High Compression Engines/Pump gas
Burning Brakes
Joined: Sep 2004
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From: Vernon British Columbia
St. Jude Donor '05
First C3 General Poster!
thats not a yes or no question...as usual...depends on the rest of the setup. Higher RM+PM detonation is always more of a concern, but if it isnt doing it at 4500 it shouldnt be doing it at 6500 either.
Race Director
Joined: May 2004
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Originally Posted by 69ttop502
On a related note here. Lets say I am running 11 to 1 with a big cam on premiom pump gas. Because the cam is so big I have no detonation but because this cam is so big it will rev very high. Since it is filling the cylinders so well would there not be a point where I could get detonation at the higher rpms. I have always wondered this.
You have no detonation because your compression is lower than you think it is! The engine will breathe better wit a longer duration cam and therefore rev higher. The longer duration lowers your compression due to escaping of intake and exhaust gases.
Longer duration cams also cause mixing of intake and exhaust gases that will therefore be less 'flammable' with therefore decreased possibility of detonation.
BTW lower octane fuels are more flammable than higher octane and therefore more prone to detonation.
That is how I read it and is most likely wrong.
Would anyone else comment on the above and set it straight?
I got the horse by the ****....but I'm hanging on
Race Director
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Originally Posted by mandm1200
Lingenfelter's book was published almost 9 years ago. I didn't read it so the CR he suggest is probably more than just a simple one sentence line. Compression Ratio is something that can be calculated; both static and dynamic. Of the two, dynamic is the more important. However, static CR is more of bragging rights. There are race engines running more than 15:1 static compression.
I do suggest reading books, more than the Lingenfelter book, and use that information as a guide line if they can back up what they suggest with data and theory. If it's their own opinion without adequate data, then take it with a pinch of salt. I ran 11:1 back in the late 70's on cast iron heads without detonation on premium pump fuel. There is no hard set number you can go by in regards to static CR. If you start to crunch numbers, you'll start to see the dynamic CR start to drop dramatically when a cam has a longer duration. For example, almost a full point of compression is lost when comparing a cam of 260 degrees to one with 288 degrees of duration.
I do suggest reading books, more than the Lingenfelter book, and use that information as a guide line if they can back up what they suggest with data and theory. If it's their own opinion without adequate data, then take it with a pinch of salt. I ran 11:1 back in the late 70's on cast iron heads without detonation on premium pump fuel. There is no hard set number you can go by in regards to static CR. If you start to crunch numbers, you'll start to see the dynamic CR start to drop dramatically when a cam has a longer duration. For example, almost a full point of compression is lost when comparing a cam of 260 degrees to one with 288 degrees of duration.
I only mentioned the numbers as an example for all the "KNOWNOTHINGS" like ME
. It is just a ballpark number to give an idea to whomever is interested.Obviously it is WAY more complicated than that.....as seen through posts like the "What engine other than Chevy would you use in your Vette" People will argue forever and a day and that is the beauty about the complicity of the combustion engine and its quirks and quarks.
Anyway, I'm talking way to much crap and would love to know more about all of the above!!
Melting Slicks
Joined: Dec 2000
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From: Wichita, KS
You can read about dynamic compression ratio and the relationship to the camshaft duration here http://cochise.uia.net/pkelley2/DynamicCR.html
Compression ratio as used by most people is static compression. This does not take in to acount the fact that compression can not really start because there is still a valve open as the engine starts on the compression stroke. Dynamic compression takes in to acount the valve movement. The end result is that almost all pump gas engines are running between 7 and 8.5 to 1 dynamic compression ratio.
Compression ratio as used by most people is static compression. This does not take in to acount the fact that compression can not really start because there is still a valve open as the engine starts on the compression stroke. Dynamic compression takes in to acount the valve movement. The end result is that almost all pump gas engines are running between 7 and 8.5 to 1 dynamic compression ratio.
Burning Brakes
Joined: Oct 2001
Posts: 1,146
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From: Milford NH
Originally Posted by Summerfun
One of the big engine builders (like Lingenfelter) suggested not to use more than 10,5:1 on aluminum heads and 9,5:1 on cast iron.
You have to take a lot of variables into consideration as a lot of people have mentioned in their posts to be able to accomodate the higher compression without doing damage.
Go and read a book or two of one of the big well known builders and take everything else you hear with a pinch of salt! File it in the back of your mind for cross referencing.
1. The quoted compression is the static compression, what happens inside the engine during operation is dynamic compression, this is where the cam shaft comes into play. Larger camshafts require a higher compression because the valve overlap DECREASES compression.
2. Aluminum heads helps because they dissipate head better then cast iron.
3. High quench design is required to better mix the air/fuel mixture. (Zero deck clearance with flat top pistons).
4. Computer control fuel injection (this is probably the biggest factor). The computer is smart and can control air/fuel ratio along with timing.
5. I think I have read somewhere that having a cold air intake helps also, not sure if that is true.
6. Good quality plugs will help a little.
Le Mans Master
Joined: Aug 2003
Posts: 7,720
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From: Chattanooga TN
As mentioned above electronics and knock sensors help. My wife had a 96 LT-1 Caprice with 10.5 compression and you could not make it knock on 87 octane. The Caprice/SS LT-1 has iron heads. My Vette and my 83 pickup both have stock L98 aluminum-head 90-91 TPI engines with 9.5 compression. They ping on anything less than 92 octane. I think combustion-chamber design has a lot to do with it too.
Melting Slicks
Joined: Jan 2004
Posts: 2,672
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From: New Cumberland PA
Originally Posted by Summerfun
Would you mind explaining a bit more about static and dynamic CR?
Static Compression, SC, is fairly simple. Take the volume when the piston is at BDC and divide it by the volume when the piston is at TDC. Some things needed to known to figure this: bore and stroke of the cylinder, bore and thickness of the head gasket, size of the combution chamber of the head, and the piston to deck clearance. Head chamber size is in cc's while other measurements are in inches. Something must be converted so that you are using similiar terms. The easiest thing is to find a web page that allows you to just type in nubers and have it do the calculation.
Dynamic Compression, DC, is almost identical to the above with the exception of the fact it takes into consideration that compression does not start to build until the intake valve is closed. If the intake valve closed at BDC, then DC equals SC. All cams have the intake valve open as the piston starts to travel upwards in the compression stroke. Now for an example: if the stroke of the engine is 3.50" and the intake valve remains open for 90 degrees (1/2 way of the compression stroke), then the effective stroke is 1.75". This would cut the compression ratio by almost 1/2.
When the intake valve is open just a few degees after BDC, it has very little affect on the dynamic stroke. The reason for this is the as the crank is starting to turn from BDC, the bottom of the connecting rod is moving more horizontal than vertical. As far as stroke is concerned, the horizontal movement has no affect; the only concern with stroke is it's vertical movement. This delay in closing of the intake valve allows the incoming mixture to fill the cylinder. As the crankshaft contiunes to roatate the relationship of this horizontal versus vertical movement changes. At 45 degrees they are equal and at 90 degrees there is no horizontal movement, only vertical. The reason for the above is that it's important to understand that as the duration of the cam is increased, the longer the valves are left open, the greater the affect it will have on dynamic compression.
Race Director
Joined: May 2004
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Originally Posted by mandm1200
First of all I am not an expert, engineer, or professional engine builder. With that said, I can only apply information that I believe to be correct.
Static Compression, SC, is fairly simple. Take the volume when the piston is at BDC and divide it by the volume when the piston is at TDC. Some things needed to known to figure this: bore and stroke of the cylinder, bore and thickness of the head gasket, size of the combution chamber of the head, and the piston to deck clearance. Head chamber size is in cc's while other measurements are in inches. Something must be converted so that you are using similiar terms. The easiest thing is to find a web page that allows you to just type in nubers and have it do the calculation.
Dynamic Compression, DC, is almost identical to the above with the exception of the fact it takes into consideration that compression does not start to build until the intake valve is closed. If the intake valve closed at BDC, then DC equals SC. All cams have the intake valve open as the piston starts to travel upwards in the compression stroke. Now for an example: if the stroke of the engine is 3.50" and the intake valve remains open for 90 degrees (1/2 way of the compression stroke), then the effective stroke is 1.75". This would cut the compression ratio by almost 1/2.
When the intake valve is open just a few degees after BDC, it has very little affect on the dynamic stroke. The reason for this is the as the crank is starting to turn from BDC, the bottom of the connecting rod is moving more horizontal than vertical. As far as stroke is concerned, the horizontal movement has no affect; the only concern with stroke is it's vertical movement. This delay in closing of the intake valve allows the incoming mixture to fill the cylinder. As the crankshaft contiunes to roatate the relationship of this horizontal versus vertical movement changes. At 45 degrees they are equal and at 90 degrees there is no horizontal movement, only vertical. The reason for the above is that it's important to understand that as the duration of the cam is increased, the longer the valves are left open, the greater the affect it will have on dynamic compression.
Static Compression, SC, is fairly simple. Take the volume when the piston is at BDC and divide it by the volume when the piston is at TDC. Some things needed to known to figure this: bore and stroke of the cylinder, bore and thickness of the head gasket, size of the combution chamber of the head, and the piston to deck clearance. Head chamber size is in cc's while other measurements are in inches. Something must be converted so that you are using similiar terms. The easiest thing is to find a web page that allows you to just type in nubers and have it do the calculation.
Dynamic Compression, DC, is almost identical to the above with the exception of the fact it takes into consideration that compression does not start to build until the intake valve is closed. If the intake valve closed at BDC, then DC equals SC. All cams have the intake valve open as the piston starts to travel upwards in the compression stroke. Now for an example: if the stroke of the engine is 3.50" and the intake valve remains open for 90 degrees (1/2 way of the compression stroke), then the effective stroke is 1.75". This would cut the compression ratio by almost 1/2.
When the intake valve is open just a few degees after BDC, it has very little affect on the dynamic stroke. The reason for this is the as the crank is starting to turn from BDC, the bottom of the connecting rod is moving more horizontal than vertical. As far as stroke is concerned, the horizontal movement has no affect; the only concern with stroke is it's vertical movement. This delay in closing of the intake valve allows the incoming mixture to fill the cylinder. As the crankshaft contiunes to roatate the relationship of this horizontal versus vertical movement changes. At 45 degrees they are equal and at 90 degrees there is no horizontal movement, only vertical. The reason for the above is that it's important to understand that as the duration of the cam is increased, the longer the valves are left open, the greater the affect it will have on dynamic compression.
Drifting
Joined: Sep 2003
Posts: 1,325
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From: FRANKLIN GA
I'm suprised no one has mentioned Quench.
I'm running 11.94:1 on pump gas with the timing retarded a little, and the carb a little rich, and a long duration cam, but I still don't think I would be able to run pump gas at all if I didn't have so tight a quench (.037). A very tight quench in the .035 to .045 range will allow you to run at least a half a point higher, than you would normaly be able to run, with everything else being equal.
I'm running 11.94:1 on pump gas with the timing retarded a little, and the carb a little rich, and a long duration cam, but I still don't think I would be able to run pump gas at all if I didn't have so tight a quench (.037). A very tight quench in the .035 to .045 range will allow you to run at least a half a point higher, than you would normaly be able to run, with everything else being equal.
