Older bayless dual dcnf intake question

Kevin Cozzo

True Classic
hey guys, i'm looking at a set of dual dcnfs with intake to finally buy...birthday is coming up, and I found a setup that hopefully will work good, for a decent price, as I can't afford new, even tho I would really like that. The carbs are weathered well, and I'll probably rebuild em, and clean em up, and have a one piece airbox made by a local sheetmetal guy that has been awesome so far for custom stuff.
here's the question- I asked the seller about the intake, which has no markings, and he said he got it at bayless, made for an X...Does anyone know if those made BITD were the exact angle, or a compromised angle? I know many have said they had no problems with the split angle thing...any opinions on this would be welcome also
And of course if anyone has a pair of DCNF's with a water jacketed sprint intake for under 650, I'd be interested in that also, LOL...I missed the ones Matt had... grrrr
 
Hi Kevin,

I have run the split-angle PBS manifold for over 20 years now, on 3 different engines with no problems.

I am curious, why do you want water jackets?
 
but Ulix, you have no reference point to compare them to... if that's all you've known. Split angle manifolds WILL result in a compromise of the float level, no if's or buts.

SteveC
 
AHHHH...you guys aren't helping! hahahaha I found a setup on Ebay (if no one outbids me) I'm sure I can make it work as seller has stated, and others, such as Ulix, that they've had no problemos. just wondering if the old PBS manifolds were split angle...
Steve, you asked why I wanted the water jacketed version....the truth: I don't know...I kinda figured that it would be as close to the original design. Of course, I never figured out WHY they put water jackets in the inlet manifold...Is it to heat up quicker, or to keep it cooler?
 
PBS is a "split the difference" manifold... IMO Kevin hang out for one that is the correct angle, or have the manifold you buy re machined to make it correct if it isn't... float level is kinda critical!

and no you can not make a wedge spacer ... that just doesn't work... anyone that suggests that has no idea...

to correctly re jig the manifold requires the stud holes on the carb mounting face to be welded up, the face re machined at the correct included angle, and then the stud holes re drilled and tapped.

Water jacket is for fast warm ups, in very cold climates...I don't think in TX it's going to be of any advantage to you TBH.

SteveC
 
Last century I made a wedge to go between the intake manifold and carb of my Honda SL-125 dirt bike so the bigger carb would clear the frame tube. As Steve says, it's not really a good idea and the mounting studs will then be at the wrong angle.
 
I had a pair of DCNF with a phenolic wedge spacer. It serves me well for years. I admit it had to be done correctly since the studs holes had to be rethreaded to sit in the right angle. But if done right, it will work fine. So yes, the best choice is to get the right manifold, but if you can't find one or you got a really good deal on a 128 one, I would consider a wedge spacer but with the related modifications.
 
PBS is a "split the difference" manifold... IMO Kevin hang out for one that is the correct angle, or have the manifold you buy re machined to make it correct if it isn't... float level is kinda critical!

and no you can not make a wedge spacer ... that just doesn't work... anyone that suggests that has no idea...

to correctly re jig the manifold requires the stud holes on the carb mounting face to be welded up, the face re machined at the correct included angle, and then the stud holes re drilled and tapped.

Water jacket is for fast warm ups, in very cold climates...I don't think in TX it's going to be of any advantage to you TBH.

SteveC

Hey Steve, I just found your ad on ebay, might be contacting you...you should toot your own horn...didn't know you had these intakes. I want this to be easy, was hoping to find a complete setup, but alas thinking ordering your correct angle intake, as thinking about it, yes correct float angle would be better. I did find a pair of 36 dcnf's....I have always thought that dual 40's are overkill, but that was before I realized how the dual carb thing worked- one barrel per cylinder, not just more fuel, but also airflow. My engine is a 1.5, has larger intake valves, and a eurospec cam, nearly flat pistons, and slightly shaved head...wondering if I should wait for a pair of 40's now. I was always under the impression that for that much carburetion, the engine would have to be heavily modified....even a pair of 36's should still be more flow than a single carb tho, correct?
 
36's or 40's... really depends on the rest of the motor which size is best... with the stock euro cam I would opt for 36's to keep the air speed higher at lower revs, it will be more responsive and won't need to hit revs before it starts to run nice.

I don't like to do the hard sell Kevin, otherwise it may seem to some that I'm telling what I'm selling ... I'm more about guiding people along ... I always say the advice comes free, it doesn't mean you have to buy anything from me..if people do then that's all good.

36's are usually a little cheaper too... if your not going racing but want the pick up and feel (and sound and visuals) of twin carbs then the 36's will do nicely.

Check whatever you buy...
1) are a matched pair...you'd be surprised how many mis-matched odd webers there are out there in the marketplace
2) find out the details of the carbs innards.... choke sizes, auxiliary venturi size etc...you might be lucky and get something that is very close to requirements, but if they are not at least you'll know what your up for when they arrive. buying four chokes and four aux venturis to setup the "cheap" carbs you bought could make them quite expensive in the end
3) have the correct carb interlink (balance) setup... these are hard to find and new they are quite expensive...
4) haven't been abused by a ham fisted mechanic... make sure all the parts are there and nothing is broken or missing... if you can get pics or look inside the carbs and make sure they havent got any signs of corrosion and things like the float pin tabs aren't snapped off... this is more common that you'd like to think.

I'm away in Thailand for another 6 weeks or so... but if your keen on a manifold PM me and we can get the ball rolling

SteveC
 
I'm with Steve about the 36's. Even with a modified engine unless you really intend to go racing, 40's are overkill and maybe harder to tune for "relax" driving.

There was a time where a lot of X1/9 with DCNF's and owners were deceived about them. You can't expect the DCNF's to transform your car unless the remaining parts are matched. I mean, you probably need a bigger cam, some head modifications (bigger valves and inlets/outlets), headers, a little more compression...

Just a pair of DCNF'S will be there mostly for the look and the sound. But you have to start somewhere!
 
I'm running 32mm chokes on my DCNF 40s so I'm not sure it would perform much differently than 36s. Also, there are different flavors of DCNF which can affect how much clearance you need vertically. I'm using the flattop version where only the aux venturis stick up so air is unimpeded when using a low profile air intake.
 
The size of the chokes is what matters most and which must be matched to the rest of your engine and to its intended use.

I have not yet found a good explanation what the difference would be between running 36s or 40s both with, say, 30 mm chokes.
Weber suggests the range of choke sizes that work "best" with a carb size. But that doesn't answer the above question.

Has anyone read anything on this subject?
 
The size of the chokes is what matters most and which must be matched to the rest of your engine and to its intended use.

I have not yet found a good explanation what the difference would be between running 36s or 40s both with, say, 30 mm chokes.
Weber suggests the range of choke sizes that work "best" with a carb size. But that doesn't answer the above question.

Has anyone read anything on this subject?

Yes, Weber suggests the range of choke sizes that work best with each carb body/butterfly size...as it affects how well fuel is atomised and held in suspension, for best results the "quality" (i.e. small fuel particle size and well homogenised mix) of air/fuel mix makes more difference that the nett quantity...

If it made no difference to the overall effectiveness of the carb, then Weber would simply need to make a "one size fits all" carb and we would simply alter venturi sizes to suit our applications... but that isn't the case...is it?

SteveC
 
IMG_0237[1].JPG

This is a chart that Weber made that I got out of Pat Braden's book. Same chart is in John Passini's Weber book. John says to err on the smaller side for rally which I would take to mean street use too and larger for racing. It just addresses venturi size not carb size. a 1500 would have a 375cc cylinder. Even John admits in his book that this chart is not the last word and a lot depends on the tuning of the motor. I would guess for the same size venturi, say for example a 32, a 40 carb would create a larger venturi effect than the 36 carb due to the greater difference in neck down.
 
well then that answers that...need to change my ad...THX, I always wondered
Actually, it is not for cold-start/warm up. When the car is started, the coolant is just as cold as the rest of the motor so it doesn't heat up the manifold to pump cold anti-freeze through it. It is there to heat the manifold after the coolant is warm. The expansion of a gas/fluid across the throttle plate is accompanied by a large drop in temperature. I can't hardly recall thermodynamics anymore, but I think it is derived from the first law. Think of how a pneumatic tool gets cold in your hand when you use it. A cold manifold can causes the fuel to come out of suspension and condense along the walls of the manifold and un-atomized (liquid) fuel doesn't burn nearly as well as atomized fuel does. Also, if the weather conditions (humidity and temperature) are just right, the heat absorbed can cause icing of the throttle plate which is why even fuel injected cars sometimes have heated throttle plates. Google "carburetor icing" and "heated intake manifold" or "heated throttle plate" for more information. The Fiat SOHC motor being counter-flow obfuscates a lot of the need for the water jacket as the exhaust manifold is in such close proximity to the intake but it is still helpful particularly in cooler climates. Would you notice it gone? Maybe not in Texas but you might in Minnesota. It certainly does not hurt anything to have it which is why just about every car on the planet left the factory with a heated intake manifold from about 1920 (way, way, way before anyone thought about emissions) to the last throttle-body injected cars in the 1990's. And as mentioned, some still have a heated throttle plate to this day. Discount that if you must but a lot of smart people have thought a lot about this for a lot of years.
 
Actually, it is not for cold-start/warm up. When the car is started, the coolant is just as cold as the rest of the motor so it doesn't heat up the manifold to pump cold anti-freeze through it. It is there to heat the manifold after the coolant is warm.

I would have thought the coolant passages were to cool the manifold due to its proximity to the exhaust manifold. I thought it was another item to help reduce that temp similar to the the stock heat shield that tends to rust away. Why would you want the air to expand before entering the engine? Wouldn't this be the same reasoning as a "cold air intake" that always adds 15 hp?

I guess to partly answer my own question: most engines are now direct injected. There is no risk of the fuel condensing.
 
Actually, it is not for cold-start/warm up. When the car is started, the coolant is just as cold as the rest of the motor so it doesn't heat up the manifold to pump cold anti-freeze through it. It is there to heat the manifold after the coolant is warm. The expansion of a gas/fluid across the throttle plate is accompanied by a large drop in temperature. I can't hardly recall thermodynamics anymore, but I think it is derived from the first law. Think of how a pneumatic tool gets cold in your hand when you use it. A cold manifold can causes the fuel to come out of suspension and condense along the walls of the manifold and un-atomized (liquid) fuel doesn't burn nearly as well as atomized fuel does. Also, if the weather conditions (humidity and temperature) are just right, the heat absorbed can cause icing of the throttle plate which is why even fuel injected cars sometimes have heated throttle plates. Google "carburetor icing" and "heated intake manifold" or "heated throttle plate" for more information. The Fiat SOHC motor being counter-flow obfuscates a lot of the need for the water jacket as the exhaust manifold is in such close proximity to the intake but it is still helpful particularly in cooler climates. Would you notice it gone? Maybe not in Texas but you might in Minnesota. It certainly does not hurt anything to have it which is why just about every car on the planet left the factory with a heated intake manifold from about 1920 (way, way, way before anyone thought about emissions) to the last throttle-body injected cars in the 1990's. And as mentioned, some still have a heated throttle plate to this day. Discount that if you must but a lot of smart people have thought a lot about this for a lot of years.
Ideal gas law: PV=nRT
 
I would have thought the coolant passages were to cool the manifold due to its proximity to the exhaust manifold. I thought it was another item to help reduce that temp similar to the the stock heat shield that tends to rust away. Why would you want the air to expand before entering the engine? Wouldn't this be the same reasoning as a "cold air intake" that always adds 15 hp?

I guess to partly answer my own question: most engines are now direct injected. There is no risk of the fuel condensing.

Nope. Not for cooling. Heat shield is for the surrounding parts. Phenolic spacer is for the carburetor. Intakes like to be warm.

The throttle plate expands the air i.e. there is less pressure after the throttle than in the atmosphere so the gas/fuel mixture expands as it passes the throttle plate and absorbs energy as it does so. Or are you thinking I meant that the heat of the intake expands the air? There is a difference between a cold air charge and a cold manifold. I would be quiet surprised; no flummoxed* if there was a measurable increase in power from defeating the coolant circuit in an intake manifold on a Fiat X1/9.

Yeah, with the popularity or port injection and now direct injection, there is no danger of condensing fuel on the runners but there is still the possibility of the humidity in the air turning to water ice on the throttle plate so some cars still heat just the throttle plate.
* I just like the word
 
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