So to sum it up, if you wanna drag race are you better off with short LSA, and long duration. or long LSA and long duration?

I use Performance Trends Engine Analyzer to "look" at various cams. You can see the *slight* shift in power at the high end on the large LSA cams.

Decreasing lobe seperation:
1. narrows the powerband
2. moves the powerband lower
3. makes the idle choppier
4. makes more power
5. pumps up the midrange

Increasing lobe seperation
1. widens powerband
2. makes the idle smoother
3. takes away from the midrange
4. produces less maximum power

Racers usually use tight lobe seperations. The only exceptions are Pro-Stock, where 500+ ci are revved to the moon with cams so HUGE, tight LSA's produce too much overlap, and nitrous/blower/turbo applications where overlap sends all that extra fuel mixture out the tailpipe!

Tight lobe seperations increase overlap and can help increase velocity in overly huge intake ports (4bbl 351C). This will pump up the midrange, and make the engine feel less "lazy". Some 351C drag race cams have lobe seperations as low as 102-104 degrees. Automatics generally need a tighter lobe seperation for extra midrange.

Small engines with big ports LOVE tight lobe seperations, large engines with small ports work better with wider angles between the lobes.

A tighter lobe seperation also allows you to use a slightly larger cam in a lower rpm range. If you want maximum valve timing without revving the engine to the moon, a tighter lobe seperation can make it happen.

It's the dynamics of the tight (low) lobe seperation that produces the extra power.

Tightening the lobe seperation increases the cranking (effective) compression. The piston can only compress the air it traps in the cylinder. Long durations and wide lobe seperations push the intake closing event into the "compression" stroke. If air is escaping out the intake valve as the piston travels upward, it's not getting compressed, and cylinder pressure goes out the window! Narrowing the lobe seperation helps build more compression by making the intake valve close sooner, enabling the compression stroke to capture and compress more air. Advancing the camshaft will do this as well.

Some people mistakenly believe that compression is lost during the OVERLAP period... WRONG! Overlap occurs when the exhaust valve is closing and the intake is just starting to open... Nothing is being compressed yet! NARROWING the lobe seperation pulls the intake closing event out of the compression stroke. Advancing the cam will make the intake valve close sooner as well.

Here's an image that will help explain how increasing lobe seperation DECREASES cranking cylinder pressure. You can see how spreading the lobes further apart puts the intake event further into the compression stoke. This causes the valve to be open longer into the compression stroke... bleeding off cylinder pressure.

Here's another site with some lobe seperation info.

http://www.fordmuscle.com/archives/2001/04/camtech/

Just be sure to use an extremely free flowing exhaust with extra tight lobe seperation angles. Their scavenging effect from their extra overlap NEEDS some free flowing tubes, or the exhaust will just get backed up into the cylinders... Really bad exhaust systems will actually make the carburetor get all black with carbon. Open er' up, and let er' fly! I really like the dual 3" woofers that occupy the underside of my car.

Also ask yourself how high you are willing to rev the engine before deciding upon a camshaft.

And a VERY detailed thread with all kinds of great info, go here...

http://www.fordmuscle.com/phpBB/viewtopic.php?topic=1588&forum=1

Good Luck!

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SCOOP

2.26 60'S
14.9 @ 90.86MPH

65 coupe,351w,C4,Big Boss 429 hood scoop,8" 3.40 TracLoc.

The other thing that gets me is: what the heck is the difference between a oval track cam and a bracket racing cam? and some sites even list them dually ie: "good 1/4-3/8 mile oval grind, good bracket racing"

Mike that is a very good post but I have a question. The statement of tight LS angles work best for good flowing heads. My experience seems to indicate tight angles work best for heads with marginal breathing. By that I mean the port volume and air delivery ability are pretty closely matched giving higher velocity flow. I first became aware of using the tight angles on a friends 283 SS engine and the 104 centerlines worked well enough to set the record with that combination. That same center in a Boss 302 was a dog (could have been other problems there).

I hear the PRO Stock racers are using centerlines in the 118, 119 degree area but I have no knowledge of how well their heads flow these days.

I have been doing some engine modeling on the computer and then let the computer design the cam. What I am seeing is the same engine with increasingly more air flow keeps getting the centers spread out.

Any comments?

As an aside to the guys trying to pick a cam and run high compression. The tighter the centers are the more valve to piston interference you have and the more the piston will need cutting.

John

As far as using tight lobe seperations with restrictive heads in racing classes...

Exhaust racing out of the head while BOTH valves open (overlap) helps pull the air/fuel charge through the intake manifold. A decent exhaust port and a good set of headers can really help pull more air/fuel through a restrictive intake port.

Tight lobe seperation angles also produce more cylinder pressure. You'll see superstockers, and others that are compression limited running insanely tight lobe seperation angles... especially if the engine is required to breathe through restrictive, stock heads.

351C's have overly HUGE intake ports, and the extra overlap (when used with a nonrestrictive exhaust) helps provide extra intake port velocity, which is just what the doctor ordered for these engines.

As far as Pro-Stockers are concerned, I'd say their cylinder head designs are pretty much optimized. They have no compression ratio limits, and with the size of the cams they are using.... plenty of overlap is a given.

Good Luck!

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Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

quote:

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Originally posted by n2oMike:

351C's have overly HUGE intake ports, and the extra overlap (when used with a nonrestrictive exhaust) helps provide extra intake port velocity, which is just what the doctor ordered for these engines.

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All I have ever ran in my clevelands have had between 106 and 108 lsa's. I have a solid camshaft that has an lsa of 110, but I have not used it in anything.

I have a restrictive exhaust system on my 4v cleveland: 1 ?? header primary tubes. I switched to little larger duration and to a 112 LSA from a 110, and the engine seems to have woken up across the board. Whereas before I would wait for the power to kick in at 3500-4000 RPM?s, it seems to be there now at 2500 RPM?s. It just doesn?t seem to have any flat spots. Maybe it is slower, I don?t know, but I?ll find out this Saturday at the track.

Anyway, I think there may be something to the notion that a generous flowing head with a smaller/street exhaust system might not respond well to narrow LSA?s, like below 110.

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'69 Mustang Fastback, 351C, Holley 950 HP, New Parker Funnelweb intake, ported 4-v quench heads, 10.75:1, UD pulley, windage tray, Top Loader, 4.30 gears 9" rear
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