing. A post of your dyno would be helpful. Belt slippage does come to mind. I am still waiting for my computer from SRT, so I have been out of the picture for over 2 weeks now. I will be using a Greddy Emanage computer so time will tell if this solves my a/f issues. I am also installing a 2.87 pulley as well. When the parts do arrive one day, I will post my results after the dyno. I am glad I am only paying 50.00 a dyno session.
Lee

klynchgs4

i'm an accountant, not a mechanic - i don't know a whole lot here, but what i do know is when i installed the 2.87 pulley my car put down 400hp to the wheels (when cold) and 380hp after it warmed up (up from 351hp). now i'm just waiting for them to figure out what's wrong with my tranny (now replacing clutches, t/c and adjusting the valve body)

Dr.G

iTrader: (1)

to update and answer some of your ?'s....same dyno was used, no timing adjustments were made, and new belt with no slippage (the boost was there, jumped from 5 to nearly 8 with the new pulley). I left out of the shop in a hurry to get back to the office so i don't have the dyno sheet on hand to post but i'll try to get it asap to post up. but i can say that it was a steady climb in hp to the top and a pretty flat line across 12 on the A/F line.

BA_GS400

iTrader: (1)

the other semi-obvious thing to at least ask here, not that I'm doubting you or the shop, is to ensure both runs are "corrected" numbers for the weather conditions.
most people know that, but for those that don't I wanted to put it out there.....

looking forward to the dyno sheet!

aslowdodge

Is it possible that the heads are maxed out and creating a bottleneck?
On my viper going from a 5 to 8 lb pulley didn't add very much because of the heads. The engine actually experienced boost creep in whiich the boost increase partly due to not being able to flow efficietly through and started to back up..

On the lower rpms up to 4500 power and torque increased but fell off as the heads became ineeficient.

I am assuming you have a positive displacement blower like I have on my viper? My is made by autorotor in sweden . It's best know as the kenny bell blowers for mustangs.

If you have a centrifical like a vortech or ati then forget what I said as they don't seem to have this problem

BA_GS400

iTrader: (1)

I'm pretty confident that it's the centrifical type.
I would have thought our exhaust manifolds were the bottleneck. They totally blow for airflow.....

morris

iTrader: (2)

Hes running a RMM kit as the base, it is centrifugal.

TTurboPimp

iTrader: (12)

Its not the air flow of the heads, or lack there of. When SRT was making their SC kits, there were other GS' that were pushing more than anyone has pushed with the RMM kit.....and this was without any porting or polishing.

klynch....not that I am doubting you, but I would love to see the dyno sheet where your putting down 400 to the wheels with the RMM kit. If you are, then you are the only person that I am aware of that has managed to do this. I tinkered with every possible combination with that kit, and had the best tuning done with alot of support items that most have not done.....still was well shy of 400.