V2Rocket

Thinking about modifying an 8v head by enlarging slightly the combustion chamber to lower compression a little bit. Stock is supposed to be 9.5:1, my head is milled a bit so probably around 9.6. I'd like to bring it down to maybe 9:1 for my supercharger but also to keep a good amount of off-boost driveability (don't want the sluggish of feel of 951 8:1 pistons).

I seem to recall that the head material was pretty thick and I imagine not much would have to be taken out for a half-point drop.

Is this something feasible without causing major damage or should I avoid it altogether? If so, how much volume would I need in the head?

billthe3

Figuring out the compression ratios aren't that hard and if you have the head off and can measure the size of the combustion chamber you should be able to figure out how much material you would have to remove to lower the compression ratio however much.

To get the compression ratio you need to know the stroke, the volume of the combustion chamber in the head, the volume of the combustion chamber in the top of the piston (the amount of space available from the deck surface to the top of the piston at TDC) and the thickness of the headgasket. You figure out the volume taken up by the stroke and headgasket by getting the surface area of the diameter of the cylinder and then multiplying that by the thickness of the stroke or headgasket. For figuring out the compression ratio its just a matter of doing a simple formula.

(cylinder head volume + piston volume + headgasket volume + stoke volume)
(cylinder head volume + piston volume + headgasket volume)

Have you looked at all into finding a thicker headgasket to use? That would be a relatively easy way to lower the ratio some, though I doubt it would be more than a point or two.

When I was working on my head it looked like there was enough material there to take away some without any issues, however you have to realize that altering the shape of the combustion chamber will affect the flow dynamics of the cylinder and negatively affect how well the valves flow.

If you've got all the numbers you can insert different numbers in the formula to figure out what you would need to change to get the ratio you want to achieve, either through a thick headgasket or different pistons or different head chambers, etc. Once you know how much of a volume difference you need, its just a matter of making the changes.

DANNiE

I used to hate "CCing" Cylinders... I used to have to do this on the Vipers all day..! What a pain in the A$$ to get right on the first go around...

First I used to find the largest combustion chamber... then id have to polish it out by sanding off all the burs and "hot spots" or flash casting tips then once all smoothed out CC check it again and take note of CC to go and match all the other Combustion Chambers... one good tip is using a small propane torch to heat the surface of the Chamber to see the hot spots...

billthe3

Where do you work Dannie?

I had a fun time doing this stuff on my car, but admittedly it was my car and has 6 less cylinders, plus I'm not doing it every often.

V2Rocket

im imagining you have like a dremel with a fancy attachment and slowly remove small amounts of material until they all cc the same at the desired volume..

is the trick to try to take evenly from the entire chamber? i dont want to mess up the machined surface and i dont want to mess up the plug/valveseats either


bill as for headgaskets i have searched a bit and the general consensus here at least is that could be dangerous as it eliminates the factory quench area and at a point can alter cam timing which is critical on the interference engine. not sure i want to go down that path. i also have considered getting some mods done to my pistons but they are kind of thin so not sure i wanna do that either. dont want to put out $800 for a set of custom pistons either.

DANNiE

To CC the heads you need the Valves in and closed; so basically no cams installed... the valves are steel and you dont really have to worry about messing them up if you are replacing them with new ones after but if they are going to be the ones you are using try to avoid them; also if you change valves after CCing double check to see if your still in good spec note that some valves have dished areas to change the CC amount too so a set of flat valves will be less CCs than a set of dished valves...

i used 1/8 inch thick plexiglass with a very small hole in the middle of each cylinder to inject the gel solution -i dont remember what it was called the boss just always had it kind of reminds me of hand sanitizer- ive heard of guys using the green rubbing alcohol too tho since its easy to see in the chamber; you will need to use your head bolts and find some nuts i would just use some spare nuts and bolts to bolt the plexiglass down to the head... use a very accurate syringe this is how you find out how many CC's the chambers are when you inject the gel/solution...

i used like 600 grit wet/dry sand paper the idea isnt to actually polish the chamber but to remove the Burs and high spots that create hot spots from when the head was cast you'll be surprised how the smallest tip of a piece of flash casting can heat up and cause pre-detonation... you will see them glow red with the hand torch; not a cutting torch just a plain flame for like light pluming soldering...

yes you kind of want to evenly sand the chamber; but stay away from the surface edge... its a good idea to take a magic marker and draw a line below the surface edge as a stay out area -i used to draw a line right below the surface edge and then another one right below about 3mm's- but remember to clear any high or cast flash hot spots first be careful around that surface edge tho it is the sealing point to the block after all...

take your time it doesnt take that long its just a pain cause if your a stickler and want all the exact CCs it takes time cause you do a lil sanding and put the plastic on inject see where your at and do it again till your to the # you need... remember to always start with the largest CCed cylinder first and then match all the others to it... most times though they are all very close in size... i do remember tho that heads that came from the machine shop after being milled sometimes we 3 or 4 CCs out of the others... most i remember seeing was 6cc...

Aluminum is soft and dosent take a whole lot of pressure to remove material from; we never used a dremil cause it makes low spots its best to do it by hand and feel the imperfections under the sandpaper as you go across the chambers... 600 should be good in the case of where it was 6cc to the largest sized cylinder i used 400-450 grit to speed up the process as i got closer like within 2-3cc's i went back to the 600 and higher grits like 800 or 1000 when you get up to a 1000 tho it begins to polish...

the guy i learned from was great at this and really had it down packed im glad I worked along side him at Doug Levin's... Doug, supercharges vipers well he doesnt really do all the work Craig does and whoever is helping him does lol and Mike does the machine work; I actually have Mikes # he does a great job but hes here in south florida...

Another thing we used to do was "o-ring" the block... Doug had bought a tool that you bolt down to the block and just set its depth and turn the handle and it cut a groove into the block; then we used thick piano string/wire i dont remember the note but it was solid metal and the way it came off the roll it was round already it was pretty easy to do... O-ringing the head was very tricky because the space between the chambers is tight once you start putting grooves between them; we usally sent the heads to mike for that and did the blocks ourselves... note that we used custom copper gaskets and APR head studs; in our case we'd have to use Raceware cause we have porsches...

FRporscheman

Spencer, if you want 951 pistons I have a set for sale with 136k miles.

Are you going to intercool your SC setup? If not, then lower comp is better .

8:1 is good, then you can use a really high-psi pulley .

And they're forged, not cast like the NA ones!

V2Rocket

yes to intercooling

no to the 951 pistons, too sluggish feeling off boost, id like to maintain some semblance of drivability hence the 9:1 which isnt quite so far a drop

FRporscheman

Have you driven many 951s? I ask because I expected them to have lag as well, but both cars I drove felt ok (stock and 2.8 stroker with 8:1 and a small turbo). The latter not only felt "OK" but it felt like a v8.

V2Rocket

driven 2 951s both with mods, one with a larger turbo and one with the stock k26. lacked the urgency in motion (if you can call it that) of the NA when below 3000rpm.

billthe3

But you'll have a s/c on yours so you won't have to deal with the lag issue.

67King

Been there, done that, have a head I'm afraid to run on an old 2.3 Turbo Ford sitting in my garage. And it is iron. I'm just afraid my chamber walls are too thin.

I guess the things I'd look at are trying to find out where you NEED to be, rather than where you want to be. Honestly, 9.1:1 on a blown engine still seems a bit high. If it were a small bore 4V, it might be okay. But the bore size is working against you, and the spark plug location is also working against you.

Anyway, from there you can make a better assessment. How much boost you are running, are you intercooling, etc. I'd expect you probably really need to take out more than a half a point. But, if you just go for that, figure you've probably got maybe 40 square cenimeters of surface area in the chamber (estimate of half of piston surface area)? And you need to take out 4CC's? The valves probably take up half of what is left, so you may be looking at 2mm's? That is a LOT of material. If you've got a thick chamber, you might be okay, but if you've got a thin one, it is asking for trouble. There is a certain amount of variability designed in to account for mold wear, core shift, and other things that make sand casting imprecise.

You could help the situation by sinking the valves further in, but that will hurt flow.

Of course, you can help all of this by running race gas, and retarding timing. But, I think you'd be a lot happier with it if you dropped in some dished pistons (i.e. turbo pistons), and went from there.

V2Rocket

not lag issue just off-boost driveability. cruising around town at 15mph i dont want to be hitting full boost but i do want to be able to motor around easily.

67King

Centrifugal or positive displacement?

V2Rocket

eaton so positive displacement