Camshaft overlap for plenum manifold vs ITBs

[naukkis79]

In my opinion, at full throttle the ITB and plenum manifold FI systems are basically identical. If it doesn’t run well at full throttle with a plenum manifold, it’s not going to run well at full throttle with ITBs.

Yep. It's about throttling and when not throttled throttle blades won't do anything. You can use plenum in ITB-systems too if wanted.

[ptuomov]

With plenum manifold and high overlap cams, the instability in throttling at low and mid range rpms comes from the following positive feedback process: higher cylinder filling >- stronger exhaust suction wave -> higher cylinder filling. The volume between intake valve and throttle plate determines how freely this positive feedback cycle can escalate.

[naukkis79]

With plenum manifold and high overlap cams, the instability in throttling at low and mid range rpms comes from the following positive feedback process: higher cylinder filling >- stronger exhaust suction wave -> higher cylinder filling. The volume between intake valve and throttle plate determines how freely this positive feedback cycle can escalate.

That chain isn't unstable. Unstability comes like this -> higher cylinder filling -> rev rise-> higher manifold vacuum -> more EGR reverse flow together with lower airflow from intake-> misfire-> revs drop -> manifold pressure rises-> EGR reversion becomes combustible and together with higher manifold pressure cylinder fill is much bigger ->revs rise and so on.

Rev rising is only tuning problem.

[ptuomov]

With plenum manifold and high overlap cams, the instability in throttling at low and mid range rpms comes from the following positive feedback process: higher cylinder filling >- stronger exhaust suction wave -> higher cylinder filling. The volume between intake valve and throttle plate determines how freely this positive feedback cycle can escalate.

That chain isn't unstable. Unstability comes like this -> higher cylinder filling -> rev rise-> higher manifold vacuum -> more EGR reverse flow together with lower airflow from intake-> misfire-> revs drop -> manifold pressure rises-> EGR reversion becomes combustible and together with higher manifold pressure cylinder fill is much bigger ->revs rise and so on.

Rev rising is only tuning problem.

I disagree.

I disagree with you on whether a process in which, at a constant throttle angle, higher cylinder filling in this cycle leads to even higher cylinder filling is unstable. It’s unstable in the sense that minor deviation from the current state will lead to further deviations in the same direction. It’s not growing without bounds, of course, but enough to make a car unpleasant to drive.

As additional facts, you see the same instability on a dyno holding the rpm. Holding the exact same rpm at slight throttle opening in a dyno, the torque generated is unstable with big overlap cams and plenum FI manifold. Therefore, the engine speed increasing in actual driving is a symptom and not the cause of the surge. It’s also the case that this happens without any detectable misfires and with either a speed-density or MAF systems keeping the AFR in a reasonable range. So misfires are not the cause, but they can be a symptom, especially in at alpha-N load measurement method is used.

Exhaust gas recirculating in itself is also not a problem as long as it’s consistent given the measured load and engine speed. Exhaust gas recirculating is only a problem to the extent that it contributes to the instability, for example, by reducing the exhaust pulse and reducing exhaust suction during the overlap.

[Rick!]

So, from your first post asking for an overlap value that avoids part throttle surging on a cross plane V8 and your dislike of the surging in a daily driver to your most recent post with personal dyno experience of part throttle surging, it seems you have the data already to make your decision. It seems your looking for validation of your hypothesis from some of the folks here. Please let us know when you install a different cam and fix the issue and also let us know if your hypothesis is correct about the root cause of the surging.

My earlier remarks hold true for both part throttle and WOT - you create a torque hole/poor cylinder filling/diluted cylinder filling, it affects that rpm range whether at PT or WOT, in my experience.

[naukkis79]

I disagree.

I disagree with you on whether a process in which, at a constant throttle angle, higher cylinder filling in this cycle leads to even higher cylinder filling is unstable. It’s unstable in the sense that minor deviation from the current state will lead to further deviations in the same direction. It’s not growing without bounds, of course, but enough to make a car unpleasant to drive.

How so? If revs are just increasing engine is accelerating - that isn't usually very unpleasant.

As additional facts, you see the same instability on a dyno holding the rpm. Holding the exact same rpm at slight throttle opening in a dyno, the torque generated is unstable with big overlap cams and plenum FI manifold. Therefore, the engine speed increasing in actual driving is a symptom and not the cause of the surge. It’s also the case that this happens without any detectable misfires and with either a speed-density or MAF systems keeping the AFR in a reasonable range. So misfires are not the cause, but they can be a symptom, especially in at alpha-N load measurement method is used.

Full misfire is worst case scenario - some traditional V8 hot cam tuning is to make idle to 8-stroke, it could be stabilized pretty well. Much more difficult situation is where burn speed and completion varies, so only 50% of mixture gets burned, or burn happens so slowly that stroke won't generate normal amount of torque. Those all variables changing continuously makes engine state very unstable.

Exhaust gas recirculating in itself is also not a problem as long as it’s consistent given the measured load and engine speed. Exhaust gas recirculating is only a problem to the extent that it contributes to the instability, for example, by reducing the exhaust pulse and reducing exhaust suction during the overlap.

There isn't any measurable exhaust suction in part-throttle. Manifold vacuum greatly exceeds suction that is coming from quite low exhaust purge. In idle intake manifold has something like half of atmospheric pressure, exhaust instead have about full atmospheric pressure. So when both intake and exhaust valves are open at overlap there's a massive flow from exhaust to intake manifold. In common plenum that flow can go from other cylinder to other - and as intake tract length varies usually a lot from cylinder to cylinder, part of that is how plenum ports are relative to cylinder overlap and intake strokes.

[ptuomov]

So, from your first post asking for an overlap value that avoids part throttle surging on a cross plane V8 and your dislike of the surging in a daily driver to your most recent post with personal dyno experience of part throttle surging, it seems you have the data already to make your decision. It seems your looking for validation of your hypothesis from some of the folks here. Please let us know when you install a different cam and fix the issue and also let us know if your hypothesis is correct about the root cause of the surging.

I’m just absolutely dumbfounded how much overlap the NA ITB V8 that I own can take compared to how little overlap other NA plenum manifold V8 systems can take and remain enjoyable to drive at part throttle. I am choosing a cam set right now for another street car engine that will have a plenum manifold and trying to understand how much overlap I can have without hating it. That’s why all the questions.

[ptuomov]

How so? If revs are just increasing engine is accelerating - that isn't usually very unpleasant.

There isn't any measurable exhaust suction in part-throttle.

Manifold vacuum greatly exceeds suction that is coming from quite low exhaust purge. In idle intake manifold has something like half of atmospheric pressure, exhaust instead have about full atmospheric pressure. So when both intake and exhaust valves are open at overlap there's a massive flow from exhaust to intake manifold.

In common plenum that flow can go from other cylinder to other - and as intake tract length varies usually a lot from cylinder to cylinder, part of that is how plenum ports are relative to cylinder overlap and intake strokes.

When you drive a car at speed limit, you use maybe 30hp and have the engine at part throttle. Now, the car starts surging and you have to lift the gas pedal a little to bring the rpm and speed down. Then the car may start suddenly decelerating, and you have to give it more gas. Rinse and repeat. I find this unpleasant, and I believe 99% of people do so, too.

The level of pressure or vacuum at the chamber when the intake valves opens isn’t terribly relevant. What’s relevant is how much that vacuum changes when the volumetric efficiency changes. I think there’s a significant relationship there.

I don’t subscribe to the theory that say 650cc cylinder feeding thru a 45-50mm diameter intake runner with total induction length of 40cm is going to have a lot of instability caused by unstable and variable amount exhaust gas recirculating from one cylinder to another. Sounds unlikely to me.

Exhaust recirculation is not the problem, in my opinion.

On a ski trip so back to the festivities...

[hoffman900]

So, from your first post asking for an overlap value that avoids part throttle surging on a cross plane V8 and your dislike of the surging in a daily driver to your most recent post with personal dyno experience of part throttle surging, it seems you have the data already to make your decision. It seems your looking for validation of your hypothesis from some of the folks here. Please let us know when you install a different cam and fix the issue and also let us know if your hypothesis is correct about the root cause of the surging.

I’m just absolutely dumbfounded how much overlap the NA ITB V8 that I own can take compared to how little overlap other NA plenum manifold V8 systems can take and remain enjoyable to drive at part throttle. I am choosing a cam set right now for another street car engine that will have a plenum manifold and trying to understand how much overlap I can have without hating it. That’s why all the questions.

Single cylinder bike engines are the worst for this, but due to what I described in my first post, AFR can swing like 2-3pts over the entire curve due to the carburetor seeing the bad pulses with the good pulses, and them not being dampened out by a plenum. There is a reason you almost never see stand-off on a 4bbl wet plenum race engine.

Most ITB carbureted engines will have a huge hole in the power curve somewhere for this very reason. Not as severe as a single cylinder as the bad returning waves are attenuated throughout, but still.

Bikes can get away with this typically, especially the single cylinder stuff, because they’re ~250lbs and have 5spds worth of gears topping out anywhere from 70-100mph.

Drive a Weber I4, with 260 + @ .050 / 104-106* centerline cams and you’ll be surging and bucking around until you get to 3500rpm or so.

EFI helps fix that because the system isn’t dependent on metering fuel by depression across the needle/jet. When the bad waves are propagating, it isn’t metering the fuel twice.

[naukkis79]

I don’t subscribe to the theory that say 650cc cylinder feeding thru a 45-50mm diameter intake runner with total induction length of 40cm is going to have a lot of instability caused by unstable and variable amount exhaust gas recirculating from one cylinder to another. Sounds unlikely to me.

Exhaust recirculation is not the problem, in my opinion.

When there's a pressure ratio of 1/2 and valves are both open there's massive flowing happening. And with too much EGR burning limits are easily met.

With VVT it's super simply to find out how much of exhaust recirculation can be used at low rpm part throttle - with high flowing 4v heads not much at all with lights loads and those induction lengths are usually much longer than 40cm. And result for too much EGR is unstable burning.

[naukkis79]

Drive a Weber I4, with 260 + @ .050 / 104-106* centerline cams and you’ll be surging and bucking around until you get to 3500rpm or so.

EFI helps fix that because the system isn’t dependent on metering fuel by depression across the needle/jet. When the bad waves are propagating, it isn’t metering the fuel twice.

It's not about double metering, it's about big fixed venturi carbs inability to mix fuel into wildly pulsating low speed airflow. WOT at low rpm cannot sustained, but give it enough pump shot and engine runs low rpm WOT just fine as long as pump shot lasts. Here we mainly did dirt rallying with Webers/Dellorto's and those drivability problems didn't realize, engine got into working rpm range before pump shot end.

With those classic carbs driver just have to understand that with wild cams and big carbs you just can't floor it with big gear at low rpm - you though could get sustained full power out of engine with part throttle. One part of excitement with those things, give it someone not experienced with them and they just kill engine to center of traffic lights.

[ptuomov]

Bob - I totally see the metering issue and, although I don’t understand carbs, I can see why for example the location of the carb in the intake runner including trumpet/stack could make a difference. It’s my understanding that with a street car, you always want the throttle plate as close to the intake valve as possible in FI ITB systems.

[modok]

Complicated question with an easy answer.

The ideal amount of overlap at WOT is unchanged.
The sensitivity to exhaust reversion is increased.

So....if you WERE using a short overlap cam to improve part throttle and idle performance..... you don't need to do that with IR.

If you were running a exhaust system that works terrible at low rpm, with IR you will become suddenly aware of it.

[modok]

Bob - I totally see the metering issue and, although I don’t understand carbs, I can see why for example the location of the carb in the intake runner including trumpet/stack could make a difference. It’s my understanding that with a street car, you always want the throttle plate as close to the intake valve as possible in FI ITB systems.

Just had an odd thought
It is popular to use a "dual plane AIR GAP" manifold on a v8
If all that is said here is true....
then
The gap should not be below the throttles, it should be inbetween the throttles and the carburetor body.

Shame neither one of us has any motivation to try that.