lars

The CCC (Colorado Corvette Crazies - the Ultimate Corvette Tuning and Beer Drinking Fraternity) got together this last weekend for a little tuning and beer tasting. As part of our tuning session, we had a BG Speed Demon 850DP that belongs to Forum member "lavetteman" (Joey). Joey has been having trouble getting the 850 to idle and run right on his BB C3, and was afraid it might be too much carb for the car.

Now, we've all heard the horror stories of installing a carb that's too big: You can't tune it right; it will bog; it will run rich; you'll loose throttle response; your dog will die, etc. So we decided to set up Joey's 850 and run it on a well-build small block...

With the 850 all set up on the bench, we yank'ed the BG750DP off of Co_Sledgehammer's (John's) '64 409 small block and slapped on the 850, jetted just 1 size smaller than stock jetting on both primary and secondary sides. With mechanical secondaries, the 850 was probably going to make the SB fall on its face....

The 850 actually produced an outstanding idle quality with responsive idle mixture screws and instant off-idle throttle response. Smashing the throttle produced no bog whatsoever, and the car showed an absolutely brutal level of performance with no adverse side effects at all. In fact, we're thinking of keeping Joey's 850 and telling him it got lost in the mail... Seat-of-the-pants feel is that the 850 actually runs stronger and smoother than the smaller 750 on this modified small block. Spark plugs all burned a nice gray color... just perfect.

So much for the myth of the oversized carb... not that I think I'd recommend running 850's on stock small blocks, but this big carb sure does run nice...

GrandSportC3

great information!!!

Brettmc

That's great! Now that its tested on a big small block, can you guys run it on a small big block? Any idea why it wont run on Joey's vette?

Brett :thumbs:

MotorHead

That's very good info, if you take a look at my sig you will see many parts on my "L48' that are supposed to kill it, but it runs perfect from idle to 6000RPM with a 3.08 rear end. :D

lars

I set it up a bit differently from the way Joey had it set up. Joey and I have also talked about some other tuning issues on his car that are not related to the carb. Once those issues are ironed out, this carb should run really well on his BB.

Chris69

Well doesn't that just throw all the equations and tests by the likes of Lingenfelter, CHP and others out the window. Just when I thought I had it all figured out, I obviously continue to know nothing!! Back to step 1. :lol: :lol:

:steering:

lars

I clearly wouldn't advocate running an 850 on most small blocks and throwing all the carb sizing rules out the window. But this little comparison test does show that it's possible to tune a big carb for a small application without the problems of stumbling, bogging, rich-running, etc. that are so commonly rumored. If you have such problems, it's probably not entirely due to the carb...

427Hotrod

Great info Lars...

I've been saying for years....you can make a "large" carb work just fine without too much effort if you don't get too far out of the window and make sure all the basics are in place...timing curve...proper cam etc.

I've used 850 DP's on cars with auto's and 3.08 gears and they worked beautiful. Just has to be set up right.

A few years ago I tested one of those 750 Dominators that I had just rebuilt for a buddy on my 427 in the Vette. I removed the 1050 I had on there, which by all rights was way too big for an oval port, 427 and installed the 750. While it ran fine.....no problems....there was a definite drop in performance with the smaller carb.

The Mfgs. encourage you to use smaller ones because they are more "idiot proof" meaning they are less likely to act weird.

But I LOVE to race guys who subscribe to all that stuff.

Engines like air and whatever you can do to feed them will be good!


JIM

lars

Hey Jim! Good hearing from you. The TX flag is still proudly displayed in the garage... I hear you're running in the 10-range... great job! In your post you touch on one of my favorite subjects: I always tell people that 90% of all carb problems are ignition setup problems. What you say is true: if the rest of the car is set up right, you can get away with just about anything on the carb sizing, and erring on the big end is not all bad!

gkull

I keep all my old Chevy High performance mags. I was just rereading an article about whether to tunnel ram or not. Dec. 00 issue. On stroker small block they did the best with a single 1020 cfm competion fuel systems carb four barrel on a single plane. Forget the 650 - 750 myth about small blocks

The secret was in the fuel delivery of the downleg style boosters. Then they went to 1820 cfm of tunnel ram and it delivered 11.7 more foot pounds from 3500 - 7500

WA 2 FST

Hearing this report makes me wonder less and less why my 950 HP Holley runs perfectly on my mild 454. Everyone except Jim and Chuck Harmon said the carb would be too big. It runs a tad fat at WOT during the summer, but driveability and idle are excellent, and now that its getting cooler its getting faster. :)

thanks for taking the time to post this info.

LAvetteman

Check it out, my carb is famous!!!! :lol:

Brett, my guess is my old 454, which was in my 71 LONG before I bought it, has issues. No telling how many hours it has on it, some probably pretty rough, and no telling how well it was built to begin with.....

Here are the facts that concern me:

When I did a compression test months ago, I had even compression between all 8 cylinders, but many forum members commented how low they seemed to be.

I have installed an O2 sensor and A/F meter to try to aid in tuning. WHen I start her up cold she steadily climbs from lean to rich and reaches full rich within 1 minute of cold startup. No matter what RPM or load range I run, she stays at full rich. The demon 850's come with 85 primaries and 93 secondaries. So I try jetting down to 78 primaries and 85 secondaries. She seemed to run cleaner, but A/F meter still showed the same readings.

The old Holley 750 that was on the BB when I got it had 73 primaries and 83 secondaries. She ran ok with that setup, but the entire carb was worn and leaking fairly bad.

When I puchased the 850 last December I posted asking what should I buy, the 750 or the 850. Lars and a few others suggested the 750. Corey, a few others suggested the 850. One member with a yellow 70-72( can't remember who he was) said he had a similar set up, had tried the 750, but got much better perfomance out of the 850. SO thats what I ordered.

My guess, the 454 is worn and not running up to its potential, not sure though.

Problem is, I don't have the time, funds, or the interest to do any major repairs to her, I am afraid she will have to go, soon.

blunsford

I just bought a street avenger 770 for my 454. Now I'm wondering if it's too small. Anyone???

427Hotrod

Great to hear I'm still represented in the garage! I keep up with you guys here on the Forum and at least you know I'm there in spirit!

Yeah, I made it into the 10.80's first time out. Only got one pass before being asked to leave....pesky little rollcage issue!

I'll be going back soon set on "kill" and I won't make it an "easy" check out pass like last time! If weather cooperates maybe I can hit some good times. Of course running with 3700 ft showing on weather station would be great to you guys! I want some of that "cave" air!

See ya,


JIM

killervette666

Im running a 750 cfm Speed Demon with a Performer RPM intake manifold, on an other wise stock L-82, can i tune it to work??? were planin on gettin cam/lifter kit as well as port matching heads :confused:

WA 2 FST

I just pulled a SA 770 off of mine when I swapped to aftermarket heads. I can't remember your combo exactly, but it will not make as much power as mine now... in fact, it may be very close to what I had (~400-425hp flywheel). So I say run it. The 770 carb did just fine for my 454 HO crate motor. It just wasn't going to cut it for the mods I just did.

AKRAY4PLAY

i find your testing results interesting. when i dynoed my motor, i played with several carbs and set-ups. i have a healthy 406 sb, and went by, and against all the books. the books got me the best numbers on the dyno. i ran a 650, 750, 835, 850 and 950 Hollies, a couple Edelbrocks and even ran two BG Race Demons, 750 and 850. the best results came from the BG 750. what i found out was all carbs are not made equal. the larger venturis in the Holleys made less power do to restricted and turbulent flow. the Edels would not supply enough fuel on top and the BG's were a little rich on the bottom. i think it was well worth the money for the three days on the dyno. i learned a lot, but am by no means an expert on carbs.
what exactly are your set-up tricks?? i would love some insight on how you tune. i could always use some more power to play with. Brian

Chuck Harmon

This article compared a vast number of carbs on a SBC of unknown size . It shows the best carb to be the 950 cfm HP from Holley's pro shop. For what it's worth, it is exactly the same carb I use. More than a few have told me it is too big for my 427. Oh well :rolleyes:

Chuck