high rpm vs low rpm

[Execut1ve]

very good, if I am to consider camshafts I need to be educated on camshaft attributes and how they bear on this issue

[Frank Lee]

FWIW: Ford 302 has 3.0" stroke; 1920 rpm gives 960 ft/mn piston speed.

If 1000-1200 is "optimal piston speed for efficiency" then that rpm range for the Ford 302 is 2000-2400 rpm.

Wish my F150 had a tach; I kinda know where the sweet spot is just by putting every mile on 'er since '94.

[Execut1ve]

so I was 80 rpm out of that range. I'll take that I would round to 2k anyway as I wouldn't even want to try reading 20 rpm on the stock tach

[pounsfos]

here have some "light" reading. google is your friend

Camshaft - Wikipedia, the free encyclopedia
HowStuffWorks "How Camshafts Work"

[euromodder]

Originally Posted by Execut1ve

I don't think regearing to higher gears is an option since it already struggles a bit maintaining highway speed, likes to downshift on all but the slightest hills.

There you have it : go for improvements in the (low to mid) rpm range at these speeds.

They'll allow you to keep in the higher gear longer, drive slower while still maintaining good FE (when overall engine efficiency drops because of lower rpm, driving slower will not gain any mpg anymore), or even gear up in the future.


I'm sort of in the same boat.
@100 kph / 62mph and 2000 rpm in 5th (top) gear, driving slower doesn't help anymore to get better mpg.
Even a small incline seriously increases the FC.
The car can't be driven efficiently @ 90 kph (which is the official extra urban speed limit here) - it's consuming more fuel than @ 100kph.

[Jim-Bob]

Camshaft theory is a fairly extensive subject to get into, but the basics are: Lobe separation, Lift and duration. Now lift is how far above the base circle the cam lobe's peak is. I (and the base circle is the portion of the lobe that the lifter is on when the valve is closed) It affects how far off the valve seat the valve will go at peak lift. Peak valve lift is in turn affected by the rocker arm ratio. If you had .500 lobe lift with a 1:1 rocker ratio, the valve would lift .500 off the seat. if you had a 1:1.6 rocker ratio ( you do), it would be .800 valve lift. (note that these numbers are for illustration only as only radical race engines have that much lift!). Duration is the amount of degrees of crank rotation that the valve is off the seat. This can be measured as gross duration or duration at 0.050 lift ( meaning from where the lobe is 0.050 off the base circle to where it closes to that figure). Lobe separation angle (LSA) is the included angle between the peak of the exhaust lobe and the intake lobe. This affects overlap, which is the number of degrees that both valves are open at the same time. Wider LSA figures do not fill the cylinder as well at higher RPMs, but do build more cylinder pressure lower down in the RPM range. Thus, a wide LSA cam will have more low end torque than a narrow LSA cam, all other things being equal. This affects the dynamic compression of the engine which is different than the static compression ratio of the engine. While static compression ratio is important, it is the peak cylinder pressure that makes all the difference. Remember that the compression ratio is the measurement of the volume of the cylinder and combustion chamber at bottom dead center (BDC) compared to the volume at top dead center (TDC). How much pressure is actually trapped in the cylinder is a function of the camshaft and cylinder head port design and will determine how the vehicle drives. This is also why many modern cars with double overhead cam engines can vary the lobe separation angle via the use of a solenoid valve that changes the relative positions of the two cams. Some also have two different cam lobes that switch at a given RPM as well (Honda VTEC being the best known of these) to extend the power curve. However, your 1962 engine design is not that sophisticated and you will have to choose carefully the cam that best suits your needs and matches the rest of the components of the vehicle.

Try looking at Comp Cams online catalog ( http://www.compcams.com/ )for some ideas as to what is available. They should also have a good section on their site about theory as well. I used one of their Xtreme Energy cams (XE262) in my Chevy 355 and it made a huge difference in both FE and acceleration. To chose that cam I used a dyno sim program and entered a lot of parameters (including flow bench data I found for my heads online) to get the most accurate match possible and maximize torque. A similar program is available to download for free off their site, but it probably only deals with their cams. Look at their truck cams, especially the smaller ones to get an idea of what is available to improve mileage and driveability. Your particular engine should be equipped with a roller cam, so all you need is the cam and a timing set. The old lifters can be reused. If it were a flat tappet cam then the lifters would need to be replaced at the same time.

[Execut1ve]

very helpful post jimbob! the camquest tool (on the comp cam site) lists the attributes slightly differently: what is the lobe centerline and how does it relate to lobe separation angle mentioned above? I realize I'm probably oversimplifying here, but could you condense the 3 attributes you mentioned into their effects on low end torque vs high rpm horsepower? thanks

[some_other_dave]

Originally Posted by Execut1ve

It is said that the best fuel econ occurs at 80% of max torque rpm, which would be 1920 rpm.

That sounds like a garbled version of what I have seen around here. That best economy (for acceleration) occurs at about 80% load (roughly equal to producing 80% of rated torque at the RPM) at around 2000 RPM.


For cams: Lobe center and separation are pretty much different ways of saying the same thing.

Overlap (where the intake and exhaust valves are open at the same time) is good for high RPMs and bad for low RPMs.

More duration (how long the valve is open) tends to be better for high RPMs versus low, in large part because high duration usually means more overlap.

Lift, I'm not so sure about. I believe that if you go with too much lift you wind up with less velocity, similar to if you use too large-diameter a pipe (e.g., as an exhaust primary). But obviously if you have very little lift you won't let enough air into or out of the combustion chamber.

And these sorts of things also depend on the rest of the configuration of the engine; things like valve shrouding (how close the ports are to the edges of the chamber), chamber size and shape, piston crown or dish, spark plug location, cooling, yadda yadda yadda yadda.

Some of this stuff really is still a "black art". You want to promote velocity and mixture quality at some RPM/throttle combination. The best engine guys can figure out an approximation of the different things to do with the engine to promote that, but then they have to experiment and test to get things right.

-soD