Bell mouth ports?

A

Al Seim

Well-known member
Next exciting inquiry -

On at least one factory high performance engine (Ford SVT 2.5 w/ extrude honed ports) the head intake port inlets are slightly bellmouthed (maybe a 1/8" radius?) presumably to make things less sensitive to misalignment upon assembly.

Given the difficulty of exactly port matching head and intake (or exhaust) ports - I wonder if this is not a good idea? Especially on LP engines, where - on the intake side anyway - simply stepping up the size is not an option.

Has anybody (ideally a flow bench owner..) tried this? In other words, is there any significant reduction in flow if one puts a bellmouth radius on the downstream side og an intake port? It stands to reason that there would be a gain - if slighltly misaligned. So if there is little if any loss when correctly aligned this would seem like a good idea...

Al Seim
1.6 VW Scirocco
www.Race-Technology.com
 
If I understand what you are describing, it should help to prevent reversion. This is explained in Vizard's book in the section on exhaust and states improved flow with these kinds of mismatched exhaust ports. These kinds of steps are also suppose to be useful on carburated intakes by helping to get fuel off wetted walls back into the air/fuel stream again.
 
Might work on intakes but looks like it is the wrong side for exhausts. "Belling" the header is more what Vissard is describing there.
Does this meet "port matching" as allowed in LP? Might pass as a "crude" job of matching.
 
Frogeye":2mjsd7vv said:
Might work on intakes but looks like it is the wrong side for exhausts. "Belling" the header is more what Vissard is describing there.
Does this meet "port matching" as allowed in LP? Might pass as a "crude" job of matching.
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Of course, direction of flow is important to which side is larger. If it was on the wrong side it could actually restrict flow.
 
Al, on the MG 1275 heads, they are somewhat already like this due to the ports working their way around the pushrod bores in the head. The 1275 have locating rings to locate the intake manifold to the head, both the intake port and the head side of the intake have steps on them for the locating ring to sit in, what I do it make sure there is a smooth transition with the locating ring on both the intake and intake port. I think the idea is to make the 1" as good as you can as far removing lumps and bumps, and make the transition from the locating ring have no ledges to have to get over. I see no benefit in going alot bigger in the 1" allowed area than the rest of the port, this would do nothing but create a stall area, in short you would not want a the 1" gasket porting considerably larger than the rest of the port, as it would slow down velocity. The gasket porting alone in our heads does not create a big benefit, but some gains, I don't know if I ever flow tested just the port matching alone, but after a good valve job is done, and the port matching, cfm numbers in our heads jumped a couple of points, of course different head designs would produce different results. I think the main thing to find the premium, best flowing core you can get your hands on, and I don't care what kind of engine we're talking about, there will better and worse flowing head castings.
 
My thinking was more along the lines of desensitizing alignment by means of adding a fairly small bell or radius (say, a 1/8" radius or a 30 degree taper extending 3/16" into the port), rather than a deliberate / significant step in area that mould have significance wrt reversion.

(Given that we only have an inch into the port to play with, I seriously doubt that anything useful could be done wrt reversion on the intake side, just a guess though)

The idea being that a perfect match would be best, but that if a perfect match is unlikely, then a transition into a very slight bell or radiused entrance would be better than a sharp lip facing the flow. That much, I think, is obvious.

What I'm unsure of is how much worse a perfectly-aligned downstream slightly radiused transition is than a perfectly aligned and perfectly matched transition. Anybody know?

WRT legality - I would not think that anyone would consider a very slight taper inside the first 3/16" of the port anything illegal, would they? The intention here is easing a match, not anti-reversion, so I'd say we were still within a reasonable definition of port matching....
 
Al Seim":gsmvsf82 said:
WRT legality - I would not think that anyone would consider a very slight taper inside the first 3/16" of the port anything illegal, would they? The intention here is easing a match, not anti-reversion, so I'd say we were still within a reasonable definition of port matching....
Click to expand...

Al, you are given 1" on both sides, what you do with it, is up to you. Yes I think if it is difficult to match exactly on given head and intake, then being just tick bigger on the head side would be alot better than have a ledges there.
 
Hap - makes sense.

fwiw, I just spoke to my engine machinist, who builds mostly big drag race engines. He routinely makes the head port entrance about 1/16" (on the radius or outline) bigger than the intake manifold outlet just to avoid a step the wrong way.

Al
 
Fuel injection may or may not be different, depending on the location of the injector.

In addition to a step in the intake, I wonder if it would be beneficial to have a step from the carburetor (Weber and SU) to the intake manifold. Open plenum, carb. V8 racers use "shear plates" to keep reversion from effecting the carburetor signal. :ask: Granted, you would have to be careful you don't get carried away and cause problems by making the port too big.

I'm also surprised I haven't seen any bellmouths for SU's that look like this http://www.rockymountainatvmc.com/produ ... ilyId=1075 . You can change the vertical location of the foil to suit where the piston rides the majority of the time. Have it extend into the carb. as close to the piston as you can. However, unlike a slide throttle, a different needle profile will probably be needed as the increase in air velocity through the carb. will make the piston rise faster.

Even the SM guys were playing around with flow straighteners http://specmiataclassifieds.com/SMF/ind ... pic=1892.0
I'm sure they could take it one step farther with an extension to the flapper valve in the afm. However, I don't know if you could tune one to take advantage of the increase flow.

Also, the 3rd post describes what seems like a really great way to gasket match. http://speedtalk.com/forum/viewtopic.ph ... sket+match
 
Bob, all the MGs (anyway) intakes are machined bigger on the carb side of the intake from the factory, with a bigger taper cut, so thats kinda already happening, also with the limited prep rules, which is what we are speaking of, no other mods to the intake or carbs are allowed, of course full prep cars have the allowence to do alot more.
 
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