Effect of final drive on acceleration

[wdfifteen]

Originally Posted by Shaun @ Tru6

Where I live fun equals 2nd and 3rd all day long. I think a 2.73 will be a lot more fun than a 2.20.

Yes, it will. The 2.73 will give you zippier acceleration.

[Gogar]

I think you should finish your car up and get it together with the auto; there's just no telling what might have made that other fellas car feel terrible, even if it was "mechanically fit" on paper.

If you come across a five speed trans at a good price in the meantime, grab it and sit on it.

[Jim Bremner]

2.73 will be fine

My vintage Mustang had 4:11 gears but once you had it in 5th gear the ratio was 2.79 I ran 245/50/16 tires and at 5,200 rpm it was pushing 152 mph. I wish that I could have found a longer track to see if I could get it up into the 180's The motor made peak hp at 6,400 rpm


1st gear ratio times final drive ratio you want to have around 10 or 11 to one with my Tremic the first gear was around 13 to 1 so 2nd gear was before you crossed an intersection. Tremic now has a tight ratio box that I would have bought.

[flipper35]

I went from a 2.73 to a 3.55 in my Cobra replica and it does make it easier for my wife to drive and I can stay in a higher gear in town. It doesn't pull as hard with the 3.55 though and if I were running the 1/4 mile I would have to shift into 4th. The transmission ratios are 2.44, 1.77, 1.34, 1.00.

If I were running on a road course or drag strip I would go back to a 2.73.

[masraum]

Originally Posted by flipper35

It doesn't pull as hard with the 3.55 though

Huh? More gear means torque multiplication. More torque means more shove. How does more shove not equal more pull? It should pull harder in every gear with a lower rear.

[HardDrive]

Ya, I think you've got that turned around.

[jyl]

I had a '65 Mustang with the 289 HiPo that would not go much over 100 mph, but would roast the rear tires all the way to redline in 2nd . . . that was fun. Stuff had been welded to the rear axle housing then cut off . . . wheelie bars?

Back in the day, friends had 928s when they were still nearly new. One friend's 2V auto was a dog. Another's 4V manual was a rocket.

I'd get your car running, decide if you live it, then swap in a manual. Note there are, or used to be, supercharger kits for these.

[flipper35]

Originally Posted by masraum

Huh? More gear means torque multiplication. More torque means more shove. How does more shove not equal more pull? It should pull harder in every gear with a lower rear.

It should, but this is a lightweight car with ample horsepower. It winds up quicker of course, but it really pulls really hard with the taller gear. Odd I know, but what can I say.

Edit, I should say it winds up nearly as fast with the taller gear as the shorter gear.

[Tobra]

On the shocks. There is an outfit on this coast that does revalving/rebuilds on the Bilstein shocks. I have a set on my car and it is amazing what custom valved shocks do for a car.

[rattlsnak]

Originally Posted by Shaun @ Tru6

Where I live fun equals 2nd and 3rd all day long. I think a 2.73 will be a lot more fun than a 2.20.

You are correct.

[masraum]

Originally Posted by flipper35

It should, but this is a lightweight car with ample horsepower. It winds up quicker of course, but it really pulls really hard with the taller gear. Odd I know, but what can I say.

Edit, I should say it winds up nearly as fast with the taller gear as the shorter gear.

Honestly, I think that's one of those things where the butt-dyno thinks one thing, but what's really happening is something else.

[flipper35]

Very well could be. I just know it doesn't matter because my wife did not like the fact it idled at 18mph. Made it hard to drive in town for her. If I get the chance, I will run it at a track and if it gets in the 10s I believe you. Then again, it has much better tires now so I would have to run it with the 2.73 and 3.55 again with the same tires. it was traction challenged and could run 11 flat.

Also, I googled if others had the same and the Roadkill guys went with a taller rear to be quicker, but that was with an auto.

[javadog]

Shaun,

Your friend is dead wrong.

I spent 20 years driving an '85 US 928 with a five speed. The final drive we got in the US was simply too tall.

I also have seat time in a Euro 928S with about the same power and the shorter-geared 5 speed. Way more fun.

No question I'd run a lower final drive. You should, too.

st gear in a US 928 5-speed of that era feels like 1st in a similar vintage 930. Tall. tall. tall... lower the final drive and it will still be plenty tall; you won't be shifting too often.

JR

[masraum]

Originally Posted by flipper35

Very well could be. I just know it doesn't matter because my wife did not like the fact it idled at 18mph. Made it hard to drive in town for her. If I get the chance, I will run it at a track and if it gets in the 10s I believe you. Then again, it has much better tires now so I would have to run it with the 2.73 and 3.55 again with the same tires. it was traction challenged and could run 11 flat.

Also, I googled if others had the same and the Roadkill guys went with a taller rear to be quicker, but that was with an auto.

I'm definitely no expert, but I would be surprised if it hurt your 1/4 mile unless it really kills your off the line traction or you redline top gear before you reach the traps. I could certainly see it harming a 0-60 if it caused an extra shift. I know that sometimes gearing can cause a problem if you are at a shift point very close to the traps.

[Shaun @ Tru6]

Short post, extremely long day first going to my chrome shop, then test driving this pristine 91, then to meet Bob of Anchor Atlantic in southern Jersey to pick up my 928 heads.

Net net, I have no idea why anyone would want an automatic 928.

More tomorrow.





Love the GW

[masraum]

Originally Posted by Shaun @ Tru6

Short post, extremely long day first going to my chrome shop, then test driving this pristine 91, then to meet Bob of Anchor Atlantic in southern Jersey to pick up my 928 heads.

Net net, I have no idea why anyone would want an automatic 928.

I have always liked 928s.

I always thought the same thing (RE: auto 928), or any vehicle with an automatic trans.

But, now that I'm older, I can see the merit. I love my Boxster S 6 speed. I'd rather have that than a boxster with a tip or PDK.

BUT, I could definitely see the merit of having something like a Panamera, Cayenne, Mercedes, etc.. that was less of a sports car, and more of a touring car with an automatic for when I'm driving with the missus. She does love to go for a drive with me in the Boxster and is OK for me letting it out a bit, but there are also times when a slightly more relaxed, easier drive is nice.

[dewolf]

I've had seven 928's. Two S4's. Both auto. One an 89 and the other a 90 with PSD. Both of them would light the tyres off the line no problem. If the trans isn't adjusted correctly for first gear starts on full throttle then it will seem like a real slouch.

[DanielDudley]

Auto 928s can be fun to drive, but you have to hold them in gear like a manumatic, and drive them like you stole them.

[Shaun @ Tru6]

I made this chart. Let me know if it's not really viewable. Downloading and opening up bigger helps. I think the problem with the auto 928s I'm driving is they all start in 2nd. Still, my E320 seems more fun to drive.

[masraum]

Is the third line up from the bottom (89-91 928 manual) a little screwy in 4th and 5th?

If I'm reading your chart correctly, it looks like you're saying that in 4th at 6k you'll top out at 174, but in 5th @ 6K, you're at 161. I think you might want to check the formulas in your spreadsheet.