OST-020: Variables

[oldeskewltoy]

My client for OST-020 sends me an email asking me for a head. He explains to me he has a specific aftermarket piston that has a STATED compression ratio of 8.6 to 1, and he his target compression ratio of 9.0 to 1. He further informs me that the block and pistons have already been machined to go together.

Now why did I title this Variables?? Well, lets look @ the variables for this build and see............


Variables:

With 39cc chamber as manufacturers specs calculate too
#1 - bore 3.189", stroke 3.031", gasket thickness .028", deck .005", piston volume -4.1cc, chamber 39cc = 9.36 static CR

Odd... isn't it...(#1) the manufacturer of this piston shows an advertised compression ratio of 8.6 to 1, but math doesn't lie... and using THEIR stated specifications... doing the math - the compression ratio isn't 8.6, but rather 9.36!


#2 - 3.224", stroke 3.031", gasket thickness .028", deck .005", piston volume -4.1cc, chamber 39cc = 9.53 static CR

THIS is my clients piston. The oversize (going from 3.189 to 3.224) is the reason for the added .17 in the final compression number.


Now... if you looked carefully you will see that they use a 39cc chamber, while the stock chamber volume on a 4AG is 36cc...

With 36cc chamber(stock volume before deshrouding)
#3 - 3.224", stroke 3.031", gasket thickness .028", deck .005", piston volume -4.1cc, chamber 36cc = 10.1 static CR

OUCH... a bit higher then my client would like....



So... now that we are working with KNOWN variables.... with 37cc deshrouded chambers, a 1.6mm gasket* and client measured deck

#4 - 3.224", stroke 3.031", gasket thickness.06299", deck 0.0", piston volume -4.1cc, chamber 37cc = 9.18 static CR

* - HKS metal 1.6mm gasket

You will note the gasket thickness has "grown" from .028" to .063"... and you might be thinking that this is a bit large for squish... and to some extent it isn't ideal, BUT it isn't bad either.... for example... a stock head gasket is .047", the stock deck on a 4AG .015"... simple math and you get .062". So for all intentions... we are using the stock squish.

So 9.18 is perfectly doable, and should yield a nice static compression ratio for a boosted 4AG.



Ok, now that the math is out of the way... we can begin the head. The client asked for a smallport head, this one was one of my inventory.



The first thing to note on this head is the massive amount of deposits in the coolant passages (below the chambers in above photo) Note the coolant passage that lead between the exhausts valves is so clogged you can't hardly see them



Now here is the same head after very thorough cleaning (including a .005" re-surface to accommodate the use of the HKS metal headgasket)... all of the coolant passages that lead up between the exhaust valves are now fully cleared and the passages themselves have had all the scale removed. This should keep this head well cooled.


All of the exhaust guides were bad and were replaced, the intake guides were all still within specification. Anytime you replace a guide you need to machine the seats to properly align the valve contact area so the valve seals, this is not a 3 angle valve job.

Interesting note... I have found more fwd heads with guide wear then rwd heads... at least so far.

This head will receive the following... matching bowls and seats, deshrouding and balancing chamber volume to 37cc, exhaust re-taper and polish, intake port blending, and roof reshaping, overall deburring and polishing cam journals, and hand lapped valves.






and this builds first multi-panel photo showing the differences in the exhaust ports from old used, to cleaned and prepped for porting.

More to come.......