Altering deck height for varying cc heads
 

My heads range from 90.4cc to 91.6cc and from my calculations on my build, this could bring me from ~10.5:1 - 10.7:1 with my current deck height I measured from #1.

The deck height with one 0.25mm base shim was at 0.81mm, I've read this is probably a bit too close for comfort, so I'll order 6 0.5mm shims to help bring this up (CR down effectively). I was able to measure this as my pistons do have a slight flat edge that I was able to measure with my calipers. Should I also be conducting the solder method to check my piston clearance (heads were flycut 0.18mm)

Im open to input on my deck height, however, my question is: should I be worrying about my amount of variance in head volume stated above and compensate for it through deck height (if possible), or just leave them as-as. A couple points in CR seems a bit high.

bore diameter: 98mm
stroke: 70.4mm
piston dome volume: 42.2cc
deck clearance (#1): 0.81mm

You can't vary the deck height by using different shims per cylinder. If you did that your heads would not be level as the cylinder heights would be different. This could lead to the cam binding in the housing once everything is torqued up.

I hope someone else has good advice on what to do about the head volume variation. It does seem like a bit much. Are you sure the were all fly cut the same? Measurement error?

yeah, that makes sense. didn't seem like that would be a good approach. :|

Im not able to verify if they were, but they've all been stamped as such and I trust they were done correctly.

If you have the right equipment measure the dimension across the head between the cyl. mating surface and the cam tower mating surface to make sure they are all the same. Or have a shop check them.

I did not have the heads chamfered for the larger bored (98mm) cylinders, is that what you're referring to in the measurement above? will this non-chamfered edge decrease my CR or cause detonation??

That's a LOT of compression for a street engine and I hope you are using twin-ignition.

I would recommend making the effort to even out each chamber's volume as that will really help prevent detonation. Remember, there are significant differences in cylinder head operating temperatures!

You should have each head cc'ed and then matched so they are all the same. THEN, measure CR so you know precisely what you have as that effects how much ignition timing you can use.

Im going to use a 0.50mm shim instead of the 0.25mm ones, which will drop me to ~(10.2:1 - 10.4:1). I will be using a twin plug XDI2 setup, so I will have control of the advance.. thanks for the input!

lots to study yet on the ignition system.. :)

"I did not have the heads chamfered for the larger bored (98mm) cylinders, is that what you're referring to in the measurement above? will this non-chamfered edge decrease my CR or cause detonation??"

No. I'm talking about the distance between the cam tower mating surface on top of the head thru the head down to the cylinder mating surface. If this dimension is not the same for the heads on one bank then the cam tower mating surface for the three heads won't be in the same plane.

I think I'd have that edge chamfered to prevent a potential hot spot. Chamfering it will lower the CR because it will increase the volume of the the chamber.

I will double check this when I return in 1 week. Thanks for the advice.

That will tell you if they were all cut the same, assuming the heads all came off the same engine. If that's the case and that dimension is not equal then the flycuts weren't equal which will affect the CR and could cause cam binding. The valve depth controls the CR besides the flycut. The valves can be set deeper on the high CR heads.

I had my heads twin plugged AND cut for the 98mm chamfer. I cc'd my heads and got 91.1 cc The typical volume for an unmolested 3.0L head is ~90cc.

So I agree i'd definitely look at the head cut depth and also look at your second plug location in the combustion chamber. Are all the 2nd spark plug ports cut/blended into the chamber generally the same?

Kevin,
Just looking at some pics of twin plug heads I haven't seen any radiusing of the plug pocket into the chamber at the hard edge? At least on the ones I looked at. I'd think that would be beneficial for flame front propagation? He may be able to balance the chamber volumes by doing that. I would also chamfer the hard edge on the flycut since he's going with a bigger bore. I'm just learning these engines but the same tricks that work on bikes, snowmobiles, Chebbies, Fords and even some diesel tricks should work on these.
Steve

thanks again guys, I dont have pics of all my heads, just two.

these are at 91.5cc and 91.0cc (you can see the stamps on intake).. but I do see how (and how some have suggested) that I can chamfer the 2nd plug bevel on the larger heads and reduce the overall average of the heads. It was made with a smaller 12mm diameter.

Im curious to see my lowest cc head (but Im on a plane and will report back in 1 week!)

As far as chamfering the edges to smooth out the transition to larger bore, I wonder how necessary this is, since my pistons do have that slight edge for the squish band. Maybe it'll even help with turbulence and be less a concern for hot spots?

https://www.dropbox.com/s/owxkyucx2c...jpg?dl=0&raw=1
https://www.dropbox.com/s/owxkyucx2c...jpg?dl=0&raw=1
https://www.dropbox.com/s/5ff7oqtm93...jpg?dl=0&raw=1

Just looking at the pics it looks like the cyl. band is wider than the squish band on the piston. I think I'd chamfer the chamber back to the inside edge of the squish band on the piston. The volume removed by the chamfer can be cal'cd to determine the effect on CR. Another thing is that hard edge between the valve pockets and the piston dome, I'd radius that. If you get a burrette you should be able to get them close by playing with the plug pocket radii.

boost,

I agree with what you're saying about using the plug area to balance the volume. Seems like the best place to do it since i've heard that the factory blending of the original spark plug can vary by as much as 0.7cc!


cgarr does a lot of heads for people on this forum. If he did yours, you can be 100% sure he meas'd your heads and cut them all to the same depth/thickness. As far as the 2nd plug blending goes, I can see they were done minimally to preserve reducing the chamber volume. That 12mm plug choice is for best clearance getting to the spark plug from the outside AND reduces disturbance to the combustion chamber.


For pistons like these with the "step" at the edge, the chamfering isn't a must. That step around the piston is what lets us get away w/out chamfering the head. If the pistons had no step on the side, yes there would be some clearance issues there with the piston and deck clearance would be really tight. That said I think the chamfer is good for making a smooth transition from the cylinder to the head. Only seems to make sense when you're going from a bigger hole (cylinder) to a smaller hole (combustion chamber) that you blend that transition smoothly?

My pistons were set on a 90cc chamber for my target CR, but one came back at 91cc. I ending having to massage around the spark plug port to even them out also. This dropped my CR from 10.8:1 to just under 10.68:1 as calculated.

what sort of tools/techniques could I use to finesse these park plug pocket areas?

Im going to use one of these with an air die grinder and gently massage the outside area of the 2nd plug pocket.

I may chamfer the edge too, while Im at it!

https://www.dropbox.com/s/bhs74aiknf...jpg?dl=0&raw=1

here are my measurements before I go for the "pruning"

1: 91.5cc
2: 91.6cc
3: 90.4cc
4: 91.0cc
5: 91.6cc
6: 90.8cc

I'll re-stamp them once Im done getting them all sorted.

When you say you might chamfer the edge, are you talking about the edge of the plug pocket or some place else? I'm not sure I'd do the edge between the chamber and the mating surface by hand.