There is some information floating around on the net that the CFM ratings of carburetors has been inflated. All of the carburetor companies now use a higher vacuum to up the CFM's that they advertise. For example Holley takes a 750 body and puts on an 850 base and calls it a 950. uzzled: I know that there is more to airflow than just bore size but a bigger hole will flow more air if all the other variables are the same. Sure would be nice to know that apples are being compared to apples here. I do know when I was trying different carbs this summer there was no difference between in performance between the 830 and the 950. Both ran a best of 106 mph. Did not get to do a dyno comparison. I have talked to some people that know their stuff and they just love an old 850 Holley. It has a bigger throttle body than the 950! So which one is the "bigger" carb.
Doug, Holley is the first manufactor that used the cfm method all the rest used the physical '' area'' of the bores. I would trust Holleys measurements above all others. BUT [there is always a b-u-t in the mix.] With carbs, there is no absolute. Once , I traded my 850 sq.bore holley to a guy for a 950 holley 3 bbl. My car was a 65 nailhead 401 skylark and his was a 68 396 camero. All our buddies said it wouldnt work for me. When we got the cars back to running both cars ran much better. I dont mean just a little, both cars would fry the tires. i was happy and the other fella was extatic. You can trust Holley. If you want a holley to perform really well, just buy one of thier books and set the carb up just like they say. they have already done all the little tricks and found out how well each one works. the local guys think I have some big secret about tuning holleys. I do. The secret is this; build it just like holley says.
Doc, Thanks for the reply. It does seem that most of the top tuners want to work from a Holley platform. I understand part of the reason is that they can get parts to customize at a reasonable cost. I think that I am going to try and find a 1000HP and work from there.
doug, i did something different, i went with the old holley 3 barrell that was made in the years 68-69 for nascar. they were in two sizes 950 cfm and 1050 cfm with vac secondaries. They work very well.
Doc, Just wondering how you changed secondary jetting ( if at all) as the 3 bbl used specific jet plates? I ended up modifying an old 4781 secondary metering plate to match the 3 bbl booster position and now use screw-in jets. Curious! Jay
Jay, i did not change the secondarys at all on either the 950 or the 1050. They seem to do just fine jetted the way they are from the factory. The only thing that I have changed is the sec. diaphragm spring, and then all I did was to stretch it a little. Somewhere, I have a chart that gives the sizes of the jet plates in thousanths so they can be cross matched.
Hey Doc, while looking for something else, I found my old Holley info and noticed that there were only 3 jet plates available for the 3 bbls (34BP-5113-3, -4,& -5). Didn't look like much of a selection-seems like it would be hard to "dial-in" without a drill set unless the standard 4 bbl plates(34BP-2007-x) would interchange! Would you happen to know as I have never checked? Jay
Well, Jay, I tend to trust holley. If there is only three plates there probably is a reason. I have found them to be right most of the time. That said, there is only one way to tell if the 4 bbl. plates will work. get them side by side and compare. Very carefully, I might add. Personally , I wish that holley would not have put jet plates on any of thier carbs. I prefer the old style jet block for the secondaries. That way, Ican jet the fronts leaner and add what I take off the front to the rear jets to keep the same fuel ratio. But, I doubt that there would be a need for more than the 3 plates that Holley specifies because the rear barrell is assisted by the power valve. when the secondary is opening the power valve kicks in and richens the mixture. The 3 bbls came with the rio accelerator pump so there is no mod needed there. The 3bbls were created and set up for nascar racing, they were very well set up by holley already.
Doc, help me out as I have never seen or can even see how a power valve can be utilized with a jet plate. I'm no expert but I have worked on quite a few-including the 3 bbl. Thanks. Jay
Jay, The power valve is on the front side, but it doesnt come into play untill the carb is opened up enough to make the vacume drop to the level it is calibrated for. This is certainly in play when the secondary starts to open. The power valve works off of vacume only, so when the throttle is opened if only slightly the vac drops , if only for an instant, and the power valve follows the vac. indication , if only for an instant. If the throttle is held open then the power stays open untill the throttle is closed enough to cause the vacume to go up.
Ok Doc, I'm with you. I was thinking you had installed a power valve on the secondary side while retaining the OEM jet plate & I couldn't see it working without a lot of modification. From what I gather, you do all your mixture control on the primary side and let the secondary side fall where it may. If I remember correctly, the power valve channel restrictors are much smaller on the 3 bbls than the 4 bbls and would have to run larger primary main jets to get the A/R ratio in line. I really didn't care for that, that's why I modified mine the way I did. Still run a power valve on the primary side, drilled the PVCRs to the 4781 specs & reduced main jet size on the primary side accordingly for improved idle quality, throttle response, & mid-range drivability. Modified a 4781 secondary metering plate to fit the 3bbl secondary main booster placement,installed it in place of the jet plate and whala, A very balanced, tuneable,& streetable 1030. Thanks for your response. Jay
You know, a lot of guys get all concerned with the cfms of a carb and forget about the air cleaner. The air cleaner is the villan that steals cfms more than the carb. as often as not. Take a hard look at the total pkg. [carb plus air cleaner] .
Getting back to the original question, are carb ratings inflated? Did anyone here go on the Holley factory tour in Bowling Green at the Nats about 10 years ago? It was one of the neatest things I've witnessed. The tours were given in groups, with various questions being asked along the way to the "tour guide". Cfm ratings came up, and while I can't recall if anything was said about 4150 series, the dominators were addressed. Holley guy said 1050 dom flows 1090, 1150 flows 1190, but the 1250 flows 1250. By the way, I can't remember if this was before or after the HP series were introduced.
