FiveElements

Hey here. Jim here with an 89 car. I'd like to put 18" wheels on that are 8" front 10" rear. What offset & tire size is ideal if I want to add a little height for taller gearing? And how can I calibrate the speedo to match the difference? (I want to drop the rpm at 100kph because the 4speed auto is just too short of gearing. 2700 rpm is just too high at 100kph. I'd like it to be more like 2200. This car already has upgraded intake & exhaust. The 928 is in despite need of 1 more gear). I also plan to install a 88 trani for the taller gearing. I know the 89 trani down shifts smoother and faster but I notice no substantial difference from my 87 car)

pcar928fan

Sell the '89 and get an '87 or '88!

SERIOUSLY! You will hate the lack of pick up though! I got a '90 trans in my '88 and WOW did that make the car quicker! You are correct about rpm being a bit higher than on the earlier S4's though. Prior to this trans my '88 turned right at 2200 rpm at 60.

Sorry I can't help you on the wheel question much. The best 18" wheel fit that you can get from factory Porsche wheels are either the base 18" Panamera wheel or the CIII wheel from 911's. Those are 8" and 9" for the PanAM wheels (and I LOVE them on my GTS btw) and 8" and 10" for the CIII wheel.

FiveElements

Ya, but I like the 89 car interior and brakes. Also my current 89 has new everything. My 87 car has no substantial change in torque or performance even with the taller gears. The 89 car has enough improved intake & exhaust upgrades that putting the 88 trani will not be so bad.

FiveElements

I also think the ratios are different for each gear in the 88 & 89 trani. Does someone know where to find that chart?

TheoJ

does this help?
http://jenniskens.livedsl.nl/Evolution/928specs.xls
I also made a spreadsheet to calculate ratio, teeth, meshing, effect on speed etc. Did this a while back to find out what was humming in my diff. Need that?

FiveElements

Wow. This is great. Thank you!

mark kibort

havent we beat this up before , so so many times.

No, the gearing wont make a difference in acceleration , "overall". remembrer, gearing only optimizes your ability to put the power to the ground over any given speed range. since no one has spoken of speed ranges, there will be NO difference. (because you are not changing the HP of the engine)
BUT, if you are talking speed ranges say, 55 to 80mph, or 50 to 73mph, then each gear box will hit those ranges with different efficiency. In other words, if you goal is to optimize from 55 to 80mph, the 2.2 S4 box is best. if you want to optimize from 50 to 73mph, then the GT transmission would be better.

Ive raced both at the track, with the same output engine and tires and spun near the exact same lap times. (just for your reference, 2.75 vs 2.2 gear boxes)

Now, the 18s are a great rim size. and to answer your question directly, there may be absolutely no difference in diameter with 18 vs 17s. there is little even going from 15 to 18" !!!!! why because the tire profile changes with the bigger diam. rims to keep the diameter relateively constant. 24.5 is the general overall diameter. I run up to 26" rear tires 305x35s.

keep in mind, the RPM vs MPH will always never change as they are tied together. however, your actual MPH maybe a little faster than before . actuallly , since the 928 already is slower than actual, the 26" tire actually makes the Speedo accurate.

FiveElements

Thanks Mark. This is good information.

Speedtoys

Wow..people line up DEEP for the 89 auto transmission.


Swap it.

GlenL

I'd say so. He's not talking lap times, by the way.

I suggest getting some tires to hold up in the body. It's pretty tight. Going to 25" diameter instead of 24.5 is a 2% change in RPM. You'll want a lot bigger (33"!) to see the change you're getting at. Craigslist has used tires or a friendly tire store that'll work with you.

It's a final drive change that you want. Whole tranny or gears at yer option.

Bjbpe

Manual transmisions are a lot more fun. My '87 S4 5 speed hits 65 mph at just around 2000 rpm (for what it is worth).

FiveElements

Yes, but is it easy to put in a clutch & 5speed in an 89 auto? Anyone do that? Are all automatic 928s missing the mounts to even add a clutch petal?

Ethre

There are a couple of threads on Auto-Manual conversions. Pretty big undertaking.

I switched from 225/50 to 225/60 on rear. If I remember the calculations right that is a ~5 mph speed shift at highway speeds. That it definitely noticeable in terms of engine speed on the highway.

FiveElements

I've got the Cup 2 wheels now in the back with 255/40/17. I think I'll swap the trani & go with a 18x10 996 rim in back.

mark kibort

its more like a 5% change in diameter and that would work out to 2.5mph at 50mph. and you wont be able to see it, as the RPM and MPH are tied together.