ezstriper

I know you gave cam specs, but who's cam ? as I stated in another thread you can change or "fool" the engine a lot by moving things around and still keeping the same lift/duration...example, you can add or remove compression with the cam design, now won't be computed figure like a 125cc dome with 130cc head, but ck with a compression gauge and can change a bunch...both ways

Budman II

iTrader: (1)

EZ, it is one of Bob's cams, but he did not grind it for this specific engine. He ground it for another OSO member, whom I bought it from. It had never been run in an engine when I got it. He actually had two identical cams ground and only used one. It was ground specifically for a 502 with stock 088 heads, RPM Airgap, and Stainless Marine exhaust. I looked at the specs and thought it would be an ideal cam for what I was planning to build at the time, as I was going with the same heads and induction back then. Then later on I switched to the AFR heads and Dart single plane. In numerous conversations Bob had mentioned that this may not be a suitable cam with SC. He said you have to be careful comparing specs between cams, especially a roller to a flat tappet, because a roller gets the valve up off the seat faster and there can be a lot more area under the curve where the valve is off the seat. If I had planned this project out with Bob from the start instead of backing into it like I did, I'm sure that he would have designed me a cam that worked with my combination from the get-go. As it is, I think my real problem is the way the water is going into the pipes, and that is how I plan to deal with it for now.

Just as a hypothetical, what effect would advancing or retarding a cam have on reversion? SB, MT, others want to weigh in? Just curious.

SB

iTrader: (1)

Merc Blue has their Crane Cams installed 5° advance in the HP500/500EFI/525EFI for a reason. Closes the exhaust valve sooner. A wider lobe seperation angle (LSA) closes the exhaust valve sooner also.

Remember, the exhaust valve is still closing After Top Dead Center (piston going down) on the intake stroke.

Atmospheric pressure doesn't care what valve is open, just as long as it's open. It tries to fill the growing void left by the piston going down. So...air/exhaust/water coming back thru the exhaust into the combustion chamber is exhaust reversion.

mike tkach

iTrader: (3)

good explination sb.timing events can cause or lessen reversion.maybe your post will help some understand that.

fbc25el

Longer connecting rod"s help prevent reversion also.

Budman II

iTrader: (1)

Ran it again, with one side having the regular tip with restricted top water holes, and the other side having the silent choice diverters with a 4" extension pipe cut at an angle to extend the dry part of the exhaust almost all the way to the butterfly. This allows most of the water to drain right down the Y-pipe, with a little bit going out the tips. Ran it and both sides were pretty wet. Damn. So I ran it with the seawater pump belt removed an no water coming out the exhaust. Held a thin piece of paper at the tip and observed it being shredded as it pulsed back and forth. Put my hand back there and it was sucking it back pretty hard. I had visions of "motor" yelling "told ya so". Then I had an idea, and almost on a whim, put both muffler tips with the salisbury flappers on there and ran it for about 45 seconds. Boom - both sides dry as a bone. Ran it again and pulled the headers - pipes still dry. Can it be something as simple as that? SB, your earlier quote about collector length versus primary length is going through my head. Or was it just a matter of the salisbury flappers dampening the back pulse?

SB

iTrader: (1)

The flappers don't dampen the pulse per say, they try to stop it. Positive pressure opens them, there no pressure or negative pressure closes them.

Remember, they are there to close down when engine not running and/or wave hitting back trying to go into the pipes.

The closing isn't as quick as the reverse pulse, but they do help.

So, in this regard, ie: trying to close when 0 or negative pressure, I guess they do dampen...but more the return energy wave..as it's trying to be a one way valve of sorts.

What I mentioned earlier was about collectors dealing with energy waves. Exhaust physics show that as an energy wave 'hits' the atmosphere, there is a resultant opposite wave that shoots back.

Camshaft and exhaust designers try to make it so your exhaust valve is closed, or almost closed, when this happens. It appears exhaust is more important in this regard.

A wrong collector length can ruin your day.

Again, if your header collectors are 4" and the rest of your exhaust is 4", then the rest of the exhaust count's as part of the collector.

I'm at work and trying to get out of here, so excuse me if my typing and 'talking' or lacking. LOL.

SB

iTrader: (1)

FYI: I've never run or tested without flappers, so I never thought about it ...at all.

Learn something new everyday...thanks....

Edit in: and yes, mufflers can effect your 'tuning lengths and etc' also.

Budman II

iTrader: (1)

SB, thanks for the reply in the middle of a busy day. Sucks having to work on a Saturday. Hope you enjoy the rest of your weekend.

I'm dropping it in tomorrow. Going to play with it a little more this evening, and decide which collector to use. Gotta say the silent choice ones with the 4" extension pipe running through them are looking pretty promising.

thirdchildhood

iTrader: (6)

My 525, which has a "bigger" cam, has been running fine for years with the CMIs and silent choice diverters and 4" tips with internal flappers. The Y pipes are kind of a safety feature if any water gets past the flappers since it will run down the Y pipe instead of going over the hump.