Any Dyno Gurus?

[kountzecobra]

OK, lets see if anyone can help me. Last week I took my Cobra to a local University's auto shop lab to dyno test it. The 5 gas "sniffer" showed me a little rich, so I went down one jet size on both carbs. Mixture, etc. looked good after that. Still a hair rich, but I like it on the safe side anyway. Now my question: This chassis dyno does not read horsepower but does read torque in ft/lbs. Naturally the printer wasn't working, but I had 2 students watching the numbers during the runs. Highest peaks were 528 533 535 and 540 ft/lbs. All peaks were in the 5100 to 5300 rpm range. Does this relate in any way to horsepower? Looks like pretty high numbers to me. I really just wanted to know about the air/fuel mixture on the dual fours but am curious about the torque numbers. Any enlightenment would be appreciated.

[Bob In Ct]

David:
The Formula is HP = (torque X RPM) / 5252. Don't question it, it's the gospel. At 5,252 RPM the torque and HP lines will cross.

Bob

[Tom Kirkham]

Horsepower = RPM x Torque / 5252
At first glance it would appear that your engine would be making about 535 HP. But since the torque was measured at the rear wheels and I assume that you were measuring engine RPM and not wheel RPM that this is not the case. What you need to due is divide out the rear end ratio, and the transmission ratio and multiply the torque converter slip to get the RPM the rear tires were turning. My guess is top gear with a 1:1 ratio, a rear end ratio of 3.5 and a torque converter slip of about 5% (pure wild guess) your wheel RPM would be: Engine RPM/(3.5)*(1-.05) or RPM/3.68. So your car was putting down to the ground about 145 HP. How this relates to your car can be seen at http://vettenet.org/torquehp.html

[Bob In Ct]

David:
Go to Yahoo advanced search and type in the words "torque", "horsepower" and "primer". There is an excellent article.

Bob

[David Kirkham]

My brother never ceases to amaze me.

David

[A Snake]

Don't want to rain on anybody's parade, but Hp and torque loss from engine to rear wheels should not exceed 30%. Even a bone stock 1970 351C 4V (300hp advertised) will measure over 200 hp at the rear wheels with a C6 and 3.55:1 rear gears and stock size tires.

Al

[Bob In Ct]

David:
If you are measuring engine RPM the formula will yield the HP at the rear wheels. It does not matter what the rear end ratio is, the tire size or the gear. That being said, I would expect the best numbers to be in 4th gear (1:1) would all fluids warm.

If the engine is turning 5,252 RPM and the torque reading is 450 FT-LBS, you have 450 HP at the rear wheels.

Bob

[kountzecobra]

This is interesting but I am still a little confused. I would be surprised if my mildly-ported iron-headed 351 Cleveland really put out 535 horsepower. This is big block horsepower territory. Any more comments, I like these kind of discussions. Maybe I can learn something. Seems like the rear gearing should come into play like Tom suggested?

[Bob In Ct]

David:
The dyno is oblivious to the gearing, the tires and the rearend ratio. All it knows is the torque being applied and the RPM of the engine. If it measures 400 HP, it's at the rear wheels by definition. No doubt 400 at the rear wheels is more at the crankshaft.

Bob

[Tom Kirkham]

Bob you need to reread his post. David said torque was measured. Not power. Bob if what you said were true then having the car in a lower gear (to increase the torque to the rear tires) would increase the power. What a transmission or any geartrain does is trade speed for torque and vice versa. If the power were measured at the input speed of the geartrain and the torque measured at the output of the geartrain then the geartrain would become a power increasing device by the gear ratio of the gear train. Even in an ideal world (one that has no frictional losses) power in = power out. Hope this helps

[George Snyder]

I am not an expert by any means, but have run one of my cars on a dyno many times.
It seems to be an acceptable fact that drive train loss for manual is about 15% and 18 for automatic. You could also add 5 to 8% for engine accessaries, as water pump, alternator, etc to yeald BHP at the crank. I have seen this confirmed from guys at other Forums that did both to their engines, before and after.

The size tire, gear ratio has no bearing on the numbers. Manuals are always run in 1.1 ratio.

They also are not very accurate. Some Guys use coast down numbers when calculating rear torque and HP numbers. A chassis dyno is a great tool to measure changes that are performed on the same engine. Cam change, carb settings etc.
The only way to get true numbers is with an engine dyno.

kountzcobra,
I agree with you that the numbers sure do seem high for that engine. Remember they are at the rear wheels and you would then add your percentages for crank!
They are a lot of fun and are a great tuning tool, especially the air/ fuel ratio graph, which was your intentions.

The other rules for torque/ hp are: hp = 25% 0f torque @ 1,300, 50% @ 2,600, 755 @ 3900, always.
Bud

[kountzecobra]

There is a shop nearby that installed a new chassis dyno a couple of months ago. Just for sh!ts and giggles, I think I will give that one a shot just for a comparison.

[Tom Kirkham]

David,
Have you run your car on the new dyno yet?

[kountzecobra]

Haven't had a chance yet Tom. Hopefully this week I can make an appointment to do that. I have been playing with timing and carb jets and it runs real strong, so I would like to see some dyno numbers now. Hopefully I can break that 200 rwhp mark

[Jahred]

Interesting Excerpt from the referenced site.........

The Case For Torque
Now, what does all this mean in carland?
First of all, from a driver's perspective, torque, to use the vernacular, RULES :-). Any given car, in any given gear, will accelerate at a rate that *exactly* matches its torque curve (allowing for increased air and rolling resistance as speeds climb). Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it. Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context. 300 foot pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would be *double* at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where horsepower and torque always come out the same.

[Bob In Ct]

Johnny:
300 FT-LBS at 4000 RPM is for far more valuable than 300 FT-LBS at 2000 RPM!

Picture a wheel over a well with a 10 pound bucket of water hanging on a 2 foot wide wheel. It takes 10 FT-LBS of torque to raise the bucket up from the well. Who is stronger, the guy who can turn the wheel 100 RPM or the guy who can turn the wheel 200RPM? They both exert the same torque. The power of the guy at 200 RPM is greater. Did you ever wonder why Formula 1 cars turn their engines so fast?

Bob

[kountzecobra]

Well, my cobra and I have the local chassis dyno reserved at 8:00 AM Saturday for as long as I need it. Going to try a few different things and get a few numbers. Checked over everything this week and put in a fresh set of plugs. I guess I'll see what happens after the dyno-flog. I'll be happy if I can get 275 to 300rwhp. I'lll post my findings tomorrow.

[Steve R]

Great thread Dave. Looking fwd to the results.
Steven

[OZCOBRA]

There is only ONE complete chassis dyno , Dynamic Test Systems Australia,Coming to a state near you!!!!! It may sound a little biased, but this is the only system to give a TRUE and complete analysis of your engine and drivetrain.Check it out at www.dtsaust.com.au
Regards Paul

[Jahred]

Just wanna make sure I am not missin somethin........


"Johnny:
300 FT-LBS at 4000 RPM is for far more valuable than 300 FT-LBS at 2000 RPM!

Picture a wheel over a well with a 10 pound bucket of water hanging on a 2 foot wide wheel. It takes 10 FT-LBS of torque to raise the bucket up from the well. Who is stronger, the guy who can turn the wheel 100 RPM or the guy who can turn the wheel 200RPM? They both exert the same torque. The power of the guy at 200 RPM is greater. Did you ever wonder why Formula 1 cars turn their engines so fast?"

It would be my understanding that torque is what gives rise to acceleration and if you can get the same torque at 2000 vs 4000 RPM you are well ahead of the game........(or at least ahead of the poor SOB that took 4000 RPM to reach the same level of torque).....

Re your example, they don't both posses the same torque.......if they did, everything else being equal, they'd be doing the same RPM. The only conclusion that can be reached, is that in your example, the driver which pushed the objectd to 4000 RPM, again, everything else being equal, had alot more torque than that which drove the same weight to 2000 RPM. ??????

Some questions never die........