What RPM Do You Shift At?? Options

[Brad Roberts]

I think its the cam. The injection is more than capable of handling 20+ more hp.

We picked up about 300-400 RPM more useable HP+Torque by running a better valve spring.


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[Bleyseng]

So better valve springs and chromemoly pushrods?

[Brad Roberts]

Actually just the valve springs. The pushrods are really light from the factory.

The valve springs dont do anything except keep the valves from floating above 5k RPM. Basically better valve train control to get the max out of the stock cam.


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[Demick]

If you want to be scientific about your shift point, here it goes:

First, you need a torque curve for the car. I will use Geoff's curve that he posted (I reposted it below so you don't have to look back and forth).

The 914 torque curve is almost flat up to about 4300rpm (I will assume it is flat at 108 ft-lbs).

Second, you need to know how much your gear ratio changes for the shift. For autocross, people are most interested in the 2nd to 3rd shift point, so that is what I will use here. Stock 914-4 transaxles have a 1.889 ratio 2nd gear, and a 1.261 ratio 3rd gear. This means that your gear ratio changes by about 33% when changing from 2nd to 3rd gear. Two important things happen in this change: 1. Your RPM will drop by 33%, and 2. For a given torque generated by the engine, your torque at the rear wheels will drop by 33%. Since it is the torque at the rear wheels that actually accelerates the car, this is a very important number. This also means that we need to make 33% more engine torque in order to maintain the same acceleration when we shift from 2nd to 3rd.

Now, back to the torque curve. Pretty much wherever we make the shift, we will drop back into our 108 ft-lbs of torque region. In order to have gained 33% torque in this transition, we need to have had about 81 ft-lbs of torque before the shift. On the graph, this corresponds to about 5400 rpm. This is the ideal shift point from 2nd to 3rd gear to maintain maximum acceleration.

Working through the same math, ideal shift points for 3rd to 4th is 5300rpm. 4th to 5th is 5250rpm.

Note, this applies to Geoff's '76 2.0, and will change dependent on the torque curve for your car, and any difference in gear ratios you have.

Makes sense?

Demick
'74 2.0

[Curvie Roadlover]

So, how fast should you be going to use 5th on the freeway?

[SirAndy]

So, how fast should you be going to use 5th on the freeway?

depends on your wheel size ...

i only use 5th for cruising. with the current speed-limits, i wouldn't actually need 5th, could stay in 4th easily ...

[porsche735]

Demick,
I agree with your logic. That is exactly what I do on the track, just shy of redline. From about 5400 to redline, you really don't get much extra pull, an upshift serves you better. Of course, it is dependent on your specific torque curve. Running headers, different valve springs, different cam, higher compression, etc. will change things a bit, so use teh dyno run from those changes...

Chris

[Jeff Krieger]

This discussion makes me think of the new cars equipped with continously variable transmissions. They're supposed to always be in the right "gear" at the right time but they're probably not nearly as much fun as a nice shifting manual tranny - like in a BMW or Honda or something.

[Dave_Darling]

Resurrecting an old thread here. One of the 944 guys did some good work recently on the subject. He used a spreadsheet to graph things for him, using his 944's torque curve. He graphed the total drive-wheel thrust versus road speed for the various gears, also including the ring and pinion and the tire size.

The interesting thing to note is what happens when you change tire diameters...

http://members.rennlist.com/944sandmore/sh...ftingpoints.htm

--DD

[Demick]

Interesting to see it presented graphically like that. Helps to understand the tradeoff on tire sizes.

However, tire diameters change how fast you will accelerate in a given gear, and how fast you can go in that gear, but have no impact on the optimal RPM to shift gears. Shift RPM is determined solely by the engine torque curve, and the transmission ratio change between the 2 gears, as I explained a few posts back.

Demick
'74 2.0

[Aaron Cox]

i like to shift at 5l or 5.2k

[Queenie]

This topic came up on the other BBS and I read lots of interesting opinions. I'm wondering what are the thoughts of the folks on this BBS. It seems to me I have been shifting too early and cruising in too high a gear. I was cruising at about 2400, maybe I shoulda been down a gear and running a higher RPM. Also, on a stock 2.0 at about what mph on the freeway do you take 5th gear? (IMG:style_emoticons/default/driving.gif)

2400 seems very low to me. I am rarely below 3000 unless I've spaced out on shifting (hey, it happens) or am expecting to accelerate very soon. Cruising speed is always 3200-3500, regardless of gear. And I usually crank it up to nearly 4000 before shifting into 5th.

I know that my speedometer doesn't read correctly, so I drive mostly by the tach.

[Curvie Roadlover]

Thank you to the recent posters on this thread. Since I first posted this thread way back when, I have really begun to pay attention to my tach more and now I'm shifting at about 4500 to 5000 and cruising at 3000. If you could see my avatar better, you'd see that I have the tach turned so that the red line is at 12:00 o'clock. I used to shift by sound without even looking at the tach, but I was always shifting too soon. Now I watch the tach. I guess that when I've done it this way long enough to get used to the different sound at the higher RPMs I'll be able to shift by sound again (only at the right RPM, not too soon)
rrrrrrrrrrrrRRRRRRRRRRRR now! (IMG:style_emoticons/default/driving.gif) rrrrrrrrrrrrrrrrrrrRRRRRRRRRRRRnow! (IMG:style_emoticons/default/driving.gif)

[TimT]

to just add to the confusion......

I shift as soon as I get into the rev limiter.....on my 3.2 its only 6800 rpm (Ill have to check the chip)

I dont look at the tach much..

[mskala]

Thanks Dave, that is a good way to graph it. I just put my
supposed torque curve to it, and it showed shift at redline
(6300) or higher (IMG:style_emoticons/default/blink.gif) for everything except 4-5, which
is ~5000.

[redshift]

If I am cruising less than 75, the car usually stays in 4th, and it may stay there @ 85, depends on how much fun I am having.



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