911 turbo tranny gear question

[jbrown]

I have an 83 sc with a 3.5 ( stroked for a 3.0 liter) turbo motor in my car. It has sc cams higher compression , k27hf turbo running 11lbs boost, has about 400rwhp and torque. I bought a 911 turbo short bell housing tranny off of Imagine auto about 3yrs ago. The tranny has short gears in it. about 50-60 in 1st, 80 in 2nd, 110 in 3rd and about 135 to 140 in 4th. I have the tranny out for a clutch so I wanted to change 4th gear to get the revs down alittle bit at cruising speed.
Right now at 80 I am at about 3400 rpms in 4th. Way to high for me. So I called Jim at patrickmotorsports and he came up wit a new 4th gear that would have me at 3000rpms at 85 which sounds good. The question I have is- I was told if I shift at 6000 rpms in 3rd, It would drop down to 3800rpms in 4th. Is that to low. Will I lose alot of performance. What do you gear heads think.
Thanks John

By the way the new gear would give me 85 at 3000rpms which sound great to me, I just dont want it to fall on its face when I shift into 4th when I am on it. I get full boost at 2700rpms.

[930gt-40r]

I dont think you will have a problem if it goes down to 3800 in fourth since you are well into the range of boost. It will never be as responsive in fourth as you have it set up now, but it will still be fine at 400 horse.

[911st]

Remember you will be doing 110mph when you shift to 4th.

If it is a street car an all your battles are mostly done by then you will have a great close ratio race transmission up to 110 and then 4th will be your over drive.

If you mostly do DE's on the track and you shift into 4th under load at 110mph and 3800rpm it is going to dog on you.

What rear tires are you running? If they are 16/245's they are about 24.5" tall. If you go to 17/275's you will be at about 25.5" tall which might reduce your cruse rpm about 150 rpm.

I would not want to be at 3000 at 85. I would spend to much time there and get ticked to often. If I am going 85 I want to be in battle mode and know I am moving along.

I also look to how my acceleration is going to happen on the freeway which here is about 60-70mph and sometimes 80. If I am at the point where boost is available in high gear most the time I can scoot around cars without having to drop a gear when cursing at 60.

If I want to get around a car fast, I want to be able to drop a gear and be solidly in to the HP curve.

If you have not tried the transmission I would put it in and give it a go before over-thinking things and see if you like it. It might be just fine. To me it sounds near perfect as it is.

[jbrown]

I have decided to change 3rd gear also. Making it alittle taller makes the gear ratio perfect.
I have had the tranny in the car for 3 years now. The car flys with it but I do alot of highway cruising and it is way to low for that. This should help me out.
I run 18" rims, I dont remember the size of the tires

[Matt Monson]

To do it right you're going to change 3 gears. Your 2nd gear is currently too short. With the right 2nd in there, you would want to go with a taller 3rd, and then the 4th that's been suggested to you is probably fine.

Do you know the actual ratios and the final drive? If you do I would be happy to chart it.

[jbrown]

I dont know the ratios I do now the speed of the first 3 gears at 6000rpms.
1st- 48
2nd- 84
3rd- 123
forth I am not sure.
I was told if I ran 2nd to 6000rpms when I shift to 3rd I would be at 4300rpms and basically the same at 6000rpms shifting into 4th.
I though that sounded pretty good.

[930gt-40r]

I would just change 4th- I would love to have short 1st-3rd and then have a tall 4th if it werent a track car.
Idealy I would have a 6 speed, but thats not in the cards for now.

[Matt Monson]

Originally Posted by jbrown

I dont know the ratios I do now the speed of the first 3 gears at 6000rpms.
1st- 48
2nd- 84
3rd- 123
forth I am not sure.
I was told if I ran 2nd to 6000rpms when I shift to 3rd I would be at 4300rpms and basically the same at 6000rpms shifting into 4th.
I though that sounded pretty good.

Based on those numbers, I'm pretty sure you've currently got stock 1,2,3 ratios in there with a slightly shorter than stock 4th gear.

Do you know the ratio on the 4th gear that's been suggested? It looks like he may be suggesting that you put the stock 4th back in there...

[pkracer21j]

Sounds to me like you have stock gears and a short R/P in it...8:39 or 8:41. My 77 runs out to about the same speeds. Shes good for 160 or so before the limiter cuts in.

[jbrown]

The speeds I gave you are with the new 3rd and 4th. It looks to be good. I am going to go for it. I know the old 4th is definitely a custom gear I have the tranny open and my mechanics says the numbers on it are not a stock 4th gear.

[jbrown]

If I remember right 160 sounds about right for redline in 4th. The new 4th gear

[RarlyL8]

I am familiar with your engine setup and the gearing issues.

With the HFS you should see 1.0bar by 2800rpm. The torque curve and boost threshold of a 400whp engine using that turbo will complement taller gearing.
I have a custom 4th and 5th gear in my 915 for that specific setup.
The new gears allow the car to drop to 2600rpm @70mph. During a hard run through the gears the velocity and torque carry the engine through the final gear change right in the meat of the torque curve. Car pulls like a freight train.

[jbrown]

rarlyl8, I agree with you. These gears are to short for my use. I was shifting so fast. My motor makes great power and I think it will benifit from the longer gears.

[Tippy]

I swear, my car pulls harder in 3rd and 4th than 2nd due to being in the ideal torque range having a G-50.

I wish I had taller gears all around.

[forwheeler]

jbrown, I sent you a pm.