The V8 Owners Forum -

JSF55 wrote:Ah right ! the holleys and webbers are rated diffrently so 390 holley is actually bigger than a 500 webber, i read it somewhere !

Well here's your answer http://www.edelbrock.com/automotive_new ... _chart.pdf

Edelbrock 500 cfm ; aplication - 225 till 327 cid and don't forget the Ve (volumic efficenti) ratings of the engine which will be in the 75-80% range.

I'm sure that RPI have that 0.080" made to suit the standard rover application, the kit they list for the 500, 1486 , doesn't contain that jet, Paul, yep that was the info u sent, i'm hoping with a few mods to the engine i can get away with 0.083" in mine, john

the 500 has smaller primary throttles than the 390 making it more sutable for smaller engines which is why you can get away with it without any problems on a 3.5. On the secondaries its too much carb but as they are vac operated they will only open to what the engine wants. In most cases the 500 performs better at low rpm than the 390 purely because of the smaller primaries and of course has a higher ceiling if you go for more.
Your manifold is a pretty good choice for a mildish 3.5-3.9 or high torque engine but if your do change the engine to larger then your going to have to change it. The runners/ports are just too restrictive to make good power on a larger engine.

Those 080 jets are std parts, some more stuff here which Summit have if no-one here does. FWIW when I swapped a 390 for a Weber I found:

Better mpg, more so in town
More tractable, could pootle in top easily
None of that arcane faffing with Holley float settings & petrol all over
Lost the instant 'snap' the Holley had when stamping on the pedal at 10-20mph, the car used to really 'jump'

IainB (I think) mentioned these dyno people, though I never went there in the end, the guy knows his 4-barrel carbs and will tweak them:
Autocraft
Unit 9 Houghton Regis Industrial Estate
Houghton Regis
Dunstable LU5 5QH
(01582) 866 688

cheers,
Bill

Hi Bill
Does this guy have a rolling road?
I want a tune up and power readings
Regards
Dave

Hi Dave, yup they have a r/r and the chap knew his Holleys & Webers & could set them up. Not sure whether they had jets/needles etc to hand though, best ask, if you want them to fettle. As I say I didn't go there, so best ring & make sure the guy is still there,
cheers,
Bill

JP. wrote:

JSF55 wrote:Ah right ! the holleys and webbers are rated diffrently so 390 holley is actually bigger than a 500 webber, i read it somewhere !

Well here's your answer http://www.edelbrock.com/automotive_new ... _chart.pdf

Edelbrock 500 cfm ; aplication - 225 till 327 cid and don't forget the Ve (volumic efficenti) ratings of the engine which will be in the 75-80% range.

Notice that it says "general displacement guidelines" and not "your motor will run like a sack of poo if you use it on an engine 5% smaller than our guideline minimum size"

And also notice it doesn't say "remember to correct the sizes we publish to your closest guesstimate of volumetric efficiency" I guess they have taken all that into account already.

It is not an exact science, maybe the 500 is a touch too big, but many people run happily on Weber 500's on stock 3.5s.

I believe the Holley 390 was actually developed to make big inch engines run as fast as possible, in Nascar racing, when they tried to limit horsepower by restricting carb size?

This could be a load of toss so feel free to say so But - carbs basically draw the air they need (assuming the carb can supply enough of it); so the max theoretical flow isn't that important. As they're vacuum sensing devices really, what's important is the venturis are of a size/shape that keeps decent intake velocity & gives a strong signal to the jets. So given that, if a Holley 390 is OK on a 3.5, then the Weber is at least as capable since it has smaller primaries, and the max cfm isn't so important.
Is that reasonable or gibberish ?

Alley Kat wrote:This could be a load of toss so feel free to say so But - carbs basically draw the air they need (assuming the carb can supply enough of it); so the max theoretical flow isn't that important. As they're vacuum sensing devices really, what's important is the venturis are of a size/shape that keeps decent intake velocity & gives a strong signal to the jets. So given that, if a Holley 390 is OK on a 3.5, then the Weber is at least as capable since it has smaller primaries, and the max cfm isn't so important.
Is that reasonable or gibberish ?

Think you nailed it pretty good

Alley Kat wrote:This could be a load of toss so feel free to say so But - carbs basically draw the air they need (assuming the carb can supply enough of it); so the max theoretical flow isn't that important. As they're vacuum sensing devices really, what's important is the venturis are of a size/shape that keeps decent intake velocity & gives a strong signal to the jets. So given that, if a Holley 390 is OK on a 3.5, then the Weber is at least as capable since it has smaller primaries, and the max cfm isn't so important.
Is that reasonable or gibberish ?

I've been trying to think of a satisfactory way to say that myself actually.

The 500cfm measurement is at full flow, so irrelevent at idle, or half throttle, or even 3/4 throttle. Maybe at WOT it is too big, but not until then.

Better than being too small, anybody?

I actually have my throttle cable set up so I don't get the last 20% of movement. More of a design error really, than a potential cure for an oversized carb.

Marki
The main reason I bought the dual port manifold was, I was told, that it was good for small throttle openings and that it was the lowest profile available. (Bonnet clearance)
Is there a similar low manifold that would be good for a big engine?
I must expand on my idea of a big engine though. Unlike you chaps I do not want massive power/speed, as I do not want to spend 6 grand on the suspension needed to improve my MGB to be able to use it, but I would like about 200bhp on a very low stressed motor that will last me until I can't dress myself.
Regards
Dave

yes what you were told is right in both cases but it really wont support the bigger engines. You could easy get 200bhp from what you have, 3.9/4.0 would be nicer but I certainly wouldn't go above a 4.6 for that kind of power as the costs just aren't worth it in your case.

If your stuck you could try this place http://www.webcon.co.uk/
They also have a rolling road.