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Having just finished a swap, I'm looking forward to this thread.  Still have to get a whole new exhaust system (with AFR gauge) and air socks in place before I can hit the dyno.  (And Tune of course!).  Stay tuned...

1976 Fjord Blue BMW 2002

2002 Topaz Blue BMW M3 Convertible

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40mm vs 45mm

If you want high rpm power 45's are better, but you will probably lose a little of the snappy performance at low rpm and the turning is a bit more touchy.  40's are snappy around town and probaly best for most street cars but run out of breath on the track.  The best dual carb for a warm street engine is the discontinued Weber 42 but they are hard to find.  I have also had good luck with Mikuni 44's but they are not made any more either. 

 

At the risk of a controversy --  We often suggest the weber 38 down draft as it will out run 40's on the track and be almost as good at street speeds.

 

Jeff

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40mm vs 45mm

If you want high rpm power 45's are better, but you will probably lose a little of the snappy performance at low rpm and the turning is a bit more touchy.  40's are snappy around town and probaly best for most street cars but run out of breath on the track.  The best dual carb for a warm street engine is the discontinued Weber 42 but they are hard to find.  I have also had good luck with Mikuni 44's but they are not made any more either. 

 

At the risk of a controversy --  We often suggest the weber 38 down draft as it will out run 40's on the track and be almost as good at street speeds.

 

Jeff

 

With the 38 down draft what manifold do you suggest? Also, on a properly built 292 cammed engine, what do you typically see for peak horsepower/torque and rpm ranges between the  40,45 and 38 carbs?

 

Your response is appreciated

Dean

Edited by 2002dean
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  The best dual carb for a warm street engine is the discontinued Weber 42 but they are hard to find. 

 

what is the value of a 42?  the 40's can take venturis bigger than the smallest that go into a 45, so there is already a several mm overlap in the range of the 40's and 45's. 

 

you have far more carb experience than i Jeff, but i thought it was not about the carb BODY size (40 vs 42 vs 45) tuning is about the range of venturi and jet sizes the bodies can accommodate.

Edited by mlytle

2xM3

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I had this conversation before I chose the side drafts.  I was sitting on a 40DFAV, a Lynx SSD manifold but no 45mm carb, and the dellortos but no manifold(s) or linkage.  After talking over the pros and cons of each, and deciding on the motivating reasons for chosing one over the other, I decided to buy a manifold for the Dellortos.  1. I told myself that to be a true, back in the day type, 2002 owner and tuner, that I HAD to do side drafts at some point in time.  And 2.  I own a coffee shop and we host Cars and Coffee every Saturday morning, this calls for sexy.  Sidedrafts = Sexy.  And so far, the Dellortos have been silky on the street.

 

Regarding the 38/38 out breathing the 40s, that's interesting. Considering the obvious throat count disparity. (ouch! Too much math.. head hurt)

 

But I do know that my daughters friends use 38s on their Chevette dirt track cars and they are WOT 2.0L motors. 

 

 

40mm vs 45mm

If you want high rpm power 45's are better, but you will probably lose a little of the snappy performance at low rpm and the turning is a bit more touchy.  40's are snappy around town and probaly best for most street cars but run out of breath on the track.  The best dual carb for a warm street engine is the discontinued Weber 42 but they are hard to find.  I have also had good luck with Mikuni 44's but they are not made any more either. 

 

At the risk of a controversy --  We often suggest the weber 38 down draft as it will out run 40's on the track and be almost as good at street speeds.

 

Jeff

Edited by eurotrash

2002 newbie, and dead serious about it.
(O=o00o=O)
Smart Audio Products for your 2002

 

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These are my random observations -- as they say "your milage may vary"

 

Based on a street M10 9.5 to 1 & 292 cam --  As to peak HP  40's, 45's and the 38 will make about the same power may be with a slight edge to the 45's.  The choice depends on how the car is to be used.  40's are good at street legal speeds and ok for short blasts.  They are even pretty ok for drive schools.  If you want to get more out of the car at the track 45's are better or even the 38 down draft.

 

One cylinder draws air/fuel at a time.  So the cylinder sees two 38mm barrels vs one 40 or 45mm barrel.  The down draft does not have a nice dirrect path like the side draft, so it has some disadvantage but it will flow more so the engine will be able to rev higher.

 

Some real world numbers-  SCCA ITB 2002's are required to have completely stock engines (8.6 to1 etc) with a 32/36 carb.  A careful fresh rebuild nets 105-110 hp at the wheels and will easily rev to 7200rpm on the track.  In VARA we have a special class for 2 barrel cars which is quite popular.  My #13 car with 11 to 1, 314 cam and 38weber made 143hp at the wheels.  Some other cars in the class with compression at 12.5 are making 155 at the wheels.  These engines will pull right to 8000 although we normally shift at about 7500.  The exact same engines with 45dcoe's will make 15 more hp.

 

If a 40 and 45 carb have the same chokes the 45 will flow more under the same vacuum conditions.

 

Carbs work off the vacuum created by the venturi (choke).  If the difference between the venturi and carb body is small the vacuum signal is weak and tuning can be more difficult.  It also follows the changing venturis changes the vacuum signal so flow through the jets changes.  Bigger venturi needs a bigger jet to get the same flow.

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These are my random observations -- as they say "your milage may vary"

 

 

Good info, Jeff.

 

Another data point: The 38 downdraft Weber has shown to provide equal or greater torque at the lower* end of the racing power band (*somewhere around 5K rpm or so), which may be attributed to the longer intake runners on the downdraft manifold. Depending on track / what car is being used for, this may be favorable.

 

That said, I just finished setting-up a hot street engine with dual 40's. Fun stuff:

 

https://www.youtube.com/watch?v=Ln1WlaUNVBQ&feature=youtu.be

 

(now, I can only straighten out the whole google/youtube account mess I've created....)

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If a 40 and 45 carb have the same chokes the 45 will flow more under the same vacuum conditions.

...and the 40 with 38mm chokes won't meter very well at small throttle openings!

Likewise, a 45 with the smallest chokes you can buy (34's?) will be kinda twitchy at very

small throttle openings, simply because it's hard to GET a very small throttle opening (compared with a 40)

 

But they do ok on the track.  I've used both, and while I pretty much agree with Jeff, I think the falloff with a 40 starts a little

higher- you can (barely) make it into the 6's before they start really to diverge, and while a 40 will certainly SPIN past 7 under full load,

it'll be very restricted compared to a 45.

 

t

Edited by TobyB

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Alright, I'll add my modest experiences onto this.When I first bought my car almost 10 years ago it had:

Once overbored bottom end

Ported E12 head with a 292 cam

9.5:1 CR

MSD ignition

Weber 38/38 downdraft

The previous owner had put it on a Dynojet dyno and the results are attached here. You can see that peak hp was 99 and peak torque was 111. Not that bad actually. Obviously these numbers are at the wheels, so perhaps something like 120-125hp at the wheels??

 

A while back I rebuilt the motor and now it has a 2.3L bottom end from an S14 but still uses the identical head as before, no additional work has been done to it other than fresh valve guides. I'm also now running 45mm ITB's and Megasquirt EFI. I finally put it on a dyno a little while back to do some tuning and that dyno sheet is also attached. As you'll see, peak hp was 131 and torque was 139, so perhaps 160hp-ish at the wheels?? This was also a Dynojet, but obviously at a different location, very different days, different operators, etc... Not empirical of course, but fun food for thought perhaps.

 

 

post-32732-0-98984000-1397277816_thumb.j

post-32732-0-47363600-1397277830_thumb.j

post-32732-0-98984000-1397277816_thumb.j

post-32732-0-47363600-1397277830_thumb.j

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. Not that bad actually. Obviously these numbers are at the wheels, so perhaps something like 120-125hp at the wheels??

. As you'll see, peak hp was 131 and torque was 139, so perhaps 160hp-ish at the wheels??

I think you meant crank/fly wheel, not wheels.

'71 2002 Malaga, fun weekender

'70 2002ti Colorado, Restoration/money pit

'74 2002 turbo in my dreams, sideways...

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  • 3 weeks later...

40mm vs 45mm

If you want high rpm power 45's are better, but you will probably lose a little of the snappy performance at low rpm and the turning is a bit more touchy.  40's are snappy around town and probaly best for most street cars but run out of breath on the track.  The best dual carb for a warm street engine is the discontinued Weber 42 but they are hard to find.  I have also had good luck with Mikuni 44's but they are not made any more either. 

 

At the risk of a controversy --  We often suggest the weber 38 down draft as it will out run 40's on the track and be almost as good at street speeds.

 

Jeff

 

Are they really about the same output? (38/38 vs DCOE)

-Mattio523

 

1976 BMW 2002

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