Given stock TC88 combustion chambers, how much milling of head gasket surface equals how much increase in compression ratio? Can anyone give at least a rule of thumb such as

.020" = .25 increase in compression

I just pulled the figures above out of the air to illustrate what I'm looking for in case it wasn't clear from the question, which is clear in my mind, but seemed less so when I put it into words...

What I'm looking to do is increase compression ratio by .25 to .5 without running the risk of creating too little piston-to-head clearance. Certainly there are thinner head gaskets available, such as Cometic's .020" (I'm running Cometic head gaskets now, but in the stock thickness, which I think I remember was .050" before being compressed), but I don't want to worry about the p to h clearance in case that's an issue.

 

It's not a linear relationship since the chamber sidewals have some taper. However, you can figure that .007" cut will reduce chamber volume by 1cc. I doubt that you really want to use a .020" Cometic head gasket as the head gasket sets squish which should be between .030"-.035". Don't think Cometic makes a .020" head gasket for Twin Cam motors. You don't really have to worry about piston to valve clearance until you start cutting .060" off the heads and using a .030" head gasket; anything in that range should be mocked up and clayed to check clearance.

You can achieve your goal by just changing head gasket thickness from .050" to .030". Go to the BigBoyz website and use their compression calculator to verify.

 

Thanks, djl, that's exactly the kind of answer I was looking for. I'll do a little more digging on the site you suggested as well.

 

Try to stay with a .030" gasket as the closer you can put the piston to the head during normal operating temps, the less chance of detonation you have.
And as djl has already pointed out, .007" is the thumb of rule, that equates to 1cc, but when re-surfacing, as with most all precision machining operations, ALWAYS measure twice, and cut once.
Scott

 

Thank you, Scott, for your input as well. I'll give a little background as well as the results of my digging. First of all, the reason for this inquiry is that I built this motor (or more accurately, top end) after doing what I felt was a fair amount of research and talking to a number of folks who are reputable, and had what I thought was going to be a pretty well performing combination. That was: 10.5:1 compression, ported heads and S&S 585 cams. I got this cam used, and for a good price, but only after I'd settled on it as the choice for this engine. The fellow that ported the heads was less than thrilled with the cam choice, but said he thought it'd be "OK", at least.

When I went to my local performance shop and stated my goal of building a 10.5:1 compression motor and told him what cam I had the owner said 10.5:1 is a little too much for that cam, that it should have no more than 10.25:1. This struck me as splitting hairs, and in fact, contrary to conclusions by others I'd talked to, including the head porting fellow, tech people at S&S, and others. But I decided to follow his advice. That said, he machined the domes of the pistons, allegedly to acheive 10.25:1 compression. I put the bike together, including compression releases in the heads, and it makes pretty decent power, but not like I expected. And the compression releases rarely close without goosing the throttle and sometimes even pulling them closed by hand. So I have suspected that the compression ratio is not 10.25:1 as I was led to believe.

Fast forward to now. The motor has 6 or so thousand miles on it since the top end has been put together. A couple of baggers have come out of this same shop with 98" motors, 10.5:1 compresion and some super secret camshafts (.590" lift, but otherwise secret) and the damn things do power wheelies. Mine is, as I said, a little disappointing. Granted, mine is 95" not 98", but it seems like I ought to be getting better results than I am. The shop owner says it's because of the different cam grind, but won't reveal the cam he's using. I'll buy the goddamn thing from him if he tells me what it is, but he won't give it up. So I have pressed him a little, and inquired what do I have to do to my motor to get similar results, and after asking me what my compression ratio is again tells me maybe we have to mill the heads and go with a thinner gasket to bump up the compression and get one of these super secret cams. Well, this pisses me off, because like I said, I feel like a quarter point is splitting hairs, and this seems to confirm that the compression isn't as high as he says, which I've suspected all along, and whe I went to him in the first place and stated my goals, it was very clear what kind of performance I was looking for from this bike.

I plugged some figures into the compression calculator. I don't remember what the deck height was when I put this together, but using .010" and flat top pistons results in 9:1, so that seems reasonable since 9:1 is the advertised stock ratio. Using those figures, and guestimating that the remaining dome volume is 6cc (it started as 11.3cc), that results in a static compression ratio of only 9.7:1. Now I am royally pissed off, because if I mill the heads .020" and use the .030 gaskets I can barely get to 10.25:1. This assumes that .020 results in a reduction of chamber volume of 3cc, using the rule of thumb stated above.

I'm very much tempted to pull the heads, mill them and put back tagether with the .030 gaskets and see how it runs with the cams I have. I can do all that in my home shop, so it would have minimal cost, which is a major factor.

Thoughts?

 

Before I did anything, I would run a compression check to measure the cranking compression. You can then back into the static CR by plugging in numbers for chamber volume but you do need to know your head gasket thickness and the dome volume.

.010" is way off for your deck height; typical for a TC motor is .003" to .005" down in the hole; positive deck height. I can email you a .xl file compression calculator that you can play around with; can't send it via the forum. PM me your email address and I will send it to you. You can always use the BigBoyz but having your own file is more convenient.

If your figures are correct, you will need to mill about .050"-.060" of a set of stock cylinder heads with the 6cc pistom dome to get to a CR that will make those 585s happy with an intake close of 45*. Piston to valve clearance will likely come into play in that situtation. You may be able to open up the valve reliefs to get the clearance you need. Howwever, if I were you, I would chalk this up to experience, don't use that builder again, get a set of .010" over cast flat tops and start over with the heads and make them work with the 585s.

Of course, if you go the teardown route, you can measure and find out exactly from where you started.

I wouldn't worry about power wheelies; half technique, could be lower primary gearing but it's not becuase of some super secret cam. Just focus on getting a "happy" place for those 585s.

 

All is not lost, by a long stretch.
If you are looking to have the last laugh on this, give us a call, and we can go over a couple of things.
Scott

 

djl, PM sent. Scott, I'll give you a call and see what your thoughts are. I think I ougt to be able to make something much better out of this by milling the heads and using a thinner gasket. It doesn't run bad now, but it certainly doesn't have the power I expected or that I think it should, not by a long shot. I have a couple of other things outstanding with this shop, parts for another bike, but I think he has lost a customer. WHich is a shame, because he's convenient as hell, and the only one that is.

 

First of all, I would like to thank Scott for taking time to discuss this with me on the phone and being so free with information. You and your brother were both extremely helpful. I believe you have won a customer, though my budget does not currently permit a major upgrade.

I used djl's compression calculator to "back in" to some of the missing values, having done a compression test and filling in the blanks with the known values. I went back and looked, nd discovered that the head gaskets installed are actually .030" and for reference I used .005" as deck height, even though I can't find my notes so I don't know the actual deck height. I came up with 180 psi on the compression test for the front cylinder, a little less on the rear, average 178. Using the intake closing figure of the cam and 85cc as head chamber volume (because that's what the bok says; I hear they vary from 86 to 88cc) I come up with 3cc as piston head volume and a static compression ratio of 9.74. According to S&S this should be enough to support this cam, but again, it isn't what I set out to put together. Not sure just what 'm going to do about this just now, but I do have a set of S&S 510 cams on the shelf that I might put back in for the rest of the summer. It's cheap, costing me only a few gaskets, and will work better with the low compression I have than the 585's, at least on the bottom of the RPM range.

I'll figure out what to do about raising the compression ratio over the winter, I guess, or at least when I get a few bucks squirreled away for the purpose.

I had a bit of a heated discussion with the shop owner yesterday about this whole issue, and he insists that what he has done with regard to the piston dome is appropriate for the 585 cams and what I said I wanted out of the motor, which, aside from my stated goal of 10.5:1 compresion I also had the caveat that I didn't want to have a grenade, and wanted to be able to jump on it and ride it 1,000 miles without having to worry about pump gas, etc. He has developed his own spreadsheets for calculating corrected compression ratio, etc, and states that "you can't use the ones like on the Keith Black website", etc. djl's spreadsheet, which he was also extremely willing to share, is pretty comprehensive. I don't know why one can't use the online versions, too... the math is the math, regardless, it seems to me that it depends on the accuracy of the info available to enter in the boxes. No one gets to change the formulas. He also stressed that we did not cc the motor, since I didn't have the $ for all that, which is all true. I still think the compression is too low for the perormance I expected, and certainly lower than what I expected even after machining the piston domes (I expected a "few thousandths" to be taken off the dome, not half of it). With all that said, I believe he is devout in his belief that he did at least what he thought was the right thing according to my riding habits, what I told him I wanted and his calculations. This guy has been in business since '72, has a great deal of engine building experience, has done some winning at the drag strip, and has built some serious hot rods, both in baggers and bar hoppers. So I'm sure he knows what he's doing. But I didn't get the results I wanted. Maybe if I had gotten what I wanted, I'd have a pinging, overheating monster. I just don't think so.

I'm still going to get this thing back up to 10.5:1 compression in the near future, one way or the other, and see what it does. I hope I don't regret it, but if it pings, I can always go back to what I have now... right? I think I"m going to cc the heads when I pull it apart, even if I don't do anything about equalizing chamber volume or anything, just so I know exactly what I have. Based on the input i received from Scott and his brother, Craig, as well as the tech people at S&S, who are always extremely helpful, I'll come up with a plan.

 

The only way you get to 10.5:1 static CR is by reducing chamber volume, increasing piston dome volume or some combination of the two. In either situation, piston to valve clearance may come into play. So, be sure and mock it up and check.

A few years back there was a popular build called the "Hippo" build; named after the creator who passed away a while back. It consistenly made 100/100 numbers and was stupid simple. Bore to 95" for SE cast flat top pistons, .030" head gasket, a set of SE Performance heads (the early ones) out of the box and a set of SnS 570 cams. The SE Performance heads were milled .050" or thereabouts and provided the compression needed; I believe the chambers were 76cc. Valve to piston clearance was not an issue. The TDC lifts of the 570 and 585 are identical but with domed pistons, you may need to open up the valve relief. All I am saying is be careful; mock it up and clay the top end as part of the assembly process.