Cooling efficiency, speed and exhaust wrapping

Bob_pipedream · #1 10-13-2010, 05:30 AM

Now here is a thought... (also used speed in title to get Bob to read this)

When the air goes through the cabin heatbox it picks up heat from the exhaust and also cools the exhaust. Could that cooling action have the effect of increasing back pressure and thus increasing EGT on the cylinders on that pipe?

I have my carb heat box and cabin heat box on the same exhaust that deals with Cyl 3&4 - both 3&4 have high EGT and high CHT and very clean pipe, whilst 1&2 have low CHT and EGT and soot in the pipe. This is partly due to airflow, but I am beginning to wonder if having the boxes on the same exhaust is not overcooling it in two patches to create back pressure...

By the way, I know this may be due to other issues such as fuel distribution etc. I have also realised that I did not seal the end of the cowl ramps too - But just thinking on other lines now...

Now if I go and wrap the exhausts, not only do I decrease cowl temperature, but also I speed up the passage of gas through the exhaust which aides cylinder scavenging.

If cowl and CHT temps go down, then perhaps I can decrease cowl intake size and even exit points which could aide performance?

Or is everything I said wrong and the result of too much piccililly in my sandwich?

WingsOnWheels · #2 10-13-2010, 05:37 AM

Since there is soot in the exhaust on 1&2 it sounds like those cyl. are running a bit richer than 3&4 which would also explain why they run cooler.

Bob_pipedream · #3 10-13-2010, 05:41 AM

Yes with carb the fuel distribution does get a bit messy and I am pursuing that - she is a new engine and there are a few things I have still to do. Soot may have been caused by dirty plugs that I have now cleaned.

However my train of thought here is that wrapping the exhausts and maybe even the intake pipes, may create better efficiency and lower cowl temps enabling a reduction in cowl inlet size. Not sure if anyone else has tried this (but hope so).

Just found this too (its about increasing efficiency etc.):

http://www.eaa393.org/Presentations/...tPaser2006.pdf

Also found lots of older discussion fro 4-5 years ago - anyone got new information - particularly about this ceramic thing as I like the sound of that... What about only coating the inside of the pipe?

Mike S · #4 10-13-2010, 07:34 AM

You should do a bit of searching on this site for wrapping the exhaust--------

IIRC, lots of folks have had bad experiences with that----weakens the metal, and the pipes fall apart.

nucleus · #5 10-13-2010, 09:14 AM

I like ceramic coatings, started to use them on my turbo RX-7's, it makes a huge difference in the engine compartment heat. My mistake on the plane was that I stepped down in temperature rating, Wankle's have notoriously hot exhaust; which made me think the 1200 degree coating would be alright for the plane, but it really is not, so the outer coating is coming off now. No big deal functionally as the interior coating is high-temp and intact, and it does most of the work.

A big downside to the coating it that once you have your exhaust coated it can never be welded, so make sure your fitment is good first.

When I look at my engine install vs. others without the coating it is clear that it makes a big difference in radiant heat output, fewer nearby items with that scorched look.

My cabin heat works very well for an RV (warms three toes instead of two!); I did all the tricks I learned here, like a spring in the heatbox and blocking off most of the intake.

Never use the wraps on exhaust, they greatly shorten exhaust life and at the same time prevent visual inspection. Bad combo.

Hans

RV8RIVETER · #6 10-13-2010, 10:39 AM

Actually it depends on the coating some are not weldable and some are. Make sure you understand the specs of any coating you choose.

I think the only downside to coating the exhaust is cost.

Wraps are a bad idea on bare pipe. They have been shown to trap moisture and promote corrosion. I don't know of anyone who has wrapped coated exhaust pipes. If the coating was applied well, there should be no exposed metal to be vulnerable to the trapped moisture.

jrs14855 · #7 10-13-2010, 05:54 PM

I have seen suggestions that wrapping the intake pipes on the lean cylinders only will help balance the egt.

Mike S · #8 10-13-2010, 06:19 PM

I suspect it is just the opposite.

Lean cyl= lots of air, not enough fuel.

Wrapping the intake tube will--(should?) lower the internal temps, which will make the air inside more dense.

Dense air = greater amount than less dense air, therefore, it will cause the cyl to go leaner.

Or, am I just rambling

Bob_pipedream · #9 10-14-2010, 01:03 AM

Now wrapping the intake tubes sounds interesting as no downsides to that (my intake tubes are Stainless and without the fluctuations in heat should not suffer corrosion). I suspect that I have two lean cylinders due to seemingly standard bad fuel distribution and I suspect that differing temps under cowl helps cause it.

I too would suspect that wrapping the rich cylinder tubes would be the way to go?

Like this ceramic idea too - has anyone any idea of if it may improve performance or better EGT balance and smoothness?

DanH · #10 10-14-2010, 07:05 AM

Wrapping an intake tube won't change the actual fuel/air ratio with a carb setup. The fuel was added at the carb so temperature affects the mix, not just the air. However, it may change combustion behavior because heat helps evaporation and atomization.

Changing exhaust pipe temperature only changes the velocity of the pressure waves within the exhaust gas, thus changing the timing of negative wave arrival at the exhaust port. Correct wave timing helps scavenge the cylinder. You can also change timing with shifts in pipe diameter or length.

Pipe wrap may lower in-cowl air temperature, which could be both desirable and undesirable. Lower temps are better for fuel boil issues, easier on the alternator, etc. With some work higher temps can be turned into thrust, or more accurately, less cooling drag.