air suck'n calculation?

[scott]

are there any of you guys out there with an indepth undestanding of air flow dynamics?

i've a web 32/36 dgv and i've calculated the area of the barrels going into the intake manifolds. i've also have the small single snout plenum that k&n sells. the area of the snout is just a hair smaller than the area of the carb's two barrels combined. a 32/36 has 2.824 sq in of area and the snout has 2.405 sq in.

i'm using this plenum to hook to a remote air filter (snorked)

am i choking the intake to the point i'd notice lost hp? should i consider this other plenum that has a snout with an openning messuring 11.045 sq in?

which ever plenum i use i'll be sucking air down a 3" dia smooth walled pipe with a couple of gentle turns. pipe will be about 4' long. the area of a 3" openning is 7.07 sq in

and i know exhaust affects this too. i've already went to a 1 3/4" dia exhaust and a series iii exhaust manifold.

[greenmeanie]

From memory, on these engines it is the head that is the limiting factor. There is a sharp 90° turn from the intake gallery onto the back of the intake valve that leads to flow separation and shrouding among many other limitations. Now you know why I was trying to get you to section that head.

Many years ago there was a good article in one of the landy rags comparing various carb flow numbers with a stock engine. Again from memory the flow capability of the 34ICH was adequate so even a slightly strangled 32/36 should be OK.

Bear in mind flow is not merely the cross section of the intake but also the velocity of the air stream.

[scott]

thanks, i've still got that head and you're welcome to come slice it up. yea i know there are lots of choke points, and things that slow air flow by causing disruptions, like sharp turns, long runs, rough surfaces. it's just that i'm sure i gained a bit with the exhaust, and a 2.5 cam, and an elec ign, and the 32/36 web, 8.5:l head, .06 over pistons. i just don't know how much each added and i don't know if this last thing i'm going to do will add to the easier breathing and would the next thing that i don't do be a lost opportunity for more power. and yes it seems almost futile to be talking about power in a 2.25.

[CliftonRover]

it would not hurt to find a bigger plenum obviously, but I would also attempt to minimize the length of intake pipe, in the engine bay. for normal road driving you could position the intake to suck cold air from the front of the car. I am thinking of my fluid mechanics class and how else to help... let me brainstorm a little. but there is only so much you can do for a 2.25

[Rineheitzgabot]

Call Mike Pierce at Pierce manifolds. Quite the master when it comes to Weber carburretion.

[scott]

rine, funny you mention peirce, i just came from his web site. clif the length is to get the air filter above the windscreen. and i recalc the openning of my small plenum, it's actually 3.14 sq in this is bigger than the barrels combined.

[jp-]

If you're worried about power loss, just try this: Run the vehicle without the snorkel attached, then with snorkel attached. Compare seat of pants feel. I doubt you'll notice any loss, if there is some. Avoid the complex math whenever possible, that's my motto.

True, the head design could be better on the 2.25, but the manifold design is horrid. Anything you can do to straighten the flow of air to the chamber is better (pierce manifold) and removing extra heat is a must. The original design of the exhaust bolted to the intake manifold is fine for cold weather, but awful for keeping the fuel charge cool. I made an isolator plate between my two manifolds.

[scott]

jp, badvibes suggested a quick change set up. having a section of the snork under the hood removable and sliding a k&n cone filter on to the snout of the plenum and hook the snork up only for fording. i have a new exhaust manifold with a little flap that direct the exhaust away from or towards the intake manifold. i made my own actuator that ties into the choke cable to replace the automatic one that i couldn't find anyway. this manifold had gentler radius too compared to the the original. still got finish painting the bulkhead and rewiring job before i cn put the 2.25 back in to actually see what all this crap has done. math is fun but i agree with you on doing it by feel

[jp-]

jp, badvibes suggested a quick change set up. having a section of the snork under the hood removable and sliding a k&n cone filter on to the snout of the plenum and hook the snork up only for fording. i have a new exhaust manifold with a little flap that direct the exhaust away from or towards the intake manifold. i made my own actuator that ties into the choke cable to replace the automatic one that i couldn't find anyway. this manifold had gentler radius too compared to the the original. still got finish painting the bulkhead and rewiring job before i cn put the 2.25 back in to actually see what all this crap has done. math is fun but i agree with you on doing it by feel


Flap or no, the design still lets too much heat get to the intake manifold.

[scott]

jp agreed, how did these things ever run in the sahara? tell me about this isolator.

[greenmeanie]

Ah now gents, be careful about what you think about manifold heat. On a wet flow manifold such as this heat up to a certain point is desirable. A lot of people who take the echaust heat away using a device such as the Pierce exhaust manifold suddenly find their truck has a horrible hesitation on acceleration. I'll credit inliners.org for the following explanation as they say it a lot better than I can. THis was written for Chevy 6 pots but the same principles apply.

The most frequent complaint I have is from members who complain that since they’ve installed their multiple carbs, the engine hesitates and stumbles at low speed on initial normal acceleration even after warm-up. The most blamed culprit is the darn Rochester Carburetor. Usually it is falsely accused!
Usually when installing dual exhaust manifolds or headers, the production exhaust heat supply to the inlet manifold is eliminated because (except Edmunds and most recently Clifford) there’s no provision or instructions to provide heat…Why?? I don’t know and could only guess. In any event heat will be required to achieve good driving response and reasonable fuel economy. Here’s why:
As liquid fuel enters the manifold from the carburetor, the vacuum vaporizes the fuel and causes a chilling effect on the walls of the manifold much like the chilling effect of spraying an aerosol on your skin. Now you have a cold manifold. If you do not supply a continuous supply of heat the manifold will remain cold and even build frost in some conditions. At this point, if acceleration is attempted, the vacuum will drop, fuel will no longer vaporize, and will condense on the cold manifold walls until they are fully saturated with wet fuel – this takes about three seconds, during which time no fuel is going into the engine ( and thus no power or a "sag") After the walls are fully saturated with fuel the air flow finally picks up and floods some of the cylinders but not all of them because liquid fuel is notoriously bad for equal distribution.
More fuel (bigger jets) will only slightly help this problem and actually worsens the over-rich condition and spark plug fouling and fuel economy.
The solution: Moderate, and continuous heat supply to keep the walls of the inlet manifold warm and fuel in vapor form. Exhaust heat is fast but requires a butterfly valve in one manifold to force the exhaust flow. Water heat is slower but very clean and not corrosive to aluminum manifolds. This method utilizes the water pump to continously supply warm water to a passage underneath the manifold. It is sometimes necessary to weld a tube to the manifold but be sure to obtain intimate contact between the coolant and the manifold wall and floor. Simply tack welding a closed wall pipe to the bottom of the manifold will not result in sufficient heat transfer.

[scott]

mmmm, so heat is kinda like beer, not enough and your slow and unhappy, too much and...hey wait a minute! what am i saying? there's no such thing as too much...

i've alway wondered about this tube that runs through my spitfire intake. i though routing the coolant through it was to compensate for the exhaust mainfold being right underneath it.

greenie, do you know of the temprature range that arasolize gasoline converts to and stays vaporized? and is it safe to say vaporized fuel in manifold good, in carb or lines bad?

[greenmeanie]

Scott,
It is not just as easy as a set temperature as the thermodynamics of the problem involve pressure change due to vaccuum etc. As there are, however, a lot of coolant heated intakes out there on the V8s I would say the most convenient temperature that works is in the region of 180°-200°. It may not be the lowest temperature but it is the most readily available.

Also bear in mind for a coolant heated intake that heat transfer has a lot to do with surface area. Merely running a pipe or welding it to the underside of the intake doesn't do much as the contact area is so low.

Creatign a proper coolant gallery along the bottom of the intake and then installing a pierce exhaust manifold would probably be the ideal set up but ultimately I don't think you'll see much of a noticeable power gain for all that effort.