Transaxle gear ratio question

[Kurt]

Gents-
when my tach gets to 4000 it feels like the engine going to blow up and/or world is coming off its axis.. !!
I have a 331 engine, open tube exhaust, an a ZF transaxle with the following gearing:
1st 2.23
2nd 1.47
3rd 1.04
4th .846
5th .705
diff 4.22
I have other cars, like a CSX Shelby Cobra, so I know "loud, brute, raw" etc. cars. But this is REALLY LOUD.. and the last thing I want is to do any damage....
any ideas/thoughts ? Or is it just me and I should just let it wind up....
Thx- Kurt

 

[Rick Muck- Mark IV]

Gents-
when my tach gets to 4000 it feels like the engine going to blow up and/or world is coming off its axis.. !!
I have a 331 engine, open tube exhaust, an a ZF transaxle with the following gearing:
1st 2.23
2nd 1.47
3rd 1.04
4th .846
5th .705
diff 4.22
I have other cars, like a CSX Shelby Cobra, so I know "loud, brute, raw" etc. cars. But this is REALLY LOUD.. and the last thing I want is to do any damage....
any ideas/thoughts ? Or is it just me and I should just let it wind up....
Thx- Kurt

You really wanted the 3.77 diff.

 

[Jac Mac]

Gents-
when my tach gets to 4000 it feels like the engine going to blow up and/or world is coming off its axis.. !!
I have a 331 engine, open tube exhaust, an a ZF transaxle with the following gearing:
1st 2.23
2nd 1.47
3rd 1.04
4th .846
5th .705
diff 4.22
I have other cars, like a CSX Shelby Cobra, so I know "loud, brute, raw" etc. cars. But this is REALLY LOUD.. and the last thing I want is to do any damage....
any ideas/thoughts ? Or is it just me and I should just let it wind up....
Thx- Kurt

A 331 should be just getting happy at ~4000 RPM if its a fairly warm build.. Your 4.22 R&P with 0.705 5th gives you an effective final drive ratio of ~2.97/1 for comparison to your Cobras etc. Ricks 3.77 R&P recommendation would give you ~2.65/1.... you sure your numbers are correct??..is your Tach set for 8 cylinders?

 

[Graham Endeacott]

You need to figure in the rolling radius of your rear tyres for these figures to mean anything at all.

Regards,

 

[Jac Mac]

You need to figure in the rolling radius of your rear tyres for these figures to mean anything at all.

Regards,

Yeah, I read his other posts, he has Avons on 15" & Pirellis on 17" but no sizes listed, but given that ~2.97 final drive ratio it should be good for ~160 mph @ 6k on a 26.5 rear tire dia or 80 mph at 3k. ( Both 5th gear )... makes me wonder if the noise of concern at ~4000 is just getting up on the cam or another issue. Not enough info..

 

[Graham Endeacott]

Yeah, I read his other posts, he has Avons on 15" & Pirellis on 17" but no sizes listed, but given that ~2.97 final drive ratio it should be good for ~160 mph @ 6k on a 26.5 rear tire dia or 80 mph at 3k. ( Both 5th gear )... makes me wonder if the noise of concern at ~4000 is just getting up on the cam or another issue. Not enough info..

Good point.

Regards,

 

[Kurt]

Gents-
Thank you for your responses...
its a 3 year old Olthoff build.
I got the #s off the plate on the transaxle itself.. so I'm assuming the #s are correct
Tach set for 8 cylinders? that don't know-
My rears are 26.5 and my fronts are 24.5...
thx Again- Kurt

 

[Erik Johnson]

The 4.22 ring and pinion is going to make your engine rev quite high at speed, as mentioned about 3000 rpm at 80. The fact that you are running wide open exhaust on the car makes me ask the obvious question, what did you expect it to sound like?

I would check all of your mounts to make sure that they are all still in place and not rattling around, also check your transmission mounts, they can sometimes transfer noise into the cabin if they are solid.

If all the mounts look good, and your engine is running well, then perhaps you might consider a muffler to help take the edge off?

Erik Johnson
(720) 899-6005 phone

 

[Mike]

Kurt, what do you mean? It's loud or their are mechanical noises being made that sound destructive? Mine is open exhaust too and it really sang at 4K rpm on up. Sounded awesome.

Erik, I see you're in CO. What do you do on GT40s? Thanks

 

[Kurt]

Thx guys-
Erik- what did I expect ? lovely loudness, like on my cobra. But the gt40 make the cobra feel and sound like your uncle's Lincoln town car ;-)
I will check all mounts and will do on the exhaust- I have the muffler tubes- ill try them. thank you
Hi Mike- hope you've been great.. no- doesn't sound damaging mechanical noises, and I don't mind the loudness- I love it.. I just feel like it sounds like its revving too high for the speeds I'm going... 3500 at 80mph sounds/feels like 6000rpm.
and it makes me ask- am I going to blow this motor if I hit 120 mph,,,,

 

[Jac Mac]

This going to sound cheeky, but I will just ask anyway.... have you found 4th & 5th gears yet...eg perhaps you have a gear selection issue! or a slipping clutch... if your getting 3500 @ 80 mph that sounds likely on the info you have given..

 

[Kurt]

Jac Mac..
Not cheeky, my friend.. you make a good, logical point...
buy, yes, that 80@3500 is while I am in 5th....

 

[Jac Mac]

On 26.5 rear tires you should have 79.20 mph @ 3000 RPM.

So if you coax it up to 80 mph on light throttle it should have only 3000 RPM, if you then stand on the gas while still in top/5th and the RPM goes up with little or no speed increase then you have a slipping clutch problem.

If you currently have the Pirelli tires fitted check the dia on them and I will re-calc the speed.

 

[Julian]

Changing the 5th gear ratio is a common upgrade on Pantera's to provide a more reasonable 2600 rpm @ 80 mph for highway cruising (and a lot cheaper than the 3.77 R&P swap).

Julian

 

[Kurt]

Jac- My rears are 26.5 and my fronts are 24.5...
I will check the 80/3000 ratio and do the slipping clutch as you described.. thank you

Julian- will look into that..
thanks again guys...
Kurt