overlap generalities....

[LSOHOLIC]

On an EFI setup, what are the general signs of to much overlap ??

On the dyno or at the track....
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[LSOHOLIC]

Maybe a better way to word it would be.....what are some general signs of over scavenging on an EFI motor ??

[LSOHOLIC]

Wow....I must be wording this incorrectly.

I'm headed to the dyno with a new cam and wanted to understand the symptoms of an over scavenged motor in the usable rpm range, but in an EFI format (no signal). The new cam has more IVO & EVC than any I've even ran.

I assume in a worst case senario I'd see vacuum @ WOT.

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[Ozonkiller]

I'm very interested in what people have to say. I'll be heading to the dyno this summer to test this very thing. If this thread is still alive, I'll post my results.

[ProPower engines]

It basically depends on the fuel management system being used.
As in if it can learn fast and if the parameters can be infinitely altered to suit operation conditions and tuneable to get the max power under all conditions.

I get guys that want to upgrade to more cam etc etc etc but do not want to spend the money on the system to allow that so that ties my hands to min. upgrades where a fully tuneable system allows unlimited changes to all parameters

[My427stang]

I have only seen issues with idle, basically the O2 sensor thinks its richer than it really is

Most guys fix it by going open loop at idle if they need to. However, as a single reference, my 489 FE with an EEC-IV based computer has 65 degrees overlap measured at .020 tappet rise and it's lashed pretty tight and it has no issues in closed loop.

I am not sure how it would look on a dyno in regards to scavenging at higher RPM, you may be able to tweak injector timing and slope to mask it a little.

[ProPower engines]

Now to your post

What system are you running and why are you suspecting this issue.
Any thing stand out on the dyno when testing??

[user-23911]

Differences in cams are noticable at low revs and idling. You're usually going to get more overlap on a cam with more duration. That's unless you're playing around with VVT and your duration will stay the same but overlap is adjustable.

With a cam that has both more overlap and more duration, you'll lose intake vacuum at idle compared to a normal one. You'll get less low RPM torque and more high RPM torque.It pushes the powerband up the rpm range.
As to how well it behaves, that depends on how well it's tuned.

Most engines, the overlap is fixed with the cam so you're not really going to get it wrong.
DOHC, you can usually dial it in.

[My427stang]

I thought about this last night a bit, I am not sure that EFI or no EFI makes a difference in the scavenging effect once you get to any significant RPM

In the end, the question lies, Is the addition overlap filling the cylinder more than atmospheric pressure alone or doing that AND evacuating some too? (over scavenging)

That's a function of cam design, head design, intake, chamber and exhaust relationship. However I "think" that hemis and canted valve engines are more susceptible to it, not LS motors, but that is just a hunch from what I have read. I am not a hemi guy

However, to go back to my injector timing discussion, in theory, if an injector was timed to fire late and fast, it could put the fuel in at a time that is affected less by scavenging. At least I think it may be able to, potentially over fueling in the later period of cylinder fill. However, at RPM injectors reach a duty cycle that almost becomes a continuous fog that makes specific shot timing have less authority. What you see here is me talking myself out of the idea

Low rpm though, to combat a diluted mixture, especially idle, this would and does work. Some tuners start injection against a closed intake valve in theory to use the heat to further break up fuel, others time the injector to be fully ramped up as the intake opens to have a clean fully atomized charge. There are lots of techniques, but in the end, adjusting injector timing really only works if 1) you can adjust it with your controller, and 2) rpm is low enough that it has a defined beginning and end in the port.

How much cam are you actually running and can you adjust your injector timing?

[wyrmrider]

stock manifolds and exhaust?
headers and free flowing duals?

[Walter R. Malik]

There are to many OTHER parameters to be considered which might not match one another ... RPM ... total cam durations .. intake charge velocity ... exhaust system & tuning ... are just a few.

[CamKing]

Over scavenging plays tricks on the O2 sensor. It can't tell the difference between the unburnt oxygen left in the cylinder from combustion, and the oxygen that's flowing from the intake straight out the exhaust during overlap. The computer sees this extra oxygen, and changes the fuel mixture to try and burn it. The computer thinks it's lean, even when it's not.

[peejay]

I am not convinced that overscavenging can even be possible at idle/low load.

However, it sure al heck is possible that a misfire can and will fake the O2 to read lean.

[CamKing]

It can happen at any RPM, but I was talking about mid to high RPM's.
I just tried to explain this to a customer, but he didn't believe me.
He's tuning with the O2 sensor, but the cam I sold him, had more duration then anything he had run. When he tuned it to the same O2 numbers he was use to, it didn't run, and showed no heat in the exhaust. As he went leaner, the power picked up, and the exhaust temps started to come up to a normal range. He got it to the point, it was making good power, but he said he couldn't run it with that tune, because he said it was too lean. I tried to explain that it really wasn't lean, it was just the O2 sensor picking up the oxygen from the added overlap, but he didn't get it.
If you measure the Air CFM in, and the Fuel lbs/hr, you can see how the O2 sensor can fool you.

[LSOHOLIC]

I really did not want to make this thread about a specific combo......and I'm only concerned with the usable rpm range (rpm drop to shift point) not idle quality or other rpm parameters.
I will be testing collector extensions on the dyno as well...per Pipemax calcs.

So.....should over scavenging be a concern ?

My main concern is losing cylinder pressure out the exhaust and hurting the ram inertia effect @ rpm. Thus, killing VE @ rpm.



But, for those that the specific combo would help....
hydraulic 346"
15* head
12:1
pump E85
usable rpm range 5200 - 7600
25* of overlap @ 0.050"
----------------
~exhaust info~
1.575" exhaust valve (80 grams)
0.372 lobe lift on a 254 hydro lobe @ 0.050
50* seat
84cc D-port
1 7/8" primary/ 26" long
merge collector w/ 2 1/2" choke to 3" collector extension

HERE IS A PIC OF THE HEADER..............

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