915's vs 906's

[darkside]

I know 67 915 heads will give you higher compression but 906's have bigger valves,They are both hp heads. What heads would you go with if you was building a 440 hp engine ?

[max]

although majority of the 1967 915 heads have small valves there was an hp version of them and they had the same valve sizes as the 906's.

it depends on what you plan doing with the rebuild. if you plan on installing new pistons then i would correct the compression with the new pistons instead with the heads.

[defiance]

The 915's are closed chamber.  If all else is equal, closed chamber has superior combustion characteristics (but tends to lead to lower fuel mileage and higher emissions).  The valves can be reworked, compression can be changed with pitons, etc... but a head is either open or closed; can't be changed as far as I know.
Given that, 915's are considered more desireable...  As long as you're willing to get them set up to work with your engine.

[Chryco Psycho]

I would use the 915 heads , & install 1.81 exhaust valves & 2.08 or 2.14 intake valves 

[Ghoste]

Changing valve size will also give you a good excuse to install hardened seats while you're at it.

[RD]

but a head is either open or closed; can't be changed as far as I know.

you can make an open chamber head a closed chamber by milling the surface of the head.  you have to mill the intake side of the head also to compensate for the loss in deck height in order for your intake to fit properly.

[tan top]

wasn't  there two different  915 heads, ( different size exhaust valves ) the 915 head with the larger Ex valve went on the 67 440   R/T / supercommando  motor , could  be wrong on this though

[max]

wasn't  there two different  915 heads, ( different size exhaust valves ) the 915 head with the larger Ex valve went on the 67 440   R/T / supercommando  motor , could  be wrong on this though

no, you are right, the 67 440 hp engines used a 915 head with a 1.74 exhaust valve while the other non hp 915 heads used a 1.60 exhaust valve.

[toupee]

I hope nobody minds me digging this post up from the grave, but I'm just curious what is meant by

The 915's are closed chamber.  If all else is equal, closed chamber has superior combustion characteristics (but tends to lead to lower fuel mileage and higher emissions).  The valves can be reworked, compression can be changed with pitons, etc... but a head is either open or closed; can't be changed as far as I know.
Given that, 915's are considered more desireable...  As long as you're willing to get them set up to work with your engine.

Aside from the bump in compression what else is there to compensate for?  I've heard the 915's are good for about a 1/2-3/4 point of compression.   As long as you don't go over 10.5:1 you should be ok on high octane, right?

[mikepmcs]

I know where you can get a set of 915's for $1K

[defiance]

I hope nobody minds me digging this post up from the grave, but I'm just curious what is meant by

The 915's are closed chamber.  If all else is equal, closed chamber has superior combustion characteristics (but tends to lead to lower fuel mileage and higher emissions).  The valves can be reworked, compression can be changed with pitons, etc... but a head is either open or closed; can't be changed as far as I know.
Given that, 915's are considered more desireable...  As long as you're willing to get them set up to work with your engine.

Aside from the bump in compression what else is there to compensate for?  I've heard the 915's are good for about a 1/2-3/4 point of compression.   As long as you don't go over 10.5:1 you should be ok on high octane, right?

My understanding may not be perfect, but I believe the seats are not capable of working with unleaded fuel out of the box, so some minor valve work is required to make them work with modern fuels.  Someone correct me if I'm wrong...?

[firefighter3931]

 As long as you don't go over 10.5:1 you should be ok on high octane, right?

That's really pushing it for iron heads....a safer static compression ratio is 9.5:1 


Ron

[Musicman]

 

[toupee]

 As long as you don't go over 10.5:1 you should be ok on high octane, right?

That's really pushing it for iron heads....a safer static compression ratio is 9.5:1 


Ron

Thanks for setting me straight, Ron & Musicman.

I know where you can get a set of 915's for $1K

Aero Head Racing has rebuilt 915's advertised for $599 with all new parts.   They aren't ported, though.  I don't think they have bigger valves either.

[mikepmcs]

Oh, these were originals that the guy who we bought our drag car had in his shed.  I had no idea what they were worth just told him I'd let ya'll know at some point.

v/r
Mike

[John_Kunkel]

I've heard the 915's are good for about a 1/2-3/4 point of compression.   

The '70 440 with 906 heads uses the exact same compression height piston as the '67 with 915 heads. Advertised compression for '70 is 9.7 and for '67 10.1