If you had an engine with 15:1 compression....

[Pacfanweb]

I found a compression ratio calculator here, but I don't know the cc on the B1 heads, can't come up with the 15:1 figure. But all else being the same, going from 4.375 to 3.75 stroke drops compression about 1.6 points or so.

Exactly, and you'll be losing a lot more power, since you'll have lost compression AND cubic inches. You need to find out what the heads were CC'd at that put the compression ratio at 15-1. If they were very small, you should be able to use different heads and get a lot closer to where you want to be and still keep your cubes.

 

[CantedValve]

Using the calulator linked above (this one), a 4.375" stroke, a 69cc chamber, 0 deck height, and a .030 gasket, I get a compression of 14.95:1. if I change the stroke to 3.75" I get 12.96:1

It all depends on your assumptions... but you are right, it will take more than just stroke to get to 10:1

 

[desertdweller]

As Pacfanweb mentioned, dished pistons.

One thing that hasn't been mentioned yet is camshaft. Depending on the cam grind you choose
you can bleed off some of the cylinder pressure by finding a cam that has a shorter lobe seperation.

I would probably start looking at around 108 degrees. Call some manufacturers and ask them for
advice on it.

Oh and bigger heads


DD

 

[JC]

So it looks like if I can find a .090 head gasket, and get around 90cc combustion chamber volume, I'll have around 10.5 compression. I think it can be done

JC

 

[JC]

Hoo boy....just found where the B1 heads can be flat milled to 47cc, or angle-milled to 37cc(!) I think this is where the 15:1 squeeze came from :Q

JC

 

[desertdweller]

Originally posted by: JC
So it looks like if I can find a .090 head gasket, and get around 90cc combustion chamber volume, I'll have around 10.5 compression. I think it can be done

JC

90 CC Mopar Heads


DD

 

[crab]

Think I could cram that bad boy in my 1990 Ram Wagon (van)? Haha Dodge Ram Wagon R/T

 

[Pacfanweb]

Originally posted by: JC
Hoo boy....just found where the B1 heads can be flat milled to 47cc, or angle-milled to 37cc(!) I think this is where the 15:1 squeeze came from :Q

JC

That's what I was thinking. You can't get that much compression with just flat tops without some small chambers.
Use some standard CC heads and you're right where you want to be, cubes and all.

 

[PG]

I don't understand the original post. Does the engine run or not with that compression? Where did you get the car? How do you know it's 15:1?

Compression ratio only tells part of the story. You can run a high compression ratio with a big cam because big cam profiles have enough overlap to bleed off some pressure.

 

[JC]

Originally posted by: PG
I don't understand the original post. Does the engine run or not with that compression? Where did you get the car? How do you know it's 15:1?

Oh, it ran all right....in my friend's race car. He finally split a cylinder wall, got tired of pouring money into the car, and sold me the engine minus the B1 heads and intake. He also gave the aluminum rods away as souvenirs, not good to run aluminum rods on the street anyway. It's 15:1 with the small-chamber B1 heads, that's what Dick Landy said. I tend to believe him

JC

 

[Eli]

Originally posted by: JC

Originally posted by: PG
I don't understand the original post. Does the engine run or not with that compression? Where did you get the car? How do you know it's 15:1?

Oh, it ran all right....in my friend's race car. He finally split a cylinder wall, got tired of pouring money into the car, and sold me the engine minus the B1 heads and intake. He also gave the aluminum rods away as souvenirs, not good to run aluminum rods on the street anyway. It's 15:1 with the small-chamber B1 heads, that's what Dick Landy said. I tend to believe him

JC

Aren't just about all connecting rods made out of some sort of aluminum alloy?

 

[Evadman]

You can run aluminum rods on the street, just remember that they stretch a little under load, and make sure you have the clearance. ( well, sorta anyway )

Those heads have to be angle milled. probably a 0-.120" grind.

The only way you are going to get back to anywhere near a good compression is with new heads. Make sure they are compatable with the pistons you have. If the heads were angle ground, then the pistons were probably custom clearanced to clear the valves. They may need some more clearancing when the heads come back up.

 

[JC]

Originally posted by: Eli

Aren't just about all connecting rods made out of some sort of aluminum alloy?

No, most rods are steel. Only short-lived race engines use aluminum rods.

Originally posted by: Evadman

You can run aluminum rods on the street, just remember that they stretch a little under load

Sure, as long as you don't plan on running any real length of time! If they were long-term durable, NASCAR engines would use 'em

JC