Camshaft overlap for plenum manifold vs ITBs

[ptuomov]

This is a question about a naturally aspirated cross-plane V8 that is used as a daily driver and that runs on premium pump gas.

How much more camshaft overlap can the car tolerate with ITBs vs plenum manifold while still being a nice to drive daily driver that idles well? I’m guessing that the same question applies to comparing a single plane four barrel carb manifold vs. four independent runner Webers.

I hate a car that randomly surges with part throttle cruising.

[hoffman900]

I believe less.

The shared plenum dampens the pulses by the time it gets to the carburetor, thus leveling out the AFR curve. An ITB / Weber sees all the bad pulses along with the good.

Calvin has given the example of exhaust tricks that work on a single plane 4bbl intake that won’t work on the same engine with Weber’s.

[Belgian1979]

I have an ITB system, though efi. The car was ran with a single plane and carb and the same cam before the use of the itb efi system. It does run a lot better but that may be purely due to the effect of efi being less dependant on vacuum.

In general the vacuum when measured through a vacuum chamber is similar to that of the carb+single plane setup.

[ptuomov]

I believe less.

The shared plenum dampens the pulses by the time it gets to the carburetor, thus leveling out the AFR curve. An ITB / Weber sees all the bad pulses along with the good.

Calvin has given the example of exhaust tricks that work on a single plane 4bbl intake that won’t work on the same engine with Weber’s.

To me, that doesn’t make sense.

The reason why any system surges at part throttle is that a small random increase in volumetric efficiency increases the strength of the exhaust vacuum at overlap. This further increases volumetric efficiency and the strength of the exhaust vacuum during overlap.

The plenum system is a runaway train in this positive feedback setting with a lot of volume between the intake valve and the throttle plate. In contrast, ITB system with the throttle plates close to the intake valves create a large local vacuum that stop this runaway train.

This is true for FI systems. I suspect it’s also true for carb systems, but I’m both too young and too old to deal with carburetors in 2020.

[Geoff2]

I have four Weber 48 IDF carbs on a 455 Pontiac, street driven, been on 15 years. There is no comparison between an IR set up like this & a 4 bbl. Instant throttle response, MUCH smoother idle. The smoother idle allows you to run more cam for a given idle roughness. And very forgiving with detonation. When I ran 4bbls, I had to be careful with the dist advance curve detonation on hot days, even with 98 octane fuel [ Australia ]. With the Webers & no other changes, I have 35* locked timing & run 91 octane; it has NEVER detonated. Fuel economy with Ws is about the same as 4bbl. Only possible downside, & I have not done a comparison, is top end HP might be down slightly. DV tested aftermarket 58 IDA Webers on a 427 Ford & they made 702 hp & 650 ft/lbs of tq....so not too bad.....

[stevek]

This doesn't answer the question, but maybe useful anyway.

I have a 514 BBF with 8 x 55mm EFI ITBs sitting on top of a Blue Thunder IDA manifold.
This engine runs on LPG (propane) not Gasoline. The injectors fire above the throttle blade and there is no wall-wetting with LPG.

The cam is not huge:
Comp Cams SFT
270/280 total
240/246 @.050"
.34"/.347" lobe lift
108LSA on 108ICL
So what's that - about 82 degrees total overlap and 27 @.050"?

Idle is currently set @ 1000rpm and when warm, vacuum is around 60kPa.
I can easily pull this motor down to 700rpm at idle. I can, but I don't - because oil pressure and vacuum brakes.

I moved overseas for work and never finished the tune or dynoed it. In the very short time I did have it running, I managed only a few miles around the neighborhood up to about 3000rpm. The throttle response was amazing, especially for such a quick & dirty tune, and I did not notice any surging at any load or RPM up to that point.

[RevTheory]

I think what Bob is talking about is a situation where you have an exhaust that isn't working right at a certain rpm and you have a returning wave that hits during overlap that actually pushes the charge back up through the intake. In a plenum manifold, there's always another cylinder on its intake stroke to take some of that in but on an ITB, it's going straight up through the carb causing fuel stand-off.

There was an excellent youtube video showing fuel misting up through the air filters at a certain rpm but I haven't been able to find it for years now.

[naukkis79]

To me, that doesn’t make sense.

The reason why any system surges at part throttle is that a small random increase in volumetric efficiency increases the strength of the exhaust vacuum at overlap. This further increases volumetric efficiency and the strength of the exhaust vacuum during overlap.

The plenum system is a runaway train in this positive feedback setting with a lot of volume between the intake valve and the throttle plate. In contrast, ITB system with the throttle plates close to the intake valves create a large local vacuum that stop this runaway train.

It's about intake manifold vacuum, intake manifold vacuum will reverse-flow exhaust gases into intake manifold. Put enough overlap to cam and with common plenum intake you have zero vacuum in intake, even when throttle is closed. With throttle blades near intake valve and runners isolated from each other intake manifold volume becomes so little that even when reverse flow zeroes manifold vacuum at overlap cylinder volume can pull plenty of vacuum to later in intake stroke making it possible to flow fresh air into engine - and have some vacuum in carb to flow fuel into that air too.

With cams that have ~150 degrees overlap it's usually not possible to get engine idling below ~3000rpm with common plenum intake, with ITB setup they can be set to idle pretty well at under 1000rpm.

[naukkis79]

I think what Bob is talking about is a situation where you have an exhaust that isn't working right at a certain rpm and you have a returning wave that hits during overlap that actually pushes the charge back up through the intake. In a plenum manifold, there's always another cylinder on its intake stroke to take some of that in but on an ITB, it's going straight up through the carb causing fuel stand-off.

There was an excellent youtube video showing fuel misting up through the air filters at a certain rpm but I haven't been able to find it for years now.

Returning wave always makes mist above velocity stacks at low rpm full throttle use -it may complicate carb tuning. With big solid choke ITB carbs engine probably just can't take WOT at low rpm. CV carbs or FI makes that problem go away but with traditional Weber or Dellorto setup your right foot is part of carb tuning.

[ptuomov]

To me, that doesn’t make sense.

The reason why any system surges at part throttle is that a small random increase in volumetric efficiency increases the strength of the exhaust vacuum at overlap. This further increases volumetric efficiency and the strength of the exhaust vacuum during overlap.

The plenum system is a runaway train in this positive feedback setting with a lot of volume between the intake valve and the throttle plate. In contrast, ITB system with the throttle plates close to the intake valves create a large local vacuum that stop this runaway train.

It's about intake manifold vacuum, intake manifold vacuum will reverse-flow exhaust gases into intake manifold. Put enough overlap to cam and with common plenum intake you have zero vacuum in intake, even when throttle is closed. With throttle blades near intake valve and runners isolated from each other intake manifold volume becomes so little that even when reverse flow zeroes manifold vacuum at overlap cylinder volume can pull plenty of vacuum to later in intake stroke making it possible to flow fresh air into engine - and have some vacuum in carb to flow fuel into that air too.

With cams that have ~150 degrees overlap it's usually not possible to get engine idling below ~3000rpm with common plenum intake, with ITB setup they can be set to idle pretty well at under 1000rpm.

That does not explain surging at idle or part throttle cruise. That also doesn’t explain why the surging happens with fuel injection systems regardless of the load measurement method, although the way it surges and stalls is different depending on how load is measured.

I think I understand what’s happening inside the engine. I was asking for practical quantitative experience from people who’ve been doing this for a long time. How much more overlap can an ITB FI engine take compared to plenum manifold FI engine to still be enjoyable to drive?

[naukkis79]

That does not explain surging at idle or part throttle cruise. That also doesn’t explain why the surging happens with fuel injection systems regardless of the load measurement method, although the way it surges and stalls is different depending on how load is measured.

With excessive exhaust residuals you got many different running problems as actual burning varies wildly from cycle to cycle.

I think I understand what’s happening inside the engine. I was asking for practical quantitative experience from people who’ve been doing this for a long time. How much more overlap can an ITB FI engine take compared to plenum manifold FI engine to still be enjoyable to drive?

It can tolerate all overlap you could have. Of course something stupid like keeping exhaust valve open to IVC is excluded but you can dial both exhaust and intake valve to as close to piston as you dare. I have not tried any long-overlap non-itb setups in decades but as nowadays engines have VVT I have checked how much overlap those long-runner common plenum intake FI setups can take at low rpm - not much at all, totally undriveable with lot's of misfires etc.

[ptuomov]

That does not explain surging at idle or part throttle cruise. That also doesn’t explain why the surging happens with fuel injection systems regardless of the load measurement method, although the way it surges and stalls is different depending on how load is measured.

With excessive exhaust residuals you got many different running problems as actual burning varies wildly from cycle to cycle.

How much more overlap can an ITB FI engine take compared to plenum manifold FI engine to still be enjoyable to drive?

It can tolerate all overlap you could have. Of course something stupid like keeping exhaust valve open to IVC is excluded but you can dial both exhaust and intake valve to as close to piston as you dare. I have not tried any long-overlap non-itb setups in decades but as nowadays engines have VVT I have checked how much overlap those long-runner common plenum intake FI setups can take at low rpm - not much at all, totally undriveable with lot's of misfires etc.

That’s useful. So an ITB FI engine can take the overlap that it wants/needs to make midrange torque and still be nice to drive. I own an NA ITB V8 engine with long tube headers, and it has cams that probably wouldn’t even allow the same engine to start with plenum intake manifold and exhaust manifolds, so what you say is consistent with my (limited) experience.

In terms of surging, I’m referring to persistent surging or dying down at fixed throttle angle, which is not created by an individual misfire on its own but a positive feedback process across cycles.

[Rick!]

So the surging to be avoided is always the symptom of a high overlap cam?
Not a function of incorrectly calibrated spark advance or marginally inadequate fuel calibrated engine combo?
If the the engine power characteristic has a torque hole in it that is substantial,
then the surging is just running up and down on the torque hole ramp.
I don't know how one would tune that out, ITB, plenum or otherwise except by changing valve events.
Changing exhaust pulses can help out a little but it's still like "stepping on the balloon" where you gain
a little in the problem area at the expense of performance in another.
ITBs or Webers on cammed 4 cylinders amplify this issue when the combination is off and
you get a bunch of "gargling" until you climb onto the linear part of the power curve.
If you've ever put a vacuum gage on an cammed ITB 4 cylinder, you'd wonder how the needle stays attached to the spindle.
V8s are a bit better but I've never experimented purposely with a high overlap cam.
I don't recall any of the old 6C Ferraris having surging issues but I only worked on a few of them.

Analagous to 2-smoke performance, even with E-Tech injection, you have to use a guillotine in the exhaust port to promote low speed driveability and still reach WOT power goals as a full time high exhaust port creates torque holes that no amount of reverse pulse can fix.

[ptuomov]

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.

In my opinion, the main positive thing that FI ITBs do to a street car is to make it run way better at part throttle than a plenum manifold setup if both (a) there’s a lot of camshaft overlap and (b) it runs great at full throttle already.

There may be other problems that cause persistent surge and then dying down at light throttle cruise, but the high overlap cams combined with a large volume between the intake valve and the throttle plate is the main one that I’m aware of.

[naukkis79]

In terms of surging, I’m referring to persistent surging or dying down at fixed throttle angle, which is not created by an individual misfire on its own but a positive feedback process across cycles.

Problem with large overlap cams and common plenum intake is that engine throttling is unstable. When there's near zero vacuum in throttle basically stops working, throttle blade movement won't change measured fresh air to engine but instead changes relative egr to fresh air ratio - and as both are near same pressure flows relative egr and fresh air ratio in cylinders start to vary, which not only vary amount of combustible mixture but also that mixture afr itself starts to varying. Carbs react to varying flow rate and FI systems can't detect per cylinder airflow.

With ITB it's still adjustable how much fresh air every induction cycle gets, so it's also calculable to FI system to get correct AFR and engine can be throttled to desired speed. A tiny bit of roughness can be found in idle and very light throttle as egr to fresh air ratio can be a bit too EGR centered, so tightest emission control measurements might still be hard to pass.