What does this mean?

There's your answer!..... When it "comes on cam" it's working the way Enzo intended! [Although, I'd still like you to describe how it feels ]

Seriously, a good question, and somewhat difficult to answer in "plain English" - As evidenced by the # of views versus the # of answers......

Below 4000 (in your engine) the overlap in valve opening/closing points is wasting fuel, feels "lumpy", and doesn't generate "serious" power. At 4K the gases (inlet charge and exhaust waste) are starting to move at their designed velocities, and the engine is working as intended - Life feels good, and it's "on the cam"..... Too many RPM and the valves bounce, too few and you're not in the "sweet spot".

Or something.

Cheers,
Ian

 

When you plot horsepower vs. RPM (and torque vs. RPM .. they differ) in many engines there is a fairly abrupt change in the slope of the HP (or torque) curve. This means that with a small increase in RPM there is a relatively large increase in HP (or torque) .. and you feel this as you accelerate. Its that kick-in-the-butt point. Some call this coming up on the cam since this increase often can be attributed to increased breathing .. that is, more flow through the engine via the valves (cam-operated). There are a lot of factors that contribute to those curves so the "on the cam" expression is more of a slang expression. Motorsports writers like it a lot.

 

I'll do my best to not make a fool of myself. I'd start by saying it should really be referred to as coming into the “power band” I think since similar things occur on two stroke engines and it's not just an affect of the cams alone. Spark timing and port design has a lot to do with power curves as well. Think about skeet shooting or the great line in Apocalypse Now when the guy asks the guy hanging out the helicopter gunner " how do you sit up here and shoot women and children" and the gunner answers " it's easy, you just don't lead them as much". On engines we are going to put into a minivan we want the power to be smooth at low RPMs such that you don't have to rev the engine up to 3k RPMs to pull into a parking spot at Kmart.

The goal of making power inside an engine is to burn as much fuel and air mixture as possible. The higher the RPM, the more times you can pump the cylinders full of this mixture and explode it. Ideally you open the intake valve, allow the piston to suck in as much mixture as it can and then close the intake, compress the mixture and light it off. Then you would wait till it forces the piston all the way to the bottom of the stroke and then open the exhaust valve and force everything out. Works great at low RPM but to get the most out of a high RPM race engine you put in cams with high lift, long duration and overlap and heads and intake manifolds with large ports designed to flow high volumns of air. Overlap means one valve is not closed all the way when the other begins to open.

One of the reasons for the long durations and overlap is imagine the air/fuel mixture getting sucked into the engine by the piston going down so fast that the air going past the intake valve has momentum built up and even after the piston reaches the bottom of it's travel air is still rushing past the valves. Similar effects with the exhaust as well. Everything is happening so fast that it is important to get the valves open sooner and stay open longer basically "leading". Sort of like aiming a few feet ahead of the clay pigeon knowing by the time you react and pull the trigger and the pellets travel from the end of the barrel to the target. Works great until a slower moving target comes along ie: the women and children.

As far as the port size is concerned imagine inhaling a cigarette and blowing the smoke through a clear drinking straw. Imagine what that would look like as the smoke travel through the straw, the smoke would move pretty fast and exit the other end of the straw as a fast moving stream of smoke. Now try the same thing blowing the smoke from your mouth into a large diameter storm drain pipe. As soon as the smoke left your mouth it would begin to slow down and swirl and mix with the air sitting around inside the pipe already and eventually a bit of the smoke would exit the pipe. The large ports in racing engines work great at high RPMs pushing 800 cfm but at low rpms the air/fuel mixture sort of acts like when you blew the smoke into the 2 ft diameter storm drain pipe.

The newer computer controlled EFI engines can change cam timing from lower to higher rpms but most engines can't. On high RPM motors the cams are setup to get the most fuel/air mixture in at high RPMs. At slower RPMs these engines are very flat on power since the valves are opening way too soon and staying open too long and the ports in the motor are so big the air traveling through the intake system are so large the air/fuel mixture travels at too slow a velocity to gain any momentum. The result is a motor that makes very little power until you get the RPM range up to the point that everything starts to work together and the air speed inside the engine gets up to the point that it flows more continuous instead of a choppy start/stop type motion. You can get more low end power by using smaller intake and exhaust ports and open them a bit later and get rid of the overlap but then at high RPM's the motor is choking for more air.

Something to think about. Our race engines are setup to spark at 40 degrees BTDC. That means that as the crank is rotating we set off the spark to light the fuel 40 degrees of crank rotation before the piston reaches the top of it's compression stroke. The piston is still going up and has not fully compressed the fuel/air mixture when we set off the spark and the piston will still continue to the top of it's stroke as it's already starting the controlled explosion of the fuel. We do this becuase we find we have to ignite the fuel a little early at high rpms since everything is traveling so fast inside the engine. Your normal street car probably sparks at around 5 degrees before top dead center at idle and maybe 20 degrees BTDC at higher rpms above 2000

 

as the inlet valve slams shut - it sends a high pressure wave back up the intake tract at sonic velocity.

between that high pressure wave and the closed valve, the incoming fuel/ air mixture is being 'packed' into a more dense charge of air & fuel.

at the 'sweet spot' the sonic wave hits the plenum (a seemingly 'infinate' atmospheric pressure) at a particular rpm.

the wave then 'reflects' an opposite, low pressure wave back down the track at sonic velocity.

at the 'sweet spot' the low pressure wave hits the valve as it is opening to then 'suck' the mixture into the cylinder.

bottom line: at the sonic sweet spot, more air & fuel gets crammed into the cyl = more hp

same thing happens on the exhaust side.

basic acoustics & resonant frequency manipulation.

fun stuff....

Rgds,
Vince
PS: sonic = speed of sound = 'more or less' a constant speed

 

When your skeet shooting you don't aim at the target or else the target will be gone by the time you get off the shot. You aim a few feet ahead of where the moving target is so when you pull the trigger the clay pigeon and the lead pellets arrive at the same location at the same time. The faster the target is moving the further ahead of the target you have to aim.

 

Actually that line is from "Full metal jacket"

 

Dear Ferraristi,

KKRace,

...very good write up!


Shamile

Freeze...Miami Vice !

 

KKRace: good write-up. A bit politically incorrect with analogies that I too have a hard time translating over to internal combustion motors (even though I like war movies and target shooting) but hey you are spot on with the timing aspect.


Here's my attempt at plain English:

Cams: their job is merely opening and closing the intake and exhaust valves for the combustion chambers - and at the right times.

"Air-fuel-mixture flow into and burnt fumes flow out of" the engine is a complex dynamic process of ~10,000 scientificly quantifiable variables and perhaps 3 to 7 magical spells, depending on which race shop built it.

The efficiency of this flow is very much determined by the timing of the valves opening and closing. As RPMs change, so do the dynamics of the flow - and not in a linear and easily predictable fashion.

Traditional cams are not adjustable, so they can only be shaped to provide one pattern of valve timing - and this one pattern will only work PERFECTLY in one narrow RPM range. It may work prettty well in the neighboring RPM ranges a little lower and a little higher, so the effective "sweet spot" can be a bit bigger.

Result: the motor runs poorly until it's accelerated up to the "sweet spot" RPM range. As Kevin pointed out, minivan motors are built with cams to have a sweet spot suited to parking lots, and their spots are not that sweet. Ferraris (and many good sports cars) have much sweeter spots located much higher in the RPM ranges.

Once it's in The Zone the motor doesn't have to work hardly at all to suck in air as the flow dynamics are working to cram as much air-fuel-mix (and very well atomized too) in as the motor can stand, and it doesn't have to work hard to push the burnt fumes out as their dynamics cause them to scream right out. Result: High Efficiency. Almost ALL the considerable work (power output) the motor is doing is transmitted throught the drivetrain right to the wheels, causing your grin to widen.

We're skipping over volumes of intricate exotic combustion chamber science and everything about spark timing here, BTW, because none of that works without the right cams anyway.

To sum, the traditional transition from low-RPM dog to sweet-spot screamer is called "coming up onto the cam". It is, as you know, a joyful occasion.



I suspect it's an old English expression due to the style of phrase... but have no historical trivia to back that up.



Variable valve timing (i.e., Honda V-Tech kicked off the trend and now every car manufacturer has their own alphabet soup for it...) is where it's at. Modern motors can always be on the cam!

- Josh