Poor-sha

After 5 year of track abuse my trans (which is probably the only stock part left on the car) has finally gone. My car is at Katech and the trans is going off to RPM for a rebuild. I figure this is a good opportunity to change out some of the gear ratios and want to see what folks think.

The car is a full track attack motor with the GM stage 3 cam and sees 80-90% track duty. While I hardly ever drive it on the street I would like to be able to so making it as streetable as I can without giving up track performance is a goal.

For reference here are the stock Z06, ZR1, and Z51 gear ratios (1-6):

Z06 (MM6)
2.66
1.78
1.30
1.00
0.75
0.50

ZR1 (MH3)
2.29
1.61
1.21
1.00
0.82
0.68

Z51 (MZ6)
2.97
2.07
1.43
1
0.71
0.56

Given that 5th gear is a huge drop and is something I do use at VIR I think that the ZR1 5th is a good call along with the shorter 6th since the cam likes a little more RPM at cruise anyway. The question is what should I do for 1-3 (4th is always 1:1)?

I'm leaning towards the Z51 gearing to give the car a little more punch off the line as well as make it easier to get started from a stop. I can think of turns where a shorter 3rd would be good (oak tree) and bad. Most of my days are at Summit Point main or VIR full.

So what do you all think?

AU N EGL

Sean

In that case look at Rockland Standard Transilla unit

http://www.rsgear.com/products/trans...tranzilla.aspx

Poor-sha

Thanks Tom for the suggestion. The gear options look pretty similar. Anyone else have any suggestions?

Solofast

Don't just look at the ratios, calc the speed in each gear and that will tell you more than a gear ratio number.

The Z51 first gear is a waste, you already have enough power to spin the tires all the way thru first with the taller stock gearing, so why do you want to go shorter?? With the torque available in second you hardly need first after you launch anyway. In the old 4 speed days the "close ratio" four speeds had a tall first and closer spacing between gears. They sucked for drag racing, but were the hot setup for road courses.

A shorter second and third "might" give you some more punch off of corners, but you need to look at where you drive and decide if that's a help, or, do you find yourself between gears and get nothing out of it.

There could be an advantage if you went to the ZR1 gearing in that your 2nd and third are closer, and you might be able to find the right gear a bit easier since there is more overlap between the gears, so you aren't coming off of a corner in the wrong gear ever.

spazegun2213

I agree with what Solofast said and will add that if you graph the speed in each gear (via rpm's) over your dyno graph, you'll get a great idea where each gear change will put you in your power band.

I'm of the opinion that the reason the zr1 is SO much faster than the z06 is the gearing. That being said I have gears identical to the zr1 in my race car and LOVE them. While I shift a lot more, you accelerate much faster and I never feel like I'm out of my power band.

YMMZ but that close ratio trans is worth a look!

Bill Dearborn

Aren't there two different choices for the ZR1? At least GM indicates that when they say you get one transmission with the Carbon/Z06 edition and a different one with all the other combinations.

Bill

Poor-sha

Thanks guys, keep it coming. I'm leaning towards the ZR1 set now. The only thing that concerns me is that tall 1st gear. With the cam the car is hard enough to get going and like I said I still want to think I'm going to drive it on the street.

Bill - I believe the only difference in the two ZR1 sets is one has a taller 6th gear for better mileage.

spazegun2213

So what exactly are you afraid about with the tall first gear? its not going to be drivable? not mild mannered? I mean GM put that transmission in a street car, so I would not hesitate to toss it in another street(ish) car.

topmover

Sean
I think Ive met you once and I know your on cornercravers as well. I've noticed your post on the various sites the past couple of days and havent jumped in as I dont have great interest or knowledge on some of the more technical stuff as I'd prefer to drive rather then engineer or wrench. However you and I seem to frequent mostly the same tracks. I have a ZR1 and a C5z both of which I track. Im not sure if the ratios in the C5z is the same as the C6z but both my cars make very similar HP and TQ numbers. My Z06(when it runs) is quicker but mainly due to weight(I think) as I indicated Im not an expert and it may have more to do with the TQ band however I like the gearing in my ZR1 much better. With the ZR1 I have more flexibility with 2nd and 3rd and the power seems so much more flat.

0Louis @ LG Motorsports

The car will be much faster with the Zr1 box. What clutch is in it? Cruise RPM in top gear will be high. 2100-2300 rpm.

First gear is tall, so itll be tricky if you have a light weight, small diameter clutch.

Poor-sha

Clutch is an LS9X. My concern with the tall first gear is mainly because of the stage 3 cam which isn't very happy below 2k RPM. I need to go find that site that will calculate speed at RPM for a given gear again but with the tall first and cam I might not be able to ride around at less that 25 MPH without constantly clutching in and out.

Everett Ogilvie

Sean here is my spreadsheet graph of what may be my optimal choice of gears. Note that redline in 6th is about 189 or so. This box is extremely close ratio and is based on a proven sequential set of ratios in a pro racing class. Note how the rpm in each of the high gears (4th, 5th, 6th) is centered about the max power point of the engine;



With these gears if I come off turn 14 at Road America in 2nd, all I need to pick up on the long uphill front straight is 100 mph to hit 187 !

dvandentop

you crazy SOB Everett!!!

let me know when the sequential is on order

Everett Ogilvie

LOL

Poor-sha

I thought I'd close out this thread with my final decision and observations now that I've gotten a few track days on the new setup. I ended up going with the MH3 (ZR1 close ratio) gearbox that was built by RPM and had Katech put in a Quaife diff while they were at it.

The result, I am very happy with the choice of the MH3 gear especially with as much as I agonized over the decision. The only downside is that the taller 1st gear and big cam do make it more difficult to get the car going from a stop and the car doesn't really like to drive at less than 20 MPH. However, once you're moving beyond that speed the car is fantastic.

On the street the close ratios and shorter 6th actually makes it easier to drive the car at an RPM range that is low but high enough to eliminate any surge. With the cam and original gears I'd often have to drive in 5th on the highway to get it smooth.

Obviously the track is where this really shines though. I've done a day at Summit Point Main, and 4 days at VIR (full and grande) since the change. This gear set seems superior in every way but here are some specific notes.

Summit Main:
Still a 3/4 track but the taller third really helps in the run up to turn 3 where I was typically running out of RPM in the stock C6Z gearbox. Also, it helps in the run up to 9 where it lets me get the car settled before having to shift in to 4.

VIR:
As everyone knows this is also a mostly 3/4 track but again the taller 3 really helps in the lower esses where I can get the car settled before the upshift in to 4th. In oak tree 2nd gear is definitely the right gear now whereas before it always seemed like 2 was too short and 3 was too tall. I can get comfortably in to 2 and then run up to 5th on my way up the hill. Of course, the shorter 5th was one of my main reasons for picking this gear set and it works well. I am definitely carrying more speed on the back straight and am no longer up near the rev limiter in 4th saving me wear on the motor. Finally, coming on to the front straight again the longer 3rd helps me get on to the straight and out of the turn before I have to upshift.

All and all I'm very happy and the only thing I would change is a shorter 1st gear of it was possible.

Joy of 6

I am getting a new RPM tranny for the same reason you are. I talked to TJ there and was wanting to change 5th and 6th in my MN12. He said cant do as the shafts are different. I ordered the level 5. So call him and see what you can fit in your gear box or just order a new one that fits. I opted for the stock MN12 box for one reason is that I have 3>90 rear end, so stock 5th ratio works ok for me