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

[ByronRACE]

Why use a chassis dyno? Confirmation of the known and/or measurement of the unknown. The former, is what causes most problems because it causes controversy. There is usually a gigantic discrepency between most engine builder figures and what is actually measured; even when accounting for drivetrain losses. I've personally dyno'd over 1600 Ford powered cars on a dynojet 248 and can count the number times on one hand that a SBF or BBF actually exceeded 1rwhp per cubic inch on pump gas without a power adder. Compare that experience with the number of engine advertisements with power figures that would EASILY exceed 1rwhp per cubic inch in a car, and you have discovered the root of the problem...marketing fudge factor; big numbers sell engines.

As far as dyno accuracy is concerned, I've compared engine dyno to chassis dyno directly, with a variety of transmissions and have a very good idea regarding 5spd drivetrain losses for various transmissions (within 5%). I've also then taken the same car from the chassis dyno to the street, and produced horsepower/torque functions using dataloggers and simple physics...mass, time, speed...and produced the same data (within 5%) of the chassis dyno.

So, for example...

If you had an engine dyno that showed 600fwhp, and put that engine into a car with say a T5 transmission and 8.8 Ford rear, and did a 4th gear pull that that read 400rwhp... and your engine builder is telling you it's because of the water pump, alternator, and drivetrain losses...there's more to the story.

600fwhp
- 13-15% for drivetrain losses (90hp max)
- 5% for max accessory losses (30hp max)
- 5% fudge factor for tire slip, calculation error, whatever (30hp max)

So worst case, the chassis dyno should read at LEAST 450rwhp for an engine advertised at 600fwhp. In reality, it should read higher because the error isn't truly cumulative as shown above; some losses are related.

Most of the time, a 600fwhp advertised engine will dyno at 400rwhp or below. And, the same 600fwhp engine as tested on your own engine dyno will read far less than 600fwhp; which is even harder to explain away. This is all too common and expected.

In your case, after reviewing the dyno graph, peak torque is at at 5200rpm and peak power has been reached by 5800rpm. As advertised, the engine was supposed to reach peak power by 6700rpm. Either the intake/exhaust geometry has changed dramatically; or the cam or cam timing has changed dramatically since it was dyno'd. If you corked the intake with air filters, it would not change the location of the peaks by nearly 1000rpm; it would just change the amplitude of the high rpm portions of the graph. Same is true for exhaust corking (mufflers). My guess is it's not the same cam, or perhaps not the same cam timing. Furthermore, changing the cam timing after achieving a peak number on an engine dyno isn't unheard of...it's done to produce more bottom-end torque to make the car idle better and be more fun to drive in the rpm band you use the most. You can make a better high rpm number by retarding the cam a bunch, but it may not idle / drive worth a damn...so you compromise. If I had to guess, something like this is what took place. Either that, or the cam is the same and they had a very different intake/exhaust system on the car when it was on the engine dyno. Race gas and timing advance would help amplitude as well, and shift the RPM slightly higher...but I wouldn't expect 1000rpm out of fuel/timing alone.

Either way, you' still have a great running engine approaching 1rwhp/cubic inch at the tires...and that's nothing to be ashamed of.

[DAVID GAGNARD]

When I was dynoing my motor, as we were bolting it on the dyno, the operator began asking me specifics about the engine, I told him the cam specs/compression etc.,etc., he then asked what I EXPECTED out of the motor........I told him I would hope for 475 hp, but would be very happy with an honest 450hp with a good a/f ratio(it made 472hp@ 6400rpms)........He told me I would NOT leave disappointed, cause I was being very realistic about my motor!!!!!!!!!!! I DID NOT leave disappointed at all.........

He said he has dynoed a ton of supposedly 600hp pump gas motors that barely made 400hp and made for some very angry owners........

Keith;even as such, you still have one heck of a motor and car, if YOU are satisfied with the performance, that's all that counts, regardless of any dyno #'s..........

David

[neverwouldof]

Originally Posted by DAVID GAGNARD

When I was dynoing my motor, as we were bolting it on the dyno, the operator began asking me specifics about the engine, I told him the cam specs/compression etc.,etc., he then asked what I EXPECTED out of the motor........I told him I would hope for 475 hp, but would be very happy with an honest 450hp with a good a/f ratio(it made 472hp@ 6400rpms)........He told me I would NOT leave disappointed, cause I was being very realistic about my motor!!!!!!!!!!! I DID NOT leave disappointed at all.........

He said he has dynoed a ton of supposedly 600hp pump gas motors that barely made 400hp and made for some very angry owners........

Keith;even as such, you still have one heck of a motor and car, if YOU are satisfied with the performance, that's all that counts, regardless of any dyno #'s..........

David

I am satisfied with the car by all means, i just wanted to make sure i got what i paid (57k) for..... it sounds like i did and i just need to have it looked over. It has a big hesitation when you get on throttle... not sure if sitting for two years had any effect on lines or pump. The carbs were rebuilt though. we'll see but i'm more optimistic after all the help on here.

Keith

[John McMahon]

Keith,

The car wasn't dormant for two years, just not run a lot as I understand it. I don't know who Fred used on the carbs, he initially had them rebuilt at Holley when the car was built. I would havesome go through the whole drivetrain on that chassis dyno and get a new baseline established. It also has about $75K in it, so you got it at a good price.

Clutch pedal: That's how thay are on Contemporary's, my 289FIA was that way, you may be able to adjust the release point, but all the pedals are hung by the same brakcket, maybe shorten the pedal?