cc heads
 

Hi guys,will a difference of less than 1cc in head volume between all heads make a noticeable difference in how the engine runs?Thanks,T.C.

Using the compression ratio spreadsheet with the following inputs (I get to choose because you didn't specify an engine, so I revert back to SWB :))

80mm Bore
66mm Stroke
1mm Deck Height
38.7cc piston dome volume

With a 72cc chamber you get 9,7 to 1
With a 72.5cc chamber you get 9,5 to 1

Two tenths of a point in compression? I would say that definitely makes a difference.

That's all theoretical of course. In reality, all the cylinders have different temperatures, airflow is different depending on the induction system (my old weber manifolds have very strange looking ports on the outboard cylinders), fuel may be different depending on the precise jet size, you may experience spark scatter based on the ignition system etc. etc. All these deviations from theoretical perfection mean that it will experience less performance in the real world.

Which is not to say you shouldn't strive for perfection-- you do the best you can and then when reality intervenes you're starting from a better point.

I would say that the heads should all be the same. If not, what is the good reason why not? I can't think of a good reason.

What are the facts of your situation?

1975 2.7 90 bore 70.4 stroke I,m shooting for 1.25-1.5 deck height 40mm webers w/34 venturis.8.5:1 cr flat topped RS pistons.I don't have the equipment to sink a valve.How do I go about removing enough material from chamber.Thank You.T.C.

TC

Before you start machining the heads to get them all within 1 cc, you should be sure your measurement technique is repeatable to better than 0.25 cc or you will end up chasing your tail.

heads
 

Thanks Tom.

Given the modest specs of your street motor, I can't imagine that increasing the cylinder volume of some of your cylinders to get them all within less than 1cc would be worth the effort. If you are worried about inconsistent idle or something, you might try arranging your heads in some kind of order based on the firing order.

Sort of like putting the heaviest piston and rod nearest the crank, and working outward, when dealing with balance. I'm not sure just what you'd want - higher compression head firing just before lower through the firing order? I'm assuming the heads all vary somewhat in the tenths of a CC range. But bringing down the CR on all but one to get a match seems like a lot of work for a street motor. Of course you could then have all the heads decked enough to get back up to the 8.5/1 (pretty modest) CR. But again, a lot of work for a rather modest benefit.

Porsche has all sorts of specs and tolerances for its motors, but looking through one of the little white spec books for an SC did not reveal a cylinder head volume tolerance.

Here is how I cc'd my heads

http://forums.pelicanparts.com/911-engine-rebuilding-forum/348387-bouffant-hairdos-miranda-grace-slick-these-heads.html