At tale of 2 engines and their transmission.

[vcho455]

Good afternoon Gearheads,
I am hoping to spark a discussion of transmission ratios that doesn’t center around overdrive ratios.

I am in the process of reconfiguring my 72 Firebird and inconceivably things have not gone according to plan. The car has a Doug Nash 4+1 crash box behind my stroker 455. The engine made 535 HP and 468 TQ when built with torque above 400 ft/lb’s between 2500 and the 6200 redline. I am planning on replacing the Doug Nash with what should be a far more streetable Tremec TKX. Along with the transmission swap I have a new engine in the works also. It is to be a 303 CID engine with some KRE aluminum heads with a horsepower goal of 475 HP and a red line of 7500 RPM. No longer is the Firebird to be a just drag strip toy. Upgrades to the suspension are in the works with the goal of autocross and maybe some track days at PIR. The car is riding on Nitto 285/40/17’s.

The schedule was to have things back together and running by spring. The hitch in the get up currently is the non-availability of certain custom parts allowing me to assemble the new engine before spring. Since I’d much rather have the car running when the rains stop next year instead of not come spring I want to have the new transmission installed behind the old motor and when the parts become available again assemble the new motor hopefully over the summer and install it next winter. This is where I’d like to start the discussion.

Which TKX to use? The advertised high RPM ability of the TKX is the draw for me. That being said I have seen very little online as to whether the transmission lives up to its advertised 7500 RPM limit.

I would like to find the best rear end ratio for the best TKX gear set. The current 3.08 rear gears in the 12 bolt will have to go. But what would be the best ratio to use with what TKO ratio?

I have studied both TKX gear ratio combinations at length and believe I may know what I should do but, I want to hear some other opinions on the subject. I did extensive searches of this site for discussions around wide ratio vs close ratio gear boxes or how to select custom ratios but, to my disappointment none of the threads I found talked about this subject from a competition standpoint. Highway drivability seems to be the biggest concern. That doesn’t really concern me, that is why we have trailers. I will say that my Doug Nash combined with the 3.08 rear ratio has worked well in the past for me.

As I understand it for autocross one wants to maximize the time in second gear on the course to reduce the number of shifts per lap. The new engine will be able to do that better, with the higher redline, than the old one with its 6200 redline.

Please feel free to respond my assumptions on this or any other statement including my choice of the TKX.

Obviously as in everything there will be compromises especially with 2 very different engines. But is it going to be impossible to have one transmission ratio and rear end ratio work reasonably well with both engines?

I have used the gear ratio charts here: https://weddleindustries.com/gear-calculator for much of my research and later I found this site to be beneficial too. https://www.blocklayer.com/rpm-gear.aspx

[Kenova]

Can I assume your Doug Nash has a 3.27 first gear?
A TKX with the 2.87 ratio and a 3.50 rear gear will give you an overall first gear ratio of just over 10:1 which would be close to what you have now. Probably a little more than you want for the big engine but pretty good for the small one. You would have to look into whether or not the 1.87 second gear will give you the rpm/speed range you want on the course. The 0.81 overdrive would be fine for modest speeds on the interstate.

Ken

[vcho455]

I apologize for taking so long to reply. It’s all that life stuff getting in the way of responding.
You have taken a different tack to the problem than I was considering. I was thinking of using the lower 1^st gear ratio set ( 3.27 1.98 1.34 1.0 .72) with 3.31 gears. This would give the 3.27 box a first gear overall ratio of 10.82 which would be slightly higher than your recommended 10.19 but probably not noticeable to me without a back to back comparison.

The top speed in 2^nd gear with the big engine would be 76.1 mph with the 3/27’s and the 3.31’s
With the higher 1^st gear set (2.87 1.89 1.28 1.0 .68 ) and 3.55’s it would be 71.0 mph. Maybe enough difference on the right day on the right course to matter. But I am not any kind of expert on auto-crossing as my only experience was many years ago in my old Alfa Spider.

The MPH difference with the smaller engine would be even greater so there may be something to be gained there. 92.1 MPH vs 85.8 MPH.

The 2 differences between the gear box and rear end ratio combinations I see are the percentage of pull that each gear creates and the drive shaft rpm.
The calculators show the % of pull for each set up to be as follows:
With the 3.27 box and 3.31 rear ratio 2^nd gear=24.0% 3^rd gear=27.2% 4^th gear=23.5% 5^th gear=25.3%
With the 2.87 box and 3.55 rear ratio 2^nd gear=16.1% 3^rd gear=22.5% 4^th gear=19.5% 5^th gear=26.5%
I am under the impression that the more equal the percentages are the mph gained in each will be the same. This seems to be a good thing to me but, most of my experience has been 1/4 mile based.

The other difference I see is with the 3.31’s would be a reduction of driveline rpm which should result in the vehicle being smoother at any given MPH. I believe this would be more of an NVH and component lifespan plus more than an actual performance gain at the speeds I expect to see.

I am interested in your take on my musings.