Mk-4 Gearing

[Kevin M]

Jim (J-6),

I am trying to figure something out and would greatly appreciate your help.

In one of your posts, you mentioned that your car ran a 2.50 ring & pinion at LeMans.

What ring & pinion ratio did it use at the other races where they didn’t have to gear for top speed?

Second, I believe that the T-44 had the close ratio gears. Is first 2.32, 2.22, or something else?

And last, if you happen to know without having to go and measure, what is the diameter of the rear tires when the car is fitted with the proper racing rubber?

Thanks for your help,

Kevin

 

[Jim Glickenhaus]

My car only ran at LeMans. J4 won Sebring but I don't have those set up sheets. At LeMans my car ran:
Ring/Pinion 3.09:1
1st 2.32:1 5.57:1 overall
2nd 1.69:1 4.21:1 overall
3rd 1.29:1 3.21:1 overall
4th 1:1 2.50:1 overall
Tires were 1010x15 F (25.10 dia)
1235X15 R (27.35 dia)
psi cold 34F 40R
I now run a final drive 4.56:1 (overall) in 4th
and similar sized tires at 27psi F and 30psi R. (The more modern tires seem happier at these psi)

 

[Kevin M]

Jim,

Thanks for the reply. I am a bit confused by the numbers though.

The list shows a ring/pinion 3.09:1, but further down it shows 4th as a 1:1 with a 2.50 overall, which would imply a 2.50 ring & pinion?

Kevin

 

[Bob Putnam]

There is a set of quick-change gears in the back of the trans.
[image]http://www.erareplicas.com/mk2/qgears.jpg[/image]
Don't know, something is broken, will fix, Ron

 

[Jim Glickenhaus]

These #'s are off the race engineers set up sheets. The T44 also has a set of quick change gears at the back of the box. The ring and pinion isn't changed but the quick change gears change the overall ratio. The sheets could also be wrong but I believe the final ratio was 2.50:1 at LeMans which got them 223mph at 6400. Difference in tire PSI is interesting as well. When I try the PSI they used car feels terrible.
I'll ask Lee Holman about the box.

 

[Kevin M]

Thanks again for your help.

Kevin

 

[Ron Earp]

So with your specs (tire diameter and gearing) I get the following with a 2.50 rear gear:

Final 2.50
First 2.32 95.52
Second 1.69 131.13
Third 1.29 171.79
Fourth 1.00 221.61
If one observes 6800 RPM as max RPM. This does not take in account the increase in tire diameter at speed due to centrifical forces so 6400 RPM at 222mph might make more sense. First is almost good to 100mph!!!!

But with your super short 4.56 gear:

Final 4.56
First 2.32 52.37
Second 1.69 71.89
Third 1.29 94.18
Fourth 1.00 121.50

You are limited to 121 mph at 6800 RPM!!!

R

 

[Kevin M]

I think that 100 mph in first gear was perhaps intentional for more than one reason.

I’ve been wondering why, with as much torque as the Mk2 and Mk4s had, none of the drivers has ever complained about the car being at all hard to drive. (At least in anything I have read)

What occurred to me is that by using such a tall 1st gear, they greatly reduced the torque reaching the rear tires at speeds below 60 to 70 mph. Call it a simple mechanical torque control system.

That would explain why cars with modern 5 speed gearboxes and much lower first gears have more trouble with spinning the rear wheels. They may be seeing 50% more torque at the tires than the original cars.

Kevin

 

[Ron Earp]

Might be more than just mechanical traction control. Power is another thing that I've always found interesting about any of the race cars (or cars for that matter) from the 60s.

Horsepower back in those days was rated in gross terms, not SAE terms that were brought into play in the early 70s, thus they are seemingly over rated by today's standards.

You can verify this by any number of means but one of course it to look at trap speeds of cars back in the day. Sort of low for the horsepower they are supposed to be making. Any of course we see this on the dyno too.

With modern technology it is no simple feat to pull 400 hp from 302 inches, pulling out all the stops, but of course the race engines from those days are rated at 425 hp gross and the gross rating might explain it.

I've always wanted to see what some of those motors would actually put out on a dyno to see the real numbers. Same with some of the modern "Supercars" that are claiming some fairly high hp numbers for the motor but the trap speeds dont' back it up.

I suppose in the end it doesn't matter what they make, just what they did, but I'm highly interested in motors and motor technology hence the question.

R

 

[Kevin M]

I would love to see a Dyno pull on one of the original “390 hp” 289s. It would be very instructive.

I have always assumed that the numbers you see on a modern engine dyno (like Westech where a lot of the magazines seem to test) are close to the old “Gross” numbers because the conditions are the same: No Air Cleaner, Cold inlet air (not hot under hood air), open Headers or Headers through Race Mufflers, no Water Pump or Alternator drive, etc.

The big question is if dyno correction factors are equivalent to what they used 40 years ago.

If anyone has any information on this, I for one would love to hear it.

As for production cars, they are rated with full intake and exhaust systems as run in the vehicle and the accessory drives in place with all accessories. I think the correction factors assume higher air temperatures as well. (Ron, you would be familiar with the effect of density altitude on power being a pilot).

There is no question that a ZO6 Corvette engine would show a lot more than 405 hp if it were run a dyno in the same condition that most motors are dynoed.

Kevin

 

[Ron Earp]

I would like to see these dynos of some of the older motors too. I have seen some of the "hot cars" of the time but none of race cars of the periods. One Ford 427 motor in a Galaxie rated at 425 gross hp (A 65 I think but I'm not too good with years in those, nice Galaxie too) I noticed was only able to make 282 at the wheel on a local dyno here in Benson (my little 4 cly Lotus make 291 on the same dyno). So, that motor was more like a 325hp motor rated with current standards. Nothing wrong with the motor either, strong and stock.

I would be suprised if a 390hp 289 was making 340hp by today's standards, or around 300 rear wheel.

I would say the Z06 motor is the strongest ever in a Vette. The old 454s etc. were rated in gross hp and if you check that old cars' quarter time it is apparent it didn't have that much juice.

Jim, ever dynoed J6 on a chassis dyno? It would be interesting to see for sure.

R

 

[Kevin M]

I stared building cars and street racing in the early 70s and I remember that a lot of cars with high horsepower ratings wouldn’t make 200 hp when you put them on a chassis dyno.

A simple set of headers with header mufflers and a good dyno tune to get the spark curve and carb jetting right would frequently get you 60 – 80 more horsepower at the rear wheels.

A lot of those old motors were rated something like 400 hp @ 6,000 rpm with open headers on the manufacturers dyno, but they hit peak power at more like 4,800 rpm once they installed them in a car with cast iron exhaust manifolds, 2” exhaust pipes, air cleaners taking 150 degree under hood air, etc.

It’s very hard to compare the old cars to today’s cars because of the huge difference in the tires. The old cars had way too much torque for the street tires available back then.

I had a 68 Charger with a 440, 4 speed, 4.88 Dana Posi rear end, and that thing would spin in both first and second gear with more than about ½ throttle. If you ran it today with modern tires it would certainly be at least 1 full second faster 0 to 60 and a good bit quicker in the quarter as well. Those cars were fast on the strip where we used slicks and could get them to hook-up, but they were a lot slower on street tires.

 

[Jim Glickenhaus]

Ron
Never had J6 on a chassis dyno. As for the ratio's I may be higher than 4.56. I am using the lowest drop gear but I may not be that low. Using my tach and SPA speedo I figure about 170 at 7000 in 4th. I've had this motor up to 7000 but usually shift at a lower point 6800.
Best

 

[Kevin M]

Jim,

With 27.35” rear tires, that would come out to about a 3.35:1 overall ratio in top gear.

I’d love to hear that motor at 6,800 rpm. It’s got to be an incredible sound! I’m sure it’s a magnificent car to drive if you can just control your urge to bury the throttle all the time. Something I think I would find almost impossible to resist with a car like that.

Ron, the engine in Jim’s car is basically a slightly de-tuned version of the NASCAR motors from the 65 – 67 period. Finding a GT40 Mk2 or Mk4 that has been on a chassis dyno is probably going to be hard, but you might be able to find something on the old stock cars. Lee Holman might have a lead on this, if Jim wants to ask him at some point.

 

[Ron Earp]

Jim,

It would be easy to calculate the final if you know the tire height and know the RPM at say, 60 mph, in 4th gear. 4.56 is really short (numerically high) and would seem to make the car unlivable at highway speeds.

R

 

[Jim Glickenhaus]

Ron
I think Kevin is close. It's fine on the highway but where it really shines is passing on those 2 lane roads. The time you have to spend accross the dotted yellow line is never too long.
Best

 

[Kevin M]

The shock factor on the car you are passing ought to be pretty good when they see what just went by them.

 

[Mike Dozier]

Kevin

99% of America wouldn't have a clue.
The MK IV doesn't look anything like the MK I
or the new GT. On the street, I'll bet Jim gets asked if it's a Ferrari or some kind of kit more often than not!

MikeD

 

[Jim Glickenhaus]

Most think it's a kit car. Esp. guys driving Porsches. The Babes they're with aren't so sure about that...

 

[DON SILAWSKY]

I would like to see these dynos of some of the older motors too. I have seen some of the "hot cars" of the time but none of race cars of the periods. One Ford 427 motor in a Galaxie rated at 425 gross hp (A 65 I think but I'm not too good with years in those, nice Galaxie too) I noticed was only able to make 282 at the wheel on a local dyno here in Benson (my little 4 cly Lotus make 291 on the same dyno). So, that motor was more like a 325hp motor rated with current standards. Nothing wrong with the motor either, strong and stock.

I ran my original 427 S/C Cobra on a Dynojet chassis dyno a few years ago. The motor is original/tired, like the rest of the car, so the results shouldn't be taken as gospel for a new 427 side oiler w/single 4 bbl, but they probably aren't too far off: 309 HP at the wheels, which equates to 376 HP at the crank using a 0.18 drivetrain loss (that loss figure came from GM engineers for manual transmission Corvettes). Incidentally, my '91 ZR-1 Corvette (dead stock LT-5 350CID motor, 4 valves/cylinder, 4 cams) turned almost identical numbers.