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 !

Hey guys,

To update this, I just CC’d my heads and got the deck height numbers, does this all check out?

Target CR 8.5:1.
96x66
1911

-Heads CC’d at 49.5. Let’s call it 50cc.
-Deck Height came out to .076
-No shim
-Calculated CR = 8.6:1

This sounds like a decent and safe bump. Is my desk height too high?
Seems reasonable, but I know 0.045 is a solid target.

Not looking for a high compression build. Just a strong cruiser.


Thanks!

I calculated 8.5:1 via CB performances website. Other than that it looks good. I'm not sure what higher deck height does other than give you more clearance to drop down. Someone with better engine knowledge can chime in. I just know you don't want to go too low especially with flat top pistons. I think the min on flat top is somewhere around .036 in.

Will someone with more knowledge than I please explain "squelch" . The closer the deck height you make a cool zone where fuel doesn't burn right away and therefor cools the head and prevents preignition. I usually try to provide a little more than 4 thousands Al

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.

Thank you for the fantastic response @superhawk996 ! No assumption - my heads were liquid CC’d this morning at 49.5.

I did a fresh total stock head rebuild. (I probably should have done bigger valves, but I went off some good advice from Dave D of pelican parts that stock valves will be sturdy) alas, here we are.

My engine guy still have my heads, so I can still have him port and polish.

For clarity, if I port and polish, that will make up for the loss of CC (from fly cutting to 105mm). And... allow me to add a shim .020 or so.

Thanks again for the well thought out response.

I want to say when I was calculating DH on my 2056 rebuild I had a shop mill the bottom part of the cylinder effectively bringing the heads closer to the top of the piston, lowering my deck height number.

No. Port and polish work is upstream of the valve head inside the intake and exhaust ports. It doesn't affect the CC's of the head.

Yup, that would do the trick too!

Always more than one way to skin a cat!

If my deck height is .076 and I mill .020 off the base of the jugs on the Bridgeport at work... My DH is now .056 thou, and we are good to go?
No changes in the connecting rods or anything like that.

No necessity to port.

Thank you guys for helping my grasp this.

Before you cut anything make sure you're evaluating your Compression Ratio at that new deck height target of 0.056" and make sure it's where you want it.

Per your OP

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

Estimating: if you put the deck height at .056 you're going to be around a 8.95 (let's round to 9:1) with a 50cc head.

If you want an 8.5:1 CR you're still going to have to remove about 2-3cc worth of head material.

A 9:1 CR will give you more power, but also more heat that you will need to shed.

On my first 2056 iteration my CR was 9.2:1. This was before I even knew about setting CR, and just made my deck height .040 and ran with it. I did so many wrong things with that first build! Engine was really zippy but it was a bitch to keep cool. There were other contributing factors as well.

When I redid that engine to its current config I set the CR at 8.7ish:1 (I don't remember off hand, but I have it on a note somewhere.)

Zach

Thanks a lot everyone.

I’m expecting to have done a lot wrong, but that’s part of the fun I guess. Just want to not have a total dog on my hands and keep er cool.

I’ll keep you updated.

What brand of P&C are you using? The reason I ask is that, in my 2056 build which I'm in the middle of now, the AA pistons I bought gave me a deck height over 70 with no shims. I spoke with @mark henry who recommended KB pistons and cylinders instead, and the deck came in at .040. Much better. What's weird is that I took a caliper to the jugs and the KB and AA measured exactly the same from the sealing base to the top. There must be something about the way the AA pistons are cast that makes the top closer to the wrist pin.

Using AA. (Unfortunately). How’d you guess

Funny you say that, I was looking at the KB pistons FAT uses and was considering advantages of swapping them in (same cylinders)

That explains the over .070 with no shims.

Mine were AA as well with my 2056 rebuild. Milled .014 I believe.