Do higher lift & duration cams with cam timing for more overlap change compression ratio notably?

moderate cams JWT S2 and advanced cam timing as much as will make power (tune not done yet).

 

Arent they independant of each other? Never knew cams affected compression ratio. Thought the only way to affect compression was to increase/decrease height of the piston crown or gasket mod?

 

There are actually two kinds of compression, static and dynamic.

Static compression is the most common form of calculated compression. Static compression is calculated by mathematically figuring out the compression ratio with cylinder volume... head gasket volume... and cylinder head volume.

Dynamic compression is the actual compression in a built motor and is affected by the camshaft profiles. Opening valves at different times and changing the amount of time they stay open can all affect dynamic compression.

 

thanks jb, thats what I am trying to determine...

{from other thread}
Considering this - would the dynamic compression ratio be notably higher? And would it warrant higher octane? I normally run 105 race gas at the track, but if 100 pump gas would yield the same, I'll do that...

So the question is would the cam profile I mentioned in first post combined with cam timing to maximum overlap result in higher than 10.3:1 compression ratio? Or is it impossible to determine?

 

Actually higher duration cams lower your compression. To take full advantage of very high duration cams high compression pistons are needed.

 

exactly ^

 

I don't quite understand this. Assuming higher duration cams let more air in - you are still starting with the same volume of air at bottom dead center and compressing it into the same top dead center volume of air. No matter how much air you start with, it's still being compressed in the same ratio. What dynamic affects are there?

 

http://weblog.xanga.com/AussieGSR/63...mpression.html

will take a few times of reading to digest it, but it's a worthwhile read

 

I have read several similar type explanations (found in the link)... which is what brought the question in the first place... however, I still havent got an answer in either thread

I guess no one else knows either?

 

you really should ask Jim Wolf. My guess it no, there will not be any substantial change, but I don't have a cam card for that cam to know the answer 100%

 

spoke to JWT and Technosquare... both state compression "ratio" is unchanged - but compression "pressure" is altered... and even though I mentioned "pressure" in one of my questions, I clearly didn’t phrase the question properly by asking about "ratio".

So to make it clear - higher duration cams do alter compression pressure - but not compression ratio.

Sorry for the thread.

 

OK, I just read that. There is no "dynamic compression RATIO". Compression RATIO is always the same. That article is talking about the dynamic affects of cams and how the air is compressed in the cylinder.

If you get more aggressive cams you should get your car tuned. You will be bringing in more air and so you run the risk of running too lean. As far as I know our cars don't come with wideband 02 sensors so your car has no way of knowing it's running lean.

 

Some people refer to it as the dynamic compression, others call it compression pressure. Same thing, different terminology. Factory widebands or not, the car needs to be tuned, but I know the OP has that covered already

 

yes, car will be Osiris re-tuned... and one of the maps has agressive timing which is why i run race gas... I know there is a debate about the value of race gas, but its not just the higher octane for more advanced timing without knock that I run it, it's also generally cleaner than pump gas and doesnt have the additives generally found (in CA designer gas).

 

The static compression is the highest compression a motor can generate. Static compression is calculated with the valves closed, so there is nothing that could increase the static compression.

Dynamic compression can only be equal to static compression at the best. Dynamic compression will always be either equal or less than static compression.

I have no idea about that. I'm not sure who could give you that kind of specific information.

When I was choosing cams for my engine, I called JWT. I talked to a Tech, and then an Engineer, and was passed along to Jim Wolf himself. He was very generous with his time and even questioned the designer of the specific cams we were talking about for a question or two. It was a very enlightening and worthwhile conversation.

You might want to give JWT a call like Adam recommended. You might not end up talking to Jim Wolf, but if you are seriously thinking about buying one of their cams, it would be a good investment.

 

+1 Dynamic compression is the same as compression pressure.

 

Because his dynamic compression can never be higher than static the answer to whether his compression could be higher than the static 10.3 the answer is no. However his dynamic compression could be higher with different cams than with the stock cams. Since the cams are more aggresive, more than likely the dynamic compression would be lower since there would be more overlap.

 

Thanks, yes I've had several conversations with JWT tech and went down there and met with Jim personally - you are absolutely correct, he is amazingly generous with his time and went way above what I could have hoped for in helping with my situation (wont bore anyone with all the details here) - he is a very bright guy!

I already have the cams - in fact they were recently installed (with other things) motor is going back in today - tune early next week.

 

yes, pressure will be lower... so the initial question about compression ratio as it pertains to octane level is moot at this point... however, as I stated earlier, I still run race gas for other reasons (only in aggressive map and only at the track).

 

If anything cams with more overlap would reduce compression. More overlap is designed to get more air in/out of the engine. But that means the intake/exhaust valves are open at the same time for a longer time period. There's more intermixing of the intake/exhaust charges. You accept this downfall for more higher rpm hp potential.

BUT I bet you could really minimize this downfall and maximize the potential by playing around with cam timing. Especially if they can be timed independant of each other.

Cams with more lift but with stock type durations are good for slightly more hp and good for torque. The engine is getting more air in but minimizing how much intermixing there is. Keeping the dynamic compression ratio as high as possible.

But most of this is just general info. It really depends on how the oem cams are and what the engine really responds to.

I don't think the race gas would hurt anything. If anything, it might allow you to run a very aggressive ign timing map that might otherwise ping with 93 octane.