Tall or short manifold for 46s

[Walt Fricke]

I acquired a 92x70.4 2.8 build. Has GE80 cams, 2.2L heads with 40mm intake ports. 10.5/1 J&Es. It has been running with TWM 40mm throttle bodies and the "short" manifolds which are 38mm at the head port. Head intakes were ported to 40mm, so there is a step there. It had been suggested to the former owner that the 40mm throttle bodies were costing him HP.

My plan for the moment is to run my Weber 46s. Paid for, have lots of jets, car is set up for them, etc.

For this I need manifolds to attch the 46s to the 40mm heads. The manifolds I am running are the "tall " style and 38mm at the head.

Looks like I will have to purchase the manifolds new. I imagine I'll be shifting at around 7,600 rpm, though I'll dyno it with my carbs and exhaust before being more accurate about this.

Do I want the "tall" style, or the "short" style for this engine. Room in the engine compartment for "tall," so that's not the issue.

Walt Fricke

[MBruns]

Walt, I would go with the tall ones for what you are going to do
Mike Bruns

[304065]

GE80 = Tall

[Henry Schmidt]

1.5 inches will make less than no noticeable difference.
During our engine development during the 80s with the 2.5 IMSA engines we discovered that it took 5" runner length change to see any difference on the dyno.
We were looking for 3 hp so our data was pretty accurate.
In the data developed by Jim Long a Chrysler engineer during the 50s, they felt a 3ft difference was required to produce a noticeable difference on their slant six.
In their production version Chrysler slanted their engine to the right and put the carburetor next to the left side fender.
Much like so many other hp and torque remedies, this one is a no brainer.
Of course I could be totally lost so if someone out there has contrary quantified information I would love to see it.

[Walt Fricke]

Thanks, guys

Talls on order.

Walt

p.s. - Mike, you sold me a 964 front oil cooler three years or so ago for my track 911 with a juiced 2.7. Just the ticket. I run 2 of these in parallel up front, venting through the hood, and no engine cooler. Almost too much cooling. Hot day, mile high, oil temps around 180 degrees F while racing. No thermostat - I use cardboard when the weather is cool. Great piece for this application, fittings to use AN readily available, etc.

[Walt Fricke]

Thanks, Henry.

I'm sure you are right about the practical effects on "tuning." I was just reading something on a website about this - being prepared to think very long for intake tuning. Not like the exhausts where maybe you can do some good with a couple of inches here or there? Might even have been your website. Also recalling diagrams in Smith and Wenner - little carb on longish tube? Anyway, sounds like a "can't go far wrong" choice either way.

Not so sharp a bend for the end holes with the slightly taller manifold?

Who is it makes manifolds (and TBs) with the ends straight up? A quick search only turned up one very modular system, adaptable to all kinds of vehicles.

I think I know why Porsche curved the manifolds to start with: to fit the 3 barrel carbs which were available. But when they went to MFI, surely Bosch made the stuff for them? So they could have straightened things up? Or was that, too, shared with another make?

A buddy used a wand on his flow bench and said that the flow was distinctly less on the two end holes, which is what one would expect from the extra bending the mixture has to go through. Not huge, but good race motors are a combination of lots of little tweaks to go along with good fundamentals, are they not?

What about the effect of butterfly height? Per the standard lore (Frere or Anderson), pepping up the 3.0 RSR engine involved "high butterflies." I've always wondered about that, though it was MFI, not carbs. Other than some restriction and turbulence, what effect might butterfly placement have at WOT? Maybe getting the turbulence higher lets it subside by the time the mixture reaches the valve?

Walt Fricke

[Henry Schmidt]

Hi Walt
Lots to chew on.
I will say that the high butterfly RSR intake much like the slide valve systems were over kill.
The 49 mm intake valve was incapable of flowing the air volume that these systems provided.
The 3.0 RSR made a reported 315 hp @ 8000 rpm and we were able to produce 305 @ 7200 using smaller ports and a slightly modified 2.4 MFI intake.
Bigger, taller, longer are all relative terms that need constant refining.

[gestalt1]

I thought that the tall manifolds were a fix for the exhaust reversion from high overlap cams, not for hp or torque. i would think the step from going from 38mm manifolds to 40mm ports would also help prevent reversion.

[Walt Fricke]

Gestalt

The former owner of this motor thought the step in diameter might help that way, too. Though it was more a case of a silver lining when using what he had than calculating.

Walt

[Henry Schmidt]

Originally Posted by gestalt1

I thought that the tall manifolds were a fix for the exhaust reversion from high overlap cams, not for hp or torque....edit

"Fix" with what goal in mind. Generally all changes to engine specifications are intended to improve hp or torque in a specific power band or more specifically rpm range.
The transition from small manifold to large port have been proven a small benefit to controlling intake pulses.

[gestalt1]

i guess by fix i mean that at high rpms the 906 type cams worked great but at lower rpm the reversion would make it difficult for the carbs to function and idle correctly. moving the carbs higher with the tall manifold the reversion pulse would not reach the carb to cause as much of a problem.

however, i would think using an anti-reversion type exhaust would be better because the reversion would not get into the combustion chamber or the intake manifold. i'm really just guessing and don't have much experience with this.

[Walt Fricke]

At one point, anti-reversion headers were thought to be a hot ticket. I used a set once that someone lent me. Of course, I did nothing at all to quantify what they did or did not do.

Then he altered a set of ordinary Bursch racing headers for me to have this feature. Still have them. Again, nothing quantified. At least it didn't make the engines do anything totally wierd or dysfunctional.

Not too complicated a feature to fabricate.

But one does not hear of this feature nowadays. Maybe Henry knows why?

Walt

[Steve@Rennsport]

Walt,

JMHO, but tall manifolds are the item to use with high-overlap, narrow lobe center cams. Its not about power,.....its all about controlling reversion and improving drivability between 3500 and 5500 RPM. Call me sometime and I'll share plenty of dyno stories (25+ years) on this subject.

Stepped headers have supplanted the AR headers that Cyclone used to make many years ago. I still have a set of AR's on my own car,... .

[Henry Schmidt]

Originally Posted by Steve@Rennsport

....edit.....Call me sometime and I'll share plenty of dyno stories (25+ years) on this subject. .....edit

Please Steve share dyno sheets with us. I'm dying to see the quantitative evidence of an improvement between a short and tall carburetor manifold. I have an open mind, a true thirst for real evidence in lieu of anecdotal folk lore.

[Mark McClure]

Hi, I am interested in the AR header theory, what change is made to the header to create the AR.

Thanks

Mark........

[911pcars]

Current technology for the "tall manifold" theory are the variable length induction systems used by most manufacturers (e.g. VarioRam) to optimize torque at all engine speeds.

They accomplish this with scroll-type manifolds or multi-chambered manifolds with various airflow paths that are ECU-controlled to either shorten or lengthen the intake tract via vacuum-servo "throttle" plates.

Longer airflow path for low-rpm torque/power; shorter airflow path for high-rpm torque/power.

As for the difference between the short and long Weber manifolds, I'll have to differ to the guys with dyno expertise here. I suspect the longer ones were originally used with high-butterfly throttle bodies to increase the intake tract length in an attempt to create better throttle response at lower engine speeds (low being relative). On a regular Weber setup, they probably contribute incrementally like all all other engine mods.

Sherwood

[jcge]

Some data for 2.0 & 2.5 6 cylinder engines with variable intakes (albeit Mazda KF/KL V6 24V EFI water cooled without VVT.

Reference: Mazda New Lightweight and Compact V6 Engines
Takashi Sakono, Shinobu Takizawa, Setsuo Harada, Tatsuji Ikeda, and Hiroshi Abe
Mazda Motor Corp.
(well worth a read...)

[jcge]

And the resulting torque curve for each resonant assembly - the ECU controls the flap valves to maximise the torque through the rpm's