Help Understanding

[brownslane]

The difference between a Mark 5 and a Mark 9 5-speed. How can you tell, what is the better box, etc.

Ya, I know..Tom, you have one! I know but I never figured out the differences. Anybody have the info?

part of the concern that I have is that I have gone "backwards" and installed a 12A instead of the 13B (don't worry, I'm not worried about legality. And the 12A is peri-ported,dry sumped etc. About 280 to 300 HP, from Drummond.

I guess I am worried about whether the existing gearbox will last. Hewland says the JFR is only good for 240 ft-lbs....how much is a mark 5 or Mark 9?

Thanks in advance.

[Brian]

There is a big difference between a MK 5 and a MK 9 five speed. I've run MK 5's for the past 12 years or so, not that I run much anymore. The obvious difference is the MK 5 has a 1.125" diameter layshaft to the 1" diameter layshaft on the MK 9. That makes a far stiffer installation, and the gears stay in better alignment under torque. First gear is always cut into the layshaft on a MK 5, and second gear always has an integral spacer on the layshaft gear, so that you have sufficient strength in the root of the smaller diameter gears. The tooth pitch is a lot coarser on the MK 5 gears so each tooth is a lot bigger and stronger.

You convert any MK 9 box to a MK 5 by buying the layshaft, the gears, and usually a different reverse gear hub, but there are a couple of options on that and I have a MK 5 in my sports racer with the standard MK 9 reverse (even though Oliver C. swore it wouldn't work). You can also convert a MK 4 or 6 box to MK 5. My first MK 5 had been converted from a MK 4 in 1965 with an official Hewland kit.

This is not to be confused with an HD-5 gearbox, which was a F1 box for Brabham et al if memory serves.

I think the MK 5 is good for around 180 ft lbs of torque, which depending on your engine could well be 280 hp. the limiting factors as far as I'm concerned is the input shaft is the same as a standard MK 9 and the ring and pinion is also. Both those are going to limit what you can do. If the Mazda is quite high reving you will need a low pinion ratio, and I think 8-31 or 8-33 is as low as they go. I run a MK 5 with an 8-31 behind a BMW M10 2 litre, which is fairly torquey. It suffers as all MK gearboxes do with not actually being intended to use first gear of five on track, and shifting into first on the fly and not getting caught in reverse on the one-two shift is a bit of an art. You only have the reverse gear kind-of-dog-ring for first gear. Grinding off half the dogs on the gear and resigning your self to buying new ones fairly often usually works. I took first gear out and made mine a four-speed after I engaged both first and reverse going into 5A one time. Bent the teeth on first gear pinion side.

Brian

[starkejt]

http://www.taylor-race.com/pdf/ACF14E2.pdf

[VSeaber]

Tom-

Our streetport in FS is making around 200 (230 at the crank) at the wheels but torque is significantly less than the standard FM engine. Our gearbox wear has dropped significantly compared to the 'standard' FM engine and we are running a MK9 with a spacer where first gear was.

Your bridgeport, in my opinion, should be fine with a MK5 (but close). If you have a dyno sheet we could compare-

I'd try the MK5 and see how it runs. I think, like us, you will be living after the peak of the torque curve for the majority of the use of the engine. Peak torque is important but looking at where the engine is spending its time is as well.

I can e-mail dyno sheets if you like-

Victor Seaber