Thinking out loud - 1911, Found my deck height. All good? Options

[jaredmcginness]

See update and actual numbers on the added reply.


Sorry for the novel!!

Hey guys,

I'm building a 1911. I want to make sure that I have my thoughts in the right place.

-96 Bore x 66 stroke. Fresh rebuilt 1.7 heads with a 105mm fly cut register. Stock valves

-Running a .495 lift cam, lifters to match, HD springs + counter bore .630 (IIRC) extended.

I need to CC my heads first, get the volume, then determine my deck height and compression ratio? Stock 1.7 heads are 50cc. I assume mine will end up a tad larger, maybe 52cc. (will the fly cut alter the size?) But I might be off.

Is 8.5:1 a good target ratio? Ive read a few threads and this seems to be the middle ground compression. A good target DH is .045-.065? With a minimum of 0.040 to avoid contact.

A few examples - per a CR calculator:

50cc, 0.065 = 8.7:1
50cc, 0.045 = 9.2:1

53cc, 0.065 = 8.4:1
53cc, 0.045 = 8.8:1

This in mind, depending on my head volume it should be easy to narrow down where I need to be.

Am I on the right track? New to this and been digging into some old books.

Whats a good compression ratio, that should be safe for a mild carbed (dual 36) motor and mild cam. Want this thing to last more than a summer! I dont really have a target CR, because I dont know the difference of a few decimal points!

Happy 4th (IMG:style_emoticons/default/beerchug.gif) !

[Superhawk996]

Generally speaking you're going to end up with more deck height vs. stock with the 96mm flat top piston assuming you keep the heads exactly the same and target the same compression ratio.

You're effectively squeezing more air into the same head volume. That would raise CR.

The two knobs at your disposal are :
1) increase deck height
2) start carving away the heads away to increase CC's in the head.
3) remove 0.010" from whatever base shim you're using. Since you said 0.076" without a base shim, this really isn't feasible, leaving you with knobs #1 and #2

Personally I'd rather have more deck height than less. Minimum deck height should leave some allowance for rod stretch and crank deflection, etc. That is where the typical numbers of .035" to .045" come from.

0.076" does seem a bit large for deck height. When deck height gets too large you risk losing some of the turbulent mixing that occurs as the piston comes to top dead center and is pushing all that air into a much smaller space. I don't have a lifetime of experience on this like others on the forum do since I don't build engines every day.

Bottom line, it would be safe, but maybe down on power ever so slightly if you lose some turbulent mixing.

I think Al was referring to Squish or Quench (I've not heard of Squelch)?
https://en.wikipedia.org/wiki/Squish_(piston_engine)

If it were me, I'd probably do a little material removal off the head chamber to increase chamber CC's before I went all the way to 0.076" of deck height without a base shim.

Maybe reach out to Jake, Mark, or Len -- They would have by far the most relevant experince on this topic.


When I built my original 1.7L out to 1911 I don't remember having this issue and I know I used a base shim. May be production variance in the Head CC's? Also the flycutting will reduce CC's. Material removal from the head brings the head closer to the crank centerline. If you haven't actually measured your heads - you should. Seems that you are sort of assuming they are close to to the stock CC's.