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Pedal Smasher
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Thanks for the comments; it's a fun project. I actually refer others to this forum for tuning weber 32/36 or 38/38 carbs.
If you don’t mind me asking, why did you pick the Weber DGV over a Holley 2 bbl or 4 bbl carb?

I think it’s popular among Opels because it became a trend a long time ago, probably due to Weber DGV carbs being cheaper than Holley. But Holley carbs can support engine displacements greater than 2.4L, which is roughly what the DGV was designed up to.
 

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Discussion Starter · #63 ·
If you don’t mind me asking, why did you pick the Weber DGV over a Holley 2 bbl or 4 bbl carb?

I think it’s popular among Opels because it became a trend a long time ago, probably due to Weber DGV carbs being cheaper than Holley. But Holley carbs can support engine displacements greater than 2.4L, which is roughly what the DGV was designed up to.
For one thing, the Weber bolts directly to the Opel/BMW/Pinto intake manifolds.

Throttle linkage is usually easier.

The 38 DGAS was originally fitted to German/English Ford V6 engines, either 2.5 or 2.8 liter. It should be fine on most 2.4 liter Opels.

If you’re twisting high rpms from a high compression 2.5 liter engine, then yes, the Holley 500 is an easier path at that point.
 
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If you don’t mind me asking, why did you pick the Weber DGV over a Holley 2 bbl or 4 bbl carb?

I think it’s popular among Opels because it became a trend a long time ago, probably due to Weber DGV carbs being cheaper than Holley. But Holley carbs can support engine displacements greater than 2.4L, which is roughly what the DGV was designed up to.
For one thing, the Weber bolts directly to the Opel/BMW/Pinto intake manifolds.

Throttle linkage is usually easier.

The 38 DGAS was originally fitted to German/English Ford V6 engines, either 2.5 or 2.8 liter. It should be fine on most 2.4 liter Opels.

If you’re twisting high rpms from a high compression 2.5 liter engine, then yes, the Holley 500 is an easier path at that point.
Exactly as RallyBob states the Weber is most common for BMW as well. There are a few guys that run Holley on 6cyl cars. Lately though EFI has become the norm along with turbos. Not much in the carb arena. I'm an all motor guy, and for this particular car and build the carb was the simplest answer. I already had the 38 so wanted to see if was possible to fuel the build. Also it gives that weird look to competitors at events when they realize its just a single downdraft carb.

Although the 38 bolts up using older 2002 model bmw carb manifolds, still have to die grind oval out the divider to fit the larger 38 butterflies vs the stock 32/36 most of those cars came with.

Eventually if too much cam say 316 or 336 (9mm-12mm lift; 116/132 deg overlap) it's recommended to go dual side drafts or ITB EFI. I'd imagine the carb wouldn't feed enough fuel and run out of headroom. Not only that the single would probably puke more fuel than it takes in :)
 

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Discussion Starter · #65 ·
Exactly as RallyBob states the Weber is most common for BMW as well. There are a few guys that run Holley on 6cyl cars. Lately though EFI has become the norm along with turbos. Not much in the carb arena. I'm an all motor guy, and for this particular car and build the carb was the simplest answer. I already had the 38 so wanted to see if was possible to fuel the build. Also it gives that weird look to competitors at events when they realize its just a single downdraft carb.

Although the 38 bolts up using older 2002 model bmw carb manifolds, still have to die grind oval out the divider to fit the larger 38 butterflies vs the stock 32/36 most of those cars came with.

Eventually if too much cam say 316 or 336 (9mm-12mm lift; 116/132 deg overlap) it's recommended to go dual side drafts or ITB EFI. I'd imagine the carb wouldn't feed enough fuel and run out of headroom. Not only that the single would probably puke more fuel than it takes in :)

When your engine is really breathing well there are still some tweaks that can allow the 38 DGAS to still work satisfactorily.

At some point you need to allow more fuel into the fuel bowl or you will starve the engine. I used to run a 2.5 mm Grose-jet inlet needle in my DGAS. I also upgraded all fuel lines and the fuel inlet fitting to 3/8”.

On a track-only car, you can obviously delete the choke plates. But I’ve cut the choke tower completely off the top of the carburetor before for added airflow.

One of my favorite mods on a bigger or higher rpm engine is to bore the venturies out from 27 mm to 1-3/16” (30.16 mm) and then blend it back in.

I’ve also trimmed the throttle shafts flatter (like on the older Italian 32/36 Weber’s), and installed button-head screws to secure the throttle plates.

The auxiliary venturies can be streamlined significantly. The fuel flows on one side only, so the opposing support arm can be trimmed to virtually nothing.

All in all you can get around 370 cfm from a 38 DGAS modified this way, up from 330 cfm stock. Though I don’t recommend this on a mild engine, a larger displacement engine with high compression gets a strong boost.
 
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When your engine is really breathing well there are still some tweaks that can allow the 38 DGAS to still work satisfactorily.

At some point you need to allow more fuel into the fuel bowl or you will starve the engine. I used to run a 2.5 mm Grose-jet inlet needle in my DGAS. I also upgraded all fuel lines and the fuel inlet fitting to 3/8”.

On a track-only car, you can obviously delete the choke plates. But I’ve cut the choke tower completely off the top of the carburetor before for added airflow.

One of my favorite mods on a bigger or higher rpm engine is to bore the venturies out from 27 mm to 1-3/16” (30.16 mm) and then blend it back in.

I’ve also trimmed the throttle shafts flatter (like on the older Italian 32/36 Weber’s), and installed button-head screws to secure the throttle plates.

The auxiliary venturies can be streamlined significantly. The fuel flows on one side only, so the opposing support arm can be trimmed to virtually nothing.

All in all you can get around 370 cfm from a 38 DGAS modified this way, up from 330 cfm stock. Though I don’t recommend this on a mild engine, a larger displacement engine with high compression gets a strong boost.
I'll look into the bore size of the main venturi's and consider boring them when the motor is apart again. Also the fuel line yeah I keep getting reminded of how should of ran 3/8 vs 5/16 since not really pressure but volume of flow to the bowl. Have 250 needle for the bowl and surge tank keeping head on the pump has helped a lot to get where we are now.
Yup no choke assembly at all with holes jb welded and sand profiled with the venturis and short horns.
Low pro plate screws on the butterflies here also.

Thanks for the other tips. Will not be able to help it over winter to review the carb again with the cam install.
 

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Discussion Starter · #67 ·
Some of the mods I’ve described are pictured here in my Flickr photo account.

 

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Bob, first I want to say I have looked through the wealth of knowledge you post here (and it didn't take me a full pot of coffee, only half a pot morning and about a 6pk of beers evening). I did come across your photos along with previous posts where specific 38 weber mods are mentioned. Very nice fab skills on custom air cover I'll add. Much respect.

So bears my direct question about boring the fixed venturi from 27mm to 30.1mm (1-3/16). Does it make enough difference for me to justify it? I see where you've done this to many 32/36 carbs as well for the larger 36 barrel. That does make me want to do this so I tried to use calculators and see a relationship of throttle sizing vs venturi sizing. Main thing to me is yes messing with the throttle relationship ratio vs main venturi reduces the effect of the booster ie getting on the main. This shouldn't be an issue if the motor is larger displacement and or high velocity ported large cam(high compression) etc to match. I believe we could benefit from doing this along with a few other tweaks. Motor has plenty of off idle grunt and very high velocity low end. Spends most of its time above 4k rpm. The idle range to 4k is almost a blink of an eye which also indicates the effectiveness of high velocity inlet along with combo of a short gear ratio rear end.

For the board
=================================
I'm not directing this calc content at anyone but posting this cause it's what I've found in the past 24hrs. Feel this is a decent place to park this content in hopes to contribute something here.
I feel our engine is very high velocity. Most of these calculators figure for single cylinders requirements so using ITB's vs 2BBL. Would we just add the 2BBL together since they feed all cylinders? I'd believe so.

Math of calculators gives a picture but doesn't add up exactly. As a few Pcar guys have also found. Calcs seem to ballpark it.
  • Throttle bore diameter: D = (.8 to .9) x (V x n)^.5
  • Main venturi diameter: d = (.7 to .9) x D
  • Where:
    • D = Throttle bore diameter, mm
    • d = Main venturi diameter, mm
    • V = cylinder displacement, liters
    • n = engine speed, RPm
Using our 2.3L engine per cylinder requirements
TB Dia = .85 x (.575x8000)^.5 = 57.7mm
Venturi Dia = .8x57.65 = 46mm

Another calculator respected in Pcars is Bruce Andersons 'Porsche 911 Performance Handbook' (p154 venturi and throttle sizing for webers)
Optimum venturi size in mm = 20 x ((cylinder capacity/1000) x (peak power rpm/1000))^0.5

Venturi = 20x(.575x8)^.5 = 42.9

The factory 2.3L S14 engine from BMW comes with 46mm ITB's. Sure it's fuel injected but gives an idea of throttle sizing. Issue is this is large due to the motor had twin overhead camshafts and very well ported from factory. The engine we are using is now the same displacement but very different flowing single overhead cam head on it (M10). The twin cam engine also is 16v using 37mm(x2per cyl) intake and 32mm(x2per cyl) exhaust. The single cam head on our engine is 46mm intake 38mm exhaust.

Last but not least; for more generic branding a calc for cfm of carb to engine size.
The formula for calculating how much CFM (cubic feet per minute) an engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.
Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110%.

2.3L
CFM = 140.355 x 8000 x .95 / 3456 = 308.7
According to this we have plenty of carb and would never need dual side drafts or any "mods" :rolleyes:

Looking into these calculators has really not proved much to me especially since many that build an engine like ours use weber 45mm side draft ITB's where the chokes range up to 36mm PER cylinder.

Only other thread I found is on Help Wanted: Over-boring a Weber 32/36 dgav
User and avid poster/racer on there is Graham who I respect a lot(even though I don't know him personally, isn't the internet fun). He's taken the BMW M10 engine about as far as anyone I know for a NA motor. That being said I believe they missed the on the 38 working well on 4cyl engines.
 

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Discussion Starter · #69 · (Edited)
So bears my direct question about boring the fixed venturi from 27mm to 30.1mm (1-3/16). Does it make enough difference for me to justify it? I see where you've done this to many 32/36 carbs as well for the larger 36 barrel. That does make me want to do this so I tried to use calculators and see a relationship of throttle sizing vs venturi sizing. Main thing to me is yes messing with the throttle relationship ratio vs main venturi reduces the effect of the booster ie getting on the main. This shouldn't be an issue if the motor is larger displacement and or high velocity ported large cam(high compression) etc to match. I believe we could benefit from doing this along with a few other tweaks. Motor has plenty of off idle grunt and very high velocity low end. Spends most of its time above 4k rpm.
The first car I did this to used a 2 liter engine with 11.2:1 compression. I had simultaneously swapped cams and gone from an automatic transmission to a ZF 5-speed. The performance increase was massive from before with a stock 38 DGAS, but again…multiple things changed at the same time.

This car was autocrossed and did track days. It was also my 4-season daily driver (in Connecticut) for 7 years. I drove to every event I ran. Ran it when it was 10 below zero. Never failed to start immediately.

The intake manifold was extensively ported and welded. I used a 2-hole insulated carb spacer that was 3/4” thick. I experimented with 1/4”, 1/2”, and 3/4” spacers. The 3/4” spacer was the best for overall power, the 1/4” was better ‘in town’, but honestly it was still plenty quick. It pulled from 2000 rpms quite nicely and strong to 7000 rpms. It sounded very good, and in fact most folks assumed it had twin DCOE’s under the hood.

At some point I swapped back a stock 38 DGAS because the state had begun instituting IM240 emission testing for older cars. No way it would pass with the modded carburetor! With a proper lean-out, 87 octane fuel, retarded ignition timing, and yes…a catalytic converter, I got it to not only pass emissions, but it barely registered on the meter.

After the test, but before putting my modified 38 DGAS back on, I ran the car with the stock DGAS. It ran ‘OK’, but by comparison to the modded carburetor it was a dog.

Regarding the CFM ratings. Twin carbs with individual runners will almost always run double the amount of airflow or more than a single carburetor. But therein lies the rub…a common open plenum intake can potentially produce more top end power with a smaller carb than a pair of sidedrafts.

Particularly with high overlap cams and a correctly sized plenum…a single intake port can essentially get a much bigger air/fuel charge from the plenum design than a single throttle plate per cylinder due to inertia tuning.
 
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First video doesn't show. Yeah holy old school man. Good stuff. I've an old shaky video of early 2004 autocross using a camcorder.

What do you think about the turbosport uk link at the end of my post? They mention the idle air bleed increase to 145 even 150 to get rid of overfueling with light throttle off idle. Supposedly gets rid of the flat spot. Same area the F66 tubes fix by moving the hole. I'm sure with F50 tubes the air bleed would help and use a different idle jet. Most even in the bmw world use a f50 and just over jet the idle. Seems a backwards way to band aid the issue vs just change to F66's.

I'll have to remember to post once get things all together if my port job goes successful. Thanks for all the insight Bob.
 

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Pedal Smasher
1973 Opel GT
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Maybe we need to turn this BMW owner into an Opel owner. He would fit right in with our community.
 

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Pedal Smasher
1973 Opel GT
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Thanks man! I'll take an Opel GT forum sticker and put it visible inside the racecar. Serious do you guys have any?
There are a few Opel GT race cars but the most famous of them is Tinyvette. Mike Mier owns the car and races it in the 24 Hours of LeMons. He does have stickers for it and he runs Opel Motorsports Club. I don’t know if I’ve seen any forum stickers.


You could also rebadge your BMW racecar as an Opel. Could be fun to screw with people like that. LOL. Make your competition get caught up in why a BMW has been labeled as an Opel and distract them. But, I think the 24 Hours of LeMons has more effective ways of distracting the competition...

 

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Plot thins and comes to an end. Still browsing the interwebs on this and stumbled then it stared me right in the face.

On a few forums it was mentioned that early 38 DGAS webers they had a 27mm choke and later ones have 30mm. Mine was purchased early 2000's. It was also noted the needle valve on early ones is 200 and later models 250. I put dial calipers and just laid it over top to see and sure enough mine are 27mm. I also had to change the main needle valve from 200 to 250 during my build. I'm going to bore mine now for sure. Looks like a very few (Bob and others) figured this out before the later carbs were released.

A screenshot of the current weber offerings from Topend Performance shows this for baselines.



So begs the only question. If weber followed their own base ratio rules venturi vs throttle size for vacuum; why did they put too small of fixed choke initially. Weber states throttles should be sized 1.25 x choke size. Therefore throttle of 38mm/1.25 = 30.4 choke/venturi not 27mm. A few other forums for BMW's (E9) 3.0-3.5L 6cyl using dual 38's noticed 6k rpm power falling off when modding the motor. In a nut shell it was using early vs late model carbs that the 27mm choke was restricting air flow.

So made this :cool:



I'll have it laminated and put in the race car later, but for now. . . I taped one on to get sizing large enough where it will go. Should show up in videos and give interesting conversation trackside for those who wonder wtf. Led shop lights are a little bright this morning.

 

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Discussion Starter · #76 ·
I think Weber kept the 38 DGAS at 27 mm venturi size because it was essentially a bored out 32/36 DGV. To go much bigger and retain OEM reliability requires a new lower carburetor casting.

Early on I experimented with even larger secondary venturies on the 32/36, and as a result broke thru the casting. I was still able to use the casting by epoxying the holes and blending them, but for an OEM that would be unsuitable.

So that requires new tooling for Weber. Which I’m sure they didn’t want to spend the money on, with carburetors essentially being a dying market for them.
 
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