6/carbs: ? about tall 2ndary venturi's, rich, I miss you. Options

[brant]

carbs: ? about tall 2ndary venturi's
Hi all... thanks for the help in advance.

I want to try a set of the tall 2ndary's on my IDS-3C carbs.
I'm running the stock ones and want to try the tall ones, that I've heard called "906-style"

its a race car and withoug re-hashing all of my jetting and dyno history, my questions are:

1) pierce manifold says they have 2 different types in stock a 3.5 size and a 4.5 size. I believe this measurement is in reference to bore and not height. Pierce said that both sizes are the "tall" type. Which do I want? Motor is a 1967 2.0/S motor at altitude.
(the pierce catalogue references that the IDS carbs came with 4.5 short, but I'd like confirmation before I order)

2) Does anyone know if these tall secondaries interchange on the IDA carb versus the IDS carb. I would assume so, but I once bought some F33 emulsion tubes from Pierce that do fit the IDA, and found that they would not fit the IDS. I didn't know there was that much difference between the two carbs, which surprised me. But again, before I order do people know if the 906-type tall 2ndaries will fit an IDS-3C body?

again, thanks in advance everyone!
brant

[J P Stein]

QUOTE (brant @ Nov 3 2005, 04:37 PM)

Its the reversion issue and my 4K stumble that has pushed me into the decision to try. I'm only running S cams, but depending upon how they are timed, they could still be a problem. Plus I'm running such big venturi's that I picked up a mid range stumble. brant

Steve Wiener talked me out of S cams just for that reason.(reversion) Nothing I have read since (on the net) from guys that use the S cams has made me think he was wrong (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)
Racers that keep the motor wound up shouldn't have much problems, said he.

AAAron:
Ayup ITGs.....which gets ya another pic (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

[Aaron Cox]

what is reversion? im a newb (IMG:http://www.914world.com/bbs2/html/emoticons/051103-stupid4.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

[brant]

QUOTE (J P Stein @ Nov 3 2005, 05:49 PM)

brant [/QUOTE] Steve Wiener talked me out of S cams just for that reason.(reversion) Nothing I have read since (on the net) from guys that use the S cams has made me think he was wrong (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) Racers that keep the motor would up shouldn't have much problems, said he.

When I had the small 31 venturi's I was totally able to jet the stumble out...

boy.. I only have SC cams to compair to... but boy the S cams are fun. Man they kick in! I sure like them! I want to step up not down. (in a couple of years I'm going to try some GE 80's with big valves and big RPM)

usually I can keep it wound up.
its a VERY minor issue for me.
but heck I figured if I could make it a tiny bit better why not do it during the off season.

my gearing is less than optimum, so it was only an issue in traffic, in a corner, when I would get off cam.
Hopefully the new tranny will adress half of the issue right there, and if the 2ndary venturi's help at all, then I'll be set!

pretty pics JP!

(p.s. what cams you got in there?)

[J P Stein]

The only place I've actually seen this was on a running 2 stroke dirt bike with the air box removed. There were 2 "standing waves" of fuel one about 1.5 inches and the other about 3 inches from the throat of the carb. Facinating the way they moved in & out , formed & deformed depending on the throttle opening.

Then I was reading a thing on the Pooper. Pete Lovely said that he had to form a air box out of a soft drink cup to keep the wind (at speed) from blowing away the reversion......or the car would run like crap.

When you get (as I see it) the standing waves of fuel bouncing around in an air cleaner (not enuff room overhead), it raises hell with the working intake system harmonics. Note: there is a lot of conjecture here. (IMG:http://www.914world.com/bbs2/html/emoticons/huh.gif)

[brant]

I should add that I'm not sure I actually have reversion yet. It might just be a jetting issue due to my large vents and poor velocity.

brant

[campbellcj]

I have the tall secondaries on my car but also was not aware there were different sizes of 'tall'. The engine also had IMO 'too large' venturis/chokes when I bought it (36's IIRC) and I downsized them to 32's. It still runs like crap in the midrange (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) but it's much better...

[J P Stein]

I run my engine quite rich on the idles....so rich that I have some plug fouling problems even with the MSD.....but the transition is fine. I have less overlap with the Solex cams and thus, less problems, me thinks.

[MecGen]

Hi Aaron

QUOTE

"Reversion creates other problems. In an automotive application, reversion refers to reversed air flow, or in simpler terms, it's when air in the intake runner reverses direction for a split second. The condition is caused when a burst of pressure escapes into the intake runner from the cylinder during valve overlap. Studies have shown reversion creates resonance shock waves inside the tubes which exit the open end of the tube at various rates depending on engine speed. It has also been proven that these shock waves interfere with each other when the stacks are in close proximity. Installing a free-flowing air filter on top of each stack or over the carburetor air horn eliminates these conditions."

Reversation is a bad for intake harmony

Later

(IMG:http://www.914world.com/bbs2/html/emoticons/beerchug.gif)

[Thorshammer]

(IMG:http://www.914world.com/bbs2/html/emoticons/slap.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

Reversion takes place normally at a given rpm and is "fuel stand off" meaning you can actually see a small cloud of fuel above the velocity stack. This is caused by many things, but the actual process is quite simple.
Due to one or a combination of things, a pulse is sent up the intake tract from the combustion chamber and actually makes a portion of the fuel reverse and it is contained (hopefully) by the velocity stack. Jake said he uses the shortest stack so that it just contains the reversion. This is right. All engines (non direct injection)even fuel injected engines can have reversion, although it is more rare for a fuel injected engine to have reversion. The key here is the pulse being tuned to draw the combustion chamber down at the right point, and at a given rpm, this is usually meant to enhance power at peak torque, although some engine builders do things differently. Below this rpm if the exhuast pulse is returned earlier (there is a delta for temp) then reversion can happen, from the returned exhaust pulse going up the inlet tract during valve overlap. Equally so, if the inlet valve opening is very early, then the cylinder pressure can actually push the mixture back out as well. This is why some "s" cams can have a mid range issue, because the inlet valve opening IS very early and there can be a pressure wave up the intake towards the carbs. I like to run that cam in a slightly advanced state to reduce this. But everyone reading this needs to understand that since these factory cams were produced we have learned so much more about pressure waves and valve action, its not even funny. What was not possible due to metalugy years ago, is now possible, cam lift rates that could not have been utilized, are now being so. This is very important when building a "cookbook" engine. And this is why People like Jake Raby get the money to build type four engines, No cook book shit here, just hard work and proper testing to find the RIGHT combination.

The best way to control reversion and to reduce "flat spots" is to build engines that are true packages, If you have 906 cams and are running headers that are 40 inches long, your wasting your time, probably better off putting in some E cams if you only have 40 inch headers, so remember it's a package. Brant is on the right track. BTW, the 3.5 or 4.5 refers to inches, not id or od. you want 4.5. This will increase the signal strength and make the carbs easier to tune. Trust me. 4.5. And remember these are very crude castings and must be massaged to make them work well, At least that is my opinion.


Erik Madsen

[TimT]

QUOTE

When you get (as I see it) the standing waves of fuel bouncing around in an air cleaner (not enuff room overhead), it raises hell with the working intake system harmonics. Note: there is a lot of conjecture here.

hehe Werd (IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif)

its not really conjecture though. One of the great things about fuel injection, EFI, and sequential injection specifically is that you can adjust the injector end angle such that no fuel is being released when you have reversion (exhaust closed piston moving up intake open). Ya get better efficiency, less emmisions because fuel isnt getting into the engines cycle except when it is needed.

Carbs cant do this..every time the piston descends it draws vacuum and hence fuel from the carb..this charge of fuel hangs in the manifold and gets bounced around until it is finally used

[Phoenix-MN]

Back to the original question -
I'm running the taller version and they appear to be the 3.5's. My engine is 2.5L and jetted as follows;

Main Vent. 32 mm
Tall secondary vent.
F26 emultion tubes
180 air correction jet
140 main jet
160 idle jet
"Solex grind cams"

This combination runs/pulls strong from 2000 to 7000 rpm without any flat spots or peaks.

Hope this helps,
Paul

(oop's forgot to add I'm running IDA 3C carbs, IDS I belive are different for the accelerator enrichment output)

[Thorshammer]

Tim is right.

Late injected fuel injection has the least possible reversion, and this is one of the reasons they are so tunable. And with that said, the timing event of the injection is preset and does not change with RPM and that can still produce some reversion, but with modern engines we don't see it, because it is completely contained within the inlet tract/manifold. However, you will still see reversion with a velocity stack manifold and injection at certain RPMS, although it will be reduced. And the reason that is: many FI systems do not change the degree at which the injector fires throughout the range, although some do. So (especially with race systems) you get a slight trade off when you decide on a degree of crank rotation to fire the injectors, when this timing happens it will not be perfect for all rpms, many FI tuners actually tune to inject the fuel very late in the cycle so that as lower rpms there is little or no reversion, this is common on 911 air cooled engines to also cool the inlet valves. But at very high rpms, you will normally see the engine lean out when this is set to late. In many engines FI is still a trade off of fuel distribution and powerr delivery, but then again, is it a race motor, or a street motor. If it is a street motor, you can do this because the time at high rpms is not as much as in a race motor. Then again, some FI systems will change the fuel injection timing based on rpm, and it is not fixed. which solves the problem except on valve overlap and off the gas, this changes everything. (IMG:http://www.914world.com/bbs2/html/emoticons/aktion035.gif)
If I had my druthers, I'd have injection.
Erik Madsen

[Phoenix-MN]

Also found this -

"Other performance-enhancing options for Weber-equipped cars are the tall, auxiliary venturis developed for the 906 engine. These really enhance the metering signal "seen" by the main circuit to sharpen the throttle response when using slightly-too-large main venturis for maximum power. The tall manifolds used on the 906 and 911R were developed to help contain the reversion in the intake tract from using camshafts with lots of valve overlap. Reversion causes a ‘fuel cloud’ to form over the individual intake trumpets that plays havoc with fuel mixture at higher RPM’s. Spacing the carburetor farther away from the intake valve with taller intake manifolds, increases torque and throttle response, especially with racing-sized venturis."

Dug up the invoice from AJR for the sec. vent. part # 109.100.985.01
Paul