Skip at Oshkosh and superchargers

[Coyote Chris]

I attended the lecture by Skip Holm at Oshkosh about air racing and had him explain what he meant when he talked about a “tubed” Packard Merlin. Evidently, if I understood correctly, the aftercooler is replaced by a tube and the air charge is cooled solely by the methanol injection (ADI). Skip said he has seen 138 lbs mp on Dago….I now wonder how a standard P-51D intercooler/aftercooler works. I have heard conflicting stories. Is there
a radiator with a dedicated supply of coolant in with the main radiator and does this feed a cooling core on the engine that cools both the air out of the second stage of the supercharger or is there also a way to cool the air between the two stages of the Merlin’s 2 stage super charger (an intercooler)? (Oshgosh is a tough place…I think I had to miss the lecture on the Pond racer to listen to Skip…too much to do!)
Next year, If he comes again, am taking a tape recorder so I can remember all he talked about!

[51fixer]

Re: Skip at Oshkosh and superchargers

I attended the lecture by Skip Holm at Oshkosh about air racing and had him explain what he meant when he talked about a “tubed” Packard Merlin. Evidently, if I understood correctly, the aftercooler is replaced by a tube and the air charge is cooled solely by the methanol injection (ADI). Skip said he has seen 138 lbs mp on Dago….I now wonder how a standard P-51D intercooler/aftercooler works. I have heard conflicting stories. Is there
a radiator with a dedicated supply of coolant in with the main radiator and does this feed a cooling core on the engine that cools both the air out of the second stage of the supercharger or is there also a way to cool the air between the two stages of the Merlin’s 2 stage super charger (an intercooler)? (Oshgosh is a tough place…I think I had to miss the lecture on the Pond racer to listen to Skip…too much to do!)
Next year, If he comes again, am taking a tape recorder so I can remember all he talked about!

P-51 Intercooler setup has separate core in the radiator dedicated to the Intercooler system. There is a coolant pump (just changed one in Princess) also dedicated to this system. It is located on the L/H side of the engine and uses the same drive as the generator. The intake air comes in through the carb turns 90 degrees into the 1st stage of the supercharger but travels through the fuel discharge nozzle where the fuel is added to the air. After the 1st stage wheel it then it travels through several passages then into the 2nd stage wheel. It thens travels through the intercooler housing through a finned radiator core and then into the induction tubes and rams horns where it meet the cylinders. Both supercharger wheels are on the same shaft. There is a clutch and gear setup to change the speed that the superchager spins. That is selected by either a manual switch or/ and an altitude sensing switch.
Rich

[Coyote Chris]

Re: Skip at Oshkosh and superchargers

Thanks Rich! There is only one point in your excellent description I would like to clear up. This quote/paraphrase is from the book, "Janes Fighting Aircraft of WWII" P281, Intercooler system of Merlin engines.
"The coolant is circulated by means of a centrifical pump and passes from the header tank through the radiator to the jacket situated between the two stages of the supercharger and to the intercooler matrix, placed between the supercharger and the induction pipe, and thence back to the header tank." I take this to mean that there is a water jacket used as an intercooler between the first and second stages but the there is an aftercooler (which the brits call an Intercooler matrix) after the second stage? Would this be the small "box" with tubing inside it to take the heat out of the fuel air charge headed for the intake manifold?

Thanks for your time!
Chris

[Smee Again]

Re: Skip at Oshkosh and superchargers

Chris, it sounds like you have a good grasp on the aftercooler system. One other thing to mention is that when using a "tube" motor for racing, the 1/3 of the radiator usually reserved for the aftercooler is now used strictly for engine cooling. This helps engine cooling a great deal at the power settings used for racing, and coupled with the use of a spraybar, enables the coolant door to remain closed for superior aerodynamics.

SA

[51fixer]

Re: Skip at Oshkosh and superchargers

Thanks Rich! There is only one point in your excellent description I would like to clear up. This quote/paraphrase is from the book, "Janes Fighting Aircraft of WWII" P281, Intercooler system of Merlin engines.
"The coolant is circulated by means of a centrifical pump and passes from the header tank through the radiator to the jacket situated between the two stages of the supercharger and to the intercooler matrix, placed between the supercharger and the induction pipe, and thence back to the header tank." I take this to mean that there is a water jacket used as an intercooler between the first and second stages but the there is an aftercooler (which the brits call an Intercooler matrix) after the second stage? Would this be the small "box" with tubing inside it to take the heat out of the fuel air charge headed for the intake manifold?

Thanks for your time!
Chris

Chris,
There some coolant that goes through that area. I don't think it is extensive in the area it reaches in the supercharger.
I will have to look in my original overhaul manual and see if diagrams or pics will show this. Maybe someone else will give the the exact size of the coolant passage in the supercharger.
I have pulled and disassembled the aftercooler or the beer cooler as Sparrow calls it but I haven't split a Supercharger.
Remember that the Brits don't talk right and it's hard to understand some of what they write. They did make many versions of this engine so some may different than others in some of the details as well.
I spent a month unBritizing Princess after we got her. I am still scratching my head of a couple of things.
Rich

[Coyote Chris]

Re: Skip at Oshkosh and superchargers

Wow, Rich and SA, you guys are awesome! Thanks a bunch. Cant wait to get to Reno on the first Saturday, set up camp for 9 days, and get to the pits and learn more. At Oshkosh, got to talk to the crew of the P-38 "Ruff Stuff" (which along with Glacier Girl was way cool) which apparantly is the only P-38 in the world, according to them, that has the turbos working, and learned alot also. Just cause of people like you here, I am making a donation to this forum...thanks!

Old man on red motorcycle headed for Reno...

[wingman]

Re: Skip at Oshkosh and superchargers

A couple quick scans of under Strega"s hood in 1990 and 1992. One thing I've always found interesting is how much the under-hood arrangements can change from year to year even with the same engine builder.

[Randy Haskin]

Re: Skip at Oshkosh and superchargers

A couple quick scans of under Strega"s hood in 1990 and 1982.

Strega in '82? C'mon, Neal....

[wingman]

Re: Skip at Oshkosh and superchargers

There -- it's corrected. Happy now, flyboy?

[wingman]

Re: Skip at Oshkosh and superchargers

For comparison with the above, here is a pretty much stock setup, I think.

[RAD2LTR]

Re: Skip at Oshkosh and superchargers

If you look at the Strega pics, those are without a doubt a set of race exhaust stacks, where as the Rainier Beer mustang has the stock set. (An extension of a different thread.)

Just an observation (hopefully my undoubtedly observed race stacks are indeed race stacks.)

Will

[Smee Again]

Re: Skip at Oshkosh and superchargers

There are no real "race stacks" for the Merlin race motors. The stacks used for racing are the later -9 model stacks due to their robust construction (less likely to burn off and/or crack). These stacks are currently being reproduced overseas, and thus far, have performed quite well.

SA

[Smee Again]

Re: Skip at Oshkosh and superchargers

For comparison with the above, here is a pretty much stock setup, I think.

It's pretty hard to tell from that angle. A shot from the starboard side would likely tell much more.

The first thing to look for on a racing Merlin would be the obvious induction tube in place of the aftercooler, which can be clearly seen in the Strega pics.

Next, look at the blower itself, and in particular, the backside of the blower (aka - the "hat" or "horses ass"), where the carb mounts. If it's a 2 piece hat, it's a -7 blower, and most likely not a racing setup. For racing, a one-piece -9 blower is used (as well as the wheelcase, which drives the accessories), which has a superior fuel discharge system, and thus better distribution of the fuel/air charge.

If you can, also take a peek underneath the engine, and check for several little "bumps" protruding from the oil pan. This would indicate a -9 pan, also preferred for a true racing setup, but may not always be a good indication (a -7 pan has also been used in racing applications).

Anyway, these are a few things to look for when determining a racing Merlin from a stock "daily driver" setup. Take a good look at Voodoo this year, and you will see most of the above described characteristics (with the exception of the oil pan, IIRC).

SA

[Coyote Chris]

Re: Skip at Oshkosh and superchargers

Hey, thanks SA...I will definately pay closer attention when I am bumbling around the pits early in the week...I will see if I can find a "tour guide!"

Chris

[51fixer]

Re: Skip at Oshkosh and superchargers

"The coolant is circulated by means of a centrifical pump and passes from the header tank through the radiator to the jacket situated between the two stages of the supercharger and to the intercooler matrix, placed between the supercharger and the induction pipe, and thence back to the header tank." I take this to mean that there is a water jacket used as an intercooler between the first and second stages but the there is an aftercooler (which the brits call an Intercooler matrix) after the second stage? Would this be the small "box" with tubing inside it to take the heat out of the fuel air charge headed for the intake manifold?

Thanks for your time!
Chris

Chris,
I checked the Merlin -7 O/H manual today.
There is only one area that acts as an Intercooler and that is the housing on top which the air/fuel charge passes through after exiting the supercharger and before entering the cylinders.
The water passage is on the OD of the supercharger housing. Coolant from the aftercoolant radiator core enters the coolant passage on the supercharger housing about 1/4 of the way up on the lower L/H side (B bank) of the engine. It flows upward through this passage and at the top it enters the aftercooler housing via a metal tube. It is then drawn into the aftercoolant pump and discharged downward into another line which carries it back to the radiator core.
This cooling passage is more of a cooling helper in the supercharger rather than really effective at cooling the air charge.
Rich