Hi Mike;
Hopefully your nephew makes a fast and full recovery.
I attached a bellmouth/velocity stack chart, I wish the guy I received
it from remembered the source...
The difference between a good and simple intake could easily be 10%. A
tuned intake in the tuned range even more. Any approximation could go a
long way.
Best power is probably closer to 13.5 AFR @ 27* @ WOT @ 7000rpm in a
stock 13B NA-> say 13-14 AFM.
Oil/Water Temps ~160F according to Lynn.
Best power will probably be just rich of peak EGT. I've heard of peak
HP EGT's as high as 1800F. IIRC, Lynn mention 1600F and measured @ 3"
from the exhaust port - and Lynn probably has more high power dyno/race
time than the rest of us combined.
For Al:
Top speed is the intersection of too many curves (engine HP/ load, prop
thrust, airframe/prop drag)
One answer to less than peak HP rpm (@top speed) is to start iteratively
lopping off the prop tips until peak HP engine rpm is restored for
whatever air speed.
If the top air speed is prop HP limited (for a particular V); a dive
"trading altitude for kinetic energy=HP" may allow the engine to achieve
and maintain a higher HP output and consequently top speed -> an
indication of being over-propped (assuming it is not pitch limited).
One hint, of the possibility, is that full power is less than WOT and
not completely out of whack compared to car intakes/HP/loads...
The best answer is to probably get Paul Lipps to design an optimized
prop for your engine(HP curve) vs airframe(drag curve). Expect to
fly/chop at least 2 props until it is optimized.
Again, ANY drag clean-up goes a long way at top speed.
Cross your fingers
Cary
> Sorry for my absence on a topic I started. My nephew had a
> skateboarding accident last weekend and is in the hospital in a
> chemically induced coma until his brain swelling is under control. And
> I thought car powered airplanes were dangerous. Stay away from
> skateboards.
>
> Tracy - My mixture monitor shows stoich at full throttle. I suspect an
> airflow issue rather than fuel.
>
> Al - my vacuum gauge is located in the plenum directly behind the
> throttle plates.
>
> Lynn - I agree that the TB inlet is horrible and needs a bell mouth.
> The inlet is also pretty obstructed by the cowl which isnt visible in
> the pics I sent. I think I'll make an attempt to clean this up some
> and see if it gets me anywhere. If not I'll live with it for a while.
>
> Kelly - always tough to determine if the problem is an engine issue or
> if the load is simply too much for the engine to overcome. I'm going
> to experiment with the shallow dive several have suggested and see
> what happens.
>
> Mike
That chart appears in a book I own called "Tuning BLs (for British Leyland) A series engines". Has to do with Bugeye Spite and MG Midgets. The version I found is shown as figure 6.4. So it may well be reprinted by permission, stolen, or original to another printing of the same book. Thank you David Vizard.
The numbers shown are of little value but the outcome is astoundingly different as suggested in the picture. I can detect a difference in flow up to 2 diameters from the mouth of an intake tube on my flow bench. The best flow is from the nearly flat large diameter that could be fitted to most intake logs or plenums. It is the worst when it comes to fuel standoff more common in Periphery port engines. So they might do better with injectors close to the port face. This thinking must be applied to the ends of runners at the log junction as well as the Throttle Body air inlet.
The seemingly over-rich best power mixture is due to the dismal combustion chamber shape and the flame quench that happens at each end of the rotor face. The cool rotor face and the cold rotor housing and the odd long flat wedge shape combine to put out the fire, and let unburned fuel escape the fire of combustion, only to burn while leaving the engine right about where your EGT probe is installed.
The richer mixture reduces this loss slightly, at the cost of fuel economy. About 4.7 MPG. This is a problem with the Weber that is out of its operating range at high RPM, and connot deliver mixtures below the 13s. Leaner mixtures are more difficult to light, so in the stock RX-8 you will see closed throttle ignition advance going well into the 40s to give the lean mixture time to get burned. the Other ploys are often used to do this like shutting off some exhaust gas recirculation. Higher compression and high energy multipal restrike ignition systems are some.
You might try a prop that you know has less pitch than you want just for the tests, so the engine will not be load limited. In any case a reduction in full open throttle should have some effect on RPM. So, I would dial up a little richer before the test, and perhaps 25 degrees of advance, just to be sure that combustion difficulties are not being test instead of engine flow demand.
Lynn E. Hanover
Obtuse connections Tzar
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