Pro's and con's of compression vs timing
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Pro's and con's of compression vs timing
I am building a solid roller LT1, I want as much compression as I can run on pump gas. The cam will be 260/270 @.050. 108 LSA I would like to run 12to1. Will this cam bleed off enough compression to run on 93 octane?
Next question, say if you have a 12 to 1 engine on pump gas with 32* of total timing, and thats all it will take and not ping. The other engine is an 11 to 1 with 38* total and that is it's limit. Which one will make more power? It's all about cylinder pressure, right?
David
Next question, say if you have a 12 to 1 engine on pump gas with 32* of total timing, and thats all it will take and not ping. The other engine is an 11 to 1 with 38* total and that is it's limit. Which one will make more power? It's all about cylinder pressure, right?
David
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There is a thread on dynamic compression ratio (DCR) in which Chuck Riddeck, Rich and quite a few other guys made some excellent points. The consensus seems to stand on the fact that a well built street engine will live a happy long life on a DCR of ~9.0:1.
The timing thing isn't a tell-all. It would seem at a glance, that the engine with less timing may very well have a better combustion chamber... like many of the new cylinder heads but that doesn't necessarily mean the engine is making more power. Not quite that simple.
-Mindgame
The timing thing isn't a tell-all. It would seem at a glance, that the engine with less timing may very well have a better combustion chamber... like many of the new cylinder heads but that doesn't necessarily mean the engine is making more power. Not quite that simple.
-Mindgame
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http://web.camaross.com/forums/showt...pression+ratio
Very infomative.
(what is up with this bs flood control...... lordy!)
-Mindgame
Very infomative.
(what is up with this bs flood control...... lordy!)
-Mindgame
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Re: Pro's and con's of compression vs timing
Originally posted by FASTFATBOY
Next question, say if you have a 12 to 1 engine on pump gas with 32* of total timing, and thats all it will take and not ping. The other engine is an 11 to 1 with 38* total and that is it's limit. Which one will make more power? It's all about cylinder pressure, right?
Next question, say if you have a 12 to 1 engine on pump gas with 32* of total timing, and thats all it will take and not ping. The other engine is an 11 to 1 with 38* total and that is it's limit. Which one will make more power? It's all about cylinder pressure, right?
Bret
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Posts: 2,148
From: Bay Area, CA
Originally posted by Mindgame
(what is up with this bs flood control...... lordy!)
-Mindgame
(what is up with this bs flood control...... lordy!)
-Mindgame
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From: TN
Originally posted by SStrokerAce
I'm not so sure a long life is doable on 9:1 DCR. I know you will make some awesome power with that, but it still will detonate if not dialed in right.
Bret
I'm not so sure a long life is doable on 9:1 DCR. I know you will make some awesome power with that, but it still will detonate if not dialed in right.
Bret
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From: Buffalo, New York
9:1 DCR is right on the edge, IMHO. Look at the popular combo of a CC306 and stock 54cc size combustion chamber 355. That static CR is an alarming ~11.8:1 But the DCR is ~8.7:1. There aren't many combos out there running 9:1, I like to see it a little lower. If all you can get is 91 octane, aim for 8.5:1 or a little less.
As far as a 260/270 solid roller, 12:1 may be close but not quite high enough. Detailed cam specs would help, but assuming the seat to seat is in the 300/310 range and the lsa is 112 with a 108 icl the DCR would be ~8.3 with the above combo. If you can get something better than that 91 octane swill, I'd go up a little on the static CR. Otherwise, that sounds about right.
As far as timing goes, you have to know a lot more than me to figure it out ahead of time. It's a matter for experimentation unless you have built the exact same combo before. Don't decide your CR based on the amount of timing you think you should run. That's bass ackwards for sure. Pick the cam that meets your needs in terms of rev range, drivability, complementing your heads, etc. Pick the static CR to get the desired CR with that cam, and adjust the timing when it's all put together.
Rich Krause
As far as a 260/270 solid roller, 12:1 may be close but not quite high enough. Detailed cam specs would help, but assuming the seat to seat is in the 300/310 range and the lsa is 112 with a 108 icl the DCR would be ~8.3 with the above combo. If you can get something better than that 91 octane swill, I'd go up a little on the static CR. Otherwise, that sounds about right.
As far as timing goes, you have to know a lot more than me to figure it out ahead of time. It's a matter for experimentation unless you have built the exact same combo before. Don't decide your CR based on the amount of timing you think you should run. That's bass ackwards for sure. Pick the cam that meets your needs in terms of rev range, drivability, complementing your heads, etc. Pick the static CR to get the desired CR with that cam, and adjust the timing when it's all put together.
Rich Krause
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From: TN
Originally posted by rskrause
9:1 DCR is right on the edge, IMHO. Look at the popular combo of a CC306 and stock 54cc size combustion chamber 355. That static CR is an alarming ~11.8:1 But the DCR is ~8.7:1. There aren't many combos out there running 9:1, I like to see it a little lower. If all you can get is 91 octane, aim for 8.5:1 or a little less.
As far as a 260/270 solid roller, 12:1 may be close but not quite high enough. Detailed cam specs would help, but assuming the seat to seat is in the 300/310 range and the lsa is 112 with a 108 icl the DCR would be ~8.3 with the above combo. If you can get something better than that 91 octane swill, I'd go up a little on the static CR. Otherwise, that sounds about right.
As far as timing goes, you have to know a lot more than me to figure it out ahead of time. It's a matter for experimentation unless you have built the exact same combo before. Don't decide your CR based on the amount of timing you think you should run. That's bass ackwards for sure. Pick the cam that meets your needs in terms of rev range, drivability, complementing your heads, etc. Pick the static CR to get the desired CR with that cam, and adjust the timing when it's all put together.
Rich Krause
9:1 DCR is right on the edge, IMHO. Look at the popular combo of a CC306 and stock 54cc size combustion chamber 355. That static CR is an alarming ~11.8:1 But the DCR is ~8.7:1. There aren't many combos out there running 9:1, I like to see it a little lower. If all you can get is 91 octane, aim for 8.5:1 or a little less.
As far as a 260/270 solid roller, 12:1 may be close but not quite high enough. Detailed cam specs would help, but assuming the seat to seat is in the 300/310 range and the lsa is 112 with a 108 icl the DCR would be ~8.3 with the above combo. If you can get something better than that 91 octane swill, I'd go up a little on the static CR. Otherwise, that sounds about right.
As far as timing goes, you have to know a lot more than me to figure it out ahead of time. It's a matter for experimentation unless you have built the exact same combo before. Don't decide your CR based on the amount of timing you think you should run. That's bass ackwards for sure. Pick the cam that meets your needs in terms of rev range, drivability, complementing your heads, etc. Pick the static CR to get the desired CR with that cam, and adjust the timing when it's all put together.
Rich Krause
and was the one that brought up 91 octane (at the mercy of whats availible sometimes). I'll most likely be limited to something like a hot cam or cc305. Just looking for some general guidlines.BTW I noticed your question about figuring ratios for different octane gas in the link mindgame put up, did you ever figure anything out about that?
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Originally posted by rskrause
9:1 DCR is right on the edge, IMHO. Look at the popular combo of a CC306 and stock 54cc size combustion chamber 355. That static CR is an alarming ~11.8:1 But the DCR is ~8.7:1. There aren't many combos out there running 9:1, I like to see it a little lower. If all you can get is 91 octane, aim for 8.5:1 or a little less.
As far as a 260/270 solid roller, 12:1 may be close but not quite high enough. Detailed cam specs would help, but assuming the seat to seat is in the 300/310 range and the lsa is 112 with a 108 icl the DCR would be ~8.3 with the above combo. If you can get something better than that 91 octane swill, I'd go up a little on the static CR. Otherwise, that sounds about right.
As far as timing goes, you have to know a lot more than me to figure it out ahead of time. It's a matter for experimentation unless you have built the exact same combo before. Don't decide your CR based on the amount of timing you think you should run. That's bass ackwards for sure. Pick the cam that meets your needs in terms of rev range, drivability, complementing your heads, etc. Pick the static CR to get the desired CR with that cam, and adjust the timing when it's all put together.
Rich Krause
9:1 DCR is right on the edge, IMHO. Look at the popular combo of a CC306 and stock 54cc size combustion chamber 355. That static CR is an alarming ~11.8:1 But the DCR is ~8.7:1. There aren't many combos out there running 9:1, I like to see it a little lower. If all you can get is 91 octane, aim for 8.5:1 or a little less.
As far as a 260/270 solid roller, 12:1 may be close but not quite high enough. Detailed cam specs would help, but assuming the seat to seat is in the 300/310 range and the lsa is 112 with a 108 icl the DCR would be ~8.3 with the above combo. If you can get something better than that 91 octane swill, I'd go up a little on the static CR. Otherwise, that sounds about right.
As far as timing goes, you have to know a lot more than me to figure it out ahead of time. It's a matter for experimentation unless you have built the exact same combo before. Don't decide your CR based on the amount of timing you think you should run. That's bass ackwards for sure. Pick the cam that meets your needs in terms of rev range, drivability, complementing your heads, etc. Pick the static CR to get the desired CR with that cam, and adjust the timing when it's all put together.
Rich Krause
The cam will have a 108* LSA, Installed on a 104.
I may back off the cam and compression. Cammed right 11.5 to 1 compression will as much power as needed(570 FWHP) I think .
David
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From: MD
This is somewhat a loaded question since there are so many variables that it’s very difficult to cover all possibilities.
To start with assuming nothing else gets in the way, 12:1 compression will only make roughly .5% more power then 11:1, so we’re not talking about big numbers anyway, probably smaller then you could reliably test on a dyno. Not many of us have a good reason to build an engine that runs that close to the extreme ragged edge of what you can get away with, loosing 3hp won’t kill you or loose many races.
Second, the question is based in an incorrect assumption, that an engine will make the best power with as much timing as possible before it starts detonating. That is not necessarily true (though it is in a lot of cases).
There are also many other factors that determine how detonation prone a package is outside of DCR and timing. Intake air temp, the spark plugs used, the age of the engine, combustion chamber shape and design, piston shape and design, how heavily loaded it is during cruising (steep gears can delay the onset of detonation), cooling system efficiency, how accurate the fuel metering is…
To start with assuming nothing else gets in the way, 12:1 compression will only make roughly .5% more power then 11:1, so we’re not talking about big numbers anyway, probably smaller then you could reliably test on a dyno. Not many of us have a good reason to build an engine that runs that close to the extreme ragged edge of what you can get away with, loosing 3hp won’t kill you or loose many races.
Second, the question is based in an incorrect assumption, that an engine will make the best power with as much timing as possible before it starts detonating. That is not necessarily true (though it is in a lot of cases).
There are also many other factors that determine how detonation prone a package is outside of DCR and timing. Intake air temp, the spark plugs used, the age of the engine, combustion chamber shape and design, piston shape and design, how heavily loaded it is during cruising (steep gears can delay the onset of detonation), cooling system efficiency, how accurate the fuel metering is…
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And just to throw one more on top of that......
A street engine also has to deal with deposits in the combustion chamber over time (carbon and such). A freshly built engine with nice clean pistons and chambers will have much less tendency towards detonation than that same motor with 5000 miles on it.
BTW- The old rule of thumb is 4% HP per point of compression. I won't argue with the WS6 TA about his .5% figure- it may well be much more relaistic than my old rule of thumb. In any case, I definitely agree that living on the edge of detonation isn't worth the handfull of HP you'll gain from higher compression.
A street engine also has to deal with deposits in the combustion chamber over time (carbon and such). A freshly built engine with nice clean pistons and chambers will have much less tendency towards detonation than that same motor with 5000 miles on it.
BTW- The old rule of thumb is 4% HP per point of compression. I won't argue with the WS6 TA about his .5% figure- it may well be much more relaistic than my old rule of thumb. In any case, I definitely agree that living on the edge of detonation isn't worth the handfull of HP you'll gain from higher compression.
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From: Buffalo, New York
Originally posted by Damon
And just to throw one more on top of that......
A street engine also has to deal with deposits in the combustion chamber over time (carbon and such). A freshly built engine with nice clean pistons and chambers will have much less tendency towards detonation than that same motor with 5000 miles on it.
BTW- The old rule of thumb is 4% HP per point of compression. I won't argue with the WS6 TA about his .5% figure- it may well be much more relaistic than my old rule of thumb. In any case, I definitely agree that living on the edge of detonation isn't worth the handfull of HP you'll gain from higher compression.
And just to throw one more on top of that......
A street engine also has to deal with deposits in the combustion chamber over time (carbon and such). A freshly built engine with nice clean pistons and chambers will have much less tendency towards detonation than that same motor with 5000 miles on it.
BTW- The old rule of thumb is 4% HP per point of compression. I won't argue with the WS6 TA about his .5% figure- it may well be much more relaistic than my old rule of thumb. In any case, I definitely agree that living on the edge of detonation isn't worth the handfull of HP you'll gain from higher compression.
Rich Krause
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From: MD
Daemon- I'm not sure how what you're stating is that different then my 'age of the engine' in my comments... by that I meant to include deposits and the increased likelyhood that oil will get past the rings or stem seals lowering the octane of the A/F charge (oil has an octane in the 70's).
WRT to my % loss due to compresson I'm basing it on a SAE published study that showed that not only does the power increase get much lower at higher compressions, but at 11:1 you're only at 3% total loss and it's just about impossible to get compression high enough running anything anything that could be called gasoline to get it substantially under 2% (at about 14:1).
There are places that it's worth taking things to the ragged edge to get 2 or 3hp, but this isn't one (based on the original pump gas comments). The consequences of messing this up in a 'street car' that will see some miles and pump gas are greater then the advantages of getting it 'just right.' I'd much rather err .5 or more low then .1 high (especially if you add a power adder... you can always run more boost or spray). You could also end up surprised by some newer head/chamber designs which don't need nearly as much timing or compression as older ones
WRT to my % loss due to compresson I'm basing it on a SAE published study that showed that not only does the power increase get much lower at higher compressions, but at 11:1 you're only at 3% total loss and it's just about impossible to get compression high enough running anything anything that could be called gasoline to get it substantially under 2% (at about 14:1).
There are places that it's worth taking things to the ragged edge to get 2 or 3hp, but this isn't one (based on the original pump gas comments). The consequences of messing this up in a 'street car' that will see some miles and pump gas are greater then the advantages of getting it 'just right.' I'd much rather err .5 or more low then .1 high (especially if you add a power adder... you can always run more boost or spray). You could also end up surprised by some newer head/chamber designs which don't need nearly as much timing or compression as older ones
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