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Converting my 88 to a 95 and the pistons I have are specific to my heads, so I'm not getting different ones, but they're rated at 10.5:1 compression which is a little more aggressive than I'd like for this particular bike.

My question is how much gasket thickness should I use to get that compression down to more of the 9.5:1 or 9.8:1 range?
 

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You can get an idea by playing with this Compression and Engine Displacement Calculator but without knowing all of the variables (cylinder head volume, piston dome volume, etc.), it's just a guess. A better approach may be to cut the piston dome. Otherwise you'll probably need a very thick gasket. Google squish quench area combustion efficiency if you don't already know why a thick gasket may hurt efficiency.

http://www.bigboyzheadporting.com/TwinCamComp.htm
 

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Converting my 88 to a 95 and the pistons I have are specific to my heads, so I'm not getting different ones, but they're rated at 10.5:1 compression which is a little more aggressive than I'd like for this particular bike.

My question is how much gasket thickness should I use to get that compression down to more of the 9.5:1 or 9.8:1 range?
Wouldn't be how I would do that. Add volume to the head chamber. Going to 95" requires new pistons? Why not go with a flat top and be right where you want to be? Or use a cam with a later intake close to bleed off some of the low speed cylinder pressure.
 

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Correct pistons is the way to go. But people have used thick gaskets to lower compression. If money dictated that the build went that way, I'd look at using shims under the jugs too.

Be aware that you'll need to back down the spark maps to account for the loss of quench effect because of the spacing in the squish area. The lack of quench can mean that you really need to belly out the spark curve.
 

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You are approaching it, backwards... why are you using a piston and head set. Designed to perform as designed at 10.5, and then ham stringing it??? I don't think gaskets that thick are made.... as therm an be done, but why bother? And as Don said also... the pistons are designed to fit into the Chambers at a specific clearance in order to effect the maximum effect from the combination,,, moving the piston farther away from its designed specified distance, negates any positive effect of the combination...I wouldn't want the time on the project you are starting...
 

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Discussion Starter #6
As I said I'm not changing the pistons. It's a long story but I have heavily modified HTCC heads that only work with HTCC post and there's only one option for those pistons.

Cometic sells base gaskets also for twin cams in different thicknesses so I'm going to play with that and the head gaskets. I was just looking for pointers on how much thickness I need.
 

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The HTCC is a segmented hemi and squish distance is critical.
The guys that got that right and had them flowing ended up with 115+ hp 95" motors.
Others struggled with them.
The squish band is as-cast. This needs to be cut and the heads decked to get the bore diameter right then a .030 head gasket is utilized.
If its any consolation you are probably at a true 9.8:1 right now, not the 10.5 you thought, with a stock head gasket.
 

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The squish band is as-cast. This needs to be cut and the heads decked to get the bore diameter
I don't understand ... "heads decked to get the bore diameter right", what do you mean, the diameter of the combustion chamber at the face of the cylinder head?

Or just careful matching of the 20° angle in the head?

But, I agree with you. Never freak at the advertised compression ratio of pistons. It's meaningless until you measure the size of the chamber, and how far down the hole the pistons are yourself.

It's an interesting project to do, I'm envious.

On many, if not most pistons, you can machine some off the top too. If the squish fits, perhaps you can take some of them easier?

But do the math.
 

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I don't understand ... "heads decked to get the bore diameter right", what do you mean, the diameter of the combustion chamber at the face of the cylinder head?

Or just careful matching of the 20° angle in the head?

But, I agree with you. Never freak at the advertised compression ratio of pistons. It's meaningless until you measure the size of the chamber, and how far down the hole the pistons are yourself.

It's an interesting project to do, I'm envious.

On many, if not most pistons, you can machine some off the top too. If the squish fits, perhaps you can take some of them easier?

But do the math.
Its two ways of saying the same thing, yours is a bit better..
If the head chamber is cut at 20 the deck then needs to be milled to bring the diameter size back. It is not an easy process to get the heads the same, the proper CCs where you want them, and all the geometry right. And back in the day (and still today) many realized that a nice set of heads with a bathtub that flowed like the HTCC CNC heads INTAKE with a little smaller EXHAUST could propel a 95" motor the same or better and none of the BS with angled domes. The side benefit is a piston that is lighter and has a lower CG.
 

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So, basically, what he is after is dropping 1 point of compression while still maintaining a healthy 30 (ish) thou squish, at a 20° angle?

How do the pistons work, do they have a small rim around the outside of the dome so you can check and measure the deck height?

Like someong said, lowering compression with gaskets will increase the squish, and I don't think they make them thick enough to have such an effect either. Cam sounds possible, do you understand the different kinds of compression that arise due to different cam timing? How you can reduce compression by having later intake closing cams?

Wiseco helped me out once with machining the top of their pistons. They said the crown was .183” thick, so it allowed for a good bit of machining. In that case it was 70 thou.

But then, that would have less on an effect on these "pointed" pistons.

You want to keep your squish tight to avoid all the things you want to avoid with high compression. What do they recommend for these? Whatever you add to the base gasket will go straight on the squish.

Do they look like these? Or the one below?

 

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What's actually the problem you are trying to fix?

Would you swop them for standard heads, cash your way? (A complete set up (new) went on Ebay for $1,575.00 in May).

I've been thinking about this and looking at the photos.

Correct me if I am wrong, but it strikes me the theory of what is going on here is that in order to raise the compression, they are reducing the size of combustion chamber, but if that was attempted while using flat top pistons, it would require such a small combustion chamber that would start to interfere with the fill and hold back performance. They are "High Torque" but also high rpm heads, they are designed to go up to 7,000 rpm, right? Hence they must have high compression AND high flow.

So, instead of a hard squish, like on a stock engine or an Evo, they are using the angle to increase the squish but also allow a broad chamber, instead of a tight little bathtub? It's a sort of double benefit in the design?

OK ... but stock it looks like it's just cast rather than machine cut, which means it is going to have H-Ds habitually crappy tolerances; squish, deck height and compression. It's going to be a "safe" plug and play for franchises to swop out but be less than optimum.

The proper way to do it would be to use an accurate, machine finished 20° that matches the machine finished piston, and ensuring the pistons are decking correctly. In order to achieve that, you would have to skim the cylinder head a good bit and then put the head in a fixture on a big lathe or CNC machine to cut it at 20° accurately, and probably trim a bit off the cylinders to get their height right.

Not difficult (if you have the fixtures etc), nor complex, nor even that expensive to do.

I think this Axtell did/do this for their 20° perimeter domed 11-1 compression piston kit, as do others. They charge about $110 for it. Do you Dorf?

Now, perhaps that's overkill in this case, it is a budget job, performance/tolerance don't matter so much etc. So what's the alternative?

Well, I guess that's two fold. You need to work out how many cc of material you need to remove in order to drop compression 1 point, and then how and where to remove it. Looking at the pistons, you could and it would be easier to remove some off them. If that's not enough, it looks like you could remove some out of the combustion chamber, cleaning it up as well. Not on the "20° face" but in the bowl.

But it's not good practise and it may not fix your problem.

So the first question to ask would be that ... what cc to increase combustion chamber size in order to reduce compression.

What is the stock chamber size? I am reading 84cc to 90cc in difference places. Using the Big Boyz calculator, that'd be 10cc +, which is a lot.

i'd swop them on, or try and fix them properly using someone that's fixed them properly before. It's known territory.

What cam have you in there?



Here's one Big Boyz ported or polished, it looks like they removed some metal from the chamber, e.g. unshrouding the exhaust valve.

 

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The as-cast nature of the head chamber can be + or- .040 off center and not uniform in any axis. The piston is perfect. The squish distance observed by the guys back in the day that checked them was all over the map too tight and loose. Once machined this all gets corrected. That polished chamber fixed nothing regarding squish.
 

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Interesting. I did not know that.

So, for the OP's benefit ...

assemble the cylinders and pistons, torque down, measure for deck height, machine cylinder to suit.

machine cylinder heads in a lathe to a clean 20° squish.

clay piston tops and assemble heads with gaskets to measure the actual squish

skim heads to suit.

assemble heads with valves, measure combustion chamber size and then do the mathematics to see what compression you actually have, with the cam you actually have.

choose gasket thicknesses to suit.

take it from there.

I'm still not sure what the problem is that he wants to fix, if it is real or just imagined.

Lastly, possibly, choose cams with later closing intake timing to reduce corrected compression.

And/or, while it is apart, fit compression releases.
 

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It's only 10.5:1, put it together and run it, I doubt the compression is really going to be a big factor. It's not a fire breathing 13:1. Since the heads are off, you could install a manual compression release if it makes you feel better. Or as mentioned, install a cam with later closing to reduce the dynamic compression.
 

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Interesting. I did not know that.

So, for the OP's benefit ...

assemble the cylinders and pistons, torque down, measure for deck height, machine cylinder to suit.

machine cylinder heads in a lathe to a clean 20° squish.

clay piston tops and assemble heads with gaskets to measure the actual squish

skim heads to suit.

assemble heads with valves, measure combustion chamber size and then do the mathematics to see what compression you actually have, with the cam you actually have.

choose gasket thicknesses to suit.

take it from there.

I'm still not sure what the problem is that he wants to fix, if it is real or just imagined.

Lastly, possibly, choose cams with later closing intake timing to reduce corrected compression.

And/or, while it is apart, fit compression releases.
I agree and like I said out of the box these builds barely broke an honest 10:1.
 
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