Hi Paul and others,
I was quite puzzled on that "fuel cloud" phenomenon, have never
seen/heard it before. I'm not sure if it was fully covered before, so...
I found this discussion thread when looking for info about nox boosting.
It's quite good on explaining the thing.
"Fuel stand-off - intake reversion - what are the causes?"
http://www.eng-tips.com/viewthread.cfm?qid=190526&page=1
Originally i got the name for phenomenon from this article:
http://www.carcraft.com/techarticles/116_0406_nitrous_350_chevy/index1.html
There was some papers listed later on thread:
"Intake bell-mouth effects"
http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf
NACA paper on induction manifold dimensioning
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090861_1993090861.pdf
NACA paper on induction manifold dimensioning - pipe length
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930084998_1993084998.pdf
And later, article about exhaust manifolds
http://www.ssheaders.com/header.htm
Cheers,
Petri
I first noticed this around 1965 on 800 HP small block V8 Chevy
Chaparral racing engines. The fuel mist was being blown away by the
speed of the car. This is what I did about it. I designed this
air box and had it installed on the Chaparral 2D and the problem
went away. Perhaps just out of sight and out of mind. The car went on to
win the Nurburgring road race in Germany so it obviously worked.
As far as I can tell this was the first ram air box on a race car.
Now common practice on almost all race cars today. Of course the P38
and the Me109 used similar ram air boxes years before.
Last years 245 HP 2nd gen 13B p-port had it. This years 264 HP RX8 p-port
engine had it in spades. You can find a video of it on the web site.
http://www.rotaryeng.net/intake.html
This years engine generated 20 more HP. Both in the same
7300 RPM range. Strangely this years engine had 3 inch shorter runners.
Both had downdraft Weber carbs as did the Chap. 1965 V8 Chev.
Although we did not run it above 4000 RPM I did not notice it on Marks
Suspinski 13B p-port engine when running with EFI and 25 inch runners.
That is not to say it won't do it at higher RPMs. Mark should have
constructed an air box by now.
I suspect it has to due with the carb venturi. Air must be shooting
out of the intake runner sucking more fuel out of the float bowl and
then back into the engine. I think the engine is flooding
with excess fuel. In other words the mixture is probably down
around 8:1 or worse. IMHO the engine is drowning with fuel.
The BSFC is real bad when this happens and the engine stumbles.
So too is the gallons per hour read out from the dyno flow meter.
Perhaps one way of dealing with it is to restrict the size of the
fuel hose to just enough to get peak power at 12.5:1 I am going to try
that on the low pressure fuel injection I am now building.
Paul Lamar ...No rotor no motor.
Gosh! "The Vacuum Cleaner" was a creation of the Chaparral team!
I knew that name "Chaparral" was familiar from something, but didn't
first recall what it was. All those cars are legends. You have been very
lucky, Paul. Wish i will be some day a good enough engineer - or a
millionaire - (or, preferably, both :)) to have an opportunity to work
on such a brilliant team.
Regarding the cloud-thing:
You actually mentioned stand-off on first dyno posting, but then i
didn't got the meaning of it.
After reading those war-stories of what others have gone through with
carbs, seems that they merely settle with air filters, induction box and
down-jetting the carb to adjust to over-rich condition.
But down-jetting is not feasible to cure that resonant peak - its likely
quite narrow RPM-wise, and jets are at least 1000-2000 rpm wide, i
think. If carb is going to be on engine awhile, maybe starting induction
tubes with those bellmouths and putting them on a box, and carb on top,
would make a difference for resonance peak behavior.
Now mechanical or electronic injection seems even more desirable...
Also, remembering those RPM-adjusting induction pipes on R26B, it would
give a sizeable boost on power curve if something similar could be made
for Renesis.
Maybe it could be even kept purely mechanical - by putting
spring-tensioned centrifugal weights on someplace appropriate, and
leading that RPM-induced movement to tubes by gears, wires etc. It would
be easy to add a mechanical bias-adjusting to allow for varying conditions.
Anyway, it is great that P-port Renesis is now a reality and running.
Cheers,
Petri
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