Stingray69

I knew what intake tract reversion was.

My guess is Duke's a LOT SHORTER than me.

the duke

So do the rest of us. and it has nothing to do with water reversion, which is what the question is about.

the duke

As you guys all revere CFM/SB, check and see who he agrees with

Stingray69

Duke, buddy, I revere YOU, and Zone5 and 5325user, and FormulaFastech; you are one funny scizophenic!

the duke

I'm glad that you revere me. Glad you revere the other guys too. From what I know of Zone5/Formulafastech, he has been boating for the last 3 years with no reversion, and with a Crane 741 cam. You know, the one that is so old and outdated, that its no good anymore. As a matter of fact, my engine has an off the shelf, no reversion cam too. Has your boat motor run in the last 2 years in the water? 5325 user has twin engines, with milder cams than I have, and he has run 85 mph with them. They are "off the shelf" too. He'll hit over 110 with blowers next summer.

Strip Poker 388

iTrader: (6)

http://www.offshoreonly.com/forums/s...3&page=2&pp=40

the duke

I assume that you are talking about the part in that thread about the 741??? Where he is talking about early intake opening on the 741 causing reversion?? I have to disagree. I am not a cam designer by any means, but these guys are. http://www.iskycams.com/techtips.php#2002
and they explain what causes reversion. Not early intake opening, and certainly not the intake manifold.

Here are 2 pictures of heads off a 502 with a 741 in it. These are not stock heads, and they are not stock exhaust. No signs of reversion

mcollinstn

Theory: The moving column of airflow through a single cylinder motor has mass and inertia. The flow starts and stops at the intake valve, and at the exhaust valve. The length and mass of the column before the intake valve pulses due to the start/stop and, at certain rpm will also be either amplified or lessened due to harmonics. These are affected by a host of factors such as column length from throttle plate to intake valve, and the port area. At some NON complimentary rpm, the oscillations within the port can cause a negative (or relatively MORE negative) pressure situation at the intake valve when it opens. In a case such as this, it will result in a less complete filling of the cylinder, or a MORE negative pressure situation during the intake downstroke of the piston. Exhaust primary pipe length will also have an effect on how the pulsing of the exhaust waves create a relatively higher or lower pressure immediately after the exhaust valve.

Depending on valve overlap and intake centerline, there *could* be a particular low rpm situation whereas the intake tuning could create a lower-than-average intake port pressure at the point of intake opening.
Depending on exhaust tuning, there *could* be a particular low rpm situation whereas the exhaust pulsing could create a higher-than-average exhaust port pressure at the point coinciding with the intake opening, while the exhaust is still held open. THIS WOULD be a situation that would indeed be more APT to result in EXHAUST reversion. This could indeed be caused in a borderline situation by a slight intake tract length change, which an intake manifold swap *could* achieve.

Note that the example above is for a single cylinder engine.
If we NOW consider a V8, we will invariably have some pulsations from other cylinder events coming into play. These could also cause the situation above, and may be more likely to occur in different manifold configurations.

A single plane intake may be swapped for a dual plane. In that situation, the intake pulses from other cylinders could cause the situation described in one or more cyliinders. Or vice versa, a dual plane to single plane could be the straw that breaks the camel's back.

Also, swapping from log exhaust manifolds to tubular shorty headers could be the factor that ends up causing reversion. Or vice versa. Or shorties to full length primaries.

As stated, each motor is a collection of 8 single cylinder motors operating in conjunction with each other, sometimes benefitting from the additional pulses, sometimes not.

So in THEORY, ANY change in intake length, area, exhaust length or area, or which cylinders are sharing a common plenum in the intake, or which exhaust primaries dump into which others and at what distance from the exhaust valve - THOSE CAN AFFECT whether a motor is "closer to reversion" or "farther from reversion" and also can change the rpm at which the most crucial point occurs.

So it is sophomoric to make a blanket statement like "there is no way an intake change can have anything to do with exhaust reversion".

REAL WORLD ANSWER THOUGH: It is mighty UNLIKELY that an intake swap will cause an otherwise dry (non exhaust reverting) motor to turn into a rusty lump. As with any significant change in hardware, though, a man is not wise to assume much of anything. Some cams are KNOWN TO BE SAFE. Some cams have a partial history of causing reversion in a few applications. A 741 is a cam that CAN be part of a reversion problem. LOTS of 741's running out there dry as a bone though. The devil is in the details....

mc

mcrsr

so what do you advise for a mild cam that is reversion free??? i am running very mild comp cams extreme marine cams, anyone have any experience w/ these??

Stingray69

Not being very specific about just WHAT you have for cams (very mild) or exhaust, I would suspect your exhuast isn't up to the task.