IT7 rear end ratio guestion

J

jls288gto

New member
Anyone out there using the 5.17 rear end ratio? I know the 4.88 seems to be popular, why not use the 5.17 for more acceleration? I've currently got the stock rear end ratio, I believe it's 4.02 or something close.
Thanks in advance
Jason
 
ok, help me out a little more... big difference? Are we talking no top end at all in exchange for faster acceleration? I haven't done any motor buildup yet, and need gearing to at least help keep me out of others way, hopefully.
 
the 4.88 is the best for most guys/cars, the 5.17 is 4x the price and only good for tight tracks...

The 4.4 we used was great, but only for larger tracks. I think the 4.4 is a standard in the more powerful Rx2&3

K
 
by the way, I don't mean to sound like an a$$, I'm just getting started with this car and any help is greatly appreciated.
 
you will have to use 5th gear if you go 5.11 ratio, not good, go 4.88
 
Actually Daryl the 5.12 was soooo low that 5th become quite useable at very high speeds to save some motor... It seemed to me that the car hit an aerodynamic wall and 5th was as good as 4th...

4.88 was almost the same way, the 4.4 saved you from using 5th, but you had to be better than I was at carrying speed through the corner to make it work really well.

K
 
This really depends on the track and the nature of the corners and your gearing. Since we all use stock transmissions, the only thing we can change is the rear end. We use a 4.88 but will build a 5.12 I think it is for later this year in a separate pumpkin. In CENDIF, a track like Gingerman might be a good candidate for a 5.12 as it is slow with lots of tight turns and we think we might pick up about 1/2 second on lap times, but on the longer faster tracks 4.88 is probably the best, although one might argue that some tracks might even use the 4.4. (Road America?) What we all need to do is do some calculations once we get corner entry and exit rpms and see what corners we might be able to use the extra umph and where the new gearing might cause an extra gear change with the alternative gearing. That to me is the tradeoff.

Dick Plank
 
Jason,

Where do you plan on running? If you are in So-Cal and plan on running at Willow Springs, I wouldn't even do the 4.88.

I know that's the popular gear there but I think having to use 5th gear does more harm to the lap times than the loss of acceleration AT THAT TRACK.

The ITA record (CRX) is an 94.7mph average(1:35) around a nine turn 2.5mi track. Top speed will be about 125, and the slowest corner exits to a steep down hill. The spec7/pro7 guys are around 90mph average (1:40ish).
 
Last year I switched form my 4.09 (SE) to the 4.88. For the track I am at most times, it's perfect with the stock engine. 7800 in 5th at the end of the front straight, and 8100 in 4th at end of back straight. The track will decide the right ring and pinion.
 
The 4.09 alluded to in the thread was the lowest stock ratio... Other wise 3.9 was the common final drive ratio.

4.88 pre built, pre raced are out there and available, but you'll have to ask around.
 
I'm an old conservative guy. I only have a factory authorized rebuilt engine. Cost $950 delivered five years ago. Maybe some day I will be able to get one of those special IT motors.
smile.gif
Therefore, I don't try and run high revs with it. I run a 4.88 rear gear. I have spoken with a lot of people about shift points. I tend to "short shift" at about 7200rpm unless I am in the heat of battle. At Memphis I am, on a really good lap, at about 7400rpm in 5th at the end of the front straight. At Road Atlanta its a couple hundred more. My understanding is that above 8000rpm the stock metal seals begin to "bounce." I know I'm not the fastest driver but I really haven't had anyone (RX-7) just walk away from me on the straights. Last year I got to within 2sec of Bob Stretch's lap record at Memphis in his killer 240SX. Hope to do better this year with some different springs, etc. Sorry I wandered. My point is unless you have a motor that is purpose built and really strong at the upper end (carbon seals, etc.) I have been told by two well respected engine builders that after about 7000rpm you are really just making a lot of noise and it more beneficial to shift.

Just my experience and .02.

Drive Well
 
Dyno chart for a well tuned but old and tired steel seal motor said shift at 7900. If you shift from 3rd to 4th at 7000 rpm, it drops to 4500. Power doesn't start to get good until 5600 or 5800 rpm on anything I've seen. It will last a lot longer shifting at 7000, but the area under the curve gets small and the car goes slower.

Tire size is the other option for ratio tuning. 205/60, 225/50, 225/45 are all viable hoosier options. Just a matter of trying to find the best compromise.
 
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