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Posted by: Curvie Roadlover May 3 2003, 08:16 PM

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?

Posted by: need4speed May 3 2003, 08:39 PM

I've always heard that the traditional school of thought for aircoolds is that you want to cruise on the mid to high end of the RPM range, to keep more air flowing over the heat fins. Makes it a bit of a gas hog tho.
(my Ghia with the stock 1600 dp gets 36mpg freeway).

driving with neither a tach, nor a stereo, I've learned to listen to the music of the engine. For a while, I drove with no speedo either - and I could tell roughly how fast I was driving by the engine tone, and gear.

I really can't wait until I get a teener. :loveletter:

Posted by: SirAndy May 3 2003, 09:37 PM

yepp, no numbers to tell. i drive by listening to the engine.
have not looked at the RPMs in quite a while.

but i did remove the RPM-Limiter within the first week of driving the car,
so i must hit the 5000 frequently

Posted by: Curvie Roadlover May 3 2003, 09:54 PM

I used to shift by sound as well and never really paid much attention to the tach until just recently. I was riding with other people and thinking, man, these guys really run up the rpms before shifting. Then I started thinking maybe it's me whose not running the rpms up enough! So that's when I started watching the tach. I'd like to know what people think is a good range rpm-wise for a given gear.

Posted by: SirAndy May 3 2003, 10:04 PM

i suppose that very much depends on your engine setup and the gearing of your transmission. i remember that brad had some dyno charts that showed RPM/HP ration on a stock 2.0L ...
damm alzheimer kicking in, but somehow the number 4600 comes to mind to be a good shifting RPM. you want to be OVER the top HP for that gear when you shift up so you hit the HP peak for the next higher gear right away ...

anyone here with the real numbers?
Andy

Posted by: Bleyseng May 3 2003, 10:30 PM

ask and you shall receive, Andy.


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Posted by: SirAndy May 3 2003, 10:31 PM

thank you sir,
sooooo, 4600 seems about right ...

Andy

Posted by: Bleyseng May 3 2003, 11:01 PM

I try to shift around 5000rpms when AXing, to keep in the power band. My motor is non-stock so that dyno is alittle off for a stock motor but still close. Running a motor to 6000+ is a waste of time.
Geoff

Posted by: Joseph Mills May 3 2003, 11:48 PM

I think this is a good topic. Being a relative newbie to 914s, I probably have way too many observations (read Obsession).

For both the street and AX, I usually shift by ear. When the engine sounds happy, I shift up. However, having said that, with my new 914, I have started taking note of tach readings and have started making adjustments, both for the street and AX. Here is what I have found.

For the street I usually short-shift before 3k - I like to semi-bury the throttle for 2nd and wind it a little higher - kinda fun. For 2nd & 3rd I go to 4k+. I usually cruise down a street at 4k. Keep the air flowing. It's noisy. So what.

For me the freeway is a chance to let it breath. Coming off of on-ramps I usually take it towards 5k and then slow down and merge left with traffic. Keep it above 3k and when needed, shift down for slower highway traffic. For me, 5th gear on the freeway @ 70mph is about 3300rpm.

For AX I usually short-shift 1st gear - strictly for launching. 2nd gear depends on the course (depends on the first turn). Sometimes I go to 6k (my car is not stock & winds freely to 6k). However, I know the T4 is not a high winder - it's all about low and mid range torque. Recently I have started forcing myself to short shift all gears for AX, and I'm seeing an improvement in my times - even though we all know the penalty of shifting a 914 in an AX. It's called 4 feet of linkage. Early shifting may not work for you - works for me. I'm installing an adjustable rev-limiter and shift light to help me shift before 5k instead of 6k.

I guess for the street, I'd say don't lug it around - wind it higher than you think you should. Cruise at 4k.

For AX, I say shift the damn thing. You'll go faster.

my 2.0L cents

Posted by: Lawrence914-6 May 4 2003, 05:43 AM

Might be a waste of time, and a waste of fuel... but, damn, it feels good.

While puttering around town, I probably shift aroun 4000-4500. During spirited driving, 5500 or so... once in a while I tag the rev limiter.

-Rusty

Posted by: Bleyseng May 4 2003, 10:25 AM

Rusty, you are driving a 6 which is way different! No fair!
Geoff

Posted by: Lawrence914-6 May 4 2003, 10:32 AM

Anytime you want to drive it, Geoff... all ya have to do is ask.

I'm pretty free with the keys.

-Rusty

Posted by: ! May 4 2003, 10:51 AM

I shift when the rev limitter won't let me go any farther.....

Posted by: ! May 4 2003, 10:55 AM

Off to the kids first communion....beautiful day in Santa Barbara.....


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Posted by: Bleyseng May 4 2003, 10:59 AM

Nice pic of the girls! which one is the artist? which one wants to drive at the AX?
which one is daddys girl?


Rusty, next time I am near your neck of the woods I'll take you up on that. Hey, maybe you could get stationed at Fort Lewis! Only 30 to the AX course at Bremerton.

Geoff

Posted by: Brad Roberts May 4 2003, 01:35 PM

We shift the 2.0 cars at 5K when on the track or AutoX. Typical race people say to shift 400-500 RPM over peek torque. I shift the cars at 3500 when beusting around town.

Anybody notice the date on Geoff's dyno run ??

9.16.00


B

Posted by: Jeff Krieger May 4 2003, 01:54 PM

The owner's manual for my '73 2.0 says that max. torque is 105 lb*ft at 3500 rpm. Shifting 500 rpm over peak torque would be about 4000 rpm. I'm no expert but that does seem a little low.

Posted by: Bleyseng May 4 2003, 04:46 PM

I think they mean peak torque is just before the torque curve falls off which is at 4500rpms if you look at the chart. The torque curve is really flat from 2000 to 4500 rpms without a spike that you could call "max torque" unlike the hp curve.
So I shift at 5000..........
I lost the newer dyno chart so I posted the old one. 95hp at the rear wheels is the best my car has done.
Geoff

Posted by: Brad Roberts May 4 2003, 05:32 PM

Correct.

We are running a nice header and free flow muffler. My torque curve is not the same as the factory on a 100rwhp engine.

B

Posted by: Bleyseng May 4 2003, 05:39 PM

How do you get more hp above 5000 to 6000? Impossible due to the Djet? the cam?
5300 rpms and then it drops like a lead zepplin!
Geoff

Posted by: Brad Roberts May 4 2003, 05:45 PM

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.


B

Posted by: Bleyseng May 4 2003, 05:50 PM

So better valve springs and chromemoly pushrods?

Posted by: Brad Roberts May 4 2003, 06:00 PM

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.


B

Posted by: Demick May 5 2003, 10:14 AM

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


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Posted by: Curvie Roadlover May 5 2003, 11:26 AM

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

Posted by: SirAndy May 5 2003, 11:58 AM

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 ...

Posted by: porsche735 May 5 2003, 12:56 PM

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

Posted by: Jeff Krieger May 5 2003, 01:20 PM

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.

Posted by: Dave_Darling Jul 30 2003, 03:25 PM

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/shiftingpoints.htm

--DD

Posted by: Demick Jul 30 2003, 04:35 PM

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

Posted by: Aaron Cox Jul 30 2003, 04:42 PM

i like to shift at 5l or 5.2k

Posted by: Queenie Jul 30 2003, 04:42 PM

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.

Posted by: Curvie Roadlover Jul 30 2003, 08:00 PM

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! rrrrrrrrrrrrrrrrrrrRRRRRRRRRRRRnow!

Posted by: TimT Jul 30 2003, 08:49 PM

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..

Posted by: mskala Jul 30 2003, 09:30 PM

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 for everything except 4-5, which
is ~5000.

Posted by: redshift Jul 30 2003, 11:39 PM

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.



M