two schools of thought: big air jets and small air jets...48IDA

[Jim Ratto]

All you Weber tuning experts, can you make me understand something I have been wondering about for a long time?

Why do some IDA (2bbl and 3bbl) applications run air corrector jets that are SMALLER than the main jet size? And then other applications run air jets that are LARGER than main jet size?

according to "books", the Weber rule of thumb is venturi x 4 is main and add 30-50 to get air corrector. Of course books don't make cars run, people do. But if you look at various jetting applications for cars that run IDA's (2 and 3bbl) you'll find some that require air jets SMALLER than main....obviously to make high rpm mixture rich... but why are some applications jetted that way? Which way "works"? Porsche 906 motor....46IDA3C.....42mm vents, 175 main, 145 (approx) air....
Old 904 motor, 46IDA, 170 main, 130 air jet....
But all VW guys seem to run 42mm, 175 main, F7, 210 air....etc...   why?
Jets are jets, venturies are venturies, etc.... did specific gravity of fuel change or something or?

[louisb]

Could it have to do with the fact that we don't run air cleaners. You would think that at high rmps, the air cleaner would restrict airflow.

--louis

[Jim Ratto]

906 didn't either

[nicolas]

i am not sure, but can it be that the aircorrector is used to balance the quantity (air AND fuel) that gets sucked into the carbs, in relation to the vacuum that is created into the shafts.
 and that the fuel jet is responsible for the quantity of fuel in that. the percentage of fuel and not the rate...

[louisb]

One thing to keep in mind is that probably only half the IDAs out there are tuned right.

--louis

[.]

Have you tried "square jetting" ?
 

[Jim Ratto]

Have you tried "square jetting" ?
 

yeah 155 x F2 X 155

[Jim Ratto]

check out this info from some Cobra guys..... see what i mean?

http://www.clubcobra.com/forums/archive/index.php/index.php?t-74594.html

[Tony M]

Soo many factors to look at - large cam - small cam - duration - lobe center - comp. ratio - head flow....... and like you say Jim - why ? I have had very close engine sizes but different cams - and the a/c jets were still close - But on our race motor - we used 210 mains and 180 ac  - just how it worked out

[Jim Ratto]

Soo many factors to look at - large cam - small cam - duration - lobe center - comp. ratio - head flow....... and like you say Jim - why ? I have had very close engine sizes but different cams - and the a/c jets were still close - But on our race motor - we used 210 mains and 180 ac  - just how it worked out

I wonder how 160 x  F2 x 130a/c would work

[alex d]

check out this info from some Cobra guys..... see what i mean?

http://www.clubcobra.com/forums/archive/index.php/index.php?t-74594.html

what amazes me is they are using very small vents to feed a 427! it's like if we used 40mm chokes on a 3,5 liter 4 cylinder engine!

[Dougy Dee]

I'm pretty sure the emulsion tube you are running contributes to sizing of AC and Main jetting. The Weber Tuning Guide (Black Book) has a good section on E tubes and characteristics...

[Jim Ratto]

check out this info from some Cobra guys..... see what i mean?

http://www.clubcobra.com/forums/archive/index.php/index.php?t-74594.html

what amazes me is they are using very small vents to feed a 427! it's like if we used 40mm chokes on a 3,5 liter 4 cylinder engine!

you're right.
874cc per cylinder "by the calculator" should need 49mm vents to turn 7000 rpm

I think a 7-liter V8 must pull so MUCH air through the carb that maybe the setup needs such a rich top end mixture to keep A/FR where it needs to be....  reminds me of watching the fuel flow demonstration at NHRA museum.

But then again.....   what about the little 2000cc 906 motor? 

[louisb]

check out this info from some Cobra guys..... see what i mean?

http://www.clubcobra.com/forums/archive/index.php/index.php?t-74594.html

I thought this post was rather interesting.

Well Chas, I am no cam expert, but I know now that the the vacuum produced by the engine has a lot to do with the jetting and venturi size.
In my case, even with my engine having worn oil rings (the rest is ok, with 175 readings on all cylinders in a compression test) the vacuum produced was fairly high.
If you have a strong vacuum at low rpm you can go for a larger venturi size and still have enough signal to produce the correct fuel/air mixture.
When a cam has overlap it means that the two valves will be open at the same time during the final exhaust stage, which helps scavenge the exhaust gases while drawing from the intake while the piston is still moving up. That helps fill the cilinder better, but this works only from certain rpm's on up. At low rpm the response is pretty crappy, and that is why the idle with those cams is so bad.
The vacuum with those cams is comparably weak at low and medium rpm and the velocity of the air in a too large venturi will not be enough to draw the correct amount of fuel from the spray nozzle, creating bogs and hesitations in the transition.
At high rpm the engine will suck in enough air to make this work, that is why they say that for racing the larger venturi of 42mm is ok but your low and medium range will suffer.
I do not know how your cam will behave. It is not so far off my cam.You can try taking the venturi out and turning the inside to produce a 38mm innner diameter. You can try and see what happens. If it still works ok, you bore the venturi out to the next size and see again. The worst thing that can happen is that you have to buy new chokes if you overshoot.
IMHO I believe that anybody with 48IDA's on a 427 with a mild cam must be running rich with the more popular jetting and choke (37) sizes suggested in several threads in this forum.

Is the Cobra IDA intake IR in design or is it plenum? This line really interested me.

If you have a strong vacuum at low rpm you can go for a larger venturi size and still have enough signal to produce the correct fuel/air mixture

I had this really long post about the different engines and why that would produce differing amounts of vacuum, but it basically all boiled down to different engines, different air flow characteristics, different power bands & different A/F needs. Short explanation, FE Engine = big motor, horrible head design (FE was a truck motor afterall.), power at lower rpms, tree trunk snapping torque, sewer pipes for ports, etc. I ended up thinking myself into a box and as my friend always says when I do that, KISS.

I think it all boils down to AirFlow@RPM and desired air/fuel to achieve your goal. I think I would be looking to engines that preform more like our engines for a guide to tunning carbs. (Motorcycles?)

I wouldn't get too hung up on what the book says. I doubt who ever wrote "the book" ever bolted 48s to a tea kettle er VW flat four. We could get into a theoretical discussion about carb mechanics and air flow characteristics but I am not sure if that is what you are looking for here. ANd of course, once the rubber meets the road, most theory goes right out the window.

--louis
 

[Jim Ratto]

check out this info from some Cobra guys..... see what i mean?

http://www.clubcobra.com/forums/archive/index.php/index.php?t-74594.html

I thought this post was rather interesting.

Well Chas, I am no cam expert, but I know now that the the vacuum produced by the engine has a lot to do with the jetting and venturi size.
In my case, even with my engine having worn oil rings (the rest is ok, with 175 readings on all cylinders in a compression test) the vacuum produced was fairly high.
If you have a strong vacuum at low rpm you can go for a larger venturi size and still have enough signal to produce the correct fuel/air mixture.
When a cam has overlap it means that the two valves will be open at the same time during the final exhaust stage, which helps scavenge the exhaust gases while drawing from the intake while the piston is still moving up. That helps fill the cilinder better, but this works only from certain rpm's on up. At low rpm the response is pretty crappy, and that is why the idle with those cams is so bad.
The vacuum with those cams is comparably weak at low and medium rpm and the velocity of the air in a too large venturi will not be enough to draw the correct amount of fuel from the spray nozzle, creating bogs and hesitations in the transition.
At high rpm the engine will suck in enough air to make this work, that is why they say that for racing the larger venturi of 42mm is ok but your low and medium range will suffer.
I do not know how your cam will behave. It is not so far off my cam.You can try taking the venturi out and turning the inside to produce a 38mm innner diameter. You can try and see what happens. If it still works ok, you bore the venturi out to the next size and see again. The worst thing that can happen is that you have to buy new chokes if you overshoot.
IMHO I believe that anybody with 48IDA's on a 427 with a mild cam must be running rich with the more popular jetting and choke (37) sizes suggested in several threads in this forum.

Is the Cobra IDA intake IR in design or is it plenum? This line really interested me.

If you have a strong vacuum at low rpm you can go for a larger venturi size and still have enough signal to produce the correct fuel/air mixture

I had this really long post about the different engines and why that would produce differing amounts of vacuum, but it basically all boiled down to different engines, different air flow characteristics, different power bands & different A/F needs. Short explanation, FE Engine = big motor, horrible head design (FE was a truck motor afterall.), power at lower rpms, tree trunk snapping torque, sewer pipes for ports, etc. I ended up thinking myself into a box and as my friend always says when I do that, KISS.

I think it all boils down to AirFlow@RPM and desired air/fuel to achieve your goal. I think I would be looking to engines that preform more like our engines for a guide to tunning carbs. (Motorcycles?)

I wouldn't get too hung up on what the book says. I doubt who ever wrote "the book" ever bolted 48s to a tea kettle er VW flat four. We could get into a theoretical discussion about carb mechanics and air flow characteristics but I am not sure if that is what you are looking for here. ANd of course, once the rubber meets the road, most theory goes right out the window.

--louis
 

if an engine is running carb throat per cylinder, it seems to me it would almost HAVE to be IR. I don't think Webers would run right on one big open hole under them. No sense in the expense of 4 IDAs dropped on a plenum Vicotr Jr thing. Just use two Holleys and try and make it run half as good as a IR'd Weber motor.

[Jim Ratto]

The Porsche engine charateristics.......

80mm x 66mm
10.3:1
45mm intake x 39mm exhaust valve
intake cam timing/lift: 388' duration x .481"
exhaust cam timing/lift: 360' duration x .414"
210hp @ 8000 rpm
46IDA 3C: 42mm venturi, 170 main, 145 air, F24 e tube, 70 idle

[Rick Meredith]

All the Cobra Weber manifolds I've seen have been IR.

[Jim Ratto]

what I'd like to find out is: WHY?

A carburetor works off of simple principles of differences in pressure. Sure, more air might get yanked through a Weber on one motor over another, due to displacement, rpm, flow characteristics whatever

But WHY are two Otto-cycle motors using such different, almost opppsite...jet stacks? I am wondering, do the "method" of running 170 main x 205 air in a F11 or F7 tube is "leaving something on the table"? What makes it "right" for one motor to run at one extreme end of spectrum and one motor to run at other extreme end? Just sheer air volume being ingested? Well, guys, again, look beyond the 427 Ford then and look at the 2.0L (120c.i.) 6 cylinder Porsche.....ONLY 332cc per cylinder.

Maybe the VW way of jetting is simply based on "IDF's are jetted like.... 140 main, 200 air....bla bla bla....so IDAs are the same..."

What makes us VW guys, who truth be told, tune by seat of pants in variable conditions, right on....and when we seem to be so different from "other" IDA guys? Do laws of physics make a left turn for VW guys?

Sorry, but I really want to get it. Somebody give me a dyno and a month to do this.

[Jim Ratto]

ANd of course, once the rubber meets the road, most theory goes right out the window.

--louis
 

I'm not sure I agree. Theory has its place. And specific examples of how other set ups have been tuned, running similar, if not SAME carbs I don't think so much as theory. Reading that Cobra post, OBVIOUSLY some of those cats know what's going on with their Webers. They don't seem to be the type that say "well joe blow said run a XXX main and since I don't really know HOW it should run....I think it runs great."

Too much of that in the VW world that I have seen in last 17 years.

[Rick Meredith]

Thought I had... would bore vs stroke play into this?

[Rasser]

When i had my old 1914 (w130  -  1,25 rockers  - 10.8:1CR  - 40mm dells - 1-5/8" header, 40-37,5 heads  etc. etc.   ), I used 37mm vents and on the dyno we came out with 160/160 jets (57 idle),  can´t remember what E-tubes we used.
We tried switching to longer stacks and the mixture leaned out at topend and the powerband moved further up rpms.

The height of the stacks definently has something to say.
I would think that the height of the stacks, lenght of the manifold and E-tubes are what makes all the difference... off course together with differences made by big cams, ccm etc. etc.

[louisb]

I think the VW has had a lot of stuff bolted to it that was never designed specifically for a VW in order to go fast. My guess with the F7 or F11 tubes, the carbs originally came with F7s right? Back when people first started bolting them on, there probably was not a whole lot else to use. So they went with what they had, made it work best they could, and it sort of became the standard. Not to many people stray from the norms. I bet if most peeps knew what you were running in your IDAs they would be surprised. I do agree that I think there is room for improvement. However, you may not be able to find that improvement without optimizing the whole package. I don't think the limitation is the flat four design. That is evident from the other thread. I think the limitation is that we take stuff, throw it at an engine and then try to make it all work together. Not much you can do with a cam, header length and intake length as they are sort of set. (long or short, 1 5/8 vs 1.34 etc, FK-8, FK-45, Web86c. Not to mention most were designed 30 years ago.) Even most "custom" head porters use a few basic designs with minor variations. So the carb becomes the crutch to try to make it all play nice.

As to the problem with theory, unlike the theoretical world, the real world is not perfect. I think it can lead you in the right direction, but it won't get you perfection.

The Porsche 904 may be similar in size but I still don't think you can make a comparison. Still too many variables. bore, stroke, valve timing and overlap, rod ratio (if you put any stock into that, I do), IR length/size, valve size, port size/length, exhaust setup, intended rpm range. If you built a VW engine with everything the same as the Porsche engine then you end up with a Porsche engine, minus 2 cylinders. And it would probably run with the Porsche jet stack.

45mm intake x 39mm exhaust valve
intake cam timing/lift: 388' duration x .481"
exhaust cam timing/lift: 360' duration x .414"
210hp @ 8000 rpm

I find this part interesting. The cam specs, the max rpm and the valve size. That is a pretty big intake & exhaust valve for an 80 bore. Wonder how that effects the flow through the heads?

The Ford 428 FE has a 105mm bore, 53mm intake & 42mm exhaust (Edelbrock heads) . That site was down so I couldn't find cam specs.

I am betting that their IR length is shorter than the one we typically use.

I don't know. I think I am well past the limits of my meager capabilities here.

--louis

[Rick Meredith]

I am betting that their IR length is shorter than the one we typically use.


--louis

I know it is!   

[alex d]

Jim do you know which emulsion tubes were used in the 904?

[Jim Ratto]

Jim do you know which emulsion tubes were used in the 904?

I have various books that give jetting for 906, 904, 356 4 cam

[Jim Ratto]

When i had my old 1914 (w130  -  1,25 rockers  - 10.8:1CR  - 40mm dells - 1-5/8" header, 40-37,5 heads  etc. etc.   ), I used 37mm vents and on the dyno we came out with 160/160 jets (57 idle),  can´t remember what E-tubes we used.
We tried switching to longer stacks and the mixture leaned out at topend and the powerband moved further up rpms.

The height of the stacks definently has something to say.
I would think that the height of the stacks, lenght of the manifold and E-tubes are what makes all the difference... off course together with differences made by big cams, ccm etc. etc.

that's interesting....longer stacks made it run leaner

I would think the taller stack would contain the standoff better and the engine would ingest it, keeping af/r in the correct realm.

[Jim Ratto]

I think the VW has had a lot of stuff bolted to it that was never designed specifically for a VW in order to go fast. My guess with the F7 or F11 tubes, the carbs originally came with F7s right? Back when people first started bolting them on, there probably was not a whole lot else to use. So they went with what they had, made it work best they could, and it sort of became the standard. Not to many people stray from the norms. I bet if most peeps knew what you were running in your IDAs they would be surprised. I do agree that I think there is room for improvement. However, you may not be able to find that improvement without optimizing the whole package. I don't think the limitation is the flat four design. That is evident from the other thread. I think the limitation is that we take stuff, throw it at an engine and then try to make it all work together. Not much you can do with a cam, header length and intake length as they are sort of set. (long or short, 1 5/8 vs 1.34 etc, FK-8, FK-45, Web86c. Not to mention most were designed 30 years ago.) Even most "custom" head porters use a few basic designs with minor variations. So the carb becomes the crutch to try to make it all play nice.

As to the problem with theory, unlike the theoretical world, the real world is not perfect. I think it can lead you in the right direction, but it won't get you perfection.

The Porsche 904 may be similar in size but I still don't think you can make a comparison. Still too many variables. bore, stroke, valve timing and overlap, rod ratio (if you put any stock into that, I do), IR length/size, valve size, port size/length, exhaust setup, intended rpm range. If you built a VW engine with everything the same as the Porsche engine then you end up with a Porsche engine, minus 2 cylinders. And it would probably run with the Porsche jet stack.

45mm intake x 39mm exhaust valve
intake cam timing/lift: 388' duration x .481"
exhaust cam timing/lift: 360' duration x .414"
210hp @ 8000 rpm

I find this part interesting. The cam specs, the max rpm and the valve size. That is a pretty big intake & exhaust valve for an 80 bore. Wonder how that effects the flow through the heads?

The Ford 428 FE has a 105mm bore, 53mm intake & 42mm exhaust (Edelbrock heads) . That site was down so I couldn't find cam specs.

I am betting that their IR length is shorter than the one we typically use.

I don't know. I think I am well past the limits of my meager capabilities here.

--louis

I think in the 1970's, most IDAs came with F2 tubes. When I bought mine (2nd hand, had been stored for 15 years or so) they had 37mm chokes, 150 main, F2 tubes and 130 a/c. To be honest with you, I don't think they had ever been run, as they were surgically clean, no stains, all the nylox were clean, etc. Of course, I chucked the 37 chokes in a box and went straight to 42mm....dumb. I think I then tried all kinds of stupid jet stacks, not making them work right until I went to 40mm, 160 F2, 195 air. Anyway it 'felt' right....who knows if it was.

Yeah the Porsche ran very generous valve diameters, but were angled as they are in hemi-design chambers, not side by side like neighboring twins in our antique VW chamber heads. Eventually I think they went 49mm intake, so if that tells us anything how they made more power... the RSR cams got even hotter than these 906's too....imagine that.

Yep I think you are right, louisb, there are many variables in carb setup, and most certainly (when just comparing CYLINDER to CYLINDER..... since each venturi, main, air, emulsion... could care less what it's neighboring counterpart is doing when on IR manifold....unless you have way crazy standoff) the lowly VW and the mighty Porsche are clearly different. But-  again, carbs work on depression, so air goes into void, depression in vent tells fuel to flow, it does, gets aerated with air from a/c jet...etc. What makes the Porsche, Cobra, (and take a look @ Sarge's jetting from '70's) "wrong" for todays big cc motors? Or are they in fact wrong?
Seems like every other "hi po" Weber model on Ferrari, Alfa, etc.... little 30 or 32mm vents....130 main, F11 tubes, 210 airs...
But not the IDAs.

[louisb]

You know Jim, you had me up half of last night turning this problem over in my head lol. Unfortunately, I didn't have any great revelations to add. I do however, think that today's practice in jetting VW engines is a result of Cargo Cult Engineering. (See something work for someone else, don't know why it works, mimic it anyway and hope it works for you.) But I think that is common in a lot of hot rod car cultures. (The other being Voodoo Engineering. Its all black magic so send it to some Witch Dr. that works some Voodoo and they work.) Without a dyno and a bunch of spare brass I doubt we will know the answer anyway.

--louis

[Jim Ratto]

You know Jim, you had me up half of last night turning this problem over in my head lol. Unfortunately, I didn't have any great revelations to add. I do however, think that today's practice in jetting VW engines is a result of Cargo Cult Engineering. (See something work for someone else, don't know why it works, mimic it anyway and hope it works for you.) But I think that is common in a lot of hot rod car cultures. (The other being Voodoo Engineering. Its all black magic so send it to some Witch Dr. that works some Voodoo and they work.) Without a dyno and a bunch of spare brass I doubt we will know the answer anyway.

--louis

good point man....

for what it is worth.... I ordered a 4th 160 main and a set of 130 airs. I have a nice flat straight section of road, and a stopwatch. If I can find the time, I am going to check my thoughts. 

[thirdrock]

good topic jim and louisb  who knows? all that air at 7000 rpm plus needs alot of fuel in mixture