Holley carb choice on 350 sbc

[ian]

I need some advise please on the best carb to use on my 350 sbc. I have a low-rise dual plane Holley Contender intake manifold. My engine is pretty standard at present, and will remain so until I have completely sorted my DAX after its back on the road. But eventually it will be tuned for big torque and HP. I was thinking of a Holley 650 double pumper (maybe a 600 d/p?) but I've just been offered a Holley 750 cfm with vacuum secondaries, almost unused and complete with Holley twin inlet pipes, for 100 quid. Considering that my chevy engine isn't going to be massively tuned until after I have got it on the road and properly sorted, do you reckon a 750 cfm carb may be a bit much? Any pointers on vacuum v. mechanical secondaries? And are there any pitfalls to watch out for when buying these Holleys used? Thanks in advance. Ian

[robert]

Ian

The 750 will be far too much for a mild 350, you would be more suited with a Vac Sec 600 for the moment, maybe a 650 depending on HP.

Unless you are going to be doing a lot of trackdays and being a real hooligan, then steer clear of a double pumper, as your fuel bills will be a damn sight higher then with a Vac Sec.

The Vac will work on the actual load applied to the car sensed by surprisingly enough "vacuum". The double pumper works on your right foot and can be used anytime, however will mean higher fuel bills.

Hope this helps.

Robert
Forum Admin

http://www.cobraclub.com/flags/UK.gif

[mikey]

Ditto Ditto Ditto to above. Performance with a 600 or 650 will be better due to higher mixture flow speed. Steer clear of mechanical secondaries as another major problem is cylinder washdown that increases wear.
Mike.

[robert]

Ian

Please see the following tech stuff on the club site.

http://www.cobraclub.com/html/carbs.htm

http://www.cobraclub.com/html/airflow.htm

Robert
Forum Admin

http://www.cobraclub.com/flags/UK.gif

[wilf]

And now for a different opinion (whaddya mean your'e not surprised?) - ditto on the carb size and ditto on steering clear of mechanical secondaries and double pumpers for a road car.

BUT... think about an Edlebrock (Carter/Weber) carb - they are so very much easier to set up and do not have gaskets below the fuel level, nor do you have to worry about fuel level setup. Far easier to change jets/rods, no power valves to blow.
I spoke with more than one person at Stoneleigh who were moving away from Holley's because of the reasons above.

Just my 2p's as ever

Wilf

[robert]

But then it wouldn't be authentic, unless you went for the optional four twin choke down draught Weber. }> :P }> :P :+ :+ :+

Robert
Forum Admin

http://www.cobraclub.com/flags/UK.gif

[ian]

Thanks guys for your input. Nice to see Rob and Wilf fight over carb choice too Which Edelbrock would you recommend Wilf, bearing in mind the engine will be mildly tuned for a while? 600cfm or maybe 650 cfm? What are they like in comparison to the Holley for mpg (not that mpg is a big worry for me!) I assume there are no issues fitting an Edelbrock carb to a Holley manifold. Thanks. Ian

[callum]

As a Chevy lover I go with Wilf. You want an Edelbrock 600 CFM carb (Part# 1405 for the manual choke). The simple sum is engine cube x max rpm divided by 3456 times the volumetric efficiency of the engine. So a 350 Chevy spun up to 6000 rpm gives you 2100000 over 3456 = 608. A very good engine is 95% efficient - a stock motor about 80%. Say yours is 90% and that gives you 547 CFM at full chat. All the concerns about fuel wash and gas velocity hold good. The Edelbrock/Weber is a neater, updated design, less fiddly and easier to keep in tune. Authenticity is too high a price for boring reliability unless you want to be a martyr and put up with all 1960s technology!:P

[wilf]

Ian - Callum beat me to that recomendation, but I would come at it from a different direction - Edelbrock do 600cfm and 750cfm carbs, nowt in between.

And Rob and I are not arguing about carbs - we just get good results in different ways, all we can do is say what we know works, and let folks make up their own minds.

You can get good results from Holleys or Edelbrock carbs, all I know is which is easier to tune. (although let's face it, once you've tuned your carb, are you really going to continue to fiddle with it???).

Wilf

[mikey]

I tend to agree with the Edelbrock choice now. I'm on a Holley 750 vs and they are fine with a standard to mild tune. With a hot cam where you need to up the tickover, with the holley the metering system at tickover and start relies on there being very little gap between the primary butterfly and body. Opening the throttle to achieve the tickover screws this up. You can compensate by utilising slight increase in throttle stop on the secodary or by drilling a small hole in the butterfly on the primary side allowing the stop to be taken down but it is a compromise. Unfortunately efficient metering is needed to get a clean pull at lw revs. The Edelbrock metering is not as prone to the metering situation.

If the engine is mild or no tune you will not suffer the above so price may play a part.
Mike.