A
Anonymous
Guest
I know that different cams are designed for more torque but how does that work? Could someone give me an explanation as to how torque is created in terms of the intake and exhaust strokes? Thanks.
Torque is what everyone with a working truck wants and what a inline six is all about.
A camshaft determines a large part of where the power band has its peaks but also what the power curve looks like as in how steep the curve looks.
You are most likely aware the cam determins how long a valve is open (duration in crank degrees) And also how far it is open ( Lift).
The main factor as to steepness of th torque curve is the lobe seperation.
A wider lobe sep like 112-114 degrees will give a wide flater torque curve.
A narrower seperation like 106-108 will give a sharper increase in the torque and a more limmited power band.
All else being the same a wide sep will idle with more vacum and have less lope.
A camshaft determines a large part of where the power band has its peaks but also what the power curve looks like as in how steep the curve looks.
You are most likely aware the cam determins how long a valve is open (duration in crank degrees) And also how far it is open ( Lift).
The main factor as to steepness of th torque curve is the lobe seperation.
A wider lobe sep like 112-114 degrees will give a wide flater torque curve.
A narrower seperation like 106-108 will give a sharper increase in the torque and a more limmited power band.
All else being the same a wide sep will idle with more vacum and have less lope.
Two things make torque - displacement and/or compression.
Displacement is set at a physical maximum measurement when you build the engine, as is the Compression.
But a cam can change the effective displacement and/or compression - the values that the engines actually operates at when running. The point at which the intake valve closes is the single most important event in valve timing - at least as far as displacement and compression are concerned.
For example, imagine a single cylinder engine with a physical displacement of 100 cubic inches, and a static compression ratio of 10 to 1. When the engine is running, the intake valve on any cam won't close off the cylinder at the dead bottom of the compression stroke - it does it later, which means that the actual amount of stuff getting compressed is less than the max 100 inches, and so the actual (dynamic) compression ratio will be less than the measured 10-1. If the intake valve closed sooner, the engine would effectively displace and compress more, and if it were later it would displace and compress less. So you see that by playing around with the intake valve closing point, you can increase or decrease the effective displacement of the engine (within practical limits).
Also keep in mind that high-rpm = high horsepower, and that horsepower is mathematically derived from torque and rpm.
"Racing" (high-rpm/hp) cams tend to close the intake later, which is why they need really high static compression ratios to make up for the "bled off" compression. They rev high, but run like poop at low rpm.
"Torque" (low-rpm) cams tend to close the intake sooner, to increase the effective displacement and compression - which is why they don't want/need really high compression ratios, or they will detonate themselves to death. They run great at low rpm, but simply can't breathe well enough to rev very high.
Clear as mud?
Displacement is set at a physical maximum measurement when you build the engine, as is the Compression.
But a cam can change the effective displacement and/or compression - the values that the engines actually operates at when running. The point at which the intake valve closes is the single most important event in valve timing - at least as far as displacement and compression are concerned.
For example, imagine a single cylinder engine with a physical displacement of 100 cubic inches, and a static compression ratio of 10 to 1. When the engine is running, the intake valve on any cam won't close off the cylinder at the dead bottom of the compression stroke - it does it later, which means that the actual amount of stuff getting compressed is less than the max 100 inches, and so the actual (dynamic) compression ratio will be less than the measured 10-1. If the intake valve closed sooner, the engine would effectively displace and compress more, and if it were later it would displace and compress less. So you see that by playing around with the intake valve closing point, you can increase or decrease the effective displacement of the engine (within practical limits).
Also keep in mind that high-rpm = high horsepower, and that horsepower is mathematically derived from torque and rpm.
"Racing" (high-rpm/hp) cams tend to close the intake later, which is why they need really high static compression ratios to make up for the "bled off" compression. They rev high, but run like poop at low rpm.
"Torque" (low-rpm) cams tend to close the intake sooner, to increase the effective displacement and compression - which is why they don't want/need really high compression ratios, or they will detonate themselves to death. They run great at low rpm, but simply can't breathe well enough to rev very high.
Clear as mud?
I have a tattered old copy of a Dyke's Instruction Manual. The covers and many pages are missing, but the newest information I can find in it is dated 1941 so I presume it to be pre-WW2.
One engine illustrated is a Continental Model "L4" Red Seal four-cylinder flathead and is described as being intended for truck and tractor use.
Bore is 4.5" and stroke is 5.5" for about 350 cubic inches. Max rpm is 1300 for a rated 43 horsepower.
Cam timing is as follows:
Intake opens 10º After Top Dead Center
Intake closes 20º ABDC
Exhaust opens 36º BBDC
Exhaust closes 5º ATDC.
This is gross timing when the valve first begins to move, NOT the modern method of using .006" or .050" of lifter rise.
Note the complete lack of overlap. The exhaust closes 5º ATDC, and the intake doesn't move for another 5 degrees. There is a 5º duration when neither valve is open.
This manual has a listing of many older engines and gives the intake opening event for purposes of checking cam timing but only gives info for a few engines on exhaust events and intake closing.
From perusing these data it is obvious that the really slow-speed engines had really late intake opening events. They also used very low compression ratios.
GMC built a series of large truck engines (gasoline), the largest having 707 cubic inches from six cylinders with a bore of 5" and stroke of 6". Compression was 4.42 : 1 Keep in mind that gasoline was around 60 octane. This engine developed 550 lbs/ft of torque at 1000 rpm. Max horsepower was 173, rpm not listed.
For max torque at low speeds, cam timing needs to be really short. The closest I have found is the Economaster grind from www.reedcams.com
Joe
One engine illustrated is a Continental Model "L4" Red Seal four-cylinder flathead and is described as being intended for truck and tractor use.
Bore is 4.5" and stroke is 5.5" for about 350 cubic inches. Max rpm is 1300 for a rated 43 horsepower.
Cam timing is as follows:
Intake opens 10º After Top Dead Center
Intake closes 20º ABDC
Exhaust opens 36º BBDC
Exhaust closes 5º ATDC.
This is gross timing when the valve first begins to move, NOT the modern method of using .006" or .050" of lifter rise.
Note the complete lack of overlap. The exhaust closes 5º ATDC, and the intake doesn't move for another 5 degrees. There is a 5º duration when neither valve is open.
This manual has a listing of many older engines and gives the intake opening event for purposes of checking cam timing but only gives info for a few engines on exhaust events and intake closing.
From perusing these data it is obvious that the really slow-speed engines had really late intake opening events. They also used very low compression ratios.
GMC built a series of large truck engines (gasoline), the largest having 707 cubic inches from six cylinders with a bore of 5" and stroke of 6". Compression was 4.42 : 1 Keep in mind that gasoline was around 60 octane. This engine developed 550 lbs/ft of torque at 1000 rpm. Max horsepower was 173, rpm not listed.
For max torque at low speeds, cam timing needs to be really short. The closest I have found is the Economaster grind from www.reedcams.com
Joe
Yow! If you assume it had a pretty flat torque curve and kept making 550 lb/ft, it'd hit 173 hp at 1652 rpm. Even if the torque fell to 450 lb/ft, it'd only take 1980 rpm to make 170 hp.Lazy JW":2pa81hru said:
With a bore/stroke that big, I'll bet it redlined just over 2000 rpm.
I found in another section of the book (it is a mess) that stated the 707 redlined at 2100 rpm. These engines had 7 mains, overhead valves, two-barrel carbs, two heads (three cylinders each), the exhaust manifold was three-piece with expansion joints. Big engines for big trucks, before diesels were in vogue for such useage. I would love to see one sometime.
Joe
Joe
Hi Joe,which one of the Reed cams were you referring to(stock number)?Thanks.
Leo
Hi Joe.Thanks.Leo
My 1950 Chilton's manual has some data for tractor tune-ups. There is a partial cam spec listing for various models.
My 1942 John Deere Model A tractor shows the intake opening at 10º ATDC and the exhaust closing at 5º ATDC. Note there are 5º where both valves are closed.
This appears to be about typical valve timing for engines rated at less than 1500 rpm. One engine opened the intake at 18º ATDC. My Johnny Popper is rated at 975 rpm.
These slow-speed engines typically have a long stroke. My two-cylinder John Deere A has a 5.5" bore and a 6.5" stroke.
Apparently the designers wanted the piston to get a good start downward before popping the intake valve open to get a good, strong vacuum pulse through the carb. This would also help to keep flow velocity up.
This almost universal late intake opening on slow-speed engines caught my attention. When the rated rpm climbs over 2000 then the cam events appear a bit more "normal" but are still pretty conservative by modern standards. The stock Ford 300-6 cam is a veritable whopper in comparison.
If you are really serious about low-speed torque, don't put a very long duration cam in, but open the valves quickly when it is time.
Joe
My 1942 John Deere Model A tractor shows the intake opening at 10º ATDC and the exhaust closing at 5º ATDC. Note there are 5º where both valves are closed.
This appears to be about typical valve timing for engines rated at less than 1500 rpm. One engine opened the intake at 18º ATDC. My Johnny Popper is rated at 975 rpm.
These slow-speed engines typically have a long stroke. My two-cylinder John Deere A has a 5.5" bore and a 6.5" stroke.
Apparently the designers wanted the piston to get a good start downward before popping the intake valve open to get a good, strong vacuum pulse through the carb. This would also help to keep flow velocity up.
This almost universal late intake opening on slow-speed engines caught my attention. When the rated rpm climbs over 2000 then the cam events appear a bit more "normal" but are still pretty conservative by modern standards. The stock Ford 300-6 cam is a veritable whopper in comparison.
If you are really serious about low-speed torque, don't put a very long duration cam in, but open the valves quickly when it is time.
Joe
jamyers":1ym1qfoo said:
Yup
His Johnny Popper didn't have electric start I reckon. Niether does mine. No problem starting cold but when it's hot...
I learned to spray a quick shot of ether in the compression release petcocks, then spin it. Usually starts right up even hot.
Those things will lug down so low you can actually see the flywheel clearly. I killed mine one time and it re-started itself
I was plowing with the two-bottom trail plow, got hung up on a rock, grabbed the hand clutch just in time. Tried to ease it over the rock by simultaneously pulling the trip rope with my right hand, hanging on to to the steering wheel with my left hand, and pushing the clutch forward with my righ foot
Musta been quite a sight :roll: Gave it too much clutch which lugged it down too far, and it died. I actually SAW the flywheel spin backwards. As I was grabbing for the clutch with my hand, the whole tractor rocked backwards, then rolled forward, giving it just enough impetus to turn the engine forward before I released the clutch and it fired up again 8) My wife said she heard it die and start right back up. I'll probably never duplicate that feat as long as I live.
Joe
His had electric start - musta been one of them 'modern' ones. I know what you mean about watching the flywheel, my brother and I used to try and see who could idle it down the lowest, and I swear you could actually see the flywheel aalllllmmmmmooooost stopping as it went by tdc.Lazy JW":22dvn9xo said:...His Johnny Popper didn't have electric start I reckon. Niether does mine. No problem starting cold but when it's hot...
...
Those things will lug down so low you can actually see the flywheel clearly. I killed mine one time and it re-started itself
Killed it and it restarted? I'll bet you can't do that again, either. (brings to mind the saying about "one-armed paperhanger")
Static. I don't even know at what number of degrees they are set. There is a cast mark on the flywheel that reads LH IMPULSE. You line that up with a mark on the block, turn the magneto until the impulse trips and lock 'er down.
All of the old tractors I have worked on had no advance, just the automatic retard from the magneto impulse mechanism. Dunno if the newer gas models had advance or not.
Joe
All of the old tractors I have worked on had no advance, just the automatic retard from the magneto impulse mechanism. Dunno if the newer gas models had advance or not.
Joe
SuperMag":3gdxiaxe said:
Advance curve, what's that?
I don't think they revved high enough to even think about needing advance.
Ever see/hear a "Hit-N-Miss" engine? Grandpa had several of those hanging around, used to fire them up just for fun. OSHA would have a stroke if they ever saw one running. The starter handle was simply bolted to the flywheel, and it was just a blur sticking out waiting for you to get near so it could beat your leg off at the knee.
Safety guard? We don' need no steenkin' safety guard!
A
Anonymous
Guest
My Uncle has a `47 JD model M. He always stuck a tin can over the exhaust to keep the rain out. It was always fun as a young teen to watch him start it up and pop the can off 4'!!
As far as the post, Your cam must match your head flow, bore/ stroke and compression. They're all relevant.
As far as the post, Your cam must match your head flow, bore/ stroke and compression. They're all relevant.
