Explain camshaft specs to me........

[JollySipper]

I know what over-lap, lift, and duration are, but the specs are always confusing to me. I don't understand what a performance cam measurements would be compared to a more mild cam. Could you guys maybe give examples of, say, a street profile versus an all-out top fuel camshaft?

Thanks!

[Ace-Garageguy]

The differences are pretty straightforward really, though the specific numbers would depend on the specific engine.

In general, more lift and duration make more power. Lift opens the valve wider, duration keeps it open longer.

Overlap is to a certain extent a function of those, and keeps both intake and exhaust valves open at the same time, in order to facilitate cylinder filling and more complete exhaust purging, using the speed of the outgoing exhaust to, in effect, pull more fresh charge in.

Lots of lift and overlap tends to make engines produce power higher in the RPM range, and lose power down low. Same for lotsa overlap.

A cam is ideally carefully matched to the engine specifics, like desired operating RPM, intake manifold design, fuel delivery system, header / exhaust system, etc. These variables are all closely interrelated, so component selection and tuning is best done with the aid of a dynamometer.

A low-RPM high-torque cam for a smallblock Chebby may have something in the neighborhood of .375" of lift, and will idle happily at 600 RPM and work well with an automatic trans.

A peaky race cam for the same basic engine, that won't idle well or pull enough vacuum to operate power brakes, may have around .525" of lift.

[Ace-Garageguy]

Hmmm...I'm getting the dreaded "THIS TOPIC CAN NO LONGER BE EDITED" horseradish.

Anyway...

Cams for supercharged engines, or different fuels like alcohol, natural gas, etc. have different design characteristics from gasoline burning normally-aspirated engines too.

It is a very complex subject. 

[Ace-Garageguy]

Here's some great info directly from Lunati...

http://www.lunatipower.com/Tech/Cams/CamSpecTerms.aspx

http://www.lunatipower.com/Tech/Cams/CamProfileTerms.aspx

And this is what's available from Lunati JUST for the smallblock Chebby. Read through the specs and intended applications, and you can make sense of the numbers.

http://www.lunatipower.com/Category.aspx?id=58

[JollySipper]

Thanks, Bill! There is some very good info in those links, written where the average person can understand it........

I'm wondering..... you said a higher lift camshaft doesn't operate well at lower RPMs. Could you maybe "cheat" by using a milder cam, but with solid lifters that you could push to higher revs to get more out of it?

[Ace-Garageguy]

I'm wondering..... you said a higher lift camshaft doesn't operate well at lower RPMs. Could you maybe "cheat" by using a milder cam, but with solid lifters that you could push to higher revs to get more out of it?

That's a very good question. The answer is complex, kind of yes and no, so I'll give you a simplified version. Camshaft and valvetrain design were still pretty much a black art back in the 1950s when the first generation of US-made OHV V8 engines were introduced. There were lots of combinations experimented with, and lots of compromises. With the advent of advanced computer modeling and "polynomial" cam profiles, things have changed dramatically.

It's safe to say that hydraulic lifters were originally designed to eliminate the necessity for periodic fine adjustments of valve "lash", or operating clearances between valvetrain components, and in the process, also eliminating the clicking noise that usually came from these clearances. Solid lifters are usually still used in maximum-output applications, because they tend to control valve events more precisely under extreme conditions. The lighter weight possible with a solid lifter also allows higher revs without valve "float". Though float can be controlled to an extent with higher valve spring pressures, those higher pressures also create more friction and can waste horsepower.

Hydraulic lifters are said to "pump up" (though there is still a considerable amount of argument about this phenomenon), and they can certainly "leak down" or "collapse", and all these have adverse effects on valve lift and timing. Solid lifters, of course, suffer no such problems.

Today, there are several technologies available to provide both variable valve lift and variable valve timing...while the engine is running...to tailor the valve events to what the engine is being asked to do at any given moment. The video below explains some of what's available in production engines.

From Road&Track...

"Manufacturers like to use a lot of acronyms on their cars. Variable valve lift (VVL) and variable valve timing (VVT) are two of the most popular ones. These systems sound pretty similar, but what do they actually do? Luckily, there's a real engineer here to explain it to us.

Variable valve lift is mainly used to enhance performance. Instead of having one cam profile for the entire rev range, a VVL engine has two: low-lift and high-lift. Under regular conditions, the engine will use the low-lift cam to operate the valves, but under higher load, a solenoid switches the engine over to the to the high-lift cam (or cams), increasing valve travel and therefore, performance.

Variable valve timing, on the other hand, is used mainly for emissions control. Essentially, it allows the engine to advance or retard the valve timing using oil pressure. This allows for more control over how much air-fuel mixture is in the cylinder (less under light-load driving, more when power is needed), controlling temperature and emissions.

But that's only a very basic explanation. Watch as Jason Fenske of Engineering Explainedwalks us through VVL and VVT in much more interesting detail."

Edited October 26, 2017 by Ace-Garageguy

[Ace-Garageguy]

And for anyone interested in delving deeper into the intricacies of valve train choices, design, and especially the solid vs. hydraulic lifter debate, here's a link to another great article.

http://www.hotrod.com/articles/mopp-0312-solid-vs-hydraulic-lifters/

 

[Dragline]

All good stuff that Bill posted. I'd add that fuel burning drag cars have relatively mild cams. With a 14:71 supercharger pushing Oxygen laden Nitro down into the cylinders a radical camshaft is truly not needed. In fact, exhaust timing is more important than intake on those cars as scavenging is paramount to lessen hydraulic lock since the engine is force fed.

[blunc]

speaking of hydraulic lock, I heard a story about an engine that wouldn't crank over, the guy then replaced his battery and then starter before figuring out his problem went much deeper...water in a cylinder.

[Ace-Garageguy]

speaking of hydraulic lock, I heard a story about an engine that wouldn't crank over, the guy then replaced his battery and then starter before figuring out his problem went much deeper...water in a cylinder.

Here's one for you...back in the '80s I had a VW Rabbit or Sirocco come in on the hook. Guy ran through a deep puddle, going pretty fast, and the engine quit and wouldn't even crank afterwards. Seems the air intake was pretty low on the car, and it sucked in enough water to hydraulic-lock one cylinder. Bent the con rod. 

[unclescott58]

This is very interesting. And this brings up a question of lifters I've wondered about for years. It's my understanding that hydraulic lifters are self adjusting. Solid lifters are not. If this is the case, what does adjusting the lifters do? And how is it done?

[Ace-Garageguy]

This is very interesting. And this brings up a question of lifters I've wondered about for years. It's my understanding that hydraulic lifters are self adjusting. Solid lifters are not. If this is the case, what does adjusting the lifters do? And how is it done?

Most automotive valve trains are designed to have some clearance (commonly called "valve lash") between parts that transmit motion to the valves, in order to compensate for things like different expansion rates of different materials.

For example, an aluminum cylinder head will expand at a different rate as it reaches operating temperature, and a different overall amount, than the cast-iron block it might be bolted to, and the steel valve train parts will also expand differently.

Adjusting this clearance so all the valves go through identical motions once the engine fully warms up, and without binding, can be done in several ways.

Hydraulic lifters do it automatically.

Non-automatic means of adjusting lash include threaded parts and locknuts on the lifters themselves, or on the rocker arms, or by replacing valve "pads" or "lash caps" that come in varying thicknesses. Some BMW engines use a small cam device on the rocker arm that's rotated to obtain the correct clearance, and then locked in position with a jam nut. Some engines, like the old  Aston Martin 6, have no provision for adjustment, and the lash is achieved by VERY careful measuring, assembly and light machining of the parts involved while the entire head is removed from the engine.

Clearances may be adjusted hot, or cold, according to the manufacturer's specs, and may involve using a wrench, screwdriver and feeler gauges, or may require removal of the cams to allow replacement of the pads, whose required thickness is determined by a mechanic using feeler-gauges and micrometers.

The manual operations require a great deal of care and precision, and can vary immensely from engine family to family.

 

Edited November 9, 2017 by Ace-Garageguy

[10thumbs]

When I hear 'cam' I think often about a buddy from my youth days.  A Chevy guy. 

He had an obsession with cams, and was able to disassemble a motor and rebuild as well.  Just his motors were weak, and never ran well.  Probably built in cams that were not supported from the rest of the motor.  Heads and such.

"Three quarter cam".  lol.

"Here my new Isky 505"....I'll never forget those times.

Ha!....another favorite is 'diy head porting'.

 

 

[Ace-Garageguy]

When I hear 'cam' I think often about a buddy from my youth days.  A Chevy guy. 

He had an obsession with cams, and was able to disassemble a motor and rebuild as well.  Just his motors were weak, and never ran well.  Probably built in cams that were not supported from the rest of the motor.  Heads and such.

Lotta that around. There's always the guy who buys the biggest cam and bolts on two-750 double pumpers & a tunnel-ram. Stock engine otherwise, hooked to an automatic trans. Step on the gas and....nothing. Bog. Pop. Spit. Fart.  

[Dragline]

I always loved those guys. We called em bolt ons around here.

[10thumbs]

Shade tree mechanics in my area.   A home that's mobile, and 5 cars that aren't!  

"That thing must have a cam in it"....

[JollySipper]

I once had an early '80s S-10 with a 305..... it couldn't get out of it's own way. Whoever did the engine swap left the single muffler exhaust, and put on something like a 750 cfm Holley carb! It sounded good at idle, tho, with it's big, lopey cam...........

[blunc]

the problem is usually not just one thing.

if you're changing engine innards, modifying one part can yield small to mild improvements in performance...changing lots of parts without applying the science and proven knowledge will usually result in a waste of money, time, and sometimes a persons life.

you can switch from a two bbl manifold and carb to a four bbl manifold and carb, and performance will improve but the cam in the block may be best for only two bbl carbs and the resulting best case fuel mileage.

the same scenario with single exhaust will also be limited by the exhaust...but just bolting on any dual exhaust system will not always yield best results.

electronic fuel injection is a totally different beast with it's own pros and cons.