OK after reading many good threads here about cams and seeing alot indicating that one needs to focus on how much overlap is needed the question arises. How much overlap do you need for a combination? We all know you can get the same overlap values many ways (long duration with high lsa, smaller duration with low lsa, etc. etc.). But it seems as though some suggest that a combination needs a certain amount of overlap. What things come into play on how much overlap is needed. Obviously rpm band and cubic inches. What about compression, intake setup, head flow, exhaust. I assume these are all factors as well. Let's hear some good generalization on the subject.

 

There is no such thing as "need" when it comes to cam specs. Its more of a matter of "want" vs "need".

-do you sacrifice drivability?
-do you sacrifice idle quality?
-do you sacrifice fuel economy?
-do you sacrifice valvetrain life due to excessive rpm ranges?
-do you prefer more throttle response at low rpm?

etc.... etc....

There is no such thing as the perfect cam. There is only a perfect cam based on what the individual's desired goals are. Its more important to understand what LSA/overlap does, not try and put your finger on an actual value to define.

Tony

 

Good points to consider. How about from the stand point of obtaining max power in a given rpm band with no thoughts given to anything else, except valve train life. Lets say 6000-7500 with whatever motor combination you want to through out there.

 

Remember that the .050 value of overlap doesn't tell the whole story, the overlap area itself can change based on ramps and lobe choices as much as it can on the duration and LSA specs.

Bret

 

I was thinking about posting something like this the other day. With that said, what does it take to achieve a "matched" set-up? Taking nothing into consideration but max power in the givin RPM range. Say max rpm of 7500 since it was already stated. I know no one is going to throw out there secrets, but a broad generalazation would be nice.

 

Good point Bret, I'm just trying to keep things at .050" right now for simplfication purposes. Anyone?

 

A good example of component interaction is re the cams for early Z28s. The '70 1/2 cam was 'detuned' quite a bit from the '67 -'69, to allow its use with an automatic, but it turned out this cam made more power than the original in either the 302 or the 350 when used with the stock exhaust. With open headers, the early cam was ~20+ HP stronger in either engine...

 

Good exhaust is the key here too.... Without the right exhaust tuning the cam can be junk if there is too much overlap. One reason why the standard shelf cams don't have a ton of low LSA and overlap IMHO

Bret

 

I think we can take compression out of the parameter list. Overlap is our point in
time when gas flow provides gains when both valves are open...therefore
we don't need compression.

Overlap from my understanding is designed around these (to name a few) variables:

RPM
Intake runner length
Exhaust runner length
Valve size and combustion chamber design

All the above having immediate effects on pressure changes and wave tuning.

The purpose of overlap is to take advantage of resonant points within the intake
and exhaust system while the piston is not providing much help.

Here's a visual analogy with water for those that can't envision pressure changes:

When your tidal wave of water is speeding toward the door, you want that
door opening as the tidal wave reaches the door jamb.

Just before the above instance occurs, you want to tear down the dam and
let the water left in the chamber to get the hell out.

As that water leaves, it creates a suction (pressure depression) in the chamber...sort of like this vacuum that pulls leaves behind a car when it speeds
by.

When that door opens and water is already flowing down the header tube,
it also wants to suck more of that tidal wave along with it.

That's part of the cycle. Now the valve is closing and chops off the exhaust
wave giving it a wavelength.

On the other side of the chamber, intake valve closes, some of that tidal wave hits the back of
the valve and bounces backward...creating yet another wavelength.

NOw there are two opposing waves flowing in the opposite direction in their
respective intake and exhaust runner. Some of that energy is acoustic, some
of it is pressure.

If the wavelengths match a resonant length of intake/exhaust pipe, then the
reflected pulse will create a sucking effect at the exhaust port and a pushing
effect at the intake port.

If the RPM is correct and the valves happen to open at the correct time, we're
going to have some kick *** charge moving into the chamber during the next
overlap period.

The overlap is going to be very dependent on a specific RPM to make the most power above and below the peak RPM within the gearing window.

 

Just from reading about a ton of different engine combos over the years, playing around with engine sim packages, and reading David Vizard, I would tend to "generalize" that most street cams do not have enough overlap for optimum volumetric efficiency (torque and horsepower) for a given RPM range. That statement assumes that you have an efficient, tuned exhaust system (long tube headers).

In other words, most street performance cams reduce some of the overlap in order to improve idle and drivability for a given maximum power RPM.

I have never read of a "rule of thumb" for best overlap figure based on engine parameters. I think that's because it's much too dependent on other factors - intake parameters, exhaust parameters, cylinder head flow, rod ratio, compression ratio (smaller combustion chamber = stronger communication between exhaust/intake). To really optimize, without actually testing a few hundred cams on a dyno, you need pretty advanced engine simulation software.

 

to me overlap is a bad thing ..... with fuel injection if the vac in the motor drops the map will see it and the computer will think there is a load and try to up the fuel then the o2 sees the fuld and ajusts back down and you go back to square one and start over this can be a hard thing to program around becouse when you do firt put a load on the motor the vac will jump up so then the motor will lean out ...... so to me part of why the factory put a wide lobe center low overlap cam is to help the fuelinjecton work with little problem though most hot rod types will live with more problems them just joe driver

 

Understandable... but there is a limit to how much the system can handle in terms of overlap. That's why quantifying that limit is usefull in terms of cam design and selection.... we can put more overlap in the motors than we need in all honesty and the computer can deal with that.

Bret

 

Or just tune the motor in open loop and forgedabout oxygen sensor control and adjust the problem area of the map range via the ve table.

Seems to me that more overlap is good if you have the proper setup to take advantage of it. In my case it will be 1 7/8" to 2" to 3.5" merge collector kooks headers, borla 3.5" xr-1 race mufflers, and a 3.5" h-pipe single side exit oval doctor gas exhaust tubing, an sb2 intake, and a 1550 cfm 4150 throttle body.

Also, it seems as if the intake valve closing is almost 90% of the determining factor in where your optimal power band and peak power will be made. After that is chosen it seems as though the other three events can be played with to shape the power band a little defferently gaining or loosing power after your peak and making more or less in your optimal power band.

 

I completely agree with the part on intake closing point but to run in open loop makes the car not much more the a carb becouse as the air changes the amount of fuel will not but then the motor you discribed .(though hot) I would worry about how it will drive or is it mostly a toy ( by the way I am very picky about how my car dives most are not so much ) large airflow TB and big injector sizes and open loop make part throttle very hard to deal with you have to give the motor to much throttle to make it go then it leaps to life .... I have been thinking on mine of adding a second set of injectors to add the extra fuel under boost so I can keep the first injector size down to help how the car drives off boost

 

Car will have between a 4000-5000 stall with a th400 and 4.56 gears. I think it's size (434) will keep the driveability just fine. This is only a drive approximately 100 miles a week maybe kind of toy. I could run closed loop if I wanted but I'm not sure those oxygen sensors are going to deal well with the kind of cam I may decide to run.

On a side not I have a customer with an 01 z06 that I tuned which runs open loop all the time and he has no issues what so ever with driveability. His car is only bolt ons though.

 

I have driven cars on the street with 5000 stall and in some ways it is funny when you can blip the throttle and rev the motor a little but not effect the movment of the car when crusing along ...... though driving a bad *** car gives the driver a lot of attention ( not always good ) my bottom line is time is a little short to get to work and getting it warm so you can drive and deal with it in traffic is a pain to the point that you just don't want to do it you have lost your driveability .... when even the wife wants to drive your car (becouse of the attention I am sure ) and it can still do low 10's then that car is a driver

 

You just gotta marry the right women, ha ha. I'm not worried about my wife driving my car (first off I don't have a wife, not at age 25). If I was worried about smooth operation and fast times I'd be building a turbo motor. Wooohooo!!!

 

being a power adder sort of guy I tend to cach flak from N/A sorts but in the last no bull comp here where you have to drive 30 miles in traffic then race heads up we (best freinds car ) came in second with a 10.20 at 135 mph ( we are at 5000 ft and the air was like 8000 ft 100 degree day) in the 1/4 and we where the only car that did not have a tralier following the car to go home on and after the race we where the only ones to drive home .... also in looking at the group of cars all where power adder none of the N/A guys showed up

 

I am of the understanding that turbo cams dont run any overlap, they actually run negative overlap, lets have more talk on turbo cams which it is said they are differentfrom a supercharger cams. might go in a defferent thread. RON in Daytona

 

Nice description Adrenaline_Z. Ha, I was just thinking about this on the way to work this morning and you took the words out of my mind.

With overlap it's all about plenum, not compression. Sure, the cam creates the overlap but if you cant get the wave out with intake and exhaust you lose.