Gents:

I need to replace my carb. I'm looking at a Barry Grant Speed Demon 650 - my question is: Vac secondaries or mechanical? It seems that mechanical secondaries are indicated for manual transmission. Why?

Engine: 1970 LT1 (rebuilt to run on pump gas 10:1 compression)
Mallory billet distributor with Pertronix ignition (no vac advance...)
Manual Transmission
Bored .030

Cam specs: 234/244 465/488
Hyd. lifters (I know...shoulda kept the mechanicals!)
Double hump heads - ported/polished
Stock high rise alum. intake port matched
2-28 springs
Stock crank (polished)
Stock rods

Original carb:
Holley part # 80508
CFM: 750
Jets: (F)72, (R)plate 134-21
Power Valve: 4.5
Type: 4160

I want to get away from Holley. I'm pretty sold on the 650cfm. I need an electric choke too. It's either gonna be Barry Grant or Edelbrock...

Thanks!

 

650 CFM should be a good size for your engine. I would stay with vac. secondaries. Mechanical are better for racing purposes, but are not needed for the street. And vac. will be more economical.

The problem with electric chokes is they don't cool off at the same rate the engine cools. So you pull into a store lot, and 15 minutes later try to restart the engine. But the choke is cool and closes, while the engine is still warm.

Plasticman

 

In my '62 I have an LT-1 short block. Dart Iron Eagle Platinum heads, Headers, 340-dual point dist, and a Holley #3310, 750 Vac. Secondarys. I never run a choke and never had a starting problem even on a cold day after its been sitting for a few weeks. The combo runs fantastic for me

 

You didn't mention your rear end ratio. A lower ratio (higher numerical ratio) will be more tolerant of a larger carb or mechanical secondaries than a 3.08 ratio.

Ditto other's views on the vacuum secondaries. Sticking with them will allow you to 'floor it' at a lower rpm without causing problems (coughing, spitting, backfiring, etc.) It will essentially allow you to get away with a slightly larger carb (& make slightly more peak power) without driveability problems.

The vacuum secondaries allow the airflow thru the carb to control the secondary opening so it doesn't happen before the engine can handle it.

An old Holley book I have says the minimum WOT rpm for a 350 with manual tranny and a 650 double pumper is about 1200 rpm......might be ok if you go with mechanical secondaries, but will misbehave when floored at a lower rpm. They also point out that even at 100% volumetric efficiency (not likely) that your 350 only consumes about 700 cfm at 7000 rpm. I think you'd be better off w/vacuum secondaries and maybe a 700 or 750 cfm carb if you'd like....although I bet you'd have a hard time measuring a power difference from the 650 carb.

Best Regards,
Bob S.

BTW - Don't mention to the BG tech people how much clearance you have (or don't have) between the carb air cleaner flange and the air cleaner lid........they'll swear you need to cut a hole in the hood and raise the air cleaner for proper airflow!! I went thru this with them a year ago on my ERA Cobra, and it seems they're ignorant of the Holley carb installation details on Corvettes.......

 

I'm running a 750 VC SD on my 350ci. I removed the choke and linkage. It did not appear to me it would clear the air cleaner base without modification. No problems starting without it. Cold starts? One pump of the pedal, hit the key and it lights right off. I hold a high idle with the pedal for about a minute then it is fine. Hot starts? Just twist the key and it's running.

 

Thanks for the input!

My rear end is a 3:14 or a 3:40 (can't remember - I counted the teeth and did the math three or four years ago -) I'm sure someone will straighten me out -

point is: rear end is fairly low geared -


Here's my pick
Barry Grant 650 CFM Speed Demon 132-1282010VE (polished)
Vacuum Secondary
Electric Choke
Mild 302-427ci Engines -
http://www.barrygrant.com/demon/default.aspx?page=12

I wouldn't call my cam mild but maybe it is compared to crazy, pro streeters and such....

 

I've got a Holley DP 650 w/ mechanical secondaries and electric choke on a 350ci and my cam is I think a little milder than yours. It's definitely the right carb cfm wise - had it rebuilt and rejetted recently. The 650 will be a lot more streetable than the 750. When I go WOT it's almost always above 2K rpm and it responds nicely.

Let us know how that BG works out for you - sounds like a good choice.

 

The 750 will work nicely but the 650 will work just fine. I would skip the choke though with where you live. I've had my 650 mechanical secondary for two years with no choke.. I've never missed it. On the BG when cold tap the gas turn the key and idle it at 1500 for a few minutes and it will warm up and run very quickly. Once the car is hot only a turn of the key is required.

 

I too have the 650 SD with mechanical secondaries and no choke. True, it's a bit cold natured when cold, but it only takes a minute or at most two at 1500 RPM to get it warmed up sufficiently. I can floor it at any RPM with no bad behavior so wouldn't worry much about having mechanical secondaries if you decide to go that way.

 

i totally agree on rear end. i have a 3.08 and a sd 750. first with that rear end, you will not be able to light em up, and hills are a bit of a problem. it is a sacrifice, i like the highway cruising speed with that rear end, but hate the lite em up and hill problem. thinking of jumping to a 3.50 or thereabouts rear end. i have heard the 650 is better for my rear end, but dont think i will be able to spin em with that rear end. once started, the engine flies, acceleration is unreal, so check your rear end, and decide.

 

If you go to a Barry Grant carb from a Holley the original drop base air filter will not work as I understand it. Is this right?

 

I use a carter withmechanical linkage and an airflow valve on the secondaries (to prevent) lean out with my lead foot at low rpm.

 

Ok, that would stink. I don't use the stock air cleaner - I use a K&N aftermarket unit - is the diameter of the top part of the carb different or is it a top to bottom clearance issue?

I'm gonna call Barry Grant today and get the low down.

 

no a drop base will fit. you gotta hammer a little, so if its original, which i doubt since you are going to a sd, dont worry about it. the main problem with messing with this stuff is hood clearance. i have to use the low profile edlebrock foam filter, turned it 90 and i have clearance. that will prob be your main issue.

 

On your motor with your cam, 650 is perfect. Bigger is not always better.

 

I think the carb diameter and carb height are both different from the Holley. I see you have a small block hood. There is going to be an issue with intake height combined with carb height combined with air filter unit height and your hood clearance. I went through this a bit putting a zz383 with Vortec heads under my small block hood. I used an Edelbrock Vortec Performer with a Holley 750 Street Avenger carb and my original 66 drop base air filter unit and a 3" K&N filter. I have about 1/4" clearance with this combination.

 

The original drop base for my '66 L-79 fits the BG carb on my Cobra just fine.

What intake do you have? The height may be a problem. I know the 302 Z/28 manifold was taller than the L-79. The '70 LT manifold likely is also. If your manifold is taller than the standard aluminum L-79 manifold, you may be forced to use a 2" tall filter element rather than the original 3". (This is what my L-79 currently has to run with the Edelbrock carb a PO installed.)

Bob S.

4/10/06 Edit (Just for the record) - My statement above about using a 2" element is wrong: The Vette air cleaner base gives the most 'drop' of any base I know of. Using a 2" element with it would not allow sufficient lid clearance above the carb. My earlier statement should have read: If your manifold is much taller than the standard aluminum L-79 manifold, you're in trouble. Sorry for the misleading info.

 

Mine is very tight already (running the GM aluminum high rise manifold with the Holley and a 3" K&N. I can't go to a thinner filter because of the Holley's horns. I have to believe that overall height is similar to Holley - I'm going to look it up and measure mine too - back in a minute.

 

I used some tin foil in five areas on my air filter lid including the wing nut. Closed the hood. I was a 1/4" at the closest point. You are going down the same road I went down!

 

The Speed Demon is 1/2" shorter at the top of the horn than my Holley and 1/4" shorter at the shoulder.

It looks like it will work - we'll see...I'm ordering the set up as we speak.