20 Valve Blacktop Race Engine

[Trekka273]

I am not 100% sure what gearbox it will be mated to yet.

My previous silvertop was mated to a VW 'box and we never really had any problems with it (aprat from crown wheel & pinion damage, etc) and it revved to 8000 RPM.

Basically it will come down to a choice of three, Porsche, Renault or VW. Even though it doesn't aid super quick shifting, I don't think there will be too many problems so long as I take some time with the changes and don't rush them.

[Starfire]

The alpha transaxles are worth considering. Quite nice little units

[takai]

I think youll find that they are Alfa.... And it could be a pain doing the clutch stuff for an Alfa, especially in a tight space like an offroad buggy.

[Trekka273]

In all the years I have been involved in offraod, I have never seen an Alfa transaxle used.
The 'boxes of choice are the VW, Renault, Porsche, Albins 'Box and a few Fortin's and Mendeola's. There is no way in hell that I can afford the Albins, Fortin or Mendeola 'boxes....
Very large tyres are used, which put huge stresses on the crown wheel & pinion. On my last car I ran 32" Hankook 4WD tyres. Albins make gears to suit all the VW, Renault and Porshe 'boxes, and you can also get them to make crown wheel & pinions as well.

[off-road]

In my opinion a full race kombi box will not cope with the motor you are planning.Revs is the problem because revs create the heat that destroys kombi boxes.If you cannot afford a Porsche or a Renault stick to a standard 20valve as that is the limit of a kombi.If you weren't hurting them in the past you weren't driving it hard enough.As you stated earlier in this post things have stepped up in the past few years.Buy the best trans you can afford and build a car and power plant to suit it.

[Trekka273]

Sure, revs cause a problem with the build up of heat, hence why we ran an oil cooler on the trans, it ran top quality synthetic oil and was stripped after almost every race we did.
You can drive the car hard without necessarily hurting the transmission... there are a number of cars around still running much more powerful engines than a 20 valve motor and also running high revs.
Like I said earlier, the 'box will be decided later on, after I have finished the engine.

[off-road]

I don't agree but pm me if you want to discuss it further as I don't want to get into a disagreement on the open forum as I'm just trying to help a fellow off-roader.

[DigitalPho3nix]

Whatever box you decide to go with, get a set of 'dog' gears for it. This will make it very strong and reliable. Plus it gives you rally car cred when you have a satisfying *clunk* when changing gears...

[Trekka273]

Off-road, I understand what you are saying, and I am not disagreeing with you entirely. Is the VW 'box getting towards its limits? Very much so. But like I said, I am not worrying too much about what 'box I am going to run at this point in time. Down the track I will be looking at that and working out what the best option will be. Chances are it will not be a Mombi 'box.

[30psi 4agte]

I know what you mean by the Albins box being $$$$$$ my m8 just bought for his buggie cos he was fed up with blowing VW ones with his sr20.

Anyhow. Adjustable cam gears are a good way to adjust the engines powerband. I have had some good results with bringing the power down from the 9500 rpm area to 8500 by adjusting cam timing., it has also fattened the torque range up a bit too

[Starfire]

Maybe a silly question, but why not use a reduction gear between the engine and the box? Surely this would reduce the speed that it has to run at.

[Billzilla]

Whatever box you decide to go with, get a set of 'dog' gears for it. This will make it very strong and reliable. Plus it gives you rally car cred when you have a satisfying *clunk* when changing gears...

Dogs aren't gears, they're a selection mechanism.

[DigitalPho3nix]

Thought dog gears where the ones that are straight cut instead of helically cut like normal road cars...

[oldcorollas]

Thought dog gears where the ones that are straight cut instead of helically cut like normal road cars...

nope, thats "straight cut gears".. the dogs are instead of synchros.. google ftw for pics
http://www.rallyplanet.com/media/pho...hotowpaXm3.jpg
the six lumps on the face of each gear.

[Ben Wilson]

In my opinion a full race kombi box will not cope with the motor you are planning.Revs is the problem because revs create the heat that destroys kombi boxes.If you cannot afford a Porsche or a Renault stick to a standard 20valve as that is the limit of a kombi.If you weren't hurting them in the past you weren't driving it hard enough.As you stated earlier in this post things have stepped up in the past few years.Buy the best trans you can afford and build a car and power plant to suit it.

Logically then, if the big revs are killing the 'boxes, building up the motor for strong midrange should be OK. I've never seen off road racing for real, but if the tracks are even slightly technical in nature, a motor built for mid range will be a lot easier to drive and in many cases faster than one built for top end.

I've never seriously worked on a 20v, but for a 16v, I'd be looking for cams around 264-272 duration* with as much lift as I could get away with. Tune the inlet and exhaust for around 6k or so and you should have a nice thing.

* - The extra flow at low lift will have the effect of extending the duration of the inlet cam, a larger exhaust cam may be a good thing. I'd concentrate any porting on the exhaust, a stock silvertop flows heaps more on the inlet than the exhaust, I'm sure there are gains to be made there.