lars

Gary (SIGNGUY) finally received his tuned and tested 750 mechanical secondary Speed Demon system from me. For those of you following the earlier threads on this subject, there has been some lively discussion regarding carb sizing: If you go by some of the "recognized" formulas, a 350 only needs a 600-650 cfm carb. If you go by my actual dyno testing results, a 750 works just great - especially the mechanical secondary carbs.

So Gary decided to offer up his slightly modified 350 C3. He ordered a 750DP Speed Demon carb from Summit, along with a recommended list of supporting parts (heat shield, filter, throttle cable bracket, etc) and had it all drop shipped to me. I modifed the parts and set the carb up. After test running the system on my own 350 C2, I boxed it all up and sent it out to Gary. He received the system on Saturday and had the car running on Sunday.

According to Gary, the car is now a whole new animal: From a slow rolling start, with the clutch out, punching the throttle to WOT breaks the rear end loose in a cloud of tire smoke - the car would never do this with the 600 cfm carb. Throttle response is crisp, and bottom-end response is instant and brutal.

Here are Gary's own comments. He has photos with this, but at the time of this posting his server is down so the photos aren't posting right now. He'll be correcting this later:

well...i got her on and the 1st thing i should say is...
YES...a definite improvement over the 600 cfm...no doubt about it...
i notice it mostly at the bottom end...the acceleration is noticably better...
a ton more "put you back in seat" type power...
it is also a LOT smoother than i was ever able to get the Edelbrock to run...
i did everything just as Lars stated in his nicely detailed instructions and it was a snap...
i AM however having a slight problem that is NOT the carburetor and i will discuss that in a moment...
1st...some pictures...
the first thing i did was remove all that was no longer needed and did a nice clean up job...
it was nice to be able to get at some of those tight areas that are impossible to clean with the carb on the engine...
ie...there were some gas stains on the aluminum intake that Simple Green Automotive took right off and made look new again...the 1st two pix show it naked, cleaned and ready for the Demon...






the next two pix show the plate setting on the intake with the carb spacer below it...






then it was time to set the Demon into place...here she is in all her splendor...





then the throttle cable bracket...i HIGHLY recomend this to anyone doing this mod that has a cable operated throttle...it is SWEET and fully adustable...




the next two pix show how i had to slightly "modify" the air cleaner base with a ball peen hammer to clear the two big "screws" on the top of the bowls...
(sorry i dont know what they are called so please forgive my ignorance there)
it wouldnt quite sit down flush on the base...it wanted to kind of rock side to side on them...
this little "mod" did the trick nicely...






the next two pix show the custom fuel line that Lars made for me...
Lars, ive gotta tell you that it fit like a glove! just as tho it was made right there on my engine...
no problems at all...it looks very professional and added so much to the looks of the install...
thanx so much for that Lars!





when doing the final hook-ups i opted to attch the vac advance to the direct nipple (can i say that?)
instead of the ported because mine always ran better with it hooked up that way...
and after trying this carb both ways today, i found that it works better that way with this carb too...

and this brings me to "the problem"...

as Lars suggested, i did the test run with no vac lines attached and everything was perfect...
it ran sweet right out of the box...perfect...
then when i T'd off of the direct vac nipple to run my headlights and a/c it did change the performance...
test running it 1st without hooking the accessories up was brillant beacuse had i hooked everything up at the beginning, i would have not been impressed with the carb...thinking that it had a problem...doing it this way, i now realize that i may have a small vac leak somewhere (???) causing a slight hesitation "off the line"...
if it is sitting still idling and i feather the trottle, there is a slight "hitch" at the very bottom rpms...
almost like a burp...
once i get beyond that...no prob...its business as usual...

i DO know that it is NOT the brake booster as it runs perfect with that hooked up...
so that only leaves the accessory line...
when i redid my engine bay and had the dash out i completely went thru that vac system so i highly doubt that the problem is there...
however...my headlight vac has always been suspect as when the car is not running, if i pull the headlight opening switch below the steering column, my headlights do not open...they never have as long as ive owned the car...leave that little switch down and start the car and the lights do open.
also...if i drive the car with the lights on and shut the car down before shutting them off...the do not go down without the motor running...
they otherwise work perfectly whenever the car is running...
because i am "sharing" the direct vacumm between the timing advance and the headlight vacumm, could this be causeing my stumble "off the line"? is it because i have a vac leak somewhere?
are the two related?
now, keep in mind that it is not a terrible stumble...its quite minut actually, however it runs so flawlessly without the vac acc being hooked up, i want to correct the problem...
any ideas or comments about this will be greatly appreciated...

bottom line...it is NOT over carbed...this Speed Demon works perfectly on this mildly modified engine...as a matter of fact it feels like it needed this all along...almost like it was thanking me for finally giving it the fuel that it needed all along...
my motor is finally complete...

Lars...you ARE "the man" and i thank you greatly for helping me with this and bringing life to my engine...





[Modified by lars, 10:38 AM 3/8/2004]

Stewart's74

Awesome. I wonder if the Speed Demon would affect the performance of my car that dramatically? I currently have a Lars-buerator Q-Jet :)

lars

Stewart -
I have found that the Speed Demon will change low rpm and mid-range throttle response (for the better). WOT performance will be very close to identical if the Q-Jet is jetted correctly since both carbs are the same size. But since you don't always operate at WOT, but you do operate in the mid-range, the overall "feel" of the BG carb makes the car feel completely different.

GTR1999

Sounds good, I'm sorry I didn't get one. I hope the new Holley 670 st avenger I did get performs well.
Gary

427V8

Lars,
So you are saying that my 460hp 434smallblock is vastly undersized with a 750 vacuum sec speed demon?
I'm running a performer Airgap intake, and have a cam with 108 degrees lsa, 239 degrees duration and about .500 lift...

I did have a 850 on a supervictor but couldn't get it to run well at all.
But I have never claimed to be a carb guy!


[Modified by 427V8, 11:48 AM 3/8/2004]

BSeery

am I the only one that sees nothing more than a bunch of red x's ?

Stewart's74

Thanks Lars. More $$ to spend :rolleyes: . Maybe I'll have one by June 19th.

lars

427 -
Not to open a whole can of worms again, but the numbers I have personally seen while dyno testing, and the numbers I have achieved at the drags, indicate that engines in the 302-400 range run best with a 750. Above 400, an 850 runs very well and will produce higher hp numbers than the smaller carb. We have run 850 Speed Demons from Forum members on our small blocks out here in Denver (during setup and testing), and they run great. The best 1/4-mile ET I ever got out of a 355 SB Nova drag car was with a 1050. The guys at Westech Performance (the big dyno group in L.A.) and the guys at Dougan's Engine Service have had the same results regarding carb sizing and agree with this philosophy. This sizing does not correlate at all with the sizing calculators put out by Holley and BG. But their main objective is not to get your car running fast: their objective is to maximize profits by reducing warranty claims....

This is not to say that you can't set up a very strong running engine with a smaller carb. Heck, the NASCAR boys run Holley 390's, and they're pushing 500 horse or so.

lars

BSee -
Yes, there are red X's for now....

"...Here are Gary's own comments. He has photos with this, but at the time of this posting his server is down so the photos aren't posting right now. He'll be correcting this later:"

BSeery

So..... I skipped a part.
I suck at speed reading.

1Bad69

hee hee...i'm getting all giddy reading these posts about BG carbs..can't wait for the 850 Lars Special... :thumbs:

Older Than Dirt

Me too! Will a 850 work good with a ZZ430 with ROD 6 and Hooker headers-sidepipes, JCL baffles? :)

SmokedTires

Demon has a great section on the site here on selecting the correct carb for your application. It looks like I'll be ordering the Mighty Demon 750 for mine :D

Glad to hear your vettes running so strong signguy, the engine looks great !!! :cheers:

shoptek

Lars,
I too concur (sp?)!!!! The Qjet CFM's were just what the Dr. ordered.
My mildly worked 350 WOKE UP with the Qjet you preped and tuned for me as opposed to the 600CFM Edelbrock I'd been running. The throttle response is IMMEDIATE.....The mid end is STRONG, lots of "pull"....
I can't wait to completely test WOT!!! :D :D

Thanks again :cheers:

Cali,68,L-79

LARS and or signguy. Please post all the parts numbers you used for this setup to include the fuel line.

David

1Bad69

David...i believe the part list is in the other post about the same title...


[Modified by 1bad69, 7:36 PM 3/8/2004]

Cali,68,L-79

1bad69 thanks for IM. Quite a bit of money there. Well if it's in the name of speed i'm all for it.

:thumbs:

OkeeGuy

1st of all i would like to thank Lars as well as others of you that have made kind remarks and/or given compliments...

for those of you that arnt up to speed on this conversation, there is a lot of info concerning the whole project here... http://forums.corvetteforum.com/zerothread?id=767841

frankly, there has been a lot of talk on here about wether or not the BG Speed Demon or other like carbs are "too much" for our engines...
admitedly, i am not an automotive tech of any kind...
but i DO know when my car has better performance than it did before a mod and i am here to tell you that my car performs WAY better than it did with a 600 cfm Edelbrock carb on it...
there is simply no comparison...

i have been told by a BG tech that i am now "overcarbed" and that the "numbers" do not support such a claim but all i can tell you is that IN MY CASE, the numbers are wrong...
numbers are fine and dandy but i live in the real world and in the real world
the carb swap made a ton of difference...
one thing that i dont understand about the "numbers" that say that all our cars need is 600-650 cfm is...General Motors put 750 cfm carbs on our cars...
why do you supose they did that? hhhmmm???

this may not be the carb of choice for some of you but i am here to tell you that, for me, it made a world of difference...
the job cost me a little over $600 but i was able to sell my Edelbrock carb and accessories for $200....so in my case it was a $400+ mod and one that was worth every penny...

there is a complete parts list on the other thread if you are interested in knowing what exactly was purchased...
i think that it could be done a little cheaper if need be because i DID go for a couple of extra "goodies"...

Cali,68,L-79

I'm tossed between going stock and buying one of those bad boys. I've been eying those BG's for four years. so about $600. i also have an Eldebrock model 1400 and with a L-79 I know I can go with more carburation then your 79 but alas I have to come to a decision on which way to go. Question does that carb come polished?

Eric R

:iagree: and thank you. When seeking the last HP or the seat of the pants grunt, you cannot go wrong with your selection. BTW I am also over carbed running a BG750 and proud of it. A BG will outflow a Holley and tunes well out of the box.

LARS, you are doing a great job. Please think about expanding your papers to include carb mods for people with larger cams. Such things as drilling the butterflies when the transfer slots are exposed past limits and opening the idle air bleeds and so on. There is so much more to this than what has been covered. Also a quick paper on carbing a "daily driver" could be a real bonus for the guys wanting to get into a BG or Holley on a smaller scale.

Thank you for keeping this thread real.