General engine performance question-(dreamlist)

[FinallyGotOne]

So what is the most beneficial engine changeout or mod to a 79 SC? How can I get my 0-60 speed much quicker?

I think we all know that the main issue with the 79SC is the gearing, and cam (correct?)

Boring out to a 3.2 litre is about $10,000, and adds about 30 or so horses. But keeping the same cam and tranny it seems would limit my overall quickness...

So can i change my gearing to make my car quicker without having to be driving around on the freeway at 6000 rpms at 70mph?
And how much pull will I notice from that if i did do that?

OR am i at an empasse no matter what tranny I use or gearing simply due to the fact that the cam is tamer?

So this brings up, can I somehow put in a higher performance cam, or will this entail a bunch of other work??

If I were to change out to a 3.2 L engine as some have done, won't I need to change out the 915 transmission to see a good improvement?

It just seems so annoying to me that the 79 sc basically has no acceleration in 1st gear without taching out, and I must be going maybe 30-40 miles per hour tached out, and needing to shift up to 2nd. BUt the car really dislikes that rpm range of shifting.

NO I'M NOT RACING MUSTANGS FROM LIGHT TO LIGHT, OR SUBY STI'S. But at an autocross that quickness will really improve the overall times. I run the same speed as boxsters and really think with a lighter car i should be quiicker.

I have the 74 911 headers and muffler, short shift kit, and dropped the weight of the front and back bumps and tea tray spoiler-replacing all with fiberglas. Remember this is my daily driver so I cannot delete the pasenger seat and go to uncomfy racing seat etc...

ADVICE??? I know there are some real gear heads that can help me..

[longhornchris04]

Big-bore CIS are "relativly" common, try searching the forums for more. They do a decent job of getting more torque and earlier which will help at the low end. That said, it means you are looking at a rebuild with new P&Cs, so if you go that route you might as well up the compression, at least to the single-plug limit. Hopefully a 915 expert will chime in on the torque/HP limits of the 915.

As for a hot-cam, the best available for CIS is probably the 964 cam, which adds a bit more top-end, at the expense of the mid range. This obviously involves a top-end rebuild. The problem is the way CIS meters the fuel, hot-cams cause trouble, so there just aren't many options.

Beyond that, you will have to start looking at EFI conversions. These will let you run much more agressive cam profiles, but more agressive generally means more high end at the expense of low end.

You may want to consider getting a new ring and pinion for your 915... unfortunatly as a daily driver this will hurt your fuel economy. It also means cracking open the transmision, but more torque and easier access to the power at high RPM w/o being too fast to handle the course.

There's also the "poor man's" solution of running smaller diameter tires if available (don't know what rims you have) as these effectivly change your gearing.

If you are running 225/50/16s on the back, going to a 45 series will be a 3.5% difference, a 40 series would be a 7% difference. This of course keeping in mind that different size tires have other implications.

[FinallyGotOne]

I am running 17 inch cup wheels with a 45/40 profile tires. I keep hearing about Taller gears. the rennsport site suggested taller gears could feel like 50 extra horses.
So that brings up the questions. AT what speed is this okay for. I mean it's great to be 0-60 lightning quick, but if I cannot reach 100 miles an hour, than really where s the fun in that??
I wont take it faster than 130 ever again.
Yeah, hopefully some 915 geeks will get in on this discussion...

[longhornchris04]

Hmmm, already running 45/40s... I don't think you can get any lower profile w/o problems.

Do you have the Bently manual? I'm at work so I can't look anything up (and my home computer is on the fritz, so I can't help when I get home).

I think the Bently has the gear ratios in it, our host may as well. That, plus your tire knowledge and a quick google for a transmission spreasheet should give you enough info to list redline speeds for each gear.

The host has a decenct sheet in the tech section, but I don't know what the ratios are for the '79 915 (i.e. what gears you actually have), and your tire height is different (close, but different). Of course, you may be able to determine your ratios by driving and noting RPM and speed (assuming your speedo is calibrated).

As for the "feels like 50 extra horses" you get the double benifit of higher RPMs/more power (primary) and slightly more gear multiplication (secondary). For autocross, this may be the cheapest path to victory.

[Steve@Rennsport]

Originally Posted by FinallyGotOne

I am running 17 inch cup wheels with a 45/40 profile tires. I keep hearing about Taller gears. the rennsport site suggested taller gears could feel like 50 extra horses.
So that brings up the questions. AT what speed is this okay for. I mean it's great to be 0-60 lightning quick, but if I cannot reach 100 miles an hour, than really where s the fun in that??
I wont take it faster than 130 ever again.
Yeah, hopefully some 915 geeks will get in on this discussion...

Hi,

With your taller 17" tire/wheel package, your overall gearing has become taller and that means fewer revs at a given speed. This also exacerbates the larger RPM jumps between gear changes that makes the car struggle more to accelerate.

Its not helpful for any car, but especially ones at higher altitudes,...

Properly chosen, closer ratio gears keeps the car in its best, most productive power band without making the engine scream at cruising speeds. One doesn't necessarity need a 150+ MPH car as much as one that gets to 120 in a hurry,... .

[FinallyGotOne]

Renn, you can suggest a better gear set up? Do I just replace tranny gears, or rebuild tranny completely? I tried to call you guys earlier today, I'll try tomorrow thx