At lower rpm, the overlap period allows raw fuel to go right into the exhaust system, increasing emissions. It also allows exhaust to back up into the chamber.

Of course, decreasing the lobe seperation, or increasing the duration of the lobes increases overlap.... which is why these cams 'thump'.

NOW... as the rpm picks up, the air/fuel charge gets more momentum in the intake manifold... and as long as we have a VERY FREE FLOWING EXHAUST, the fast moving exhaust actually draws a vacuum, and helps draw in the intake charge. The longer we keep both valves open, the longer this effect takes place. When the exhaust valve closes, the 'vacuum' stops, but that extra momentum of the air fuel charge is still there... which helps fill the cylinder. We use this 'vacuum' to help pull the intake charge through the intake runner, then slap the exhaust valve shut to trap it in the cylinder. It's how we can get more than 100% volumetric efficiency. We can effectively 'supercharge' a naturally aspirated engine with a HIGHLY EFFICIENT exhaust system and the right camshaft.

Good Luck!

Thanks for the info

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SCOOP

oddly obsessed with big scoops on little Mustangs

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

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SCOOP

oddly obsessed with big scoops on little Mustangs

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

quote:

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Originally posted by Fastymz:
So for all motors it's the same ? It doesn't matter on size or heads or compression ?

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I'm just trying to understand if there is a set rating for all motors for wide or narrow overlap.

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SCOOP

oddly obsessed with big scoops on little Mustangs

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

quote:

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Originally posted by Fastymz:
Yeah I worded that poorly. I was trying to ask, does a wide and small overlap measurement always fit those parameters and is not effected by cubic inch's, or can a larger motor get away with more over lap etc.

I'm just trying to understand if there is a set rating for all motors for wide or narrow overlap.

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You and I pick cams with a small enough overlap that it will idle on the street.
Overlap is the only thing that determines how rough or smooth an engine idles.
Overlap is the result of duration of each lobe and the lobe displacement angle.
See this link for a better explanation
http://members.uia.net/pkelley2/Overlap.html
The scavenging that is possible in certain circumstances is due to the total intake and exhaust tract design.
If you really want to know a ton of cam info, I believe Vizards SBC cam/valvetrain book is the only one out there except for maybe some automotive engineering text books.

Les

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 WIDENING the 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.

Tight Lobe seperations work by taking advantage of increased overlap. The exhaust flowing out the port at very high speeds creates a vacuum that helps draw in more intake charge. This is very handy with overly large intake ports (4 bbl 351C) to help with intake port velocity... as well as engines with overly small intakes or carbs. (to help draw the intake charge through the small ports/carb)

However, a GOOD, free flowing exhaust is required on engines with cams having tight lobe seperations. Without a good exhaust, the exhaust just doesn't flow out and create a vacuum to pull the intake charge in like it should. The exhaust just backs up into the cylinder and makes the car run like crap. Extreme cases will even have the exhaust back up into the intake, which will turn the inside of the carb black with soot. I've seen it...

In a nutshell...
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
6. stinks up the exhaust

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

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