Disappointing Dyno Results - Help!

[Mark Swanson]

'99 GTS, HMS Cat-Back, TNT Air Induction/K&N Cones, 6,000 miles

Max RWHP = 363.5
Max RWTQ = 411.4

I dynoed with several other VCA members yesterday and my numbers were the only truly unexpected ones. Even a stock GEN I motor had 380 RWHP! Strange thing about my HP graph is that it wasn't smooth. It was smooth up to 3700RPM where it was at 300HP. Then flattened out and actually dropped a little from 3700RPM to 4300RPM and then continued upward to 363.5 at 5400RPM. The miles per hour also levelled out at 3700-4300RPM, although I never let off the gas.

Any ideas on what's going on?

Torque curve peaked at 411 at 3700RPM then decreased through 4300RPM then back up to 390 at 4700RPM, then slowly back down to the end of the test.

CF = 1.02, SAE Corrected HP, Dynojet Model 248C, Fan running

 

[Martin D]

Mark,

I had a similar experience a while back. I went to the Dyno for Baseline and only pulled 398 to the tires on the first run and then a best of 388 on the next two pulls. The funny thing is, I had similar upgrades - Borla Cat back, TNT air intake, and 70mm TB's. Now having said all that, when I ran the car in the Quarter, I turned 122 with just those mods & 345 gear. So the moral is this, if it's fast, it really does not matter what your RWHP is. After all, I was expecting 430 to the tires after running 122. Anyway - I hope this helps. I also would suggest you get rid of the TNT induction - go back to the stock intake and see what your numbers are then. Also, I can tell you that the Hennessey Airbox is awesome... you may want to consider it.

Regards,

 

[Frank Parise]

Mark, your numbers are way too low, definitely outside of any acceptable parameters for a Gen II motor. You should expect 400-410 RWHP in showroom stock condition, without K&N filters. Add K&N filters and you should expect 415-425 RWHP. You probably already know this, so let me ask a few questions.

The dyno could be suspect, but you said your results were the only surprise. Did you do the run in 4th gear? Do you have a stock 3.07 rear gear? Do you have results from the other GEN II Vipers with stock or similarly modified cars?

There may be some inefficiency in your cat back exhaust or your air box, but I would be surprised that it robbed you of over 60 RWHP.

No acceleration between 3700 and 4300 RPM doesn't make sense. It doesn't sound like your car was running normal, and I wonder if you have a fuel delivery or air intake issue. Was it spitting or sputtering during the test or did it sound normal? We recently did a dyno in Phoenix and determined afterwards that one of the cars was only running on 9 cylinders. Have you tested to make sure your throttle bodies open completely at WOT? Are you sure all of your spark plug cables are securely connected?

Your Viper Tech can hook up the DRB to your car and diagnose anything unusual going on. I'm no expert, but I would be very concerned about these dyno results. Something ain't right!

 

[Bill/APEX Motorsports]

Mark, we've found that the Vipers are _very_ sensitive to heat-soaking on the dyno. Last week, I bought a $500 high-velocity fan that blows basically right into the radiator opening, just to keep the coolant temperatures down where they belong. Vipers can lose as much as 20-30hp because of increased coolant temperatures.

 

[Mark Swanson]

Follow-up Notes:

(1) I think the 363.5RWHP/411.4RWTQ dyno run was either a problem with the dyno, heat-soaking, bad gas, or other.

(2) I went to a different dyno shop a few days later and posted a 398RWHP/453RWTQ first run and a 386RWHP/437RWTQ second run. However, I still had a flat spot on the HP curve from 3900-4200RPM.

(3) I went to my Viper Tech and he reflashed the computer and changed the air-fuel mixture setting.

(4) I redyno'd at the same place as the results from (2) above and recorded 400RWHP/447RWTQ first run and 414RWHP/465RWTQ second run. The HP curve's increase slowed down at the 3900-4200RPM mark but it didn't completely flatten out like before.

All runs except the last one had a fan in front of the car with the hood open. The last run had the fan at the side of the car blowing across the engine.

Is the problem solved or should I continue to pursue this?

Given my mods I would have expected a little more HP, although the 465RWTQ seems OK. Also, the HP curve probably shouldn't "slow down" although it's better than a "flat" line.

I also did an air-fuel ratio on the runs in (4). Interestingly, the ratio levels out at 3500-4000RPM, just before the HP curve slows down. In the first run it leveled at 12.4 and in the second run at 13.3. Overall, the air-fuel ratio in the second run drops from 14.6 at 2500RPM to 13.3 at 3500RPM to 12.0 at 5000RPM to 11.3 at 5600RPM. Are these ratios too low for the Viper?

Am I analyzing this too much? After all, with a 13% correction factor (divide by 0.87) I have 476 flywheel HP, and 534 flywheel TQ from the last run.

Response to Martin
You didn't like the TNT cones? Roy said they definitely added HP and that's what they use on their 700HP cars. Seems like heat-soaking on the dyno could be a problem with these, but how about everyday driving or on the drag strip? It seemed like throttle response improved when I changed to the cones.

Response to Frank
All the runs were in 4th gear and I have stock gears.

Also, the guys running the dyno thought they saw a little bit of black smoke at the 3700-4000RPM level during my runs in (2) above.