Carb questions?, Will idf 40's keep up? Options

[914_classic]

I'm starting a 2.0 build and I have set of idf 40's but I don't know if they'll keep up. It's going to have the biggest cylinders and pistons I can buy in it with a knife edged and balanced crank, port and polished heads, big cam... Blah blah blah. I have a set of IDF 40s and I am wondering if they will be able to keep up. Any help would be appreciated.

[Mike Bellis]

The biggest would make your engine a 3.6L. Is that your intention? If so, no, carbs are too small. But it would also depend on cam, heads, stroke and budget. If you spend $20k on a full race engine or just a rebuild with larger pistons. Too many variables to say for sure. Need more info on your plans.

[jmill]

We need a little more info than that. First off, is this going to be a low RPM DD or a high RPM track car? How big is big and how fast are you going to spin it?

You'll need to calculate your venturi size based on cylinder capacity and RPM. Once you determine your venturi size does it fit in the IDF 40 carb body? Also remember that venturi selection is going to be a compromise between high speed performance and low speed drivability.

[John]

Is this a 4-cyl engine? Are you using dual 40s?

For reference, on a warmed over 2.0 that we did many moons ago, we found that we could not get the choke size that we wanted with the 40s. A better choice would have been 44s choked down just bigger than what was available with the 40s.

I think the limit on choke size with the 40's was 32 when we ran with a 4-cyl car.

The last season we ran it, we had the stock 28 chokes bored out to 34 and had the machinist attempt to taper the choke similar to what they were. (they are a parabolic shape)

The car ran the best the last season. Then we decided to go to a 3.2 and be done with it.

[VaccaRabite]

I'm starting a 2.0 build and I have set of idf 40's but I don't know if they'll keep up. It's going to have the biggest cylinders and pistons I can buy in it with a knife edged and balanced crank, port and polished heads, big cam... Blah blah blah. I have a set of IDF 40s and I am wondering if they will be able to keep up. Any help would be appreciated.

before you spend thousands on parts and end up with a trainwreck of an engine, please research what makes power without killing the engine with heat.

40s will be fine for a 2.0. Most would say they would be idea. 44s will work, but will give you all the power in the top end.

Zach

[914_classic]

I am an engine builder and have enough knowledge of aircoolers not to melt the thing. its just going to be a large displacement four that derived from a 2.0 four cyl. I get almost all of my performance stuff at cost so it will be done big and be done right. id like to know more about the formula and whatnot for calculating the correct venturi size. this will be a higher rpm racer. its headed for prepared rear wheel drive rally cross and eventually historic class in rally america.

if the 40s wont feed these they will go on my 1.7 to get her running (serious electrical issues going on in the d-jet system).

thanks so much for your help

[Jake Raby]

If you are looking for hi RPM then high volume will be necessary. The 28 vented 40mm IDF is barely good for a 2 liter engine. Going up to a 32mm vent makes it good for a 2056 that will turn 6.250K RPM. Going to a 34mm will give you a solid 300 more RPM. After the 34mm venturi the carb loses it's ability to atomize and the circuitry gets all screwey and it won't tune at the transition points.

I have built race engines that HAD to use a 40mm carb and could use up to a 38mm venturi, BUT I found that the engine made more power, tuned easier and was faster with a 36mm venturi. One of these engines would turn 9K, but was only 1832ccs.

There may be some formulas for calculating this; however I have yet to find one that was worth wasting the time on. In general, if you are looking for Hi RPM performance just start with a 44 IDF and the carb flows 290 CFM/ barrel right out of the box Vs. the 40 IDF @ 212 CFM. The 44 will tune easier, run better and can retain a smaller venturi/throat ratio to net better mixture quality and progression between the circuits. Making the smaller carb flow more requires enlarging the venturi thus reducing the air speed that breeds atomization..
If you have to run 40s for some reason, I have a set of race prepped Solex 40 P11s that are already maximized for hi revs and they are for sale.

Through my development I have found that a pair of 40s are maxxed out at 2143cc, the 44s take us from a 2165 clear to a 2.7 liter engine, then we use 48s from that point on up. My record for a 40mm carbed engine is 207HP and 8,200 RPM. Record for a 44 IDF is 2666cc @ 221 HP and for a 48 is 284 HP from a 2.9 liter twin plug daily driver. All but the 2 liter was running pump gas, not race gas.

Bigger is not better, BUT if you plan on heads flowing more than 190 CFM, a cam with more than 290 duration@ .020 and more lift than .480 on a displacement of greater than 2100ccs you will be better suited with a 44 IDF.

[914_classic]

That was exactly what i was looking for, thanks so much! I may steer clear of a stroker crank but larger pistons and cylinders is the plan along with flowed heads and a healthy cam, its looking like 44s will be the plan. are DCOE's worth trying? Im a pretty big watercooled fan and a lot of the watercooled racers who run carbs use DCOE's.

twin plug heads!? my father made his own dual plug heads for an old triumph 650/750 hybrid drag bike he built in the 70's and it works wonders for that bikes power band, was it hard (relatively speaking)? the historic class in Rally America is pretty loose about engine mods and dual plug heads would be pretty beneficial for rally i think.

[jmill]

Are DCOE's worth trying? Im a pretty big watercooled fan and a lot of the watercooled racers who run carbs use DCOE's.

DCOE's are side drafts. They work great for watercooled cars because the intake port is on the side of the head. IDA's and IDF's are down drafts. The aircooled heads have the ports pointed up perfect for a down draft application. Unless your going to turbo it why would you increase manifold length or add a 90 degree angle in the mix to screw up flow?

Here's the venturi chart I reference. It's only a guide. I'd go with Jakes real world experience.

[914_classic]

Thanks! I'm not super familiar with carbs, I'm an efi guy but carbs are usually a cheaper alternative and get plenty of performance gains. If I was going 100% all out I'd run Autronics efi but it's super pricey and carbs should do plenty.