915 v 906

[LR1970]

Hi, can anyone tell me the compression drop I might have if I swap 915 heads for 906 heads? I have a 67 R/T with about 10:1 compression, I have a set of reconditioned 906 heads with the valve inserts for unleaded fuel I would like to use instead.

Thanks

 

[Photon440]

Some places say almost 1 point. I haven't done it myself. But here's a whole article on different heads:
http://www.moparmusclemagazine.com/techarticles/5115_cylinder_heads/photo_02.html

 

[69Bee]

Without measuring a cylinder chamber volume of each head, and plugging that into the formula with other data from your lower end, it is anyone's guess as to what compression ratio you will have. It will be between 8 to 12 to one; that should cover all bases. Measure the cylinder heads and fill in the blanks to be completely sure.

1) Cylinder head volume
2) Head gasket volume
3) Deck height volume
4) Piston dome volume (+ for valve reliefs, and - for a dome)
5) Cylinder swept volume

Know all of these numbers and apply the proper formula, and you will have your answer. As far as assuming printed cylinder head volumes, always assume they are approximately 5cc above that. The number in the book is a target, and most head I have measured were about 5cc's higher.

 

[tallhair]

Thanks Photon

 

[Meep-Meep]

Installed on a 440 the chamber difference yields about .4 point, assuming you are comparing 79.5 and 86 cc chambers. Add the thicker head gasket on the open chamber and you can see nearly 1 point drop. A block with a huge bore chamfer (like the 77 block in my car) will reduce the CR even more.

 

[cryplydog]

the chry corp. went to the open chamber head for a reason, while many want the 915 head for particular reasons also many want the 906 I belive for the same reason you do. better tuning and streetability! as an old hot rodder I like both for equally different reasons. damn, I hope all that makes sense. good luck. and don't discount that 906.

 

[Banzaiii67]

I am not a engine builder by any means, but would anyone argue this..

Dome pistons + Open chambers = Good Combination

Flat tops + Either Chambers @ zero deck = Good Combination

Dished Pistons (valve reliefs) + Closed Chambers @ zero deck = Good Combination

 

[Meep-Meep]

The open chamber design was introduced to help stop detonation. I know, I know, quench and all, but there is still quench with the open chamber design. My personal opinion is that the larger quench distance allowed the flame front an easier path to completely burn whatever mixture might have been present on the other side of the cylinder.

 

[67Satty]

I've always read that they introduced the open chamber head in '68 to reduce emissions. And I believe the only way to get quench with the open chamber design is to have domes on the pistons while not having the pistons too far down in the cylinder. From what I understand, quench is either there if the piston-top to head number is there (.035" - .055"?) or it's not there once you get past .055" or so.

As to how important it really is, everyone online will say if you build a 440 without quench it will be a dog and detonate but that hasn't been my experience. I hope to have the money to throw some Stealths on my motor by this summer in order to have some real-world ET comparisons between bone-stock open chamber 452s and OOTB closed chamber Stealths. But then, I will have three variables that will have changed: 50 pounds less on the front end, better flowing heads, and quench vs. no quench.

I guy on Moparts reported picking up several tenths from doing nothing but swapping stock 906s for stock 915s, so the only thing he changed (besides maybe a little more compression) was quench in place of no quench and he picked up several tenths just by having quench according to him.

 

[Meep-Meep]

There is still quench with the open chamber heads. The question is about the amount of swirl that is produced with open vs. closed chamber based on said amount of quench. The closed chamber with a tighter quench might produce more swirl, but will also add compression. Several factors that seem to contribute to combustion dynamics and it seems that altering just the chamber shape will bring unintended consequences (as in increased or decreased compression).