Overlap...intake stroke or exhaust stroke.
11-04-2013, 09:27 PM
#1
Overlap...intake stroke or exhaust stroke. Was looking at a few different popular grinds and comparing their valve events. Saw quite a variation even with close specs, so was wondering what the difference in where the overlap is located in the timing events.
For example, do you want the overlap to be while on the exhaust stroke or on the intake stroke.
Example A:
Intake valve opens @ 4* BTDC and exhaust valve closes right @ TDC. So in this instance, the 4* of overlap are while the piston is going up on the exhaust stroke.
Example B:
Intake valve opens right @ TDC and the exhaust closes @ 4* ATDC. So in this intance, the 4* of overalap are while the piston is going down on the intake stroke.
Which one of those examples would promote a better timing event? Or do you want a little bit of both, say 2* on the exhaust stroke and 2* on the intake stroke?
For example, do you want the overlap to be while on the exhaust stroke or on the intake stroke.
Example A:
Intake valve opens @ 4* BTDC and exhaust valve closes right @ TDC. So in this instance, the 4* of overlap are while the piston is going up on the exhaust stroke.
Example B:
Intake valve opens right @ TDC and the exhaust closes @ 4* ATDC. So in this intance, the 4* of overalap are while the piston is going down on the intake stroke.
Which one of those examples would promote a better timing event? Or do you want a little bit of both, say 2* on the exhaust stroke and 2* on the intake stroke?
11-05-2013, 12:53 PM
#3
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My understanding is: example B will help the engine carry past peak better. So I've seen that as favourable on these boards. That and the "equally split" example. But I don't know if there are any drawbacks to it on the low end as opposed to example A. I am also staying tuned.
Last edited by dirty_old_chevy; 11-05-2013 at 02:03 PM. Reason: elaborated a bit
11-05-2013, 02:33 PM
#4
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Neither has an advatange over the other. It depends on what is trying to be accomplished.
Different valve size, port sizes, engine bore, crank, rods, camshaft lobe characteristics, exhaust, intake manifold, desired powerband, weight of car, transmission, stall, gearing, rear gear, etc. All play a roll in where the overlap event takes place. If overlap is even needed.
When you decide that you even need overlap, how much should be determined based on need to get a desired engine efficiency along the powerband vs. sound, driveability, or tuneability. Most guys just select goobs of overlap and they don't need the high amounts as the intakes they are using will be the most limiting restriction. Also with the excessive overlap they are wasting energy and fuel and increasing intake port recovery time.
Many guys only look at how much overlap. Its not how much but where it occurs for the particular combination. So to answer your question, there i no one clear cut answer, and if they try to tell you this, run fast.
Example, where I select the overlap to occur for cathedrals is totally different for most squareport combos.
Different valve size, port sizes, engine bore, crank, rods, camshaft lobe characteristics, exhaust, intake manifold, desired powerband, weight of car, transmission, stall, gearing, rear gear, etc. All play a roll in where the overlap event takes place. If overlap is even needed.
When you decide that you even need overlap, how much should be determined based on need to get a desired engine efficiency along the powerband vs. sound, driveability, or tuneability. Most guys just select goobs of overlap and they don't need the high amounts as the intakes they are using will be the most limiting restriction. Also with the excessive overlap they are wasting energy and fuel and increasing intake port recovery time.
Many guys only look at how much overlap. Its not how much but where it occurs for the particular combination. So to answer your question, there i no one clear cut answer, and if they try to tell you this, run fast.
Example, where I select the overlap to occur for cathedrals is totally different for most squareport combos.
11-05-2013, 08:11 PM
#7
Neither has an advatange over the other. It depends on what is trying to be accomplished.
Different valve size, port sizes, engine bore, crank, rods, camshaft lobe characteristics, exhaust, intake manifold, desired powerband, weight of car, transmission, stall, gearing, rear gear, etc. All play a roll in where the overlap event takes place. If overlap is even needed.
When you decide that you even need overlap, how much should be determined based on need to get a desired engine efficiency along the powerband vs. sound, driveability, or tuneability. Most guys just select goobs of overlap and they don't need the high amounts as the intakes they are using will be the most limiting restriction. Also with the excessive overlap they are wasting energy and fuel and increasing intake port recovery time.
Many guys only look at how much overlap. Its not how much but where it occurs for the particular combination. So to answer your question, there i no one clear cut answer, and if they try to tell you this, run fast.
Example, where I select the overlap to occur for cathedrals is totally different for most squareport combos.
Different valve size, port sizes, engine bore, crank, rods, camshaft lobe characteristics, exhaust, intake manifold, desired powerband, weight of car, transmission, stall, gearing, rear gear, etc. All play a roll in where the overlap event takes place. If overlap is even needed.
When you decide that you even need overlap, how much should be determined based on need to get a desired engine efficiency along the powerband vs. sound, driveability, or tuneability. Most guys just select goobs of overlap and they don't need the high amounts as the intakes they are using will be the most limiting restriction. Also with the excessive overlap they are wasting energy and fuel and increasing intake port recovery time.
Many guys only look at how much overlap. Its not how much but where it occurs for the particular combination. So to answer your question, there i no one clear cut answer, and if they try to tell you this, run fast.
Example, where I select the overlap to occur for cathedrals is totally different for most squareport combos.
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11-05-2013, 08:27 PM
#8
I would speculate that most experienced cam guys are merely going to pick valve events based on similar combinations to yours that they have seen work well in the past. While some people will try to philosophize why a certain valve event works, nothing is going to to trump those real world results for my dollar. I personally would be most persuaded by somebody who could point to a solid result on a similar combo.
12-03-2013, 08:45 PM
#9
Missed this one quite a while ago now. Agree with Speedtigger on this. Also
PredZ who seems to know his way around valve events usually runs reverse
split cams and believes that equidistant overlap is best as the idle will then
have a distinct "rapid popcorn"***his words not mine*** sound. He always
maximizes his static and dynamic compression ratios as well as exhaust
efficiency. Every engine combination is just that a collection of details and
the ones that really run have ALL the bases covered.
PredZ who seems to know his way around valve events usually runs reverse
split cams and believes that equidistant overlap is best as the idle will then
have a distinct "rapid popcorn"***his words not mine*** sound. He always
maximizes his static and dynamic compression ratios as well as exhaust
efficiency. Every engine combination is just that a collection of details and
the ones that really run have ALL the bases covered.
