Carb Size Question

[ksamson]

Hello Roadsters, I have a 351W stroked to a 393 cui. I currently have a 650 CFM Quick Fuel. It has been suggested that I go to a 850 CFM. Does anyone have a similar set up?

[OVCobra]

As always it depends...650 on a 393 seems a little on the small side but it would/should have great throttle response at the expense of top end. I am running a 347 (same carb) so I have no direct experience but for others to comment, would be helpful to know details on intake (single or dual plane, make/model), heads (port volumes, make/model), cam (duration, lift, make/model) and practical redline (forged or cast crank). Either way it should be a rocket!

Dave

[skullandbones]

Just by checking the carb calculators out there, you will find that your displacement is too large for a 650. It might be ok for a 302 but not yours. 700 to 750 is more like it. A lot of people get caught up in the "bar bragging" of their displacement and carb size so bigger is always better for that. I had a 750 Holley double pumper on a 427 and it never starved for fuel. So matching it with the engine is essential. Good luck, WEK.

[ksamson]

Thank you

[Boz1911]

I'm running a 750 double pumper on my 408 and so far it seems to a good match.

[blueoval_bowtie_guy]

I'm running a 750 double pumper on my 408 and so far it seems to a good match.

Same here.

[maczter]

650 is a little low, but an 850 sounds way too big. You running manual or vacuum secondaries? Try checking a carb calculator such as http://www.csgnetwork.com/cfmcalc.html. If you use the one on Holley's website, it will recommend a vacuum secondary unless you are going full race. I have used many double pumpers on auto equipped cars, but for manuals in light cars, the vacuum secondaries have treated me best. For the record, I am running a Quick Fuel 750 DP on my SBC 383/700R Auto, and it is making over 420 HP. Do not have it on the road yet so I cannot tell you the performance, but past experience has been good.

[edwardb]

650 sounds a little light for your 393. I have a QF 650 DP on my 306 with AFR heads. Perfect combination, and seems to be about the size most use with that displacement. For your 393, I would guess a 750 would be about right. Maybe a bit more. But like any carb engine, the setup and tuning will be what really dials it in. The carb calculators tend to shoot a little on the low side for these cars IMO.

Interesting the mechanical vs. vacuum secondaries subject came up. This is another one of those widely debated topics, especially on the other forum, probably right behind power steering and power brakes. Based on my observations and personal experience, mechanical secondaries are the way to go. Properly set up, they are very streetable and provide excellent throttle response and performance. Perhaps slightly less fuel economy, but last I checked that's not why I drive this thing. I went from a Holley Street Avenger vacuum secondary on my Mk3 to a QF 650 DP, and it was like putting in a new upgraded engine. And I did everything to get the SA properly tuned. Just wasn't a good match, and never could get the secondaries to respond properly. Many others cited a similar experience, so I bit the bullet and replaced it. Best upgrade to the car I've made. High powered light cars with manual transmissions are candidates for mechanical secondaries.

[esff32]

I have a 351W and I have a 750. Throttle response is good however I haven't had much time behind the wheel to give you a better idea.

[maczter]

I agree with Edward on loving the DPs. You can customize the size of the shooters as well as the cc amount and the cam for the stream pattern. Yes, there is a bit of an art to properly getting it right. Get one with a good four corner idling system, I think the QF'S all do and you will not get a better accelerating carb. IM me and I can point you to some great reading material. Best carb I EVER had was one rebuilt by the Carb Shop in Rancho Cucomunga CA in the 90s, killer job, and ran liked a rapped ape.

Jack

[Mesa Mike]

Custom built pro-systems 780 on my 5.0. Carb should take in much more than displacement. call pro-systems and they will walk you through it.

[CraigS]

I agree on the DP carb. I spent 3 yrs working on my BG vac secondary and felt I had it pretty good and was happy w/ the performance. But when I bolted on a QF DP it felt like I had gained 35 hp.

[mike w]

Rule of thumb is to double the displacement of your engine (393 x 2 = 786) and then ratchet down to the nearest carb size which would be 750 or 780 CFM.

[Avalanche325]

With modern heads (depending on what you have), roller cams, etc. You can move a lot more air, and make a lot more power, than the same sized engine used to. Therefore, I agree that most of the carb calculators out there are undersizing an engine with good aftermarket heads.

I have a QF Q-series (DP) on a 347 with TrickFlow 185 CNC heads. The calculators all say that is way too much, but QuickFuel (talked to two different techs) and Fordstrokers all gave me the same suggestion.

So, "it depends" is the answer until we hear about your heads, intake and cam. With a nice set of heads, you will use all of an 850 and probbly a little more.