Cam Lift/Duration & Torque

[Stallion]

I was reading a book on restoring and modifying a C3, and the author said to get best torque results, he was going to go with a high lift and low duration cam.

Why is this? Why would this give you more torque? I would see the high lift would be to get more air into the chamber, but why low duration? :confused

Thanks!

TR

 

[JohnZ]

Long-duration cams have more "overlap" - degrees of crankshaft rotation where both the intake and exhaust valve in any cylinder are open at the same time (intake almost closed, exhaust just starting to open); this bleeds off some cylinder pressure, and results in less efficient combustion at low rpm, thus less torque. Normal production cams have less "overlap", and build more cylinder pressure at low rpm, thus more low-end torque.

High-performance cams have to be matched to the engine's other components so performance is optimized for one set of conditions, with corresponding trade-offs under a wider range of other operating conditions. Cam design is a VERY complex subject.

 

[Dream_Driver]

John is correct again )

Very complex subject. Your best bet would be to call a good cam manufacturer and ask what they think you should use. They will ask you gear ratio, weight, engine size and output, transmission used , and what the intended purpose of the car is for.

But to answere another ? of yours the reason behind having more lift than duration is to get more fuel and air into the combustion chamber for more power but still have lower duration to keep low end torque,
You can also do this another way. If it is a SB in ? you can keep the 1.5 rocker arms on the exhaust and put 1.6 on the intake so you will have more lift on the intakes. But before doing such things make sure your valve springs do not bind and that the slot in the rocker arm will not bind as well against the rocker arm stud. You can get a set of stamped performance rocker arms from chevy pretty cheap that have long slots and these will operate well up to about 488 gross lift.

 

[Stallion]

Long-duration cams have more "overlap" - degrees of crankshaft rotation where both the intake and exhaust valve in any cylinder are open at the same time (intake almost closed, exhaust just starting to open); this bleeds off some cylinder pressure, and results in less efficient combustion at low rpm, thus less torque. Normal production cams have less "overlap", and build more cylinder pressure at low rpm, thus more low-end torque.

High-performance cams have to be matched to the engine's other components so performance is optimized for one set of conditions, with corresponding trade-offs under a wider range of other operating conditions. Cam design is a VERY complex subject.

I see what you mean, John. But knowing this, why would anybody want a longer duration cam? Is there any reason for that lack of pressure?

Thanks so much, John!!

TR

 

[JohnZ]

Long-duration performance cams hold the intake and exhaust valves open longer than a standard cam, thus admitting more air-fuel mixture through the intake valve per cycle, and the exhaust valve is open longer, allowing better exhaust scavenging; result? More power. Downside? The power and torque are at higher rpm, so street driveability and efficiency suffers - there's no free lunch; anything you gain with a given cam is offset by something you lose - that's why cam selection has to be made by considering many different factors, not just by picking "big numbers" out of a catalog.

The reason for reduced cylinder pressure at low rpm is that both valves are slightly open ("overlap") for a longer part of the cycle than with a standard cam, which bleeds off some cylinder pressure as the piston comes up on the compression stroke; this is the phenomenon that lets some engines with very high static compression and radical cams run on pump premium gas without detonation where a similar engine with slightly lower static compression and a lower-overlap standard cam will detonate on the same fuel - due to a higher "dynamic" compression ratio with the latter combination, which builds higher peak cylinder pressure at low rpm due to the reduced period of overlap.

 

[Stallion]

I understand, John. The cam you get should depend on what you do with the car, and then you can fit the cam to yours need accordingly. Thanks, John!!

TR

 

[grumpyvette]

its very simple once you understand a few concepts
first look at this chart
http://www.iskycams.com/ART/techinfo/ncrank1.pdf
then these two cam cards

http://www.crower.com/misc/cam_spec/cam_finder.php?part_num=00242&x=52&y=10

http://www.crower.com/misc/cam_spec/cam_finder.php?part_num=00244&x=31&y=9

the first cam starts compressing the cylinders volume at 38 degrees abdc the second one starts compressing the cylinders volume at 45 abdc
the differance in effective displacement for those cams in a 350 engine with 5.7" rods is 323cid for the milder cam and 312 cid for the slightly more aggressive cam.
the differance in effective rpm levels is about 500rpm from 220@.050 to 234@.050
so the milder cam will tend to build tq about 500rpm lower in the rpm range and build that torque burning about 11 cubic inches more cylinder volume. that does not seem like much untill you realize that 3500rpm x 323 inches /2=327 cubic feet of fuel air mix burned for the smaller cam vs 3500rpm x 311/2=314 cubic feet of fuel air mix burned for the slightly larger cam at the same 3500rpm level thats why the lower duration cam makes more tq lower in the rpm range, but thats not the whole story here!!.... the slightly wilder cam may prove to produce less tq, at that 3500 rpm range but it potentially produces more powerstrokes of efficiently filled cylinders 500 rpm higher in the rpm range, thus if your geared correctly to take advantage of the higher tq peak, the slightly wilder cam potentially produces 359cubic feet of fuel air mix burned at 4000rpm.
what youll find is that as the cams timeing gets longer in duration the engine s torque peak moves higher in the rpm range effectively sacrificeing low rpm power for slightly lower power per stroke to gain more strokes per second, this trade off tends to favor the larger duration cams pulling power only if the cars gearing keeps the engine in its most efficient rpm range, but theres a problem or two, as the rpms go up the gas mileage goes down, and at about 4000rpm-5700rpm depenging on the parts used a point is reached where the cylinders just can,t fill effectively due to lack of available time even if the ports and valves are made larger, so the trade off of more strokes per minute but less efficient stroke starts to gradually become less effective. notice on the dyno belo that happends at 5000rpm

then this chart
http://www.babcox.com/editorial/ar/elements/30228b.gif

what youll figure out if you look closely is that the engine has compression in the cylinders only after both valves are closed and that up to a point any increase in the engines rpms tends to increase the airspeed in the ports which increases the cylinders volumetric efficiency but at some point the time the valves stay open becomes so short that the cylinders don,t have time to fill completely.
torque is basically a ballance between effectively filling the cylinders and the number of power strokes you can get per second, look at any dyno chart and youll see that the torque production keeps climbing untill the cylinders can,t fill effectively due to increaseingly shorter time available to fill them due to higher rpms

 

[Stallion]

Okay, I see what you mean with the trend of filling the cylinders and torque. So max torque is usually at a lower RPM because of this filling pace?

 

[grumpyvette]

look at the dyno graph again

on this engine the cylinder filling efficiency is maxed at between 4000rpm and 5000rpm, above 5000rpm the time available to fill the cylinders is getting short enough that they can,t fill completely so the ballance between the power per stroke and the number of strokes has started to fall, if you really look into it this engine maxed out at about 4000rpm but between 4000rpm and 5000rpm the increaseing number of power strokes above 4000rpm was compensateing for the slight loss in cylinder filling efficiency up to about 5000rpm, by 5000rpm the cylinder filling efficiency had started to fall off so rapidly that even the increasing number of power strokes of lower tq produced could not keep up and power starts to fall off, by 6000rpm its becomeing hopeless as the cylinders are producing significantly less tq per stroke.
while its very true that the individual power strokes may produce more tq at low rpms the useable tq (cylinder pressure)is only applied from piston to crank during about 24 degrees of the 720 degree repetitive cycle, its the ballance between the volumetric efficiency the engine has filling the cylinders and the number of power strokes per second, thats where gearing comes in,... if your geared correctly youll be able to keep that engine in the most effective rpm range.
EXAMPLE
it makes no sence to build an engine for your corvette that produces max hp in the 5500rpm-7000rpm range like this dyno shows

even if it does produce 500plus hp if the transmission shifts at 5500 rpm and gearing in your corvette keeps you in the 1000rpm-5500rpm range 90% of the time!
an engine like this one below would make your vette FAR FASTER

 

[JohnZ]

In a nutshell, that's what makes Vipers run the way they do; they make GOBS of torque, over a very wide band, and don't need to rev to the moon to make power. The Viper V-10 makes 535 lb-ft. of torque at 4200 rpm, has over 400 lb-ft. at only 1600 rpm, and is still nearly 500 lb-ft. at 5500 rpm. Power peak is at 5800 rpm, with fuel cutoff at 6250. They're "torque-monsters".

The other end of the scale is a Formula 1 engine; 180 cubic inches, 750hp at 17,500 rpm , with next to no torque, and the gearing has to keep it in a power band between 13,000 and 17,500 rpm. Valve springs don't work due to harmonics over 9,000 rpm, so they use a 2,000 psi closed-loop compressed nitrogen system to close the valves instead of springs. Big power at obscenely high rpm, next to no torque, and 7-speed transmissions to keep the engine in its power band.

 

[Stallion]

I understand, so you want to get the most amount of torque throughout the longest range of revolutions. Thanks guys!

TR

 

[grumpyvette]

"so you want to get the most amount of torque throughout the longest range of revolutions"
thats the first half! followed by

WITHIN THE RPM RANGE YOUR CARS GEARED FOR SO THAT YOU SPEND 90% OF THE TIME BETWEEN THE TORQUE CONVERTERS STALL SPEED AND THE TRANSMISSIONS UPPER RPM SHIFT LIMIT.

EXAMPLE.
IF YOUVE GOT A TRANS THAT SHIFTS TO THE NEXT GEAR AT 5700RPM,AND A TQ CONVERTER STALL THAT ALLOWS A 2200RPM LAUNCH ENGINE SPEED ALL THE PARTS IN YOUR ENGINE BUILD-UP SHOULD OPERATE IN THAT 2200RPM-5700RPM WINDOW OF 3500RPM.
look at this chart again
http://www.babcox.com/editorial/ar/elements/30228b.gif
a quick check shows that cams in about the 215-235 intake duration range are suitable for that gearing (depends on where you want max power and what displacement and other parts are in your combo!
the closer to stock your engine is the closer to about 215 degrees intake duration you should be!, add a stealth ram, and good heads to a 406 and your hurting potential hp unless your useing a cam with about 235 intake duration

 

[Stallion]

Okay, I understand now. Wow, there is a lot to factor in when you are designing this. I like it a lot. It's really interesting to me. I could see myself doing this kind of work for the rest of my life. Very very interesting, maybe so because it applies to me so much with my Vette, and I just love learning about her.

TR

 

[JohnZ]

Torque is what matters in a street engine (vs. the extreme Formula 1 example above that has lots of horsepower, but only in a very narrow high-rpm band, only suitable for a race motor), and what counts relative to torque is what's called "torque bandwidth" - the highest possible level of torque all the way through the rpm band you use in normal driving - in mathematical terms, "the most area under the curve".