lars

Building, tuning and playing with Forum Members' parts puts me in the unique position of being able to compare a wide variety of parts and components on a back-to-back basis: I have my '64 350 Roadster set up in such a way that it can be easily run with any of the popular carbs, so I actually test run every single Forum Member carb that I build and set up. This is actually a lot of fun...! In addition to test running a wide variety of carbs, I also test run distributors with a wide range of curve setups to get back-to-back comparisons of the differences.

There have been a lot of discussions regarding carb sizing here on the Forum. There is a wide range of opinions, and a few people actually get a little emotional about it.... I also notice that the carb tends to get blamed for most poor performance problems: If a car stumbles, it's the carb. If it breaks up, it's the carb. And it's usually because the "carb is too big." There seems to be the perception that a carb that's too big will cause poor throttle response, off-idle stumble issues, flat spots, and a variety of other odd symptoms and problems.

I have used my 350 as a testbed for Carter AFBs (less than 600cfm), Rochester Q-Jets (750 and 800 cfm versions), Holley 390, 590, 600, 650, and 750s (both vacuum and mechanical secondary), Edelbrock Performers (600-800 cfm), BG Speed Demon 650 & 750 (I use a BG Speed Demon 750 double pumper as my standard setup), and BG Mighty Demons (650 & 750).

Right now, I have Forum member "1bad69" (Mike's) brand new out-of-the-box BG Speed Demon 850 double pumper mechanical secondary on the "Forum Testbed" 350. I did a quick bench setup on the carb, bolted it up, and took it out for a drive....

...I'm gonna' go change my Depends now, because I think I just pee'd my pants... this is just too much fun..! Who the heck ever said that a big carb produces flat spots, hesitations, and off-idle stumbles...?? With a good bench setup and a little bit of tweaking, this 850 DP carb produces absolutely instant and brutal off-idle throttle response, incredible drivability, and tire-smoking WOT performance with all 4 barrels open to the full 850 capacity. There are no flat spots, no hesitations, and no stumbles - it performs impeccably and flawlessly, in spite of being a clear case of "over carburation."

(Mike - I can't wait to get this thing bolted up on your 454... this thing should freakin' scream...).

The point of all this is not that I'm advocating 850 Demons (which are rumored to flow quite a bit more than their rating) on 350 smallblocks. A 350 is clearly and obviously over-carbureted with an 850 by a significant margin. Rather, the point is that the problems of stumbles, hesitations and poor performance are generally not caused by "improper carb sizing." A correctly set up carb, virtually regardless of size, can be made to run incredibly well on virtually any engine. If you have hesitation problems, it's not because your carb is too big - it's a tuning issue.

Other things I've noted over my years of tuning these things: The smaller carbs are much more forgiving regarding a bad setp: You can tune and jet a small carb anywhere across a broad range, and it will still run within an acceptable range. The bigger the carb, the more sensitive it gets to the tuning parameters, but it will still run and respond extremely well. The 850 on my car right now is one of the best running carbs I've tested in a long time, but the low airflow requirements of my 350 only allows me to crack the idle speed screws open 1/8 turn. With this small of a throttle opening at idle, it's difficult to keep the primary & secondary throttle angles correctly balanced - it takes a little finesse and a "feel" for where the throttles are at. Idle quality and off-idle throttle response is easily destroyed by getting the settings out of whack. But once correctly balanced and set, the carb will perform perfectly. I'll be showing Mike how to get this set right when we install this 850 on his 454 this summer during the CorvetteForum Toronto Tuning for Beer Tour. Stay tuned for photos and writeups on this event...! :cheers:

Now, quit screwing around here on the Forum and get back to the garage and do a little tuning on those carbs!!

...and don't ever try tuning a carb without a beer.

1Bad69

Lars...I think i may have pee'd myself also just reading about it...damn, i can't wait for June to roll around...awesome job...thanks... :thumbs:

:party:

MotorHead

I have to agree with everything you said. I think if you put a big carb on a small engine it will run fine if tuned properly, I believe if the motor isn't pulling the CFM's the carb can deliver it simply doesn't use it, it does not cause stumble or bog. A guy I know is running a 1050 Dominator on a 350ci street/strip motor.

Hmmm I wonder what carb I need for the MiniRat :lol:

standup

That's great info, thanks :cheers: ..........just curious if you think there is any advantage in running a larger carb (greater than 600cfm) on a stock 350?????

Also, what do you think about the Barry Grant 3x2 "Six Shooter" setup for a stock 350?????

Brettmc

I agree wholheartedly with Lars. The Holley 850 DP on my 468 runs great. I've learned a great deal on how to tune it from Lars. I've made several changes to it and have to re-tune it regularly. I think it's got to the point that I make changes just so I can re-tune the carb!!

Brett :thumbs:

The Dude

Lars is right on the money. (And he even sounds sober.) I was running the same 800 CFM Holley DP on the 355 I pulled in favor of the 406. It was great--even performed BETTER than the Lars-prepped Q-Jet I had in the now-defunct loaner program a couple of years back.

Now, repeat after me:

There's no such thing as over-carburetion--"they" are often WRONG.

MotorHead is absolutely correct. I can't for the life of me figure out how some people believe that a carburetor "knows" what engine it's sitting on. Ab-so-funnin' ludicrous... :lol:


[Modified by The Dude, 5:23 PM 4/5/2004]

74droptop

Lars how will a 750dp with mech sec perform on a 427sb motor.Will it be enough for the big inch motor.I currently have this carb on my 383 seems to run ok I'm sure if you tuned it things would be better.How much do you charge to set up a carb??

AGVI

Sounds awesome! Mike, I want a ride!!! :hurray:

Cali,68,L-79

:withstupid:

lars

74drop -
A 750 DP will run very nicely on your 427....
....but an 850 will tear your head off...!

1Bad69

i guess i should start looking for a deal on some new tires...

75Rag

I am soooooooooooooo looking forward to the Lars Toronto Tuning for Beer Tour ...... feels like Christmas ....... how many more sleeps?

Stewart's74

:iagree: Hell yeah! I cant wait till June!

Sigforty

Hey Lars when are you making your next pass down South here to Louisiana. I missed you last time, but will be ready if you ever come back.

DJ Dep

Hey Lars...are you saying you are gonna dump all those crappy Quadrajunks and come over to the dark side? :D :reddevil LOL

Deo

Cali,68,L-79

Crass

1Bad69

crass

adj : (of persons) so unrefined as to be lacking in discrimination and sensibility


:shocked:

lars

Not exactly... but I am saying that there is more than one way to skin a cat... :yesnod:

mandm1200

I have a few thoughts in regards to this post/topic.

The impression I got from reading this is that bigger is better. How did I arrive at that? "I'm gonna' go change my Depends now, because I think I just pee'd my pants" was the statement. Is poster stating that this carb is one of the best he tested (maybe the best) or did I read too much into it?

I consider myself to be logical and analytical so going so far overboard with an 850cfm does not seem practical. I do want to note that this is not intended to flame or bash anyone, just another viewpoint. If an 850 is better than a 650, why isn't a 1050 better than an 850; or why not just go with two 1050's. The point I am trying to make is that there is a certain size carburetor that will be the best if the same type/model from the same manufacture are compared. Only one will be the best at obtaining the most horse power over the broadest rpm band, some will be 'good' and others can be called 'better'. Since the Holley and BG are different, a 750 maybe optimal from one company while the 700 or 800 may be best from another. The human factor has to be eliminated and actual data taken. This is why there are engine dyno's. They record real values. Another way is to go to a track and make half a dozen passes, make a change and see what the difference is.

I don't think anyone would disagree that changing the size of the carburetor will have an impact on the dyno results. Depending on what the driver/owner is looking for will also have a great deal to do with the size of the carburetor that will be ultimately chosen. Many times it will be a give and take; lose a little on the bottom end and pick up some hp and torque at the higher end. My belief is that there is some reason why the manufactures did not run 850's on a small block back in the late 60's early 70's during the horse power wars.

At what point is a carburetor too big? Although it is not an exact science, the maximum cfm an engine can be calculated. A 350ci at 6000 rpm's will roughly need 600cfm. In other words, the engine will consume 600cfm of fuel mixture running at 6000rpms. It will not breath in 850cfm unless the rpm's are rasied to 8500. The cfm of the engine will be relatively constant no matter what the size of the carburetor is bolted on, unless you install one that is too small for the rpm that the engine is running at.

I agree that an 850cfm can be made to work on a 350ci. I don't question that. What I really want to know and see is the 850cfm out performing the 650cfm at both the bottom end and the high end. How responsive it is compared to the 650. From my experience 20 years ago on a 400 plus horsepower 350ci, there was quite a bit of difference when running a 650, 700, or 750 Holley mechanical double pumper. The 650 was much snappier at lower rpm’s, but tailed off a close to 7000 rpm's. The 750 Holley was sluggish at lower rpm's but ran fine up to 7500 rpm's. At lower rpm's I am referring to under 4000.

So if anybody wants to share their dyno sheets here showing the hp/torque curves with carburetor swaps it would be much appreciated. I was planning on using a 670 Street Avenger for the 388ci engine (60 over and 3.750 stroke) I am putting together. The 670cfm may be too small.

Mike

1Bad69

i will post my dyno sheet later in the summer after our dyno day...there is a holley 750 on it now...
dyno sheet for that is in my sig. in the web page...


[Modified by 1bad69, 6:27 PM 4/6/2004]