C2 overbore-higher compression?

[Justin N.]

Dear Members,
My original block had to go .60 over to correct the misaligned(crooked) .30 over the previous Bubba family member had performed. I forgot to ask my machinest today, but does this increase the compression? I figured 9.7 to 1 with the 64cc 'Camelhumps' and the flat top pistons. I assume this is standard bore.??

Thank you much!

Justin #41362

[William V.]

Re: C2 overbore-higher compression?

Justin

overboring increases the displacement. By going 60 over you increase to approximently 337 cu/in. If the 60 over pistons are of the same design as the old units, the compression ration will stay the same.

[Mike M.]

Re: C2 overbore-higher compression?

if you aren't familiar with cc'ing a block/head to determine the comp ratio, have the machinist do it for you. if you want to increase c.r, and haven't got the 60 over pistons yet, consider the sp h perf pistons and then you should have the assembly cc'd to determine the c.r. if you are staying with flat-tops, the cr is probably close to that of an original vette engine, mike

[Joe L.]

Re: C2 overbore-higher compression?

Justin-----

All other things being equal, the increased bore will produce a higher compression ratio. However, the increase will be quite small. From standard bore to 60 over, I'd guesstimate somewhere in the .2 range.

[Brian Monticello]

Re: C2 overbore-higher compression?

Justin,
If you are interested in compression ratio, there are several more important factors to consider

1. Piston to deck clearance
2. Combustion chamber volume
3. Piston type (flat, dome, dish)
4. Head gasket selection.

You really need to measure 1 and 2 (or change) to determine a true static compression ratio and select 3 and 4 to get you where you want to be. Of course you could deck the block to changge #1 and unshroud or mill to change #2. There are many variables to consider.

I've seen some SUPER complicated CR calculators and I've seen some that are much too basic. This one is a good mix

http://www.csgnetwork.com/compcalc.html

Have fun and do your HW!
brian

[Brian Monticello]

Re: C2 overbore-higher compression?

One more note - please don't assume your heads are 64cc. Over the winter I CC'd a set of 461s that were originally 62CC and they measured 56-57 CC. Also CC'd a set of 462's that were supposed to be 64CC and they were 67-68 CCs.

4.06 bore
3.25 stroke
4.06 head gasket
.039 thick head gasket
62CC head
flattops
.03 piston/deck

is 10.0:1 CR

if I drop to 56 CC we go up to 10.7. The difference between 10.0:1 and 10.7:1 is the difference between racing gas and pump premium. Be sure to measure everything!

Using this calculator (to add to the comments by Joe) and the config above, I get a 9.88:1 with a 4.03 bore so the additional 30 overbore increases the CR fractionally.
brian

[Justin N.]

Re: C2 overbore-higher compression?

Dear Members,
Thank you all for much insight! I'll see the machinist this Monday and check the size of the chambers, they're #291's. I believe these came off 68 327's and 69 350's.

Justin #41362