Crankcase compression for experts

[Chaz]

Stock MontyMan "pro/gas" powerhead ...

This is the "waterski" 6 guzinta 1 pipe ...

[FORBESAUTO]

Very interesting and thought provoking thread.
There's a lot to be taken from this even though i,m building a V4 trying to get a strong 6500 RPM not for all out speed.
Just a strong reliable lake motor.
How does polished exhaust (mirror shine) compare to a smooth finish?
Longer for carbon to bind to ?
Faster flow ?

On the cc volume, one size don't fit all. If your not gonna turn crazy rpm, a tighter cc is usually beneficial. You can widen your ports in cylinders to get more flow to get more charge in cylinder and increase cc pressure to bring velocity back up at low rpm. It'll also allow the cc pressure get below atmospheric pressure sooner at bdc and allow reeds to open sooner to maintain flow through crankcase to cylinder before transfers close, then fill cylinder volume more efficiently once ports close. At really high rpm levels you can get to a point of deminishing returns where the extra pressure causes more drag than you are gaining from the stuffing. Also you can lose low end power if your cc pressure is more than needed and allows too much charge to escape through exhaust port at low rpms.(usually the case with high blowdown degrees between exhaust port and transfer openings and a strong scavanging exhaust set up). So in a nut shell you'll need to experiment with your particular engine/porting design.

[FORBESAUTO]

Stock MontyMan "pro/gas" powerhead ...

This is the "waterski" 6 guzinta 1 pipe ...

Chaz, can you build me one of those mids for my bass boat? That'd be cool as crap to be at the landing waiting on your flight in a tournament with set up!!

[powerabout]

Thats way way longer than stock, like has doubled the length of the exhaust.
I wonder how that looked on the dyno compared to without it?
Stock has nothing under the adapter as the collector is in the middle of the case not at the bottom.

[Chaz]

Chaz, can you build me one of those mids for my bass boat? That'd be cool as crap to be at the landing waiting on your flight in a tournament with set up!!

Yeeesss . Besides cool , it knocks 100+ pounds off of the outside of the transom ...

Not for the faint of wallet ..

Next opening is sometime in March ..

Month ( or however long I can stand motorhome livin in the Keys ) vacation in May ...

powerabout

Thats way way longer than stock, like has doubled the length of the exhaust.
I wonder how that looked on the dyno compared to without it?
Stock has nothing under the adapter as the collector is in the middle of the case not at the bottom.

Well Power ya caught me ... how did you know that's a big azz center console with a 30" offshore mid ...

Chaz = thinkin , V-6 JohnRudes only came 20" and longer . I could have swore the exhaust come out the prop hub ...

[FORBESAUTO]

Yeeesss . Besides cool , it knocks 100+ pounds off of the outside of the transom ...

Not for the faint of wallet ..

Next opening is sometime in March ..

Month ( or however long I can stand motorhome livin in the Keys ) vacation in May ...

Dang it! I was hoping uncle Chaz was gonna hook a brotha up lol

[FORBESAUTO]

To add a little twist to the crankcase volume controversy. You always hear how by lowering the volume it helps low end, but it also can help upper rpm power to a certain extent. By lowering the cc volume (1) cc pressure is increased allowing cc pressure to be above cylinder pressure at the point the transfers open at higher rpms (2) increased velocity helps charge and push burnt gasses out quicker at higher rpm (3) also allows cc pressure to drop below atmospheric pressure at bdc sooner allowing reeds to open and create a continuous fresh charge flow all the way through transfers until they close by reducing area to evacuate (a larger volume can act like a storage tank). It can hurt low end also by overdoing those three points at a slower piston speed, but to counteract it you can open a booster(s) port a few degrees earlier to help bleed off some pressure at low rpm and that little bit won't make any difference at higher rpms. I feel you won't to try and have the cc volume to where it can achieve just below atmospheric pressure at bdc at your desired rpm to get the best cylinder charge in the last half of transfer port duration. You just have to play with it and see what works best for your application.

[powerabout]

nosub also said in testing engines and increasing cc comp you would lose VE as it started to heat the mix up too much at high revs
I wonder if he will start reposting one day???

[spybot]

Not about outboards but have a look
Some of you may find this interesting
https://www.youtube.com/watch?v=O8e92MgD3LA&t=414s
Andy

[Mark75H]

Not about outboards but have a look
Some of you may find this interesting
https://www.youtube.com/watch?v=O8e92MgD3LA&t=414s
Andy

Not to be rude, but this discussion is so far beyond that video. Maybe you should start reading from post #1 to get a grasp on how far up this is.

[spybot]

I have followed this from the start Mark
I understand where the discussion is you may have missed my first entry ?
The video was intended to introduce new members/readers to grasp the theory of cc stuffing and its effects.
No offence intended nor taken
Andy

[Chaz]

To the conspiracy theorists and passers of useless inaccurate information ...

Suck / blow on a 15 ~ 0 ~15 psi gauge , thats what I thought .. couldn't move it off of zero.

Now hook it up to the crankcase .. pumps between 5 - 7 psi positive .

Got any logged data that shows vacuum anywhere from transfer opening to where the piston starts slowing as it is approaching BDC ... ???????

Got facts ???? __________________________

At least build a poor mans Spintron with an electric motor and a strobe light ... before ... naaaa ... keep up the self deception .


Chaz = thinkin I do like how the theory has went from scavenging meant nothing . To that's all that matters now ....

Tip of the day ... plant your seed now , you too will have a grassy knoll by summer ...

[FORBESAUTO]

The point where the charge flow will go from intake through reeds directly to cylinder that is evacuating needs to happen as close to bdc as possible. Any latter than that and you are filling cylinder with a charge that is (1) hotter than the would be charge directly from the outside (2) there will be a dead spot in flow at the transition point going from positive cc pressure to below atmospheric pressure where the flow will resume from externally. And you won't this transition to happen at the point where the piston is moving its slowest, which is at bdc. It cannot possibly happen very much prior to bdc due to the fact of the downward movement of the piston is gonna increase cc pressure regardless of all outside influences. What I'm saying is that a too large of a volume can also cause the transition point to be too late due to the fact of it taking longer to evacuate the larger area and too little volume can also cause it to be too late because of so much pressure it can't all get out in time, then you have the problem of the hotter than could be charge entering the cylinder. And the required amount of volume needed changes with rpm, scavanging capabilities of exhaust, port durations and flow capabilities.

[Mark75H]

Jennings mentions an intake valveless 2 stroke powered generator in the TTH from the stoneage of modern 2 stroke understanding. My experience with my reed valve converted Kawasaki 350 triple was the same; with a bunch of blown out/broken reeds the motor was impossible to kickstart, but could be hill started and would run above idle OK ... and screamed on top end just fine. Couldn't start it, wouldn't idle, but would still run 110+

[Mark75H]

I don't get the part of what you are saying about Jennings's work being outdated. Its not outdated, it remains the foundation that all the rest is built on. Granted, the tower is now very high, but it still stands on the same foundation.