jrhone

DCOE 40s or 45s???

26 posts in this topic Last Reply

Recommended Posts

I have a modded engine and a set of DCOE 40s that I may want to install at some point. Right now I have a Weber 38 on it. It’s got 9.5:1 pistons, 292 cam, headers and 123 Tune distributor. At what point should you look at DCOE 45s instead of 40s?  

 

 

Share this post


Link to post
Share on other sites

You will want 40's for that application.

 

The point I look at 45's is when we are talking a chunk more aggressive than your current spec.  You can get 45's to run, just that the tuning window is pretty small and you will be choking them down as far as they will go.

  • Like 1

Share this post


Link to post
Share on other sites

I agree with Andrew, the low speed throttle response, mid range torque and drivability will be much easier to tune with the 40's.  Driving around with 34mm chokes in 45's is a bit of a waste.  You also will not need to port the intake manifolds to fit the 40's. 

  • Like 1

Share this post


Link to post
Share on other sites
Quote

At what point should you look at DCOE 45s

 

...at the point where the 40's are slowing you down!

Which, I agree with Byron and Andrew, they won't be.

Plus, you own them.

 

t

Share this post


Link to post
Share on other sites
41 minutes ago, Preyupy said:

You also will not need to port the intake manifolds to fit the 40's. 

I thought DCOE 40s and 45s shared the same base gasket, and most all dual DCOE manifolds fit either one without any extra porting (enlarging port ends, vs simple port-matching).  And enlarging manifold intakes was only required for 48/50/55 DCOEs...

Share this post


Link to post
Share on other sites

Thanks. I was pretty sure 40s would work for my engine best but I do have a spare M10 that I will build at some point and that will be bored and stroked and I was kind of wondering when I should start looking at 45s or FI

Share this post


Link to post
Share on other sites
3 hours ago, visionaut said:

I thought DCOE 40s and 45s shared the same base gasket, and most all dual DCOE manifolds fit either one without any extra porting (enlarging port ends, vs simple port-matching).  And enlarging manifold intakes was only required for 48/50/55 DCOEs...

The port at the gasket between the carb and manifold is 40mm.  If you put 45mm carbs on it you need to open the manifold by 5mm or you will have a step (the wrong way!!) and you will have a horrible time trying to make it run correctly.  This is the first thing I check when someone shows up with 45's on a BMW, you can't believe the number of times this has been overlooked over the 40+ years I have been playing with DCOE's on M10s.  The bolt spacing is the same and the holes are centered the same but that is the BIG difference between a 40 and a 45.   

  • Like 2

Share this post


Link to post
Share on other sites

Thanks, Byron.  Didn't realize that - makers some sense, though I'd assumed it went the other way.  (i.e. most aftermarket DCOE manifolds were already 45mm inlets).  So for 45s you need to trim the base gasket hole?  I thought I'd only ever seen gaskets that matched the 45mm opening, and manifolds that matched the gasket hole...  

 

Tom

Share this post


Link to post
Share on other sites

Gaskets and manifolds I have seen are ~45MM. There may be 40MM manifolds out there. The biggest difference between the 40 and 45 DCOE carbs is the choke size they can accommodate. They call it a choke for a reason. It does seem logical that the 5MM difference downstream from the choke will make some kind of difference in airflow, but I have never seen an objective comparison test of the two, Anyone?

Share this post


Link to post
Share on other sites
7 hours ago, jrhone said:

It’s got 9.5:1 pistons, 292 cam, headers and 123 Tune distributor.

 

I run that same setup with one DCOE 45, the 40's should be good to go!

 

Mark92131

 

 

IMG_1290.JPG

Share this post


Link to post
Share on other sites
19 minutes ago, Chris_B said:

Gaskets and manifolds I have seen are ~45MM. There may be 40MM manifolds out there. 

 

There are plenty of 40mm-specced DCOE manifolds.  The IE manifold, the old Korman manifold, at least one casting variety of the Canon DCOE manifold, and the original 2002 Ti OE manifolds (at least the three sets I've looked at).  

 

Hope that helps, it is certainly something to ask prior to buying a manifold(s).

Share this post


Link to post
Share on other sites

Like most questions of ‘hogging’ manifolds you are at risk of robbing a lot of mid range for a bit more top end. 

 

Definitely no steps between the carb and the the manifold or manifold to head but a bit of a ‘restriction’ (within reason) will help the velocity and atomisation of the charge. 

Share this post


Link to post
Share on other sites

When you reach the limits of your 40mm side drafts cant you bore out the chokes ?  I am running 40mm mikunis and pretty much same engine specs. i was having issues when the engine was maxed out with not getting enough air flow. So, I bored out the chokes to 35mm  and re jetted seemed to do the trick it now pulls hard all the way up to redline. ... Also webber 40s flow better than mikuni 40s .....so ive read

Share this post


Link to post
Share on other sites

On a Weber, the venturi (choke) is a replaceable element.  No boring.  Usually, some real tuning,

though, as the venturi is the primary element in determining fuel draw into the carb.

 

Simeon, I mildly(in the case of 1 racecar, only) disagree about the effect of diameter of short ir manifolds on torque-

my car came with 45 Cannons with 40's, and thinking, like you, that it was costing me, I tried a set of

ti manifolds.  Very close to the same length, smaller diameter.  With rules- mandated 32mm

venturis, no effective difference in lap times. Yes, it's not unlikely there was a little torque/hp tradeoff...

When we were allowed 36's, the Cannons were possibly better- but if so, not by much.

The air horns mattered more, but by far the biggest variable was the airbox!  I gained- and lost-

well over a second in the search for an airbox that filtered AND sucked.

 

DCOEs-  art, science AND luck, and they bolt on!

 

t

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now