How much CFM for a 2v head?


Well-known member
hey i was wondering how much cfm would u recomend for a aussie 2v head, would 390 be a enough? thanks
More than likely, but it does depend on what engine size you've got. Most of the time a four barrel 390 would only be using the two small primaries, which would only need 180 cfm or so. If you put heavy dampening on the secondary circuit, you'd seldom run into the 390 cfm potential, except when giving it some curry!

For ease of fitting, though, its hard to beat the Autolite 2100 or 350/500 cfm Holley #2300. Remember too that cfm on most American two barrels was rated at a higher pressure, so that 350 or 500 rating may be more like 250 or 350 cfm respectively. Most engines can handel more than 30 percent carburation oversupply.

Carby Calculations:-

If you take the cubic inch displacement, then multiply it by the safe maximum rpm it gives you a number. Divide that by 3456. Then multiply it by 80% , which is the likely volumetric efficiency. Result is often about 245 cfm for a 200 reving to 5000 rpm, and 325 cfm for a 250 reving to 4500 rpm. A 2-bbl 350 (really a 248 cfm carb) for a stock 200, a 2-bbl 500 (really 354 cfm) for a stock 250.

Further Info:-

Techno junkies! Anyone who's interested in this sorter airflow (hot air more like!) could look at the technical links by clicking on to the Ford website label, go to the technical section, and hitting on Bowling & and Grippo formuaes (
Here's a better way to estimate your airflow requirement.
For every flywheel BHP you need about 2cfm of carby flow, this applies to a single carby, not for triple twin throats such as Dellorto's.
So if you putting the 2v head on a 200ci with a bit of cam, other improvements and are trying for say 125 honest BHP something around 240cfm will suffice, so a 350 cfm Holley is plenty. (lest you think this is low, the stock 200 would struggle to give 85bhp)
Another thing is that the stock 250-2v manifolds isnt set up for 4 barrels, and whay have the complications anyway.
I can tell you from firts hand experience with 250-2vs that a 500 is TOO big with a cam, ok on a stocker, 350 is OK on 200ci.
The Schjeldahls' compression calculator also suggests CFM when it does a calc for you.
Nothing beats hands on experience! I don't want to put people crook.

I have only delt with Holdens and Valiants. The 500 cfm 2300 I have is just a throttle body on a propane car, so that doesn't count as a sucessful petrol 250 with a 2-BBL Holley. :cry:

My friend, a died in the wool Holden man, ran a 650 cfm 4MV Q'jet on a Cain manifold on his HZ 3.3, and it was passable, but had a stock cam.

I have heard that Holleys are touchy with respect to total cam duration. Apparently there is physically less venturi in a Holley, rather than, say, a Rochestor 2-Jet (not Varajet). Some people have said the 350 and 500's are not compatible with durations of over 280 degrees. My thinking is that its a fuel standoff problem, or a lack of hi velocity air mixture flow on overlap. There is no room to fix this on an Aussie 2V, unless you make a bonnet bulge for a spacer. My XE onlt has 188mm from the manifold to the base of the bonnet. Nearly all of it is taken up by the 84 mm carb and 12 mm adaptor I have; the rest is 74 mm deep air cleaner, and stupid blo**y 28 mm bump where the gas goes in, and where the mixture is adjusted. 2V's on XY/XA's run a 39 mm deep air cleaner, and no space for an adapter.

Question 1. What sort of problems does over carbing with a cammed 250 2V seem to cause?

When Danny, my old neighbour, had his 265 Hemi, he found the 500 was nice on that engine, but the ex 1966 390 LTD Autolite was better. But the 500 had no problems, and he ran an E38 spec cam.

Question 2. Not an issue because of a bigger engine? 272 degrees not too much to create a propblem?

Street Machine did a very public engine build on a 3300 XT5 with YT alloy head engine in 2001. Its rear wheel horses were 152 net with a trimatic. Drive train losses are around 1.3 for a corrected dyno to corrected net flywheel horsepower. They were forking out 200 net flywheel horses, and uesd a 500 cfm 4412 Holley and a cam. There was no reported problem with pig rooting, but they were doing dyno runs, no driveablity.

Question 3. Have you found it to be a driveability issue with cammed 250 2v's, or is it worse than that?

Oh yeah, question 4, what is Schjeldahls' compression calculator. Is that the really good little number they use in the Tech section, or is it somwhere else?
Addo, addo, addo!

Thats quite good, a 59 minute response time.

Yes, I've seen it before. But I didn't see that it was that smart. I work out compression my self, by calculator. The carb air flow rating for a 250 reving to 4500 rpm with volumetric effieciency of 80% by formuae is 259.89 cfm. It says 260 cfm. This is one nice programe!

Harping on like a stuck record, as is my custom, (I'm a Kiwi!), just remember that a 500 cfm carb is in fact at 3.0 inches of Hg, not 1.5 inches like the 4-bbls are rated at. The conversion for is 3.0 divided 1.5, then square root the answer. That gives 1.414 off the 500 cfm figure. By calculation, 354 cfm. So a 500 cfm Holley may be a little too big, but most 2V's can go over 4500 rpm easily. And I've seen 500 cfm 2-bbls run on Cortina quite well as long as the idel restrictor jest is sleaved to 16 thou!

Oh, by the way, hope you got my e-mail. My Scottish Mother in-law ( the old dragon) says she cant remember Gallic, and that Wilsons in Canterbury don't do Singles down here. (Private Joke??)

I think I'll stick to stopping carburation reflux problems, rather than looking to higher octane solutions.
Nah, no e-mail yet... Uisge beatha is "Water of life" - you can see the first word is like "whisky". And I wasn't going to suggest you try single malt alcohol injection for anti-knock. :LOL: And Gallic is Français. Ghàidhlig is what I think you meant...

That program has no way of evaluating the rod ratio, so knocking situations are another thing to consider. Still, it's great to plug in some details, nominate your VE, and check the HP at various RPM.

Correct me if I'm wrong, but one of the advantages of a long rod motor is the appreciable change in piston velocity as TDC is approached, due to the long rod describing a greater arc. This is one of the significant ways the motor is more knock resistant. (Just wanted to get an expert opinion.)

Anyway kstang, if you run a Holley, dyno tuning will get it pretty well sorted. It's the "fuel curve" that eludes people's comprehension. Don't just consider two static situations - WOT and idle. You'll rarely be using it at either. It's the meat (or savoury pressed tofu) in the sandwich that matters here.

How's this for disjointed? LOL.

Regards, Adam.

Cheers, Adam.
I'm going off on a tangent a little here, but the effective compression with any short rod engine is inreased. I did some calcs in the early 1990's...I'll see if I can find them. They don't line up with US Hot Rods talk on the benefit of short rod engines, but they do reinforce there statment that poorer rod length to stroke ratios are better on the street. With cars like the T3 Tickford 5.6 litre and the outrageously long stroked 105.8 mm stroke 5.4 Duratech V8, Ford seem to aggree with this idea too.

There is a relatively simple trig equation using the exhast and intake open/closure degrees which can calculate a value of effective compression, not the full static compression. An XE carby 4.1/250 Falcon with a 5.88 inch rod will have a static compression of about 9.35:1, its effective comprssion is more like 7.6:1. If you increase the rod to Aussie post 1970 3.3/200 length, that is 6.275 inches, the effective compression drops. Could be 7.4:1. This means detonation is less likely in a long rod engine.

However, long rods engines aren't all good. The loss of effective compression hurts low speed torque, so a long rod engine can carry more static compression to bring the effective compression up to the same 7.6:1 that a stock rod 4.1 has. This means that a short rod engine can carry more duration than a long rod engine, and still maintain good low speed torque.

Fords Australia has got the low speed torque thing sussed with there big sixes and V8's. The amount of copmression they carry is impressive. How about a rise from 288 Nm (212 lb-ft) of torque in the early XC days to 385 Nm (284 lb-ft) with the latest Intech I6? On a worse grade of fuel with over 6 points less octane. Fab Ford Magic!
XECUTE":2ees5ots said:
This means that a short rod engine can carry more duration than a long rod engine, and still maintain good low speed torque.
This could explain the performance and appeal of a cammed up V8.

I'm trying to visualise a velocity/time graph for long vs. short rods. Wasn't aware the effective compression changed (but if I think hard about that graph and engine timing events, it may tally). I thought it was simple :LOL: - the charge was being slammed less hard into the now-closed chamber. If you looked for a reason to why the engine knocked less, the differing turbulence might be a consideration in this idea.

Having said that, the increased "hair-trigger" effect of a shorter ratio may also be some of the appeal.

And then there's the mongrel stroker 235/ex186 TO-4 engines pulling 9's. Seemingly defies logic, doesn't it?

Bottom line? I'd hate to be stuck having only two capacities and one manifold to play with. Poor buggers.
about two years I had a 2V head and intake shipped from Oz. Before it was shipped I had it modified. The guy I got it from (and I hate that I have forgotten his name, cause he was very nice and trustworthy) had the the Stromberg flange removed and a Holley spreadbore flange welded in. They did a great job I must say. I purchased a Holley 0-8007
390 CFM 4bbl. This way it is a direct bolt-on and no funky adapters. It has vaccum secondaries that can be adjusted to open at different engine speeds. This is exactly what Kevin is asking about.
well i started with a standard 465 on my 250 2v, went well. i then fitted a 465 that had the air horn machined and k&n stub stack fitted. i also fitted a metering plate to the secondarys so i could tune the secondarys better. i also fitted a quick change spring kit the vac secs. i found that with the air horn off and stub stack i had to go up in jet sizes as it ran too lean=more air flow. biggest problem i had though was the flat spots.
i then bought a 600dp as it was cheap. only 5yrs old at the time(still new). i had lent it to all my mates and still hadnt been on my car, so i fiited it and the difference is incredible. no flat spots, no hesitating just pulls like train all the way to 5500. 1st-2nd chips and sideways.2nd-3rd chirps tyres. btw these tyres are new.
so no 600dp is not too big. :devilish:
Noticed you said you had a dp 600 meaning a "Double Pumber" 600 four barrel carb back (have mechanical secondaries) in February when you posted this. Have you still got it, have you had any problems. Do you know what cam you have in it your 250 2V (understand if you dont want to say).
I spoke to my carb re-builder yesterday he said this was a very good carb and can run very efficiently but tend to cost more $$$ to purchase. I am surprised you say the difference between the 465 and the 600 dp was "incredible" as a 465 would normally be considered more than sufficient for a 2502V.
Id be Interested to hear anything you have to say on it.
yeah i still have the 600 double pumper on the 2v and never had any probs. i have seen quite a few of 600dp`s on ebay recently, shouldnt be too hard to get one. I originally built the engine about 96-97 and cant remember the size cam..sorry, should have the cam tag around somewhere though. i remember choosing a mild cam as the others made power further up in the rev range and as we know ford 6`s dont rev that much, i wanted bulk torque down low. on a couple of long trips recently i found the fuel consumption was around the same as the 465vacsec but no more flat spots and hesitations. keep in touch tim, if theres any more you want to know on 2v`s let me know.