Compression Ratio and Quench question.
Melting Slicks
Joined: Dec 2005
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You can't judge piston depth with the naked eye when you're checking for .001-010", you need a straight edge and a feeler gage.
Thread Starter
Melting Slicks
Joined: Sep 2004
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Motorhead, how do I check dynamic compression?
I did have stamped numbers on the block from what I can remember. I remember finding that the block was a 4 bolt 80-85 probably originally a 305 from a truck.
Also, gkull you might be on to something with the spark issues. My tach does jump around inconsistently after 4K rpm (as I mentioned in another post) I've really just been shifting by ear. Which is a shame because I have an autometer tach and MSD dist, should be no problems.
I did have stamped numbers on the block from what I can remember. I remember finding that the block was a 4 bolt 80-85 probably originally a 305 from a truck.
Also, gkull you might be on to something with the spark issues. My tach does jump around inconsistently after 4K rpm (as I mentioned in another post) I've really just been shifting by ear. Which is a shame because I have an autometer tach and MSD dist, should be no problems.
Thread Starter
Melting Slicks
Joined: Sep 2004
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What were the other cylinders pressures? What I'm wondering is if the 150lb cylinders were within 10% of the others. If the other cylinders aren't within 10% (~165) there are mechanical problems.
You can't judge piston depth with the naked eye when you're checking for .001-010", you need a straight edge and a feeler gage.
You can't judge piston depth with the naked eye when you're checking for .001-010", you need a straight edge and a feeler gage.
Melting Slicks
Joined: Dec 2005
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DCR is a lot more involved to figure. I found a web site that will calculate this but I'd check it against others. Motorhead or others may have one they've used and verified the accuracy.
http://members.uia.net/pkelley2/DynamicCR.html
http://members.uia.net/pkelley2/DynamicCR.html
Le Mans Master
Joined: Sep 2001
Posts: 7,739
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From: Unreconstructed, South Carolina
Based on info provided ... static compression is about what Matt G found ... I see about 9.3 to 9.4:1 with NO machinework to deck & piston 0.025" down in hole. Suggest your machine shop's 10.2 quote is suspect.
handy, accurate scr calculator:
http://www.21cgt.com/FMWebCatalog/frmConversion4.aspx
-edit-
W/ typical 0.040 hg, quench is probably non-existant. Thinner GM composite head gasket 10105117 (0.028" x 5.8cc) will get your quench closer and should yield about 9.6 to 9.7:1 scr.
Some guys claim thin steel shim hg felpro 1094 (0.015" x 3.2cc) works OK w/ their Alum heads ... I dunno if OK ... but would probably yield a good 0.040" quench and about 9.9 to 10:1 scr.
handy, accurate scr calculator:
http://www.21cgt.com/FMWebCatalog/frmConversion4.aspx
-edit-
W/ typical 0.040 hg, quench is probably non-existant. Thinner GM composite head gasket 10105117 (0.028" x 5.8cc) will get your quench closer and should yield about 9.6 to 9.7:1 scr.
Some guys claim thin steel shim hg felpro 1094 (0.015" x 3.2cc) works OK w/ their Alum heads ... I dunno if OK ... but would probably yield a good 0.040" quench and about 9.9 to 10:1 scr.
Last edited by jackson; Oct 9, 2007 at 10:18 AM.
Le Mans Master
Joined: Oct 2002
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From: Las Vegas - Just stop perpetuating myths please.
Low compression pressure.
For example my c.r. is only 8.8 (due to miscalc on my part - wrong piston numbers) and my compress is 180-190psi all cyls. I got good compression because a got a good matching cam. Sorry for the bad news but u really need a smaller cam at least or tighter heads worse case.
But something doesn't make sense torquevette - how u got 383" from a 305" block. As far as i know u can't bore a 305" block out to 4" bore let a lone 4.030" for a stroker.
I have my doubts now with what that machine shop sold u.
cardo0
But something doesn't make sense torquevette - how u got 383" from a 305" block. As far as i know u can't bore a 305" block out to 4" bore let a lone 4.030" for a stroker.
I have my doubts now with what that machine shop sold u.
cardo0
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Le Mans Master
Joined: Dec 2003
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From: Fremont CA
Quench is VERY IMPORTANT to get correctly around 0.040" is the figure you want. Many shops use a deck sander for surfacing blocks (I wouldn't use a shop that uses this method), my shop uses a grinder with a BHJ "Block True" fixture to get the decks square/equal heigth to the crankshaft/camshaft centerline of the block. Small Blocks have about a 9.025" deck heigth and it is common to deck to "zero deck" of 9.00" whan blueprinting a motor. If your quench is too big (over 0.048") there are many different head gaskets on the market to get it around the desired 0.040" dimension.
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Melting Slicks
Joined: Sep 2004
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I forgot to add, the dynoshop also predicted that my cam might not have been degreed properly. Is this possible?
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
So who has dyno numbers for 2 identical motors, the only difference is one has .040" quench and the other one has .060" quench I would love to know how much more HP I am going to gain on the 427ci small block I am building right now ?
torqvette I get 9.4 SCR and 7.2 DCR, 7.2 DCR is way too low and the reason it does not make the power it should
The cam it too big for the compression ratio
torqvette I get 9.4 SCR and 7.2 DCR, 7.2 DCR is way too low and the reason it does not make the power it should
The cam it too big for the compression ratio
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
tight quench causes fuel atomization - for a faster burn rate. Less wasted fuel going out the exhaust pipe. every extra foot pound of TQ helps
Le Mans Master
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From: Seattle WA
St. Jude Donor '14
Vizard zero-decks virtually every engine he builds; that's good enough for me. 
As noted previously it's not about power - directly - it's about detonation sensitivity. The ability to run 36 degrees of advance is key to power; having to pull back the timing on pump gas means you didn't build for the street.
As noted previously it's not about power - directly - it's about detonation sensitivity. The ability to run 36 degrees of advance is key to power; having to pull back the timing on pump gas means you didn't build for the street.
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
That is a good answer, the best I have heard yet and makes sense.
If I run into detonation problems I would be looking into it. When you run a big cam you start to get into piston to valve clearance problems with reduced quench. I know I could not get mine any lower I was at the limit with .060". GM has been putting out motors for 50+ years with .060+ quench.
If I can advance the timing to 40 degrees on my 406ci and run 91 octane gas and make near 600HP then I am not going to worry about quench. Alot of theory is written about the subject yet I don't see any real world data.
If I run into detonation problems I would be looking into it. When you run a big cam you start to get into piston to valve clearance problems with reduced quench. I know I could not get mine any lower I was at the limit with .060". GM has been putting out motors for 50+ years with .060+ quench.
If I can advance the timing to 40 degrees on my 406ci and run 91 octane gas and make near 600HP then I am not going to worry about quench. Alot of theory is written about the subject yet I don't see any real world data.
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
That is a good answer, the best I have heard yet and makes sense.
If I run into detonation problems I would be looking into it. When you run a big cam you start to get into piston to valve clearance problems with reduced quench. I know I could not get mine any lower I was at the limit with .060". GM has been putting out motors for 50+ years with .060+ quench.
If I can advance the timing to 40 degrees on my 406ci and run 91 octane gas and make near 600HP then I am not going to worry about quench. Alot of theory is written about the subject yet I don't see any real world data.
If I run into detonation problems I would be looking into it. When you run a big cam you start to get into piston to valve clearance problems with reduced quench. I know I could not get mine any lower I was at the limit with .060". GM has been putting out motors for 50+ years with .060+ quench.
If I can advance the timing to 40 degrees on my 406ci and run 91 octane gas and make near 600HP then I am not going to worry about quench. Alot of theory is written about the subject yet I don't see any real world data.
An ideal timing curve would actually pull timing out over 6000 rpm
Le Mans Master
Joined: Jun 2004
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From: Seattle WA
St. Jude Donor '14
Definitely agree that it would be great to see back-to-back dyno tests with just a gasket change.
Also agree quench is something that needs to be planned into the build and carefully measured on the extremes - 99.999% of the engines I build are < 1.2 HP/CID, so I haven't run into any issues personally with a zero deck. I have some builder friends that are building more extreme beasts, and they have milled the reliefs to get to .040 quench. But again, nothing objective you can grab onto.
The General used the steel shim gaskets on some of the HP beasts; with an .015 thickness and the pistons .025 down in the bore that's .040.
Not sure if there's benefit to be found beyond 32-36 degrees - most of the dyno runs I've seen even on the hot engines don't seem to drop power after 36 degrees.
I dropped an email to CarCraft asking for another one of their dyno flogs around this - maybe we'll get to see if there's anything really to it.
Also agree quench is something that needs to be planned into the build and carefully measured on the extremes - 99.999% of the engines I build are < 1.2 HP/CID, so I haven't run into any issues personally with a zero deck. I have some builder friends that are building more extreme beasts, and they have milled the reliefs to get to .040 quench. But again, nothing objective you can grab onto.
The General used the steel shim gaskets on some of the HP beasts; with an .015 thickness and the pistons .025 down in the bore that's .040.
Not sure if there's benefit to be found beyond 32-36 degrees - most of the dyno runs I've seen even on the hot engines don't seem to drop power after 36 degrees.
I dropped an email to CarCraft asking for another one of their dyno flogs around this - maybe we'll get to see if there's anything really to it.
Thread Starter
Melting Slicks
Joined: Sep 2004
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So who has dyno numbers for 2 identical motors, the only difference is one has .040" quench and the other one has .060" quench I would love to know how much more HP I am going to gain on the 427ci small block I am building right now ?
torqvette I get 9.4 SCR and 7.2 DCR, 7.2 DCR is way too low and the reason it does not make the power it should
The cam it too big for the compression ratio
torqvette I get 9.4 SCR and 7.2 DCR, 7.2 DCR is way too low and the reason it does not make the power it should
The cam it too big for the compression ratio
Motorhead, what SCR should I aim for? And do you know what the resulting DCR will be? Do you still say 10:1 not higher? Also, it might be relevant to mention that I will be using nitrous in the near future.
Last edited by enkeivette; Oct 10, 2007 at 03:25 PM.
Pro
Joined: Jul 2000
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From: Suffolk New York
An answer, you're my hero! When I get my POS daily driver working again I will be pulling the heads and having some work done.
Motorhead, what SCR should I aim for? And do you know what the resulting DCR will be? Do you still say 10:1 not higher? Also, it might be relevant to mention that I will be using nitrous in the near future.
Motorhead, what SCR should I aim for? And do you know what the resulting DCR will be? Do you still say 10:1 not higher? Also, it might be relevant to mention that I will be using nitrous in the near future.
High Compression and High DCR is not going to be good, you'll be building a new motor!
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
An answer, you're my hero! When I get my POS daily driver working again I will be pulling the heads and having some work done.
Motorhead, what SCR should I aim for? And do you know what the resulting DCR will be? Do you still say 10:1 not higher? Also, it might be relevant to mention that I will be using nitrous in the near future.
Motorhead, what SCR should I aim for? And do you know what the resulting DCR will be? Do you still say 10:1 not higher? Also, it might be relevant to mention that I will be using nitrous in the near future.
I would bump it up to at least 10:5 : 1 and near 11:1 would be best if you are keeping that cam or are going to run a similar cam DCR will be around 8.5:1 and it will make the power it is supposed to.
Guys, when I mentioned 40 degrees advance, I had it set there by mistake one time, was running around for some time before I caught it. Never pinged or anything, it was actually over 40 degrees when I put the timing light on it and caught it. I usually run mine in 32-34 degree range
Last edited by MotorHead; Oct 10, 2007 at 07:14 PM.