High CHT's resolved and what worked.

[maus92]

I have a cooling problem in my RV-8A. Cylinder #3 runs very hot on T/O - I've seen that temp spike to ~450 deg before transitioning to cruise/level flight where it settles down to about 360 or so. I should explain that I'm the 3rd owner of the aircraft, and have been modifying it over the past 10 months. Part of the mod process was to replace the non/malfunctioning EGT/CHT gauge with the integrated G3X system, and this is when I discovered the hot cylinder.

Mitigation steps thus far:

1. Replaced the stock baffling with continuous silicon strips. When not in flight, you can see the side baffles drooping away from the cowling, while the rear baffles are compressed against it. Presumably when airflow enters the inlets, the side baffles unfurl to seal the gap. The underside of the cowling has telltale staining - not unlike the Shroud of Turin - that mark where the baffles are rubbing/sealing, and clean gaps where they are not.

2. Seal off the inboard cavity of the inlet ramps formed in the cowling. This was suggested by another builder on my field, so I closed offf the side of the ramps closest to the spinner with fiberglass.

However, there are other issues that I probably need to address, based on the pressure differential discussion in this thread.

There are six blast tubes installed. One on the lower inlet ramp plate for the alternator; two for the mags; one for the battery; one for the fuel pump; and one for the gascolator, all coming out of the back baffle plate. Two questions: Are these blast tubes actually effective, and considering that each of these openings might account for -1" of pressure differential, might they be part of my problem?

The heating system has a 2" scat tube that runs from the back baffle plate, to a muff around an exhaust pipe, to the heater box on the fire wall. When the diverter is closed, it dumps the heated air into the rear of the engine compartment. Based on this thread, it seems to me that this arrangement might have a negative effect on pressure differential.

Anyway, comments are appreciated!

CA

[Rupester]

Given the thousands of RVs with the identical cabin heater scat tube arrangement, I doubt that is a major contributor to your problem.

[DanH]

Originally Posted by maus92

There are six blast tubes installed. One on the lower inlet ramp plate for the alternator; two for the mags; one for the battery; one for the fuel pump; and one for the gascolator, all coming out of the back baffle plate. Two questions: Are these blast tubes actually effective, and considering that each of these openings might account for -1" of pressure differential, might they be part of my problem?

Charley, here we have a perfect example of where a few measurements would be worth 1000 opinions.

I would not hesitate to fly with those blast tubes blocked (my own airplane has none), so there is an opportunity to obtain "with and without" temperature values if you wish. Likewise, upper and lower plenum pressures, with and without the blast tubes. Send an email if you want to do it and I'll forward a how-to paper.

[rvbuilder2002]

A blast tube is only effective if it it's outlet is very close to the object you are trying to cool, or it is routed to some type of shield/shroud that surrounds it.

Plane Power engineering has said their Alt does not require cooling... that they have tested to well above the temp that the alt gets exposed to where it is located.

The batteries we use have done fine for many years with no indications that the heat they are exposed to is a problem.

A few people have found that cooling the gascolator has helped with a vapor lock problem, but my opinion is that the vapor lock issue is usually caused by something else (I.E., a hot gascolator is not the root problem). The majority of RV's flying with a carb. and gascolator have no gascolator cooling, and don't have vapor lock problems.

Do a simple test.
Block off all of your blast tube inlets with tape and see if it makes a noticeable difference. If not, then you know that you have other issues to resolve (which you may have anyway, but all of the blast tubes could be amplifying the problem).

Edit: Looks like Dan types faster than I do (not hard).