Before and After Dyno - Intake Valves

[GR8_ASP]

Completed a second dyno test on my SRT. I did a dyno test 1 week before replacing the intake valves and a second set 1 week after the repair. The graph is not easy to read but the 3 lower curves are the "before" condition and the top 3 are after. The last run, number 6 (red), was done with the air filters removed. I wanted to see how much restriction the filters provide. The effect was minimal. I do not see any significant gain in changing the filters alone.

More to come for sure but I needed a baseline. You may note I did not gain 25 hp or 40 lb-ft. I gained about 20 RWHP and 22 RWTQ. The RWHP gain was close, considering driveline losses. The torque stills seems a little low to me.

I continue to see the significant variability in the graphs. It is fairly repeatable with the peaks around 100 rpm apart. Any ideas what is causing this? Tests of Gen I and Gen II just before and after my first 3 runs did not show this variation.

 

[knuk]

So bottom line is that replacing the valves did not give you the performance that you paid for? This is my concern so I asked DC if they would dyno the car after the valve replacement and if it is not up to specifications what would they do? I was told that they are confident that the performance will be there so they will not dyno the car.

 

[DEVILDOG]

Choppy! Single beehive valve springs?

 

[Greg Good Cylinder Heads]

To be completely honest with you guys, I am surprised that a backcut on an intake valve is even worth 20 horsepower.

I would like to get my hands on one of the "bad" valves so I can flow test it and compare it to a good one on the next set of Gen 3 heads I do. I have been porting heads for a while now and I experiment with valve job angles constantly. I have yet to see enough flow gain in a backcut on an intake valve to pick up 40 pounds of torque. A backcut "usually" does not help flow all the way through the flow curve. It usually only helps the mid lift flow numbers, and only minimally at that, on a 45 degree valve job like on the stock Vipers. A 50 or 55 degree valve job is a different story. The backcut can make or break the flow.

I suspect the bad valves are also causing some air/fuel seperation as well as a flow loss.

 

[Joseph Houss]

Here's the original statement "....with intake valves that did not receive the specified intake valve machining, resulting in a potential loss of about 25 horsepower"

Key word is potential loss, not specific numbers. Looking at your chart, your peak HP is about 442? Factor in a driveline loss of 12% ... you're at 502 HP. That's a number that can't be beat by all but a few Italian supercars!!!! ... and your torque numbers seem awesome (are you supposed to factor in driveline loss? That would make your SRT somewhere around 535 lb/ft!!!)

Completed a second dyno test on my SRT. I did a dyno test 1 week before replacing the intake valves and a second set 1 week after the repair. The graph is not easy to read but the 3 lower curves are the "before" condition and the top 3 are after. The last run, number 6 (red), was done with the air filters removed. I wanted to see how much restriction the filters provide. The effect was minimal. I do not see any significant gain in changing the filters alone.

More to come for sure but I needed a baseline. You may note I did not gain 25 hp or 40 lb-ft. I gained about 20 RWHP and 22 RWTQ. The RWHP gain was close, considering driveline losses. The torque stills seems a little low to me.

 

[GR8_ASP]

Joe not disagreeing. The horsepower increase is around what I was expecting, based on the stated effcts in the letter I received (25 HP loss). The torque increase wasn't as I gained around 20 ft-lb and the letter indicated a 40 lb-ft loss. What does that mean. I don't know. The overall results are in the neighborhood of what the 500hp/525 ft-lb stats would indicate. Maybe a little on the weak side but in the range.

The vibrations or wavy curves are more of my concern. Not seen anything like it and I believe it does detract for performance as the car really sees the area under the torque curve, following both the high points and the low points. It was noticeable that the wave heights did decrease with the new valves. Also I am not sure if they are engine or driveline based, but they do appear to be time based.

 

[Viperzilla]

Here comes another debate, not trying to make one or let it get heated. But I've heard to factor driveline loss, you use 1.15 for manuals and 1.2 for autos. I've heard this from friends and from a highly regarded tuner on this board. Just another FYI I guess you can say.

 

[Mike Brunton]

Actually you don't use 1.15 - you divide by .85. It's a different number.

450RWHP * 1.15 = 517.5BHP
450RWHP / 0.85 = 529.4BHP

And Sean Roe did a test and I believe quoted about 11% as a good driveline loss number. 11-12% seems to be accepted in the Viper community. So that would be dividing by .88 or .89 depending.

It does seem like 25HP and 40lb-ft are somewhat optimistic (to say the least) for a barely noticeable mis-cut on a valve.

I'm very very curious why the graph is so jerky. Viper SRT - where were they grabbing the ignition signal from on your car? When I've seen dynoes like that, it often indicates a back tach pickup or some serious play in the driveline (a motorcycle with a loose chain will have a dyno curve like that)

 

[Bill 96 GTS]

Most dynos have a range of curve smoothing. Something like 1 thru 5 degrees of smoothing. My guess is this is normal with minimal smoothing selected.

 

[Viperfreak2]

Bill, good point. The only issue would be a back to back pull between a Gen II and III. If the Gen II didn't show lumps, and the III did (same settings) what would that tell us?

 

[Viperzilla]

There have been other SRT dyno sheets looking like this (pulls on seperate dynos by seperate cars)
Thanks for the clarification Mike.

 

[RC Viper]

When I did my SRT prior to the valve replacement it looked exactly the same
way. In fact, with no prior knowledge of any valve issue the operator approached me and told me that "something was wrong; most likely ignition related". I have yet to dyno the car after the repair. It is important to note that day we had just dynoed several other Gen II cars and did not have the issue on any of them. I should add that I had the only Gen III that
was dynoed that day.

 

[GR8_ASP]

Randy is correct. The dyno operator was also concerned in my case. I can state that the day of the first measurements A Gen I and a Gen II were tested just before and a Gen I just after mine. They did NOT exhibit the "bumpy" curves.

For efficiency I have used 12% loss at the peak power speed. That seems to be pretty close. The efficiency loss decreases with lower engine speed as some of the losses are velocity (or velocity squared) related. At peak torque it is closer to 10% loss. To use a %loss you multiply the flywheel HP by (1-eff loss). To get back to flywheel you divide RWHP by (1-eff loss). In this case RWHP / 0.88 = FWHP. For runs 4 & 5 it is FWHP=439/0.88 = 499 FWHP. If the average of the oscillation height is subtracted (to get a more average peak power) it would be 496 RWHP. Both in the ballpark of the stated 500.