CR vs DCR?
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
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DCR IS a real thing....it's what it's all about.
BUT...measuring/calculating it is a pretty tough thing. If I followed all the calculations....my motor could use well over 12.5 compression and run just fine on pump gas. I may not be the brightest out there...but I can tell you..that just ain't happening!
One huge factor that is often overlooked other than excellent tuning when running at the edge...is volumetric efficiency and RPM. Eventually....a really good set of heads is going to start getting those cylinders really full as RPM goes up. It's entirely possible to operate an engine below that range where the heads are really doing their best and stay out of detonation. The large port heads etc I think helps that equation. The nice small port ones will probably bring it on earlier. So what an engine does under full power at 5500 rpm will be a LOT different at 6500-7000 rpm when the race parts are doing what they are designed for.
Also, real world conditions like temps, altitude etc all come into play here. I lived in Denver for a few years and WAS running nearly 13.0 compression in a 427 on the street with pump gas using iron heads and a big by large solid roller cam. But back in TX at seal level and at 100* outside....no way.
Also, take all the magazine tests with a grain of salt. Dyno testing with controlled environments and temps is a LOT different that real world street use.
But I personally wouldn't build anything with 9.5 compression unless it was a boat maybe and I wanted to hammer it on *dock gas*. Iron heads can safely be in the 10.0 range easily. We have a 383 in our group with an honest 10.2 using double hump heads and a tight 107 LSA cam that has 210-220 psi cranking compression....and it does fine.....even with a close ratio trans and 3.08 gears. And it runs killer too!
But my 540 is much lower than that on cranking compression (11.06 static). I race/dyno and drive on 93 octane just fine. Race gas does nothing for it. I could probably sneak it up a little higher..to the 11.2-11.5 range..but that little bit might put me in a position to have to retard timing...and I don't think it's worth it.
BTW- my motor has the pistons OUT of the hole at .009". Total quench of .028. The 383 has them OUT of the hole .002 with a total quench of .038.
JIM
BUT...measuring/calculating it is a pretty tough thing. If I followed all the calculations....my motor could use well over 12.5 compression and run just fine on pump gas. I may not be the brightest out there...but I can tell you..that just ain't happening!
One huge factor that is often overlooked other than excellent tuning when running at the edge...is volumetric efficiency and RPM. Eventually....a really good set of heads is going to start getting those cylinders really full as RPM goes up. It's entirely possible to operate an engine below that range where the heads are really doing their best and stay out of detonation. The large port heads etc I think helps that equation. The nice small port ones will probably bring it on earlier. So what an engine does under full power at 5500 rpm will be a LOT different at 6500-7000 rpm when the race parts are doing what they are designed for.
Also, real world conditions like temps, altitude etc all come into play here. I lived in Denver for a few years and WAS running nearly 13.0 compression in a 427 on the street with pump gas using iron heads and a big by large solid roller cam. But back in TX at seal level and at 100* outside....no way.
Also, take all the magazine tests with a grain of salt. Dyno testing with controlled environments and temps is a LOT different that real world street use.
But I personally wouldn't build anything with 9.5 compression unless it was a boat maybe and I wanted to hammer it on *dock gas*. Iron heads can safely be in the 10.0 range easily. We have a 383 in our group with an honest 10.2 using double hump heads and a tight 107 LSA cam that has 210-220 psi cranking compression....and it does fine.....even with a close ratio trans and 3.08 gears. And it runs killer too!
But my 540 is much lower than that on cranking compression (11.06 static). I race/dyno and drive on 93 octane just fine. Race gas does nothing for it. I could probably sneak it up a little higher..to the 11.2-11.5 range..but that little bit might put me in a position to have to retard timing...and I don't think it's worth it.
BTW- my motor has the pistons OUT of the hole at .009". Total quench of .028. The 383 has them OUT of the hole .002 with a total quench of .038.
JIM
Race Director
Joined: May 2005
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From: Millington Illinois
St. Jude Donor '08-'09
MotorHead, I'm in the same boat with you, in that I've noodled every detail since the first custom stroke, full-on racing engine I built nearly 3 decades ago. Back in the day, DCR wasn't something I gave much thought to. I knew it existed, tho I didn't know what it was called or what it should be, but I've gone back and found that the static CR / cam combinations of the better street build-ups I've done weren't too far off from these DCR rules of thumb which are coming, more or less, into acceptance. Apparently the cam techs I've paid attention to over the years didn't just fall off the last turnip truck to drive by, tho I wonder about some of them nowadays...
I'm still with you on <7.5:1 DCR leaving a good chunk on the table, particularly with SB's. When deciding to step the cam selection up for my pump gas, solid roller 496 BB, I elected to allow the DCR to fall to about 8:1 to leave some margin for top end cylinder pressures - something I figure I can afford to do since torque certainly won't be lacking. Such reasoning aside, 8.25:1 DCR should be a good target for most alum head hot rod street engines, but mid-high 7's is fine for "milder" builds.
I'm also OK with cranking pressures considerably above what is commonly advised in the popular press, as long as everything is right. It's not that precise of a reference point anyway, so I don't get too caught up in exactly where the needle points, unless it's way off from what I expect. Figure I'll likely see 200psi on the 496...
However, I feel it should be noted that the vast majority of those umpteen-million GM mills with >.060" quench operated no where near the limits of 92 octane gas, and that Mr. Yunick (who knew more about Chevy engines than about anyone who ever lived) found that a good tight quench is indeed important in truly high-performance engines. IMHO, accepting Smokey's case on this matter, until someone of similar stature shows that things have changed, shouldn't necessarily define anyone as unknowledgeable, by whatever means they may have come by it.
No offense taken, and none intended. Just my story and I'm sticking with it...
I'm still with you on <7.5:1 DCR leaving a good chunk on the table, particularly with SB's. When deciding to step the cam selection up for my pump gas, solid roller 496 BB, I elected to allow the DCR to fall to about 8:1 to leave some margin for top end cylinder pressures - something I figure I can afford to do since torque certainly won't be lacking. Such reasoning aside, 8.25:1 DCR should be a good target for most alum head hot rod street engines, but mid-high 7's is fine for "milder" builds.
I'm also OK with cranking pressures considerably above what is commonly advised in the popular press, as long as everything is right. It's not that precise of a reference point anyway, so I don't get too caught up in exactly where the needle points, unless it's way off from what I expect. Figure I'll likely see 200psi on the 496...
However, I feel it should be noted that the vast majority of those umpteen-million GM mills with >.060" quench operated no where near the limits of 92 octane gas, and that Mr. Yunick (who knew more about Chevy engines than about anyone who ever lived) found that a good tight quench is indeed important in truly high-performance engines. IMHO, accepting Smokey's case on this matter, until someone of similar stature shows that things have changed, shouldn't necessarily define anyone as unknowledgeable, by whatever means they may have come by it.
No offense taken, and none intended. Just my story and I'm sticking with it...
Well said! 
My most recent build figures out to 10.4 static, 8.25 DCR but quench is .039, Dewitt's Radiator, Aluminum 195 heads, hydraulic roller. I know I could pull more power out of it with a bigger solid roller, bump up the compression, bigger heads but chose a more street friendly setup and left some power and compression on the table.I built a 440 Mopar about 5 years ago with a buddy of mine. 11.5 to 1 solid flat tappet, 1050 carb, aluminum heads. It was NA and dialed in on the dyno. Ran unbelievebly strong. pulled over 600 HP and almost 700 ft. lbs.torque. When it blew up and we tore it down the tops of the pistons had eroded , some worse than others and 2 had broken wedging pieces between the rest of the piston and head. Broke one head and chewed the other up. Screwed up the cylinder walls on 2 cylinders and cracked the block through to a water jacket. I know it was detonation and honestly believe it was broght on by improper quench ( didn't deck the block to save the #s). Never heard a thing until it came apart and it was well tuned and maintained but was apparently detonating at higher RPM. I had the same discussion with him during the build that we are having here but he insisted that he wanted to maximize his power. This was a rebuild of the original engine for his 70 Roadrunner.
It was a costly lesson for him (over $10,000 and lost his #s matching block) and I. If you are not building an all out race engine, err on the side of caution, leave some power on the table, and try to maintain streetability. There is always someone faster. Sorry for the hijack.
Last edited by 63mako; Nov 30, 2007 at 09:46 AM.
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Great discussion
As far as detonation at higher RPM's I had the motor out and put on an engine dyno and it was pulled in the 6500-7000RPM range and if there was a problem these guys would have know, the motor was also still making power above 6500RPM.
Now I agree with you guys, for most of us here building mild street motors it is better to be conservative especially if you want a plug and play motor and don't want to be messing with it all the time.
230psi is on the edge however I think you can get by with 240 psi on 94 octane. I am pushing the edge and will be even more so on the new 427ci in my basement. 63Mako, I will shoot for a .040" quench too, I never really meant there wasn't anything to a tight quench, as a matter of fact I would have done it on my current 406ci but the piston to valve clearance would have been past the limit.
I would like to see some real world tests on the exact same motor, one with .040" and then with .060" quench and see if one makes more power than the other and also see if one runs into detonation problems. So for all I have seen is theory and no real tests. I like to see things to believe them.
As far as detonation at higher RPM's I had the motor out and put on an engine dyno and it was pulled in the 6500-7000RPM range and if there was a problem these guys would have know, the motor was also still making power above 6500RPM.
Now I agree with you guys, for most of us here building mild street motors it is better to be conservative especially if you want a plug and play motor and don't want to be messing with it all the time.
230psi is on the edge however I think you can get by with 240 psi on 94 octane. I am pushing the edge and will be even more so on the new 427ci in my basement. 63Mako, I will shoot for a .040" quench too, I never really meant there wasn't anything to a tight quench, as a matter of fact I would have done it on my current 406ci but the piston to valve clearance would have been past the limit.
I would like to see some real world tests on the exact same motor, one with .040" and then with .060" quench and see if one makes more power than the other and also see if one runs into detonation problems. So for all I have seen is theory and no real tests. I like to see things to believe them.
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
I know I am leaving 75-100hp at the top end on the table because of my heads and cam choice, etc. While I realize it is a big consideration and compromise, I still want plug and play. When I am done with the engine, I need to be done. I will be terribly disappointed if I have to fiddle with it all the time to make it run reasonably.
People need to read my posts to understand what I am after. Then offer advice based on what I want in my post.
I am not after what most people are. I think I am going to end up with a fantastic, well thought out, balanced selection of components which will give me one heck of a fun low end torque machine that anyone can own and operate. Even a guy all thumbs with no mechanical aptitude will be able to own this car and have a good mechanic maintain it.
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Now you got it. This thread, my threads, have never been about maximizing power. well, yet anyway
I know I am leaving 75-100hp at the top end on the table because of my heads and cam choice, etc. While I realize it is a big consideration and compromise, I still want plug and play. When I am done with the engine, I need to be done. I will be terribly disappointed if I have to fiddle with it all the time to make it run reasonably.
People need to read my posts to understand what I am after. Then offer advice based on what I want in my post.
I am not after what most people are. I think I am going to end up with a fantastic, well thought out, balanced selection of components which will give me one heck of a fun low end torque machine that anyone can own and operate. Even a guy all thumbs with no mechanical aptitude will be able to own this car and have a good mechanic maintain it.
I know I am leaving 75-100hp at the top end on the table because of my heads and cam choice, etc. While I realize it is a big consideration and compromise, I still want plug and play. When I am done with the engine, I need to be done. I will be terribly disappointed if I have to fiddle with it all the time to make it run reasonably.
People need to read my posts to understand what I am after. Then offer advice based on what I want in my post.
I am not after what most people are. I think I am going to end up with a fantastic, well thought out, balanced selection of components which will give me one heck of a fun low end torque machine that anyone can own and operate. Even a guy all thumbs with no mechanical aptitude will be able to own this car and have a good mechanic maintain it.
AWilson: I think we're on the same page. When I was designing my build, I looked at the same stuff. I kept wondering how much hp I was leaving on the table, but in the end, I followed the advice of Jackson and cardo0 and built an engine with sufficient power, but also more reliable because it was not a high compression machine. I don't have the time or money to do this thing over and over again
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Pretty tight Jim. But that will let you run more DCR safely.BTW, John's iron head, 10.25 comp, 778 HP 548 is on it's way there (runs on 91 octane)!
Let me know how it does.
Last edited by 63mako; Nov 30, 2007 at 05:39 PM.
Le Mans Master
Joined: Apr 2007
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From: Graceland in a Not Correctly Restored Stingray
AWilson, yes we may seem to go astray, but I hope these sidebars may be of use to you and to others following along, as well. While it's true that engines which aren't on the edge don't see the same conditions as racing engines, I believe you'll have a leg up for doing milder, yet still high-performance buildups if you know more about what's going on at that edge.
Back on quench, even if I could I'm not going to spend millions of dollars to re-prove the findings of respected experts in the field, but so the reasoning is better understood by anyone scratching their head over this... Basically, to oversimplify, without a reasonably tight quench, flame propagation is less likely to burn any trapped A/F mix which isn't squeezed into the primary area of the combustion chamber, whatever the performance level of the engine. This unspent mix represents not only untapped power, but also results less than optimum fuel economy, as well as increasing the pollutants exhausted into the atmosphere, which means an engine that doesn't run as clean as it could. I'm not going to get all "green" on everyone, but given the choice I doubt many of us would consciously decide to throw money away on gas that's not going to result in anything except for a bigger carbon footprint. Just food for thought...
Back on quench, even if I could I'm not going to spend millions of dollars to re-prove the findings of respected experts in the field, but so the reasoning is better understood by anyone scratching their head over this... Basically, to oversimplify, without a reasonably tight quench, flame propagation is less likely to burn any trapped A/F mix which isn't squeezed into the primary area of the combustion chamber, whatever the performance level of the engine. This unspent mix represents not only untapped power, but also results less than optimum fuel economy, as well as increasing the pollutants exhausted into the atmosphere, which means an engine that doesn't run as clean as it could. I'm not going to get all "green" on everyone, but given the choice I doubt many of us would consciously decide to throw money away on gas that's not going to result in anything except for a bigger carbon footprint. Just food for thought...
Last edited by TheSkunkWorks; Nov 30, 2007 at 07:59 PM.
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Melting Slicks
Joined: Mar 2002
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From: Beringen
I think with quench one should also look at the type of engine. Meaning if you have a big bore engine, piston rock could be more and there COULD be a chance of the piston hitting the head. Also it depends on clearances in the rod big and small end.
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
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Yeah...mine slightly *polishes* the head,,but I spin her up there pretty high sometimes!
JIM
JIM
Le Mans Master
Joined: Oct 2002
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From: Las Vegas - Just stop perpetuating myths please.
I'm still waiting for a DCR formula and who is Pat Kelly?
Sorry S489 but my software blocks your links for some reason.
Well if DCR calcs where truly that important then why doesn't Comp Cams or Crane Cams or any of the other major cam mfrs have one on there website. Vizard doesn't even print a complete formula in any book nor does Smokey Y. nor Lingenfelter though they discuss its importance. Heck anyone can post something on the internet for free but once they consider that something that is published for profit could well have legal liabilities it becomes hard to find one published in a book. Heck i could even post something i noodled up but how do i know its correct?:o Why not let others prove out for me?
No i won't down load someone else's spreedsheet onto my computer when i can make my own - if i only had a good formula.
Pat Kelly sounds like an honest enthusiest trying to help others and does have some good information posted. But no, i don't trust the free-be stuff from the net.
I guess what i'm saying is that don't bet the farm with a DCR calc and don't try to prove the guys publishing static c.r. numbers wrong with it. Just because MotorHead has a working combo that challenges the convention is no guarentee yours will. I just see the DCR calcs as mostly unproven estimations and perpetuated as a way to run unreasonble c.r.'s or the only way to choose a camshaft.
Now i feel soo much better.
cardo0
Well if DCR calcs where truly that important then why doesn't Comp Cams or Crane Cams or any of the other major cam mfrs have one on there website. Vizard doesn't even print a complete formula in any book nor does Smokey Y. nor Lingenfelter though they discuss its importance. Heck anyone can post something on the internet for free but once they consider that something that is published for profit could well have legal liabilities it becomes hard to find one published in a book. Heck i could even post something i noodled up but how do i know its correct?:o Why not let others prove out for me? No i won't down load someone else's spreedsheet onto my computer when i can make my own - if i only had a good formula.
Pat Kelly sounds like an honest enthusiest trying to help others and does have some good information posted. But no, i don't trust the free-be stuff from the net. I guess what i'm saying is that don't bet the farm with a DCR calc and don't try to prove the guys publishing static c.r. numbers wrong with it. Just because MotorHead has a working combo that challenges the convention is no guarentee yours will. I just see the DCR calcs as mostly unproven estimations and perpetuated as a way to run unreasonble c.r.'s or the only way to choose a camshaft.
Now i feel soo much better.
cardo0
Thread Starter
Drifting
Joined: Jun 2006
Posts: 1,912
Likes: 12
From: Schaumburg (Chicago) Illinois
Sorry S489 but my software blocks your links for some reason.
Well if DCR calcs where truly that important then why doesn't Comp Cams or Crane Cams or any of the other major cam mfrs have one on there website. Vizard doesn't even print a complete formula in any book nor does Smokey Y. nor Lingenfelter though they discuss its importance. Heck anyone can post something on the internet for free but once they consider that something that is published for profit could well have legal liabilities it becomes hard to find one published in a book. Heck i could even post something i noodled up but how do i know its correct?:o Why not let others prove out for me?
No i won't down load someone else's spreedsheet onto my computer when i can make my own - if i only had a good formula. Pat Kelly sounds like an honest enthusiest trying to help others and does have some good information posted. But no, i don't trust the free-be stuff from the net.
I guess what i'm saying is that don't bet the farm with a DCR calc and don't try to prove the guys publishing static c.r. numbers wrong with it. Just because MotorHead has a working combo that challenges the convention is no guarentee yours will. I just see the DCR calcs as mostly unproven estimations and perpetuated as a way to run unreasonble c.r.'s or the only way to choose a camshaft.
Now i feel soo much better.
cardo0
Well if DCR calcs where truly that important then why doesn't Comp Cams or Crane Cams or any of the other major cam mfrs have one on there website. Vizard doesn't even print a complete formula in any book nor does Smokey Y. nor Lingenfelter though they discuss its importance. Heck anyone can post something on the internet for free but once they consider that something that is published for profit could well have legal liabilities it becomes hard to find one published in a book. Heck i could even post something i noodled up but how do i know its correct?:o Why not let others prove out for me? No i won't down load someone else's spreedsheet onto my computer when i can make my own - if i only had a good formula.
Pat Kelly sounds like an honest enthusiest trying to help others and does have some good information posted. But no, i don't trust the free-be stuff from the net. I guess what i'm saying is that don't bet the farm with a DCR calc and don't try to prove the guys publishing static c.r. numbers wrong with it. Just because MotorHead has a working combo that challenges the convention is no guarentee yours will. I just see the DCR calcs as mostly unproven estimations and perpetuated as a way to run unreasonble c.r.'s or the only way to choose a camshaft.
Now i feel soo much better.
cardo0
Le Mans Master
Joined: Oct 2000
Posts: 7,831
Likes: 5
From: Austin TX
Well, since I'm on Firefox right now I guess I do...
Pat Kelley is a member of the Chevelle forum who was cool enough to spend some time to create the application.
It ain't rocket science, just some nice programming...
The first time I heard of DCR was in the book "How to Build Max Performance Chevy Rat Motors" by Ed Staffel.
He has all the info you need in there to check Pat's application if you are so inclined.
Pat Kelley is a member of the Chevelle forum who was cool enough to spend some time to create the application.
It ain't rocket science, just some nice programming...
The first time I heard of DCR was in the book "How to Build Max Performance Chevy Rat Motors" by Ed Staffel.
He has all the info you need in there to check Pat's application if you are so inclined.
Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
Sorry S489 but my software blocks your links for some reason.
Well if DCR calcs where truly that important then why doesn't Comp Cams or Crane Cams or any of the other major cam mfrs have one on there website. Vizard doesn't even print a complete formula in any book nor does Smokey Y. nor Lingenfelter though they discuss its importance. Heck anyone can post something on the internet for free but once they consider that something that is published for profit could well have legal liabilities it becomes hard to find one published in a book. Heck i could even post something i noodled up but how do i know its correct?:o Why not let others prove out for me?
No i won't down load someone else's spreedsheet onto my computer when i can make my own - if i only had a good formula. Pat Kelly sounds like an honest enthusiest trying to help others and does have some good information posted. But no, i don't trust the free-be stuff from the net.
I guess what i'm saying is that don't bet the farm with a DCR calc and don't try to prove the guys publishing static c.r. numbers wrong with it. Just because MotorHead has a working combo that challenges the convention is no guarentee yours will. I just see the DCR calcs as mostly unproven estimations and perpetuated as a way to run unreasonble c.r.'s or the only way to choose a camshaft.
Now i feel soo much better.
cardo0
Well if DCR calcs where truly that important then why doesn't Comp Cams or Crane Cams or any of the other major cam mfrs have one on there website. Vizard doesn't even print a complete formula in any book nor does Smokey Y. nor Lingenfelter though they discuss its importance. Heck anyone can post something on the internet for free but once they consider that something that is published for profit could well have legal liabilities it becomes hard to find one published in a book. Heck i could even post something i noodled up but how do i know its correct?:o Why not let others prove out for me? No i won't down load someone else's spreedsheet onto my computer when i can make my own - if i only had a good formula.
Pat Kelly sounds like an honest enthusiest trying to help others and does have some good information posted. But no, i don't trust the free-be stuff from the net. I guess what i'm saying is that don't bet the farm with a DCR calc and don't try to prove the guys publishing static c.r. numbers wrong with it. Just because MotorHead has a working combo that challenges the convention is no guarentee yours will. I just see the DCR calcs as mostly unproven estimations and perpetuated as a way to run unreasonble c.r.'s or the only way to choose a camshaft.
Now i feel soo much better.
cardo0
numbers are just example, not a real engine - meant to display equations only . . . thanks
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
For anyone interested, I've started a new thread on the relevance of DCR and the accuracy of empirenet's calculator...
http://forums.corvetteforum.com/show...post1562991580
http://forums.corvetteforum.com/show...post1562991580
