Anyone Running 8.5 Dynamic Compression Or Higher ??
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Team Owner
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From: Summerland B.C. Canada
Anyone Running 8.5 Dynamic Compression Or Higher ??
I'm looking to get the most out of my 355 rebuild and am curious if anyone is running 8.5 DCR or higher without detonation problems. (pump gas) 
My set up at this time is roughly 8.8 DCR, .044 quench. I will make adjustments as necessary to chamber volume and/or cam timing.
My set up at this time is roughly 8.8 DCR, .044 quench. I will make adjustments as necessary to chamber volume and/or cam timing.
Melting Slicks
Joined: Oct 2002
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From: Cincinnati Oh
I don't think it's doable (generally 8.2:1 is the max for pump gas, but depends on the combo I guess).
I run aroun ~8:1 DCR and have a little trouble with 93 octane in the heat... I'm pretty sure you'd need race gas and/or severely retarded timing to run that kind of DCR (and you would need SERIOUS static CR or a really small cam???).
What are the specs of what you're looking at?
I run aroun ~8:1 DCR and have a little trouble with 93 octane in the heat... I'm pretty sure you'd need race gas and/or severely retarded timing to run that kind of DCR (and you would need SERIOUS static CR or a really small cam???).
What are the specs of what you're looking at?
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Team Owner
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From: Summerland B.C. Canada
The numbers are a result of the machine work. .030 over bore, block decked .020, .039 head gasket, 58 cc cambers. I'm working on the heads now and will be unmasking the valves and polishing the cambers. This will reduce my DCR and i will adjust the chamber size more till i get to a safer DCR. A 64cc chamber gets me to 8.25. That is my target but i thought i would ask if anyone is running higher and what setup they have to do so.
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From: Good health is merely the slowest possible rate at which one can die
St. Jude Donor '04-'05-'06-'07
That sounds very interesting. I like your quench quest. I think that it'll pay big dividends in overall efficiency. Depending on certain final measurements, I may squeeze mine as tight as under .035". Yes, I know that defies conventional wisdom, but we'll see.
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From: Summerland B.C. Canada
Originally Posted by CentralCoaster
How do you figure out DCR?
If it factors in valve events, does it also factor in VE and engine speed?
If it factors in valve events, does it also factor in VE and engine speed?
But i'm doing every little thing i can to make it more efficient. 
Instructor
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The formula which i have always seen for figuring out dynamic compression. Only really figures in the intake valve closing point to the equation, so thus the reduced stroke, and compression. Always coming out with a fixed number.
But isnt there a point at a certain rpm where the intake charge starts having enough momentum, not to be pushed out of the cylinder by the upward rising piston. This point it is sure volumetric efficiency is increased, but what happend the the dcr. You are no longer backflowing and your getting the full or even more cylinder charge? dcr changes?
But isnt there a point at a certain rpm where the intake charge starts having enough momentum, not to be pushed out of the cylinder by the upward rising piston. This point it is sure volumetric efficiency is increased, but what happend the the dcr. You are no longer backflowing and your getting the full or even more cylinder charge? dcr changes?
Melting Slicks
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From: Cincinnati Oh
I've used online calculators to determine DCR - it is very much dependent on the "static compression ratio" factors like bore, stroke, and chamber volume - but it uses valve timing to calculate how much ACTUAL compression you get as a result of the cam you use (hence dynamic).
So it puts a number on why you can run a higher static CR with larger cams, but cannot with smaller ones (because the valves close earlier, and don't allow as much bleed off as a large cam would).
If you build it with a metal shim gasket, and realize that you are in PING city... then the easy fix would be a thicker gasket. But then your quench goes out the window.
Personally I shoot for 0.35-0.40" quench depending on the types of rods and pistons used (forged items expand more and require greater clearances).
I wouldn't go above 8.2, just what I've read though.
So it puts a number on why you can run a higher static CR with larger cams, but cannot with smaller ones (because the valves close earlier, and don't allow as much bleed off as a large cam would).
If you build it with a metal shim gasket, and realize that you are in PING city... then the easy fix would be a thicker gasket. But then your quench goes out the window.
Personally I shoot for 0.35-0.40" quench depending on the types of rods and pistons used (forged items expand more and require greater clearances).
I wouldn't go above 8.2, just what I've read though.
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Thread Starter
Team Owner
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From: Summerland B.C. Canada
I read 8.3 as being the max for a pump gas car, but engine temps really come into play when you climb higher in DCR. I don't want a pinger, thats for sure, but i do want to squeeze every bit out of this build 
Safety Car
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From: Glenbeulah Wi
The paper I read recommended staying at 8.5 to 1 or less. I have not run anything higher then 8.5 to 1 on pump gas so I have no actual data to help you out. My 355 runs 8.2 to 1 with no issues on 93 octane fuel.
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From: Summerland B.C. Canada
Originally Posted by bjankuski
The paper I read recommended staying at 8.5 to 1 or less. I have not run anything higher then 8.5 to 1 on pump gas so I have no actual data to help you out. My 355 runs 8.2 to 1 with no issues on 93 octane fuel.
Looks like 8.25-1 shouldn't cause me any grief so i will continue to use that as my target number.
Instructor
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Originally Posted by Ramanstud
I've used online calculators to determine DCR - it is very much dependent on the "static compression ratio" factors like bore, stroke, and chamber volume - but it uses valve timing to calculate how much ACTUAL compression you get as a result of the cam you use (hence dynamic).
. Why is the compression reduced from Valve timing? Because the intake valve staying open after bdc causes some intake charge to be pushed back out of the cylinder by the upward rising piston.
But what happens at higher rpms when the air charge has enough momentum to actually overcome the degrees of late intake closing and actually keep filling the cylinder until the valve is completely closed. Now at this point is the dynamic compression ratio still lower than the static ratio?
Melting Slicks
Joined: Oct 2002
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From: Cincinnati Oh
To be honest, I haven't a clue as to how the DCR changes relative to RPM. I would think that it would actually be less at higher RPM because the intake charge just doesn't have enough time to be pushed out of the exhaust valve as it did relative to a lower RPM duration. Also, you'll be getting LESS air at higher RPM (and less air to compress means lower CR/DCR).
Again, the details are above my head, but that's my best guess.
Again, the details are above my head, but that's my best guess.
Race Director
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From: The Top of Utah
If you were getting less air, it would be the same as closing the throttle. The engine would quit increasing in speed. This does happen, eventually. In the meantime, the air that the pistons was pushing back into the intake manifold at idle, causing that lovely, rumpity rump idle, gains inertia (energy, mass x velocity) as the rpms increase. The weight and speed of the incoming air has the energy to resist the expulsion forces of the rising piston (to a point), and pack the cylinder with more air/fuel to make more power. That is the whole reason high rpm, high horse power, engines have cams with so much duration.
RACE ON!!!
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Team Owner
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
VE = DCR / SCR
right?
DCR will exceed the SCR anytime the VE is over 100%. DCR for the TPI is probably highest at the torque peak, where the tuned runners are optimized, but probably still not over SCR.
Using only valve events is like measuring VE at zero rpm. No air charge inertial losses or gains.
right?
DCR will exceed the SCR anytime the VE is over 100%. DCR for the TPI is probably highest at the torque peak, where the tuned runners are optimized, but probably still not over SCR.
Using only valve events is like measuring VE at zero rpm. No air charge inertial losses or gains.
Le Mans Master
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From: Castro Valley Calif.
St. Jude Donor '03 thru '26
Here's the DCR calculator I use...........the download is toward the bottom of the page. I'm around an 8.3, but I kept my quench at .040 and don't have any problems. One thing that will help reducing DCR is retarding the cam timing
Le Mans Master
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From: Castro Valley Calif.
St. Jude Donor '03 thru '26
Originally Posted by Caboboy
Here's the DCR calculator I use...........the download is toward the bottom of the page. I'm around an 8.3, but I kept my quench at .040 and don't have any problems. One thing that will help reducing DCR is retarding the cam timing
Whoops
: http://cochise.uia.net/pkelley2/DynamicCR.html
Thread Starter
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From: Summerland B.C. Canada
Originally Posted by Caboboy
That the calculator i'm using. Knowing your up to 8.3 makes me feel safer about my 8.25 target area. Maybe i should aim for 8.31 so i can "one up" you
Le Mans Master
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From: Castro Valley Calif.
St. Jude Donor '03 thru '26
Originally Posted by AGENT 86
That the calculator i'm using. Knowing your up to 8.3 makes me feel safer about my 8.25 target area. Maybe i should aim for 8.31 so i can "one up" you Sounds like you're gonna be ready for the good weather Dan