Power & Theroy

Post Reply
Don
Posts: 46
Joined: March 24th, 2007, 3:40 pm

Power & Theroy

Post by Don »

I have a question............2 indentical vees except one has external ducts to the heads / cylinders and oil cooler. The other uses the VW fan and a gutted generator (only function is a bearing holder for fan). The first vee will have a larger forntal area by the same cross sectional area of the VW fan inlet on the second vee. Assuming no load from generator would there be a significant difference in power required to push the vees through the air? The reason for the question is if I want to run a vintage vee in regular racing is it worth the effort to remove the fan and make up custom ducting or just gut the generator and keep the fan; all else being equal (in theroy any how)?

any comments?
thanks, don
BobShedd
Posts: 57
Joined: July 16th, 2006, 10:34 am

Re: Power & Theroy

Post by BobShedd »

Don, Just a few random thoughts that may or not answer your question. I race a Monoposto legal vintage vee ( fully functioning charging system ), I aso build engines for many friends / competitors and have spent many hours testing vee engines on the dyno.
1) I do not believe the horsepower drain from the generator is measurable, the HP drain is the fan. The drain from the fan is considerable - some think it to be in the range of 6 to 10 HP at racing speeds. I intend to do a fan / no fan test one day soon when I get caught up ( grin ). I don't believe that gutting the generator does anything execpt make it a little lighter. If you are running with a club that follows Monoposto Racing rules then the generator must be working. Make sure your generator bearings are very good, some generators turn very hard sucking power. Run your fan belt very loose - 1 to 2 inches of deflection.
2) Removing the fan and adding cooling ducts, if done correctly will make more power and a faster Vee. Come up with lots of air to the engine with very little in the way of scoops sticking out - think NACA ducts. Lots of air to the heads, probably need a cylinder head temp guage ( CHT ) to make sure you are keeping the heads cool enough. Cool heads and warm oil make horsepower.

Hope this helps, Bob
SR Racing
Posts: 1205
Joined: June 24th, 2006, 1:58 pm

Re: Power & Theroy

Post by SR Racing »

Not real easy to answer it all. But, at about 5000+ RPM the fan/gen takes about 5 HP. (the majority of that is the fan, assuming the battery is fully charged) Removing the fan/gen and cooling via duct work will introduce drag but without knowing the type of ducting it is difficult to determine the loss. The secret is to cool using air that is going to be a drag already and introduce as little additional drag as possible. NACA ducts are fine, but difficult to get them to cool as well as other means. Whatever cooling you use, you also want it can scavage from the car as easy as possible. And Bob is correct, a CHT is helpfull to determine how well your stuff works.

I think you will find that no matter how much drag your air ducting introduces (within reason) , it won't be as much as the fan/gen until you get well past 100 mph. With aero drag quadrupling for every doubling of speed it becomes a big issue up there. The fan/gen doesn't care how fast the car is moving. :). For example, at 60 mph your fan could take 5 HP and the ducts only 2. at 150mph your fan could still take 5 HP, your ducts might take 25 hp..
Don
Posts: 46
Joined: March 24th, 2007, 3:40 pm

Re: Power & Theroy

Post by Don »

Thanks very much this is the information I was looking for, very helpful. Now............ how can I get 150 mph out of my vee hummm.........................

Appreciate this,
cheers, don
Dietmar
Site Admin
Posts: 650
Joined: June 24th, 2006, 11:56 am

Re: Power & Theroy

Post by Dietmar »

Don:

Just an additional comment: if you do decide to retain (run) the fan, be sure to use the 16 blade (early) version.
Dietmar
http://www.quixoteracing.com
smsazzy
Posts: 703
Joined: June 24th, 2006, 5:56 pm

Re: Power & Theroy

Post by smsazzy »

Don wrote:Now............ how can I get 150 mph out of my vee hummm.........................

Appreciate this,
cheers, don
Easy, push it off a cliff.
Stephen Saslow
FV 09 NWR
sharplikestump
Posts: 183
Joined: January 12th, 2009, 2:28 pm

Re: Power & Theroy

Post by sharplikestump »

Both Bob and Jim have far more experience with the vintage setup, but I will add this.....for what it's worth:
Early fan, stock 7" pulley, nuthin-special belt, very little time spent aligning stuff, stock non-charging gen.

3,000 rpm syphons 1 hp.
6,000 rpm GOBBLES 6-10 hp.
On the subject of running a loose belt: Doing it by running a large or skinny belt on a gen. pulley with none or very few shims can help the power some, but if you do it by adding many shims to the gen. pulley, you have now changed the ratios, and once spintrifical force kicks in, you are spinning that gobbler all the faster....bigger losses.
Mike P.
(please remember "for what it's worth").
Post Reply