Dale,
I have been monitoring this discussion with interest, lots of good
theories being put forth, but as in many other areas, the real world is
very different.
The amount of gasses that are being expelled would require much more air
flow past the exhaust, to have an adequate venturi effect, than we have
at our speeds, I suspect. I think what you suggest, wave scavenging, has
more application to our world. (We know that there is a vacuum formed in
the exhaust pulses, some have used this for a long time to vent the
crankcase, at times so effective as to pull in gaskets. This theory is
also used to scavenge the crankcase on the modern car (Pulse Air
System).
I tried some experiments with wave scavenging many years ago at the
drags. We built headers with pipes (about a 60" collector) that extended
above the rear wheels on the roadster. We began cutting the pipes
shorter as we progressed with runs. The long collectors gave a
tremendous boost to power off the line, but the top end suffered. As we
shortened the pipes the low end power diminished and the top speed came
back.
There is a formula for header lengths that has been used for years (we
have discussed this before on this net).
Except for putting the exhaust out the back of the car, or underneath if
skirted, (both low pressure areas) I personally don't think you will
find an improvement. Even in the fender wells I think you will find that
the exhaust stream will extend well beyond the side of the car. I have
been told that the exhaust exiting the side of the car has the effect of
an extended 2X4 10 feet long on the aerodynamics. In other words, an
increase in frontal area. Another thought, if the exhaust exits under a
skirted car, wouldn't that nullify the ground effects?
Tom, "just some pondering", Redding CA 2:30PM PST
Dale & Eleven Thomas wrote:
>
> I believe the original idea is that exhaust is a high pressure gas, forced
> out of the cylinder by compression. If one vented the gas under a panel that
> had high speed air rushing over it, it would help with the scavenging of the
> exhaust. The air running over the panel would leave a low pressure area
> under the panel, high pressure flows toward low pressure and voila ! Of
> course theory and practice are two different things.
> There is allot more to wave scavenging, (which is what we are really
> talking about) then just venting high to low pressure. as the wave of high
> pressure runs down the pipe (the eggs), it causes a low pressure area behind
> it. Also as the piston is going down on the intake stroke, usually the
> exhaust valve is still open and it draws some exhaust (another low pressure
> area) into the cylinder diluting the fresh intake charge (YIKES!!!).
> Anyhow, I am getting more technical than necessary. the main thing is to
> squeeze every drop of help/horsepower out. And scavenging the exhaust better
> maybe one free way of doing it.
>
> Dale" chief" Thomas
>
> -----Original Message-----
> From: FastmetalBDF@aol.com <FastmetalBDF@aol.com>
> To: landspeedracer@email.msn.com <landspeedracer@email.msn.com>
> Cc: dferguso@ebmail.gdeb.com <dferguso@ebmail.gdeb.com>;
> dhaller@techline.com <dhaller@techline.com>; land-speed@autox.team.net
> <land-speed@autox.team.net>
> Date: Tuesday, January 04, 2000 4:31 PM
> Subject: Exhaust Extraction --- it's exhausting
>
> >John, Dave, Douglas ( --- my son ), and list: The effect we're discussing
> >here, the utilization of the air flow past the open ends of either
> individual
> >pipes from each cylinder, or collecters which team up these runners into a
> >large diameter pipe ( if the collector is circular --- some are not ), to
> >gain an extraction effort on the exhaust gases --- is known as the
> Bernoulli
> >principle. We've all probably used spray guns on our cars and other
> >projects, and that spray gun, or your wife's or girlfriend's perfume
> >atomizer, use this principle to "lift" , or pull, the fluid involved into
> the
> >airstream which is rushing past a small orifice at the upper end of the
> tube
> >immersed in that fluid.
> > This principle, it would seem, could be used to good advantage to aid
> >flow out of the cylinders, thus enhancing the fuel mixture flow INTO the
> >cylinder, thereby reducing the TIME required to "change" each cylinder.
> This
> >results in increased HORSEPOWER, as H.P. is a measurement of work done in a
> >GIVEN TIME. One H.P. = 550 foot - pounds of work PER SECOND.
> >We know we need LOTS of H.P. to push our racecar --- be it a "brick" or a
> >sleek lakester or 'liner --- to really fast speeds. My youngest son
> Darrell
> >is a great fan of this theory that WAY TOO MUCH HP IS STILL NOT ENOUGH !
> I
> >often tell him he goes by the " sledgehammer " approach to speed.
> > I believe that in order to actually realize any useful gain by
> >"extraction", that individual stacks, mounted at 90 degrees to the
> airstream
> >going along the sides of the car MIGHT work the best, but I think that to
> get
> >this on pipes of , let's say from 1 3/4" dia or so on a smaller engine, to
> 3
> >1/2" or so on a bigger mill --- that the vehicle would have to be traveling
> >at a speed VERY FAR beyond it's ultimate capability. To reduce the
> diameter
> >to obtain a much smaller cross - sectional area at the pipe end, hoping to
> >better its relationship to the flow going past, would obviously be TOTALLY
> >Counterproductive, due to the resultant INCREDIBLE BACK PRESSURE created !
> >These are just my thoughts --- I can't see any useful application of
> >Bernoulli's law here. Maybe Mayf could dope all this out on his
> computer ---
> >I sure can't. Remember, those exhaust gases are already under tremendous
> >positive pressure to escape --- ever stand 10 feet or so from (JUST an
> >UNBLOWN smallblock) and feel the " slugs " of air hit your pantslegs from
> the
> >old " straight out stacks " ?? Many of us have ---that's LOTS of
> >pressure, and hard to improve on ! Besides, like Dave Haller likes the
> >collectors coming out the front fenders on his altered --- "cool !" he
> says
> >---well, we ( Doug , Darrell and I ) like the looks of ours - same place -
> >on the Black Radon altered, and I, myself, don't care if we lose a few
> >horses in the design! MAYBE we can spare a few -----
> > Bruce Ferguson Black Radon Engineering Racing Team
> >
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