Joel5.0

I always read the famous "a 600/650/700 cfm Holley is too big of a carburetor for a 302 SBF" responses here. I have had a "little" experience with SBF, SBM, SBC and import piston/wankel setups (1.2L - 2.2L) N/A setups that have "required" dual Weber side-draft carburetor setups, Holley 500 CFM, Holley 600/650 CFM carb setups (to name a few) for better performance w/out affecting/hindering driveability.

Being out of the import scene for many years, I wasn't able to show what I meant..... and my responses regarding the utilization of an 850 CFM DP Holley on a little daily driveable '65 Mustang I had moons ago, or a 1.8L Toy with dual side-draft Webers, were usually "dispatched" as hearsay, impossible, etc. The following pic is of a local 1.3L 13A wankel engine setup, or a 79.3 CID engine, street driven, in a classic R100 frame setup.



What carburetor size (with the oil induction tubes mod) is on top of it?....... and it has some SBC and SBF setups in its "victims list" BTW.... does it rev past 8,500 RPM's?.... yep. Is it also driven to go buy a loaf of bread and groceries?.......

Portmaster

Well being I can't spot a secondary metering block on the rear (just from the photo) and the air horn has been milled off I'm going to guess it's a Holley 4160 3310 750 cfm. It's a Rotory engine so cubic inch displacement isn't as relevant. 8500 rpm isn't that big a deal for a rotory engine. Whats the point? What does this have to do with a 5.0 pushod engine?

Joel5.0

Yep... the "jury" is still out trying to define cubic displacement for the wankels.....1.3L (79.3 ci) advertised or 2.6L (158.6 ci) when compared to 2 rotations of the eccentric shaft or 3.9L (238 ci) based on 3 rotations required for the 3 faced dual rotor config to go through a full Otto cycle. Lets use the latter, or 238 CID....... how can displacement not be relevant for carburetor sizing, if the formula to select carburetor size is based on it and RPM's? Isn't the wankel a 4-cycle engine?

Anyway........yep, a 750 CFM it was. Before this is taken to the 1,050 CFM or 1,150 CFM stratosphere for streetable 302 setups, ... based on my experience working on smaller streetable engine setups (wankel, pushrod or OHC)..... I always see a trend to under provide the 289 CID+ windsor setups in the carburetor dept..... would I use anything "smaller" than a 650 CFM on a 289?.... ..... why?.... if Mr. Carrol Shelby specified a 715 CFM carburetor for the 289 GT-350's.... why is a 700/750 CFM carburetor "perceived" as too big? ..... why have every streetable setup I've worked benefited from a a "bigger" carburetor....... performance and driveability wise.

Does using a too big of a carburetor (700/750/800) CFM Holley on a 302 with mild mods cause a problem? ...nope. Will the combo benefit performance and driveability wise?.... based on my experience, and depending on the mods, that is the case. Aside my junk, I have a current customer with a '85 Mustang, untouched GT40 heads and a 750 CFM VS Holley..... he always gets asked how much N2O he is using.... I wonder why? ..... he then turns the A/C on to drive his junk back home.

mjr46

what is the 85 complete set up Joel?? and what does he run??

Joel5.0

Performer intake, B-303 cam, GT40 heads, Holley 750, LT headers which were on the car when he bought it 4-5 months ago. I only tuned the setup and installed a MSD Digital 6 plus box + a Holley blue FP. He has not taken it to the track yet, but has taken care of some SBC's, other Mustangs and imports on the street (something I don't recommend BTW). It is now in need of a clutch replacement.

mjr46

wow...just a lil ole performer intake, that's all that's on my 92, I'm about to take the 600 off of it and put my old 650 I have sitting around and tune it accordingly to see if it picks up any steam

67mustang302

Part of the problem is that the CFM ratings can be rather arbitrary. Flowed with or without boosters in place, wet flow vs dry flow etc. You could have 2 600cfm carbs and one of them might actually flow a real world 50-100% more, depending on how they're rated.

The other thing is how sensitive the metering/booster combination is. I'm running a 650 DP Quickfuel on my 302 now, with annular boosters. The thing pulled so much freakin signal at WOT that the boosters were coming on at 1,800rpm and drowning the **** out of the engine with fuel. Still dialing it in, but a lot closer now, and it'll run at WOT from 1,000rpm in 5th without problems, obviously not in a hurry when it's doing it though. Carb technology has changed a lot, and if you know what you're doing and how carbs work you can get a large carburetor to work really well.

Most people just don't understand carburetion though, and I usually recommend people run "the right size" or a tad smaller. Carbs will run better slightly on the small side than slightly on the large side, when in the hands of the average joe. Most people would just hurt themselves trying to get a large carburetor to work right.

Joel5.0

CFM ratings can be arbitrary, no question about that....... I see them more like a "model" description however, when limitations are imposed using the "model" for sizing, it could be a problem. Add that to the results I've seen/experienced/worked on..... and "over-carbing" does deliver. Mr. Carrol Shelby knew about it, others as well, and the latest real life example continues to confirm it....... a little 331 stroker setup, Canfield 195's, Vic. Jr. intake, custom Tight Lash Solid Roller (TLSR) CI Cam, 4.10 gears, C4 transmission with too tight of a converter for the track (it's a street setup that is driven to the track), no transbrake, LT's and full exhaust that had a custom carburetor made for those specs + driveability....... size or model?......... a Pro-Systems 950 cfm.

After driving it to the track for the first time....... 11.03 ET @122 MPH leaving off idle at the stage...... I don't think torque down low is an issue there.

67mustang302

Haha, the trick is Pro Systems carb. Those guys definitely know what they're doing with carb technology. It'd prolly go slower with some out of the box 950 from Holley or something though.

In the end as long as you can shear and atomize fuel, the size becomes less relevant. That used to be the EFI advantage, you could run unrestricted induction and the injectors did the atomizing for you, rather than needing a bunch of carb signal to do it. Modern booster and metering technology means that you can run much larger carbs than in days gone by and still get good atomization, or even better atomization than EFI.

Pro Systems "carb sizing calculator" is now CIDxRPM/2820 x VE. They say the 2820 rather than 3456 accounts for modern technology in a carb, and will size it larger than traditional and get you closer to where you need to be. And with annular boosters and better metering systems you can go even larger and be fine.

Portmaster

Thats pretty impressive. Do you recall what the 60' time was?