Supercharger + High Compression Heads. Let's talk.

[Jason in TX]

Ok, so I've read that it is actually advantageous to not run higher compression heads on a flathead when adding volumetric pressure to the combustion chamber; that it is the recipe for pre-detonation.

But I also want to cool factor of running a vintage supercharger with some finned aluminum heads. I'm told that at idle this supercharger puts out almost no pressure, but as you accelerate, you may get 2.5 - 3 lbs of boost at most (less than a roots blower)

The heads I'm considering are rated at 8.75:1 or maybe they are 9:1, but I'm not fully sure.

Am I going to actually hurt the performance of drivability, or will it pretty much be no big deal? This will be on an otherwise stock, rebuilt 1940 Ford Flathead.

[Tim Ayers]

Have you read Joe Albin's book on supercharging a flathead? If not, that is required reading.

Also, I think since Joe has written his book, Edelbrock has come out with lower compression finned heads. May want to to check those out as well.

[LSGUN]

What kind of supercharger are we talking about? I have a McCulloch, but yes generally with any form of forced induction you will want a lower C/R.

BTW I sent you a PM a few days ago.

[Dave/Green Bay]

Jason: I have a 35 21 stud with a 39 McCulloch that we are putting on. I think my supercharger is much the same as the picture you posted several weeks ago. We are using the Eddie Meyer Heads that Don Orosco in repoping. The Head were somewhere in the 8.5 range. The boost of the supercharger is around 4 lbs. We decided to take material out of the heads to bring the compression down to under 8 so we won’t have pre-detonation issues. The overlap of the cam is also very important as to make sure that you’re not blowing fuel and air when the valves are open. We used a MAX 1
Walt DuPont on the Barn ran one of these on his 40 coupe for years and was very helpful sharing his experiences with his setup. Dave/Green Bay

[Jason in TX]

@Dave/Green Bay - GREAT TIP on the MAX 1 cam. Thank you. I have seen Walt's photos and have tried searching for his discussions on exact set up, but I guess I missed that.

@Tim Ayers -
I was just looking at his website reading his Tech Tips page. http://www.roadrunnerengineering.com/tech%20tips.htm

I'm finding that the normal common boost for a blower is around 5-6# Because this blower is centrifugal, it is not as efficient and should be lower. I suppose it will be somewhat of a trial and error set up.

He also talks about removing a little material from heads to get a desired CC, but focusing on the area above the valves, and not degrading the Ricardo design.

Compression Ratio for a Blown Flathead

The blower boost can be tailored for good performance and reliable operation with flathead compression ratios from 6.5 to 9.0:1. Optimum supercharged street performance is achieved with compression ratios of approximately 7.5 to 8.0:1. A 7.5:1 compression ratio gives better fuel economy than lower ratios while providing better volumetric efficiency and minimizing the octane requirements associated with higher ratios. For a 276 cubic inch engine, this translates to a head combustion chamber volume of about 75-80 cc. Stock iron heads come with this size chamber. Custom aluminum heads are also available. Some high compression aluminum heads, like the Offy 425, can be modified to achieve the desired volume and promote better breathing by removing about 0.06 to 0.12 inches of material over the valves (not over the piston). Some material can also be removed in back of the valves to the gasket line. Be sure and leave a 0.125 in. or larger radius in the inside corners to prevent stress risers and promote more flow.
For normally aspirated street flatheads, a compression ratio of about 8.5:1 gives good results. See the December 2002 issue of Street Rodder for a discussion of compression ratio vs. breathing for normally aspirated flatheads. A reprint with corrected plots is available from Roadrunner. A chart for estimating the compression ratio of the Ford flathead can be accessed from the Home page.

@LSGUN

This is the blower. It's a 1937 McCulloch. First year of production. It mounts to a stock 1936 Aluminum intake.

[Dave/Green Bay]

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Jason: Would it be possible to take a close up of the tag on the supercharger. My didn't have one, we have found a place to make them but need dimensions and numbers. Thanks Dave/Green Bay

[Dave/Green Bay]

Jason: We also took a 35 intake and lowered the mounting bolt and cut out about 2 inches of material on the front to lower the supercharger. Mounted on a stock intake they are way up in the air. We're trying to fit under the hood. If your interested let me know and I can get some pictures.

[Jason in TX]

I don't have dimensions of the tag with me here at work, but here is a picture I have saved. In the "model" box, 37 is for the year, F is for Ford (they had M for Mercury) and 85 is for 85HP.

So, yours would say 39F85 or 39M85, depending on if you are doing a Merc or Ford engine. I would imagine the M would be much more rare.

[Jason in TX]

I would love to see some pictures. I actually bought phenolic spacers to make up for the gap in height, but you may have a better idea to lower the blower! Save room, right??

Originally Posted by Dave/Green Bay

Jason: We also took a 35 intake and lowered the mounting bolt and cut out about 2 inches of material on the front to lower the supercharger. Mounted on a stock intake they are way up in the air. We're trying to fit under the hood. If your interested let me know and I can get some pictures.

[Dave/Green Bay]

Jason: We actally had a spacer piece made at a foundry to take up the space between the supercharger and the intake manifold. The mold could be used again if you are interested. We made it out of aluminum and did extensive maching to make it fit and look good. I will get pictures. We are mounting the generator on the driver side behind the supercharger.

[Jason in TX]

I suppose I could also drill another hole in the front mounting support higher if it didn't interfere with the triple pulley.

I was considering mounting a double pulley generator on the driver's side head and using the generator as the tensioner keeping the belts tight.

[Ol' Ron]

Edelebrock makes a lo compression with a CNC combustion chamber. excellent for use on blown engines.

[PeterC]

H&H sells Navarro "Blower" heads to accomodate a Blower - I am running a S.C.o.T Blower with those heads with excellent results.

[Flat Ernie]

Honestly, I think you'll be OK with the older centrifugal blowers and "high" compression heads. You can always do a bit of machine work over the valves to reduce compression further - it'll help flow a bit too.

For trial & error, play with your pulley sizes because ultimately, how fast you spin the blower is what determines your boost

[S.Duncan]

Ron;are the Edelbrock heads available in 59a style as well as 8ba?

[Flathead]

I wouldn't spin one of those McCullough blowers any faster than they originally intended. You could strain the gears and overspeed the impeller. Different than a roots style.

[Jason in TX]

Yeah. It spins 6 times the pulley speed. That is the way it is set up.

[Ol' Ron]

Duncan
I'm not sure, give them a call. I think they have 72cc.

[Flat Ernie]

Originally Posted by Jason in TX

Yeah. It spins 6 times the pulley speed. That is the way it is set up.

Well, spin it 5 times the pulley speed and it won't make as much boost!

[dgkipp]

We are setting up a motor for street use with MCF heads and we went with a 72 thousands head gasket made by Cometic to get the compression down. you might want to give them a call.