Highest CR on pump gas?
05-30-2013, 02:52 PM
#41
So how do bigger cams with later closing intake valves make more power than smaller cams with earlier closing valves........According to your calculator a stock cam would have a higher compression ratio than an aftermarket cam which means there's more air in the cylinder whould would equal more power.......
If your static compression is high and you have a stock(ish) cam, then your dynamic compression could easily be above the sweet spot for the engine. To avoid detonation, the tune would have to pull excessive timing.....compromising performance.
On the other hand, a more aggressive ('larger') cam would lower the dynamic compression. If the 'larger' cam lowers the dynamic compression into that sweet spot, more power will be made providing you have a good, solid tune.
Additionally, if you have moderate (or low) static compression (like <10.5:1 in a LT1/4) and a 'big' aftermarket cam, then you will lower the dynamic compression below the sweet spot of the engine. Doing this will make your engine a sluggish, weak performer, no matter how well it's tuned. This is what people mean when they say that "bigger is not always better"......
KW
Last edited by KW Baraka; 05-30-2013 at 03:04 PM.
05-31-2013, 08:45 AM
#43
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So how do bigger cams with later closing intake valves make more power than smaller cams with earlier closing valves? If the dynamic compression ratio was related to how an actual engine works in the real world you would want a cam that closed when the piston is at the bottom of the stroke. According to your calculator a stock cam would have a higher compression ratio than an aftermarket cam which means there's more air in the cylinder whould would equal more power.
Those calculations assume the engine is operating at 100% ve with no scavenging going on or anything like it. The whole thing just started so internet experts had another talking point to try to make themselves look smart.
Those calculations assume the engine is operating at 100% ve with no scavenging going on or anything like it. The whole thing just started so internet experts had another talking point to try to make themselves look smart.
The question was about safe compression ratio and then effect of valve timing entered the discussion. Some people had questions about it and how it ties in to running a certain static compression. Dynamic compression calculation answers that. I don't really understand the argument you are making.
05-31-2013, 08:52 AM
#44
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If the cam is already sized correctly for the current compression, a half point static compression increase is worth maybe 1.5% hp increase max.
05-31-2013, 02:14 PM
#45
TECH Resident
10.5:1 is stock compression, the .026 head gasket your considering will get you close to 11:1. The reverse cooling on the LT1 allows these engines to run higher compression with a good tune and premium fuel.
Do you live at a higher elevation? In the midwest we have 93 octane, areas at higher elevations have lower octane premium blends because NA engines have lower octane requirements in the less dense air as you increase in elevation.
Do you live at a higher elevation? In the midwest we have 93 octane, areas at higher elevations have lower octane premium blends because NA engines have lower octane requirements in the less dense air as you increase in elevation.
05-31-2013, 08:13 PM
#46
The question was about safe compression ratio and then effect of valve timing entered the discussion. Some people had questions about it and how it ties in to running a certain static compression. Dynamic compression calculation answers that. I don't really understand the argument you are making.
I've never seen a serious shop/engine builder talk about it, just the "internet experts".
With everything being equal how does a bigger cam with a later intake closing point make power than the stock cam if there's less air being compressed and mixed with fuel? According to the calculator the engine is sucking in less air.
05-31-2013, 08:31 PM
#47
If you have E85 available in your area, just switch to that. It can tolerate very high compression and is much cheaper than race gas. Just built a 13.5:1 385 HR that is getting installed this week built for E85. Been running it for 4+ years on my stock shortblock and later with mild cam at 11.7:1.
06-01-2013, 07:21 AM
#49
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The dynamic compression ratio assumes the cylinder is filled 100% with air and the exact amount if air was being pushed back out of the cylinder before the valve is closed which is not the case and can will vary greatly between cars.
I've never seen a serious shop/engine builder talk about it, just the "internet experts".
With everything being equal how does a bigger cam with a later intake closing point make power than the stock cam if there's less air being compressed and mixed with fuel? According to the calculator the engine is sucking in less air.
I've never seen a serious shop/engine builder talk about it, just the "internet experts".
With everything being equal how does a bigger cam with a later intake closing point make power than the stock cam if there's less air being compressed and mixed with fuel? According to the calculator the engine is sucking in less air.
Are you familiar with a thing called "RPM" and how HP is calculated? You are looking at one component of the way an engine operates and choosing to look at it one dimensionally for the sake of starting an argument. I never saw anyone make the claim that smaller cams make more power yet here you are arguing it. The original question(s) were what is a safe static compression ratio for a given octane and why.
A given engine setup makes a certain amount of torque which is just the capability for doing work (hp). In order to actually do work and not just have the capability, the torque must be applied in a specific amount of time. So essentially you can have two 9:1 compression engines with two different cams that make the exact same torque or have the capability to make the same hp. But, because of the longer duration camshaft's ability move the torque production to a different RPM range it can make more hp. Why? Because gas inertia is identical in both engines. So, both engines have the CAPABILITY to make the exact same PEAK hp but only one has the ABILITY to make more due to the laws of physics. You install a blower on the small cam engine and you just bypassed one of those laws and now it makes more power. But, you're no longer talking about identical engine setups when you make a comparison between the two.
Any serious engine builder knows exactly what dynamic compression is and how to use it. As stated earlier, there are known values for safe dynamic compression to run at all octane and temperature levels. A serious engine builder will not build you a 12.5:1 engine and then put a 256 duration cam in it (unless you said the engine would never be run over 3000 RPM or 20 degrees timing) and you wanted maximum torque), and he will not build you an 8:1 compression engine in it and put a 305 duration cam it. Reason being is that when you know what you are doing, you know that with a certain static compression how much cam is needed to keep cylinder pressure at it's safest peak point which is what dynamic compression tells you. There are plenty of things that engine builders know that you may not see them talk about. Serious engine builders build for application and desired performance, not peak anything.
Once again, I don't understand why you choose to look at a calculation that answers a simple question and then argue some other unrelated aspect. It's the equivalent of joining a conversation about how a Chinese abacus works and saying "I've never heard of anyone counting higher than five so that thing doesn't work, is wrong, and doesn't apply to the world I live in!"
Last edited by SSellers; 06-01-2013 at 07:42 AM.
06-01-2013, 10:16 AM
#50
My point is dyanmic compression ratio doesn't take in to account all the other important variables in the engine. You could build an engine with a dynamic compression ratio of 9.5 that runs fine on 87 octane or one with 8.5 that won't even run on 93. A blanket statement such like don't go over 9.0 on a street car is useless.
06-01-2013, 12:54 PM
#51
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My point is dyanmic compression ratio doesn't take in to account all the other important variables in the engine. You could build an engine with a dynamic compression ratio of 9.5 that runs fine on 87 octane or one with 8.5 that won't even run on 93. A blanket statement such like don't go over 9.0 on a street car is useless.
Exactly why it was stated that there are charts available that show the range of safe dynamic compression given OCTANE and TEMPERATURE. Who made blanket statements?
Read Pat Kelley's page about it especially at the end where he says "Using this information: DCR is only a tool, among others, that a builder has available. It is not the "end all" in cam or CR selection. However, the information provided is very useful for helping to match a cam to an engine or an engine to a cam. It is still necessary to match all the components in an engine and chassis for the best performance possible. Pairing a 305º cam with milled 882 heads just won't cut it even if the DCR is correct. The heads will never support the RPM capabilities of the cam.
The information given here should be used as a guideline only. There are no hard and fast rules. It is up to you, the engine builder, to determine the correct build of your engine. And remember, unless accurate measurements are taken, these calculations are approximations."
http://members.uia.net/pkelley2/DynamicCR.html
If you have any arguments with it, take it up with him. Of course you will not get a response as he has passed on.
Last edited by SSellers; 06-01-2013 at 01:00 PM.
06-02-2013, 10:32 AM
#52
My point is dyanmic compression ratio doesn't take in to account all the other important variables in the engine. You could build an engine with a dynamic compression ratio of 9.5 that runs fine on 87 octane or one with 8.5 that won't even run on 93. A blanket statement such like don't go over 9.0 on a street car is useless.
Engine power is all about cylinder pressure and cylinder filling CAPABILITY. Most cars that run really well have over 100%VE at a given RPM range, never across the board. Where you want to achieve the max cylinder filling is what makes a combo really run hard.
As stated earlier quench goes hand in hand with DCR for a pumpgas car, the tighter you can squeeze the more of a complete burn you get cutting down on detonation if the tune is correct. DCR is a camshaft dependent number.
Also having NO sharp edges in the chamber is a very important part of running on the edge with pumpgas.
You saying DCR is some fabricated "talking point" for message board bullshit artists? Pot meet kettle.
This is a pretty good article to read.
http://cochise.uia.net/pkelley2/DynamicCR.html
