dual vs straight pattern cams

[lil red]

just doing some research for the future stroker build.

anyhow, as stated what is the major difference between the 2 different pattern cams and what dictates using them in a build. I notice that comp, voodoo, etc use a split pattern and mp uses a straight with the larger duration cams.

which is better - alot of people on here recommend both. Is it based alot on exhaust design or the fact that nwer heads such as rhs, eddys may have a better exhaust design/flow or scavenge more efficiently vs the older x or j's when the mp cams were designed.

just wondering - as always thanks guru's

[moper]

For me it's always the combination of ports and support systems. If you're able, the best tool for cam choice is knowing what the ratio is. I don't believe there is a "perfect ratio" but more of a perfect combo of ratio and cam for a given engine/package. I'm virtually always running a split pattern of some sort. And not always "more" on the exh side. As the ratio gets closer to "1:1" then the "bonus" of the longer duration is less and less. You can also have similar results by running more rocker ratio but not to the same extent as changing the cam lobe.

[lil red]

moper - if the bonus of a larger duration camshaft is lost when the ratio cloese on 1:1 , does this mean that if 2 camshafts of the same duration (size) are compared a camshaft with a split profile will be larger?

does this affect the range where torque would be produced?

(bear with me 'im learnin')

[mullinax95]

Interesting

subscribed

[lil red]

mullinax - when u use the phrase "subscribed", is it a polite way to join in to the thread - if so I think i'll adopt the same.

[RustyRatRod]

I've always used a general rule that says engines with headers like single pattern and manifolds like dual. Course it aint set in stone.

[OldmanRick]

Originally Posted by StrokerScamp

I've always used a general rule that says engines with headers like single pattern and manifolds like dual. Course it aint set in stone.

Yea i always believed most of the off the shelf dual pattern grinds were mainly for the purpose of being a crutch for a weak exhaust port/system.

[lil red]

Originally Posted by StrokerScamp

I've always used a general rule that says engines with headers like single pattern and manifolds like dual. Course it aint set in stone.

interesting....definitely a factor with the express truck's chocked exhaust

[RustyRatRod]

Originally Posted by lil red

interesting....definitely a factor with the express truck's chocked exhaust

Maybe Justin will chime in. He's a beegenius about this stuff.

[Abodybomber]

Most of the engine buildups I notice tend to run narrower splits,or single patterns when exhaust flow flows 70 to 75 % of intake flow.This is generally a starting point I have noticed overtime.Of course,this is just one way to skin the cat.All changes with turbos,blowers,and spray.

[Idaho]

I have zero expertise, just lots of interest

If I'm thinking right on this, equal duration when the head flows 25% less through the exhaust port makes sense because exhaust flow is enhanced by the residual pressure of combustion.

Perhaps I'm stating the obvious but grasshoppers like me need to think through that kind of thing.

[moper]

Originally Posted by OldmanRick

Yea i always believed most of the off the shelf dual pattern grinds were mainly for the purpose of being a crutch for a weak exhaust port/system.

That's exactly what it is, but modern thinking is it's not only a crutch per se. It's a way to maximize the physics of the intake and exhaust cycles to fully evacuate the cylinder and help overfill on the intake side. There are heads that have a really good (beyond 80%) ratio where the builder designs the cam with less lift and duration on exh than intake side because on the exh side is only gas being pushed out rather than air and liquid being sucked in if things are working right. Dual pattern simply means the lobes are different between exh and intake. Sometimes the affect may be accentuated by runnign staggered rocker ratios too. On a custom cam the individuality of a lobe could be rate of lift, durations at various lifts, rate of closure, and/or lobe centerlines.

"if the bonus of a larger duration camshaft is lost when the ratio cloese on 1:1 , does this mean that if 2 camshafts of the same duration (size) are compared a camshaft with a split profile will be larger?
It's not so much that it's lost, it's just less of a boost. Duration is one small part of a cam lobe, and more specifically, duration at a given lift is what you're talking about... That's an even smaller piece. So which is larger is not able to be determined unless you know more about the four lobe designs themselves. Generally, what you're ferring to as a dual pattern - one where the exh has more lift and/or duration - will behave as a smaller cam when in a running engine. Meaning it will idle cleaner, respond better, and make more torque at lower rpms and exhibit a flatter torque curve than a single pattern cam of similar advertised size.

does this affect the range where torque would be produced? Any change in cam design will affect torque. Rememebr torque is the only empirical value we have. Horsepower is only torque over a given time. If there is no other change, and the engine has a full muffled exhaust, a manifold or header design compromised by having to fit a chassis, and the limited size of a factory designed port, then in my opinion the typical split patter of more exh duration and lift is preferred. As is normally the case, there are as many opinions as builders so I'm sure there's other ideas too. But that's mine.

[OldmanRick]

[QUOTE=moper;1969461729] It's a way to maximize the physics of the intake and exhaust cycles to fully evacuate the cylinder and help overfill on the intake side.

Read some years ago about engines achieving more that 100% VE (volumetric efficiency). I guess this would be part of that formula.

Nice post moper, Thanks

[lil red]

thanks moper - good explanation

your explanation of idle and cam characteristics makes sense, I remember using the mp .474 years ago and for the size it sounded much larger than what it was - but performed well

[Bad Sport]

Originally Posted by lil red

mullinax - when u use the phrase "subscribed", is it a polite way to join in to the thread - if so I think i'll adopt the same.

When you make a post in a thread, under additional options you can subscribe to the thread. You will get email notifications when a new post is added.

[OldmanRick]

Originally Posted by lil red

thanks moper - good explanation

your explanation of idle and cam characteristics makes sense, I remember using the mp .474 years ago and for the size it sounded much larger than what it was - but performed well

Yea lil red, i loved that cam. Way back when, guys would say they wanted "one step larger then stock" and were looking at the 68' 4spd grind and i would always tell'em if your going in there use the .474. Don't think anybody was dissapointed. I ran one in a otherwise stock hi comp. 340, and picked up 3 mph in the quarter. Think it was #xxxx992, but i could be mistaken, was longer ago then i care to remember

[lil red]

Originally Posted by OldmanRick

Yea lil red, i loved that cam. Way back when, guys would say they wanted "one step larger then stock" and were looking at the 68' 4spd grind and i would always tell'em if your going in there use the .474. Don't think anybody was dissapointed. I ran one in a otherwise stock hi comp. 340, and picked up 3 mph in the quarter. Think it was #xxxx992, but i could be mistaken, was longer ago then i care to remember

The original motor in my express had the 68 4 spd camshaft from the factory - the 360 ran pretty good when i first bought it for a low compression/horsepower motor