Webber 38/38 w/ Cold Air Intake

[Fletcher]

In November of 2020, I discussed upgrading my Solex to a Weber 38/38 w/ modifications necessary to retain the cold air intake and still be able to close the hood.  Bill Watson of Road Rockets at Hyperion Raceway, Sonoma took on the project.  The product of his efforts arrived this weekend.  I haven't attempted to install it yet.  I'm curious what thoughts you sages have on it. 

 

Modifications as he described them to me are as follows:

 

• Carb re-jetted for optimum 2002/M10 performance
• Phenolic insulator base plate installed (to keep the cold air cold)
• Aforementioned adapter for cold air intake w/ custom doubler plate
• Custom EGR port block-off plate

 

[Son of Marty]

You're on the right track, It should fit fine, looks like real nice work by Bill also.

['76mintgrün'02]

I'd try to hunt down a pair of original mounting brackets that attach to the rubber isolators on the outer rim of the housing.  

 

@Shiparino took a pair off of his engine when he removed a bunch of smog stuff.  Maybe those are for sale.

Tom

[Mike Self]

2 hours ago, '76mintgrün'02 said:

I'd try to hunt down a pair of original mounting brackets that attach to the rubber isolators on the outer rim of the housing.  

+1.  Without those brackets, the entire weight of the air cleaner is focused on those four small (6mm) bolts that surround the carb air inlet and will stress the carb's pot metal.  Get the brackets.  The factory wouldn't have spent the money to install 'em if they didn't think they were necessary. 

 

Also...the Weber's throat openings (where they meet the air cleaner body) are rectangular, while the opening in the air cleaner is oval.  For optimum air flow, they should match.  And make sure there's some sort of gasket to seal the joint between air cleaner and carb.

[Fletcher]

From Bill:

 

”It is important to use the three short, double-studded rubber mounts n the circumference of the filter housing to provide additional stabilization for it”

 

and

 

”We should discuss the fit into the two original brackets(which you should take off the old intake manifold.”

[theNomad]

Seems like it's on the right track. I agree on sealing the cleaner to the carb and the mount brackets. 

Have you measured HEIGHT? Hood clearance?

 

I wonder about the whole Weber adapter opening being oval vs square as well.. Logic and physics says a larger opening is needed but will the current oval actually limit usable air? Probably only at wide open throttle at 6k rpm. Not sure it'd matter in real world.

 

The original solex and air cleaner design uses the convex bowl to smoothly guide air into the carb 360*.

I've thought the best way would be to maintain that is reshaping/transitioning the inner metal to the square carb opening.

 

You want cooler air, but also enough usable air volume.

The single air cleaner inlet oval is also a smaller area than the carb intake oval. This may not matter if the intake of air is ramming from a larger scoop as the air speed will increase as its pinched down, but if its going through the stock summer/winter box I'd say that's the volume limiter. 

 

What's a "doubler plate"?

Also, care to share the tune? (and engine specs) for us 38/38 guys?

 

Edited September 5, 2021 by theNomad

[Fletcher]

11 hours ago, Mike Self said:

Also...the Weber's throat openings (where they meet the air cleaner body) are rectangular, while the opening in the air cleaner is oval.  For optimum air flow, they should match

Bill:

 

”put considerable time into locating a plate with two holes, matching the the carb and the manifold, only to be acutely disappointed when the package arrived with an oval opening which is the most common gasket and insulator. Rather than further delay the delivery, that common sub-optimal part was installed. It is the only part of the entire assembly that is not optimized.”

[Fletcher]

1 hour ago, theNomad said:

but if its going through the stock summer/winter box I'd say that's the volume limiter. 

I wondered about this part.  Is there a replacement for the stock S/W box to increase airflow? Thx

[esty]

what you have is the adapter from the square weber top to the oval 02 air cleaner opening...to use a weber with the stock air clearer that is the only option i've ever seen...i've used them on every car i've ever owned and was unaware the air intake was not sufficient..

 

unless you are building a car to drive in the Indy 500 where nano seconds make a difference i believe all of the above is hyperbole

 

['76mintgrün'02]

Maybe you should show the rest of the photos you posted in your other thread on this topic, since they show gaskets and spacers and such.

 

https://www.bmw2002faq.com/forums/topic/287545-what-else-should-i-do-while-i-have-the-intake-manifold-off/

 

I wouldn't worry about the S/W, or oval shaped restrictions unless it was a RacEcaR.

 

I do wonder about the clearance between the breather tube on the underside and the carburetor.  I've seen them make contact if the air cleaner is set too low.

 

Tom

[Fletcher]

1 hour ago, '76mintgrün'02 said:

Maybe you should show the rest of the photos you posted in your other thread on this topic, since they show gaskets and spacers and such.

 

[John76]

15 hours ago, Mike Self said:

For optimum air flow, they should match. 

 

What?....you can't get round air into a square hole??

[Milo]

Weber 38/38 ?

Why not a 40/40 ? They make that one as well.

 

Both of the above carbs are an awful match for the 2002. You really narrow your power range where you loose bottom end while dumping a bunch of gas. And a longer duration cam and header only compound the problem. A progressive 32/36 is the best solution, but it is still not as good as a stock Solex 32/32, which delivers optimum performance and gas mileage at about 21-22mpg in the city. A 32/36 will deliver about 17-19 mpg-city while the 38/38 will give you about 15-16mpg.

 

Due the rules associated with the racing classes in the US, many drivers / builders opt to use a 38/38 because it places them in a more economical and competitive class. The classes associated with side drafts are often "Open" which translates to a lot of competition and big $$. It's really a pretty ridiculous method of classing cars, but that's the way they do it. They won't allow you to run a car with side drafts and a mild motor in any class , but the top "open" class, even though the factory built thousands of "ti" variants. So, people opt to use the 38/38 as a compromise even though the factory never  intended these motors to be outfitted with this carb.  While I don't race, that's my take on the matter. Maybe some of the track guys can chime in on this topic. I have some short lived experience with the 38/38. And, no matter what I did to it or how I jetted it, I ended up with a narrow power range and awful mileage.  

 

I know a guy who installed a 40/40. And, that was even worse than the 38/38. It was all or nothing.

 

The side-draft 40s such as the Solex PHH or Weber DCOE actually work better than the 38/38. They can be tuned for any engine once you spend $$ on all the jets, emulsion tubes, chokes, pumps etc. and find the optimum combination. These carbs deliver a more consistent and wider power band than the 38/38, although they are difficult to synch. and require more frequent tinkering. You can get them to deliver decent mileage at constant speed on the highway, but once you get in the city and into the hills, your mileage drops like a rock. The factory 1800ti is rated by the factory to deliver 16mpg - 23mpg., with 16mpg being more realistic. And, that's with the skinny tall tires. The wider and shorter tires most of us are using will further eat into this. Add a cam and a header and you'll be down in the 13mpg and 14mpg range. 

 

In comparison the stock 1600-2 is rated to deliver 22mpg-28mpg. But, I've found that 22mpg is more consistent. You can get better by cruising at a steady 65mph-70mph highway, but at faster speeds it will drop down to 22mpg due to the low 4.11 gearing. This is with modern wide rubber. I think the skinny tires, as originally equipped, would help. 

 

Edited September 6, 2021 by Autostrada

[Fletcher]

No idea why 38/38.  It was Bills suggestion and as he builds high performance BMWs for a living at Road Rockets, I took him at his word and this is what we built.  

[Milo]

3 hours ago, Fletcher said:

No idea why 38/38.  It was Bills suggestion and as he builds high performance BMWs for a living at Road Rockets, I took him at his word and this is what we built.  

 I'm actually a part of the minority who advocates against this carb. The prevailing trend among a lot of 02 performance engine builders is to use the 38/38 with one version or another of a long duration cam and header etc. Few people these days build the motors for economy as they no longer use these cars for everyday transportation.