Cam Profile effect on Compression Ratio?
09-15-2002, 10:38 PM
#1
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Cam Profile effect on Compression Ratio? I've only seen this mentioned indirectly here over the years. How does a Cam profile affect the Compression Ratio of an engine?
Why do larger cams allow you to run higher compression ratios? Am confused. Someone plz help me out here. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />
Thx!
Why do larger cams allow you to run higher compression ratios? Am confused. Someone plz help me out here. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />
Thx!
09-16-2002, 01:30 PM
#2
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Re: Cam Profile effect on Compression Ratio? Good question - ttt for ya.
I've heard that a smaller LSA creates more dynamic compression, but I would like to hear the full story.
I've heard that a smaller LSA creates more dynamic compression, but I would like to hear the full story.
09-16-2002, 03:17 PM
#3
Re: Cam Profile effect on Compression Ratio? Compression ratio is not cam related. It is volume related to the cylinder, piston top, and heads.
Total cylinder pressure at any given rpm is a function of the cam and how much compression gets bled off.
Question 2 is still a valid question and I'll leave it to those who are more articulate then me to explain it. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />
<small>[ September 16, 2002, 03:19 PM: Message edited by: Ragtop 99 ]</small>
Total cylinder pressure at any given rpm is a function of the cam and how much compression gets bled off.
Question 2 is still a valid question and I'll leave it to those who are more articulate then me to explain it. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />
<small>[ September 16, 2002, 03:19 PM: Message edited by: Ragtop 99 ]</small>
09-16-2002, 06:33 PM
#4
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Thread Starter
Re: Cam Profile effect on Compression Ratio? ttt
I've heard larger cams 'bleed off' compression and allow you to run more compression? Is it something to do with overlap? How does the cam choice relate to how much compression you can run?
TIA
I've heard larger cams 'bleed off' compression and allow you to run more compression? Is it something to do with overlap? How does the cam choice relate to how much compression you can run?
TIA
09-16-2002, 11:35 PM
#5
Re: Cam Profile effect on Compression Ratio? </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by MelloYellow:
<strong>I've only seen this mentioned indirectly here over the years. How does a Cam profile affect the Compression Ratio of an engine?
Why do larger cams allow you to run higher compression ratios? Am confused. Someone plz help me out here. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />
Thx!</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">O.k. I'll take a shot at this.
First there are two types of compression.
1. Static Compression, which is what we talk about 10:1 on a stock LS1. Which is what is stated as the compression ratio above.
2. Dynamic Compression, Dynamic Compression is always less than the standard compression ratio, and ranges usually from 6 to 9 for most engines. You've probably heard that long duration cams need higher compression ratios this is because the long duration cams with late intake valve closing events also require a higher static compression ratio to get the dynamic compression into the 6 to 9 range.
The formula for Dynamic Compression Ratio is:
Dynamic Comp Ratio = (Clearance Vloume + Cylinder Volume @ Intake Valve Closing) / Clearance Volume
(Clearance Volume is the Combustion Chamber: Piston dish/dome, Chamber, deck and gasket volume)
So basically the larger cam has more overlap which will let more intake escape the combustion chamber thru the exhaust valve. This in turn will actually take air/fuel out of the combustion chamber and keep it from being compressed. Therefore it takes away from the compression.
You get more high rpm hp with the overlap, but you also need more dynamic compression and the only way to get that is too add static compression.
I hope that helps.
If not i'll see what else I can tell you to clear it up.
Bret
<strong>I've only seen this mentioned indirectly here over the years. How does a Cam profile affect the Compression Ratio of an engine?
Why do larger cams allow you to run higher compression ratios? Am confused. Someone plz help me out here. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />
Thx!</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">O.k. I'll take a shot at this.
First there are two types of compression.
1. Static Compression, which is what we talk about 10:1 on a stock LS1. Which is what is stated as the compression ratio above.
2. Dynamic Compression, Dynamic Compression is always less than the standard compression ratio, and ranges usually from 6 to 9 for most engines. You've probably heard that long duration cams need higher compression ratios this is because the long duration cams with late intake valve closing events also require a higher static compression ratio to get the dynamic compression into the 6 to 9 range.
The formula for Dynamic Compression Ratio is:
Dynamic Comp Ratio = (Clearance Vloume + Cylinder Volume @ Intake Valve Closing) / Clearance Volume
(Clearance Volume is the Combustion Chamber: Piston dish/dome, Chamber, deck and gasket volume)
So basically the larger cam has more overlap which will let more intake escape the combustion chamber thru the exhaust valve. This in turn will actually take air/fuel out of the combustion chamber and keep it from being compressed. Therefore it takes away from the compression.
You get more high rpm hp with the overlap, but you also need more dynamic compression and the only way to get that is too add static compression.
I hope that helps.
If not i'll see what else I can tell you to clear it up.
Bret
09-20-2012, 09:21 PM
#6
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im a digger but this question is what im trying to find out and since its been 10 years, im sure some more specific anwsers can be posted here.
im wondering how some thing like how a 228R cam would effect stock 5.7L LS1 with the standard 10.1:1 CR?
im wondering how some thing like how a 228R cam would effect stock 5.7L LS1 with the standard 10.1:1 CR?
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09-20-2012, 10:58 PM
#8
Compression ratio is indeed related to the camshaft.
You build for static compression which is area under the curve without valve events taken into consideration. Static compression increases volumetric efficiency on all strokes of a four stroke design. Better swirl on intake, more complete emptying of the cylinder on the exhaust phase which is removing carbon, un-used fuel atoms, and used oxygen atoms, and a faster burn on the compression stroke which enables more power with less timing advance due to the increased pressure.
Now on this note, on the compression stage, where the intake valve begins to close on the compression stage is where dynamic compression begins to build. The earlier the IVC event occurs the more area under the curve that is gained in torque for a specific RPM based on advertised duration. The later the IVO event occurs the less area under the curve used to create torque for the compression phase. If the IVO occurs just at the right moment as another cylinder is at IVC greater than 0.006" then the piston can scavenge through the manifold from another cylinder. Dual plane intakes increase torque by isolating the chamber banks and to prevent or minimize this intake scavenging effect in mid RPM ranges. If this happens then the cam is not correctly chosen for the RPM range the motor is intended for.
Early IVO helps with scavenging from the exhaust valve event from backpressure built into the exhaust system. This is beneficial because it helps to pull an intake charge into the combustion chamber. Its efficiency is based on exhaust efficiency and overlap of IVO and EVC. Where it happens in relation to valve events, your piston location, and the LSA, is your ICL or installed center line. The more compression a motor has, the wilder the overlap and durations of both exhaust and intake opening events can be. This shifts your RPM range higher for maximum HP but can limit your torque due to the early opening/late closing of the intake valve during the 4 stage process. Wider LSA's table top a max RPM motor for torque located elsewhere under the curve depending on the durations of the camshaft which depend on the efficiency of the motor which directly relate to static and dynamic compression ratios.
Installing a cam on a lower number ICL than a higher than advertised LSA will bring in torque earlier and build torque throughout the low RPM range. This is the same as looking at it through the degrees of IVO - the sooner it opens the more air and fuel mixture that is brought in - closing it late enough to get scavenging benefits from the exhaust duration really comes into play especially if the exhaust is not limited to efficiency in design or too large for the application.
You build for static compression which is area under the curve without valve events taken into consideration. Static compression increases volumetric efficiency on all strokes of a four stroke design. Better swirl on intake, more complete emptying of the cylinder on the exhaust phase which is removing carbon, un-used fuel atoms, and used oxygen atoms, and a faster burn on the compression stroke which enables more power with less timing advance due to the increased pressure.
Now on this note, on the compression stage, where the intake valve begins to close on the compression stage is where dynamic compression begins to build. The earlier the IVC event occurs the more area under the curve that is gained in torque for a specific RPM based on advertised duration. The later the IVO event occurs the less area under the curve used to create torque for the compression phase. If the IVO occurs just at the right moment as another cylinder is at IVC greater than 0.006" then the piston can scavenge through the manifold from another cylinder. Dual plane intakes increase torque by isolating the chamber banks and to prevent or minimize this intake scavenging effect in mid RPM ranges. If this happens then the cam is not correctly chosen for the RPM range the motor is intended for.
Early IVO helps with scavenging from the exhaust valve event from backpressure built into the exhaust system. This is beneficial because it helps to pull an intake charge into the combustion chamber. Its efficiency is based on exhaust efficiency and overlap of IVO and EVC. Where it happens in relation to valve events, your piston location, and the LSA, is your ICL or installed center line. The more compression a motor has, the wilder the overlap and durations of both exhaust and intake opening events can be. This shifts your RPM range higher for maximum HP but can limit your torque due to the early opening/late closing of the intake valve during the 4 stage process. Wider LSA's table top a max RPM motor for torque located elsewhere under the curve depending on the durations of the camshaft which depend on the efficiency of the motor which directly relate to static and dynamic compression ratios.
Installing a cam on a lower number ICL than a higher than advertised LSA will bring in torque earlier and build torque throughout the low RPM range. This is the same as looking at it through the degrees of IVO - the sooner it opens the more air and fuel mixture that is brought in - closing it late enough to get scavenging benefits from the exhaust duration really comes into play especially if the exhaust is not limited to efficiency in design or too large for the application.
Last edited by SS10Tech; 09-21-2012 at 01:14 PM.
