milling the head...calculations - Honda Prelude Forum

ludebeginnings · 09-14-2003, 04:22 PM

I'm building up my motor (H22) and trying to figure out how much I need to mill to get a final CR of 11:1. bottom end is staying stock, just being refreshed. head is reworked and i think the only thing that will affect cr is the deshrouding. I'm going with crower stage II cams, skunk2 high comp valves, skunk2 valve springs/titanium retainers.

Using some basic calculations I figured that milling it .035" would get me just about where I want to be.

.035 X 2.54 = .0889 cm
3.14 X 4.35^2 X .0889 = 5.2821408
539.25 - 5.2821408 = 533.967859
53.925 - 5.2821408 = 48.6428592
533.967859/48.6428592 = 10.9773123
or about 10.97:1 CR.

My question for those in the know... how much if any will the Skunk2 High compression valves increase the compression? I've seen some posts where people say it will increase compression ration .2-.3 but have yet to confirm it with any reliable source. .2-.3 sounds high, but I assume that they raise the compression of the chamber to some degree as they are flat w/no dome, therefore decreasing the volume of the chamber.

I'm not planning on building the block, and likely won't for a year or so. I am switching to a manual tensioner for the timing belt. I will have cam gears so that I can time it properly.

Let me know if I'm looking at the correct amount, or if I'm horribly wrong. thanks

NXLude · 09-14-2003, 06:33 PM

Your probably gonna be cutting things very close with that much shaved off the head......the skunk valves bump it up .5 i think

Satan_SRV · 09-14-2003, 08:28 PM

so you increased your cc size to 53.925? I was pretty sure the cc was 53.3

ludebeginnings · 09-15-2003, 02:04 AM

Satan> figured that by the displacement of 2157 divided by 4.

NXLude> wow...you really think it changes CR by that much, that would be nice, then I'd have to mill less.

I'm going to check w/crower & skunk2 tomorow, I'll let you all know what i find.

thanks

71dsp · 09-15-2003, 04:32 AM

I think you've skewed the math a bit. Here's what you need to do, and unfortunately, I don't have all the data you need.

You know that compression ratio = volume of air in cylinder at BDC/volume of air in cylinder at TDC : 1.

So, what we need is the bore and stroke of the engine, headgasket thickness, piston to deck clearance, combustion chamber volume, and piston dome volume (use a negative number for a piston dish).

The air in the cylinder at BDC is [(bore/2)^2 * 3.14159 * (stroke + headgasket thickness + piston to deck clearance)] + combustion chamber volume - piston dome volume.

The air in the cylinder at TDC is [(bore/2)^2 * 3.14159 * (headgasket thickness + piston to deck clearance)] + combustion chamber volume - piston dome volume.

2157 / 4 does NOT work because 2157 / 4 is just (bore/2)^2 * stroke * 3.14159. Using that number neglects the head gasket thickness, piston to deck clearance, and the combustion chamber volume.

The piece of data I don't have is the piston dome volume. We can back into it though because I do know:

bore: 8.7cm
stroke: 9.07cm
headgasket thickness: 0.030" (typically) i.e. 0.762mm
piston to deck clerance: .020" (typically) i.e. 0.508mm
combustion chamber volume: 53.8cc

Using my little Excel spreadsheet, I get that the piston dome volume must be about +1.5cc to yield a 10.01:1 compression ratio using the above data.

Now, let's say you're going to mill 0.035" off your head. That's like effectively having a 0.000" headgasket thickness, and a 0.015" piston to deck clearance. Again using the data above with these two changes and a 1.5cc piston dome volume yields a CR of 10.88:1. Not quite 11:1, but very close.

Vapor · 09-15-2003, 05:58 AM

You also have to take into account the material you're having removed during the deshroud job.

Billy: do you have a source for the H22 piston to deck numbers?

marcucci · 09-16-2003, 05:38 AM

Quote:

Originally posted by Vapor
You also have to take into account the material you're having removed during the deshroud job.

You must cc the head to be anywhere near accurate. You would be surprised how much material can be lost to a simple deshroud. Add any shaping and you will find you need high CR pistons or valves to keep from having to mill the head .050 or more.

ludebeginnings · 09-16-2003, 06:28 PM

well, I contacted my machinist & he stated that he didn't have to take off much material at all when deshrouding it, he figured maybe a 1-2 cc difference as a result. I also contacted Skunk2 & they stated that the High Comp valves I have will increase the compression around .5

contacted my machinist and he is going to cc the head for me for free since I'm getting so much work done. Now I'll be able to ack like i know what I'm doing...lol

marcucci · 09-16-2003, 09:21 PM

I'm not going to bother with that calc on the H22 but 2 cc's I know is enough to make a .2CR difference in my 2.6L Isuzu motor. That's significant enough to consider when putting the thing back together (i.e. fixing it with a light mill).