PMag blast tubes
 

Hello all,

Recently installed 2 Pmags which recommend blast tubes but I think the two blast tubes I installed are contributing to my hot CHT issues. Anyone have any experience running Pmags without blast tubes?

Thank you!

Charles

Mine has always had a blast tube installed for the single PMag. Given the temperature requirements of the PMag, I think you'd be making a mistake deleting the blast tubes.

Unless you went with really large blast tubes, the tubes don't steal a lot of air, relatively speaking. My guess is that the timing curve on the PMags is your underlying problem. Using the search engine, you can find a fair amount of data on the PMags, and their over-advanced baseline advance curve, which causes high CHT's. A review of the PMag manual will show you the option of installing a jumper to slightly retard timing. That's an easy change to make and might help confirm the root cause of your problem.

More likely advance than blast tubes
 

If you added two Pmags and used the recommended blast tubes and have noticed an increase in CHT I would guess that nearly all of the increase is due to advanced timing and only a tiny amount due to the air you are stealing for the blast tubes. I've experimented with blocking off the oil cooler air and the heater air from the back of the baffling on my plane and it is hard to tell the difference in CHT. But every degree of timing I advance the ignition increases the CHT by around 3 degrees F (you get a different answer LOP and ROP). First thing to do is make sure the jumper is in so that the timing is not so advanced.
Cheers
Nige

Which timing curve? Heat peak ...
 

Do you installed the jumper?

Which curve do you run?


Someone here measured the temperature on the Pmags ... the peak he got about 10-15min after shutting down the engine ... So it is questioned if the blast tube are really necessairy.

Timing is first, but look elsewhere for root cause and solution.
 

While the timing change maybe the initial cause, I would suspect the root is a sealing system around the engine needs to be improved. Pay particular attention to the lower center baffle seal plate. It was horrible on my factory new M1B. I have the SJ cowl with smaller inlets and still have way more air than the engine needs. I can open or close the oil cooler on the #4 baffle and nothing changes. Not upper plenum pressure or CHT's. Just oil temps. This tells me that I have excess air volume and pressure available mainly due to a very tightly sealed engine flow paths. Maybe too tight but that is off topic.

Keep in mind it is a cooling system. Inlets, baffle sealing, engine heat rejection, engine flow loss sealing, and upper/lower cowl pressures are tell tale signs of the balance. If timing increases heat rejection, then sealing to get better utilization will re establish the balance to restore the CHT

Bill, could you elaborate on your issue with lower center baffle seal plate?

I assume you are talking about the intercylinder baffle? It is a complex shape part under the cylinder, held in place by a hook and a clip that straddles the upper side of the cylinders.

What was wrong with it? It seems like a stock part that is presumably the same for everyone. Was yours constructed or installed wrong?

Not speaking for Bill, but the intercylinder baffles on my factory new 540 were horrible as well. Probably 99% of builders would run them as is, but I found mine needed a lot of bending, speed tape and RTV to completely seal the gaps.

+1 on the intercylinder baffle. Look hard at the front and rear baffle walls near the cylinder bases too.

Leaks are cumulative.

I've often wondered after a conversation with another builder .. the highest and potentially most damaging temps happens after you land and shutdown the engine .. wouldn't simply opening the oil door after every flight do more to protect the mags than the blast tubes? (no expert, just curious)

Short answer - no.

While running, the engine is producing heat that needs to be dissipated, the heat will migrate to the pMags via the case, oil, and convection. The pMags will themselves dissipate a portion of that heat. The hotter the pMag gets, the more heat it will dissipate until it reaches a balance of heat in versus heat out. With a blast tube on the pMag there will be a new temperature balance such that the same amount of heat is dissipated but at a lower pMag temperature.

When shut down, the engine is no longer producing heat but the heat there will continue to dissipate as the engine cools. If you start off with a hot pMag case (no blast tube) it will get hotter before it cools. If you start off with a cool pMag case, it will also get hotter but as you are starting from a cooler temp, the peak temp will be cooler than with no blast tube.

Opening the oil door will of course help on the engine cool down, but that is not the only consideration for how hot the pMag gets.

Carl