I'm sitting here (at work of course ) trying to figure out exactly why increasing the stroke of an engine will make a big cam idle smoother. Its not like any of the timing or anything is different, I dont get it.

Is this taming effect linear? i.e. will a 350 with a 218/218 cam idle the same as a 396 with a 246/246 cam?

Also, is it correct to infer an emissions connection? will a 396 burn cleaner than a 350 with the same cam?

thanks guys,
Matt

 

can't answer your question but thought it was weird because i just was wondering the same thing is math class about 20 minutes ago?? weird huh???

thomas

 

I don't think there's any direct correlation between duration and displacement as far as idle quality is concerned.

Generally the reason "bigger" cams (those with more duration and usually more overlap) are needed with larger displacement is because with a given head flow, you need the valve open longer to get sufficient air for the bigger displacement. Increased duration and tighter lobe separation gives more overlap. It's that overlap, when both intake and exhaust are open and the gasses backup the intake, that give the rougher idle, and forces higher idle rpm. The larger displacement (achieved from bore and/or stroke) is a little more tolerant of the "bad gasses" from the overlap because they are a smaller % of the total displacement.

Don't get things backward: a cam should be chosen to maximize the engine's displacement, compression, head flow, intake and exhaust system, and a bunch of other things, not the other way around. Choose the cam last.

IMO, the more desirable way to make a stroker (or any larger displacement engine) work is get more headflow. You can then use a milder cam and still get the top end power plus low-mid range torque.

 

I think that it will in certain circumstances, but not all.

In the instance that you have a carbed 350 smallblock and change the stroke, the the longer stroke gives the intake airstream slightly more velocity because the piston is moving faster down when it's at 90'. So in turn you get a stronger signal at the carbuerator which would seem to "tame" the cam, but in effect it's just letting the carb mix the fuel better.

If you had a 218/218, 550 lift cam in a 350 small block, and then put the same spec cam in a 427 big block, it should have close to the same performance characteristics (except that the 427 would have more power and torque) because although the bore and stroke is bigger, so are the valves, runners, and everything else. If everything increases the same, the cam should perform the same....meaning the torque and HP curves will be similar, except the larger motor will have more power and torque while following the same curve. (IE, peak torque might be at 3500RPM for both motors, but the 427 will have more everywhere.)

I think with fuel injection, as long as the heads can flow the air, if a longer stroke makes the cam tamer, it's probably more a function of the leverage on the crank that the extra stroke provides letting the engine idle easier because the crank is easier to turn. The intake and exhaust events still happen at the same time.

For example, I had the LT-4 hotcam in my LT-1. It had a really nice idle. the Duration was 218/228 and 525 lift with a 112' LSA. I then built a 454 and put a cam with slightly more aggressive characteristics. It has a 224/234, 544 lift with a 110' LSA. The engine idles somewhat rougher than the small block did (btw...it sounds mean!) and runs really strong. I attributed this to everything increasing in size, not just the stroke. It's funny because everyone said that it wasn't enough cam for the big block, but I went with it anyway and in reality it's almost too much for the street.

But oviously this isn't a scientific explanation... so maybe I'm off base here.... but this is just from my experience.

It sort of makes me wonder how many of those big blocks out there that run like crap are because they have a 250' @ 0.050" cam, rectangle port heads, and they only spin them to 5000RPM when the peak power is somewhere around 8000? Who knows...

-Dave C. '97 Z28

 

So on a related note, why will higher compression "tame" a larger cam, and a lighter rotating assembly tend to make cam surge worse?

 

Yep, I'd agree with this - given the same desired powerband, the cam size should stay the same with the intake/head/exhaust flow increasing or decreasing in proportion to the displacement change. But, if drivability remains acceptable with a bigger cam in a bigger motor and the area under the curve increases, then bumping up the cam duration on a bigger motor is a good way to take advantage of the additional cubes. This won't give the same results as paying proper attention to airflow (and will most likely be more difficult to tune and more sensitive to other variables in the system), but it might also be a cost-effective way to grab a few more ponies.

With regards to the orginal question, a bigger motor pulls more air at a given engine speed, and therefore can maintain acceptable velocity (and therefore cylinder filling), even if the cam is sized for a different part of the operating range.

 

A higher compression motor will tame a cam with less LSA or more intake duration because your making up for the lack of cylinder filling at low rpm by compressing it more. So in most cases it will idle better.

I lighter rotating assembly will make most engines seem rougher because you lose the momentum of the mass of the assembly. It also causes the engine to be pulled down between shifts more, but the engine can accelerate faster once the drivetrain is locked together (IE... by the clutch)

-Dave C. '97 Z28

 

Basically what Dave said, everything being equal increasing the static compression also increases the dynamic compression. This helps offset the tendency of a larger cam to bleed of cylinder pressure.

Here is a good article on the subject if you're interstested.

 

Thanks Jason.