engine and drivetrain Intake fabrication/theory questions (long)

[Guest Anonymous]

I’m in the process of fabricating an intake manifold based on the same “crossover” concept as the Lynx cast design and I have a few questions for either owners of this manifold or people very familiar with intake design. It will be a steel unit, about 12-15 pieces which will assemble together keeping the ID all smooth and consistant. There will be one 45mm Weber DCOE carburetor fitted to the unit. My first and biggest concern is that of individual manifold tube lengths. It appears that the Lynx Crossover manifold’s setup has varying lengths, meaning that the distance between the head and the carburetor gaskets for cyls. 1,2,3&4 all vary. Venturi A will serve cyl 1 &4 while B serves 2 &3. the distance between 1 & venturi A is significantly less than the distance between A & cyl 4. Likewise with 3 & B being almost a direct shot and 2 & B being longer. Is this the case and is it a problem or something I should not factor into design?

I will be using all = ID tubes to keep the velocity the same between all intakes however I’m concerned that the individual lengths are different. 1 & 4 are much longer than 2 &3 – won’t that make 1 & 4 leaner and 3 & 2 run rich or am I confused? Am I OK as long as I use the same size ID and stagger the firing order with the two venturis? Another idea I thought of which doesn’t really appeal to me is that of volume. Because the distance from Venturi B & 3 is shorter than B & 2, would it be of any value to increase the ID of the #3 tube offering the same volume as B to #2? My thought would be no because larger ID would = less velocity which would mess things up to have 4 different sized intake tubes going to the head. Am I really over thinking this? If I’m going to use a 45DCOE would it be best to use 45mm ID tubing or something that will match the diameter of the intake valve itself?

Finally, would it be better to put smaller individual vacuum ports in each chamber, each with it’s own one way valve (for the brake booster) or would it be best to just run off #4 cyl which would in effect be #4 & #1 cyl.???

Thanks for your input. This will be going on engine #3622929. I’m in the process of acquiring a shell to build so I don’t have a car or car VIN to share with you yet.

[c.d.iesel]

my thought is WHY a single DCOE - with the amount of work involved for the LYNX method - when you should just use a LYNX.

for your project,

more power and tuning would be possible with any straight length (x4) and using duel-DCOE's . Much easier to fabricate ? no?

many books about the subject :

[2761377]

here's a well used analogy for you- an internal combustion engine is basically just a large air pump. the more efficient the air movement, the more horsepower- (for a given displacement).

this analogy probably only articulates your gut feeling- unequal length intake (or exhaust) airflow is PROBABLY less efficient than equal length.

regardless, c.d. is right- get a dual weber set-up, or go with a polished peanut manifold(or hogged out 2 port)- fabrication may be less expensive but these existing systems will most likely work way better. imho

robert

[Guest Anonymous]

I don't want to go to a twin 40DCOE setup because I have heard they are really bad on fuel milage and also even worse for winter driving. This is a year round car.

I've already got the 45DCOE and have already made both mounting plates and have most of the tubes sorted out. I'm doing this so it's not like I'm paying tons of money to have this done it's just that it's hard to figure some of these things out. I assume that running a single 45DCOE will be WAY better than a 38/38 and probably put out almost as much as the twin 40 setup. Can anyone confirm any of this?

[Tommy]

There's not much you can do for the different lenght runners. Still it's better than going to 1-2, 3-4 which would allow equal lenghts easier. I would start with 45mm tube and then branch to 2*40mm which should match the head quite nicely.

Anyway I'd try to keep the diameters equal because velocity has big effect on chamber filling. Drag of 2 longer runners will make some difference anyway. I would not be too worried about runner volumes. If the flow could be considered constant the volume should not effect. Is that oversimplified?

Not much of a help but that's how I would do. I planned it sometime before I found a Lynx.

Tommy

[Guest Anonymous]

I'm worried about different lengths because having 4 different lengths will simply deliver 4 different mixtures to the cylinders which is never that good. Does the Lynx manifold address this issue or are they different lengths as well? I'm not building a forced induction or FI car so I don't need to worry as much about exact mixtures and precise delivery but I don't want to grossly overlook anything either. I have a feeling that if I just go with my gut feeling which was/is exactly as suggested, 40mm tubes with 45mm @ the carb end, i will be satisfied with the results.

Am I correct that if the manifold length to cyl #1 is twice the length of #3 then #1 will recieve a leaner mixture than #3? This is why ideally every chamber in the manifold should ideally be exactly the same length, bend, flow pattern, etc.??

[Tommy]

That's the way it is in Lynx. Difference in length is considerable. Id'say 3 times between shortest and longest. Still I haven't noticed much difference in mixture by observing plugs. I don't know if the length actually effects mixture that much but it effects on flow and that way filling of chamber. More air is sucking also more fuel from carb - correct? Of course identical for every cyl would be optimum -> ITB or dual carbs.

By no means I'm an expert in this kind of discussion. If someone knows better let us know.

Tommy

[Guest Anonymous]

Still going to go ahead with the Lynx style intake but I'll try to lengthen some where I can and shorten others where I can. I've started mocking things up with flexible nylon tubing that was pre-cut at different lengths. Now I'm trying to work out the bends as well as lengths but it's pretty hard unless you space the carb quite a bit away from the head. What's the optimal length from the venturi to the valve?

[peteinjp]

If your really worried about the lengths why not make a common plenum like a tii or 320 or just about any other bmw engine and run 4 equal length runners from there. Or maybe even just adapt one of those plenums and runners to work- they are after all designed for this particular engine and you can be sure that the lengths are appropriate. Maybe a 320 plenum could be adapted. However I'd try to find out more about why it is important to have equal lengths. My header is unequal lengths (supersprint) with the supposed idea that 2 cylinders will resonate well at lower rpm and 2 will resonate better at higher thus spreading the power over the range a bit. Also don't forget that you need to fit the filter and that brake booster is not leaving much room if you distance the carb further away from thee head than say a set of dual carbs.

The dual carbs are not fuel hogs depending on tuning. Of course they are usually tuned to go fast so there goes the milage. However without serious mods the single 45 seems like plenty and will cost less to tune in terms of jets etc.

http://www.google.com/search?hs=TFd&hl=en&lr=&client=firefox&rls=org.mozilla%3Aen-US%3Aunofficial&q=variable+length+intake+&btnG=Search

[Benetton]

I assume that running a single 45DCOE will be WAY better than a 38/38 and probably put out almost as much as the twin 40 setup. Can anyone confirm any of this?

If you are going to run a downdraft carb like a 38/38 or a 40/40 you will not notice much of a difference if you switched to a single 45 dcoe. As for a single 45 putting out as much as a 40 i think you a little confused. IT all depends on the venturi size(which you can change) you can run the exact same venturi size, ie 33mm in both 40 and 45 dcoe's. Yes with the single 45 you are going ot run larger venturi's but you will still have less hp than dual carbs.

[Guest Anonymous]

Just came across this and it caught my eye.

Since the 1-4, 2-3 lengths are paired and they not influenced by the other matched pair, you really only have to focus on making each pair of tubes equal and then, if needed, tuning these two pairs differently. Making the VE of different cylinders in a single motor unequal is actually a goal for some engine builders. It can be used to broaden the torque curve at the cost of lowering the peak. My point is don't get hung up on the fact you can't make them all even. Work with what you can do.

A very important concept to include with equal lengths of the overall tube is equal lenghts around the inside of a single tube. It is worth keeping an eye on at least. You don't want huge discrepancies around the tube or you will surely have points of 'shear' in the flow column. The start of the tube is the valve seat proper, the end is the end of the intake trumpet.

David

[chrome68]

I have a thought, the 2 barrel downdraft can flow a 40/40 right? why not look into adapting that to a sidedraft? I dont think their too long from the block to cause that many issues in room. perhaps really hogging it out, and making an adapter elbow of some sort? kinda like the adaptor plates for ford & chevy to use other carbs on the original manifolds. 02's are simple machines and run great, why not try a simple idea and see if it runs great? I've been thinking about the same thing myself. now that there seems to be nowhere to find a single sidedraft, wich is a great option for many power VS. gas daily drivers. we have to find somewhere to get our stuff.