Dynoed my 427so Cobra today.... interesting to say the least!

[snakeeyes]

Dynos are a great thing, but I think we all tend to place too much stock in the numbers and not enough in how the car actually runs. The post about exhausts and IRS robbing power are dead-on. Research on this site and elsewhere and you'll find that side exhausts, which we all love and believe to be much more efficient than an under-car setup, can actually be a detriment if not carefully designed,

As far as your numbers' I can only offer those of my Beck Lister for comparison, The car has a Corvette IRS, which does rob power vs. a live axle because of the multiple u-joints, the angle of the driveshafts, etc. It also has side exhausts. The builder dynoed the engine on an engine dyno--542 hp/ 508 ft-lbs.

A year or so later, after I got the car together, I had it dyno-tuned on a chassis dyno. I don't have the numbers in front of me, but I think it made around 417 hp and 395 ft-lbs.--losses of close to 25%. I had been expecting about 15%, which the figure you often see, so I was pretty shocked--until I drove it. It ran like a motherf_____. As I said, I think the numbers are much less important than ensuring the car's tuned right. Numbers may give you bragging rights, but they won't keep you from getting waxed by the kid in the chipped Subie WRX.

Get the car tuned, get it to a dragstrip, then decide if it's making adequate power (and getting it to the ground) or not.

[Anthony]

Originally Posted by snakeeyes

Dynos are a great thing, but I think we all tend to place too much stock in the numbers and not enough in how the car actually runs. The post about exhausts and IRS robbing power are dead-on. Research on this site and elsewhere and you'll find that side exhausts, which we all love and believe to be much more efficient than an under-car setup, can actually be a detriment if not carefully designed, As far as your numbers' I can only offer those of my Beck Lister for comparison, The car has a Corvette IRS, which does rob power vs. a live axle because of the multiple u-joints, the angle of the driveshafts, etc. It also has side exhausts. The builder dynoed the engine on an engine dyno--542 hp/ 508 ft-lbs. A year or so later, after I got the car together, I had it dyno-tuned on a chassis dyno. I don't have the numbers in front of me, but I think it made around 417 hp and 395 ft-lbs.--losses of close to 25%. I had been expecting about 15%, which the figure you often see, so I was pretty shocked--until I drove it.

Get the car tuned, get it to a dragstrip, then decide if it's making adequate power (and getting it to the ground) or not.

Obviously, your 25% loss is not only drivelline loss, but also loss do to the exhaust system, and maybe even the air cleaner as well, so it maybe it's appropriate. In addition, from your post, it appeared that the chassis dyno was not the same guy who built/dyno'd the engine on the stand, which may also enter some fudge factors in determining % hp loss.

Like you said, I think to get optimum performance, you need to have it chassis dyno'd and TUNED, as there can be large performance gains with the engine set-up right. It beats taking the car the strip, and repeatedly drag racing it, adjusting things to try to determine the best state of tune. Probably easier on the car as well.

By the way, I guess you're running a sbc? What is it's displacement ?

[DAVID GAGNARD]

I do believe both engine and chasis dynos are a good thing.......there is plenty of hp/tq to be gained or lost when using a dyno for tuning vs. seat of the pants tuning or guess work........

there used to be a dyno about a mile from my house owned by a good friend, and dirt late model racer.........he ofton would call me on a saturday morning when dynoing race motors to go and watch........on one 635hp race motor I have seen a total gain of 62 hp after a dozen pulls.......this was done with timing and jetting changes and changing out the carb spacer!!!!!!!! they started the session with the timing and jetting that the engine builder suggested and worked from there..........
On my motor,a timing change of 2 degrees made a difference of 8 to 12 hp, nothing else changed, my motor likes 32 degress total and when we went to 34 it lost 12 hp, going to 36 it lost an addition 20 hp, that is something I never would have found without a dyno tune.......

Chasis dynoing can give you an idea of what if any changes you may want to make to the driveline/exhaust system, it has been shown a number of times just how much hp some exhaust systems can "eat"........
I will always dyno my race motors and if I can find a chasis dyno I can get my car onto, I'll chasis dyno it also...........

David

[neverwouldof]

Originally Posted by DAVID GAGNARD

I do believe both engine and chasis dynos are a good thing.......there is plenty of hp/tq to be gained or lost when using a dyno for tuning vs. seat of the pants tuning or guess work........

there used to be a dyno about a mile from my house owned by a good friend, and dirt late model racer.........he ofton would call me on a saturday morning when dynoing race motors to go and watch........on one 635hp race motor I have seen a total gain of 62 hp after a dozen pulls.......this was done with timing and jetting changes and changing out the carb spacer!!!!!!!! they started the session with the timing and jetting that the engine builder suggested and worked from there..........
On my motor,a timing change of 2 degrees made a difference of 8 to 12 hp, nothing else changed, my motor likes 32 degress total and when we went to 34 it lost 12 hp, going to 36 it lost an addition 20 hp, that is something I never would have found without a dyno tune.......

Chasis dynoing can give you an idea of what if any changes you may want to make to the driveline/exhaust system, it has been shown a number of times just how much hp some exhaust systems can "eat"........
I will always dyno my race motors and if I can find a chasis dyno I can get my car onto, I'll chasis dyno it also...........

David

When you adjust timing on your engine, at what RPM are you running the motor at?

Thanks, Keith

[snakeeyes]

Originally Posted by Anthony

Obviously, your 25% loss is not only drivelline loss, but also loss do to the exhaust system, and maybe even the air cleaner as well, so it maybe it's appropriate. In addition, from your post, it appeared that the chassis dyno was not the same guy who built/dyno'd the engine on the stand, which may also enter some fudge factors in determining % hp loss.

Like you said, I think to get optimum performance, you need to have it chassis dyno'd and TUNED, as there can be large performance gains with the engine set-up right. It beats taking the car the strip, and repeatedly drag racing it, adjusting things to try to determine the best state of tune. Probably easier on the car as well.

By the way, I guess you're running a sbc? What is it's displacement ?

It's a Rodeck 404 cid alloy block with Victor Jr. heads, manifold, and Street Demon carb.

You're absolutely right as far as the different dynos. Both were Dynojets, but other than that everything was different. The engine dyno was in Wisconsin and the chassis dyno was at Keith Craft's Plano shop. I have heard that the Craft dyno tends to be conservative, but have no idea if that's true. I do know that, when Rob dynoed it, I had him remove the sidepipes and check the hp difference. Even with rejetting I think it only picked up 10 hp or so, so I think my pipes are pretty efficient. The air cleaner is a pretty large K & N with a K & N lid, so I think it's pretty efficient, too. I'm running synthetic fluids in the tranny and diff, so I feel I've minimized frictional losses there. It has a bimetal 'Vette flywheel so it shouldn't be losing much there.

I guess the conclusions I draw are that either the builder's dyno was very optimistic or the chassis dyno was very pessimistic. The Corvette tuners all use a 15% loss factor and it seems to be accurate; given that my diff is Corvette I would think it would be similar for me. Maybe the M-22 Cal Metal built for me is still tight and needs some breaking in (1000 miles on drivetrain).

Whatever the case, as I said I think you should use a dyno as a tuning device, not worry about the numbers so much, and judge the car by how it gets down the road.