Ir intake manifolds and carbs

[Little Mouse]

Why is it that on high rpm carb motorcycles it's all IR to make power, but not used on V8's much in racing
What are the upsides and downsides to IR intake manifolds.

[frnkeore]

For American V8's, it a 4 fold reason.

1. There are NO carbs designed for it, other than Webers and they don't have spacing that fits all engines. Ford did make on in '69/'70 but, try to find one. I have only ever seen, just one, in real life.

2. It's very expensive, 4 Webers and a manifold, will set you back, at least $2500. More like $8000+ if you could find a Ford set up.

3. Most racing rules, don't allow them.

4. Plenum technology has come a LONG way, mostly because of unlimited resources in NASCAR.

But, IR is superior!

[Ken_Parkman]

And a 5th reason is getting a big enough IR carb. A small V8 is 600 cc per cylinder, and a decent V8 is 1600 cc per cylinder. The technology has a difficult time scaling for a high output V8.

[Caprimaniac]

On Jap-Crap 4-cyl's because Space? Easier to fit straight to the head. A plenum w/ single carb would take up more room.

In car racing there are IR EFI manifolds used in roadracing. Australian V8, as far as I have figured out. Might also be in use in US roadracing teams. IR Carbs and Ford were mentioned.
Ford were developing engine for the GT40 and they used the weber IDA setup, I am not sure on the exact timing here, but the IR setup was swapped- as far as I know because reliability- for a single 4 manifold/carb. The weber setup might have been scrapped befor the first race, in the developing stages.

[Little Mouse]

IR has always worked from idle to high redline on all brands of engine cycles since my first honda 50 at 11 years old. Does not matter if it's an old BMW boxer, moto guzzi, Ducati. There is no such thing hardly ever made in engine cycle regardless of engine configuration with a plenum. But then engine cycles have always been very high hp for cu. Size.

[frnkeore]

The Ford carbs that I'm talking about are the inline 4v, D0ZX9510A & B (A=875 cfm, B=1400 cfm @1.5). The A has 11/16 throttles and the B is 2 1/4.

I think they were only made for about 2 years but, they are what we need today IR carbs. Of course today you can have IR, EFI for about $2500.

Enjoy the pictures of "yester year" and what could have been, "if only"

Those were my days.

[MadBill]

And a 5th reason is getting a big enough IR carb. A small V8 is 600 cc per cylinder, and a decent V8 is 1600 cc per cylinder. The technology has a difficult time scaling for a high output V8.

Umm...yas, not too many IR carbs sized for a 780 c.i. V-8!

[FC-Pilot]

And a 5th reason is getting a big enough IR carb. A small V8 is 600 cc per cylinder, and a decent V8 is 1600 cc per cylinder. The technology has a difficult time scaling for a high output V8.

Umm...yas, not too many IR carbs sized for a 780 c.i. V-8!

If that is decent size, then what is a big one? LOL.

There are IR efi setups out there. I am afraid many just don’t like getting too far away from the tried and true.

Paul

[Racer71]

Midget sprint car and silver crown use IR but with mechanical or electronic fuel injection. Dirt late models still run carbs though

[dannobee]

Didn't Cadillac just win 24 hours at Daytona with an IR setup on an LS engine?

[modok]

A high powered motorcycle needs to be tractable at lower rpms

[David Redszus]

If you can use IRs, why even consider the use of carbs instead of EFI?

Giant leap backwards.

[modok]

well, I guess.
a lot of us like OLD technology, otherwise we would not be here.

[enigma57]

Worked on the problem of building an IR setup for the 427 stroker small block some years back. As you say, finding Weber carbs large enough to do the job is the issue. I did it this way......

Calculated baseline and figured I needed at least 46 - 47mm chokes (main venturii) and 58mm throttle bores to do the job based on displacement, designed redline RPMs and size of intake valves. The late Gene Berg made oversized IDA carbs as large as that for VW racers but buying 4 of them along with a new IDA intake to suit a small block Chevy was way out of my price range.

I had at the time a couple of the old Offenhauser dual quad cross ram intakes. The ones that are nearly identical to the Weiand and Edelbrock crossrams of the same period. I also had quite a few Weber 42 DCNF carbs here (my favourite to tinker with).

So I calculated open cross section of a 46mm choke tube and corresponding throttle bore sized to support it and realized that I could obtain sufficient flow by supplying each isolated runner with both throttle bores of a smaller 42 DCNF carb fitted with 34mm chokes. Made sense. After all, hot rodders in the '40s and '50s did that with the smallish Stromberg 97s on log plenum type intakes (keep adding carbs until you have enough flow). Why can't you do the same with IR intake?

So in the spirit of 'Do the best you can with what you have', I took one of the Offy crossrams......





Milled the tops off both plenums. Welded in port divider walls of 1/4" thick aluminum to isolate each runner. Made new tops from 3/8" aluminum plate and welded them to what had once been the 2 plenums. The DCNF carbs are very compact. By removing the cold start devices and covering the holes with small blanking plates, I could 'just' line up 4 of them in a row on each side......





Yes, 8 Weber DCNF carbs on a 7 litre V-8 (4 on each side), each carb supplying a single isolated runner with both its throttle bores. You have to love tinkering with Webers and be a total carburettor masochist to sort out (jet, balance and sync) 8 carbs having 16 throttle bores on IR intake. It worked, though. Ran very well considering.

Thought about it later and if doing such an insane thing again...... I would look around for one of the original Dean Moon crossrams. The ones made for DCOE sidedraught carbs with nicely sized runners welded to the intake base, not the el cheapo ChiCom knockoffs with undersized ports you see so often on e-Bay now of days......





Only I would run 8 Mikuni HSR smooth bore, sliding valve carbs. Mikuni make 42mm, 45mm and 48mm versions of their HSR carbs......



Depending upon the intended use of the engine and how high you were going to spin it...... The HSR48 or even the HSR45 should do the job. That would be a nice IR setup.

Just a thought,

Harry

[David Redszus]

well, I guess.
a lot of us like OLD technology, otherwise we would not be here.

I certainly can't argue with personal preferences. I have some OLD technology engines with carbs as well.

But hopefully that does not limit the understanding of newer technology.

In fact, sometimes I wonder how many actually understood OLD technology.