Huge Split Duration Camshaft for Stroked L92
This is a 234-240 112 +0 XE-R cam in a 403. The smaller bore shrouds the exhaust valve a bit more than on a 418. You can tell from the torque curve this car drives like a big electric motor.
TECH Resident
Joined: Jul 2007
Posts: 844
Likes: 0
From: Florida
I realize that the intake ports way outflows the exhaust ports on my stock (Unported) L92 heads. My cam was custom designed by Comp Cams in Dec 2006. The specs are 236/248 duration @.050; .601"/.615" lift, 112 LSA. However I've never seen a camshaft with a 12 degree split.
-Geoff
Staging Lane
Joined: Sep 2007
Posts: 50
Likes: 0
From: East Oakland, CA
I agree... looking at the cam that Rick at Synergy spec'd for me... Cybernco's isnt that far off. And the split should help with an unported exhaust port. Also... Rick's intake does have more lift than the exhaust... I assume this also helps with shrouding of the valve, as it spends more time over the horrible .400-.500 range where the shrouding hurts flow.
White_Hawk made a good suggestion...try that.
me too..i think the exhaust ports actually flow pretty good in the low to mid lifts..i'm thinking they don't flow as bad as everyone thinks..just have to put the right cam in there...
Teching In
Joined: Feb 2007
Posts: 47
Likes: 0
From: Illinois
On a 4" bore engine, i don't think a huge split in duration is necessary.
I've seen smaller, less split in duration cams, with these l92s untouched, put down more RWHP than, huge cams, huge split duration, ported to hell l92s.
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FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
With regards to "both valves acting as exhaust valves" - that only happens at lower revs. Once you get 'er wound up, it's the scavenging effect from both valves being open at once that fills the cylinder since the incoming charge is actually pulled into the cylinder from a combination of piston movement and negative pressure in the exhaust. Without that, there's no way a performance engine could even approach 100% V/E.
At 7200 RPM, each valve is opening and closing 60 times EVERY SECOND - there's no way to get enough of a pull on the intake ports with just negative pressure from the downward moving piston. At this speed, the engine is actually "trapping" air in the cylinders when the intake closes. In addition, keep in mind that inertia has a great deal to do with what's going on in the intake manifold. Air in motion tends to keep moving - when you close the intake valve, the air wants to keep moving, so it "piles up" and builds a small amount of pressure against the back of the intake valve. At lower revs, there's not enough inertia to build any pressure, so the diluted charge sneaks back into the intake a bit. Once the revs build and the air gets up to speed, the dynamics change.
I've heard more than one engine builder call the overlap/scavenging effect as the 5th cycle of a 4 cycle engine. It's a bit like timing - it doesn't make sense that we fire the plug 35* or so before the piston reaches the top of it's stroke, but it works.
At 7200 RPM, each valve is opening and closing 60 times EVERY SECOND - there's no way to get enough of a pull on the intake ports with just negative pressure from the downward moving piston. At this speed, the engine is actually "trapping" air in the cylinders when the intake closes. In addition, keep in mind that inertia has a great deal to do with what's going on in the intake manifold. Air in motion tends to keep moving - when you close the intake valve, the air wants to keep moving, so it "piles up" and builds a small amount of pressure against the back of the intake valve. At lower revs, there's not enough inertia to build any pressure, so the diluted charge sneaks back into the intake a bit. Once the revs build and the air gets up to speed, the dynamics change.
I've heard more than one engine builder call the overlap/scavenging effect as the 5th cycle of a 4 cycle engine. It's a bit like timing - it doesn't make sense that we fire the plug 35* or so before the piston reaches the top of it's stroke, but it works.
You can't just run a lot of overlap and then make an engine breathe with bad intake ports or too small an intake port for the cubic inches. The heads need to have the right size ports vs your engine size to make good VE at the rpm you choose your engine to peak at. A cam will never fix that but can only maximize what your heads can already do. A bad cam will certainly hurt you though.
This is compounded by large intake valves and smaller proportionately exhaust ports and is also made worse by lower compression which generates even more exhaust volume and higher EGTs. The worst is a turbo engine with smaller exhaust housings where you can have double the exhaust pressure or worse than manifold pressure and they you will hit a wall with this problem and wisening LSA and descreasing duration will actually help you make more HP at higher rpm! The opposite of a normal NA engine.
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
Quote:
Originally Posted by Greg Good
.....What we're dealing with here is a simple situation where air flows from a high pressure area (the intake port) towards a low pressure area (the incompletlely filled cylinder). ........ Backflow before peak torque is not the real problem with big intake valves. It's at the beginning of the intake stroke where the intake valve just opens (while the engine is still on the exhaust stroke) that causes the problem. A bigger higher flowing intake valve will allow more exhaust into the intake port.....
How does exhaust flow from the cylinder into the intake port if the pressure in the manifold is higher than that of the cylinder?
Originally posted by Greg Good
...... keep in mind that during the first phase of overlap (the part before TDC) BOTH valves are in effect exhaust valves......
Maybe with a plugged cat or some other obstruction.
Please note..... not trying to pick on any individual here, I feel these statements represent beliefs held by many on this forum.
Maybe these beliefs need some more thought......
Originally Posted by Greg Good
.....What we're dealing with here is a simple situation where air flows from a high pressure area (the intake port) towards a low pressure area (the incompletlely filled cylinder). ........ Backflow before peak torque is not the real problem with big intake valves. It's at the beginning of the intake stroke where the intake valve just opens (while the engine is still on the exhaust stroke) that causes the problem. A bigger higher flowing intake valve will allow more exhaust into the intake port.....
How does exhaust flow from the cylinder into the intake port if the pressure in the manifold is higher than that of the cylinder?
Originally posted by Greg Good
...... keep in mind that during the first phase of overlap (the part before TDC) BOTH valves are in effect exhaust valves......
Maybe with a plugged cat or some other obstruction.
Please note..... not trying to pick on any individual here, I feel these statements represent beliefs held by many on this forum.
Maybe these beliefs need some more thought......
I don't think Greg cares if you "pick" on him as he knows exactly what he is talking about. He was just trying to help out with understanding why opening the intake too early before TDC on something with a very big intake valve can be bad and he's totally right. When you see engines with much larger intake ports and valves proportionately to the exhaust you will also usually see less overlap used along with a later intake opening point and an earlier intake closing point. The worse your exhaust system is the less overlap it will take before bad things happen. Then depending on your exhaust system and compression and fuel and exhaust system and header sizing you may or may not open the exhaust valve earlier or later than normal.
I agree with a lot of what you have written on just everyone always opening the exhaust way too early on any random engine but exhaust reversion with these big intake valved heads is very real and does seem to hurt power as well as running too much overlap unless you have much better exhaust and don't open the intake too early.
TECH Apprentice
Joined: Oct 2007
Posts: 301
Likes: 20
Erik, I'm not arguing with you, just trying to learn:
If exhaust reversion is a problem at higher revs on race engines, how do they achieve 100%+ VE at higher revs? At those speeds, there has to be something other than the downward travel of the piston and inertia of the incoming charge to fill the cylinders.
Please fill me in, I'm trying my best to learn and understand this stuff.
If exhaust reversion is a problem at higher revs on race engines, how do they achieve 100%+ VE at higher revs? At those speeds, there has to be something other than the downward travel of the piston and inertia of the incoming charge to fill the cylinders.
Please fill me in, I'm trying my best to learn and understand this stuff.
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
Erik, I'm not arguing with you, just trying to learn:
If exhaust reversion is a problem at higher revs on race engines, how do they achieve 100%+ VE at higher revs? At those speeds, there has to be something other than the downward travel of the piston and inertia of the incoming charge to fill the cylinders.
Please fill me in, I'm trying my best to learn and understand this stuff.
If exhaust reversion is a problem at higher revs on race engines, how do they achieve 100%+ VE at higher revs? At those speeds, there has to be something other than the downward travel of the piston and inertia of the incoming charge to fill the cylinders.
Please fill me in, I'm trying my best to learn and understand this stuff.
The extra VE is from the extra air and fuel packed in the cylinders after BDC when the pistons have turned the corner and are coming back up the bore. This is acheived mostly via port velocity and intake tuning.
You can have a great exhaust port and smaller intake port and overscavenge and see fake better VE numbers due to all the extra fuel and air blown out the exhaust during overlap.
Real VE should show higher tq at that rpm and better BSFC. If it's higher tq but worse BSFC then you are probably overscavenging.
Again we're not talking all out race engines here with these deals but most all out race deals again with big intake to exhaust valve heads have cams with a 20-30 degree split.
