Hi.

I want to install a spool and as tall as possible gears into my C7 Z06 differential. Anybody come across anything worth using? Also, does anyone know if I can use C6 Z06 parts to do the job? I'm using a Holinger RD6 gearbox with any gear ratio I want but 4th gear has to be 1:1 which means my rpm drop from 4th to 5th and 5th to 6th is more than I would like it to be. Taller diff will close these ratios up nicely.

Thanks.

Kerry.

 

Kerry 100% the c6 stuff will not work I just did a wavetrac from RPM give Todd a call they are a vendor in here & are the best to deal with due to this is all they do is trans & rearend & race what they put out.

The wavetrac get that cheap azz rearend set up out of this car plus I has a 2:73 when with a 3:42 for my A6 car. Robert

 

Thanks Robert.

Not sure what you're saying in the second sentence. Could you clarify please?

Kerry.

 

So much opportunity to learn here.

What ratios do you plan to run for 5th and 6th gears?

The final drive (ring and pinion gears) doesn't change the RPM drop when shifting. It only changes the road speed (not RPM's) when you shift. The RPM drop is the same no matter what final drive gears you have. The only way to change the RPM drop is to change the ratios for 5th and 6th gears.

And if you plan to drive it anyplace other than a drag strip (even if you plan to drive rather than trailer it to and from the drag strip), a spool is going to be really challenging to drive and very hard on other parts (drive axles, CV joints, tires, even wheels to name a few parts).

 

Hi.

I am gearing the car to hit 320mph in top gear and 4th gear has to be 1:1 therefore the taller the diff gear I run the lower ratio 5th and 6th will be and therefore the less rpm drop when shifting. So to clarify, I realise diff ratio makes no difference to rpm drop but will affect the 5th and 6th ratios I require to hit my speed target. This car is being built to run in a straight line only, almost always on salt.

Kerry.

 

OK, for a "land speed record" type setup, several things are needed to calculate all the needed ratios.

• Engine red line RPM? Or even more importantly, peak hp engine RPMs.
• Revs per mile for the tires you're using

I don't think "any ratio you want" is quite what you can achieve with that Holinger gearbox. They advertise "an extensive range of ratios available." That's not quite "any ratio you want."


Something in this setup is going to have to be custom made. If I were setting it up, I'd get the ring and pinion custom made. I'd pick some sensible ratios for 5th and 6th gears, and then calculate out a ring and pinion that puts the target speed at the peak hp RPM's.



Some quick calcs with my spreadsheets shows that with a stock sized tire (782 revs per mile, which is what a Michelin Pilot SuperSport ZP P335/25ZR20 is rated at), at 6750 RPM's, with a 6th gear ratio of 0.67:1, you'd need a 2.42 ring and pinion set for a speed of 320mph. I'm pretty sure 0.67x:1 should be within the "extensive range of ratios available" from Holinger for 6th gear, but you should check.



Once you've got 6th and the ring and pinion selected, you'd want a fifth that's slightly taller ratio than the square root of the 6th gear ratio. For a 6th gear ratio of 0.67x:1, I'd shoot for 0.79x:1 to 0.80x:1 (but never exactly 0.80:1) for 5th gear.



Of course, all my calcs here are based on guesses for tire size and for peak hp RPM's. I can plug your actual numbers into the spreadsheets and get something more exact for your own application.



BTW, Holinger advertises on their website that they can make one-off ring and pinion sets to your specs. So this is do-able with them. They may also be able to point you to a spool, or make one for you for this project.

 

You are to be commended as a forum member for going the extra mile to help out another.

 

There are also a few other calculations you need to do to be sure you'll be able to reach your target speed.

You need to make sure that, whatever RPM's it drops to on the 5th to 6th shift, at the speed you're traveling at that point, you need enough horsepower to overcome aerodynamic drag + rolling resistance at that speed (after the shift to 6th). If you drop too many revs and fall too far down the HP curve, it won't accelerate at all after the shift. This is why you really need to manage that RPM drop, even if it means you need a custom ring and pinion set to accomplish your desired top speed.

 

Ahhhh, because race car... I was wonder about the need for a spool... I had a mini spool in my GT3 road racing car... Made driving in the paddock interesting. lol

And 320 mph? I'm getting visions of the Acme rocket company, and hearing Beep beep sounds.... ;-)

 

From looking at what you are wanting to get to now, You are over my head but a spool is for sure is what you need in this car. I don't see these 2 small gears set up in the rear holding up at all. Good luck man.

 

So we agree then? Tall diff. Close 4th, 5th and 6th. Minimum rev drop. Peak hp at 8000Rpm. Tyre will be a 30" tall Goodyear LSR tyre. Sorry, tire.
My spreadsheet shows with the auto diff ratio of 2.56 I'll run to 554kph (target 500kph) which allows for the 10% wheel spin as advised. This will give an RPM drop of only 800rpm between 4th, 5th and 6th. Nice.

Question is, does the auto diff ratio fit the manual diff housing? Are the diff housings the same?

Thanks.

Kerry.

 

Whatever I end up running, those spider gears will definitely not be making the trip to Utah! Nobody seems to be able to supply a spool. The search continues!

Kerry.

 

You could just weld the spiders...

 

C6_Racer_X much appreciated.

 

*Meep Meep

 

I don't know for certain, but I suspect the diff is the same, and the ring and pinions should interchange.

What I do know for sure is that the aftermarket sources that offer 3.90:1 and 4.10:1 ring and pinion sets for C7's offer a single part number for each ratio, and don't differentiate between automatic and manual transmissions. That bodes well for the differential parts and ring and pinion parts being interchangeable.

Those 30" GoodYear LSR tires sure are tall and skinny. Really skinny. 3" wide tread skinny, mounted on 6.5" skinny wheels. Managing traction as you accelerate is going to be very challenging. Finding the right amount of aero downforce to keep them well planted without overloading will be a bit of a challenge as well.