cam event finer points

First if you are carbed you have to stay with a dual plane intake or overlap will kill your idle smoothness and require higher rpm idle.

Fuel injection intakes are usually always single plane, that is the reason for the excessively wide LSA on FI cams, to crutch the overlap for a smooth idle.

Each engine has an ideal LSA based on cubes/valve size. Most 355s it is 106* or so. That will give you the maximum torque, but if you get the duration too long, the idle will still suffer some with the dual plane intake.

Spreading the LSA to 112* reduces the overlap 12* saving the idle quality, reducing mid range torque, flattens the curve, and adds a little top end power. This is usually done when a too long duration is chosen for the application.

Adding duration while keeping the LSA the same does not increase torque, it only moves the peak to a higher rpm.

You probably need to stay down about 200* with a single pattern cam timed with some advance to get a real smooth 500 idle. You probably need to keep your overlap down to 40* advertised duration.

Roller cams with less than 220* at .050 rarely make more power than an equal flat tappet since the flat cam can accelerate the flat lifter so much faster the first part of the lobe.

I bet the big cam companies have a solid recommendation for marine engines. I know CompCams has a marine listing.

xntrik said:

First if you are carbed you have to stay with a dual plane intake or overlap will kill your idle smoothness and require higher rpm idle.

Fuel injection intakes are usually always single plane, that is the reason for the excessively wide LSA on FI cams, to crutch the overlap for a smooth idle.

Each engine has an ideal LSA based on cubes/valve size. Most 355s it is 106* or so. That will give you the maximum torque, but if you get the duration too long, the idle will still suffer some with the dual plane intake.

Spreading the LSA to 112* reduces the overlap 12* saving the idle quality, reducing mid range torque, flattens the curve, and adds a little top end power. This is usually done when a too long duration is chosen for the application.

Adding duration while keeping the LSA the same does not increase torque, it only moves the peak to a higher rpm.

You probably need to stay down about 200* with a single pattern cam timed with some advance to get a real smooth 500 idle. You probably need to keep your overlap down to 40* advertised duration.

Roller cams with less than 220* at .050 rarely make more power than an equal flat tappet since the flat cam can accelerate the flat lifter so much faster the first part of the lobe.

I bet the big cam companies have a solid recommendation for marine engines. I know CompCams has a marine listing.

Click to expand...

I'd love to do a 106 LSA, but that would have way too much overlap, even on a 194 duration cam. Stock performance thru-hub cams seem to spec out to about -12 to -14* overlap measured @ .050". Not sure how that translates to advertised, but the 212/218 cam I'm considering specs out to -9* overlap @ .050". I think that's close enough. I certainly can't imagine reversion problems on a 212/218 with 112 LSA.

I'll plug in something like a 204/214 on 110* or 108* and see how the overlap numbers and hp peaks compare. I also don't want my peak HP to be too far below 5000 or I'll be lugging the engine too close to the peak TQ and peak TQ will be at or below planing speed. That costs fuel and is harder on the outdrive, but I'm not talking big differences. Right now I'm picturing 300-330 hp with the 212/218 @ 112* cam. Maybe I can duplicate that with less duration and less LSA.

So, if I reduce the duration, that will get me less idle RPM, but then if I reduce the LSA proportionally, will I lose the ground I gained in idle speed? I guess what I'm asking is; I know that reducing duration will net a lower possible idle, but how does overlap affect it?

Thanks for the input, keep it coming.

You might glean some input from this article.....

http://www.popularhotrodding.com/en...allblock/0611em_vortec_small_block/index.html

Obviously a lesser cam with wider LSA will be needed.

Take a look at Comp's marine recommendations. They might have a good idea from their experience.

I never had to deal with smooth 500 rpm idle and 330 hp @ 5000 from a 355 before.... :sweat:

Remember that widening the LSA lowers the amount of lb/ft of peak torque and duration controls at which rpm the peak occurs... spreads the peaks farther apart....

in essence flattening the curve.... and that more rpm makes more hp.

because hp is a mathematical calculation made from torque x rpm.... :sweat:

:welcome: :thumbup:

Ok, thanks. I made some dyno sims based on some things. Believe it or not there are three different cams in the same engine simulated in this photo. The first is a 200/210 on 108, the second is a 204/214 on 110, and the third is the 212/218 on 112. The first two spec out to -11* overlap and the last is -9*. The last cam is an off-the-shelf comp XE256. The other two would be custom grinds.

They are all remarkably close, each one peaks at exactly 317 hp. The lower durations obviously show more low end torque. So, I guess the question is, will one have a lower idle? Since the curves are nearly identical, it stands to reason that VE is pretty uniform across the board. Overlap is very similar, so will the shorter duration cam get me a lower idle? Or will the fact that overlap remains similar make the same idle regardless of duration?

And, any thoughts on the 4/7 swap and idle speed/quality?

Really sad news... I'm drinking Bud from a can. Its left over from a camping trip last week.

You asked why retard... mainly to recover peak hp RPM. If you look at the curves in the picture above you'll see that they all peak hp at 4500-5000. That cam you listed installed straight up only makes 280 or so hp and only adds 10 lb ft. Retarding it makes 317 hp with a minimal loss of tq, plus it shifts the peak hp from 4750 to 5100 without changing the torque peak. In a boat, WOT at 5000-5200 is good. Any less and you run the risk of putting the WOT rpm too close to the peak torque rpm which is not good in boats. Boats are a big paradox. You want lots of torque from 600-2000, but peak torque needs to be above planing RPM. You want peak power at 5000-5200 for calm cruisers and up to 6000 for high performance boats.

So what we're always trying to chase is big HP in a grunt motor. Pretty much the same quest as a car, but more critical in a boat.

xntrik said:

In the graph, the "diamond" cam is the clear winner. What it gives up between 3500 and 4500 is insignificant.. (called "roller 3" below the graph.) It has the best torque and the best top end power.... irrelavent above 5252 rpm for your purposes.

Try installing them in their recommended timing position, and rerunning the chart..

Click to expand...

The diamond cam is the lowest duration; 200/210 on 108. If I install it straight up I lose 20 hp and it peaks a 4500. I'll make a screen cap of all three in the same phasing and you can compare.

Since its a custom grind I just made up there is no real recommendation, I just have it ground 1 advanced in the sim and then retarded 3 for a total of -2, but I'll put them all dot to dot and make another screen cap

The first photo (above) showed them all installed at varying retards to match peak hp. This first one (below) shows the same three cams all ground straight and installed straight. To be specific: "roller" is the comp roller grind, 212/218 on 112. "Roller2" is one I made up that is 204/214 on 110. "Roller3" is another made up one at 200/210 on 108. All provide between -11 and -9* overlap at .050". Again the diamond (roller 3) is the torque winner, but gives up significant power and RPM when installed straight. Gives up Too much for me; 5200 is pretty ideal, 5000 is OK, 4500 is just normally accepted as too low for a 19' performance boat.

The second photo compares the "roller" comp ground straight up to the "roller3" installed 4 retarded which makes it match the bigger cam almost exactly while still providing a torque advantage down low. If I put the comp cam in at their recommended 4 advanced, it loses significant power.

They won't enlarge for me.

How they are ground is of no consequence. That is only the lobe phasing compared to the gear, which when you degree a cam is irrelavent. That is.... if a cam is ground 4* advanced, you install it at the zero gear mark. If it were ground straight up, then you install it at 4* advance gear mark and the ILC timing is identical.

Varying LSAs will make a cam look retarded or advanced compared to another.
How they are timed by intake lobe centerline is what is important.

Try timing each as recommended by the LSA, and then try timing each at the same ILC.

The only thing that advancing or retarding a cam does is crutch the incorrect lobe and LSA to a more advantageous point of the cycle. If the exact correct cam is "ground" then no crutching is necessary for the best power.

I do advance new timing sets because of inevitable stretch and wear.

xntrik said:

They won't enlarge for me.

How they are ground is of no consequence. That is only the lobe phasing compared to the gear, which when you degree a cam is irrelavent. That is.... if a cam is ground 4* advanced, you install it at the zero gear mark. If it were ground straight up, then you install it at 4* advance gear mark and the ILC timing is identical.

Click to expand...

Try again on the photos.. the server was hiccuping for me too.

I understand about the grind versus the advance. Timing is timing regardless of whether its ground in or at the sprocket. I just got wordy and confusing If I install the comp 212/218 dot-to-dot it will be 4* advanced, so instead of simulating it by changing the advance/retard, I just changed the ICL to 112 to simulate it installed 4* retarded at the sprocket. They both simulate the same thing, its just I changed the grind in the simulation becuse it was easier.

Varying LSAs will make a cam look retarded or advanced compared to another.
How they are timed by intake lobe centerline is what is important.

Try timing each as recommended by the LSA, and then try timing each at the same ILC.

The only thing that advancing or retarding a cam does is crutch the incorrect lobe and LSA to a more advantageous point of the cycle. If the exact correct cam is "ground" then no crutching is necessary for the best power.

Click to expand...

I don't know how to time them as recommended by LSA. The Comp is 112 on a 108 ICL; 4 advanced if installed dot-to-dot. The other two cams I made up in my head, so how do you want me to simulate them? I guess what I want to know is; the three cams are 212/218 on 112, 204/214 on 110 and 200/210 on 108. Tell me the Intake Center lines you want me to use on each one and I'll simulate it.

I understand about the grind, but my question is not about crutching the grind with a retarded sprocket, the question is; does physically retarding the cam events (either by grind or by sprocket) a viable alternative as far as engine building goes? The curves look good to me, so if I can maintain my 320 hp at 5000 rpm by using 12 fewer degrees of duration that's great, but is it a wise move to accomplish that by retarding cam events by 4*?

K I S S
Here's what I recommend.

Run each cam at +4, 0, -4. Take the best torque curve below 5252 of each. Ignore horsepower. Do that with any chosen cam. But watch that overlap triangle.

Compare those curves one to another. Pick the best.

Remember it is just a computer simulation.

Groove those pistons.

IntakeLC is determined by the duration of the intake lobe, generally they are symetrical lobes(up and down) and the intakeLC is half of the duration. The installedLC is then determined by the LSA (LobeSeparationAngle). It gets too confusing. :nono: Just do what I said above.