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post #1 of 25 (permalink) Old Feb 18th, 00, 06:51 AM Thread Starter
Join Date: Jan 2000
Location: Cleveland, Ohio, U.S.A.
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I am retiring my 69 DZ carb and intake back on to the my DZ motor. The motor this is coming off of is a 302 clone, 350 block with 3in stroke. I'm running 11.8 to 1 compression, crane cam with the power band between 4700 rpm and 8200 rpm. I have installed an Edelbrock super Victor intake that is rated at 4000 rpm to 8000+ rpm. Now time for a carb, any suggestions ? I'm running a 4.56 rear gear. I have an old Holley 850 double pumper, but I think that is way too much. Any one played with those Speed Demon carbs ? Any input would be helpfull. Thank's Guys

[This message has been edited by Bob Cam (edited 02-18-2000).]
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post #2 of 25 (permalink) Old Feb 19th, 00, 02:36 PM
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I would suggest a 750CFM Demon Carb (double pumper). The following link will help with carb sizing. It is a function of both RPM and cubic inches. I use this same carb on my 396 but I don't rev it past 6200. The idle adjustment is a little tricky on this carb but it is VERY responsive. Stock jetting is 76 on the primaries and 83 on the secondaries although the DYNO shows for my 396 that 80 jets in both primary and secondaries work best for my combination.

Good Luck!!!
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post #3 of 25 (permalink) Old Feb 24th, 00, 12:11 PM
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Bob, want me to store that DZ carb and intake for you ? ? it doesnt hurt to try
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post #4 of 25 (permalink) Old Feb 24th, 00, 07:02 PM
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Speaking of storing carbs, would you be interested in selling 850 double pumper?
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post #5 of 25 (permalink) Old Feb 25th, 00, 04:24 AM
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I have a Holley 850 DP that is for sale. It's on my 406 SB right now, see link below. It came with my car, it's a 4150. I was looking at some holley ads then looking at the Carb again and I think it may actually be a 4150 Professional - i.e. Radiused venturis. Carb is too big for my engine to drive on the street everyday.

Shawn Peterson
1969 Z28, 406SB
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post #6 of 25 (permalink) Old Mar 1st, 00, 03:40 AM
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This post is a little late, I hope it helps. A 750 cfm would do you just fine. We've been using the Speed Demons for a while now, and we really love them. They use the same rebuild parts as the Holley, but the machining seems to be superior to the Holley. By the way, if anyone is rebuilding their Holley, Demon or BG carb, buy the BG (Barry Grant) kits. Way more parts for the money, great kits.
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post #7 of 25 (permalink) Old Mar 2nd, 00, 02:26 PM
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I'm still noodling my way thru the carb sizing exercise (and have been for many years!) I just built a stout 400 SB for my '69 - (Long rods, 10-1 pistons, Edelbrock Perf. RPM heads, RPM manifold, CC 230/236 @ .050 cam) and I have both 600 and 750 Edelbrock carbs. I've swapped the carbs now and (admittedly without much tuning effort) the motor seems to like the 600 better.

It idles a little better, is more responsive, and seems just as strong up to about 5000 rpm. The carb sizing formulas confirm this is the generally correct size for this engine.

So why does everyone runs 750's and 850's? Is it just for higher rpm power? Or do I just need to put in some tuning and jetting time on the 750 to truly appreciate the bigger carb?

'69 400SB, Richmond 5-speed; '99 HD Road King Classic
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post #8 of 25 (permalink) Old Mar 2nd, 00, 03:33 PM
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I would bet that quite a few people run carbs on their cars that are too big for their application. Bigger is not always better, especially in carbs and cams. You have to look at the volumetric efficiency of the engine. There is a formula to help determine appropriate carb size:
(CID x MAX RPMs) / 3456 = required CFMs
I got this formula from the book Tuning Holley Carburators or something like that. The important thing here is that the result will give you CFM rating for an engine working at 100% volumetric efficiency. A stock engine works at only 70-75% efficiency while an all-out racing engine with $$$ head work and such works around 90-95% efficiency. So if you have a really hot streetable engine, maybe it's around 80% or so:
(396 CID x 6000 RPMs) / 3456 = 687.5 * .8 (80%) = 550 CFMs. Hope this helps.
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post #9 of 25 (permalink) Old Mar 3rd, 00, 04:03 AM
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Definitely watch out for too big. This is where double pumpers get a bad rap. I once ran a street 400 w/a 650 DP. ran very low 13's on street tires, idled at 550 RPM, reved to 6K, got 14 MPG w/ 3.73 gears, with great response and driveability. Tuning (carb & ignition) is vital, most carbs are way rich out of the box.

[This message has been edited by JimM (edited 03-03-2000).]
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post #10 of 25 (permalink) Old Mar 3rd, 00, 08:43 AM
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Dual plane, divided plenum intakes are much more forgiving of over-carbing than single plane intakes with no plenum divider.

I always wondered why Chevy "over-carbed" the cars right off the assembly line, until I realized that factory perofmance engines like the DZ302 or solid lifter LT1s used dual plane intakes, not single planes.

For what it is worth I run a 600cfm Edlebrock on my warmed over 350 in my 79 Z28 and like it quite a bit. When I bought the car the old owner had a Holley 750 on it and the car "loaded up" big time. Could also be a poorly tuned Holley, but I knew right away when I test drove it after swapping the 600cfm carb that it was the right decision.

Most guys fixate on peak power figures instead of worrying about average power figures. The car magazines also play into this by swapping bigger carbs onto their test engines while dynoing them for some buildup article to try to get the last 5 peak horsepower out of the engine. To each their own, but I drive my car at peak power about .00000001% of the time and under peak power the rest of the time.

- 69 SS350 Daytona Yellow
- 79 Z28 tons of mods
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post #11 of 25 (permalink) Old Mar 3rd, 00, 12:17 PM
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The other reason GM could overcarb is that they used vacuum secondary carbs. Did they ever use double pumpers??
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post #12 of 25 (permalink) Old Mar 6th, 00, 06:47 AM
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I agree with both the comment on carbs being too rich out of the box and also the general tendency to overcarb. In addition, I'm at 6400 feet so I need to go leaner still than guys at lower elevations.

My car is running pretty good with this 600 and it's very responsive. It's also still way too rich. I need to spend some time with my Edelbrock book and the Strip Kit and lean it out a bit.

I'm also struggling with the choke setup. I can't seem to get the electric choke quite right. The fast idle screw either doesn't want to sit on the cam or the cam won't get out of the way when it gets warmed up? I'm going to get this a little closer and then go see a carb guy for the rest.

'69 400SB, Richmond 5-speed; '99 HD Road King Classic
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post #13 of 25 (permalink) Old Mar 6th, 00, 01:54 PM
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I talked to 2 local carb tuners today and explained my situation, i.e.; what is the proper size carb for my engine - the 600 or 750? And of course, I got one vote for each! However, over the course of a discussion the 600 guy made more sense and so I made an appointment to take the car and both carbs in and do a little tuning.

'69 400SB, Richmond 5-speed; '99 HD Road King Classic
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post #14 of 25 (permalink) Old Mar 25th, 01, 02:54 AM
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Calculate CFM for your engine
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post #15 of 25 (permalink) Old Mar 25th, 01, 08:01 AM
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Does my 'ol heart good to see some intelligent discussion on carb sizing! I''ve been building SBC's for 30+ years, and people always ask why they run so well, then they get quizzical looks on their faces when I tell them I run NOTHING but 600CFM vacuum-secondary 4150 dual-feed electric-choke Holleys (#0-80145), generally on Performer RPM manifolds, whether they're 350's or 383's. Can't believe how many guys listen to the speed shop counter guy or read the dyno articles in the magazines and run out to buy 750-850 Holleys (or even Dominators, for crying out loud), and then all you hear is how their car runs like crap. DUH!! They don't bother to learn about air velocity, vacuum signal strength for metering accuracy, or anything else that really matters for daily driving - just those BIG numbers! (which are only there for a few seconds at WOT on a dyno under carefully-controlled conditions).

Also hard to convince the "big carb nuts" that the only reason the 302's came with a 780-800CFM Holley 4053 was that it had to be homologated that way as a production car in order to be used legally in the SCCA Trans-Am cars, which spent their whole life at wide-open throttle between 5000 and 8000 rpm - just like your street car, right?

Unless you run your SBC engine at 8000 rpm all day long (look out for shrapnel!), a properly-tuned 600 Holley is just right for 99.9999999% of your driving. Speed shop guys just LOVE to sell $600 carburetors - the 600CFM Holley #0-80145 is $265.00 (Jeg's), and the comparable-CFM Road Demon is probably just as good or better, at about the same price (haven't tried one of those yet, but hear good things about them).

Keep the good logic coming! (At least half of the posts in the "Troubleshooting" section would probably disappear without 750-and-larger carbs).

'69 Z28 Fathom Green
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