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Sat Jun 09, 2012 8:19 am

What do people consider the "minimum constant engine speed" for highway cruising? I've always thought about 2800rpm was the magic number. How low can you go and the fan is still turning fast enough to cool properly? 2500? 2600? 2800?

Sat Jun 09, 2012 8:57 am

Good morning ALB! In my '71 Karmann Ghia the stock gearing is a 0.89 4th and a 3.875 R&P. With 165 rear tires @ 70 mph the engine turns about 3200 rpm. According to the Bentley manual, the fan turns about 1.9 X engine speed. So @ 70 mph the fan would be turning just over 6k rpm's. At 60 mph engine speed is about 2750, fan speed about 5250.

I would say using these "stock" fan speeds as a benchmark, I wouldn't go too much lower than that. Figure with my 165 tires the engine/fan speed is a bit slower than with stock 155 tires. VW also probably built in a certain "fudge factor" to allow for people who lug the engine down in rpm's or for certain driving conditions, hills, mountains and whatnot.

If you are contemplating using a lower 4th, like a Bus 0.82, then a smaller Porsche size generator pulley will pop up fan speed. Unfortunately I'm not quite sure exactly by how much but I believe it would be enough to cover running the engine rpm's down around 2500 - 2700 at highway speeds without overheating.

Here's Lowbuggett's version:
http://www.lowbugget.com/speedpulleys.html
They are also available at Pelican Parts (the real deal Porsche part).

My 2 centavos.

Sat Jun 09, 2012 9:16 am

My bus is geared to have the same as a 67 bug, 3000 @ 60mph.

I regularly run 40/50mph in 4th gear and see no over heating problems on my dakota digital head temp gauge.

If fact doing 60mph the gauge will be reading higher that cruising 50mph.

Sat Jun 09, 2012 10:01 am

Hey Darth, how the heck are you!! How's the job? Anyway, back to VW foolishness; as you've guessed, I'm just trying to figure out some gearing; I have a Berg 5 and haven't settled on the 4th-5th spread yet for the speedster. With a 1.21 4th (3.88 r&p), 3500 is 53mph and 62mph happens at 4000 rpm while with .82 in 5th 2500 is 57mph (2800 is 64mph). A .89 would tighten it up a little nicer (2800-58mph) but the top cruising speed will of course suffer slightly- from 80 to 74mph at 3500 (which, considering that legal highway speeds are 110kmh or about 68mph tops, probably isn't a bad thing :lol: ). I know you can cruise below 2800 rpm, but the question is how much lower? I know about the Porsche top pulley, but as the motor will be capable of 6500 rpm I really don't think speeding up the fan would be a good idea. I could be wrong, but...

Krusher- Good info; do you know what rpms it's turning at 50mph in 4th? And with the .89 gear, what r&p are you running? tire size?

Sat Jun 09, 2012 11:46 am

what does leagel highway speeds have to do with any thing?I would think 2800 would be fine.but that all depends on the motor and tune.some run hot some dont.some need extra cooling some dont.you need to figure out just what it will need.my theroy is if you can add a scoop or 2 ducted to the fan it will help push more air through the system instead of it having to pull it and cavatating more. or if it have air vents in the rong place and they may try to pull air out of the compartment instead of into it, thus hot&lower hp.
approach this from all sides you can even ducking from the nose of the car or other high pressure aera to the motor.

Sat Jun 09, 2012 1:10 pm

Mark- You wrote all that but did you even bother to figure out what the question was?

Sat Jun 09, 2012 2:03 pm

I do not know if mark knew what the question was either, but no matter, he did arrive at the correct answer.

you can adjust the fan by changing the pulleys and using different width fans

if the fourth/fifth drop is small, use can drop to fourth for hills/headwinds 55-65 mph, so the cooling is not too big of a worry, at 70+ on flat ground it may be able to be cooled just fine at very low rpm with proper ductwork, since the car is small and slippery, the air can move itself, the air can also move itself in a circle cooking the engine, it will make a difference

oh well, anyway, I'd gear it for 3700-4000 rpm @ the fastest speed you plan to cruise, OR make 4th and 5 th close enough you can drop to 4th, no sense overgearing it........that just reduces how much THRUST you got, maybe the speed limit is low but you can still enjoy short bursts of acceleration.......right?

Sat Jun 09, 2012 3:59 pm

yes I read the question, why did you forget what you asked? and I get shit fer spellen stuff rong.go figure

Sat Jun 09, 2012 5:17 pm

ALB wrote: I have a Berg 5 and haven't settled on the 4th-5th spread yet for the speedster. With a 1.21 4th (3.88 r&p), 3500 is 53mph and 62mph happens at 4000 rpm while with .82 in 5th 2500 is 57mph (2800 is 64mph). A .89 would tighten it up a little nicer (2800-58mph) but the top cruising speed will of course suffer slightly- from 80 to 74mph at 3500 ...
I think you should look at 4th, not 5th. For a street driven car, a 1.21 4th is a bad idea, IMO. Go much taller, then your gap from 4th to 5th isn't so large.

For my car, I originally calculated that a 1.09 was ideal. But when I went to order the gear, I found out that the helix angle of the 1.09 was much less than stock (noisy), so instead I picked a 1.13. Now that I've driven it, I wish I picked a 1.04 because the gap to the .82 5th is now too large for my liking.

What are your other gears?

Mon Jun 11, 2012 10:23 pm

ALB wrote: Hey Darth, how the heck are you!! How's the job? Anyway, back to VW foolishness; as you've guessed, I'm just trying to figure out some gearing; I have a Berg 5 and haven't settled on the 4th-5th spread yet for the speedster. With a 1.21 4th (3.88 r&p), 3500 is 53mph and 62mph happens at 4000 rpm while with .82 in 5th 2500 is 57mph (2800 is 64mph). A .89 would tighten it up a little nicer (2800-58mph) but the top cruising speed will of course suffer slightly- from 80 to 74mph at 3500 (which, considering that legal highway speeds are 110kmh or about 68mph tops, probably isn't a bad thing :lol: ). I know you can cruise below 2800 rpm, but the question is how much lower? I know about the Porsche top pulley, but as the motor will be capable of 6500 rpm I really don't think speeding up the fan would be a good idea. I could be wrong, but...

Krusher- Good info; do you know what rpms it's turning at 50mph in 4th? And with the .89 gear, what r&p are you running? tire size?

I was at first running 4.12 .89 185/65/15 40@2000 50@2500 60@3000 70@3500

I now moved the a 195/65/15 which runs 1mph slow at the same rpms to try to get the engine slighty more in the peek tq at my normal driving speeds.
noticed no temp changes between the tyres.

This is pushing my square bus.

In a speedster a 3.88/.82 and 24" tyres would give you 57@2500 and 68@3000 I would happily run that.

Mon Jun 11, 2012 10:25 pm

Have you used this http://www.johnmaherracing.co.uk/gearcalc.htm