The Ford Barn

jeepguy1948 · 03-05-2024, 05:34 PM

I’m trying to figure out what compression I have and what to do about lowering it for todays fuels.

Merc motor bored .100 over gives a displacement of about 290CI ?

Edlebrock Aluminum heads that were originally 8.5:1
I have no idea if the heads have ever been surfaced or if the block has been decked so I can’t factor that into the equation.
Quick math I come up with a CR of about 10.5:1. Way too high
I’m thinking maybe 9:1 or maybe 9.5:1 for a daily driver street machine? We are running non eth fuel although I don’t know the octane rating at the moment.

I probably didn’t spend as much time with the math as I should have before posting on here and I’m sure that somebody smarter than me can correct what I got wrong LoL.

No matter what the exact numbers are there is no doubt that the existing CR is too high.
What to do about that? The motor only has 5,000 miles at most on it after a complete rebuild sh changing pistons is not really on the table, anything I do is going to have to be done with the heads. Is anybody making heads with bigger combustion chambers? I really like the look of the Edlebrock heads but am I going to have to go back to stock heads? Can I use multiple head gaskets? If I can use multiple gaskets, which ones should I use? Best brand copper gaskets or conventional head gaskets. I typically use the spray on copper sealant on whatever head gaskets I use or what engines they are being used for.

There’s a lot of questions here, hopefully some of you rocket scientists can set me straight.
Thanks!

KiWinUS · 03-05-2024, 06:15 PM

I doubt compression is too high. Compression is very had to get enough with these great old gems.

Tim Ayers · 03-05-2024, 06:17 PM

Merc motor bored .100 over gives a displacement of about 290CI ?

Don't think this is correct. .125 over X 4" stroke (Merc crank) is a 276 c.i. motor.

If the heads are off the motor, I'd CC them. That may help give you an indication how much has been taken off of them in the past.

Kube · 03-05-2024, 06:26 PM

Quote:

Originally Posted by jeepguy1948

I’m trying to figure out what compression I have and what to do about lowering it for todays fuels.

Merc motor bored .100 over gives a displacement of about 290CI ?

Edlebrock Aluminum heads that were originally 8.5:1
I have no idea if the heads have ever been surfaced or if the block has been decked so I can’t factor that into the equation.
Quick math I come up with a CR of about 10.5:1. Way too high
I’m thinking maybe 9:1 or maybe 9.5:1 for a daily driver street machine? We are running non eth fuel although I don’t know the octane rating at the moment.

I probably didn’t spend as much time with the math as I should have before posting on here and I’m sure that somebody smarter than me can correct what I got wrong LoL.

No matter what the exact numbers are there is no doubt that the existing CR is too high.
What to do about that? The motor only has 5,000 miles at most on it after a complete rebuild sh changing pistons is not really on the table, anything I do is going to have to be done with the heads. Is anybody making heads with bigger combustion chambers? I really like the look of the Edlebrock heads but am I going to have to go back to stock heads? Can I use multiple head gaskets? If I can use multiple gaskets, which ones should I use? Best brand copper gaskets or conventional head gaskets. I typically use the spray on copper sealant on whatever head gaskets I use or what engines they are being used for.

There’s a lot of questions here, hopefully some of you rocket scientists can set me straight.
Thanks!

Why can't you get high octane fuel in your area? I have two cars with compression ratios of 10.5:1 and one with 11:1. All run quite well on ethanol 93.
If your engine is not running well, I'd suggest you figure out why that is. It is not because the compression is too high.

Ziggster · 03-05-2024, 06:36 PM

This might be helpful.

https://myflatheadford.com/flathead-...n-ratio-chart/

Ziggster · 03-05-2024, 06:39 PM

Sponsored Links (Register now to hide all advertisements)

Yeah, the charts in the link I posted above have nothing for 0.100” over. Must be 0.125”.

Pete · 03-05-2024, 07:03 PM

If you really need to know the comp. ratio you are either going to have to cc the chambers or cc the heads and the block with the heads off.

I'm with Tony on this, if the valves and pistons clear the heads by the proper amount as is,
you do not have too much compression. Just use 92 gas and go drive.

tubman · 03-05-2024, 07:07 PM

First of all, I will have to say that I agree with the posters who say that is very difficult to get to much compression in a flathead. I used to "cc" heads, but I have found it easier to just check the clearances and do what I have to do to get the piston to head clearance to .045"-.050" over the piston. This is the first thing that you should do regarding the heads. I have found all kinds of goofy stuff; Clearances of .120 on one end of the head and .075 on the other. Previously milled aftermarket finned heads that still had excessive clearance.

You have to know where you are before you start to do something.

jeepguy1948 · 03-05-2024, 07:52 PM

I will do some checking. I want to point out that I wasn’t proposing anything more drastic then additional head gaskets. The heads are off the motor now but when it was run last week I was having pre ignition/knock/ping even with the timing set 24 degrees total advance. This is not my motor, it belongs to a friend and I don’t know for certain how it’s going to be driven after it leaves my shop. I can lift the pedal a bit if it starts knocking but I can’t count on all the grandkids etc that are going to drive this (‘51 F-1). It has to be dumbed down a bit. Pistons are very clearly marked .100

flatford8 · 03-05-2024, 08:00 PM

If the heads are off the motor now, it’s the perfect time to check piston to head clearance with clay or aluminum foil balls.......Mark

Seth Swoboda · 03-05-2024, 10:20 PM

Running two head gaskets is not a good idea. Just because you can doesn't mean you should. Do you have problems with the pistons or valves hitting the heads? If not what have I missed? You're the first person I've heard looking for lower compression ratio. I'm in agreement with others here. Do you have a knock or pinging? Maybe run a higher octane and or check your timing. What camshaft are you using?

Ronnieroadster · 03-05-2024, 10:22 PM

You wrote even with the timing set at 24 total advance. To maybe help with this I have some questions is your initial timing in the 10 to 12 degree range? With an additional 12 degrees coming in by the advance system in the distributer? Thus giving you the total combined of 24 degrees?
What is the engine RPM when the all in centrifugal degrees take place?
Ronnieroadster

marko39 · 03-06-2024, 08:02 AM

have you done a compression test?

Ol' Ron · 03-06-2024, 11:56 AM

A block bored .100 0ver with a 4" crank has a displacement of aprox 270 Ci The offy heads have aprox 75CC. At one time I had a fromuls program in my computer that would calculate the CR, However from experience I'd say it was less than 9:1. Also 24 degrees od advance is nice in cruise, getting great gas mileage. However at WOT 20 degs would be safer. You have problems, but it ain't the CR?????
Gramps

03-05-2024, 05:34 PM	#1
jeepguy1948 Senior Member Join Date: Jan 2022 Posts: 721	Complicated Compression Ratio I’m trying to figure out what compression I have and what to do about lowering it for todays fuels. Merc motor bored .100 over gives a displacement of about 290CI ? Edlebrock Aluminum heads that were originally 8.5:1 I have no idea if the heads have ever been surfaced or if the block has been decked so I can’t factor that into the equation. Quick math I come up with a CR of about 10.5:1. Way too high I’m thinking maybe 9:1 or maybe 9.5:1 for a daily driver street machine? We are running non eth fuel although I don’t know the octane rating at the moment. I probably didn’t spend as much time with the math as I should have before posting on here and I’m sure that somebody smarter than me can correct what I got wrong LoL. No matter what the exact numbers are there is no doubt that the existing CR is too high. What to do about that? The motor only has 5,000 miles at most on it after a complete rebuild sh changing pistons is not really on the table, anything I do is going to have to be done with the heads. Is anybody making heads with bigger combustion chambers? I really like the look of the Edlebrock heads but am I going to have to go back to stock heads? Can I use multiple head gaskets? If I can use multiple gaskets, which ones should I use? Best brand copper gaskets or conventional head gaskets. I typically use the spray on copper sealant on whatever head gaskets I use or what engines they are being used for. There’s a lot of questions here, hopefully some of you rocket scientists can set me straight. Thanks!

03-05-2024, 06:15 PM	#2
KiWinUS Senior Member Join Date: May 2010 Location: Charlotte NC KiWi-L100 available here Posts: 2,963	Re: Complicated Compression Ratio I doubt compression is too high. Compression is very had to get enough with these great old gems.

03-05-2024, 06:17 PM	#3
Tim Ayers Senior Member Join Date: May 2010 Location: NJ Posts: 6,181	Re: Complicated Compression Ratio Merc motor bored .100 over gives a displacement of about 290CI ? Don't think this is correct. .125 over X 4" stroke (Merc crank) is a 276 c.i. motor. If the heads are off the motor, I'd CC them. That may help give you an indication how much has been taken off of them in the past.

03-05-2024, 06:36 PM	#5
Ziggster Senior Member Join Date: Aug 2018 Location: Ottawa, ON Posts: 855	Re: Complicated Compression Ratio This might be helpful. https://myflatheadford.com/flathead-...n-ratio-chart/

03-05-2024, 06:39 PM	#6
Ziggster Senior Member Join Date: Aug 2018 Location: Ottawa, ON Posts: 855	Re: Complicated Compression Ratio Sponsored Links (Register now to hide all advertisements) Yeah, the charts in the link I posted above have nothing for 0.100” over. Must be 0.125”.

03-05-2024, 07:03 PM	#7
Pete Senior Member Join Date: May 2010 Location: Wa. Posts: 5,409	Re: Complicated Compression Ratio If you really need to know the comp. ratio you are either going to have to cc the chambers or cc the heads and the block with the heads off. I'm with Tony on this, if the valves and pistons clear the heads by the proper amount as is, you do not have too much compression. Just use 92 gas and go drive.

03-05-2024, 07:07 PM	#8
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,319	Re: Complicated Compression Ratio First of all, I will have to say that I agree with the posters who say that is very difficult to get to much compression in a flathead. I used to "cc" heads, but I have found it easier to just check the clearances and do what I have to do to get the piston to head clearance to .045"-.050" over the piston. This is the first thing that you should do regarding the heads. I have found all kinds of goofy stuff; Clearances of .120 on one end of the head and .075 on the other. Previously milled aftermarket finned heads that still had excessive clearance. You have to know where you are before you start to do something.

03-05-2024, 07:52 PM	#9
jeepguy1948 Senior Member Join Date: Jan 2022 Posts: 721	Re: Complicated Compression Ratio I will do some checking. I want to point out that I wasn’t proposing anything more drastic then additional head gaskets. The heads are off the motor now but when it was run last week I was having pre ignition/knock/ping even with the timing set 24 degrees total advance. This is not my motor, it belongs to a friend and I don’t know for certain how it’s going to be driven after it leaves my shop. I can lift the pedal a bit if it starts knocking but I can’t count on all the grandkids etc that are going to drive this (‘51 F-1). It has to be dumbed down a bit. Pistons are very clearly marked .100

03-05-2024, 08:00 PM	#10
flatford8 Senior Member Join Date: Jan 2012 Location: Lyman,ME. Posts: 2,630	Re: Complicated Compression Ratio If the heads are off the motor now, it’s the perfect time to check piston to head clearance with clay or aluminum foil balls.......Mark __________________ I'm thinkin' about crankin' My ragged ol' truck up and haulin' myself into town. Billy Joe Shaver

03-05-2024, 10:20 PM	#11
Seth Swoboda Senior Member Join Date: May 2010 Location: Southern Illinois Posts: 3,795	Re: Complicated Compression Ratio Running two head gaskets is not a good idea. Just because you can doesn't mean you should. Do you have problems with the pistons or valves hitting the heads? If not what have I missed? You're the first person I've heard looking for lower compression ratio. I'm in agreement with others here. Do you have a knock or pinging? Maybe run a higher octane and or check your timing. What camshaft are you using?

03-05-2024, 10:22 PM	#12
Ronnieroadster Senior Member Join Date: Dec 2010 Location: East Coast in CT Posts: 1,529	Re: Complicated Compression Ratio You wrote even with the timing set at 24 total advance. To maybe help with this I have some questions is your initial timing in the 10 to 12 degree range? With an additional 12 degrees coming in by the advance system in the distributer? Thus giving you the total combined of 24 degrees? What is the engine RPM when the all in centrifugal degrees take place? Ronnieroadster __________________ I use the F word a lot no not that word these words Flathead , Focus and Finish. "Life Member of the Bonneville 200 MPH Club using a Ford Flathead block" Owner , Builder, Driver of the First Ford Flathead bodied roadster to run 200 MPH Record July 13, 2018 LTA timing association 200.921 in one and a half miles burning gasoline. First ever gas burning Ford flathead powered roadster to run 200 MPH at Bonneville Salt Flats setting the record August 7th 2021 at 205.744 MPH

03-06-2024, 08:02 AM	#13
marko39 Senior Member Join Date: Oct 2016 Location: harpursville ny Posts: 1,040	Re: Complicated Compression Ratio have you done a compression test?

03-06-2024, 11:56 AM	#14
Ol' Ron Senior Member Join Date: May 2010 Location: Chester Vt Posts: 8,860	Re: Complicated Compression Ratio A block bored .100 0ver with a 4" crank has a displacement of aprox 270 Ci The offy heads have aprox 75CC. At one time I had a fromuls program in my computer that would calculate the CR, However from experience I'd say it was less than 9:1. Also 24 degrees od advance is nice in cruise, getting great gas mileage. However at WOT 20 degs would be safer. You have problems, but it ain't the CR????? Gramps