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Thu Nov 07, 2019 3:10 pm

Hi,

I'm interested in the old practice of sizing Carburetors to the engine to achieve specific qualities in the engine characteristics. These characteristics range from Torque, Horse Power, Sound, Fuel Economy, Cool Factor and any other attributes I haven't named.

I am specifically interested in the use of larger carbs with smaller venturi and jets or running small carbs with large ventui and jets. I know this might start a keyboard war, but I would like to get information from anyone who is an avid carb tuner and can explain their results.

For example running 48 IDA carbs on a small engine (1776 90hp-100hp). I know people do this for the cool factor, sound and in some high competition classes for horse power gains at WOT. I understand you can get a set of 48 IDA carbs with small vents & jets to run well on the street, but would they run as well as a pair of 36 IDF carbs with big vents & jets when driving on the street? If they can run as well, then does it really matter the size of the carbs, when you can vent and jet them for a small engine.

I also know if you have a larger engine (2180) and run 48 IDA carbs the engine can give higher Hp, if you were to run a set of 44 IDF carbs with large vents could you have the same Hp as the 48 IDAs.

For both examples, does the torque and hp curve react differently based on the sizing of carbs? For the 1776 does the torque still come on early but die of quicker with 48 IDA carbs than with 36 IDF carbs? For the 2180 does the hP come on earlier with 48 IDA carbs than with 44 IDF carbs?

Thu Nov 07, 2019 3:38 pm

Tell about what engine you want to build first than we can answer your question. Need all info on the combo.

Dan

Thu Nov 07, 2019 3:50 pm

Back in the day, HotVWs had a book called "All About VW Performance Engines II".

Came out in 1997 or so IIRC?

There were a ton of back-to-back dyno sessions on different motors in there.
It would answer your questions very well.

See if you can't get a copy;
Best book on the subject ever written..

Thu Nov 07, 2019 4:12 pm

When you ask questions like this, you are not setting yourself up for answers very well. Like Dan said, give us YOUR combo, the one you have right now, and then ask about what carbs do what.

Your long string of open ended questions only promote 10,000 speculations and stories.

Thu Nov 07, 2019 4:14 pm

Maybe this helps

Thu Nov 07, 2019 4:56 pm

Hell yes it makes a difference.

I suggest you get a Weber tech book and read what Weber has to say. I also recommend you do not go up or down on the factory venturie sizes any more than what weber made for their carbs. The height and size of the velocity stack also has an effect on the tune. A big carb can be "made" to work on a small engine but it will never be optimal or produce the best HP & torque.

Thu Nov 07, 2019 5:28 pm

My 2276 with DRDL6(42x37) heads, FK45, 1 5/8" exhaust and 48IDFS with 40mm vent ran like a dog at lower rpms, once I put the custom 36mm vent in it became a torque monster that still pulls hard all the way through the gears.

Thu Nov 07, 2019 5:40 pm

A couple years ago I tried to tune a 1776 with stock heads and a mild cam, I think a W100 that had 44HPMX carbs on it. I was the third guy to take a crack at it. Even with a huge selection of jets in my tool box, smaller vents and an AFR I could not get it to run correctly, the transition between main and idle being the worst. After hours of fking around I gave up and suggested smaller carbs. He bought a set of 40HPMX carbs, I jetted them conservatively took it for a drive, changed the air and idle jets and it purred like a kitten. Haven't touched it since.

I race with guys at Bonneville who have IDAs on 1650cc 36hp based engines. Don't know how it works but they set records.

brad

Thu Nov 07, 2019 5:40 pm

I think I have that book and will need to read through....That was something that I have wondered about myself.

Thu Nov 07, 2019 5:44 pm

yes you need the right size vents & jetting and air tubes/emulsion tubes & jets. along with the right intakes and right exhaust and right cr and right camshaft......... or you can thriow a bunch of miss matched stuff togeather and tell your self oh hell this is the fastest car in the west.... or east....or north or south... and you may be right... but north,west south,east of what...

Thu Nov 07, 2019 5:57 pm

Venturi should be the same or slightly larger than the MCSA of the port.
Carburetor bore should be 1.15-1.33 times the venturi size

Using the minumim carburetor bore should have more charge momentum (ramming potential) at the same length, so, it might generate more torque overall, or it might not, depending on how the velocity vs length and where the harmonics line up.

Larger carburetor bore often has the advantage of better fuel sheer, and often better wet flow.

Thu Nov 07, 2019 7:19 pm

The question is open-ended to allow for people to chime in to give real examples on how they have run different carb sizes on the same engine and to give answers on what the actual driving characteristics where for the different carb sizes. I use the 48 IDA example because many people have run them on all different sized engines, even if most people would say they are too big for the engine. I also use the 2180 example because I've see a lot of people running 44IDF carbs to make more torque and they run well, but are also good candidates for 48 IDA cabs and run well but different.

Right now I have a 2161 engine, 84 crank, 90.5 piston, 40-35 valves, 10:1 comp, cam (forgot, but makes power to 7k and redlines at 8k), 1.4:1 rocker ratio, straight cut gears, medium to large porting, 1 5/8"a1 lowboy header & muffler, cb magna spark w/ dry cell coil, tall dellorto manifolds, dual 48 dellorto trijets carbs, 38 vents, jets (forgot but can check later) and lots of oil. The engine runs great it made 158hp and 165-170 torque on the dyno and I have daily driven it for the past month and it pulls hard. The carb jetting was set on the dyno and later confirmed with my wide-band sensor. The results were idles at 12.2 ish then climbs to 13.9 then mains kick in and drop to 11 then climbs to 14.2-5ish then the enrichment kicks in to bring it back to 12-13 ish and goes to 14.3.

I went ahead and took it for testing in Angeles Crest and it was surprisingly torque-y through all the gears (the 5 speed is insanely good). After the trip I talked with my engine builder about the power band, it kicks in around 4k and I want to bring it a little lower and he recommended to get a set of 36 vents and jets for the carbs.

I want to find out if the change in vents will bring the power band closer to 3k and still maintain the high RPM liner growth. Would it be worth special ordering 34mm vents or cb 36mm vents to find out or would time be saved by find a set of 45's to get that power band lower. If 45's are used, how will the engine characteristics change compared to using smaller vents or would there be no difference in the engine.

The engine being prepped for circuit track racing and some drag racing. I'm looking for two set ups for the carbs configuration.

1. One for higher speeds and can still pull in consecutive tight corners. (current)

2. More toque that will rip corners to like paper and beat the modified Hondas with no problems at the stop lights. (new vents or smaller carbs)

I know vents and jets are cheaper than buying and setting up a new set of carbs but this would be a long long term investment since the engine will be rebuilt and upgraded in the future (one engine to do it all and one engine to learn multiple power levels on).

Thu Nov 07, 2019 8:45 pm

In one of David Vizards books he talks about this subject a bit. He says that the best airflow and atomization is achieved when the venturi is at about 80% of the butterfly size. The best working size is between 72 to 80% and the maximum that will still function properly is around 82 to 85%.
He says that somewhere after the 82-85% range the signal at the booster becomes too low and leads to poor atomization.

He gives an example of a 40mm carb with a 34mm vent, he found the flow increased a bit when he went to a 36 vent but the signal on the booster dropped by 50%. He then gives and example where a 45 with a 32 vent flowed less air than a 40 with a 32 vent.

So then if you wanted to try a 36 vent you could do that in your 48s, however 45s would be the ideal choice according to DV's guidance. Small points if you consider real Dellorto 36 vents that fit 45s are available so you don't have to gamble with aftermarket vents maybe not having the proper profile.

I am sure you have seen the vent size chart in the CB books and the Solex books, but he then discusses how to calculate vent size based on displacement and rpm range.

vent in mm = sqrt(cylinder vol in cc * RPM)/2600

So on your 2161cc engine you need a 35 vent to spin to 6000 rpm, 37 for 7000 rpm and a 40 to spin to 8000 rpm.

This says noting about how big of a carb your intake manifolds, ports and valves can support.

Did you by chance get a dyno chart with your AFR shown when you had your engine on the dyno? In my engine, when the tri-jet comes on it goes rich and I keep reducing the size of the tri-jet. I would be interested to hear what your jetting ended up at when you have time.

Thu Nov 07, 2019 9:38 pm

Castroe wrote: Hi,

I'm interested in the old practice of sizing Carburetors to the engine to achieve specific qualities in the engine characteristics. These characteristics range from Torque, Horse Power, Sound, Fuel Economy, Cool Factor and any other attributes I haven't named.

I am specifically interested in the use of larger carbs with smaller venturi and jets or running small carbs with large ventui and jets. I know this might start a keyboard war, but I would like to get information from anyone who is an avid carb tuner and can explain their results.

For example running 48 IDA carbs on a small engine (1776 90hp-100hp). I know people do this for the cool factor, sound and in some high competition classes for horse power gains at WOT. I understand you can get a set of 48 IDA carbs with small vents & jets to run well on the street, but would they run as well as a pair of 36 IDF carbs with big vents & jets when driving on the street? If they can run as well, then does it really matter the size of the carbs, when you can vent and jet them for a small engine.

I also know if you have a larger engine (2180) and run 48 IDA carbs the engine can give higher Hp, if you were to run a set of 44 IDF carbs with large vents could you have the same Hp as the 48 IDAs.

For both examples, does the torque and hp curve react differently based on the sizing of carbs? For the 1776 does the torque still come on early but die of quicker with 48 IDA carbs than with 36 IDF carbs? For the 2180 does the hP come on earlier with 48 IDA carbs than with 44 IDF carbs?

If you've got the time and the carbs, vents, jets etc. to play around and give it a go that's great.
It never hurts to experiment and try different setups.
Hell I've run 34mm vents in IDAs with great results.
Right now I'm experiment with a single barrel 40 IDF ( 1/2 of the carb. ) on a 2276cc, not getting the results I was looking for but trial and error is what its all about.
Keep good detail notes as to what works and especially what doesn't.

Good luck and remember to enjoy doing it.

Thu Nov 07, 2019 10:07 pm

I would be curious as to what your jetting is too with the 48 DRLA tri jets, as well as your camshaft and compression ratio.

I worked with J-Dub, Alstrup, Wreck, and JPaull to get my 2276 dialed in. I have 48 DRLAs as well and was using 40 venturis with older hand ported 42x37 L6 heads. I found that it did not have the torque I was looking for so I downsized the venturis and went a different direction getting some 36 venturis made from Rick which can be found here:
https://www.thesamba.com/vw/forum/viewtopic.php?t=727671&highlight=

The venturis were sent over to Bergs where Mark Voegtly grooved them for me 36-40.

https://www.thesamba.com/vw/classifieds/detail.php?id=2238165

Those might me an option for you although 45 Dells may be a good option as well which come with 38 venturis.

Thu Nov 07, 2019 10:08 pm

36mm might be good, but I don't think going smaller will be the right solution. You can change other parts of the engine to move the power band around and juggle top end VS throttle response.
You could try different choke sizes for the exhaust system, maybe try a anti-reversion style collector of a few sizes, VS venturi shapes of a few sizes, see what happens.. Try different rocker ratios.
Different velocity stacks and/or spacers under the carbs

Fri Nov 08, 2019 11:47 pm

74 Thing wrote: I would be curious as to what your jetting is too with the 48 DRLA tri jets, as well as your camshaft and compression ratio.

I worked with J-Dub, Alstrup, Wreck, and JPaull to get my 2276 dialed in. I have 48 DRLAs as well and was using 40 venturis with older hand ported 42x37 L6 heads. I found that it did not have the torque I was looking for so I downsized the venturis and went a different direction getting some 36 venturis made from Rick which can be found here:
https://www.thesamba.com/vw/forum/viewtopic.php?t=727671&highlight=

The venturis were sent over to Bergs where Mark Voegtly grooved them for me 36-40.

https://www.thesamba.com/vw/classifieds/detail.php?id=2238165

Those might me an option for you although 45 Dells may be a good option as well which come with 38 venturis.

I forgot what cam we put in the engine and lost the spec sheet. But the rockers are 1.4:1 and the compression is 10:1.

Its interesting you mention those grooved venturies berg makes and does. I have a set of 32/36 grooved vents on some early 44 idf carbs, im going to use them on a strong 1776 combo. I still have to port my intakes so I can finish dressing up the long block and take it to the dyno. They where recommended to me on the grounds that the grooves will give the engine better turbulence at lower rpm to help with throttle response while allowing the cam to open up at WOT with the 36. I still need to try it, but i'm looking forward to seeing what happens and it should be a great daily engine.

Sat Nov 09, 2019 12:27 am

modok wrote: 36mm might be good, but I don't think going smaller will be the right solution. You can change other parts of the engine to move the power band around and juggle top end VS throttle response.
You could try different choke sizes for the exhaust system, maybe try a anti-reversion style collector of a few sizes, VS venturi shapes of a few sizes, see what happens.. Try different rocker ratios.
Different velocity stacks and/or spacers under the carbs

Right now its running 1.4:1 rockers but I dont remember the cam. I have the tall dellorto manifolds and the short/regular dellorto velocity stacks. I would like to try tall stacks some day and see what the actual effects are. and they look the business.

BUT?
Would going to tall velocity stacks really help bring the power on earlier, I have read into them and found they stabilize the air going into the carb but I don't understand the theories behind it. Taller stackes would create a more direct down flow into the carb, not allowing the air to enter the top from an angle rather than with short stacks the carbs would be pulling air in a polar array format converging on the auxiliary venturi. I have thought about it and can see the air-flow with tall stacks, they would be affected by the designed shape of the auxiliary vent more drastically before entering the venturi to draw and anatomize fuel more, (this could be the way it was designed to flow and would give the intended engineered results). But I can see the short stacks allowing air to converge on the auxiliary venturi (not flowing directly down from the top) to create more turbulence at the area before entering the venturui where it would draw in fuel, but would the extra turbulence add to the mixture of the fuel when drawing it through or does it hinder the draw through.

The idea above can also be asked when running at high rpm and low rpm and for an engine with a large carb and small carb relative to the engines size. When looking at a single engine and using a pair of large carbs, the engine can run more hp at high rpm vs using smaller carbs, giving the engine more torque at lower rpm. But in most findings its not the turbulence but the intake velocity thats used to prove these large carb & small carb ideas, this is disregarding other important factors like engine heat, intake manifold height, intake manifold flow capacity, head porting shapes and flow rates, valve sizing, exhaust sizing/ shape/design and cam. They are important but the carb idea holds true even if its not optimal for the engine operation. (its about the working concept and can be refined when applying it to the specific engine)

Sat Nov 09, 2019 1:04 pm

If we are talking about the intake lenght.....is that....
The length from the valve to the venturi?
or the length from the valve to the top of the velocity stack?

actually both.
IMO, the length to the venturi (manifold length) makes a lot more difference, maybe 2x as much difference as the height of the stacks.

Stacks also alter how the carb jetting behaves, and "deal with" the reversion fog. Really tall velocity stacks look cool but probably won't work right. Should be kept in proportion, for instance somewhere around 1/4 of the length.
If it is 12" from the valve to the venturi, then the top of the stack should be 4" above the venturi. It does not have to be exact but if you go to the extremes trying very short or tall, you won't find anything good. I never have

Sat Nov 09, 2019 7:54 pm

I think people, myself included, sometimes under estimate the importance of intake port velocity. Take for example a pretty warm street/strip engine with a fk87. Installed on a 108 IVC, the intake valve closes 66 degrees after bottom dead center. Considering it’s only 180 degrees to TDC, your already a third of the way there. The piston hasn’t swept a third of the volume yet, but it’s not insignificant. As the piston moves towards TDC, your only savior is the momentum of the intake column. If the intake runner is too short, you don’t have the mass. If the port is too big, you don’t have the velocity.

Another thing to consider is carbs are designed to operate with a specific relative pressure drop (delta pressure) between the carb bore and venturi. Too large of a venturi for the bore results in a low delta pressure and requires compensating with really big / unusual jetting. One last thing to consider is the density of air, it will also impact the delta pressure. So if your operating at high altitude, you need smaller venturis to maintain the delta pressure.

There’s a lot of science going on that maybe isn’t obvious or intuitive.