Valve overlap

[Old as Dirt]

If you forget about 4 strokes and read up on 2 stroke exhaust systems, you will see the real meaning of "coming on the pipe". The exhaust pulsing may pull in intake mix while the piston is at the bottom of the stroke.

I've seen small 2 stroke engine HP claims as high as 6 HP per cubic inch, normally aspirated, achieved without valves, and never any concerns about valve float.

Perfect example:
One of my Open Class dirt Kart engines..
BRC 150RR billet 2 stroke..
Under 9.2 CI (6.73hp per CI)
62hp on the dyno @ 12,750 rpm
Dual carbs(Buller Billet) on Methanol
8oz per gallon Blendzall bean oil/110 race gas

[Old as Dirt]

Nationals @ Lasoski..

[CGT]

If you forget about 4 strokes and read up on 2 stroke exhaust systems, you will see the real meaning of "coming on the pipe". The exhaust pulsing may pull in intake mix while the piston is at the bottom of the stroke.

I've seen small 2 stroke engine HP claims as high as 6 HP per cubic inch, normally aspirated, achieved without valves, and never any concerns about valve float.

Perfect example:
One of my Open Class dirt Kart engines..
BRC 150RR billet 2 stroke..
Under 9.2 CI (6.73hp per CI)
62hp on the dyno @ 12,750 rpm
Dual carbs(Buller Billet) on Methanol
8oz per gallon Blendzall bean oil/110 race gas

That's awesome. Can you give anymore details on the billet engine. Whats something like that cost?

[Old as Dirt]

I'll gather up some info and PM you.. thread is going off topic fast.
Cost?
Engine: 4800.00
Dual Carb setup: 500.00
CRE pipe, engine mount, pump, belts, etc.. 600.00

Sold off the engine and Kart, and bought a used BBC Sanger drag boat.. LOL

[408 Nova]

A basic 4-stroke engine like we’re talking about here works off of pressure differentials created by a piston. Overlap is an entirely different thing.

That's where you're wrong.
It's all about pressure differentials.
The piston motion is just partially responsible for the pressure differentials.
We have positive flow before TDC, because the exiting exhaust causes a low pressure area below the intake valve during overlap.
We also have positive flow after BDC, because the velocity of the flow in the port, and the closing intake valve, keep the pressure above the intake valve higher then the pressure below it, even though the piston is on it's way back up.

By basic, I'm talking about a near stock engine with almost no overlap. The affects of overlap on such an engine are negligible.

Everyone knows how overlap works I think, and post BDC filling. All I was pointing out was that in your original desrciption of overlap, that it was vague, because no where in it did you directly say that the exiting high speed exhaust gases causes a low pressure area behind it, and is primarily responsible for cylinder filling on the intake stroke of a race motor, if done correctly.

[swampbuggy]

Camking wrote----We have positive flow before TDC because the exiting exhaust flow causes a low pressure area below the intake valve during overlap. This siphon, suction or whatever one wishes to call it was discovered in the mid 1950's by none other than the legendary Ed Iskenderian. This was given the name 5th cycle, which was way before most cam companies cam into existence. So for those of you folks who did not know the history? The term 5th cycle is a lot more than a catchy name, it was a discovery!!! Mark H.

[408 Nova]

Could we discuss what is happening in the intake port and combustion chamber etc during the overlap cycle?

Easy. Air is flowing from higher pressure to lower pressure.
As long as the pressure above the exhaust valve, is lower then the pressure below the exhaust valve, air is flowing out of the combustion chamber, into the exhaust port.
As long as the pressure below the intake valve is lower then the pressure above the intake valve, air is flowing out of the intake port, and into the combustion chamber. It doesn't matter where the piston is.

This question and answer is what I was referring to.

[CamKing]

Camking wrote----We have positive flow before TDC because the exiting exhaust flow causes a low pressure area below the intake valve during overlap. This siphon, suction or whatever one wishes to call it was discovered in the mid 1950's by none other than the legendary Ed Iskenderian. This was given the name 5th cycle, which was way before most cam companies cam into existence. So for those of you folks who did not know the history? The term 5th cycle is a lot more than a catchy name, it was a discovery!!! Mark H.

False. Ed Winfield documented what was going on during overlap, way before Ed had a clue.
Ed came up with the 5th cycle name, as a sales gimmick. That's all. The cams were no different, or better then anything else.

[CamKing]

By basic, I'm talking about a near stock engine with almost no overlap. The affects of overlap on such an engine are negligible.

Again, that is wrong. Even a little 260/268@.006" hydr RV cam on a 112 LSA have 40 degrees of overlap.

[DaveMcLain]

Camking wrote----We have positive flow before TDC because the exiting exhaust flow causes a low pressure area below the intake valve during overlap. This siphon, suction or whatever one wishes to call it was discovered in the mid 1950's by none other than the legendary Ed Iskenderian. This was given the name 5th cycle, which was way before most cam companies cam into existence. So for those of you folks who did not know the history? The term 5th cycle is a lot more than a catchy name, it was a discovery!!! Mark H.

False. Ed Winfield documented what was going on during overlap, way before Ed had a clue.
Ed came up with the 5th cycle name, as a sales gimmick. That's all. The cams were no different, or better then anything else.

I think it was probably even earlier and in England where the increases in performance from extending duration, adding overlap were initially investigated. I've often though it would be interesting to have a test engine and start out with a cam that has Otto Cycle events, 180-180 seat duration on a 90 degree LSA in on a 90 degree intake centerline. Then just increase the duration. I wonder how far you would have to go before the engine started to get characteristics of being "cammy"? Then spread the lobe separation and repeat. Running it on the dyno to see what the power band looked like would be interesting too.

[CamKing]

I think it was probably even earlier and in England where the increases in performance from extending duration, adding overlap were initially investigated.

Probably was.
I just know Winfield had it figured out by the 1930's
The guy built his own cam grinding machine at 18 (1919)

[GARY C]

Camking wrote----We have positive flow before TDC because the exiting exhaust flow causes a low pressure area below the intake valve during overlap. This siphon, suction or whatever one wishes to call it was discovered in the mid 1950's by none other than the legendary Ed Iskenderian. This was given the name 5th cycle, which was way before most cam companies cam into existence. So for those of you folks who did not know the history? The term 5th cycle is a lot more than a catchy name, it was a discovery!!! Mark H.

False. Ed Winfield documented what was going on during overlap, way before Ed had a clue.
Ed came up with the 5th cycle name, as a sales gimmick. That's all. The cams were no different, or better then anything else.

I remember a magazine article about that, I never took as being a different cam but that the overlap period was something that needed to be taken into account when choosing or specing a cam.

[Tom Walker]

My 3 horsepower Briggs and Stratten lawn mower has some overlap. No, it is not a solid roller, all stock

[simon010]

very cool article


http://www.nwvs.org/Technical/2104EdWinfield.pdf

rgds
simon