Luis de Prat

Does anyone know what the optimum rpms are for upshifts on the 944 NA? Out of curiosity, I'd like to experiment shifting gears at different rpms to see if there's any change in overall fuel consumption.

In his book on driving, 1960 Le Mans winner Paul Frère writes the following:

When to change gear depends very much on the objective. It may be maximum performance, it may be minimum consumption; it may also be minimum wear-and-tear on the car. These various aims call for different techniques.

If minimum consumption is to be achieved you must remember that, in a gasoline engine, the specific consumption decreases as the load on the engine is increased. This means that under given conditions and for a given speed of the car, the engine uses less gasoline when it runs comparatively slowly -say in top gear- with a wide throttle opening than when it runs at a smaller throttle opening and at higher revolutions in a lower gear.

It follows that to achieve the minimum fuel consumption, the engine must always be kept at the lowest possible revolution rate without, however, letting it drop into a range where efficiency is impaired. In this connection, it must be remembered that an engine's most efficient range of operation is the one over which it produces its maximum torque. Below this range the efficiency falls off and the specific consumption rises, because the valve timing is not adapted to such low revolutions and losses occur due to scavenging of unburnt mixture and inadequate filling of the cylinders.

So, if maximum economy is the aim, gear changes must be performed in such a way that the engine is kept working under a fairly high load in the range where it produces maxiumum torque, provided this does not result in too high of a maximum speed, when the increase in drag would offset the engine's low specific consumption.

Paul Frère - Sports Car & Competition Driving

Note #1: The maximum torque for the 944 NA engine is between 3000 and 5000 rpms.

Note #2: The minimum upshifting points listed in the Owner's Manual are as follows:

1st to 2nd gear - 2850 rpm
2nd to 3rd gear - 2800 rpm
3rd to 4th gear - 3050 rpm
4th to 5th gear - 2700 rpm

Since these are not within the highest torque band, I assume they're given to avoid slogging the engine.

Your suggestions are welcome!

Zero10

I find I got the best fuel economy when I would shift at around 3000-3500.
However, when I was just coasting down a residential street or something, I would slip it into 4th, at around 1700-1900rpms, and just very light throttle to keep speed. I would always upshift once I got to speed to keep the revs down.

It's not so much when you shift, as what rpm's you do most of your driving at. I can just hammer it from every light, push it to 5 grand every shift, and it will make a pretty small difference in my fuel economy.

Although it does sound like I am repeating what that quote says.

Luis de Prat

That's a good point, re the rpms you're driving at. Frère demonstrates numerically in this book that indeed, maximum acceleration is achieved beyond the engine's maximum torque speed in the intermediate gears. In other words, if you're trying to be "fast" you will be by increasing the rpms.

However, in this thread I'm assuming that you're trying to keep the acceleration down. I say this because I instinctively shift early (i.e. around 3000 rpm) to conserve fuel.

What I'm thinking from what's explained in this book is that I may be shifting too early for actual fuel consumption, because of the reasons given above.

led

Well, around here maximum fuel efficiency will be when they open the "Tren Urbano" and you can leave your car at home away from traffic jams and idiots who can't tell a red light from a coke ad. ok I had to vent, I really hate how people drive around here. Not that the TU will make a difference but it makes for a nice comment

Bhj0887

When I filled up I was getting about 26 mpg, I was shifting around 3-4000 rpms and usually coasting along at 2500-3000 rpms, The time before that I was shifting at about 5500 for acceleration and then coasting at around 3000 rpms again, I got 24 mpg.

Cyrus951

3k on the spot for me..

Mike1982

I go to 3k then shift, I think that not abusing the engine is just a LITTLE more important then getting an extra mile or two per gallon. I know I am looking for better gas milage but I will NOT abuse my engine to make it happen.

Francesco944

You know what is absolutely true? You car LOVES high RPM's. In fact, it is actually GOOD for the engine! To rely on your torque to get around, that is POOR economy, and engine treatment! Try to keep it up there..., I suggest 'listening' to you engine, she will talk to you when you listen..., kinda like all them women eh? All that said, I suggest NO LOWER than 3000!!!
Also, if you're going to downshift, the higher the RPM the so much better it is for your clutch! When stopping, I take it out of gear, and use the brakes when I know I'm going to stop for a light. Save the clutches guys! Brakes are way cheaper and easier to replace!

Zero10

I think I may have been a little misunderstood. I believe for the most part, it does not affect mileage when you shift, but rather what gear you just coast around in.
However, after reading this thread, I rethought my driving. I started shifting at 3500-4500 on my way to the university this morning, and the car seemed much happier. I still stick it in 4th on residential roads, to keep the revs down, but I barely press on the gas at those revs, because I know it will lug if I push it more than 1/8th the way down.
Now, that said, obviously if you bag on it like a mad man, and peg the rev limiter on every upshift, then cram it in 5th in a 30 zone, obviously your mileage will suffer. I think my advice applies only in moderation.

mossy

What's fuel economy???
These are Porsches!!!!!
Seriously though, in order to get good fuel economy, the engine has to be working eifficiently.
Fuel economy is also relative to your driving style. Your "economy" shift points are irrelevent if you're accelerating hard, as your article shows (and most people have quoted).
I think the key is to make your driving as smooth as possible, whatever your aims (economy or performance).

Luis de Prat

Well, that's just it. I'm not asking about accelerating hard. If you read the text closely, the shift points are not irrelevant since there's a point beyond which you're increasing rpms for more acceleration, rather than staying within the maximum torque band where the chance of fuel economy is greater. In other words, in shifting from 3rd to 4th you have a choice between 3000 and 5000 rpms. When do you move the lever?

Bhj0887

For the best fuel economy and the best fuel economy with slight performance, you'd shift at around 3500, so when your in 4th, you still are close to the maximum torque band but your not over doing it,

Stan944

My 944 has a fuel consumption gauge. The readings are interesting: e.g. for flat-road cruising, it actually doesn't make a difference if you are in 3000 rpm in 2nd or 2500 rpm in 3rd, or so.
To me it's good, as I don't like staying in too low of a gear.

Zero10

Heh, now I'm going to stick my nose in here again.
For about 4 tanks of gas I followed stan944's idea. I would drive at 2500 in 3rd instead of 1700 in 4th. The fuel gauge actually read lower when I was in 3rd (I would apply this to roads of all speeds, not just 50 zones), so I presumed that I was actually getting better mileage.
This was not the case. I found I used about 20-25% more fuel driving at higher revs in a lower gear, despite the fuel economy gauge.

ERAU-944

yeah i go by the guage. on the other hand i have a CASIS (computer assisted shift indicator signal). there's been a thread on that, so i'll spare you. but i used to stay in 4th cruising on my local street around town (35mph, the light to shift to fifth comes on around 40), and i noticed that it shows better mileage (because the throttle is less open) in fifth at that speed, where wind resistance is fairly negligible. luis, i think what you're looking for is this: you can shift for economy anywhere the engine will be above 1500 rpm afterwards (downshift point) and you don't lug the engine. if you do that (i do sometimes, for economy), you'll get insane fuel economy. my problem is that i may start out a tank of gas driving like that, and i can't contain myself for long. even without taking it easy, and with a fair mix (60/40 hwy/city respectively) i get mid 20s for fuel consumption. if i went really easy i should be able to get 30. one day i'll be able to make myself do it

-Michael-