I just had a set of heads reworked by the machine shop and discovered that they were mildly ported with the combustion chambers hogged out. The machinist said they weren't very even (whoever did them didn't do a very good job) and he took a cutter and cleaned them up (visually). I've cc'd the heads over the past two days and found the following....


Head A: (54.8 cc) - (54.5 cc) - (54.8 cc) - (54.7 cc)

Head B: (55 cc) - (56 cc) - (55 cc) - (55.6 cc)


Understandably, it is best if all the chamber volumes are the same to keep compression the same. However, if I choose to run the heads in this manner, will it affect performance, and if so, will it be negligible? These heads are for a street car w/ a CC503 cam.

With the pistons I'm running, compression will range from 11.26:1 to 11.05:1. Is a .21 difference in compression any cause for concern?


Thanks in advance for any insight you can share!
-b

 

You're looking at about a 1cc to 1-1/2cc difference between the largest and smallest chambers, right?

Are you saying that small amount translates into just over a quarter point in CR? I didn't crunch the numbers, but 1 to 1-1/2cc difference doesn't seem like it would cause a .25 pt CR difference.

Anyway, if that IS the case, I doubt it would even be noticable.

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

I wouldn't run that much variance on a high hp motor.

 

That's a little too much - I would do a little chamber work and aim for 56cc on all of them or at least within 3/4 cc as an outside variance. If the heads were in front of me and I could easily tell were the extra metal was missing I would just fix those two bigger chambers with some weld and re-shaping because your Head A is matched up pretty good.

 

To each his own, but consider this:

Back to back tests have shown that a 1 point diffrence in CR translates into a 3-4% difference in power. Remember we're talking about an ONE POINT CHANGE.

The difference, however, isn't linear; the 4% difference is concentrated in a change from 8.0:1 to 9.0:1 and the 3% difference between 10.0:1 to 11.0:1 (IIRC)

So when you crunch the numbers to see what can be expected, from a power difference point of view, for a .2 (+/-) CR difference, things come into better perspective.

Now, your options:

You can grind to remove the material from the smaller chamber to bring it inline with the larger ones OR, add material to the larger ones to make them smaller. You could also consider using a thicker/thinner head gasket on one side of the engine, if one could be found having a 1/1.5c (or so) difference.

So, to me the matter boils down to a, basically, cost Vs benefit question. How much to spend for what gain and how **** you want to be. I wouldn't give it a second thought, but it's your call.

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!! Army Vs Navy Today!!!!

 

I would be less concerned with the chamber volumes and more concerned that whoever ported them made them flow worse than they did before the "porting". Uneven flow #s is less than desirable. If the port work was rough enough the machinist doing your heads couldn't stand looking at it and had to start cutting himself then it must have been pretty poor. I personally would either take them to a shop that has a flow bench and see if they can be saved or get another set of heads. Stock heads can be had cheap enough. Or send em to Loyyd and see what he can do for you.