wkd1

I see people are running cams with a duration bigger than 232deg
Doesnt this cause reversion..
Thanks

blue thunder

Not necessarily. The most important thing is how many crankshaft degrees after TDC of piston travel occurs before the exhaust valve closes. The other point of interest is valve overlap. I find with a flat tappet cam after about 58* of overlap reversion becomes noticable. There are other factors as well, but cam timing is most important.

BT

vette131

the type of exhaust you run & how far back it dumps water, makes a big difference how much cam you can run.

CPPerformance

Here is a very good article on reversion, it's causes and how to avoid it.

http://www.keitheickert.com/tech_sup..._Reversion.pdf

Raylar

Good explanation from one of the "Masters." Another thing to consider when selecting cam specs in a high performance marine engine is engine fuel system type. In normally aspirated engines you basically in todays world have two types, carburated or efi injected . The efi injected motor generally works better with a wider (LSA). This wider (LSA) will ususally allow more duration to be used in the cam without the "reversion" problem showing up. Some of our bigger injected NA cams have durations around 240+ degrees @ .050 and we haven't expierenced any noticable reversion in 496 NA efi applications. If your engine is carburated and your building power you will most likely use a cam with an (LSA) of between 108 to 112 degrees for a stronger vacuum signal at idle to operate the carburators idle and transition circuts. This narrower (LSA) when combined with too much duration will increase overlap, the potential for reversion in a wet exhaust and hence limit durations to more in the 230 to 235 degree range. I am starting to think that with todays growing muffler and noise reduction laws that a better muffler equipped all dry exhaust system with tip exhaust gas cooling is in order for making bigger power in most high performance boats. Maybe some of the exhaust builders will do some more R&D here?

Ray @ Raylar

wkd1

The cam I currently have in the motor is a comp cams XR300HR-10
intake = .560/248
Exhaust = .580/254
110 lobe seperation
specs are with a 106 intake center line
This cam will cause reversion....correct?

Raylar

Wkd1:

Those numbers for duration I take would be at .050." What size is the motor, what exhaust are you running and where does your water enter the tails? I also assume this is a carburated motor at what compression ratio?

Ray @ Raylar

wkd1

Yes those numbers are at .050... and the water exits right at the end of the risers.They are nothing special...aluminum center risers.I do keep planning on calling you but keep forgetting until early in the evening...

I have a gen VI short block.compression is 8:75.I have a set of aluminum edelbrock performer rpm heads 219 int/1.88 ex.I have had them polished and ported.I am running a edelbrock TM2-0 BBC Oval Port Intake Manifold( the older style victor open plunum with the twisted carb).And a mighty marine 850 demon carb.Aluminum Indmar center riser manifolds with 4" tips no muffler for know.Maybe adding a captians call later.Blue printed high volume oil pump , Dually 10 qrt. with windage tray and scrapper.And a MSD complete marine ignition system
Thanks