Correct 4 Barrel Holley size for blown SBC

[HalfarsedHalfyard]

I'm building a mild 350 cu in Small Block Chevy for a project and need some advice on the correct carb size to use.

I got hold of a document by Weiand that recommends a 700 cfm Holley for a 350 cu in SBC with a Weiand 142 supercahrger, I plan on using a B+M 144 blower, the cam I have is a Comp Cams blower spec cam but the heads I have aren't the best, its being built with a budget though so better heads can follow at a later date.

I've seen a good deal with a Holley 750 cfm carb specifically rebuilt for use on a blow engine, how sensitive would the engine be to a carb that isn't the optimum size? would this carb be ok for my engine?
The exact specs are:
#76 Primary Jets
#65 Power Valve
#31 Pump Shooter
30 cc Primary Pump
#27 Secondary Metering Plate
New Vacuum Secondary Diaphragm
New .120” Viton Needle & Seat
BOOST Referenced Power Valve
Manual Choke

I know nothing about Holley carbs and setting them up (but am ready to learn), I'm figuring that as long as its near enough and allows the engine to run, I willl be able to work from that onwards.

Any advice is appreciated.

Aran

[ian.stewart]

Not enough info, the big variable is the charger, Over driven, under driven, 1-1, we need to know more, For me 3D Injection is the best way to go, but as you say you are on a budget, TBH its a bit of suck it and see, you must get the car put on some rollers and get set up correctly by someone who knows what they are doing, probably not easy in Aberdeen, just to add to your dilemma, what about fitting mappable ignition, I know its an added nariable, but will aid the quest for more drivable HP.

[HalfarsedHalfyard]

OK, heres what I got so far.

The blower will be driven at 2.11 times engine speed. Using the info in the Weiand Superchargers leaflet I have this will give me about 7lbs of boost.
I will be aiming for a static compression ratio of about 7.0 or 7.5:1.

I would like to keep it simple on a 4 barrel carb but see where you are coming from with the injection suggestion, maybe I can look at this route in the future. I must admit to being a sucker for the looks of a 4 barrel on a roots type blower look though.

At the moment as long is gets running I can look at getting the car trailered south for a proper set-up.

[kev_the_mole]

Aran,

if you look on Mez's site at

http://www.mez.co.uk/ms1.html

you'll see a 4-barrel throttle body that keeps the 'Holley' look.

Cheers,

Ian

[JP.]

If you go the Holley route then take on of their Supercharger carbs.

These Carburetors that are intended for use with superchargers, and have a unique capability that allows their power valves to reference the intake Manifold vacuum below the supercharger. This allows the power valve to operate as it should, based off intake Manifold vacuum. A power valve provides further enrichment to the main metering system under load (low vacuum) conditions. Without this external referencing, the power valve would be "reading" the Supercharger boost pressure, which has no bearing on the engine load.

You'll experiance problems getting your air/fuel mixture correct with a standard carb.

Holley recommends a 700 / 750 cfm for your aplication so it looks like you've found the right carb.

If a carb is too big you'll experiance sluggish throtlle responce.

[Eliot]

Aran,

if you look on Mez's site at

http://www.mez.co.uk/ms1.html

you'll see a 4-barrel throttle body that keeps the 'Holley' look.

Cheers,

Ian

With those blowers, you must inject the fuel above the blower as the fuel cools and lubricates the blower - at least that's what holley told me when i enquired about running port injection with just a TB above the blower.
Bill Shurvinton should have some autonomics TB's available, which can also have injectors added to them:
http://www.mez.co.uk/turbo9.html

Oh, why such low compression? - im running 7psi on a stock 350 sbc with 9.25 c/r. I would aim for about 8.5c/r if you intend keeping your boost around that sorta level.

[Blown v8]

if you want to use the holley without an external refrenced power valve.you have 2 take a vacuum reading from the MANIFOLD . whatever the reading is eg-5.5 you have 2 go up 3 sizes ! in this case 8.5 or the best solution buy a mighty demon from barry grant 4 a blower motor cheaper then a holley and more tunable

[Eliot]

if you want to use the holley without an external refrenced power valve.you have 2 take a vacuum reading from the MANIFOLD . whatever the reading is eg-5.5 you have 2 go up 3 sizes ! in this case 8.5 or the best solution buy a mighty demon from barry grant 4 a blower motor cheaper then a holley and more tunable

Even though its suck through?

[Blown v8]

get hold of a book by SA design by dave emanuel. Called holley carburetors (sub-titled-performance,street and off-road applications) theres a whole chapter in there for blow-thru modifications ! to much for me to list here !

[Blown v8]

in the SA design book street supercharging by pat ganahl theres a whole chapter dedicated to to rebuilding a holley for blower use ! i followed the advice when i rebuilt my 600cfm! when i took it to the dyno i was only a couple of jet sizes out ! No trouble with the power valve but i did drill out the PVCRs as per book
another good read is a book by Haynes-forced induction performance tuning (a practical guide to supercharging and turbocharging)by A.Graham Bell
camshafts for a blower motor ?
That can keep for another night