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

 

[Mike Brunton]

Something is definitely wrong there....

If you are losing power at higher RPM, I'd look to the fuel system, air intake system, or ignition for your answers. Start with the easiest things and go from there. My Viper wasn't dynoed before my "tune up", but I was getting a maximum of 116mph in the 1/4, which is low for me (usually around 120).

First thing - check your air filter. Is it clogged? Or perhaps it is somehow covering part of the intake path? Clean it out just to be sure.

Second, pull your spark plugs. It's easy to do on a Viper. I did this to mine and noticed I was getting a good amount of detonation. Vipers run a bit hot, and the stock plugs can detonate. I switched to the NGK plugs. Check your plugs - if they look bad, then there's your problem.

Third - make sure you don't have a tank of bad gas. Did something happen suddenly to make it run bad - if so it could be bad gas. Is it knocking at all?

After this, see how the car feels. If it feels faster, then get it back on the dyno and see how it does. If it doesn't feel faster, then I'd probably take it into a shop to be looked at.

My '98 GTS with 20k miles and only K&N's and smooth tubes, and removal of the rear muffler produced 415RWHP and 465RWTQ - corrected. With your mods, I'd expect you to be a bit up on power over me (maybe 425-435RWHP). Looks like you're down about 60-70hp and about the same on TQ.

 

[VEETENN]

Make sure your throttle bodies are opening all the way.

 

[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 (probably not the technical term for this).

(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 John Hennessey
No baseline, unfortunately. No battery disconnect. All pulls were in 4th gear. My Viper Tech said no malfunctions were indicated anywhere, but that the air-fuel ratio should probably be changed a bit. Seems like dyno pulls are somewhat less exact than we might like. Could the fact that I rarely shift beyond 3500RPM (on the street) be impacting my "learning" computer? Thanks for the response.

Response to Mike Brunton
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 which helped lead to the air-fuel change by my Viper Tech. Thanks for the response.

 

[Mike Brunton]

Mark,

To be honest, I'm not really sure that TNT air intake really does a lot... I've heard both positive and negative results.

I'd *probably* want better #'s if I were you, but you're now in the realm of reason.

As I said, my GTS that is bone stock except K&Ns and smooth tubes produced 415RWHP and 465RWTQ. If I were you, I would be more concerned about the fact that the numbers are all over the board, and then try to figure out why they seem a little low.

Maybe the dyno place is doing something wrong? I'd be very suspicious of a 14RWHP difference between 2 runs with no changes to the car... unless it was massively heat soaked the first time or something.

I still think that flat spot in the power curve isn't good - can you post a graph?

 

[SRTim]

I recently had my car dynoed on a "Mustang" brand dyno...it said I had 360 hp and 368 torque...it also said I has 10 less horsepower with my nitrous switch on!

I reccomend using a DynoJet brand dyno...they're extremely accurate...I had a less than a 1 hp difference from run to run when I had mine dynoed on the DynoJet. The results were also drastically different. The DynoJet's results consistently said that I had 60 more hp than the first cheap dyno I went to.

Tim

 

[Mike Adams]

You should have more hp than that at the rear wheels should be 438 to 450 I would recomend a Mopar computer will make a world of difference in road feel. Also when dynoing the fan should always be on and the hood down as much as possible to allow the fan air to flow into the air intake of the hood. I did at test with hood up and hood down was 7 hp difference.

Call me if you have any questions 519-763-8473

 

[GTS Dean]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Mark Swanson:
I dynoed with several other VCA members yesterday and my numbers were the only truly unexpected ones...

(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?

<HR></BLOCKQUOTE>

The most telling note you presented was the remark about moving the fan on the last pull. I think the TNT tubes & cones are picking up hot, low density air from the radiator/fan assembly. With the fan to the side, the hot air is being forcibly displaced by the cooler ambient air, thus improving intake charge density.

Borrow a friend's K&N/smoothie airbox and do a back-to-back dyno evaluation vs the tubes.

 

[Serious Eric]

Agree. Cone filters breathing underhood air are an abomination foisted on us all by the aftermarket for many years now. Why in God's name would anyone want their engine to breathe that superheated underhood air rather than the ambient charge that comes in through the airbox? Any slight decrease in flow restriction with the cones is gonna be more than offset by the higher temps.

Switch to OEM style K&Ns in the stock airbox. Open up the airbox inlet (many examples and pictures shown lately on how to do it) if you feel you need more flow rate.