Reese04

I have a 7.5 IRS RE with 3.27 gear ratio and a TKO-500 manual transmission. I blew out the T5Z a little over a year ago. Couldn't take 300+RWHP. Now it is rather sluggish in 2nd gear. But in 3rd and up it gets it, with allot of speed. I have only had it up to 120mph, but I had most of the throttle yet to go. I'm guessing it would have a top end of around 180 but may take a bit of distance to get up that high. I'm not exaggerating!!

This year I just had 4.10 gears installed for lots of lower end take off and torque. Yep!! It does that, but the high end as far as top end speed goes, has been destroyed. With the cam I have, you definitely feel it kick in around 2400rpm. Almost like a passing gear in a automatic. Well, maybe a bit of an exaggeration. but you can definitely feel it kick in. The 4.10 definitely takes off fast, but I guess I got spoiled with the 3.27 speeds I was able to hit in 3rd, 4th, and 5th gears.

So, when this Winter hits, I'm pulling the differential, taking it back to the shop and having my 3.27 gears put back in. Yes, I will lose the 2nd gear war, but after that I'm catching up. I just read that 3.27 gears are more for forced air induction Mustangs where power at the line is not a problem, so they will be able to accelerate to a higher speed. Evidently the 3.73 gears is the gold standard, in between the 3.27 and 4.10. In my Miata, when I first completed the mod with 3.27 gears, I noticed that the speedo read 60, but I was really doing 65. It ramped up from there, where the difference between the speedo and actual speed increased with speed. So when speedo read 65, I was actually doing 72mph, and on up.. However with the 4.10 gears the when the speedo reads 80, I'm actually doing 65mph. The Rpms are higher and speed is much lower, sacrificing speed for torque and takeoff. I think I like the higher top speed better. I wished I could have the take off I have with the 4.10, and top speed with the 3.27.

tbone heller

Why not install a 3.55 gear?

Reese04

Thanks, I will consider it. Tremec has a website devoted to calculating your speed with a given gear ratio, tranny (of course one of theirs), and rear wheel size. Here is the link. https://www.tremec.com/calculadora.php It is very accurate. Better than using one of those road side radars. By the way how do you like your Kenne Bell? They have been discontinued. I have been looking for one everywhere. There is a rumor that they might make a few more, but nothing in the near future. The pandemic took a toll on their employee situation so they dropped making anything that they weren't selling allot of. I called them and talked to Mike.

engineer

Hi Reese04,

I have experimented with three separate ratios in my Miata (Ford 331 stroker 440hp, T5z, 7.5 IRS with Tru Trac). I have had 3.27, 3.45 nd 3.73 ratios. The 3.27 was quite tall and only good for cruising, car was a sluggish. The 3.45 was similar but a bit better. But my favourite ratio is the 3.73. It provides a solid kick over the 3.27 on acceleration but still leaves a very good margin for cruising on the freeway. For example the car is at 2300 rpm at 70 MPH with a 215/45/17 tyre. I find this to be just perfect with the T5Z which has a deep overdrive 0.63 ratio. You get a solid acceleration advantage in gears 1-4 and then 5th is still relaxed.

The 4.1 ratio that you had fitted was no way going to be useable on the freeway. A 4.1 ratio will force it to rev too hard at 70 MPH probably around 2700 rpm.

You want the cars gearing and camshaft operating window to align. So if you are cruising on freeway in top gear, with the engine at say 2300 rpm. You want your camshaft operating zone to be starting there too. That way the engine will run well at the cruise speed ( 70MPH) in an rpm zone that its happy with, and any squeeze on the throttle will deliver good power.

By contrast, 3.27 gears and a cam that kicks in at 3000 rpm with a deep overdrive transmission will feel rubbish on the freeway. The Engine will be off note and sluggish performance will entail, as the gearing wont let the engine get into the camshaft operating rpm band fast enough.

Thats why you see shorter diff gearing on large cam cars, to help them pick up rpm and get into the powerband faster.