Dynamic Compression Ratio

[L78fanatic]

Anyone have a concise (i.e. easy) method to calculate DCR? I'm struggling with "Quench", head gasket selection, etc. for my '67 L78 build. My SCR will be 11.5 to 11.6:1 if I worry about quench and stay with a shim gasket thickness (0.020"+/-), or go with a Fel-Pro .039" thick composite gasket (My preference, which hypothetically will yield 11:1 SCR)....I am assuming .020" in the hole since my block has not been assembled yet (still waiting on my 38.8 cc domed pistons).

I'm told DCR is more important when analyzing how well the engine might run on 93 octane than merely looking at SCR. My Howards cam is identical to a stock L78 flat tappet cam except for slightly more "lift". A friend on TC has an already built '69 L78 engine which I am following somewhat with respect to main elements of the build, including the cam, block & 840 heads, and his '69 L78 runs swell on 93 octane with the Fel-Pro .039" gasket and quench of about .063. I believe the DCR is one reason for this.

So, any help on this would be appreciated. I've reviewed the one calculation article that I found on TC, but honestly, it's a bit confusing for a novice! I believe in "kiss"!

Thanks,

John

 

[Jasons69Chevelle]

You just use piston deck height and compressed gasket height.
Quench is just piston to head clearance.

You will need the piston height out/in the bore at top dead center.
So you will need the piston before setting the quench. By selecting the the gasket thickness.
Chances are the height will be different in each bore.

.038-.043 is what I’ve read is optimal with steel rods/ hi po street engines
each engine is different though.

Dcr you will need the cam specs, altitude, and multiple engine specs
If your .02 in the bore and a .039 gasket your at .059 quench.

 

[77cruiser]

You can download & use this program.

Dynamic CR

cochise.uia.net

 

[BillyGman]

I know this thread is a little old by now, and I'm late arriving to the party here, but this is what I use...(for those who don't already know, keep in mind that "IVC" means Intake Valve Closes...

https://www.gofastmath.com/Compress...atic_And_Dynamic_Compression_Ratio_Calculator

and for just static compression ratio alone, I like to use this one below....

https://www.rbracing-rsr.com/compstaticcalc.html

...and as far as the importance of "squish" goes,(AKA "quench") it's definitely important for an engine with steel connecting rods running pump gas to have a .035" to .045" quench height to avoid damage and to avoid pinging/detonation. If you let it go too big, (such as over .055") you are more likely going to get pinging with pump gas. Here below is from an article in the Speed-O-Motive engine parts catalog. I recently had a phone conversation with the expert professional engine builder Scott Foxwell, and he fully agreed with this...

 

[Molly's 70]

Welcome Billy G Man!

 

[cage2592]

Yes Welcome ! Better late than never !

 

[BillyGman]

Thanks for the welcome. I didn't even know about this board until today.

 

[Chevelle_Nut]

Thanks for the welcome. I didn't even know about this board until today.

Welcome aboard! Yes the other site is hiding links from everybody. Spread the word, the more people the better.

 

[L78fanatic]

I know this thread is a little old by now, and I'm late arriving to the party here, but this is what I use...(for those who don't already know, keep in mind that "IVC" means Intake Valve Closes...

https://www.gofastmath.com/Compress...atic_And_Dynamic_Compression_Ratio_Calculator

and for just static compression ratio alone, I like to use this one below....

https://www.rbracing-rsr.com/compstaticcalc.html

...and as far as the importance of "squish" goes,(AKA "quench") it's definitely important for an engine with steel connecting rods running pump gas to have a .035" to .045" quench height to avoid damage and to avoid pinging/detonation. If you let it go too big, (such as over .055") you are more likely going to get pinging with pump gas. Here below is from an article in the Speed-O-Motive engine parts catalog. I recently had a phone conversation with the expert professional engine builder Scott Foxwell, and he fully agreed with this...View attachment 1022

View attachment 1023

Welcome to the forum! I started this thread and it's still very active....it takes time sometimes to get noticed! I will look over your comments and follow up.

John

 

[KwikSS]

BillyGman is a chassis plotting guru

 

[L78fanatic]

I get SCR = 11.1 and DCR = 8.82:1, again assuming .020" in the hole (I won't know what it is until the engine is assembled. I don't believe the heads gave been planed, and the block has not been decked). The cam really makes a difference in calculating the DCR apparently.

Using the separate SCR calculator gives me 11.12:1

 

[Shovelrick]

If your block is still at machine shop and assembly hasn't started yet ask if they Square'd the block or checked it for square, I'd also rec'd torque plate honing and align or line checking or boring the mains, after the shop did that to mine and assembled the balanced rotator from Wolfplace it spun like it was on roller bearings, nothing like a balanced & blue printed engine, Hey Billy welcome aboard

 

[L78fanatic]

If your block is still at machine shop and assembly hasn't started yet ask if they Square'd the block or checked it for square, I'd also rec'd torque plate honing and align or line checking or boring the mains, after the shop did that to mine and assembled the balanced rotator from Wolfplace it spun like it was on roller bearings, nothing like a balanced & blue printed engine, Hey Billy welcome aboard

Thanks Rick! I haven’t chosen an engine builder yet. The heads need machine work but the block was already done before I bought it. I can certainly have it checked again though. I definitely will have it balanced and blueprinted.

 

[Shovelrick]

I bought my 1975 "959" 4 bolt block with ARP main and head studs all machined and "Ready to go" from my mechanic friend that owns the shop that works on my DD after the race engine builder got it he callsd for the news lol