Any holley fan out there?

[chvyboy]

My question is, is a holley 870 cfm too big for a sbc?

Details that might help:
383 stroker/ 450 hp
2004-R transmission

[sduckworth13]

This might help you decide
http://www.holley.com/applications/C...bSelection.asp
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[mmiddle]

An 870CFM is awful large for any small block. Granted if you are running 350+ CID and a blower, maybe.... Generally, 750CFM will adequately supply even a well built 400. Vacuum secondary's are the key.

I vote no... way too big. Engine vacuum will suffer and not enough airflow to signal the venture boosters to draw fuel into the air stream. Drivability will be terrible at best. Now if you were to build an all out drag engine....

[chvyboy]

Lucky enough I was gifted this Holley 870, so ill throw it on and if it doesn't work as well as I want than I will buy a 770 dp
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[Syndrome]

If its new it will be worth more as it is instead of used. Might as well use the gift for what you need.

[mmiddle]

If you are dead-set on a Holley, sell the 870 and buy a 750 Vacuum single pump with electric choke. Double pumper manual secondary's are a major waste of gas.
Plus, being in CA, how are you planning to slide that past the Smog Cops?

Here in Texas, old cars only needs a safety inspection and "visual" check for smog stuff if so equipped.

[68C15]

sounds like a decent size carb, if you're running 8000 rpm

I would go with a 750 and annular boosters up front and conventional ones in the back. This will drop the cfm a bit yet increase the signal and give much better atomization. Make sure you get them tilted 3* out when you seat them. An aftermarket block would be a great improvement as well. For tuning it you will need a non-dampened vacuum gauge. That way you can see just how low the vac. gets for power valve selection.
Do you know much about tuning the transition circuit? That's where 90 % of driving is done

with time, patience, and skill you can make a Holley drive like fuel injection, run like a raped ape, and get surprising fuel economy

[Fitz]

Originally Posted by chvyboy

My question is, is a holley 870 cfm too big for a sbc?

Details that might help:
383 stroker/ 450 hp
2004-R transmission

Your engine is basically an air pump. The flow rate in cubic feet per minute is related to displacement and revs by the following formula:
Flow in CFM=(Displacement in Cu In * RPM * volumetric efficiency)/3456

a perfect VE is 1, so at 6000 rpm, your 350 cu in motor flows 600 CFM. Now we all know that no one ever gets a perfect VE (meaning it is always less than one) and that the flow is also influenced by manifold, heads and valve lift so adding a carb capable of flowing that figure with a little margin takes us to 700-750 CFM.

870 is a bit large for that motor. Dropping down to a 750 would likely perform better. Don't believe it??? My ZZ572 runs a Holley 850 CFM double pumper and is not lacking power.

[TBONE1964]

READ THE PLUGS!!!

Black plugs are bad.
Tan plugs are good.

Reading the spark plugs will tell you if the engine is running efficient or not. I would agree a 870 cfm does sound a little large. You may have to put smaller jets in it for it to work well.

[special-K]

A real simple loose rule of thumb I go by,that works,is 2 cfm per cubic inch. Technically it's 2.? cfm,but that works.

[68C15]

Originally Posted by TBONE1964

I would agree a 870 cfm does sound a little large. You may have to put smaller jets in it for it to work well.

that will just limit the amount of fuel and make it run even worse. Imagine this. you drink from a cup with no problem. now drink from a 5 gallon bucket.

[eldogg]

Is it a 870cfm Street Avenger? if so it will be close out of the box just lazy. a 770 would be a lot better but I understand running what you got. I agree with ^. Read the plugs and run at least 14 degrees (16-18 would be better) of base timing and no more than 36(most 383's with good heads like 32-34) total. If you have any question just send me a PM. I will try to help as much as I can.

[NC_John]

Originally Posted by 68C15

that will just limit the amount of fuel and make it run even worse. Imagine this. you drink from a cup with no problem. now drink from a 5 gallon bucket.

good analogy!

If the motor is stock or just mildly built I bet a 600/650 will work great. I put a 750 on what I thought was a "built 350" (the first engine I built- way too much of everything and nothing matched) and it looked fantastic when the hood was open but was way too carb.

For 99.9% of street applications, smaller is better!

[68C15]

Originally Posted by NC_John

For 99.9% of street applications, smaller is better!

BINGO!!!!!!!!!

I had an 80 K20 with a 11:1 355 and a holley (kind of). a 4776 main body (600dp staged progressive secondaries), 750 base plate, re-tapered venturis, stepped annulars in front, dog-leg conventional in back, and Percy's blocks. Although I had close to 80 hours building and tuning it, I have never seen anything work better. 17mpg at 70 with 33" tires, 3.73:1 gears and a NP465. Throttle response was stellar and so was performance. Driveability was better than expected from such a combination

[72bowtiestepper]

Originally Posted by NC_John

good analogy!

If the motor is stock or just mildly built I bet a 600/650 will work great. I put a 750 on what I thought was a "built 350" (the first engine I built- way too much of everything and nothing matched) and it looked fantastic when the hood was open but was way too carb.

For 99.9% of street applications, smaller is better!

You are correct !! I changed my big block Camaro to a 600cfm carb from the 750 I was told to use by a so called "expert" & it really woke up that motor .....very crisp throttle response with the 600 w/ vac. secondaries on the street...better gas milage to boot !

[GASoline71]

Well... my SBC sees a lot of runs above 6,000 RPM's and the 3310 on it is perfect. It's really not a "street" engine to most of you... but high RPM's, loose converters, big heads, and big carbs are "street" to me.

So I'm in the .1%

Gary

[special-K]

That's the problem with too much carb. You don't drive around at 6k.

[mmiddle]

If more is better, Then too much is not enough! Is not a good philosophy for carbs and cams.

Most street driven SBC's will never need anything larger than a 750 Vacuum.

Run what you want... to each his own.

[TobyArnot]

I just got through replacing a worn-out (puking gas) Quadrajet from PO with a Holley Street Avenger 670 c.f.m. On my basically stock 350 engine it runs and performs well. Started almost immediately and after a few simple adjustments (Checked float level - CHECK, minor tweaking of the idle mixture screws - CHECK, and set idle speed - CHECK) it runs super. I couldn't be happier. My truck runs as well as it ever has and cold-starts great. I will admit, I have run many Holley carbs and I can rebuild them in my sleep, but for now, I am super satisfied with this one. DITTO to what others have said about c.f.m. Too big is asking for problems. I worked in a "real" SpeedShop back in the day, and too much carb/too much cam was like a plague.

[Daddy Brim]

Originally Posted by mmiddle

If you are dead-set on a Holley, sell the 870 and buy a 750 Vacuum single pump with electric choke. Double pumper manual secondary's are a major waste of gas.
Plus, being in CA, how are you planning to slide that past the Smog Cops?

Here in Texas, old cars only needs a safety inspection and "visual" check for smog stuff if so equipped.

In California no smog check, no inspections on these old trucks. I think 1976 is the cut of date, not sure. All DMV wants is your $$$.

[chvyboy]

I guess my next question is if it can be jetted down?
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[68C15]

Originally Posted by chvyboy

I guess my next question is if it can be jetted down?
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yes, the same way you can take small sips from the 5 gallon bucket. I tried it a couple times and was quite unhappy. You will have major dead spots when you drive it.