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Sat Nov 09, 2019 8:15 pm

This is a weird thread. I thought it was a joke until I kept reading.

If carb size doesn’t matter then whey are there so many sizes?

As mentioned port velocity vs flow is a balancing act.

I have a 2276 that I’ve ran 40 IDFs with 36mm venturis and while it’s SUPER responsive it runs out of pep pretty quick.

Same motor with dellorto 45s and 38mm vents is amazing. Less response but it has a lot more power all over the rpm range.

On street I’ve always tried to make the carbs somewhat on the small side. Too big of carb makes it lazy. Too small and the motor will run out of breath before the cam is done.

IDFs has IDAs are like comparing apples to oranges.

The pic of the vents with the slots are an interesting idea.

Sun Nov 10, 2019 2:03 am

I see it like ports: any bigger than what you NEED will lose out somewhere. The Solex book says to keep on increasing venturi size until you see no power increase, then go back one size smaller.

Iirc, they say to use venturi=0.8 × throttle diameter, as a starting point. There's a formula, but I'm not at home now to post it.

From my unreliable memory, it goes something like this:

Square root of (1 cylinder's CCs × (RPM ÷ 1000)) × 0.8. I'll check when I get back home. Or maybe someone has the book on hand.

Sun Nov 10, 2019 2:06 am

Here it is, it's in the archives here:

https://www.thesamba.com/vw/archives/manuals/solex_carb.php

Hope that can help.

Sun Nov 10, 2019 5:31 pm

HotStreetVw wrote:
There’s a lot of science going on that maybe isn’t obvious or intuitive.
That's a fact.
OR, i guess it depends on what your intuition IS
10 years ago I didn't understand why you would ever use a throttle bore larger than the intake valve.
"Because" an intake valve can never flow as much as a throttle bore of the same size. In reality that ends up being more or less a "minimum" size.
Becase it's not just about dry flow. Also there is wet flow, and how the carburetor functions.
If you could increase the throttle bore size 2-3mm, and that lets you use a 1mm larger venturi, it's pretty much ALL gain.

Mon Dec 02, 2019 7:46 am

ekacpuc wrote: This is a weird thread. I thought it was a joke until I kept reading.

If carb size doesn’t matter then whey are there so many sizes?

As mentioned port velocity vs flow is a balancing act.

I have a 2276 that I’ve ran 40 IDFs with 36mm venturis and while it’s SUPER responsive it runs out of pep pretty quick.

Same motor with dellorto 45s and 38mm vents is amazing. Less response but it has a lot more power all over the rpm range.

On street I’ve always tried to make the carbs somewhat on the small side. Too big of carb makes it lazy. Too small and the motor will run out of breath before the cam is done.

IDFs has IDAs are like comparing apples to oranges.

The pic of the vents with the slots are an interesting idea.

This is my conundrum, the use of a smaller carb with properly sized venturi or keep my larger carbs and use a smaller venturi?

In your case, running 40 IDF carbs with larger vents 38mm or 39mm would it have given you a longer power band close to what the 45 dellortos gave you while maintaining some of the low down pull. Or even vise versa, if you ran 34mm or 36 mm vents on 45 dellortos would it have given you the low down power like you 40 IDFs but still maintained a longer power band since the carbs are 45mm? Like having the cake and eating it.

I guess the question is very open ended since it does not include the actual engine component formula (cam,, valve size, intake length, crank and piston sizes). But the comparison of IDA and IDF is the comparison of WOT drag to street partial throttle. Do down vented IDA carbs behave like street use IDF, and large vented IDF behave like IDA. so does the carb size matter or is there a subtle characteristic is the engine operation when going big carb small vent with equally small carb with large vent.

I decided i'm going to try a smaller vent and jet everything down for my angeles crest / street application, if im not satisfied then ill have to try 45's

Mon Dec 02, 2019 7:58 am

Dellorto 36 DRLA, some came stock with 32mm venturis, that's 88.89% ratio
Weber 48 IDA , most come stock with 37mm chokes, that's 77,08% ratio

go figure :lol:

Mon Dec 02, 2019 7:12 pm

Castroe wrote:

I decided i'm going to try a smaller vent and jet everything down for my angeles crest / street application, if im not satisfied then ill have to try 45's

Based on the engine that you describe I would think the 38 ventss on 48 DRLAs would be sized correctly for your situation. Did you have a chance to put a wideband on there and see what is happening? Further, which part of the power band are you not happy with? I am guessing mid range. How does the car drive and what are the manners like?

I may have missed it, did you ever share your jetting and cam details?

Wed Dec 04, 2019 5:07 pm

j-dub wrote: Castroe wrote:

I decided i'm going to try a smaller vent and jet everything down for my angeles crest / street application, if im not satisfied then ill have to try 45's

Based on the engine that you describe I would think the 38 ventss on 48 DRLAs would be sized correctly for your situation. Did you have a chance to put a wideband on there and see what is happening? Further, which part of the power band are you not happy with? I am guessing mid range. How does the car drive and what are the manners like?

I may have missed it, did you ever share your jetting and cam details?

I have a wideband. 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.

The engine runs great and hard but, from when we were tuning it on the dyno the power really comes on at 3,600-4000 rpm and peaks at 7,000 rpm. On the street the engine makes plenty of power down low and halls ass on the freeway, but I'm looking to make a little more power for the corners and street use, im going to try the vents.

Wed Dec 04, 2019 7:10 pm

Castroe wrote:

I have a wideband. 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.

Ahh yes, I remember now. It does seem that the AFR are swinging a bit wildly. Maybe try capping the high speed circuits with vacuum caps for a test to see/verify where they are kicking in. Are you using really large air correctors?

Fri Dec 06, 2019 12:59 am

j-dub wrote: Castroe wrote:

I have a wideband. 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.

Ahh yes, I remember now. It does seem that the AFR are swinging a bit wildly. Maybe try capping the high speed circuits with vacuum caps for a test to see/verify where they are kicking in. Are you using really large air correctors?

Ill have to hook up the tach to verify when the enrichment are kicking in but on the freeway i can see the AFR go lean in the mid high 14 then they drop down. I think they are 200, I know they where new out of a box when I got them, so what the factory put on them.

Fri Dec 06, 2019 5:51 am

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

Came out in 1997 or so IIRC?

did no one else suddenly feel really old when reading this? I'm only 37 but when I say "back in the day" i still think of like the mid-late 80s... :cry:

Fri Dec 06, 2019 8:33 am

Castroe wrote:
Ill have to hook up the tach to verify when the enrichment are kicking in but on the freeway i can see the AFR go lean in the mid high 14 then they drop down. I think they are 200, I know they where new out of a box when I got them, so what the factory put on them.

I believe the jetting as delivered from CB was 40 vent, 70 idle, 180 main, 200 AC and 120 power jets.

My engine, 2276, tims stage 2 heads, Webcam 110(similar to a Engle 120), 1 5/8 header. The Dellorto 45s were likely plenty but I wanted to try the 48 tri jets for fun.
In my tri jets currently I have 38 vents, 65 idle, 162 main, 185 air and 45 power jets. Cruise is in the 13s and power is in the 12s. I have played with smaller idle jets so I can cruise close to 14 however last time I was messing with it I was preparing for a road trip from from Phoenix to LA and back and it was over 100 degrees across the desert and did not want to have to worry about my engine temps.

I have also tried smaller mains and bigger power jets, for me that caused the AFRs to cycle all over the place, similar to what you are describing. Would like to play with that again in a controlled dyno environment sometime. I have also soldered closed the holes in the butterflies.

You may want to try to get your AFRs a bit more stable before you try other vents. Have you read through the wideband thread? there is gold in there.