Technical IR manifold theory

Does anyone here understand IR manifold theory and how it affects carb sizing ?

 

I'll let the experts answer your question, but if you want some interesting reading
Google McFarland torque link intake, SBC applications only, but lots of useful theory.

 

Just like how most modern-day motorcycles work (cept for Harleys).
There's NO intake "manifold", no cossover, no connection between cylinders what-so-ever. One throttle body (or carburetor in the older days) per cylinder.
This requires a much larger butterfly, a cam that will feed things properly, along with much larger ports in the cylinder head.
It's a combination of parts to be assembled with a LOT of homework. Not for the timid or the shy of bank account...

The butterflys (2) in my 1190cc EBR engine (Erik Buell Racing), twin cylinder bike engine are about 2.25"...PER cylinder. The piston diameter is 4.165" (as I recall).
That's a big throttle body. When you look at the top of the engine, you don't understand how it will work.

The Japanese 4 cylinder engines are the same, 44mm butterflys per cylinder (4), on a 1000cc engine.

The old days of Pro Stock, it was tried and abandoned because of it being so hard to tune for the full rpm range and the packaging, (8) BIG butterflys on top of an intake manifold... They ran out of space very quickly.

I know this isn't a scientific answer, but it might help to explain.

Mike

 

Induction theory applies to whatever you are building. Hell I have even taken a page or two out of the Smokey Yunics small block play book and applied it to MOPARs in my time. Same theory different color is all.

 

Yes the bigger size carbs sometimes are puzzling. The way I understand it you carb the engine for WOT at max flow of head.

 

Here's how I visualize it:
Start with a V8 with 2 barrel carb on a 180 degree manifold. Each time an intake valve opens, it sees ONE barrel of carb, and is likely undercarbed...adding more will allow it to make more power.
Go to 2-2 barrels on (or an open 4 barrel) a 180 manifold...now each valve opening sees 2 barrels of carb, and you're going faster!
WoHoo! 3-2 barrels...oops, getting to be too much, car doesn't go well at low rpm...each valve event sees 3 holes and flow has dropped way off!
Back to the lab...cut away the walls, go to open plemum. 1 carb, and valves see 2 barrels in each event!! Zoom! 2 carbs, 3 carbs...valves see 4 or 6 barrels each time, car is dropping dead below 6,000 RPM. Hmmm.
Throw away the damn manifolds, they are obviously EVIL.
Weld up some nice pipe so each port connects to one barrel of each of four carbs, and is isolated from all others.
New paradigm. The thing is now nice and smooth with more carb than you have dared to try before!! But...oops...the damn thing now has no top end, feels like a 2 barrel stocker...Hmmm...each valve is now just seeing a single barrel, just like when you started!
THINK. What does it want NOW? How about some carbs with much bigger barrels??

 

When running IR manifolds, you size the carb after your power range, the Venturi by individual cylinder displacements.
You match jets for power band, you adjust the butterfly's to start synchronize carbs to each other, by adjusting the best down to the lowest one (lowest vacuum believe butterfly).
Check for lean shots, drive it often and hard, and you will love this as much as fine women and or good music.

This is a very simple explanation, to a very complex setup! But it ain't as complicated that you can't learn it, it just take some good mechanical skills, some tools and access to google!!

My dad has an old book with the formulas to find the right stuff for your build. But if he has a book, I bet you can google it. Just like the younger generation would do it.

If I find a link I posted it, or I copy the formula from the book next weekend after Sunday dinner.

And trust me, with that formula math have never been more meaningful!!!!

 

Here is something, wrong engines, but theory is sound.

http://www.s262612653.websitehome.co.uk/DVAndrews/webers.htm

And this is damn fine, and includes some multiple carb and IR porn!!

http://jiminglese.com/index.html


I haven't found the formula, but you can read while I search.

 

We are talking butterflies but in reality what we are really after with independent runners is bigger venturi. When you up the venture size you naturally up the butterfly size.

Independent runners have a tendency to be peaky, they have a very short power band. The engine doesn't really need more air than it does with a plenum type of manifold it needs the buffer that a plenum type of manifold supplies. When you up the venture size you take away a little bit of the sting that the intake pulses make creating a sort of a buffer. it makes it easier to tune and while I doesn't really widen your power band it tricks the intake tract into thinking that it has a wider power band. You could probably get away with less carburetor per cylinder but tuning becomes a nightmare. You find yourself with an all the way on or all the way situation.

Smaller bores means higher velocity, slow your velocity down a little bit and your tuning parameters become a little wider. it takes a little bit of the peakyness out of it.

None of that makes any sense does it.

 

Yes and no.

I run dellortos 40 Dhla, on IR on my 4 banger 121ci.

40 being the size of the bore in the carb. I'm running 32 Venturi's and will most likely go to 34. Being the second biggest from the top.
So bore size stays the same, but Venturi's changes, or you might call them chokes. Bore is also the size of the butterfly, because they close the bore when of the gas.

I could have gone 45s instead, and run 36-38 Venturi's on them. But that will mean that I tun leaned and chocked to get more velocity, with out flushing the mill, or drowning it. I would also have to buy the odd, sizes parts, that ain't in store at my part house, but now I can run the regular parts for my 40s and get good and wide throttle response and get a wide powerband, with the most available jets.

The higher velocity means easier tuning, because the carb is more forgiving of the few tweaks. And if you got a higher velocity, it makes more of a gas mist, that gives you less fuel dripping and condensation on the walls of the manifold.

And as my dad keeps reminding me before it Enders the carb, it's a liquid that can't burn!!!!
When passed that Venturi's and butterflies it's a gas, and that burns fantastic. When cold you ad more fuel, because some of the fuel runs down the cold manifold, and the simplest way is to choke it up give you higher velocity at a leaser flow. And that compensated for better low end, a little badder performance.

I run setup as 2 2 barrels on a four banger and get more power, but also better fuel economy with them, compared to a stock mill with one carb. And with some cam work, bigger valves, compression bump and better ignition, I went from around 82 hp to around 120hp. And got from 10 km/l, to 11 km/liter.

How many of you have got a 50 percent power increase, got better fuel economy and gets a wider power band with more get yo and go.


.

 

My old Lotus Cortina ran 40's. [1700cc engine]
To get it to pull enough air we went to the largest venture available. There was almost no venturi effect in the carb and the engine "couldn't pull the skin of a rice pudding" below 3000rpm[there was fuel wash on the paint on the inner guard].
Luckily with a close ratio box I would never get under 4000rpm on the track.

Later we put 45's on it with the same 36mm choke size so there was a better venturi effect. It made a huge difference to bottom end power [without any loss in top end]

See ..... Bigger is better [more's law]

note: I made the classic mistake of building shit out of parts I had laying around [the carbs]

 

Bottom line....It takes a LOT of tuning with an IR setup. Venturi and throttle plate size are different animals (or fruits)

 

in The Horsepower Chain, Don Terrill shows a correction factor of 6.25 for IR engines. So if it were a typical 400-450hp small block where you would typically see in the .8 hg range with a 750 on a single plane or open plenum dual plane like an air gap at peak power WOT, you could calculate the venturi area of the 750, multiply that by 6.25 for total required venturi area, then divide by 8 and convert to mm to get a ballpark individual venturi size.

 

People say Webers are harder to set up and maintain than a 4 Barrel.
But I grew up with them, so for me they are easier to understand than a Holley.

 

Most guys on here judge how "good" a carb is by how well the out-of-box calibration matches their existing combo.

 

Here is a super rare setup I'm really happy with...

 

WOW! So, is there a Old Yellar clone in your future??!!
I just took a glance at the dyno sheet you posted, those torque numbers are IMPRESSIVE!

 

Hey George, how's things.
This falls under the heading of: just like that, but different.
Not specifically here, but in general, most of the negative reputation that people have contrived on Holley carbs over the years were from either people who never bought a new one or should never have been allowed to hold a tool.

 

I find Holleys easy to tune, cost effective and very adaptable. Honestly, if it wasnt for the price, I would LOVE to try 4 48 Dellortos on an IR intake on the 355 in my daily, I'm SURE it would pick up a BUNCH of torque in the 4500-5500 range, and not lose anything at the shift point. But hell, for what it would cost me, I could put a 6-71 on it.

 

FWIW, heres an alternate IR sizing formula from one of David Vizards books.

 

Blowers...mowers..... nothing beats the look and performance of 4 IDA's properly tuned. You want a cam with 112-115 lobe separation to prevent reversion.

 

Metalshapes, just curious, what is the LSA on the cam in your motor?

 

Well this is what I have heard, thing is, the difference in torque between a 112 lsa cam with the IR and a 108 LSA cam with the single plane and holley in a motor like my 355 will MORE than negate the 25-30 ft lbs increase I would expect from the IR set-up, making it a wash, then you have just squandered $1600 on some nice looking, tunable carbs.

 

That formula is accurate. I have a 327 .060 over(337) that will have 4 IDA's. That calls for a 36.4 mm choke. I planned on starting with 37mm chokes, and have 40's for extra tuning.

 

The throttle response is almost that of fuel injection. The butterfly on a IR intake is inches away from the head, and not mixed coming from a plenum. Jim Inglese stressed wide lobe separation, and lots of timing.

 

Metalshapes pm me the info on your cam if you want.

 

George,I'd have to look up the info on the cam.
But its pretty big.
Webers on IR's will give more low down torque, if you match them correctly, so that's why I could get away with that.
The 180 deg headers don't hurt either...

The Nailhead in the chassis isn't a Old Yeller clone, its its sistercar.
Roughly the same age as the Morgensen Spl that later became old Yeller #1, and built by the same guys ( Dick Morgensen & Boyd Hough )
OY #1 started out with a Plymouth straight 6, Mine with a Mercedes straight 6.
Dick Morgensen later swapped in a Nailhead mated to a Moss gearbox out of a XK120 ( he liked that setup, he used that in a couple of cars ) So that's what I'm putting in mine.
The Balchowsky intake is the icing on the cake...

 

From what I've found, the lists and formula's in Weber books, etc, are way too big for street use
Killing low down torque and drivability for a couple more HP at the top of the RPM range.

And you can put a set of Webers on a bone stock engine.
I have ( a 1600 cc Ford Crossflow 4 cyl )
Didn't touch the cam, timing or anything.
Picked up a lot of torque ( which was great, this was my towcar for my racer ), didn't loose any HP at higher RPM, and actually got much better mileage than it did with its progressive 2BBL.

 

I was thinking the same thing on the 180 degree headers, as a rule they dont live up to expectations on an engine with a plenum, but with an IR setup I bet they really shine. As a general rule, engines with plenums seem to be relatively insensitive to resonant wave tuning on the exhaust side as compared to IR engines, the only big gains with a plenum seem to come with collector length.
The Ol Yeller sister car is WICKED! What a score!

 

IIRC, the carbs on the engine in your Capri are on the small side of what the formula suggests, right?Vizard says in his book the formula will get you in the ballpark, but you may end up one or two sizes away on either side after testing. He also says to round down rather than up as a starting point.