This is simple... I noticed you did not list your cam at .05 and this is important to the selection to determine if you will go speed demon or mighty demon. The CFM rating is a no brainer (cubic inches x max rpm (be realistic) / 3456) A 350 chevy running a red line of 6000 rpm 350x6000 = 2100000 / 3456= 607 CFM. A motor will rarely reach 100% volumetric efficiency but on average maybe 85% so multiply this 607 CFM by .85 = 516.5 CFM. It should become clear that a 650 would more than support this application. A motor is just an air pump and nothing else... Also note that a bigger carb does not mean it will run rich. In fact it will run LEAN yes LEAN, why?, because you do not have the venturie velocity needed to pull the fuel from the boosters. Bigger venturies require a larger cubic inch or higher reving engine to support the needed requirements. If we take the same 350ci chevy but raise the redline to 8500RPM you would get these figures 350x8500=2975000/3456=860cfmx.85= 731 CFM. as you can see, a higher reving small block would need a 750CFM carb. This is true guys, ask around, call the tech centers and the guys who know carbs. Bigger is NOT BETTER.

 

Boy times have changed at least there are a few posts here that are in favour of the 750cfm carb on the small chevy when I put mine on everyone told me I was nuts.....except for MountainMotor :lol:

 

Listen to Lars!

Unless you only drive to the grocery store and never rev above 5,000rpm go with the bigger carb. I lived at 8500' elevation, half a mile higher than Lars If anything, a bigger carb at high altitude would make things worse, not better. The air was at least 30% less dense than at sea level. This would be like you guys at sea level using a 30% smaller engine.

Take the time and make the effort to get the carb and ignition setup to perfection. More $$$ are wasted with mis-matched expensive bolt ons looking for more power when a sharp tuneup would do as much or more for a lot less.

Chuck

 

MotorHead,

Even so, I have it on good authority that you're still nuts! :jester

Chuck

 

My head hurts :lol:

I think I'm sold on using the Speed Demon 750. Now if i could just figure out to use the mechanical or vacuum secondaries :crazy: :jester

Being a daily driver, won't I need a choke for the winters? It can get cold here in SC. That being the case, wouldn't I have to use a vacuum secondary, as all mechanicals I've seen are without a choke.

 

I guess I should have said that's when everyone figured out I was nuts :lol:


My Holley has a manual choke, getting it to work is the problem, I think you can retrofit them with an electric choke kit. :D


[Modified by MotorHead, 8:47 PM 2/2/2004]

 

SlingBlade,

Generally a vacuum secondary is used with an automatic transmission. This is usually because automatic cars tend to be geared quite high (low numerically) I installed a 650cfm Holley double pumper (mechanical secondaries) with my 79 L82 automatic and 3.55 gears. It worked very well. Your 3.70 gears should help even more. If you feel any bog when getting on the throttle with a mechanical secondary carb that has been adjusted perfectly, you need to roll on the throttle a little slower. The vacuum secondary carb will only open as fast as the spring and venturi vacuum pod will allow.

It is very easy to setup a vacuum secondary carb to prevent any bog during acceleration, but you may lose a bit of the instant throttle response the double pumpers are famous for. It takes a better & more sensitive driver to get the most out of a carb with mechanical secondaries. I like to think of myself as a sensitive kind of guy!

It comes down to preference. I get bored with cars that I only have to point and punch. I prefer shifting my own gears and modulating my own intake flow. A player piano may be able to play Chopin perfectly, but I would rather have a standard concert grand piano even though I can only play chop sticks. It gives me greater pleasure to learn to play Chopin myself. Neither is right or wrong, it just depends on what you like to do.

Chuck

 

Let me do one more post on the subject... notice that a couple of the experienced guys who are running fast cars lean towards the bigger carbs....

First, I just want to make sure you realize that I don’t pull this stuff out of my butt. I’ve been racing and building cars for 30 years. I put myself through 8 years of college and a PhD through street racing, with winnings totaling over $100K. I hold 2 NHRA Super Stock records (SS/KA). My Doctorate is in Medical Engineering with an emphasis in Thermodynamics (which includes concepts like internal combustion engines). I taught at the GM training center in Salt Lake as an advanced instructor for Carburetors, Ignition Systems, and Automatic Transmissions. I have been restoring and modifying musclecars since 1974, and currently own a large automotive restoration facility in addition to my job as the Manager for the Lockheed Martin Space Systems Propulsion Laboratory. I have rebuilt over 400 Q-Jet, Holley and Barry Grant carbs over the last 5 years. I am the carb builder for the 2000 and 2002 Bonneville World Speed Record Holder in the Stock Body Class. I’ve done extensive actual dyno testing with Hot Rod Magazine and Dougan’s Engine Service at the Westech facilities in Los Angeles to compare and verify the things I talk about regarding carbs and ignition systems. I smoke cigars and I drink beer. I know what I’m talking about. So here are the facts:

First, regarding vacuum secondary carbs: A vac sec carb, even with the lightest spring installed, will seldom, if ever, actually get the secondaries wide open. In dyno runs, I consistently see that a 650 or 750 vac sec on a 350 will open the secondaries about halfway, even with the lightest spring. Forcing the secondaries manually open during the dyno run produces a massive power gain. Vacuum secondary carbs are nice for 4-wheel-drive vehicles where you don’t want to suddenly pop the secondaries open while you’re bouncing across rocks and trees. They’re also nice for motor homes and trucks with horse trailers where you don’t want to force the secondaries open under low-rpm load-pulling conditions. If you’re going to pull a horse trailer with your Vette, get a vacuum secondary 650.

Carbs are cfm rated at a given “Delta Hg” pressure. In other words, with a given pressure differential between the top and bottom of the carb, the carb will allow a certain amount of air to pass through. There are all kinds of graphs and formulas for figuring out how many cfm your engine needs. But fact is, at WOT (wide open throttle), your engine does not produce the Delta pressure that the carb is rated at. So the carb flows a bunch less than its rating. You can successfully “over-carburate” an engine and actually get a performance improvement – even at low rpm. We have proven this on the engine dyno over and over. Look at the guys who are running fast: They’re not running 550 or 650 cfm carbs…. I’ve run carbs as big as 1050 on a small block, and they idle just fine with no off-idle stumble once set up right. No… they don’t run rich because they’re big…

Keep in mind through all this that GM installed 750 cfm Q-Jets on engines 250 cid and bigger (the OHC Pontiac was 250 inches with a 750 Q-Jet). I spite of common misconceptions, the Q-Jet is NOT a vacuum secondary carb: It is a mechanical secondary carb with an airvalve to allow the secondaries to come in without the use of a secondary accelerator pump (a secondary pump would cost more).

I have run back-to-back tests of 650 and 750 carbs on engines down to 302 cubic inches (take a look at this month’s issue of Hot Rod Magazine: There is an article in there by Matt King on computer simulation versus actual power. The engine in the article is a 302 Ford. We ran that engine with a 650 and a 750 BG mech sec carb. The 750 consistently produced 5 more hp across the rpm range, and the results in the article are with the 750 mech sec carb as shown in the photo). Dyno runs typically start the pull at 3000 rpm, where the engine is placed under massive load and the carb is popped right open to WOT. On every run I have made, a 302, 327, 350 and 383 will take a 750 mechanical secondary carb slammed right open to WOT at 3000 rpm without any bog or hesitation, and it will consistently produce better power across the rpm range than a 650 carb.

I run a 750 Speed Demon mechanical sec carb with no choke on my stock 350 in a ’64 Roadster. It’s jetted to its out-of-the-box specs with a nice blueprint setup by yours truly. I can run this 4-speed car at 1000 rpm and punch the throttle wide open without dropping the clutch with no bog or hesitation (this is a much more adverse condition than an auto trans car with a 2000 stall stock converter). Performance off idle is identical to a vac sec carb, but at WOT, the mech sec carb walks away from the vac carb. No contest. I have shown this on the dyno and on the street repeatedly. My 23-year-old daughter drives this car with no problem. She has never been able to stall it or bog it. Guys - please tell me you’re bigger men than my daughter.

Of key importance on any BG or Holley carb is the initial bench setup of the throttle blade angles. This does NOT come set up right from the factory on either Holley or BG carbs. This involves setting the primary throttle blades to expose .020” of the transition slot, and then setting the secondary blades to match the primary blade opening exactly. Any idle speed change must involve re-setting both primary and secondary blade angles equally to produce equal air flow and bleed through all 4 corners of the carb. Failure to do so results in poor idle quality, improper mixtures (“This big carb runs too rich!”), and off-idle stumbles (“the carb must be too big”). I outline this in my BG Tuning article – contact me by e-mail if you want a copy.

Run a 750 on the 350. Run an 850 on anything bigger than a 427. Run a 650 or a 750 vacuum sec carb on your motor home and Dodge 4X4 truck. If you can’t get the carb to run right, send it to me, and I’ll set it up for you.

If you really want to run a 650 or a 750 vac sec carb, I'm sure you'll be just fine. I'll take your money from you at the next stoplight.

 

I need a drink of water after that one.
Found that very interesting. :cheers:

 

Why thank you lars. It was very nice of you to donate your expertise for free here. I have already saved and shall try to put your recommendatons to use. Also I hope we don't seem to ignore your hard earned advice. It is just there has been so much perpetuated - some very convincing looking testing and numbers - that its hard to buck the media metality. Us novice grease monckeys rarely have access to dyno equipment and are dependent on what little has been published.
You really should write us a book. Again thanks. cardo0

 

I bow to you Lars!!! Guess I just read too many books or something.

 

:thumbs: :thumbs: :cheers: little change here, i've read a few place's that
the E. 750cfm carb has a design flaw that make's it inconsistent on the dyno
pulls, they talked about that may be why the new 800 is out , any thoughts
from anybody inthe know???

 

Lars

That was very interesting and informative, thanks Lars! :thumbs:

 

Lars, You Da Man! :thumbs:

 

Lars for President! :cheers:

 

One last question.

From the G-tech readings, I estimate that I have roughly 280-300 crank hp. This is with a stock Q-Jet and performer 2101 intake. Switching to a new carburetor and RPM intake is the next phase in the quest for more hp.

Assuming that I dial the carb and timing in, would I reach my goal of 350hp? 50hp seems like a lot just by changing the carburetor and intake.


[Modified by Sling Blade, 10:09 AM 2/3/2004]

 

The combination you're running should be good for much more than 280-300 at the crank. Your readings are either off a tad or you have a lot to gain from a little tuning and tweaking.

Getting your ignition curve right and the right carb set up is certainly good for some power - no question. During some of my CorvetteForum Tuning For Beer Tours, we have easily tuned an additional 30 horses into the engines just through timing and some jet changes.

Most critical in your quest for power is to get that timing set up right. Get an adjustable timing light if you don't already have one. Get your advance curve to slam in quickly by 2500 rpm, and make sure you're getting at least 36 degrees total timing. Make sure your distributor is in good shape, and that you're not getting any "timing scatter" at elevated rpm: timing should be rock steady at 36 above 2500 rpm. Timing and ignition accuracy is critical - spend some time on it to make sure it's right.

Once this is verified, get that Speed Demon carb set up just right. Use my BG setup paper to do the bench settings. Run your float levels to the lower of the 3 sight glass lines, and make sure your 4 mixture screws are all balanced out the same and optimized. The accel pumps are always misadjusted on the carbs coming out of the box, so set them up right. Use a reflective heat shield between the carb and the manifold, and make sure you have a good mechanical fuel pump wth a nice free-flowing fuel filter installed (don't install the filter on the suction side of the pump). Use a K&N air filter. Don't skimp on plug wires - MSD makes some very nice sets. Summit sells 'em for a good price.

Assuming that your cam is correctly installed and everything is assembled correctly, your combo should easily make power up through 6000 rpm, and you should get into the 350 horse range without difficulty. It should give you outstanding throttle response with very good drivability.

 

Very informative posts.I`m about to do some dyno runs with my newly built 461 on to the Dyno with a 950Hp carb..This thread came in just in time.
Thanks Lars, for sharing your knowledge.. :cheers: :cool: :cheers:
When does the "Tuning for Beer" Tour come to Europe? :thumbs: :cheers:


[Modified by BB wowbagger, 5:04 PM 2/3/2004]

 

WHERE did you find that worshipping smiley??? There's a wet spot on my chair where I laughed so hard!!

 

BB-
Tuning for Beer Tour comes to Europe this summer. I'll be in Oslo and Trondheim in July. I could certainly make a run over to Sweden at the same time. Let me know if there's interest.