Around the traps I have heard some talk about the fact that aircooled VWs with 5 speeds can overheat. On the face of it, to a non-mechanical person like me, this doesn't make sense. So I'd like to put forward why I think it doesn't make sense, and would then appreciate people's input as to whether my thinking is right or wrong.

Please note that none of the engine speeds and such like here are meant to be real, but are just being used for the purposes of illustration.

Say I have a five speed that does the following speeds at the following rpm

4th gear @ 3,000rpm = 80km/h
4th gear @ 4,500rpm = 110km/h
5th gear @ 3,000rpm = 110km/h

According to some of what is said, the car may overheat in 5th, even though the engine rpm is much lower than it would be doing the same speed in 4th.

I have heard it said that it is because the fan is running slower, but surely this would mean that it is as likely to overheat doing 80km/h in 4th because it is doing the same rpm. I would have thought if anything the engine would run slightly cooler because of the extra air flowing underneath the car past the sump at the higher speed.

In terms of the engine working harder, certainly at a higher speed the car is doing more work in pushing air out of the way, but I would have thought his was accounted for in the engine rpm.

I look forward to hearing your thoughts.

Cheers

Craig

Due to torque multiplication with the lower 4th gear, effective engine load is less in 4th at 80kph than at 110kph in 5th.........not to mention drag, it multiplies rapidly as speed increases. Now with a nice water cooled engine
your thermostat will keep your engine happily at correct temprature as long as your radiator cools eficiantly........A cylinder head temp gauge can be a usefull addition for an air cooled motor if you want to be sure.

A proper 5 speed conversion will give you closer ratios for a higher performance engine and 5th gear ratios will end up similar to what 4th gear was. So in this case no probs.
But the cooling issue is valid, basically it will always require the same amount of energy (measured in Btu's) to push your car through the air at a given speed, this energy cames from burning fuel, so at the same speed, it doesn't matter if it's a 1200cc stocker or a 2332 fire breather, the std cooling should do the job. VW engineers designed the cooling system to function at a designed set of parameters, change these and things get complicated. The way I see it, if you now lower the engine speed, the engines ability to make the same power changes as we may (for example) fall below the power output of the engine that is required for that speed, also the cooling fan speed is reduced thus giving us a situation where the engine is working harder but getting cooled less.....I think that makes sense .

there's a big write up on the Gene Berg website that covers this.
http://www.geneberg.com/cat.php?cPath=13_387 

btw, matt, would a good oil cooling system be a way to sidestep this a little?

Thanks Matt, its not my thread, but that certainly answers questions I had

Fair enough, I think I sorta kinda understand, in my own simple way.

Essentially what you're saying then is that a water cooled engine would also be producing more heat in the same circumstances (i.e. 5th at 110km/h), but the cooling system would step up to the job either through the fans coming on or due to the greater air flow through the radiator (by achieving greater heat reduction), or a mixture of both.

Would I therefore be correct in suggesting that this is actually a shortcoming of the aircooled concept (if you consider having lower rpms at higher speeds to be desireable (e.g. for noise, engine wear and fuel economy))? Or have I misunderstood and I'm wide of the mark

Thanks for the replies.

Not in my opinion Pete, oil is aour secondary cooling, air is primary. A bigger oil cooler would help reduce the oil temp, but the problem would be the actual heads, bigger fins (ie more cooling area) and/or more air would help but it's beyond me to advise on this one. Heaps of r&d required I'd say.

Agree with Matt, the oil temp can be as low u want but it wont effect head temp much, and head temp is what comes up when the engines under load

If you worried about fan speeds being slower... retrofit a smaller generator pulley to speed it up. Type 3 style tinwear under the cooling fins is suppose to help markedly.
Perhaps due to lack of availability of stock belt drive setups, you might want to do a synchronous belt conversion and seam weld the vanes of the fan.
The key way setup on the generator looks very similar to those of timing belt pulleys form your average toyota style DOHC engine. (i will check at home for you).

and you can get vw pulleys that will bolt straight on the crank that are synchronous.

If you want a 5 speed that runs good by a Hyundai

Daimo found his ran cooler and got better economy.
And that was with a 0.82 fifth and 3.88.(way taller than the stock L-Bug 0.93)
I agree though that if you went too far, there may be issues.

I thought Daimos was going to be too tall, but he said it is great.
Mind you it has twice the stock power too.

After going for a spin in Daimos old bug last year, I gotta say that tranny is geared to perfection but Like Dave said his engine has the torque to use it
expecting a stockish motor to run with the same setup would be kicking sqaure in the balls

Quote:

Originally posted by ian.mezz If you want a 5 speed that runs good by a Hyundai

BOLLOCKS! Aircooled VW's have a 5th gear as standard but they call it 4th! Yes thats right, 4th gear is an overdrive. A taller final drive ratio (diff) or taller tyres would create the same situation if it were true.

oh dear

The fact that a Beetle's 4th gear is an overdrive is irrelevant. What has the ratio got to go with the number of speeds?? Anyway, it's only an 'overdrive' into the diff. To the wheels, you then multiply by the diff ratio. I suppose you know for example that the auto Kombi's 3rd (top) gear is EXACTLY 1:1. So how many speeds does it have?

You can easily build a Rally Beetle 914 gearbox with all 5 gears above 1:1 - all close ratio speeds and NO overdrive. By your logic it isn't a 5-speed!

However you are right, you CAN create the same cooler situation at highway speeds with a taller diff, or taller tyres. VW did exactly that with their diff ratio, which got taller over the years. Make the engine turn slower at a given speed and it runs cooler, not hotter.

so all modern cars are geared to do cruising speed just above idle .........or is this debate about wether it's possible to OVER GEAR a vehicle reguardless of wether it has 3, 4, 5, 6 or even 36 forward speeds

Quote:

Originally posted by hellbugged so all modern cars are geared to do cruising speed just above idle

Vlad, do you realize who you are talking to? You might end up looking a little silly.

Daimo knows his stuff.

Well......Daimo.........Are you a VW person???

Priceless

.......a very embarrased one now

Hi all,

This is my real life experience with taller top gear in a bug. In the early 90`s good mate had 68 bug, 2165cc,8.6:1 ida etc. Ran awesome. Never over heated oil or heads.

Changed to irs, stock L box, still good albeit lost a little bit of zippy-ness. Broke box, fitted .82 top with 3.88. Engine always ran hot in 4th gear and felt lazy.

Swap back to .93 top and problem went away.

I think the VW engineers knew what the score was.

Again, this is just what we found with this combo.

Regards, Keith

yes that about right.

Quote:

Originally posted by hellbugged .......a very embarrased one now

Hey interesting discussion on final drive ratio guys.
I am about to start testing (and later on marketing) Subie 5 speeds for aircooled motors (fitted with my adaptor plate and flywheel combo to suit.)
I will most likely have to get a combination of gears and final drive ratios together that is 'one size fits all' for most aircooled customers wanting to fit a 5 speed.
Given that most of these would be fairly hypo 2 litre motors with either BIG cams and twin carbs or a turbo kit, what are the thoughts about getting the gearing where it needs to sit? Or is having a 'one size fits all' the impossible dream.....

Was thinking :
3.083 1st gear, 2.062 2nd gear, 1.545 3rd gear, 1.151 4th gear, 0.825 5th gear.
Final drive choice of 3.9, 4.1 or 4.4 ( I'm suggesting 3.9)
Other 5th gears available are : 0.972, 0.738, 0.78, 0.871.

Helical LSD