compression bump with head change?

[bb71challenger]

Anyone know the bump in compression I will get by changing from 906 heads to 915's?

[Moparal]

If yer using the same type of head gasket thickness, around 1/2 point.  If you have like felpro blue head gaskets on now, around .039 - .042  thick, then using the steel shim at .020  will give you a point when using the 915 heads.

[Chryco Psycho]

the 915 is 78cc
the 906 is 80-90 cc
 the compression bumps closer to 1 full point

[bb71challenger]

I am using the steel shim .020 head gasket right now. If I change heads and keep the .020 gasket I will move my engine from the 8.2(less than 8 actually) to close to 9:1?  In your guys opinion is that enough for a solid street engine without having to change out pistons? I know by bumping the compression up I am going to have to also change cams from the Engle low comp cam to something hotter. I like this one. I am running air conditioning so this would seem to me to be at the high end of what my engine could handle and not be a nightmare to deal with in terms of daily driving. What do you guys think?

http://www.lunatipower.com/Product.aspx?id=2337&gid=297

[Chryco Psycho]

440 correct ?
 ok doing some quick calculations , if you engine is now 8:1 with the 78 cc heads you will have close to 8.8 :1
 if you do not change out the pistons I would use the same cam or at least try ti first , you still have a low comp engine

[bb71challenger]

No its a 71 400. I think I will still need to get pistons. I really want 9.5:1
Do you know of some lighweight pistons that will not break the bank that will get me 9.5 with the 915's?

[Chryco Psycho]

Speed Pro had some very low comp 440 pistons that that ai think worked very well in a 400
opther then that check with 440 Source , JE , Diamond

[bb71challenger]

After I pulled my head out I remembered the part number Kenny had turned me on to, Keith Black KB-240. With felpro .040 head gasket or something similar I will be comfortably under 10:1. I think these are the ones for me.

[Chryco Psycho]

I generally avoid Hyper pistons , it is only a few $$ more to get a good forged piston & when you want more power you do not have to replace the pistons yet again

[Moparal]

Bret, how deep in the hole are you anyways as far as deck height?  You have an option of taking the engine apart, decking the block around  .030 , checking your new heads to see how much surface has alreay been cut off and maybe being able to shave the heads  .020   A quick check to see if heads have been shaved is to mic the thickness at a head bolt hole to the face. It would be very close to 1 inch.  Or to just cc them.  Suaring up the block and heads  might be a good route to choose when the wallet is tight, but the cylinder walls need to be in good shape just needing a light hone job for new ring seating. See how deep in the hole you are with your pistons.

[nqkjw]

Two months ago I did the 906 change out to 915s on my 69 440 barracuda.
The engine is 30 over and the 906s were dead stock.
The 915s got treated to new 2.14 and 1.81 valves and de burring of combustion chambers.Plus porting which is not relevant here..
I used the tin shim 20 thou head gaskets.
I measured piston height,cc'ed both heads and did some accurate calcs.
Rounded to one decimal place I was at 8.4:1,now at 9.3:1
Some real world figures for you.
It made 0.2sec improvement on the eighth.

[Moparal]

So your 915's werent shaved? and were you useing steel shim gaskets on your 906's before the swap?

[bb71challenger]

They are way down there Al. I dont remember perfectly but I do know that the compression factored in the high 7's with the .040 felpro head gasket. Pretty sad when you cant even get the advertised 8.2:1 ratio. This engine has turned into a stop gap. I have the money to finish my car but barely, and thats with not doing anything exotic at all. I figure a set of KB-240 pistons, the 915 heads, cam swap and whatever machine work thats needed. I will not spray this motor and I am not going to look for more power so I figure the pistons should do just fine for me as long as I dont detonate.

[Moparal]

What about balancing from cast to kb pistons?  The clearance piston to cylinder wall will be different but I think that it is in favor for you

[nqkjw]

So your 915's werent shaved? and were you useing steel shim gaskets on your 906's before the swap?

The 915s were not shaved.They were just surfaced.
The head gaskets that were under the 906s were 40thou.

Hope this helps.