engine and drivetrain Weber Sidedrafts Wooohoooo

[resra]

Toby, before running the side drafts I was running a wimpy 32/36 and it never fouled plugs. I changed the plugs every 8k-10k miles or so. I know its a fuel issue is because the engine is not just healthy enough to burn through so much fuel being dumped directly into the combustion chamber but after the engine rebuild it will be all fine.

Edited April 30, 2014 by resra

[M3M3]

Sorry resra, but you are incorrect in your diagnosis.

If all you did was change carbs and started getting fouled plugs, The fouling is caused by grossly out of tune carbs, not the engine being old. A rebuild or putting on even bigger 45s will not fix the issue. Get your carbs tuned by someone who has a clue, or buy a book and an o2 sensor and learn how to do it yourself. You will not be sorry you did it.

You are really a case of why side drafts are not a bolt on and go mod.....

Edited April 30, 2014 by mlytle

[TobyB]

Here's what Andrew was saying above, explained further:

 

sidedrafts allow only one barrel to fill one cylinder.  So 40's with 32mm chokes allow a 40mm

bore with a 32mm restriction to flow to each cylinder.  This works well since there's little manifold

to let fuel drop out of suspension. 

 

The 32/36 lets 2 barrels fill each cylinder.  When you're on the primary, you have one 32mm bore with

a 28 mm restriction flowing to each cylinder.  It actually works BETTER than a sidedraft in that the draw thru

the carb is almost continuous, but where it all falls apart is in the manifold- there is significant 'fallout' from the

pulsing in each runner, and going around corners isn't good for a fuel- air emulsion.

However, when your foot's on the floor, you have 32mm PLUS 36mm feeding each bore.  By my math,

you get 125 square millimeters on a 40 (discounting the choke) and 113+100 mm or 213mm on a 32/36.

Or 240mm on a 38/38.

 

So a downdraft flows more.  Really.  That's what Andrew's saying, and he's right.

 

You'll also notice that the difference between the 32 and the 38 is pretty small.  But the

38 is touted as 'a lot more carb'.  A LOT of that is that it's a simultaneous carb, not a sequential

So you get a big kick in the butt immediately, even though I noticed a drop in lap times of around a second,

at most...  And the sidedrafts are simultaneous, too, so you get a nice kick in the butt, and it makes them

lots of fun.

 

You don't make more power from more fuel- in fact, you make best power at something like 13:1 on

an M10.  If you dump more fuel in, you actually LOSE power below about 12:1.  The bottom end's plenty

stout, and can take 15 lbs of BOOST, so it can handle an atmospheric carb with no problems at all.

 

If you get the carbs tuned right on this engine, they will be right on a rebuilt motor.

If you stick poorly tuned carbs on a fresh build, they run a real chance of hurting it.

 

So I'm with Marshall- get them working right, first.  Then see if you need to spend the

money on the rest of it.  A few hundred in jets is less than the tax on a rebuild...

 

just my .02

 

t

Edited April 30, 2014 by TobyB

[LarsAlpina]

Definitely +1 on Marshall and Toby!

[INFIDEL]

Great thread, this makes me want 40s now! Badly!

[resra]

I have a set of Italian Bologna made webers complete with linkage and TMW manifold for the 2002 that I'm considering selling but the ones I have that I bought new is Made in Spain. Should I keep the Italian set and sell the Spanish one? Everyone seems to like the Italian made I'm not sure why. The new ones that are on are running with Canon manifold.

[GirtAllerton]

Ok guys I been following the Dual sidedraft set-up vs 38/38 debate pretty closely as I'am rapidly approaching that part of my build.

Here is what your loyal fans want to know:

 

Dual 40's vs 38/38. Potential Hp/ Seat of the pants feel / MPG - this car will run x-country trips / Top speed/ Drivability for street.

Here is my set-up -

Rebuilt M-10:  Mahle 10:1 pistons/ stock rods & crank.

                       292 Schrick Cam with IE Rockers.

                       MSD Billet Dist with MSD Ignition / wires/ coil

                       121 Head - To be fully ported and gasket matched.

                        IE step header and stainless exhaust.

                        5 speed & 3.91 LSD

 

Toby you came so close with your explaination but left us hanging and wanting the final conclusion. The 38s flow more but? And?

 

Thanks ahead of time and help me sleep better with the right decision....

 

G

[Kidasters]

I have a set of Italian Bologna made webers complete with linkage and TMW manifold for the 2002 that I'm considering selling but the ones I have that I bought new is Made in Spain. Should I keep the Italian set and sell the Spanish one? Everyone seems to like the Italian made I'm not sure why. The new ones that are on are running with Canon manifold.

 

the answer is here:

 

http://www.bmw2002faq.com/topic/151308-best-dual-sidedraft-carb-setup/page-3

 

Keep the italians.  Sell the Spanish ones.  On e-bay.   

[resra]

Thanks for the suggestion

[TobyB]

 

 

Toby you came so close with your explaination but left us hanging and wanting the final conclusion. The 38s flow more but? And?

I dunno.  Upta you.  I had trouble getting mine to cruise well- but I've learned a bit more since then.

At the time, my conclusion was that the 32/36 was a better street carb.

 

On the 'one day' project list is to make a split plenum to try running a 38/38 on-

one bore for 1&4, one bore for 2&3.

 

It's only time and welding, right?

 

As for what you SHOULD do, it's obvious- sidedrafts!  They are just so much sexier.

Plus, you get to buy a Synchrometer, some jets, maybe a wideband oxygen meter,

and by the time you're done, you really understand how carburetors work.

 

 

t

Edited April 30, 2014 by TobyB

[AceAndrew]

GIRT - Toby's response was in conjunction the link earlier in the thread, this is the same link again.

http://www.bmw2002faq.com/topic/152828-the-dyno-data-and-motor-build-thread/page-2

 

Given your "street-hotrod" engine specs you'll find the street useable power band between the 38/38 and the dcoe 40's very similar.  There's not really any debate, neither is "better".

 

Sidedrafts 40DCOE's
POSITIVES:
-would give you a bit quicker throttle response
-intoxicating noise
-the sexy factor
NEGATIVES:
-cost
-learning curve a little longer than downdraft

 

Downdraft 38/38
POSITIVES:
-great low budget value
-Shorter learning curve
NEGATIVES:
-not sexy

--------------------------------------------------

Kid, read a little deeper.  Any more it's a trade off given the Italian's aren't getting younger.

specifically the following ....

"I think the Spanish guys have been working on there molds (and general qc) as they seem to be a bit cleaner then they once were.  As for the Italian vs. Spanish carbs, with time marching on I'm seeing more and more corrosion on the old Italians (especially if they've been sitting for a while) so in my mind it's a toss-up depending on individual carbs and not "where they're made".  Before too long we will also have to contend with Chinese knockoffs."

[Kidasters]

Andrew -

 

inebriated gibbons

 

'nuff said.

[76ohtwo]

Anyone on here daily drive a car with sidedrafts? What's it like?

[Harv]

I did for years. 40's. Don't remember the specs. 125 mains? 121ti head with ti pistons 9.7:1? Isky copy of the BMW sport 300 cam. Would pull over 6k in 4th with a 3.64 rear. A big part is getting the idle jet size right. Then venturies and mains. And air corrector........

If the weather was better I could easily daily drive my 912 with 40IDF's. It has .060 idle jets 34mm venturies 125main 180 air corrector. That's on a 1600. Idles all day and can lug through traffic.

John

Edited May 2, 2014 by Harv

[76ohtwo]

I did for years. 40's. Don't remember the specs. 125 mains? 121ti head with ti pistons 9.7:1? Isky copy of the BMW sport 300 cam. Would pull over 6k in 4th with a 3.64 rear. A big part is getting the idle jet size right. Then venturies and mains. And air corrector........

If the weather was better I could easily daily drive my 912 with 40IDF's. It has .060 idle jets 34mm venturies 125main 180 air corrector. That's on a 1600. Idles all day and can lug through traffic.

John

Nice - I have a 3.64 open diff and E21 5 speed. I think they'll be put on when the motor is rebuilt - whoever does it will be a professional and have experience doing them so they're properly set up. I'm planning 9:5:1 pistons and a 292 cam. Should be a quick little car by next spring.