Cam Specs for Mercruiser 260hp?

[John_S]

Re: Cam Specs for Mercruiser 260hp?

the exhaust is pretty restrictive on older heads and even vortec heads have some limitation...

Actually, I don't think vortec heads improved anything of significance in the exhaust dept. Intake runners and combustion chamber were greatly improved, though.





I agree with wca and others, you are running too much total advance for a stock like engine.

 

[jtybt]

Re: Cam Specs for Mercruiser 260hp?

That's the thing...the cast-iron vortec heads flowed almost as good as the LS1 right outta the box for $212 each when I got mine from Sallee. A no brainer.

 

[wca_tim]

Re: Cam Specs for Mercruiser 260hp?

for future reference, for an off the shalf cam to use in a near stock or slightly modified 350 - especially with stock exhaust, something like a comp xm-256 grind is a good all around cam and a pretty safe bet. In addition, I think for comp cams it's recommended to go 1/2 turn past zero lash on valve lash adjustment. that'll make a little difference in power on your current set-up.

a thought on helping with your current cam at least for the short term might be to install 1.6 ratio rocker arems on the exhaust valves and leave the factory 1.5s on the intakes. I've known people who do this to further deal with the problem of running wet exhaust. higher ratio rocker arms on the exhaust valves will slightly widen the duration in addition to increase the lift on the exhaust valves - both of which will at least help the intake / exhaust imbalance. be sure you have enough clearance, but I can't imagine it being an issue with a stock engine. just a thought...

 

[dr_bowtie]

Re: Cam Specs for Mercruiser 260hp?

lash is 1/4 turn, Timing is set at 12 degrees initial with 25 mechanical in by 4000rpm. I set the total advance to be in so late to be conservative. Not sure on the heads I will have to check. I just asked the speed shop to do up some standard 350 heads for the marine motor I was building. I might be wrong on the cam I mentioned. Comp Cams lists two marine cams with similar intake specs. One is a split duration cam and that might be the one I picked(I normally would pick a split duration for auto or marine use). I can't remember and don't know where the cam card is.

37 total timing is pretty conservative for a stock motor with just a cam...if the cam is advance or even ground with an advance you still have a little playing room...

you can play around with it quite abit and see where it goes...try backing it off a little and test and then bump it up by 2 and try it and see...

as long as you dont exceed the fuel and it doesnt ping it may like more timing...

if the engine is effiecient it will make more power on less timing...these are where vortec heads shine the best...due to better cylinder filing and better burn rates they make more power with less lead and total timing...

if you are using non-vortec heads most likely you will see the motor will like more lead especially with an advanced camshaft...

make SURE the fuel mixture is correct at all parameters before you add timing...adding timing if you are lean at WOT wont change a thing no matter where it is...

best thing is to get a magnifying glass and inspect the plugs after a WOT pass...detonation will look like tiny pepper specs on the insulator...

I would honestly back the total down to 32deg and start from there...

I would also set teh curve to be all in by 3000rpm....it will like you much more if you do...

 

[Bifflefan]

Re: Cam Specs for Mercruiser 260hp?

Duration 260 advertised 212 @ .050 both intake and Exhaust

Lobe lift .293 Total Lift .440 both Int. and Exh.

110 Lobe Center Angle

106 Intake Centerline

Installed Straight up.

Seams to work fine but with my 2800lbs 20foot, Searay SRV195 I was expecting it to pull to 4500+rpm even with the 21p prop. I though 50mph without having to trim up would be no problem. Don't really care how fast it goes just want to make the engine is making the power it should. I have a new ignition system with 12 degrees initial timing and 24 mech advance all in by a high 4000rpm(being safe) and I have ruled out valve float. Maybe I am over propped but with the small boat shouldn't turning a 21p prop @ 4500+rpm's be no problem. Thanks again for your information and help.

That seams like alot of timing for pump gas. We run 38 deg on racing gas with a locked dist.
Im wondering if your getting detination and just cant hear it.
What do the plugs look like? any signs of this?
it will cause the top end to fall off if so.

 

[dr_bowtie]

Re: Cam Specs for Mercruiser 260hp?

yes but I bet you run 38deg total on racing gas AND alot more compression...

the flame rate of fuel AND the compression ratio are key factors to what amount of timing you can run...

lower compression engines use alot more timing that higher compression racing engines...simply because the timing exceeds the fuel characteristics

 

[wca_tim]

Re: Cam Specs for Mercruiser 260hp?

yes but I bet you run 38deg total on racing gas AND alot more compression...

the flame rate of fuel AND the compression ratio are key factors to what amount of timing you can run...

lower compression engines use alot more timing that higher compression racing engines...simply because the timing exceeds the fuel characteristics

lower compression, poorer fuel, slower burn front = more advance required to hit the sweet spot...

I'm assuming that compression is around 9:1. I'm always conservative when advancing timing... let's just say I have some pistons with holes in them in my past.. best to start at 32 and walk up 2 degrees at a time until you don't get any more power increase. also look at the strap on the sparkplug and see where the color change line is. that's a good indication of whether your timing is where it needs to be. the line should ideally be right at the bend on the electrode...

 

[Bifflefan]

Re: Cam Specs for Mercruiser 260hp?

yes but I bet you run 38deg total on racing gas AND alot more compression...

the flame rate of fuel AND the compression ratio are key factors to what amount of timing you can run...

lower compression engines use alot more timing that higher compression racing engines...simply because the timing exceeds the fuel characteristics

lower compression, poorer fuel, slower burn front = more advance required to hit the sweet spot...

I'm assuming that compression is around 9:1. I'm always conservative when advancing timing... let's just say I have some pistons with holes in them in my past.. best to start at 32 and walk up 2 degrees at a time until you don't get any more power increase. also look at the strap on the sparkplug and see where the color change line is. that's a good indication of whether your timing is where it needs to be. the line should ideally be right at the bend on the electrode...

your both right, i was thinking about it on the way home and reallized i didnt know what i was talking about at the time..LOL

 

[mkast]

Re: Cam Specs for Mercruiser 260hp?

Mkast,
Those numbers you posted would hurt rather than help a stock 350. I'm talking about a flat tappet cam.

The original question was what are the stock cam specifications.
The numbers I posted are out of the Mercruiser specifications.
These are for a flat tappet cam. You might look at the page referenced.
I suggest you take up the question of performance with Mercruiser and GM. If these specifications are close to your 406, why did you buy a stock cam?
Don't shoot the messenger, that doesn't change the message.

 

[wca_tim]

Re: Cam Specs for Mercruiser 260hp?

what I have says stock merc flat tapet cam pn 431-5943 that was put in most 350s is: 0.05 duration (intale/exhaust) 200/212, lift .400/.410, centerline 108/112, lsa 110.

I know that merc doesn't publish cam specs... so it is hard sometimes to be sure the information you're getting on this kind of thing is accurate. The numbers I posted earlier are from dennis moors book on high performance small block marine engines which has a few technical inaccuracies in it, but overall is really good. In addition, they match what I was given by a marine engine builder AND what I checked using a dial indicator and degree wheel myself before I found the other information. There is a gm part number cross reference and this was the cam put in the vast majority of flat tappet mercruiser 35os and many 305s - to the best of my current knowledge.

Hope this is helpful... Cheers!