More Reversion Info

[PaulO]

To think I doubted the existence of that phenomenon called reversion when I was first told about it!

Yesterday, the new engine arrived and last night I began the tedious process of swapping EVERYTHING!. When I removed the intake manifold from the "old" engine, I saw something I couldn't believe... rusted intake ports on the 4 center cylinders!!

To me that means that the reversion was so bad that it was actually sucking water/vapor all the way back into the intake ports and perhaps the intake manifold!

This really spooks me.

PaulO

[Riley]

Hey PaulO, that sounds spooky! Although I'm by no means an expert on 4 stroke engines, it seems virtually impossible to me to suck water vapor all the way back up into the intake manifold. On the intake stroke the exhaust valve is closed, and anything in the intake manifold should be under a vacuum, right? On compression both valves are closed, as they are on the power stroke. On the exhaust stroke the intake valve remains closed. It seems to me any reversion taking place would take place at the beginning of the intake stroke, when the exhaust valve might be still open due to long duration.

Am I thinking about this correctly? If not, please let me know why.

My first thought might be that you have some sort of head gasket problem or a cooling system problem of some sort. I had a Sanger flatbottom V-drive that had a cracked head, and the symptoms were exactly like yours. Not to mention the cream colored oil! But like I said, I'm by no means an expert, and would really appreciate an education on reversion!

Hope that baby flies when you get 'er back in the drink. Good Luck.....Chris Riley

[PaulO]

Riley,
What happens is:
As the piston rises on the exhaust stroke the exhaust valve is open. As the piston nears TDC, the intake valve opens. This happens before the piston fully reaches TDC on the exhaust stroke. As the piston travels its remaining few degrees towards TDC, both valves are open for some time. This is the overlap. There is some pressure in the cylinder as the the piston pushes the exhaust out against the backpressure. When the intake valve opens, vacuum is introduced from the intake manifold, across the top of the piston to the exhaust valve creating a momentary reverse of flow back into the intake manifold. That is reversion. The problem is that at higher RPMs it is a good thing. As the velocity of the exhaust gassed leaving the cylinder increases due to higher RPM, there can actually be a vacuum created in the exhaust manifold/port. When the overlap occurs at higher speed, that vacuum can actually "suck" more fuel/air mixture into the cylinder packing it tighter. This is why big overlap cams create more power at high RPMs.

PaulO

[Riley]

PaulO, thanks for the info. After I posted you this morning I got out a couple of old "text-books" and read up on reversion. It seems to be a function of cam overlap and exhaust back pressure, correct? If so, do you think a "tuned" aftermarket exhaust manifold may help the situation?

Dennis Moore writes in his book on small block chevy marine performance "When modifying a marine engine, the main bottleneck is the watercooled exhaust system." This is on pg. 38.

I know you have often stated your position regarding aftermarket exhaust systems, but do you think the stock Mercruiser manifolds could be compounding the reversion problem?

I'm interested to hear your thoughts. Thanks....Chris.

[CDMA]

Paul,

I believe all this reversion thing but is it really possible to go that far? Is it possible the salt air did it. Or was there more rust that that. From experience with cast iron if not coated it will rust in a matter of days if left outside even if under an overhang like I found out with my manifolds this winter.

Chris

[PaulO]

I am fairly sure it was the reversion since it was only the four intake ports that correspond to the four exhaust ports that showed the signs.
PaulO

[CDMA]

Logical enough. Just checkin'.

[RickR]

PaulO
Which cylinders were they?
I wonder if the longer stroke made a difference? (More time at TDC).
At any time during the engines short life did you shut her down before reaching normal operating temp and let her sit overnight?

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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

[PaulO]

The center 4 cylinders #s 3,4,5,6
The last time it ran, it only ran at or around idle for about 10 minutes until I could no longer stand the knocking!!
PaulO