Need advice on shifting gears.

[gungadin]

'76 Porsche 911 6cyl 3.0L
What is the optimal way to shift a car with a 5spd. manual transmission?
This car has a lot of torque.
Lately I've been skipping gears ie. 1-3-5 or 1-2-5.
I've been hyper light on the accelerator but allowing the RPMs to wind slowly up to five thousand in second so as not to be lugging terribly when I hit 5th.
This is do-able but I'd like to know what the accepted techniques are for shifting.
I've read that I should accelerate at 50% or more throttle but that doesn't feel quite right with this car.

[theholycow]

I don't think that a lot of the experience available at this forum will directly apply to your car, which is rather different with its carburetor and such. Instead, we'll have to help you figure it out for yourself.

What are the specs on your car? Curb weight, gearing, HP, torque, especially a torque/hp chart showing power at RPM.

I can't imagine trying to save some gas and ever getting up to 5000 rpm. My VW is very, very different from your car, but I shift at 2000 rpm if I'm in a hurry uphill -- otherwise, I shift at 1500 or even 1250 often enough. When I skip 3rd I shift at 2000.

You'll definitely have to experiment to find the most efficient shifting pattern. If you look at my gaslog for Effram you'll see how much experimentation I've done with shifting patterns and I'm still totally unsatisfied with the data I've collected, but I've been doing other experiments and will have to wait until their done before I resume shift pattern experiments.

One thing to consider, though: I'm pretty sure I remember reading about how difficult it is to replace the clutch in a car like yours, having to pretty much disassemble the whole car. So it might behoove you to go especially easy on the clutch...

[GasSavers_RoadWarrior]

If your torque peak is way up there, then you're kinda screwed whichever way you try to do it, best just lug it up a gear at a time.

[gungadin]

I wrote 5K rpm mistakenly. I do take to 4K tho. That's when I shift 1-2-5.
I can get the torque vs. HP @ RPM curves I'm sure.
Also the gearing ratios. The clutches are quite tough. The engines are hyper expensive to replace. You are looking at $8K if you do the work yourself and only farm out to the machine shop. Lucky for me the 3.0 has probably the best longevity of all the 911 engines often going 300K before rebuilding.
My '76 car is quite light compared to other 3.0L/911 cars. I drove it on the scales with a full tank the other day. 2550 with the driver.
Oil is so critical I'm afraid to go with a lighter weight. These things take nearly 11 qts when you change the oil. So that's why it's only green in color.
They actually have an allowable oil consumption spec that is going to make you cringe. 1qt./2K miles as they age. Interesting tho that since I've started my new lightfoot techniques my oil consumption is waaayy down. So that's good!
You can't believe the puzzled looks I'm getting in my rearview mirror. I can see them quite clearly as close as they often are.

[theholycow]

You think that oil consumption spec is bad, just check out what VW is saying for their 2008 engines (even for their non-turbo models): 1qt/1000 miles. In this day and age? Really?

[fumesucker]

Revving as high as 4,000 is probably counterproductive, engine friction rises very quickly with rpms, you don't want to burn gas in the engine only to waste the resulting energy frantically rubbing engine parts against each other.

Small throttle openings also are counterproductive in theory, they lead to higher intake vacuum and increased pumping losses in the engine.

Driving with greater throttle openings and shifting at lower rpms should give you better economy but you aren't going to squeeze great mileage out of that car, it wasn't designed with economy in mind in the first place.

[Ford Man]

My cars are quite different from yours but I normally try to go through all of the gears and shift in the 2,000-2500 rpm range. It works well with my cars as far as efficiency and the last clutch in my '88 Escort went just over 250,000 miles and the current one has about 175,000 on it and is still going strong. I never lug the engine I downshift when it goes to straining. My '88 Escort with 475,000 miles on it consumes less oil than the spec for the VW's.

[DRW]

I'd recommend shifting at lower rpm and using each gear. If you try to skip a gear, you'd have to rev it a little high in order to prevent lugging it in the next gear, and high revs wastes fuel. Skip shifting is not neccesarily a gas saving technique as it is a relaxed shifting method for those who don't like to shift gears as often.

The question of when to shift can only be answered by trying different rpms. Ultimately, you don't want to lug the engine. The harsh vibration when lugging is hard on the rod bearings and can lead to knock, which is even harsher on the rod bearings.

How low the rpms can go also depends on load. For example, if I'm driving slowly my car can handle 1000 rpm when the throttle is just cracked open the tiniest ammount. If I need to accellerate lightly I'll need to downshift and bring the rpms up to 1300 or so. If I need to go up a moderate incline, such as a freeway overpass, my motor likes 1500rpm or more. The less power needed, the lower the rpms can be.

[R.I.D.E.]

Try different rev limits on your shift points. Start off at 2500tops.
Not sure about the skipping gears, you may be better off to shift your range to lower and narrower.

My 76 280 ZX was set up as a high compression engine. It had good low end torque. A 3 liter Porsche engine should be OK as long as you dont go too low.

I assume you have FI. if not it may not be good advice to "lug" a carb engine. Thats a different situation altogether. Nissan had a Bosch system in 75 with the spring loaded flapper. Benz had the CIS low pressure continuous system. Not sure about Porsche.

regards
gary

[theholycow]

Originally Posted by fumesucker

frantically rubbing engine parts against each other.

This discussion is turning naughty...