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

[JC]

......like is sitting in my garage right now (440 Mopar .030 over, Crower 4.375" stroker crank, TRW flattops, yielded 15:1 with B1 heads). Besides ridiculously priced custom shorter rods, is there any way I'm not thinking of to get the compression down to 10-11:1? Seems like only 500cc combustion chambers would get this. I'm planning on using typical 440 heads, mebbe Edelbrock aluminum down the road (more like 75-80cc).

JC

 

[db]

A very thick head gasket?

 

[Hayabusa Rider]

Hope you have lots of airplane gas around

 

[oLLie]

Twin turbos AND a blower will fix your compression problems. LMK how that works out

 

[Pacfanweb]

Dished pistons.
Thicker head gasket.

You'd be surprised what a few extra cc's in the combustion chamber will do, also.

 

[PsychoAndy]

Got tolulene?

-PAB

 

[CantedValve]

You gotta reduce the stroke. Try putting in a standard rotating assembly. That will return it to the compression it should be (10:1). Actually, it will be a bit higher due to the overbore.

I dont think there are a set of heads out there that will fix this. Maybe dished pistons, but even then, they would have to be SUBSTANTIALLY dished. Shorter rods wont get it, cause the further below deck you get, the more likelyhood of detonation.

Get the stock 3.750" crank, stock rods, and stock pistons (in the appropriate size).

 

[Pacfanweb]

Oh, and av-gas won't cut it for 15-1 compression. My race engine has a bit over 14-1 and I run 114 octane Sunoco.

 

[rahvin]

If I remember correctly Natural gas burns ideally at a 15:1 compression ratio. If that is actually true I would do a CNG conversion on the car.

 

[Pacfanweb]

Originally posted by: CantedValve
You gotta reduce the stroke. Try putting in a standard rotating assembly. That will return it to the compression it should be (10:1). Actually, it will be a bit higher due to the overbore.

I dont think there are a set of heads out there that will fix this. Maybe dished pistons, but even then, they would have to be SUBSTANTIALLY dished. Shorter rods wont get it, cause the further below deck you get, the more likelyhood of detonation.

Get the stock 3.750" crank, stock rods, and stock pistons (in the appropriate size).

Reducing the stroke won't reduce compression, at least not in a measurable amount. If the compression is that high with flat-tops, then chances are, the block has be decked a fair amount. You can make up some of this by running and extra-thick head gasket. Dished pistons, like I mentioned earlier, will do it also.
The compression height of those pistions has to be pretty high. Changing to different flat tops may help. I don't think any one thing is going to get you all the way to 10-1. It's going to have to be a combination of things.

For what it's going to cost you in parts and machine work, you can buy a LOT of racing gas or octane booster.

Edit: Let me clarify something: Reducing stroke will mean you have to change the pistons also. You can't change the crank without changing the pistons. That would be basically the same as changing just the rods.
If you changed the crank, you'd need pistons with a lower pin height to go with the shorter stroke. The piston would still be in the same place at TDC. (top dead center)
All you'd have is the same exact engine with a few less cubic inches, which would, SLIGHTLY, lower the compression, but it would be a lot of work for very little change. You could change the compression more with thicker head gaskets. In fact, thicker head gaskets would be the cheapest and easiest way to lower compression.
I still believe that you'll need more than just one change to get any significant change, though.

 

[CantedValve]

whoa... his crank is over half an inch (5/8") MORE in stroke. Last time I checked... more stroke lead to more swept volume... more swept volume squeezed into the same space is gonna get you more compression.

I have missed the obvious before... but this seems pretty simple to me.

EDIT: With .030 bore and a 4.375 stroke, he has 520 CID... 80 more CID

 

[Hayabusa Rider]

Originally posted by: Pacfanweb
Oh, and av-gas won't cut it for 15-1 compression. My race engine has a bit over 14-1 and I run 114 octane Sunoco.

That is true. 15:1 sounds like Sunoco NOS formula

 

[db]

Competition 110 (leaded), Methanol, Acetone, Benzole, ....

 

[Pacfanweb]

Originally posted by: CantedValve
whoa... his crank is over half an inch (5/8") MORE in stroke. Last time I checked... more stroke lead to more swept volume... more swept volume squeezed into the same space is gonna get you more compression.

I have missed the obvious before... but this seems pretty simple to me.

See my edit.

 

[JC]

Originally posted by: CantedValve
You gotta reduce the stroke. Try putting in a standard rotating assembly. That will return it to the compression it should be (10:1). Actually, it will be a bit higher due to the overbore.

I dont think there are a set of heads out there that will fix this. Maybe dished pistons, but even then, they would have to be SUBSTANTIALLY dished. Shorter rods wont get it, cause the further below deck you get, the more likelyhood of detonation.

Get the stock 3.750" crank, stock rods, and stock pistons (in the appropriate size).

I've got plenty of steel 3.75 cranks. I'd really like to use the big arm, though But race gas just ain't gonna get it on the street!

This was a Dick Landy-built engine, made just under 900HP with a single Dominator. Pushed my buddy's all-steel '58 DeSoto into the mid-9s.

BTW, what's the reason that below-deck height increases detonation?

JC

 

[Pacfanweb]

Originally posted by: JC

Originally posted by: CantedValve
You gotta reduce the stroke. Try putting in a standard rotating assembly. That will return it to the compression it should be (10:1). Actually, it will be a bit higher due to the overbore.

I dont think there are a set of heads out there that will fix this. Maybe dished pistons, but even then, they would have to be SUBSTANTIALLY dished. Shorter rods wont get it, cause the further below deck you get, the more likelyhood of detonation.

Get the stock 3.750" crank, stock rods, and stock pistons (in the appropriate size).

I've got plenty of steel 3.75 cranks. I'd really like to use the big arm, though But race gas just ain't gonna get it on the street!

This was a Dick Landy-built engine, made just under 900HP with a single Dominator. Pushed my buddy's all-steel '58 DeSoto into the mid-9s.

BTW, what's the reason that below-deck height increases detonation?

JC

There is potential that the piston won't "push" all the exhaust gases out if it doesn't come even with the deck at TDC, which COULD leave enough hot stuff to ignite the incoming air/fuel mixture.

BTW, if you change the crank, you'll have to change the pistons, so if you have the exact same pistons with only the pin height changed, the position of the piston at TDC will not change.

 

[Eli]

Originally posted by: Pacfanweb

Originally posted by: CantedValve
You gotta reduce the stroke. Try putting in a standard rotating assembly. That will return it to the compression it should be (10:1). Actually, it will be a bit higher due to the overbore.

I dont think there are a set of heads out there that will fix this. Maybe dished pistons, but even then, they would have to be SUBSTANTIALLY dished. Shorter rods wont get it, cause the further below deck you get, the more likelyhood of detonation.

Get the stock 3.750" crank, stock rods, and stock pistons (in the appropriate size).

Reducing the stroke won't reduce compression, at least not in a measurable amount. If the compression is that high with flat-tops, then chances are, the block has be decked a fair amount. You can make up some of this by running and extra-thick head gasket. Dished pistons, like I mentioned earlier, will do it also.
The compression height of those pistions has to be pretty high. Changing to different flat tops may help. I don't think any one thing is going to get you all the way to 10-1. It's going to have to be a combination of things.

For what it's going to cost you in parts and machine work, you can buy a LOT of racing gas or octane booster.

Edit: Let me clarify something: Reducing stroke will mean you have to change the pistons also. You can't change the crank without changing the pistons. That would be basically the same as changing just the rods.
If you changed the crank, you'd need pistons with a lower pin height to go with the shorter stroke. The piston would still be in the same place at TDC. (top dead center)
All you'd have is the same exact engine with a few less cubic inches, which would, SLIGHTLY, lower the compression, but it would be a lot of work for very little change. You could change the compression more with thicker head gaskets. In fact, thicker head gaskets would be the cheapest and easiest way to lower compression.
I still believe that you'll need more than just one change to get any significant change, though.

He's got the right idea... Heh.. It probably wouldn't be advisable to stack head gaskets ontop of eachother, to make it thicker, would it? Doesen't seem like that would last. Can you get bigger(as in more volume than the ones you have now) heads?

 

[CantedValve]

I still dont see it. The math doesnt add up.

Stock stroke wont do it all, but it will do the bulk of it. I am guessing at least 3 points in comp. I cna work the total number if I knew all the specifics on his engine... but I roughed it out, and all else being equal, an decrease in displacement of 80 CID (back to normal) gets him 3 points. I could still be wrong, I am gonna double check, but it seems pretty simple..

 

[Pacfanweb]

BTW, what's the cubes on that? Gotta be close to 600.

He's got the right idea... Heh.. It probably wouldn't be advisable to stack head gaskets ontop of eachother, to make it thicker, would it? Doesen't seem like that would last. Can you get bigger(as in more volume than the ones you have now) heads?

You can buy extra-thick copper gaskets. Up to .090", I think.

 

[CantedValve]

Its not a matter of where the piston is. I assume that is equal. The swept volume increaseing (rougly) 17-20% is what I am working with. The BOTTOM of the stroke is gonna change.

 

[CantedValve]

520 CID

 

[JC]

Originally posted by: Pacfanweb
BTW, what's the cubes on that? Gotta be close to 600.

He's got the right idea... Heh.. It probably wouldn't be advisable to stack head gaskets ontop of eachother, to make it thicker, would it? Doesen't seem like that would last. Can you get bigger(as in more volume than the ones you have now) heads?

You can buy extra-thick copper gaskets. Up to .090", I think.

4.32 +.030 x 4.375 = 520CID. You can see why I'd like to use this crank

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.

JC

 

[SuperSix]

My compression ratio is only 8.5:1... But there's a lage hairdryer to fill the chanber.

 

[Kadarin]

Originally posted by: Eli

Originally posted by: Pacfanweb

Originally posted by: CantedValve
You gotta reduce the stroke. Try putting in a standard rotating assembly. That will return it to the compression it should be (10:1). Actually, it will be a bit higher due to the overbore.

I dont think there are a set of heads out there that will fix this. Maybe dished pistons, but even then, they would have to be SUBSTANTIALLY dished. Shorter rods wont get it, cause the further below deck you get, the more likelyhood of detonation.

Get the stock 3.750" crank, stock rods, and stock pistons (in the appropriate size).

Reducing the stroke won't reduce compression, at least not in a measurable amount. If the compression is that high with flat-tops, then chances are, the block has be decked a fair amount. You can make up some of this by running and extra-thick head gasket. Dished pistons, like I mentioned earlier, will do it also.
The compression height of those pistions has to be pretty high. Changing to different flat tops may help. I don't think any one thing is going to get you all the way to 10-1. It's going to have to be a combination of things.

For what it's going to cost you in parts and machine work, you can buy a LOT of racing gas or octane booster.

Edit: Let me clarify something: Reducing stroke will mean you have to change the pistons also. You can't change the crank without changing the pistons. That would be basically the same as changing just the rods.
If you changed the crank, you'd need pistons with a lower pin height to go with the shorter stroke. The piston would still be in the same place at TDC. (top dead center)
All you'd have is the same exact engine with a few less cubic inches, which would, SLIGHTLY, lower the compression, but it would be a lot of work for very little change. You could change the compression more with thicker head gaskets. In fact, thicker head gaskets would be the cheapest and easiest way to lower compression.
I still believe that you'll need more than just one change to get any significant change, though.

He's got the right idea... Heh.. It probably wouldn't be advisable to stack head gaskets ontop of eachother, to make it thicker, would it? Doesen't seem like that would last. Can you get bigger(as in more volume than the ones you have now) heads?

Don't stack head gaskets; they're not designed that way. They're made of a composite material, with metal around the cylinders and bolt holes.

And I'm still wondering how the original poster got 15:1 with flattops.. I'm not too familiar with 0.030 over Mopar 440's, but a piston with a good sized dome should only yield about 12:1. With a 15:1 ratio, it's going to take more than a can of octane boost to run the car.

 

[Pacfanweb]

Originally posted by: CantedValve
Its not a matter of where the piston is. I assume that is equal. The swept volume increaseing (rougly) 17-20% is what I am working with. The BOTTOM of the stroke is gonna change.

I did some tests with a few online compression calculators, and changing the stroke the amount we're talking about made about 1.4 pts. difference in compression.
So, that still leaves him at 13.8-1 or somewhere thereabouts. Still too much for the street. Like I said, it will take several things to reduce the compression, and I think there are better ways of reducing compression other than losing cubic inches.