Torch
Extremely dangerous with a torch!!!
- Joined
- Jul 18, 2008
- Messages
- 1,627
tighter lobe center yeilds greater overlap
Overlap allows the exiting exhaust to impart momentum to the intake charge by drawing it down and out the exhaust before the exhaust valve closes.
When the exhaust valve closes the intake charge stacks up in the cylinder under pressure before the intake closes.
This increases breathing efficiency at higher rpms but wastes fuel out the exhaust.
This is why the 302 V6 in my 1963 GMC always smelled like raw fuel at the tail pipe. The engine was a big truck engine designed to work at max rpm not designed to be lugged and the high overlap is how the 8:1 compression cylinder got filled well.
people called them boat anchors when they didn't know how they were intended to be driven. NAPA machinist told me this. I had complained it was a dog and he asked me if I was using the "gold" points ir the cheap points. he told me the Gold ponts have a tigher spring and keep point bounce at bay at the upper rpm's then went on to tell me how the engine was tuned by GM to be wound out before it was shifted.
After installing the gold points and driving it like that It was awesome.
Truck would do over 120 mph with 3:08 gears and a 4 speed now that the ignition system was not shunting the engine before it got into it's powerband.
Lift- increases the "area under the curve". the higher you lift the valve past the port's maximum flow rate, the more TIME the valve is open at max flow
Duration- The amount of time the valve is open
What you want to do with a small valve engine is to increase lift and duration as much as you can without increasing the stock overlap. Overlap is where your re going to lose mileage.
At some point no more lift or duration will cheat the small port so you don't need to go more than is effective. plus more lift requires stiffer valve springs to prevent valve bounce ie "float"
the higher the valve is lifted the more energy that is expressed when it closes
All that energy has to be stopped at the closing by valve spring pressure or it will vibrate the spring and lift the valve back off the seat when it is supposed to be closed.
Increasing duration actually slows the valve speed and helps float.
specialty cams are out there that open the valve fast and set it down slow.
stiffer valve springs and high lift require screw in rocker studs OR you can drill through the head casting into the stud and drive in a roll pin to prevent them from pulling out.
for good mileage I'd pick something between a 116 degree and 112 degree lobe center.
for keeping stock springs and valve train you can go up to around .500 lift
Anything over 280 degrees duration in a flat tappet is going to start dropping vacuum at idle and run thumpy but the powerband will be moved up the curve.
Overlap allows the exiting exhaust to impart momentum to the intake charge by drawing it down and out the exhaust before the exhaust valve closes.
When the exhaust valve closes the intake charge stacks up in the cylinder under pressure before the intake closes.
This increases breathing efficiency at higher rpms but wastes fuel out the exhaust.
This is why the 302 V6 in my 1963 GMC always smelled like raw fuel at the tail pipe. The engine was a big truck engine designed to work at max rpm not designed to be lugged and the high overlap is how the 8:1 compression cylinder got filled well.
people called them boat anchors when they didn't know how they were intended to be driven. NAPA machinist told me this. I had complained it was a dog and he asked me if I was using the "gold" points ir the cheap points. he told me the Gold ponts have a tigher spring and keep point bounce at bay at the upper rpm's then went on to tell me how the engine was tuned by GM to be wound out before it was shifted.
After installing the gold points and driving it like that It was awesome.
Truck would do over 120 mph with 3:08 gears and a 4 speed now that the ignition system was not shunting the engine before it got into it's powerband.
Lift- increases the "area under the curve". the higher you lift the valve past the port's maximum flow rate, the more TIME the valve is open at max flow
Duration- The amount of time the valve is open
What you want to do with a small valve engine is to increase lift and duration as much as you can without increasing the stock overlap. Overlap is where your re going to lose mileage.
At some point no more lift or duration will cheat the small port so you don't need to go more than is effective. plus more lift requires stiffer valve springs to prevent valve bounce ie "float"
the higher the valve is lifted the more energy that is expressed when it closes
All that energy has to be stopped at the closing by valve spring pressure or it will vibrate the spring and lift the valve back off the seat when it is supposed to be closed.
Increasing duration actually slows the valve speed and helps float.
specialty cams are out there that open the valve fast and set it down slow.
stiffer valve springs and high lift require screw in rocker studs OR you can drill through the head casting into the stud and drive in a roll pin to prevent them from pulling out.
for good mileage I'd pick something between a 116 degree and 112 degree lobe center.
for keeping stock springs and valve train you can go up to around .500 lift
Anything over 280 degrees duration in a flat tappet is going to start dropping vacuum at idle and run thumpy but the powerband will be moved up the curve.
