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!

03-05-2024, 05:34 PM	#1
jeepguy1948 Senior Member Join Date: Jan 2022 Posts: 730	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!