Yet another Iboats cam decision

J

JHuck

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Feb 10, 2009
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Ah the joys of choosing the right cam. I've spent hours and hours researching internet cam topics. I've learned about reversion, stock exhausts vs. the expensive stuff, torque power band rpms and the effects of duration, LCA and lift.

I am now at the point of dropping my 350 block off at the machine shop for prep for a 383 kit so I need to decide. Because I am going with AFR heads and the extra cubes, I will have a ton more torque than I currently have (I know, the Alpha is rated to 300 hp). I also plan to use GLM manifolds with stock type risers (I have captain's call thru hulls already installed so things are tight for exhaust space).

I think the extra torque will allow me to go with a bit larger cam. I like an idle with some lope so I am leaning toward the upper end of recommended duration (ie. 218 intake/224 exhaust). Some people seem to think this is too big while others say you can go higher and not worry about reversion. I have a roller block so Comp Cam's XE 270 HR looks like it might work (218/224 w/ 110 LCA). I actually spoke with Dennis Moore about the manifolds he sells and we got on the topic of cams. He said the auto grind 270 should work for me with stock type manifolds. That was, however, when I thought I had a flat tappet block. Would a roller version of the same grind change things much (besides the extra lift)? I am also looking at Comp's 224/224 cam on a 110 LCA. Some on the internet state AFR's great exhaust flow allows for use of single pattern cams. Any thoughts on this also?

Thanks for your thoughts!
 
N

n2ostroker

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Messages
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Re: Yet another Iboats cam decision

If you want a noticeable idle with the roller cam and a 383 I would do the xm270. With the bigger motor you can get away with a slightly larger cam. The idle may not be as lopey. I recently put a xm262 flat tappet cam in my 355 which has 218/224 duration at .050" and it doesn't have a real rough idle but it is noticable. No reversion problems with mine with the stock center rise manifolds and 4" risers with thru hull exhaust. My exhaust has a big drop right after the riser and comes out below the water line.

Check on some of the more performance oriented sights like speedwake.com. There are a couple of builders that can make custom grinds for your combo.
 
M

mtnrat

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Re: Yet another Iboats cam decision

Have a custom cam ground for your exact application. You will be happier.
 
Bondo

Bondo

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Re: Yet another Iboats cam decision

I like an idle with some lope
Click to expand...

Ayuh,.... A Lopy cam makes a boat about Impossible to Dock....

Personally,...
I think you're Nuts for going with such a Huge cam...
 
J

JHuck

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Feb 10, 2009
Messages
63
Re: Yet another Iboats cam decision

I guess that depends on one's definition of lope. I don't want a top fuel dragster, just looking for a sound beyond stock.

WCA Tim runs a 383 in his boat. He supposedly has an XM270 flat tappet (226/236 @112) and has no low speed issues. He told me he wishes he would have gone with a bigger cam now. He does have a better exhaust but regardless, he said it idles down fine.

Here is his vid. He has mufflers so it softens the tone somewhat.
http://www.youtube.com/watch?v=33aWuP1K6o8

My understanding is that a 383 makes a cam seem 10 degrees smaller so the 218/224 doesn't seem so big. I was hoping someone with Dennis Moore's book would chime in. I've got time till the machine work is done so I'll go ahead and get a copy.
 
Mkos1980

Mkos1980

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Joined
Oct 25, 2007
Messages
640
Re: Yet another Iboats cam decision

I run the LT4 Hotcam 218/228 525/525 112 and it idles super smooth. If I didnt tell you I had a cam then you wouldnt even know it. I run it on my stock 350 mag shortblock and vortec heads. If you keeping the stock exhaust like you mentioned this is the biggest one you want to run. I have dennis moores book and there is only ONE more cam bigger he says to use but it needs exhaust work.
 
J

JHuck

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Messages
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Re: Yet another Iboats cam decision

Great, thanks for the information!
 
J

JHuck

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Joined
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Messages
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Re: Yet another Iboats cam decision

Your set-up leads me to another question. Are you using short or tall valve covers with the Vortec heads? I am planning on AFRs w/ 1.6 roller rockers but have been told it might be close on clearance with short covers. Before I stripped the motor, I noticed the stock (short) valve covers were already very close to the risers.
 
S

straubtech

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Joined
Mar 6, 2009
Messages
20
Re: Yet another Iboats cam decision

JHuck said:
Ah the joys of choosing the right cam. I've spent hours and hours researching internet cam topics. I've learned about reversion, stock exhausts vs. the expensive stuff, torque power band rpms and the effects of duration, LCA and lift.

I am now at the point of dropping my 350 block off at the machine shop for prep for a 383 kit so I need to decide. Because I am going with AFR heads and the extra cubes, I will have a ton more torque than I currently have (I know, the Alpha is rated to 300 hp). I also plan to use GLM manifolds with stock type risers (I have captain's call thru hulls already installed so things are tight for exhaust space).

I think the extra torque will allow me to go with a bit larger cam. I like an idle with some lope so I am leaning toward the upper end of recommended duration (ie. 218 intake/224 exhaust). Some people seem to think this is too big while others say you can go higher and not worry about reversion. I have a roller block so Comp Cam's XE 270 HR looks like it might work (218/224 w/ 110 LCA). I actually spoke with Dennis Moore about the manifolds he sells and we got on the topic of cams. He said the auto grind 270 should work for me with stock type manifolds. That was, however, when I thought I had a flat tappet block. Would a roller version of the same grind change things much (besides the extra lift)? I am also looking at Comp's 224/224 cam on a 110 LCA. Some on the internet state AFR's great exhaust flow allows for use of single pattern cams. Any thoughts on this also?

Thanks for your thoughts!
Click to expand...

Selection of camshaft is driven by 3 main factors, CID, RPM, Flow numbers. From here then application and limits of application. In this case we have the limiting factor of water in the exhaust. We need to be careful of advertised (seat duration) and the amount of overlap that the cam has.

If we put the same cam in a 350 or a 383, the cam in the 383 will be more docile as the cubes absorb more of the stick, affectively making it smaller.

Better heads with better flow require less camshaft. In marine applications you are better suited to get very good heads as the low to mid lift numbers in flow will fill the cylinders quicker. Because of this, less cam is needed and that means less chance of reversion with marine wet exhaust.

I would suggest pick a max rpm that you want to turn, the a proper size CC on heads can be selected along with camshaft. Head flow numbers vary from mfgs and this means that camshafts will differ also. The guy building a 383 with a set of AFR's will need a much different cam from the guy building a 383 with a set of Edelbrocks, but both can make the same power properly built.

Just remember it is combination of parts, not a collection of parts.
 
Mkos1980

Mkos1980

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Joined
Oct 25, 2007
Messages
640
Re: Yet another Iboats cam decision

Using stock covers. Nothing else will fit. They JUST clear
 
J

JHuck

Seaman
Joined
Feb 10, 2009
Messages
63
Re: Yet another Iboats cam decision

Chris,

Thanks for taking the time to discuss custom cam options and head runner sizes with me on the phone. Once I'm positive on my head size, I will let you know.

Based on some of your earlier posts, I assume you might recommend a 750 cfm carb over the 650 I am considering. That is unless you think the Speed Demon 650 is provides better overall flow for its size.
 
S

straubtech

Cadet
Joined
Mar 6, 2009
Messages
20
Re: Yet another Iboats cam decision

Jeremy,
Your welcome, I enjoyed our chat and I hoped I answered all your questions and gave you a better understanding.

I am not a big fan of Demon carbs. I feel for the pleasure boater wanting good all around boating with a as much fuel economy that a boat can have the Carter/Weber or Edelbrock carbs are a good choice. If the boater is after mostly performance, the Holley carb or one of the modifiers versions of the carb are best suited.

Compare air molecules to car, when approaching a sharp curve we have to brake or we will crash. When approaching a big wide sweeping curve we still have to slow down but not as much to go around the curve. Velocity rates in these "stroker" engines at mid to low rpms increase greatly as stroke is added. Piston speed plays a part on the pull on the carb signal. A little larger carb on these engines can actually give you better fuel atomization and in turn make for a more efficeient engine. . .well as efficeient as a boat can be.

For what you want to do and the rpm range we spoke about on the phone yes 750 would be better suited.
 
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