Paul968

Hi,

I have a Euro 3.2 1985 911....... I was talking to a friend what I could do to make more power.

He said the cam is very mild from the factory and could make a lot more power if it were changed or worked on.
He explained the problem with doing the cam is how to compensate for the poor stock DME engine management.

He mentioned Mo-tec or a Map.

Does anyone have any info or has any one done this modification????

Thanks for your input.

Paul

Roamer

Paul, I would respectfully disagree with all of your friend's statements.

Yes the cam on the 911 is a little on the mild side, however most "upgrade" options only yield minimal power gains ~10hp. Worse yet, it tends to be all at the top end at the expense of some low-end torque.

Second, the Bosch DME is a very capable engine management system that when properly tuned can yield good power gains. The biggest limitation is in the air flow meter which can be swapped. Gains have been anywhere from a conservative 5-10hp up to a 35+(overly optomistic IMO). However, unless you want to build a forced-induction engine or really up the compression and operating rev range of the motor, it may not be that useful.

Same goes for the Motec, a great engine management system, but its usefulness is dependent upon the degees of engine modification. A stock motor with Motec might get you a little more, but don't expect signficiant gains. The dollar/hp ratio is very poor.

Rather than using a Motec system, you could get much more power through exhaust/headers/piston/cylinder (3.4-3.5 liter) upgrade and your ultmate hp number and dollar/hp ratio will be much better than the Motec alone. This, on american cars, usually results in around 260-270hp.

Talk with Steve at Rennsport for proven and reliable upgrade options, he's been there and done that.

Steve Weiner-Rennsport Systems

Hi Paul:

As Roamer said, the Motronic system is pretty good, given its mission to make good power and retain emissions compliance. I'd suggest several things, depending on what you want, your budget, and how you use the car.

Here's a few of the things we do:

1) A good chip

2) Exhaust upgrade with SSI's and a Monty Sport muffler.

3) Displacement increase; 3.4 litres

4) Twin-ignition

5) Extrude-honed intake manifolds

I've got some other ideas too, so feel free to call me about them.

Above all, a close-ratio gearbox is a HUGE bang-for-the-buck,....

Roamer

Steve, not to take this thread off on a tangent, but what would the anticipated gain be in terms of acceleration with modified gear ratios (to the extent you can answer this question at all)?

IMO, on Carreras, first is useless and the spacing between first, second and third are too great to be effective since the revs drop out of the engine's power band. I like the final drive combined with 5th, but am not really wedded to the other gears.

I have been thinking of this upgrade after seeing how current cars with equal hp and more weight are able to accelerate more quickly than a 911.

Steve Weiner-Rennsport Systems

Hi Roamer:

Installing and using close-ratio gears is akin to adding 50 HP to the car,....

Its hard to predict the exact change to 0-100 times, given how much driver skill plays a role and how badly one is willing to abuse the tranny to do this, but its usually worth 3-4 secs if the ratios are chosen wisely.

IMHO, this is FAR more important than any engine mod and should be done first.

Roamer

Steve,

This combined with some of the engine mods you mentioned above may be a cost effective alternative to engine swapping. Something to think about . . .

Paul968

I am interested in this option........ I have some questions:

1) Would a close ratio gear set up make it a more precise and more flowing gear box to shift??? Take away some of the vagueness.

2) Can the stock unit be utilized..... I assume so. The gears would be just added to my current tranny..... correct???

3) Are there any other major mods that have to be done with this conversion???

4) Who does this conversion / sells these parts???

5) What kind of price are we talking???


Steve Wrote:
"Installing and using close-ratio gears is akin to adding 50 HP to the car"

Sorry I think I am having a stupid moment..... are you telling me a close ratio set of gears adds 50hp??? Or do you mean once the close ratio gears are installed the needs to have another bhp to be really fun???

Thanks,
Paul

Roamer

Paul, what Steve is saying is that a change in ratios makes the hp that you already have that much more effective, the equivalent of adding 50hp to a stock set up.

Roamer

Steve, I would like to follow-up a bit . . .

In your earlier post you stated that:

"Installing and using close-ratio gears is akin to adding 50 HP to the car,....

Its hard to predict the exact change to 0-100 times, given how much driver skill plays a role and how badly one is willing to abuse the tranny to do this, but its usually worth 3-4 secs if the ratios are chosen wisely. "

I always was lead to believe that 50hp would get you the equivalent of .5 to 1 second in that 0-100 range or 1/4 mile. Still a very significant number. 3-4 seconds in acceleration times to 100 should probably take 100hp or more.

I am not trying to criticize your numbers, just get a better feel for what the true impact on performance would be with ratio changes.

If we assume that a Carrera modified with a chip and SSI's, in good running order and driven by a competent person can hit the 1/4 mile in 14 seconds flat, what would that same car and driver be able to accomplish with new gear ratios? Are we talking about a low-13's car here?

Because that would be great from my perspective, and probably other 911 owners as well. Has this ever been measured other than by "seat-of-the-pants" ?(If anyone else has measured this, could they chime in?)

Further, if you were to match the tranny to a high comp 3.4 making ~260hp, now what kind of 1/4 mile numbers are we talking about?

I do not mean to turn this into a "drag" discussion, that is not my purpose, but 1/4 times serve as a great reference for a car's overall ability to accelerate since you typically get through 3rd gear in the 1/4. I also realize this has little impact on top speed or acceleration over 100mph which is highly dependent upon drag and hp.

Thanks!!

art

One neat thing about cose ratio boxes is that they are more fun to downshift also, its easier to match revs when the gears are more closely spaced.

Art

Bill Verburg

Just to illustrate the effect of short gearing(blue) this graph shows stock US 3.2 gearing w/ 245/45x16 tires vs a short geared 915 in the same car(red). Note 1st and 3rd are the same gears.

The y axis is available forward thrust in lbs. the x axis in mph

Steve Weiner-Rennsport Systems

Hi Roamer:

You asked some good questions,....

I really get uncomfortable when asked to translate any engine or tranny upgrade into its direct effect on 0-60, 0-100 and 1/4 performance. There are simply far too many variables at work here to accurately predict what the changes will be and I hate making guesses,....That's why its called "Bench Racing",....

Just a few things at work:

1) Willingness to abuse the transmission internals by speed shifting

2) Willingness to abuse the clutch

3) Starting technique (critical)

4) RPM range and useful torque range; also called "area under the curve".

5) Horsepower

6) Tires & pressures

I will tell you that with all things being equal, and if I care not a whit about the car's mechanicals, I can saw off 2-4 seconds off the 0-100 times with a good close-ratio gearbox.

In general terms, I prefer to evaluate the overall effects on lap times with appropriate gearing and I think this makes a very big difference. The narrower the torque band is, the bigger the difference in performance,....


For the record;.....

I do not recommend pursuing drag racing activities with Porsche cars and strongly advise my clientel NOT to do so. Its very very hard on the car's components and will shorten one's wallet proportionately. I get no pleasure repairing people's driveline and engine damage from this, and there are other vehicles better suited for such things. Plus, the consequences are less expensive.

JackOlsen

0-100 sprints aside, the biggest allure of a close-ratio transaxle to me was for track use.

I have a tall second, stock third, short fourth and short fifth, so that the rpm drop change when upshifting or downshifting is only about 600 rpm. This allows me to keep my revs in the 'sweet spot' of the torque curve when accelerating, which is great for a rev-happy, but torque-challenged motor like a n/a flat 6.

The trade-off is top speed. You can't have it both ways (although a 6-speed G50 gets you closer to that goal). My gearing runs out at redline in 5th, at 135 mph with my current tires. For me, this is a no-brainer trade-off, since my local tracks don't let me see top speeds higher than this -- meaning no wasted gearing for those theoretical very long straights that I'll never see.

Roamer

Bill:

Thanks as always for a valuable and pertinent graphic!

Steve:

I agree with your feelings on "drags", and it is not my intent. You mentioned lap time reduction, could you give some examples?

Can an effective change be made to the gearing if I would like to keep 5th as is?

Also, is it necessary to shorten 1st? IMO, it is too short right now for my use.

Jack:

How is she on the street? That seems like a pretty short top gear for everyday thruway driving.

Bill Verburg

For rpm vs speed you need this chart


Jack is helped at highway speed by his rather tall rear tires. Most 911s have much shorter ones.

1st in a 915 is integaral w/ the mainshaft, big $$$