Dynamic vs Static Compression ratio
Discussion
Hi Guys
Like many of us on here I'm learing lots from the various guru's (DV, Dave @ Puma Racing and others) posts about cylinder head flow and velocities, cam event timing and LCA and the exact nature of flow benches. I too suffer from the strange desire to try and make the BL A-Series make more power than it has any right to.
On my last engine build I tried to apply some "Science" to the static Compression ratio I wanted to achieve, much to the chargrin of my somewhat more pragmatic and less engineering minded mini-mates!!
So using a calculation of the effective or "dynamic" compression ratio I downloaded from the Net I calculated the dynamic compression of a standard engine using the MG Metro cam, then I tried to match the same dynamic compression ratio with my new engine build.
I don't have the exact figures to hand, but the gist for example is that for a static compression ratio of 9.75:1 using the MG Metro cam (252 inlet/268 ex 108 LCA timed at 106) resulted in a dynamic compression of something like 8.2:1
Substituting for my favoured cam which has 280 inlet/280 ex 108 LCA timed at 106 without adjusting the static CR gave a dynamic CR of about 7.7:1, so by reversing the calculation needed to increase the static CR to about 10.75:1 to get back to a dynamic CR of 8.2:1
Perceived wisdom from the A-Series tuning sages would have suggested a 10.75:1 static compression for a 280 degree cam anyway, but I guess I wanted to both understand a bit about the whole dynamic compression thing and to challenge the perceived wisdom rather than just accepting it verbatum.
My questions are :
Am I on the right lines by trying to match the cam to the static compression to get the same effective or dynamic compression ??
Was I still a bit conservative and could I have gone higher in compression ?? I figured I would probably not run into detonation problems if I aimed for the same dynamic CR as a "factory" cam, which may be a bit naive I suppose !!
I still have some scope to deck the block down a bit, thus helping squish a bit too. I accept that the A-Series is a bit antiquated in it's design and it doesn't have the advantage of knock detection, even if like me you have mappable ignition control.
Any comments ??
Phil.
Like many of us on here I'm learing lots from the various guru's (DV, Dave @ Puma Racing and others) posts about cylinder head flow and velocities, cam event timing and LCA and the exact nature of flow benches. I too suffer from the strange desire to try and make the BL A-Series make more power than it has any right to.
On my last engine build I tried to apply some "Science" to the static Compression ratio I wanted to achieve, much to the chargrin of my somewhat more pragmatic and less engineering minded mini-mates!!
So using a calculation of the effective or "dynamic" compression ratio I downloaded from the Net I calculated the dynamic compression of a standard engine using the MG Metro cam, then I tried to match the same dynamic compression ratio with my new engine build.
I don't have the exact figures to hand, but the gist for example is that for a static compression ratio of 9.75:1 using the MG Metro cam (252 inlet/268 ex 108 LCA timed at 106) resulted in a dynamic compression of something like 8.2:1
Substituting for my favoured cam which has 280 inlet/280 ex 108 LCA timed at 106 without adjusting the static CR gave a dynamic CR of about 7.7:1, so by reversing the calculation needed to increase the static CR to about 10.75:1 to get back to a dynamic CR of 8.2:1
Perceived wisdom from the A-Series tuning sages would have suggested a 10.75:1 static compression for a 280 degree cam anyway, but I guess I wanted to both understand a bit about the whole dynamic compression thing and to challenge the perceived wisdom rather than just accepting it verbatum.
My questions are :
Am I on the right lines by trying to match the cam to the static compression to get the same effective or dynamic compression ??
Was I still a bit conservative and could I have gone higher in compression ?? I figured I would probably not run into detonation problems if I aimed for the same dynamic CR as a "factory" cam, which may be a bit naive I suppose !!
I still have some scope to deck the block down a bit, thus helping squish a bit too. I accept that the A-Series is a bit antiquated in it's design and it doesn't have the advantage of knock detection, even if like me you have mappable ignition control.
Any comments ??
Phil.
Hi Stan
I've just started to read the other thread, so I guess I'll get my answer there. I'll see how close to 200psi I can calculate, as per Dave's suggestion, then measure to back up the theory. My current thoughts are that I may be shades below that, and that at the next rebuild I should get the block "decked" to up the CR a bit.
It's all very interesting, and a sad day when you don't learn something new !!
Phil.
I've just started to read the other thread, so I guess I'll get my answer there. I'll see how close to 200psi I can calculate, as per Dave's suggestion, then measure to back up the theory. My current thoughts are that I may be shades below that, and that at the next rebuild I should get the block "decked" to up the CR a bit.
It's all very interesting, and a sad day when you don't learn something new !!
Phil.
I have over a long period of time attempted to absorb as much info as i can on compression ratio and the effects of camshaft duration etc.
As I get nearer to understanding more doubt happens in my confused mind.
I get static compression ratios, built an engine using an excel sheet I constructed to guide me, I was aware of DCR and briefly looked into it, my confusion has been about static compression Vs Dynamic and the perceived wisdom that cast pistons can only take about 10.5 to 1.
Dave (Puma Racing) then says 200psi for a 2v (Pinto in my case) so after this long winded thread hijack on my part, what do I build my engine to? 10.5 static? A lower DCR figure? 200 psi?
Currently the head and gasket combination will give 11 to 1 I can of course buy a thicker gasket of as I was about to do, remove a bit more from the chambers.
To sumerise V6 cast pistons, Cosworth rods, RL31 cam, will the pistons take 200psi/11 to 1?
As I get nearer to understanding more doubt happens in my confused mind.
I get static compression ratios, built an engine using an excel sheet I constructed to guide me, I was aware of DCR and briefly looked into it, my confusion has been about static compression Vs Dynamic and the perceived wisdom that cast pistons can only take about 10.5 to 1.
Dave (Puma Racing) then says 200psi for a 2v (Pinto in my case) so after this long winded thread hijack on my part, what do I build my engine to? 10.5 static? A lower DCR figure? 200 psi?
Currently the head and gasket combination will give 11 to 1 I can of course buy a thicker gasket of as I was about to do, remove a bit more from the chambers.
To sumerise V6 cast pistons, Cosworth rods, RL31 cam, will the pistons take 200psi/11 to 1?
It's not really about the 200 psi when cranking, it's about the compressive loading the piston takes when combustion takes place and the tensile loading when the piston is unloaded.
The tensile loading is going to limit the rev limit of the piston. The best thing to do is look at what others have found works and stick to that limit.
The compressive loading is going to be fairly light on a naturally aspirated engine, the biggest issue is getting the piston to bore clearance right. Again, use the clearances the manufacturer recommends or what a sucessful engine builder has found to have worked.
The biggest gain I would say is to get the ignition timing accurate. You really need triggering from a crank or flywheel mounted sensor, mainly because this takes out the variability you get with lash from gears in distributors run off other shafts etc. That can all add up, and it either adds up to wasted power or running in the detonation zone which will send an engine back to the rebuilders in no time at all.
The tensile loading is going to limit the rev limit of the piston. The best thing to do is look at what others have found works and stick to that limit.
The compressive loading is going to be fairly light on a naturally aspirated engine, the biggest issue is getting the piston to bore clearance right. Again, use the clearances the manufacturer recommends or what a sucessful engine builder has found to have worked.
The biggest gain I would say is to get the ignition timing accurate. You really need triggering from a crank or flywheel mounted sensor, mainly because this takes out the variability you get with lash from gears in distributors run off other shafts etc. That can all add up, and it either adds up to wasted power or running in the detonation zone which will send an engine back to the rebuilders in no time at all.
What is the overall aim of returning to the factory DCR? To my understanding, the purpose of bumping the compression with uprated cams was to prevent low end power loss.
On my particular engine, the factory 243 cam gave an effective capacity of 1416cc (1595cc swept) and changing to a 270 cam dropped this to 1294cc. Simply bumping the compression to match would theoretically still leave a hole in the low end power in this case despite the increased compression.
I don't know how much this reflects reality, but it was interesting non the less.
On my particular engine, the factory 243 cam gave an effective capacity of 1416cc (1595cc swept) and changing to a 270 cam dropped this to 1294cc. Simply bumping the compression to match would theoretically still leave a hole in the low end power in this case despite the increased compression.
I don't know how much this reflects reality, but it was interesting non the less.
I think you're exactly on the beans there.
with longer duration cams, you have the very real spectre of pumping fresh mixture out of the intake due to the delayed intake valve closure.
what you're actually doing is to ensure that the cylinder sees the same conditions at low speed that it does with the standard cam, which means that it'll be no nearer detonation. Or to put it another way, you're making the most of the fuel you have.
You have to be a little more careful at lowish engine speeds, rather than plain low, but there tends to be less detonation risk there anyway.
the cam/compression relationship is just about the most fundamental part of engine design IMO
with longer duration cams, you have the very real spectre of pumping fresh mixture out of the intake due to the delayed intake valve closure.
what you're actually doing is to ensure that the cylinder sees the same conditions at low speed that it does with the standard cam, which means that it'll be no nearer detonation. Or to put it another way, you're making the most of the fuel you have.
You have to be a little more careful at lowish engine speeds, rather than plain low, but there tends to be less detonation risk there anyway.
the cam/compression relationship is just about the most fundamental part of engine design IMO
Increasing compression to ensure reasonable dynamic compression with the longer duration cam is all well and good. But all this is assuming 100% VE, which is not the case.
For example, at 3000rpm, your engine has a VE of 100% therefor all your calculations work., but at 6000rpm, you have a VE of 110%, so in effect 10% more compression pressure. So your assumption that DCR for your longer duration cam equal to the stock compression pressures, is now maybe too high.
For example, at 3000rpm, your engine has a VE of 100% therefor all your calculations work., but at 6000rpm, you have a VE of 110%, so in effect 10% more compression pressure. So your assumption that DCR for your longer duration cam equal to the stock compression pressures, is now maybe too high.
stevesingo said:
Increasing compression to ensure reasonable dynamic compression with the longer duration cam is all well and good. But all this is assuming 100% VE, which is not the case.
For example, at 3000rpm, your engine has a VE of 100% therefor all your calculations work., but at 6000rpm, you have a VE of 110%, so in effect 10% more compression pressure. So your assumption that DCR for your longer duration cam equal to the stock compression pressures, is now maybe too high.
Bingo! This is exactly the issue I was trying to describe on the owners club where the trend is to fit long duration cams then bump the compression massively, from high 10s to over 13 in some cases, bumping the DCR and cranking pressure way over stock. For example, at 3000rpm, your engine has a VE of 100% therefor all your calculations work., but at 6000rpm, you have a VE of 110%, so in effect 10% more compression pressure. So your assumption that DCR for your longer duration cam equal to the stock compression pressures, is now maybe too high.
My argument was that this was simply not necessary, cams are just a method of increasing VE and if you look at the other methods of doing this, via forced induction, you wouldn't raise compression there. The absolute values are obviously a lot less with NA cams, but so far as I can see, by raising compression that much you could potentially harm the part of the powerband where the cams are designed to sing because you'll have to back off the ignition timing to stop it knocking due to the cylinder pressures. What made even less sense was that the engines in question were Honda VTECs so it has a different lobe altogether for 2000rpm than it does for 8000rpm, bumping compression shouldn't even be needed...
It all fell on deaf ears though and I thought I had it wrong, but good to see at least someone else thinks the same way!
The way I understand it is that ign adv should be Minimum for Best Torque.
If you have space above MBT, where adding advance does not increase torque before experiencing knock, you probably have the option of increasing compression. For me to add advance in such a case. I would need at least 6deg of space between MBT and knock to contemplate an increase in compression.
Of course, if you reach knock before you reach peak torque, then you have too much compression, or too little octane.
Steve
If you have space above MBT, where adding advance does not increase torque before experiencing knock, you probably have the option of increasing compression. For me to add advance in such a case. I would need at least 6deg of space between MBT and knock to contemplate an increase in compression.
Of course, if you reach knock before you reach peak torque, then you have too much compression, or too little octane.
Steve
stevesingo said:
Increasing compression to ensure reasonable dynamic compression with the longer duration cam is all well and good. But all this is assuming 100% VE, which is not the case.
For example, at 3000rpm, your engine has a VE of 100% therefor all your calculations work., but at 6000rpm, you have a VE of 110%, so in effect 10% more compression pressure. So your assumption that DCR for your longer duration cam equal to the stock compression pressures, is now maybe too high.
Surely any road derived erngine would have lower VE at 6000rpm than it would at 3000rpm...For example, at 3000rpm, your engine has a VE of 100% therefor all your calculations work., but at 6000rpm, you have a VE of 110%, so in effect 10% more compression pressure. So your assumption that DCR for your longer duration cam equal to the stock compression pressures, is now maybe too high.
I'd always been told that VE is proportional to torque (more air in the cylinders) so wouldn't the torque curve have started dropping off below 6000rpm? I would have thought VE would be at its max at about 3000rpm.
Or am I missing something blindingly obvious?
Nick1point9 said:
Or am I missing something blindingly obvious?
We have fitted a long duration cam and increased the compression ratio in out theoretical engine in our heads. So peak torque will be made higher up the rev-range.I guess we are just messing around with theory. Messing being the key word.
stevesingo said:
Nick1point9 said:
Or am I missing something blindingly obvious?
We have fitted a long duration cam and increased the compression ratio in out theoretical engine in our heads. So peak torque will be made higher up the rev-range.I guess we are just messing around with theory. Messing being the key word.
I wasn't having a go, I was just wondering if I was missing something when thinking it through!
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