My Desktop Dyno Figures

[CCX33911]

I was out of town and missed the discussions. I agree mostly with Andy in that the bottom line is that TQ at the rear wheels at any speed is what accelerates the car. You can turn high HP into TQ with the appropriate gear ratios. Look at the S2000. Not much torque but good HP and thus good acceleration.

The most important thing for acceleration (not drive ability necessarily) is to have the best average HP over a spread of RPM that at least covers the spread for the largest gear change that you make in your transmission. For example if your first gear is 3.3:1 and your second gear is 2.0:1 the ratio is 2.0/3.3 = 0.606. Now multiply that times your projected shift RPM (say 7k) and that will tell you what RPM will fall to after a shift. In this example, 4242 RPM. Thus if your engine makes best average HP over the 4242 - 7000 RPM range you will accelerate the fastest.

I sent in an e-mail to Motion software to suggest an enhancement to Dyno 2000 such that you could use there iterator to design an engine with best average torque or average HP over a range of RPM (not just peak numbers like current version). I think this would significantly improve the use of the iterator. I encourage owners of this program to also send in e-mails asking for this request and maybe they will add it.

Thanks
Gary

[Slide]

I must be missing something. I always thought torque is a measure of [twisting] force but horsepower is a a measure of work-force over distance and time. Even a runt like me can apply 100ft.lbs. of torque to a head bolt but I sure could not rotate that torque wrench at 60rpm! I think horsepower is what accelerates the car and the discussion should be about the trade off of horsepower peaks-easier to build at high rpm at cost of very low power at lower rpm.

[Mr.Fixit]

Torque is what pushes the car, spins the tires, shoves you into your seat, snaps your neck back, etc. HP is a function of he application of torque over a given time. You can have a high torque, low speed (thus low HP) motor, or the complete opposite. A motor with very low torque, but the ability to provide that torque at very high rpms and thus have more HP because they are pushing less hard but going a greater distance. James Watt determined that a horse could do a certain amount of work in a day, which is equal to 550 ft/lbs/sec. So you could move 550 pounds one foot per second, or one pound 550 ft per second and still be doing the same amount of work. Like on a ten speed bicycle, you can put it in an easy gear and pedal really fast with little force, or put in a hard gear and pedal really slowly with lots of force. Getting the same amount of work done both ways.

[VinceGledhill]

So.

Given all this. And a Rear Diferential ratio of 3.31-1 using UK Jaguar components, and a tremek 3550/5 Speed.

What should I be looking at for my 351W engine. Given that I am looking for maximum acceleration in a Cobra that weighs less than a tonne, with 50/50 weight distribution and 245/45/17 rear tyres (sorry tires).

Should I stroke it to 383? Remember, I'm looking to scare myself with the acceleration.

Wow. Someone said... great thread. yeah for sure but now I'm more confused than when I first asked.

No offence Andy but I'm now buzzing. Probably should have read more before asking such stupid questons. I'm even confused with the Torque/ HP thing so I've not got much hope.

Regards
Vince Gledhill
Limey Leeds UK

[flipper35]

Andy,

You give the torque for both motors at peak hp. What are the graphs for the torque curves on both motors.

Also the S2000 is only quick if you drop the clutch. A rolling start and they are EASILY blown off by a lesser car. Look in MT or C&D 5-60mph figures.

Street start 7.3 seconds in the Honda with 240hp and 153lb/ft at 2780 lbs.
Standing start 5.5 seconds.
Quarter mile in 15.1@96mph

The new Neon SRT-4 same magazine 215hp and 245lb/ft at 2644lbs.
Street start not available.
Standing start in 5.6 seconds.
Quarter mile in 14.2@102mph

I chose these because they were quick to look up and similar in weight and had the hp/tq spread. Given the q-mile I say tq wins here.

I like to have the torque numbers higher than the hp numbers. You make your hp down low, less stress on the engine and you don't have to DOWNSHIFT to pass.

Just my .02.

[MJN]

Vince

If you really want to understand how your engine combinations are going to perform under full throttle accelleration in the car take the engine outputs from the Dyno software and input them to the Dragstrip software. Now you will be able to understand how weight, traction and gear ratios play into all this and can really make a huge difference. Once you have determined your useable power band for street use, then maximise the average power under the torque curve from redline (say 6,000rpm) to what the revs drop to when you shift gear (say 4,500 rpm). The greatest average torque will give yout the most effective power for the chosen rpm range. If you read any of the John Lingenfelter books on small block C#$*ys this is pretty much what he tries to do. And we all know how successfull he has been at getting blindingly quick accelleration from his project vehicles.

What you will probably find is that the motor that looked the best (highest Torque or HP) on Desktop Dyno is not the one that will work best and give the quickest quarter mile times (or 0 -100mph)with your gear ratios and rear end ratio. Sorry to throw a wrench (spanner) in the works but as you are going to this much trouble might as well go all the way!

[VinceGledhill]

Absolutely bang on right Mike. Might as well go all the way. That sounds like a good idea. Test with the desktop drag strip.

The reason I'm trying to get it absolutely right this time. I've already built one cobra that I'm not happy with. Yeah it sounds good, it's a V8, but 0-60 times of 7 seconds don't cut it for me as a cobra. It will however be a very saleable car, and I'll post pictures on here when it is painted.

Great advice from everyone, I have the feeling that my next project is going to be one hell of a machine. Sorry Wilf if you don't like the shape of the Gardener Douglas.

Regards
Vince Gledhill
Limey from Leeds UK

[MJN]

Vince

If you can't get 4 second 0-60's (or lower) from the combinations you've been considering then it can only be due to the fact that you've been eating too many pub lunches!

[VinceGledhill]

Mike, you sound like an experienced man in the Pub lunches field. I know you shouldn't but it's just got to be done.;-)

[CobraEd]

The statement was made that it is torque that moves the car, snaps your neck and spins the tires. However it is not torque at the crank that does this, it is the torque AT THE WHEELS that does this. So if you have a higher hp car with less torque (at the crank), you still get more torque at the wheels where it matters if you use the right gears to match the engine characteristics. It is really very simple. This is hot rodding 101.

Ed

[Brian Dinsmore]

Here's my example of what's tring to be said. My cobra dynoed at 355 rwhp w 300 ft/lb torque. It's a built 289. dyno sheet Here's my ET slip The trick isn't having a ton of torque its just in getting the torque you have to the ground. Brian

[CobraEd]

Brians got it nailed!

[CobraEd]

This conversation is at the core of the age old topic of why a hot small block can run away from a big block. Torque at the wheels boys, . . . . . not at the crank.

Ed