custom cam grind choice help

[curtis73]

I'm finding several cams that are close to something I want but I don't feel confident with them. I called Comp about their marine grinds and told them about my concern of reversion with too much overlap and the tech said its ramp speed that causes reversion, not overlap. I gently set the phone down and ran out of my garage crying.
So anyway, I started fooling around with some cams and I have some general overlap questions. Forgive me, but I only have overlap numbers @ .050"
A stock flat tappet chevy Mercruiser cam specs out to about 202/213 on a 110 LSA. That makes -12.5* of overlap, and since its the stock cam I assume that is adequate. I have a Vortec block so I'll be doing a roller, but I'm just speaking of overlap right now. Comp does a 212/218 roller on a 112 LSA that specs out with -9* overlap. A little more, but they don't seem to think that reversion is an issue.
So, as long as I'm not increasing overlap, the LSA is not a factor in reversion, right? I'm looking at two grinds that I concocted myself: a 204/214 on 110 (which makes -11*) and a 200/210 on 108 (which also makes -11*).
The reason I ask is that on the simulation, a 200/210 on 108 retarded 4* makes the exact same power with a bit more torque than the 212/218 on 112, and has a little less overlap despite its 108* LSA.
The real question is... I assume the main factor in reversion is overlap regardless of other events, right? So if I am reducing duration along with LSA and keeping the same overlap, am I still OK?
Dyno sims attached showing a 212/218-112 installed straight up and a 200/210-108 installed 4* retarded. Same basic curves, 12* less duration. Sounds like a winner. Thoughts?

[Jetaholic]

I'm finding several cams that are close to something I want but I don't feel confident with them. I called Comp about their marine grinds and told them about my concern of reversion with too much overlap and the tech said its ramp speed that causes reversion, not overlap. I gently set the phone down and ran out of my garage crying.
So anyway, I started fooling around with some cams and I have some general overlap questions. Forgive me, but I only have overlap numbers @ .050"
A stock flat tappet chevy Mercruiser cam specs out to about 202/213 on a 110 LSA. That makes -12.5* of overlap, and since its the stock cam I assume that is adequate. I have a Vortec block so I'll be doing a roller, but I'm just speaking of overlap right now. Comp does a 212/218 roller on a 112 LSA that specs out with -9* overlap. A little more, but they don't seem to think that reversion is an issue.
So, as long as I'm not increasing overlap, the LSA is not a factor in reversion, right? I'm looking at two grinds that I concocted myself: a 204/214 on 110 (which makes -11*) and a 200/210 on 108 (which also makes -11*).
The reason I ask is that on the simulation, a 200/210 on 108 retarded 4* makes the exact same power with a bit more torque than the 212/218 on 112, and has a little less overlap despite its 108* LSA.
The real question is... I assume the main factor in reversion is overlap regardless of other events, right? So if I am reducing duration along with LSA and keeping the same overlap, am I still OK?
Dyno sims attached showing a 212/218-112 installed straight up and a 200/210-108 installed 4* retarded. Same basic curves, 12* less duration. Sounds like a winner. Thoughts?
How is it that overlap doesn't cause reversion? If the exhaust valve is still open when the piston is on its way down on the intake stroke, you can bet there will in fact be reversion in the works.

[241Libby]

Not an expert on this but my 460 stock 340hp OMC came with a 216/220 at .05 and 112 CL. and I did a complete rebuild last year and installed a 226/236 at 112 CL and I had some minor reversion with the stock OMC thru hull exhaust and as a result will be installing Lightening headers to hopefully solve the reversion issue.

[curtis73]

How is it that overlap doesn't cause reversion? If the exhaust valve is still open when the piston is on its way down on the intake stroke, you can bet there will in fact be reversion in the works.
Did you read my next sentence? --> "I gently set the phone down and ran out of my garage crying"
I have no faith in what cam tech lines say anymore these days.

[curtis73]

Not an expert on this but my 460 stock 340hp OMC came with a 216/220 at .05 and 112 CL. and I did a complete rebuild last year and installed a 226/236 at 112 CL and I had some minor reversion with the stock OMC thru hull exhaust and as a result will be installing Lightening headers to hopefully solve the reversion issue.
Ok, that's good info. The stock cam specs out to -6* overlap and the one you installed later specs out to +7. So, that tells me that as much as -6* seems to be kosher, at least on a big block 460. That might mean nothing on a 350

[241Libby]

Help out here. is +7 bad? Lightening recommends no more duration than 236 on the exhaust. Last thing I want to do is spend 2K on exhaust and still have issues.

[curtis73]

Not sure... You had just said your 226/236 cam caused reversion issues through the hub, and it specs out to +7. Not sure about the rest... that's what I'm trying to learn here

[steelcomp]

Curtis,
If you really want to know what's going on, talk to Chris Straub...CStraub69 here on the boards. There isn't a better cam designer in the country.

[paradigm shift]

There are many factors besides the cam in your reversion question. Exhaust has a lot to with how big a cam you can run without reversion along with how well your heads flow. Maybe I missed it but more information would help. Type of exhaust, where the water dumps in and I missed what motor 350 or BBC and type of boat and drive set up. Many people I think run too much cam for a boat. Most cam companies over the phone reccomned too much cam for me.
Infomaniac here on the boards is a good resource if you give enough details on build. He has done a few motors. Choosing a cam can drive you crazy and you will get opinion from one end of the scale to the other. Look for proven packages close to your build and better yet from someone you trust.
Don't stress over the choices and remember to have fun with it!! Good luck with your build. Keep us informed as it comes together.

[curtis73]

Vortec 350 that I'm building for an Alpha in a 19' Baja SunSport. Plan on 5000 rpm, 320-ish hp, replace my rear riser exhaust with center 4" risers to a stock Y pipe and thru-hub exhuast.