Rocket

Does more overlap "bleed off" compression

Visual Aid.

This animation doesn't have overlap but over lap would occur at the end of the exhaust stroke (4) and the beginning of the intake stroke (1). That is as the exhaust is near closing (but still open) the intake valve begins to open even though the piston is still moving up and "pushing" exhaust gases out.

Overlap occurs between (4) and (1). The Compression phase is the second stroke (2). Well before the piston moves up during the compression stroke the exhaust valve has been closed for a long time.

This animation shows the cycle of a very low performance low rpm cam timing (that is, intake valve opens when piston is going down on the intake stroke etc). In high performance cam timing valves open earlier and close later. But this animation will help people see the valve events in relation to the 4 cycles.

mar778c

iTrader: (1)

No. It does reduces the amount of fresh intake charge but the volume of air being compressed is still the same.

kgibson16

found this awhile ago and had it in my bookmarks.

Overlap and Compression- A very common idea, although for the most part incorrect, is that overlap bleeds off compression. Overlap, by itself, does not bleed off compression. Overlap is the angle between the exhaust closing and intake opening and is used to tune the exhaust's ability draw in additional intake charge as well as tuning idle vacuum and controlling power band width. Cylinder pressure is generated during the compression cycle, after the intake valve has closed and before the exhaust opens. Within practical limits, an early intake closing and late exhaust opening will maintain the highest cylinder pressure. By narrowing the Lobe Seperation Angle 'LSA' for a given lobe duration, the overlap increases, but the cylinder pressure can be increased as well. Thus cylinder pressure/compression can actually increase in this scenario, by the earlier intake closing and later exhaust opening. By increasing duration for a given LSA, the overlap will increase, the intake closing will be delayed, and the exhaust opening will occur earlier. This will decrease cylinder pressure, but the decrease/bleed-off of compression is not due to the overlap, it is due to the intake closing and exhaust opening events.

Rocket

:thumbs up: for good All Motor tech talk.

0VtecKiDD

False

Rocket

It prolly helps the motor get more out of da header (literally). VtecKiDD knows dis.

Chris Tune

whoever voted true is full of epic fail

Rocket

You'd think there would be more interest here since it involves sexy concepts like air flow, volumetric efficiency, scavenging, cam timing, dynamic compression etc.

Natural Aspirations

I was always under the impression that overlap "could" bleed off STATIC compression but when adjust properly has little/no effect on dynamic compression in the higher VE area. This is incorrect.

The point of overlap occurs during the intake stroke and is unable to actually "bleed off" any pressure. Without an efficient system the exhaust gasses can revert back into the combustion chamber and cause contamination of the intake charge. I learned something today simply today by reading this and watching the S2 4-Stroke video. More overlap can actually increase dynamic compression.

On a related side note, why is it that a cam manufacturers recommend higher compression for cams with more overlap.

Thanks Rocket!

Rocket

Generally cams that have more overlap are longer duration. If one adds duration to a cam and keeps the lobe centers the same, this in itself increases overlap.

Longer duration means later intake closing, so at lower rpms you are pushing some of intake charge out. This will make you loose low end torque which increased static CR can help bring back because higher static CR increase torque at every rpm.

Carburetor guys run long duration cams know this. With long duration cams, the intake charge will actually blow right out of the carb and create what is called "stand off". That's why it's not a good idea to look into a carb and open the throttle; you'll get a "fuel facial."

Fo-shizzay.

Natural Aspirations

So its not the overlap in a large cam that bleeds off compression it is the longer duration intake cam that closes farther past BDC and the longer duration exhaust cam opening farther before TDC.

In this scenario increasing overlap may also increase static compression?

Rocket

Among other things, yes. Read kgibson16's post, basically says that.

numberone

iTrader: (2)

Bump for the good info here.

PyroProblem

I always thought increasing overlap/smaller LSA could reduce DYNAMIC compression, hence why K series guys can run such high 13+:1 compression ratios on pump gas...
I was under the assumption they could manipulate the intake valve timing to reduce cylinder pressures and prevent knock in the lower rpms?

But in the same sense, if you increase overlap in a way that the exhaust actually helps draw in more intake charge, I guess you would be RAISING dynamic compression/cylinder pressures...
Hmmmmm....

Furyof4

Wow I learned some **** on h-t!

Great info, thank you.

shifty35

My understanding was that a given cam profile is optimized for a fairly narrow RPM range. If you look at a dyno of a well tuned motor, the torque peak should occur where the engine has its highest volumetric efficiency / cylinder pressure. That means that at that RPM, all the valves close in such a way that the maximum intake charge (largest mass of fuel and air) is present in the cylinder. Move away from that RPM in either direction and you start losing intake charge in some form or another.

Isn't that what "dynamic compression" is anyway? It's related to the *total* intake charge present, as well as the static compression ratio.

For a given SCR, if you can find a way to trap more air in the cylinder, your dynamic compression goes up...

So yes, at RPMs that are far away from peak torque, a big cam will "lower" your dynamic compression ratio. But around peak torque, dynamic compression will be higher.

Correct me if anything is out of place... I knew I should have done the Mechanical Engineering program instead of Computer Engineering...

Rocket

Looks like the start of a good educational series.

I'll see about writing a post to expand and introduce some more technical terms like BMEP and relate them to common terms like torque and HP. That way we can have a good foundation to build good technical discussion.

My goal is to have a more knowledgeable and informed community. This helps me, as I offer products based on technology. The better people understand the technology the better for me and all of us.

kgibson16

thumbs up for technical education.

mar778c

iTrader: (1)

Werd up to M.OT.U.

Getting the Tech back in Honda-Tech.

DonF

Pretty close, BUT VTEC engines have atleast 2 cam profiles. That would change cyl. fill depending on which profile the engine is on. Therefore you have 2 dynamic compression formulas to work with, 1 at lower RPMS and abother that starts at crossover. Mech. Engineering would not help unless you were designing bridges or the max. amount of cycles a connecting rod has before failure, depending on material and design strength. "overlap" and compression are RPM dependant. Large overlap loses at low RPM and gains at high RPM, so the anwer is,no on overall engine performance.

Professor15

In some programs you can take internal combustion engines as an elective f.y.i.

BIGTUNA

Intake Valve Closing angle is the most overlook part of cam profile concerning compression, overlap with a good scavenging header can increase dynamic compression via higher low lift flow. A small IVC adjustment can have massive gain depending on rpm/combo of the engine, I know it did with me.

Rocket

I've added an animated gif to the top post for peeps.

Keep da thinking caps on. More meaningful discussion will follow.

BIGTUNA

Buying a degree wheel and some dial indicator's taught me more than cruising the net ever would. I have a big notebook of all the timing events, cam profiles, dyno sheets ect. Its pretty cool when you cross over from "gimme stage whatever cam" to actually getting a feel for what your motor likes and fine tuning it from there.

I'm not saying I know allot but loading cams and turning the motor over watching the dials, writing stuff done opened my eyes to allot of things.

Vengeance

Mechanical Engineering isn't just "structural" engineering...
Who did you think designs engines in the first place? Who do you think designs the factories that they're built in?LMAO I can't believe you actually said that.Why isn't there another option for sometimes? Are you only talking NA? Max efficiency intake and exhaust with ram-air at speed? Even without the ram-air it still may be possible with a very efficient intake/exhaust setup(optimized for the same rpm). Blowing fresh intake charge out of the exhuast valves from too much overlap is well documented(which I thought you would know) so I'm going to go against everyone who's posted so far and say yes, it is possible...