4k shifting

[drmac]

I'm sure this has been done to death on the forum and also I do not want local law enforcement to see this post, but what is it about the 4k shift threshold mantra that makes it ideal? I have found that my car acts like a happy little freak accepting changes above 4k and kind of seems to glimpse askance at me when I try it too soon. Psychosexual issue?

[JohnJL]

I have no idea what you are referring to. Most of these cars wake up at 4k. If you shift at 4k you are missing 90% of the fun.

[Jerome74911S]

Originally Posted by JohnJL

I have no idea what you are referring to. Most of these cars wake up at 4k. If you shift at 4k you are missing 90% of the fun.

Ditto

[Reiver]

Depending upon your tune...mine wakes up at 3600 rpm....everything clics at higher rpm..the 915 can be shifted quickly when turning high rpm (slowly when not).
They were built for the power band so everything just comes together.

[drmac]

I think the question I was asking was why, technically, the rpm/gear ratio connection works as it does--I'd be happy with a pointer to a good link. I understand, I guess, that if I'm not shifting at 5k or whatever I'm having no fun driving, but that was not what I was asking.

[Lapkritis]

I only shift below 3k when oil temp is below 180°F. 4k shift would be normal commute shifting... Otherwise 6-7k except the turbo car that I don't want to go to jail in... I let the wastegate breathe a bit and still shift early at 5k when commuting. On ramp? See you at 7k+.

[Bob Kontak]

Originally Posted by drmac

I think the question I was asking was why, technically, the rpm/gear ratio connection works as it does--I'd be happy with a pointer to a good link. I understand, I guess, that if I'm not shifting at 5k or whatever I'm having no fun driving, but that was not what I was asking.

Your first post is a touch obscure. Explain it more. You shift right at or before 4k rpm and that is what you like?

Shifting at 7k rpm all the time is like the guys that say they can drop a 911 engine in an hour. General horse pucky for the average Joe.

Also note the crap you are getting is from Canadians. They are just stinkers.

[Neilnaz]

Bob, I think the OP is asking why the 915 gearbox changes gear better above 4,000RPM than it does when you are babying it at 2,000RPM. I have a feeling it is all about everything being warmed up (including the driver) and the 915 being a great gearbox.

I have to say that I go careful with the car before it is warmed up and it does feel slightly 'touchy'. I change gear at 3,000RPM or less and I am usually driving through town. Once the car is warmed up I am usually out of town and opening the car up......

Cheers, Neil

[Matt Monson]

Its about the speed differential between the free wheel spinning speed of the gears. In particular on the 1-2 shift if you don't go to 4000rpm the synchro will have to speed up the loose gear to make it spin the right speed to slip in easily. The less speed differential the synchro has to overcome to make the gear match the slider the faster and smoother you can shift. So many people shift too low in the rpms, which is why you see these posts where someone goes to the track and is amazed at how good it drives. Usually the solution around town is another 500-1000 rpm revs before shifting and it will be happier. The downshift gets all this attention regarding revs. The upshift needs attention too.

[Bob Kontak]

Originally Posted by Neilnaz

Bob, I think the OP is asking why the 915 gearbox changes gear better above 4,000RPM than it does when you are babying it at 2,000RPM.

I'll buy that as to OP's question intent.

It does notch in spiffier at a robust RPM.

[dentist90]

Originally Posted by Bob Kontak

Shifting at 7k rpm all the time is like the guys that say they can drop a 911 engine in an hour. General horse pucky for the average Joe.

Also note the crap you are getting is from Canadians. They are just stinkers.

Check out the exchange rates Bob... 7000 Canadian is about 5500 US now.

And I can drop an engine in a METRIC hour.

All make sense now?

[pmax]

The higher the rpm, the smaller the relative change when shifting. But the rpms seem to drop faster at higher rpms during the shift so that negates the effect. There's probably an optimal shifting point for the synchro's.

[drmac]

Thanks for the replies. Sorry I wasn't clearer with my original question--what I was asking is what Neil stated more clearly without trying to be cute about it, as I was: why, mechanically, does the car shift better at higher rpms, and is the higher rpm upshift a best practice in general?

The replies from Matt and pmax are right along the lines of what I was curious about, so both questions answered.

And yes, part of the question was to do with driving in town--I guess it's a question one wouldn't need ask if we were always driving in perfect circumstances. But it's also a novice question borne of driving modern manuals that do not demand quite the attention the 915 transmission does demand.

Best to all.

[Matt Monson]

Keep in mind the gearbox was designed to work with the engine. Your engine also likes those rpms and as one of the early replies mentioned you should rev it that much and not bog it down with 2500 or even 3000 rpm shifts if you are going to keep accelerating.

No issue with cruising around at those rpms steady state, but if you need to accelerate, downshift before you dip into the throttle.

Each gear has a slightly different sweet spot. On 2-3 if I shift my 3.2 at 6500 rpm I have to wait a second for the loose gear to slow down. If I shift at 6k it snicks in really cleanly.

[Algernon]

Originally Posted by dentist90

Check out the exchange rates Bob... 7000 Canadian is about 5500 US now.

If it takes 7000 Canadian to 5500US, does that mean Canadian is worthless (oops worth less)

[Matt Monson]

Originally Posted by Algernon

If it takes 7000 Canadian to 5500US, does that mean Canadian is worthless (oops worth less)

15 years ago I used to go up there a bunch. I loved going out to dinner, giving the waiter $20 US and getting $20 Can in change. And Cuban cigars.

[T77911S]

4k you are shifting in the power band of the engine.

police? it does not matter. i have hit 150 in my car numerous times.......but where? public roads or race track?

[Bob Kontak]

Originally Posted by Matt Monson

15 years ago I used to go up there a bunch. I loved going out to dinner, giving the waiter $20 US and getting $20 Can in change. And Cuban cigars.

Love me some Canada. Plenty of time spent in Ontario/Quebec. Never west unfortunately.

Montreal in the 80's. Chez Paree. Crescent Street. Had my first bottle of good wine in Montreal. Remember walking into a french speaking restaurant. Never said a word and the maitre d' said "Good Evening" to us. Somehow he knew.

On the downside. Algonquin Provincial Park, North of Toronto in July. Skin boring little flies from hell. I asked them to stop and they didn't listen.

[JohnJL]

Originally Posted by Bob Kontak

Your first post is a touch obscure. Explain it more. You shift right at or before 4k rpm and that is what you like?

Shifting at 7k rpm all the time is like the guys that say they can drop a 911 engine in an hour. General horse pucky for the average Joe.

Also note the crap you are getting is from Canadians. They are just stinkers.

Originally Posted by dentist90

Check out the exchange rates Bob... 7000 Canadian is about 5500 US now.

And I can drop an engine in a METRIC hour.

All make sense now?

Hahahahahaha. We were at parity not many months ago...


Mosport aint bad either. A Big Boy track for sure.

Ps I am 'merican.

[sugarwood]

I was asking the same sort of question recently.
RPM shift points of economy cars vs. racing cars

This might be worth adding here:



First, look at the RPM vs HP/Torque chart on the left.
The engine puts out peak HP around 5300 rpm (regardless of gear) and peak torque is at 4000 (lower curve)
So, you’d want to be in that 4000-5300 range ...for optimal engine performance.

Next, see the gearing chart on the right
Note that for a given speed (x-axis), you can be in any of the 5 gears (with varying RPMs, in theory)

Let’s say you shift from 1st to 2nd at 3000 rpm (top orange circle).
At that moment, you’re now in 2nd gear, and going 20mph.
This puts you at ~ 2000 RPM (y-axis) in 2nd gear. (lower orange circle...)
As per graph #1, at 2000 rpm, the engine produces very low HP/torque.

In contrast, let’s say you you shift from 1st to 2nd at 6000 rpm (top green circle).
At that moment, you’re now in 2nd gear, and going 42mph.
This puts you at ~ 4000 rpm (y-axis) in 2nd gear. (lower green circle...)
At 4000 rpm, the engine is producing lots of HP/torque.

With the higher shift, you are maximizing time spent at the higher part of the power curve.