low torque and high torque motors

There appears to be two lines of thinking when it comes to motors; high torque strokers (342, 347, 396, 408, 427) and lower torque motors (302, 306, 331).

One line of thinking says that high torque strokers are difficult to drive and have no place in a GT40, while the stroker line of thinking is just fine having gobs of low end grunt.

Will anyone share which type of motor they chose and why the chose it? Why they went with a stroked 396 or 427 with 500-600 lbs torque if having this much torque makes the car a beast to drive?

Next to colour, engine selction is the most difficult choice to make. /ubbthreads/images/graemlins/confused.gif


Thanks kindly...
 
Re: low torque and high toque motors

I'm using a 1964 427 side oiler just for nostalgia! the wife says"AND POWER" /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif
Nathan
 

Jim Rosenthal

Supporter
Re: low torque and high toque motors

I am going with a 302 motor (well- actually a 305 or so after a cleanup 030 bore) for the following reasons:
1) in a car that only weighs 2000lbs or so I don't think you need tons of power to go fast, especially with low drag
2) my transaxle has a fairly low final drive ratio, with an overdrive 5th- more torque than 350-400 ft lbs will make the car more than I can handle
3) the car needs to be street driveable
4) I would like to have the motor hold together for a while

Maybe this is no place for conservatism- after all aGT40 is a pretty over-the-top car- but I have heard it said that the chassis of a car should always be faster than the engine. Given that a 2000 lb sports racing car is not much of a safety vehicle, I am trying not to build in more speed than the chassis can handle. Not to mention the driver...
I have to admit also that I was lucky enough to find a 1968 302 block, which makes the emissions issue easier. If I were using a modern 4bolt aluminum block or something like that, I would probably want more power for all the engine $$$$$$ I was spending.... /ubbthreads/images/graemlins/grin.gif
 
Good point Jim - is the driver up to the engine?

The reason it's so hard to get insurance on Cobra replicas these days is that so many people have built them with the biggest, honkingest (is that a word?) engines they can, and then they go around a turn, mash the throttle, can't hook up all that power, and bye-bye Cobra.


BTW, did you catch the typo? A toque is something you wear on your head, eh...
 
I have a 302.
After some thought, I concluded that...

A) The GT40 is a light car. The rule of thumb says, "light car - high horsepower", "heavy car - high torque". You can't use any more torque than you can put to ground.

B) The car is historically a circuit racer and as such needs an engine characteristic that will get it around the track as quickly as possible. Too much torque while pulling out of a corner just makes the tail less compliant.

C) A low torque, high rev engine just sounds better /ubbthreads/images/graemlins/laugh.gif

Just my opinion.
 
I like torque. On the street I am using a very low rear 4:56/1. I find this keeps the revs up at low speed which keeps the car charged and the coolant flowing. (I've also over speeded the water pump and alternator) I also find this puts the power in the range you need at street speed.
At LeMans they ran a 2:50/1 rear in my car.
I have no prob. with traction.
 

Ross Nicol

GT40s Supporter
Chris
I'll go with your sounds better statement for a 302.I've just spent 3 days at Phillip Island being entertained by the sound of my car and I wouldn't swap it for the world. I can assure you a 302 has plenty of torque to whip ass out of corners as you will soon find out with your car.By the way why do Porsche owners hate being passed by a GT40 so much that they move over to block you on a very wide straight.I didn't finish the race as my gearshift broke but I did 1min 50.8sec 3 laps in a row so times are falling nicely.
Regards Ross
 

Jim Rosenthal

Supporter
Steve, you got me on the typo. However, I do get the distinction of being the first person on the GT40s Forum to have made a mistake in spelling....ha ha, not.
 
Actually, Knighton made the typo in his original post. And being from Canada, where they wear toques (made famous by Bob & Doug McKenzie), we can forgive him /ubbthreads/images/graemlins/wink.gif

[ QUOTE ]
By the way why do Porsche owners hate being passed by a GT40 so much that they move over to block you on a very wide straight.

[/ QUOTE ]
I may end up ticking some people off (so what else is new?), but this seems typical of Porsche drivers. They just don't like to be passed period, whether by a GT40 or a Datsun 510 /ubbthreads/images/graemlins/shocked.gif I don't really understand their thinking, but I guess it has something to do with a feeling of entitlement, given how much they have paid for their cars or something /ubbthreads/images/graemlins/tongue.gif
 
I have been looking into this question of motor sizing for some time and have come to a few conclusions (hopefully for the most part right). I will be very interested in hearing other people’s thoughts on all of this.

First: There are some other issues that get mixed in here. Most GT40 replicas seem to be using the Audi transaxle. This pushes people to a small (302/306) motor because the Audi transaxle was never designed to handle a lot of torque and would make a spectacular hand grenade behind a 427 Windsor stroker.

Next: The original Lola GT was designed in 1962 and the Ford GT Mk1 in 1963. At this time there were no wide tires in existence as yet. The early Mk 1 cars used 6" wide front wheels and 9" rears, and the body was designed to fit around this. This forces the use of what are very small tires by the standards of modern cars (as an example: Roaring Fourties recommends 205 front and 255 rear tires, which is a close approximation of what the early cars ran).

You will notice that a lot of the evolution of the GT40 (and racecars in general at that time) was to making room for wider wheels and tires as they became available. The Mk2 went to 8" front and 9 1/2" rear wheels, and the Mk4 went to 8" front and 12" rear.
One year later, the Gulf cars had flared rear fenders and were running 13" and even larger rear wheels. This was one of the reasons they could match the lap times of the Mk2s from previous years with considerably less horsepower.

The point of all of this is that in a mid engine car, you need large rear tires in order to handle a lot of torque. One reason J-VI doesn't have a problem with his 427 is those 12" rear wheels, if you made him run 9" wheels it might be another story.

If you look at modern cars, the Mosler MT900 (a car of very similar weight and power to a GT40) uses 265 front and 335 rear tires. The Saleen S7 (slightly heavier and with a little more power) uses 275 front and 345 rear tires. The Ferrari Enzo uses 245 front and 345 rears. Porsche GT uses 265 front and 335 rears.

The main thing is that you have to decide what you are building. If is to be a daily driven street car, to be used rain or shine and you want the look of the early Mk1 body. Use a 302/306, keep the max RPMs to 6,500 or less and the Peak HP to 400 or under. Anything more than that and you will go way beyond what the small, rain capable, rear tires can take and you will have a car that wants to swap ends on corner exits.

If you are building the car with the intention of spending some time on the track, or just want it to be the best performing car possible, then you need to go to the Gulf “1075” flairs with minimum 315/35-17 tires (335/35-17 would be better). This will allow the use of an engine with much more torque without the car becoming hard to handle on corner exits. You also need to realize that you are building a very high power racecar for the street, and not try to cut any important corners. This means using DOT Legal racing tires (Kumho VictoRacer V700, Hoosier R3S03, etc.) and accepting the fact that you cannot expect to drive your car at any real speed when it is raining. Yes, you will wear the tires out every 4,000 to 5,000 miles, but that is the price you pay for running a racing car on the street and having it be controllable.

I hope this is of some use and if nothing else I suspect it will result in some very interesting commentary when some of the other members of this forum bring their experience to this discussion.

Kevin
 

Brian Stewart
Supporter
Maybe this thread should be moved to engines or tyres? I'm sticking my neck out here and am fully prepared to be shot down in flames if I'm wrong, but, for any given rubber compound, the coefficient of friction should remain constant, regardless of the width of the tyre (tire). I was always under the impression that the width had more to do with heat dissapation/longevity than to do with grip per se.
 
[ QUOTE ]
I'm sticking my neck out here and am fully prepared to be shot down in flames if I'm wrong, but, for any given rubber compound, the coefficient of friction should remain constant, regardless of the width of the tyre (tire). I was always under the impression that the width had more to do with heat dissapation/longevity than to do with grip per se.

[/ QUOTE ]

The formula F = (mu) x (normal force), that we all learned about in physics in the contect of blocks resting on inclined plates, simply does not apply to tires. Wider tires do yield additional frictional force.

There are primarily two reasons for this. First is that rubber is relatively malleable, and tends to fill the surface imperfections on a paved surface, therefore the rubber at the contact patch is subject to both frictional and shearing forces. Furthermore, the vulcanized forms of rubber in tires has unusual friction coefficient properties that don't behave linearly and that are temperature dependant. Second, wider tires can dissapate heat faster than narrow tires, and thus make it easier to keep temperatures in the "sweet spot" of optimal friction coefficient.
 
The issue of how tires work is much more complicated than simple physics, which says that if you double the weight you double the friction. This is only true for smooth, hard surfaces that don’t interlock.

A tire does not work by friction alone; it works by a mechanical interlocking with the road surface. The rubber gets pushed down into the small irregularities of the pavement where it “Hooks” to the street by mechanical interlocking. If you picture a bunch of very small gear teeth laid out flat, like a steering rack, and another identical set above them interlocked you will get some idea of how this works. The trade off is that the softer you make the rubber, the better it squishes down into the irregularities and interlocks, but the more easily the rubber tears.

Most tire development is based on compounding rubber that is softer but at the same time more resistant to tearing. (When you do a burnout, that black line on the road is the rubber that couldn’t take the strain and tore off from the tire). One advantage of a wide tire is that because you are spreading the load out over a larger area of rubber, the stress on each square inch is lower and you can pull more lateral G’s before the rubber begins to tear.

You are quite right about a wider tire having a longer tread life (if the designer doesn’t use the extra width as an opportunity to go to a softer rubber for better grip). The reason for the longer life is the lower stress levels in the rubber and therefore less tearing away. One can see this very clearly with racing tires where you see little (and sometimes not so little) balls of rubber all over the tires after some hard laps. These are rubber that has literally been torn loose from the tread.

J-6’s comment on his Ferrari is very instructive. The 60’s were when people first began to learn this (I think Mickey Thompson was one of the first to figure it out in the drag racing world). In the span of a few years tires tripled in width. Compare a picture of an early Mk1 to one of the Gulf cars from 1969. The difference in rear tire size is shocking. The gulf car looks like it’s a running a couple of steamrollers back there.

Kevin
 

Brian Stewart
Supporter
Thanks for the input Kevin and Mark. This place, as usual, is a mine of useful information.

Cheers, Brian.
 
Re: low torque and high toque motors

351C 2V heads with roller cam, EFI, and aluminum flywheel.
415HP, 410 lb-ft (385lb-ft @ 3500rpm)
ZF-2 transaxle

Stock stroke, .020 overbore.
The engine is in a mild state of tune and should handle those power levels with ease.

As I noted in another post, with 335 Michelin rear tires, from at least 2nd up, the car does not spin the tires. It just accelerates.

I agree with Jim. Low end torque makes the car street friendly and easy to drive. It is happy to run in 5th gear from 1100 rpm up to 6K. 1100rpm is about 30 mph. Since 99.9% of my car's life will be spent on the street, it needed to have good street manners.

After driving the car, I can say that it has more than enough power for me to get in trouble with. It easily overpowers 90% of the public roads.
 
Re: low torque and high toque motors

Dave, I assume that with 335s you have the gulf flairs on your car?

What size front tire are you using and how much rear sway bar did you need to get the handling balanced?

Also, how do you like the aluminum Flywheel for street driving?

Kevin
 
Back
Top