3.55s or 3.73s???

quick35th

Registered User
So my rear diff is all apart. I have a newer trak loc, 8.8 rebuild kit and crush sleeve eliminator. I got rid of the 3.27s and picked up a used 3.55 gear set practically free. What I would like to know is if it would be smart to acquire some 3.73s and put them in while I still have it apart. Transmission is an M5R2 and I do not drive my car on the street, road course only. Gears 3-5 are most important to me to try to optimize. Engine is 306ci and I rev it to 6700RPM.

Shane
 
My 2 cents...

Well, I don't know how much HP you are making, but the 5.0L, unless really built with great heads and intake, etc. makes most of its power in the mid-range. Seeing how you road race this than I would say that acceleration definitely trumps top speed, but you don't want too much torque at the rear or you might lose it coming out of corners with all that power and gear. I would say go with the 3.55, especially with the M5R2. Just my 2 cents.
 
I am making somewhere around 324rwhp give or take a little at around 6400RPM. I have added a carb spacer and upped the jets since the last dyno so idk what it is exactly making.

Shane
 
It depends how fast you are going through the various corners and what rpm range you make peak torque in. If we assume you want to keep the rpms between 4K-6700, that means with 3.55s 3rd gear would be good for about 60-105mph, 4th would run about 90-150mph, and 5th would run about 120-200mph. With 3.73s those numbers drop to 60-100 for 3rd, 85-140 for 4th, and 115-190 for 5th. Now given those numbers, and the fact that you said you only care about 3rd-5th, and that with 325hp you aren't hitting 190mph, some 4.10s would give you 55-90 in 3rd 75-130 in 4th, and 100-170 in 5th and some 4.56s would give you 50-80 for 3rd, 70-115 for 4th, and 90-155 for 5th. Not knowing what speeds you run, or what the lower end of your rpm range is, it is hard to say, but unless you are running some sick speeds on high banked ovals, I would tend to say even 3.73s aren't enough for you.
 
with 3.27s I run out of first gear fast! I can't imagine the car with 3.55s or even 3.73. If your not using first gear then whats the point of having a 5 speed. You should be fine with 3.55 if the 3.27s were not enough.
 
The problem that I was having with the 3.27s at Mid-Ohio I was running out of gear in third before the corners and shifting to 4th to me would have been pointless because by time I shifted to 4th and got back on the gas I would have to down shift to 3rd again. So I was looking for a gear that would get me to 4th sooner but still be good on the road course.

As far as speeds go Mid-Ohio is my primary track that I'll race at but my hopes are that this december I'll be running the Rolex 24 course at Daytona. The car will have a race weight of about 3100lbs so even with 324rwhp it still is very capable on the top end.

Shane
 
I had 3.73 gears in my old 93LX w/ 5.0 - T-5 combo, worked quite well.

Hope this helps, my car wasn't a road course car though.
 
4.10 way too much with 5 speed!!

You have to remember that a 4.10 gear, mentioned above, although doable on the street, probably isn't the best for track use. Also, I would not suggest using 5th gear on the track, which you would with the 4.10's because it is an overdrive gear.
 
You need to use a gear/rpm calculator and your data logs from the track to figure this out on your own Shane. None of us have your setup or know the tracks you race as well as you do. That's what needs to determine the gearing you choose... not a bunch of guys on the net. Anything we said would be a shot in the dark at best.
 
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