I need an 906 expert

[Olle B]

Hi
I know the volym of the heads differ from 79-88?
Is there any easy way to messure the volym without special eq?
The main Q is................
I would like to know if anyone in the past done a (dont know the english term) cut of the high for the head,the car runs well before restauration,but it had 5 broken top rings!!
Now Im rebuilding the engine with new everything and its intresting to know if I shall do a cut to compensate for the 5cc ?more volym Ive got due to 3cc in piston high and perhaps 2 cc in polishing the chamber while porting the heads!
If I cut .020 I will be back to the original,but IF the head already is cutted,,,,,,,,,,,,,,,,,,,I will get to high!?
The car is for 95% used as a cruiser but since the R/T 70 was originally 10,.5? I will go by that for getting the original little hysteric behavior out of the car.
Did I mention it is a 383 HP original in a Challenger R/T.
Q nr 2.
Is it possible to messure the high of the "phase" parallell to the valves at its "edge" by putting a metalrule across the chamber and take the high with feelergauges?
It sounds like an idea,only troubble is I need someone who can send me the correct mess for an garantied untoutched head!
Any suggest,,,,,,,,,,,,anybody?
Best regards Olle B

[John Kunkel]

I would be surprised to see a stock 906 head anywhere near 79 cc's, all that I have measured were in the 88-90 range. Basic combustion chamber measuring can be performed with a graduated beaker and a plexiglass plate covering the chamber.
If in doubt whether a big block head has ever been milled, measure the outboard head bolt bosses. They were exactly one inch (25.4 mm) when they left the factory.
For every .0042" (.107mm) removed from the head surface, the combustion chamber volume will be reduced 1 cc.
It is possible to measure the deck height with a straight edge and feeler gauges but the piston must be at absolute top dead center.

[Olle B]

Thanks John
I will messure the hight,With as you says 88-90cc it could never haved been !0:1 or 10,5:1 from original!Not even with the original highdeck pistons showing 1,900 in high!
My new Keith Black are 1,908 but have a 5cc cutout.
I gain 2 for the hight so the total loss is 3!
Is it worth compensate for?
I will get a great improvement just by using this new pistons each of them 220 grams lither than original?
If I seek for the matematical 10:1 I will get a To high compessed engine I guess,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Olle B

[John Kunkel]

The '70 383 had an advertised compression ratio of 9.5-1 but, because of sloppy production tolerances, they were usually at least a half point lower than advertised.
With the +.030" KB pistons, 90 cc chambers and a steel shim gasket, you will have less than 8.5-1.
To get a 10-1 compression ratio with your setup you will need to reduce the total volume by around 20 cc. The heads would have to be milled .085" (2.16mm) to get rid of 20 cc, this isn't practical.
Half could be taken off the block and half off the head. When milling the heads/block, he intake side of the head must also be milled.

[Olle B]

Its a whole lot of cutting needed!
Im not sure but the 9,5 you told of,isnt that for the original 383?
Mine is an R/T with HP engine! The standard pistons was 1.867" in deckhight the R/T:s mounted in my car was original!!? and was 1.900".
If I calculate from that point I will lose the total of approx 5cc but gain the add due to more volym caused be the oversize,I havent calculated of it but lets say 4cc diff from before.
I was at my engineshop today and we desided to do a 0.2mm cut mostly to be sure of 100% flatness.This will give me nearly 2cc.
SO.................................
Figures in and out,lets say I lose 2cc compared to original HP whatsoever compession,thats not much!
But if I shall go for the real 10,5!!!! I understand the cutting will be dramatic!
I guess I will get problem with the pushrods and as you says intakemanifold.
If I stop by this 0.2mm I think I will get a lot of more power and better respons from engine compared to standard thanks to my KB pistons (and headporting).
Is there any resons to go for the matematical (never exist) 10,5
or will I get more troubble than raise of output.
The car is used for 99% as a cruisingcar.
Original is original but if it havent been that high all is at an academical point,isnt it!

Best regards Olle B

[DartGT66]

You must measure the piston top to deck clearance at TDC and the chambers to know. I have done the measuring simply using a plex plate on the chamber sealed with greas and leaving one corner open. Then I have put water in a glass, weighed it with a letter weight scale (accurate within one gram) took water from the glass with the thing, whatever it's called, they put vaccination or drugs in your body, available from all chemistrys for a couple of krones. It has a scale on the side, so I keep book how much water goes in to the chamber. After it's full, I weigh the glass again; since 1cc weighs one gram you can doublecheck the cc's from the weight difference of the glass.

Like said above, the Stock compression ratios are usually nowhere near the advertised ratios, the deck heights may vary a bit and the chambers stock are usually way bigger than the "specs" advertised. Overbore will slightly raise your CR, since there is more air in the cylinder that will be pushed in to only marginally larger chamber. But the effect is not very big in regular street compression ratios. I have succesfully many times milled about .040"(1mm), which is roughly 10cc, without need of milling the intake sides or problems with lifter preload. Actually the lifter adjusting range in a mopar lifter is very big, and can tolerate almost any milling you can throw on it. But you can of course set the preload to correct specs easily by using rocker shaft shims to compensate for the milling.

The most important is that you measure it first, so that you KNOW where you are going. I would try to get the true CR to about 9.5:1 and not bother for much more. If you are currently at the 8's, the raise to around 9.5:1 will make the engine response better, give a little more power and make the engine even sound "tighter". Above that you may start having trouble with pinging on pump gas and on my opinion its better to stick below that spot than above it although it may cost you a couple of horsepower.

[Jims451]

Ouch! Getting 10:1 compression with the Keith Black #162 piston will require alot of machine work.
I would start by offset grinding the crank 0.010" to get a 3.400" stroke. (the stroke increase is twice the amount offset ground.) This is not too expensive, but requires thicker main bearings.

Next the block needs to be milled 0.020" (9.960" block height) if using closed chamber heads, like the #915's or Edelbrock heads, or mill the block 0.030" (9.950" block height) if using the 1968 and newer open chamber heads.

To get 10:1 compression, with the closed chamber heads and the 0.020" taken off the block, the cylinder heads need to be milled for 80cc cylinder volume. This should make 10.06:1 compression, and have 0.0335" quench distance (clearance between piston and head) using the Felpro #1009 head gaskets.

To get 10:1 compression, with the Open chamber heads and the 0.030" taken off the block, the cylinder heads need to be milled for 83cc cylinder volume. This should make 10.00:1 Compression using the Felpro #1009 head gaskets.
I do not know what the quench distance will be, because I do not know how much the heads will need to be milled to get 83cc?