rear wheel horse power

In spite of what anyone might tell you, or a rolling road might 'calculate' there is no way to tell what the drive train loss is, and therefore what the flywheel horsepower is, based on rear wheel horsepower. Testing an engine on a good engine dyno, and then the car on a good chassis dyno, and deducting the latter from the former, is the only way. Any other way contains too much fudge & spoof to make any sense at all.

 

The factory stock 65 gt350 was advertised as having 306 HP. In a decent state of tune... still stock everything those things will show about 160-170 rear wheel horsepower. Where did the 130 HP go? The answer is that you are measuring two elephants with two different yardsticks that have not been compared to each other. It seems to suit the automotive world to never resolve this issue. I think it has something to do with selling very expensive loud mufflers to kids with Hondas.. (an industry 25 times bigger than all of Shelbydom)
"Did you hear how fast that car is"
jimbo

 

On newer Mustangs, 5 speed and 8.8 rearend drivetrain loss is around 15%. Older cars with a toploader and 9 inch seem to be closer to 20%.

 

I can tell you my 67 GT350/Paxton....dead stock 289 hipo, with Paxton...
Rear wheel dynoed at 225hp 250 torque....at a conservative rpm...
Paxton was good for about addl 80 hp....

 

There is no industry conspiracy about drive-train losses - it's simply a complex issue.

The most reliable way to measure it (and the way it's done in the industry) is to conduct 'coast-down' tests. Although even these measure only the combined effect of aero drag, mechanical drive-line friction and tire rolling resistance.

What you need is to fit the car with an accurate speed measuring device (speedo is not accurate enough) and a means of logging speed against time. Next you find yourself a long straight (and most importantly flat) piece of road. Drive the car up to a set speed (usually about 60mph), knock it into neutral and log the speed and time as the car naturally coasts to zero. Do this three times in each direction on the same piece of road to eliminate innacuracies and slight gradients.

Reviewing the data - average out the six runs so that the data is fairly reliable, and plot the decel curve. From this, by a process of mathematical integration, you can find the retardation load for any speed. At high speeds this load will be aero dominated, but at lower speeds (below 30) it will be largely tire load, and the rest drive-line load.

If you've plotted this (say in MSExcel), you could then use your engine torque characteristic and gear data to calculate your thrust curves for each gear and overlay the plots to see where the drag curve crosses the top gear curve - this will be your theoretical Vmax!

If anyone has an accurate torque curve for a stock 289 Hipo and can let me have the data, I'll have a go at putting this together and anyone who wants a copy can have it.

I'll stop now... :blush:

David

 

I have some experience of testing Mustangs/Shelbys on both engine and chassis dynos and have found that drivetrain losses for the t10/toploader and 9 inch rear are between 35 and 40 bhp. Considering the difference in rear wheel output to published manufacturers claims for power, might lead one to suspect that marketing was the prime consideration when calculating power output. However the figures Ford used were based on what was called engineering curve A = best example maximum ouput on an engine dyno with headers and no ancilliaries. This was considerably better than would be achieved once installed and the vagaries of manufacturing tolerances were factored in. You should be suspicious of claims that drivetrain losses account for large amounts of missing horsepower; that energy has to be lost as heat and 100bhp takes quite a large radiator to dissipate yet you rarely see cars fitted with trans or diff coolers.
Regarding coastdown if you do this in neutral you wont measure all the gearbox friction if the box is a manual one - you should leave it in gear and declutch.

 

Kevin,
Is your motor 1965 "stock"? No port work on the heads. HP cam. Stock coil and dizzy/w points. If so you seem to have a good one.
congrats!
jimbo

 

Thanks,
I have 69 windsor heads (got rid of the bump) C7 LeMans cam and roller rockers (which amazingly still fit under Buddy Bar valve covers) and stock (65- GT.350).. everything else. I have 210 RWH.
Gawd those 289s sound great at 7000 rpm...