The way I'm looking at it is from an intake charge velocity or lack thereof perspective.
I'm not factoring in the EFI and the signal it needs to run properly. I know nothing about the aftermarket EFI's and what they need to run properly. Could be it's a tradeoff to get them to work with a dual plane? At least when CR or cam overlap may be an issue.

If you effectively increase the plenum volume and thereby runner volume effectively, you slow down the intake charge.
I agree the dual plane increases the signal to the carb, due to the smaller plenum volume (and longer runners) since it can't "share" with the other cylinders creating a higher velocity charge and better vacuum signal due to the increased velocity.

Don't get me wrong I'm not saying milling down the center divider was not necessary for the EFI, I simply don't know.
But in a low CR or too much overlap cam scenario it would not be beneficial for below peak torque performance. The OP seems to have performance issue below 3000 RPM. The range in which slow intake velocity would be felt. And I'm not sure that this is not at least partially a tuning issue with the EFI.

Without knowing specifically all the specs of the engine we're just kinda shooting in the dark anyhow. I think more light will be shed once we can establish the actual CR of the engine. I'm curious what pistons were used. If they are 4 valve cutout, dish pistons, there is probably most of the problem.

I used a modified open spacer to join left to right sides in my dual plane intake to increase above peak torque performance. I also have 10.6CR with a short duration cam, so intake velocity and capture is not a problem below peak torque.
I understand it's not a "cut and dried" thing. Each engine needs to be evaluated individually and what performance is desired from it.

 

With a smaller cam the lift change of 1.5 - 1.6 is negligible! Math day. let's say that you have .500 lift with 1.5 .500 divided by 1.5 = .3333 That's your lobe lift. .3333 X 1.6 = 5.333 .033 inches is nothing, but higher lift has the possibility of flowing more CFM and will increase vacuum.

Look at a cam spec. card. The cam overlap is only at the start of the lobe ramps. Not up at the peak of max lift. That's why a roller cam with quicker and higher lift will create more vacuum that the same duration flat tappet.

 

I think it is a little bit of both problems . Reasonable assumptions lead me to believe the DCR is 7.7 or below.. So that is a little bit too much cam for what appears to be around 9.3 CR. Unfortunately this requires either a CR fix or a cam fix, or both. A thin head gasket is the cheapest to try. When the heads are off he can verify the actual compression. Advancing the cam may also help.

Second the MAP sensor EFI issue reading only one half of a dual plane is a real issue. Holley consistently denies it because they say it is "an inappropriate manifold for the engine combo". However when it occurs the EFI makes the "combo" act up much worse than a carb ever did. And then on some combos it works just fine. But then in the fine print they also suggest a single plane. Possibly the intake divider milling helped that issue. It did on engine masters with a carb engine, the milling added 20 lb TQ at 2500, but that one was not EFI. It may or may not exist any longer in the POs car. A way to test it is two O2 sensors, or just add a cheap 1" 1 hole spacer for an experiment, or maybe some datalogging.

 

I understand the math just fine.
Per degree of crankshaft rotation (and per degree of camshaft rotation) the 1.6 rocker opens the valve further in distance. This is what were counting on when upping the RR to 1.6.
But you need sufficient intake velocity to make it work. Just like you need sufficient CR to support the duration of the cam.
Once it opens enough distance to flow THAT starts the valve flow and if both are open, the overlap. If that overlap occurs sooner ( due to greater valve opening distance) you have increased the overlap. Beneficial if you got the CR and velocity to support it.

You have also increased the potential for intake reversion in a lowCR / velocity scenario, due to the opening of the valve in distance relative to the position of the piston. Crankshaft degrees vs valve opening distance. This reduces the volume of charge retained in the combustion chamber if there is insufficient intake velocity.
Due to low intake velocity the inertia of the fresh charge is reduced, thereby increasing charge lost back into the head by the rising piston. Once the air speeds up enough due to RPM the reversal is reduced. Low RPM/below peak torque is going to suffer the most. Peak torque RPM will also occur later.
I know it's a small difference in max valve lift, but a difference none the less, and throughout the entire travel of the valve not just peak lift.

A roller cam developers more vacuum due to later valve opening and more optimized valve opening characteristics. It can open the valve quicker not restricted by the edge riding of the flat tappet.

i didn’t say the 1.6 are causing the problem just that they are not helping. If the problem is low CR and excessive duration, which at this time that appears to be the case.

 

So the reality of a 1.6 vs 1.5 rocker is that at all crankshaft positions the valve will be further off the seat.
Not just valve opening, valve closing as well.
This means a larger opening for intake charge to escape back into the head during valve closing per degree of crankshaft rotation.
Also means exhaust valve opening and closing. So overlap is increased two fold. Both on intake valve closing and exhaust valve opening NOT in degrees open, but in the distance the valve is off the seat per degree of rotation. which means the CFM that is going to pass through the valve at any given degree of rotation is greater in either direction.
Advertised duration is usually computed @ .006" of valve lift based on 1.5 RR for a small block and will occur at a specific number of degrees of crankshaft/camshaft rotation. If we change that ratio to 1.6 we just changed the degrees of rotation that .006" of lift will occur, as well as the commonly used .050" of lift.
If flow starts earlier, ie the valve achieves .006" of lift, with less crankshaft degrees of rotation the effective duration has been changed. Same on the other side. Now the degrees of crankshaft rotation are greater to get the valve opening down to .006" of lift.
Not only that, we accelerated the movement of the valve. We increased how far it will move but in the same number of degrees of crank rotation, so it has to move faster. Is there enough spring to prevent seat bounce or spring surge?
It's faster movement is what we are banking on for increased intake flow. It can only work effectively if we have sufficient velocity of flow to "pack" it into the cylinder. Lacking that it is easily pushed back into the head with a rising piston while the intake valve is closing.
Also during the overlap period if the intake charge lacks sufficient velocity/energy it is easily sucked out the exhaust valve with the escaping exhaust. slow moving air changes direction much easier than fast moving air.
Increase the effective duration of the exhaust valve and this effect is increased.

Once everything is moving fast enough to prevent these undesirable effects it works great. The question is where does that happen?
Perhaps at an RPM higher than desired if lacking sufficient CR, or too big of runners in the head, or the wrong intake size or type. That RPM may be too high for the application if it's a street driven car.

In gkull's example if the original lift at the valve was .500 and and lobe lift is therefore .3333" then moving to a 1.6 make the valve lift .5333". Yes we can say that is only .033" of increase. But it can also be noted that .033" of valve lift is 6.6%. The percentage is a notable amount and what I would make my comparison with not just the distance increase in inches.

 

Yes reversion could be altering the AFR at low speeds, then over 2500 it "suddenly" clears up and runs great. You could see if the EFI has selections for cam overlap. The Atomic EFI I am dealing with has 3 settings. Your idle vac is low so you may want to try a higher setting. It has the potential to correct the AFR at low rpms, low vac. Easy to try. And I believe you said it runs ok at part throttle (low rpm) but poorly at WOT. Many FI systems use the O2 sensor at part throttle but revert to set tables at WOT with no O2 sensor. Matches your symptoms.
Easy and worth a shot.

 

My EFI system has 4 settings for cams, based on idle vacuum. I started on number 3, matching my Vacuum, changed to number 2. no difference, haven't tried number 4 base map as that is for a wild cam. I don't feel this cam is that wild. I have tried AFR settings up and down from fairly Fat to fairly lean. I've tried more accelerator shot. less shot . basically I've gone crazy trying everything I can think of with the EFI. O2 sensors are active at ALL running points from idle to WOT. the only time there not controlling AFR's is during warm up. system is incredibly similar to the system we used on Harley's up untill recently. I tune Harley's for a living. never ran into this. If you roll into the throttle, it runs nice. if you punch it. it just breaks up studdering all the way until about 3 grand. clears right up and pulls like a bear. It runs so good cruising and just normal driving. starts perfectly, idles perfectly. just runs like a dream, until you punch it. then bag of.........., And oddly enough, if sitting in neutral, revs perfectly!

 

Yeah "you" might not think it is a wild cam, but with the low CR & DCR & low vac the EFI might think so. It's worth a try.
Which EFI system?

 

So I spent some time on the Holley EFI forum
https://forums.holley.com/showthread...=stumbles+2000

And it really appears as if it could be a fueling issue. Do you know how to do datalogging? And determine if it is lean or rich when you have the stumble?



And that manual mentioned does a great job of
explaining spark and fuel for a "large" 234 degree cam.

Good luck trouble shooting it!
Lee

 

I'm running a Summit MAX 500, A Fi Tech made throttle body. And the Tuning is also much the same.
Funny part is, after rebuilding the engine, I was having the same issues trying to tune my carb, (A Holley).
And said stuff it, and put on the EFI. Which is so nice in so many ways I will perserviere and get this sorted.
I do believe my combination of parts, in spite of what was meant to be good advice from a good friend is flawed.
And yes to those who keep asking. if you read all my posts you will see that I have stated already the piston type. gasket type, specs on the heads and cam.
I am hoping the raise in compression along with a single plane intake will get me to a point where I can tune it.
In the short term, until I can round up all the parts. it does run fine as long as I don't have any fun. Unfortunately that's no fun.

 

Since you had similar issues with both a carb and efi, the issue is elsewhere. That leads us back to the other issue with excess reversion from cam overlap combined with the low DCR. Then enough airflow clears it out. More compression, maybe smaller cam.

 

I've been doing a lot of reading. Seems I'm not the only one on the planet who has come accross this issue.
Adjusting the dTPS max gain does seem to have some effect. but not enough. more accelerator shot or less Seems have little effect.
reversion is definitely a probable cause. Also the uneven floor in the RPM manifold and unequal length runners can be causing some real tuning issues.
I have obtained some Fel-Pro. 015 head gaskets. I have a line on a old Torker single plane manifold that I should get my hands on next weekend if this lock down ends. Not certain if I will need to add a spacer on top of this old manifold or not. Any thoughts?
I'm hoping raising the compression a bit, opening up the exhaust a bit with the 2 1/2 inch rams horns into 2 1/2 inch chambered duel exhaust system and putting on this single plane manifold will get me to a point where I can get the tune right.
Exhaust manifolds are still a month away. couldn't find anyone who has the ones I want in stock.

 

The Torquer manifolds were really garbage with NO thought about tuned runners. It's just a poor example of open manifold which was marginally better than a restrictive stock cast iron.

Small cams have very little intake manifold reversion caused by valve over lap.

 

I appreciate your insight gkull. I looked at your profile. Looks like you may know a fair bit about tuning EFI. I have tuned Harley's for year's. And I am running into issues I have never had with Harley's. First off. All Harley's are sequential port injection. Intakes are very short.
I have read a fair bit about how these Automotive Intakes with throttle body injection get "Wet" while cruising. Then on decell, they will pull the fuel off the walls. they have adjustments for "Tip out" in the Fi Tech tuning to compensate for this. What in the Harley world we would call de-cell inleanment. However, with my RPM manifold. some runners are almost twice as long as others. so I drop the throttle, then after a few seconds or so I restab the throttle. the short runners have dryed out. the long runners maybe only half dryed out. Now, when suddenly hitting the throttle hard after just letting it decell. it studders and stumbles untill it finally clears.
This isn't reversion in the intake I'm talking about. But I believe a real tuning issue with a dual plane intake with throttle body EFI. This just wouldn't happen if the injectors were spraying right at the intake valve. And, if I didn't wish to drive my Vette aggressively. perhaps I would never even see a problem.
But it's a Corvette. I want to drive it aggressively. And unfortunately I have a very limited budget.
The early Torker manifold is most likely less than ideal.
But the runners are fairly straight and fairly close to equal length.
I started this post to learn. And I have been doing a fair bit of reading elsewhere as well.
I don't think I'll ever have the fastest car in town. that's OK.
But the current hard aceleration is just embarrassing.
If you can shed more light on this I am all ears.
keep in mind this "Junk" single plane is almost free.

 

The Torker is less than ideal but not terrible......every intake on the planet is better now but for the purposes of testing, it is perfect. I would swap it and see if the stumble repeats.....if it does, then intakes are scratched off the list for the price of gaskets and sealer.
You want terrible......look at a Weiand X-Cellerator.....the original design...I think it was part # 7175. This intake is so bad that it can cause a good running engine to run like **** just by installing it.....
All this puddling and theory about runners and all that is not valid to me......what is valid is that your Throttle Body may have MAP vacuum reference only on one side of the primary......a dual plane can and will have two distinctly different vacuum signals form each side......this can trick a Throttle body into doing something it shouldn't be doing.
There is simply no reason to run a dual plane intake on an EFI throttle body.......
GM did it on TBI to "pull" on the cone harder for better atomization......but you can't compare a GM TBI to a radial venturi style TBI.......
A Team G (7530) is a better choice here but seeing on what you all deal with over there to get speed parts.....I would test with the Torker.....just make sure you flat sand the mating faces nice and flat.....

Jebby

 

Well stated Jebby!

 

I ship things to the EU and birthday gifts are not subject to tax or import fees. If Australia is the same I could mail you a Team G 7530 single plane . local cost is @ $325 with tax. Unsure on the shipping cost. It must be an early or late birthday for you!

 

I read through most of this, but I dont see anywhere where you mentioned the type of EFI this is. Assuming this is a Holley either Sniper or Terminator, there is a single port on the bottom of the throttle body that reads vaccuum. Most (but not all) dual plane intakes have 2 completely separate plenums causing the throttle body to read 1/2 of the engine only. A single plane intake works well to eliminate this at the expense of bottom end torque. Also a Edelbrock RPM Air Gap manifold (dual plane) has a lowered plenum wall that does a pretty good job with EFI throttle bodies. I have one with my Terminator setup and it runs great. You can do something similar with any dual plane by milling down the plenum wall by 1/2 to 1" with similar effects.

This should dramatically help throttle response due to fueling issues at low RPM with a dual plane and throttle body. Also, very cheap. Just some time and a set of intake manifold gaskets.

I would definitely try this before you start buying cams and other parts.

 

Poster has a tiny cam and 13.5 vacuum at idle. That means no reversion. I don't even have a vette engine that idles well under about 1100 rpm hot.

You need to get off the cam... I've done some Fitech builds and the problem usually ends up being bad grounds and not full voltage to the brain. The ECU needs power from the alternator and the battery like the starter positive. Twin leads to both power sources and negative. Which would be the battery and engine block

Yes, my friends 69 Camaro 383 H roller cam 236/244 110 mid .500 lift runs great on a edl air gap with a plenum divider with a cut down area and no spacer. His is a sub 6000 rpm build

 

Thanks for all the responses.
Oddly enough, Today is my birthday. None the less. 300 bucks US is about 450 Australian. plus fright. most likely over 200 Australian. I've had my niece ship things over for me in the past trying to save over what a lot of these companies charge. And haven't saved much. Shipping is expensive.
I think for the price of some gaskets I will give this manifold a try. I can always swap it out later if it works out OK.
As per type of injection. Thought I had covered that but perhaps I was unclear. It's the Summit MAX 500 (built by Fi Tech). sitting on a Edlebrock RPM manifold with the center divider milled out. intank pump, LS style filter regulator. power is off the Alternator. grounded to the intake manifold. Runs perfectly in ever situation except hard aceration under 3,000 RPM. Fuel pressure holds perfectly right on 60 psi.
Perhaps I'm missing something in the tuning. But I have tryed a lot of combinations, results vary only slightly until I go way rich or way lean with the dTPS settings. Then of course it gets even worse.