Counter bore

[dq409]

How much more would the compression ratio be if you had a smooth bore with out the counter bore?

And,,,, I was talking to one machinest that uses a .080 counter bore so he can use a ring compressor,,, Seems stupid,, IMO

How much would loss would cause that ?,,,dq

[CDNpontiac409guy]

DQ, I don't think that it's so much, "how much do you GAIN from leaving the counterbore"?, as it is, "how much do you lose by opening it" ?
Compression ratio for a given piston is calculated by the manufacturer based on a given combustion chamber size ( in our case, most of that combustion chamber is in the block ). If we open that up at all, we lose compression ratio.
How much ?
Would be very difficult to calculate mathematically... but a cc measurement would be easy.
NHRA calls for 65.8 cc in the block at TDC. This gives a compression ratio of about 11.2:1 on a stock cubic inch engine. As you increase cubic inches ( in my case, 423 ), you will increase the compression ratio... so, a reduced dome volume on the piston would be required.

I would never bore the counter bore, unless the over-bore of the cylinder made it necessary ( over about .060" ).

[dq409]

Originally posted by CDNpontiac409guy
DQ, I don't think that it's so much, "how much do you GAIN from leaving the counterbore"?, as it is, "how much do you lose by opening it" ?
Compression ratio for a given piston is calculated by the manufacturer based on a given combustion chamber size ( in our case, most of that combustion chamber is in the block ). If we open that up at all, we lose compression ratio.


EXACTLY !!! So by not having the counter bore you would decrease the CC`s thus increasing the compression ! YES?

And with going from a counter bore of .060 to a smooth bore that should increase the compression by a measurable amount,,, Thus free HP !!

I like it !!!,,,:D :D

[fatride]

CPG, Without looking, I swear I read that increasing the bore on a 348/409 would not increase the comp ratio because you would be increasing the combustion chamber at the same time (because the combustion chamber is in fact in the bore). The only way to increase the comp ratio on a 348/409 would be to change the shape of the piston!
:confused:

[Skip FIx]

Decking the block should increase compression ratio as it makes chamber shallower.

[jim_ss409]

Hey DQ. Are you going to go with a stroker crank? What about the cam are you going to use the one you have now? And how much compression and are you going to use the gapless rings? Or have you even decided all this stuff yet. Just curious.

[CDNpontiac409guy]

No Fat.... boring the cylinder ( up to .060" ) does not enlarge the counter-bore... which IS the combustion chamber. The cyl wall itself, of course, does not surround the combustion chamber.

[fatride]

Ok, I see. Thanks.
:cheers

[dq409]

Jim, I`m using the stock crank in this engine with the same roller cam.
The compression is just over 11.1
Having the heads reworked, new bronze guides and new Scorpion rollers.
Other then that it will be the same engine.
O,,,Yeh,,,,, Childs and Albert gapless rings,,,,,,,,,

Once this engine is is running the way it should I`ll do some work on therear end, body and interior.

Somewhere down the road,,,maybe next winter, I`ll think about doing a stroker.

I already have another block being preped and ready to go,,,,,,
I also have a set of 583`s that are cleaned and ready for new valves and springs,,,,,

Now back to the counter bore question.

Maybe I`m a little thick headed ,,,,,,,but it seems to me that the counter bore,,, at the top of the cylinder where the compression chamber is ,,,,, if it is a smooth bore rather then counter bored would make a smaller compression chamber,,,
I know that the piston rises above this area,,, but without the over cut around the piston at TDC, from the top of the block to the top of the rings, THAT area would be smaller CC`d bringing up the compression !

Am I all wet ? Or am I missing something ?

Go ahead hammer away at me,,,,,, dq

[SSpev]

Yes a small over bore (smooth) would yield a higher CR. ANYTHING that changes the volume in the combustion chamber or the cylinder will change the CR.

But consider valve shrouding and valve clearance. For such a small amount of compression is it worth it?

This brings up a question. I think I will start a new thread.

[Bungy]

DQ, Yes you are right on. An engine with the overbore at the top will have less compression then one without the overbore. .080" is a lot of metal to remove and that would definately drop the compression. Exactly how much I don't know, I would have to go measure a block to figure it out. As far as shrouding the valves or valve clearance, this would have nothing to do with either. If anything boring the entire cylinder to get rid of the overbore at the top would unshroud the valves and give more clearence.

[Phil Reed]

My question.......when you order 11:1 pistons in any oversize....don't you get "11:1" regardless of any counter bore? In otherwords, they compression ratio and dome height is figured in when they build the piston.

You don't "lose" any compression because that is engineered into the piston configuration.:confused: :confused: :confused:

[CDNpontiac409guy]

Phil, I discussed this at length with the guys at Ross. Their piston dimensions are arrived at mathematically, but do NOT take into account the potential for the enlarging of the counterbore. THAT figure ( dimension.... volume ) is assumed to be constant.

As Sspev said, the only sure way is to cc it.


BTW, you like engines that you can get an oil can in the cyl ?
I have a 460 F*** here that you've got to have:deal :p

[SSpev]

bungy, If you bore big enough to eliminate the counter bore AND put BIG valves in you can get in to shrouding. If the valve lift is HIGH enough you can also have shrouding. If this was not a potential problem, racers wouldn't use truck blocks or cut the cylinder notch in car blocks. I know, I know. Most people won't get this far...but it can happen.

Tom

[SSpev]

Phil, I have found that piston are relatively close to the same. Factory is the worst and it gets better as you buy better quality. Ross is GOOD! The same goes for rods. Piston pin height and stroke also contributes. A long rod (.004) a tall pin height (.003)and a long stroke (.003) Total .010 can change CR .25 points. If you have a hole with short parts in it, you can have a unbalance of half a point CR :eek: Valve placement, spark plug protrusion, and small changes in the head also add up. It is a time consuming process to check. But i think it is worth doing. Anyone can do it at home with a mic or good calipers.

Tom

[SSpev]

On other thing Phil, Buick 455 (4.3125) should hold an oil can too. AND... 400 Chrysler (4.34)

Oh yea... Were those head bolts really worth that much???:cry ;)

[Phil Reed]

Old story...I need some and some were available!!!!:D :D

[Bungy]

SSpev, I guess I misunderstood what you were saying. I thought you ment eliminating the overbore would cause valve shrouding. But I agree, Installing larger valves and/or a high lift cam can cause these problems. Bungy