[HankyJ]

If I can't ask this question on this board, I can't ask anyone. Here goes:

We all know what a supercharger or other forced induction system does for an engine. Why not a forced evacuation system? What would happen if you put basically a fan just after each collector in the exhaust system and make spin them faster than the exhaust pulses? Basically I'm thinking of a turbo-charged exhaust system. I expect I'm not the first person to come up with this idea, but there's probably a reason why it's either impossible or impractical. Ideas?

 

[Barret50]

Perhaps such a thing could reduce the need for overlap by increasing the scavenging effect. But how that would help Im not sure.

 

[HankyJ]

I'm thinking sort of the same thing. If you could scavenge exhaust better at higher rpm with help of the fans, it might allow you to run a milder cam for better bottom end. Sort of like a stepped tube header. Just a thought. All in theory at this point.

 

[Wildcat]

I know on rice-rocket street bikes a similar system is used in some. Although it is basically a valve that prevents backflow of exhaust. To my understanding, the exhaust is prevented from backflowing after it is ejected from the cylinder at the level of the collector. This creates a negative pressure in the proximal part of the exhaust (the part in the header tube) and sort of sucks the exhaust out of the cylinder when it's ready to unload again. Of course this is a passive system, your describing more of an active one.

Supposedly they (kawasaki?) developed a similar system on their earlier bikes but it was prone to failure. The new one is supposed to be much better.

So to answer your question, it is not a stupid idea at all.

<font size=-1>[ This Message was edited by: Wildcat on 2/20/02 8:29am ]</font>

 

[Wildcat]

I thought your idea was neat so I did some math:

In a 302 motor there is 37.75 cubic inches of air per cylinder (302/

.

For every engine rotation there are 4 cylinders that empty exhaust, in a dual exhaust setup, that would be 2 cylinders per side.

At 5,000 rpm, you are moving the engine at about 83 rotations per second (5,000/60).

(83 rps) X (2 cylinders) X (37.75 ci) =

6,289 cubic inches of air per second is being moved per exhaust side. To push that much air, that is one heck of a fan. But that value seems a bit high, let me know if you come up with something different.

It seems maybe the flow would actually be higher though because that cylinder volume (37.75) assumes atmospheric pressure. In a cylinder, you would think that there is actually a greater volume of air being crammed in that space becasue of combustion, which would cause even higher flow rates.

 

[HankyJ]

Those are interesting figures you came up with. I can hardly operate a calculator let alone do real math, but is the basic concept workable? Another poster made the point that you won't make power unless you're adding air/fuel. But I thought that pulling the exhaust gases out with force would free up room in the combustion chamber for more air/fuel, as is the purpose of large diameter exhaust tubes. I tend to think with an active exhaust pulling system you could get away with smaller diameter tubes (both on the intake and exhaust systems) for crisper, better bottom end, and breathe a lot better at high rpms. Also I suppose there's the question of how to make the fans move at such a high speed without requiring a complicate electric system. Hmmmm....

 

[rod n az]

Maybe have the fans hooked up to the driveshaft with a belt. Kinda like an alternator pulley and a crank pulley.

 

[Farmer Sprocket]

Ummm, As i remember reading, the guys from chrysler did an exhaust evacuation system back in the 50's. it was not a mechanical fan, it was based on the venturi principle. it worked so well that it sucked the intake charge right out of the chamber. The group was called the Ramchargers. I think that it would also burn up exhaust valves.

 

[Dr Evil]

wildcat-
I took your math and added some figures to get the CFM for a 302 at 5000 RPM.
6,289 cubic inches of air per second that the engine can take, as you stated above.
TIMES
60 (Seconds in a minute)
DIVIDED BY
12 cubed (to get Cubic Feet Per Minute)
Gives you aproxamently 218 CFM, right? That seems pretty low. I think something is wrong with that figure.
So if you can find a blower that puts out more than that, wouldn't it work? Or is my math wrong. I checked sears.com and you can get a blower that puts out Up to 480 cfm. Wouldn't that be plenty to add some HP?
Correct me if I am wrong before I go try it on my 351, with the the 351 #'s of course.

:bounce
_________________

<font size=-1>[ This Message was edited by: Dr Evil on 2/21/02 12:10pm ]</font>

 

[Wildcat]

I get the same value as you. 218 cubic feet per minute.

What I think is wrong is what I touched on before. All this math assumes that the cylinder volume (37.75 ci) is not compressed. What actually comes out of the exhaust is the result of an explosion in an enclosed area. In order for the explosion to push the piston down, there must be a greater volume of "air" actually being released. That is, the 37.75 ci of "air" that was in the cylinder would actually take up something like 200 ci of air outside becasue it would be allowed to expand when in the open area.

I have no idea where I'm going with this but maybe there is a lot more exhaust coming through there than I previously estimated. The old 400 cfm blower may not keep up.

But really, if exhaust would make such a difference, why not just run the motor without exhaust and let the heads dump into the atmosphere. This would provide the least amount of back pressure and least resistance to get the gases out. Sure you may doze off on the highway from inhaling too much carbon monoxide, or maybe your valves would warp from the cool air being sucked back into the motor after you stop. But in theory this should make you faster????

I really don't know. But headers must serve some sort of purpose rather than just running open exhaust ports. Why???