victor25

I have searched the forum, but have not found anything, yet. Has anyone attempted to change 5 gear into a overdrive? The Black 928s4 is a supercharged 89 5sp. As most of you know that year has a great final drive ratio for around town, but on the highway, it screams like a banshi!! Being supercharged I would like to drop the RPMs way down when I am cruising on the highway. Here in Michigan it is not uncommon to be cruising at 80mph with the speed limit being 70. My rps are quite high, the supercharger is trying to push a lot of air, and the millage is not so great. ANSWER... change to a different rear end, like an 87, or keep it, and try to change 5th gear to an overdrive setup. Ultimately I would like to drop down to about 2000rpms at 70/80 mph.

So has anyone tried this?? Is it possible?? From what I understand 5th gear drives the pinion shaft directly... Yes? So I would have to design and machine both a new 5th gear and pinion gear/shaft.

If anyone knows a thread on this please point me to it.

Victor

WallyP

I think that changing fifth gear would be totally impractical. There are two possible ways to improve your situation - taller tires and a change in differential ratios.

With a 2.2 diff, you should be at 2280 RPM @ 80 MPH. Perhaps you could find someone with a 2.2 who wants snappier performance around town and the two of you could swap transaxles.

victor25

I believe it a 2.63, and I am around 3000rpms

brutus

There is no way to "change fifth gear" it is direct drive to the pinion no gears involved with 5th at all. Your option is the 2:20 ring and pinion gearbox where due to different lay shaft gears gears 1-4 are lowered in spite of the tall 2: 20 ring gear. That 2:20 gearbox IS basically an overdrive transmission as it is not possible for a stock engine car to pull redline in fifth.

mark kibort

why would you want to get an "overdrive"?
do you want gearing like the corvette? It has a 6 speed with over drive.
I mean that tongue and cheek, because when you look at the corvette 6th gear (overdrive) vs the 928 2.2 rear end 1:1 5th gear, you end up seeing that the net ratio of the 928 is almost a little more of an overdrive to begin with .

so, compare the acutal ratios of what you are trying to emulate, and make a decision.

plus, you dont want to chug a long at 80mph at an rpm lower than 2400 anyway.

edit: so, you have a 2.54 or 2.73? if so, just find yourself a 2.2 gear box and call it a day. that will get you the overdrive you speak of. . dont worry about all the nay sayers, it will do exacty what you are trying to do, and all that will do for all the gears is shift up their range of operation by about 9%. in otherwords, if you redline at 45mph in 1st gear, now that will happen at 50mph. not a huge difference and probably better for your increased HP anyway. (i.e. 2nd would redline at 80, 3rd at 118mph and 4th at 155. the good news after 155mph, you get OVERDRVE to 220mph!! )

victor25

BUMMER... OK... Anyone want to swap an 87/88 for a 89 Diff?

Actually I guess have to swap trans too then if the pinion is part of the trans, Yes?

victor25

HI Mark
Well simply put, as I cruise down the highway I keep reaching for another gear. It just seams as the engine is turning way faster than I would like. It drives fine and sure makes for quick getup and go, but I would rather downshift to 4th, and just be able to just cruise quietly in 5. I can hear and feel the engine a lot more with this car than I could my 87. Plus with the supercharger on there I have more than enough power to pull the lower ratio, plus less wear on the supercharger, and better milage

mark kibort

Hi,

yes, i get it.

the answer is simple. you need a swap with a good 2.2. you wont be disapointed. it is the gear box for you and will be all that you are loooking for. no change in the world will give you what you want besides this gear box. 2.2 !

look at the actual ratios. 1-4 th are not that differnt , especially for street driving. you get the 5th gear you need, and i know exactly what you are talking about. ive had a 2.75 rear end car before for a while. its no fun on the hyway, due to over revs going 80mph, and with a supercharger, you certainly dont need the extr revs. the 2.2 gives you all you want and no downside, unless you are a dedicated drag racer that cars of launch out of the hole in a drag race. that can be better with a 2.75 or 2.54. but other than that, there is no loss going to a 2.2. i race with a 2.2, ive raced with 2.75, and the gear spacing which is the main difference is not chnaged.

Mk

dr bob

A REAL overdrive is a unit that does not allow the engine to reach peak horsepower RPM in that gear. There are more than a few cars that have two or more selectable ratios that meet this textbook definition. This definition includes the example MK cites, the 2.20 box. The 2.20, 2.54 and 2.63 boxes have about the same actual top speeds, but offer slightly different acceleration rates to get there. MK's RIP car was fitted with both the 2.20 and the 2.63 gearboxes for the record run attempts at Bonneville. Al Holbert found that there was no difference in top speed there, although that exercise was at altitude.

This NOT a invitation to debate the relative merits of various gearsets and how the horsepower moves. Really.

PorKen

You might try -4° to -6° (max is about ±8° in the cam gear slot) of cam retard, first.

I found that it feels 'effortless' to run at high rpms with more and more retard. Because of reduced dynamic compression? I'm running -8° on my '86.5 manual (2.2). It feels fine to roll along at 4000 rpm, or use 4th gear on the freeway. (Dyno comparison.)

I don't think you will have a problem with overlap using S4 cams with this much retard, and a supercharger. It may allow for better exhaust flow. You may also be able to add more ignition advance.

Dutch 928 fan

I drive an 84 S2 stock 310hp euro, dual distributor, no cats, but with a 1987 gearbox. i bought it with that set up and love it: if I want to thrash it I shift down I get to do insane speeds in fourth. I leave most cars on the road way behind, because in any gear up to and including fourth it pulls like crazy. Once you have left all behind in fourth, you shift to fifth and 'cruise' along. 1900rpm at 60mph, 2200 at close to 80 and...... See photo:

victor25

yep, thats what I want, a rocket through the first 4 gears, then cruise at 2200 in 5th at 80mph (thats 128kph for you euro folk...HAHA

so whos got an 87/88 rear end?

yardpro

i would be more than happy to swap rear ends....

928mac

What is it that he is just about at in the pic.
Is that 260

PorKen

About 8 mpg.