243 heads, ported not milled
Pistons: Mahle Forged Power Pak pistons with valve reliefs
Cam : 227/235 .630/.610

By changing the pistons it increased my chamber size.

Can anyone give me an answer?

 

I am running 61.5cc 243's milled .040 with a .045 gasket and 227/231 cam...SBE. I know some have milled down to 58cc with AFR 205's but I can not remember if 243's can be taken that far.

 

So if I milled .035 and got the .045 gaskets I would decrease my clearance by .045 correct. Doesn't seem like a big bump in compression though.

 

My motor is around 11.3-11.4 range on compression. With valve reliefs you should be able to go much more. I would seek advice from a builder...they will offer better information then I can.

 

I've spoken with 6 builders. All saying something completely different. All suggest milling. One said .030 one said .040 one said .035 one said .060.

Of course all recommended totally different cams for the car too. Totally different!

 

I am by far not an expert in that area but it occurs to me that you can physically mill away until you get close to hitting the pistons. I think you also must consider that you are affecting airflow in/out of the head when you get that close and basically start changing the shape of the chamber. On a low-power application the compression probably makes up for any loss in flow that might be wasted anyway. But on a high HP application, it seems that flow is going to factor in.

Reading back over that, zero help added here..I guess just saying that it's a relevant consideration other than just clearance by itself and hopefully someone that is more knowledgeable will come along and correct or corroborate.

 

I understand that the cam, heads, compression, cylinder cc's, gasket thickness all come into play. I'm shooting for 11 to 1 compression and my pistons have valve reliefs cut into them. Being a nitrous motor also, comes into play. I've read so much about cams and heads in the past week that at this point I just want it done and over with.

But the reality is, I need it to perform, stay alive and be reliable.

 

Well how much are the pistons dished or relieved? LS2 which comes with those 243 heads(with ~64cc chamber) is 10.9:1 or so. Different pistons of course. Make up the difference in milling and you can quickly get to 11:1. Simple math. Usually. Did the porter take material out of the chamber? If so, you should cc them if you are concerned with CR. Cheap and easy and you'll have the piece of the equation that is currently missing. Then math will tell you exactly what you need to do to reach your 11:1 goal.

 

The pistons have -4cc valve reliefs. The combustion chambers were not touched, only the intake and exhaust was ported.

 

You need to mock up assemble, measure ptv, then base your decision on that. I wanna say after about 45 or so the intake flanges need to be straightened to keep everything lining up. Mine are milled 40 or so with a cam that size and flat top pistons and im sitting at 90 and 70 ptv. Just to give you an idea.