Carb size ???? 383

[kenm78]

I am running a MSD ignition set-up the distributor was before tear down at ("bad speller please forgive me") 18 degrees mechanical advance inside the distributor and I playing with around 8-16 degrees initial timing I had a bad pinging when I started to play with the centrifical advance springs so i started running 91 and occationally heard the pre-ingnition so I tried our local sinclairs 93 and never heard the pre-ignition again "sounds like a bunch of rocks being tossed around inside the engine". but that is were I am so far. I rushed into the first build and as you can see I am paying double for every mistake. So Im not rushing this time...... Im still pondering on upgrading heads but that is anouther topic.. Thanks for your wisdom..

[Schralper]

With your setup get a 650 double pumper w/o choke. The only thing you might have to change are the jets. Start w/ #330 pink accelerator pump cam, screw in #1 position, #31 squirters. Idle screws 1.5 turns out. Try the jetting outta the box. You might have to go up 3-5 jet sizes.
Your vacuum is more than adequate, you shouldn't have any problems w/ brakes. The 280 cam is a lil big for low end if you don't have the stall or compression. Should make pretty good midrange, 3K rpm up.
Start w/ 16 deg initial adv, 36 total. A supercoil or similar should help.
My '68 longbed w/ 8.8:1 iron head 355, RPM manifold w/ 1" spacer, has a pretty big cam w/ 1.6 roller rockers and runs fine to 6K w/650 DP. A lil flat down low but that's the trade off w/ lower compression.

[Billla]

Originally Posted by Schralper

w/o choke

Why would any street engine not have a choke?

[crazy longhorn]

Originally Posted by Billla

Why would any street engine not have a choke?

I have to ask the same ? I cant see what little improvement in flow could possibly matter on the street , to remove the choke.....& then you have a tempermental eng in cold weather.... L

[Schralper]

Originally Posted by Billla

Why would any street engine not have a choke?

For max performance. Especially if it's not primary transport.
Looking at any hi perf carbed engine, I don't see chokes.

While the venturi are the same size there's no restriction on the primaries.
The choke and horn do restrict the flow.
Well even where I live, it gets cold. It's not temperamental unless you mean I have to sit in my truck for a minute to keep it running initially when it's below freezing. Which is fine w/ me cuz I can keep an I on the vitals.
I use to run elec. choke but had to mess w/ it often. Got sick'n tired of that so a proform carb body w/o choke.
With a K&N stubstack and filter along w/ good ignition it works fine.
If you want the most power from the engine over the whole RPM range use a DP carb.
Vac. Sec. seem to work better w/ milder engines and will provide somewhat better mpg, maybe. I've used both types, on multiple engines.
600 vac. sec and 650 DP's work real good on 350's so then w/ a 10% increase in engine size.....
You do the math.
This might help also- http://www.recarbco.com/technical/ne...ml#Gas_mileage

"My intended power band is somewhere around 1000-5500,or as high as 5800 RPM. Im a realist gas mileage isn't on my agenda but I want performance "
You'll NEVER have a 4K rpm wide powerband, more like 2K max.
Look at these and the engine specs -
http://www.compcams.com/Technical/DynoSheets/

[Billla]

Originally Posted by Schralper

For max performance. Especially if it's not primary transport.
Looking at any hi perf carbed engine, I don't see chokes.

While the venturi are the same size there's no restriction on the primaries.
The choke and horn do restrict the flow.
Well even where I live, it gets cold. It's not temperamental unless you mean I have to sit in my truck for a minute to keep it running initially when it's below freezing. Which is fine w/ me cuz I can keep an I on the vitals.
I use to run elec. choke but had to mess w/ it often. Got sick'n tired of that so a proform carb body w/o choke.
With a K&N stubstack and filter along w/ good ignition it works fine.
If you want the most power from the engine over the whole RPM range use a DP carb.
Vac. Sec. seem to work better w/ milder engines and will provide somewhat better mpg, maybe. I've used both types, on multiple engines.
600 vac. sec and 650 DP's work real good on 350's so then w/ a 10% increase in engine size.....
You do the math.
This might help also- http://www.recarbco.com/technical/ne...ml#Gas_mileage

I push back on a number of points here. A choke when fully open doesn't significantly restrict airflow, neither does the choke horn. It's not on HP engines because it's not needed...not because it significantly increases airflow. Too many people put "race" parts on a strictly street engine...and then wonder why it's such a PITA to drive. An electric choke should be a set once and forget - if you had to "mess with it often", there was a problem somewhere. The electric choke on my Holley 3310 in my '72 was last set in 1998

Double-pumper carbs are for competition only, IMHO. The point of the mechanical secondaries is to avoid even the slight delay of progressive opening of the secondaries. The point here is there is not "more power" in a DP carb....just no delay moving 1000 - 7000 RPM As you note, this KILLS milage on a street engine. Properly calibrated vacuum secondaries work FINE on a street engine.

We'll just have to agree to disagree

[kenm78]

This statement confuses me!

"You'll NEVER have a 4K rpm wide power band, more like 2K max.
Look at these and the engine specs –"

Here is what Comp Cams is advertising.
HYDRAULIC-Great for street machines, largest Hyd cam for stock converter, better with 2000+ stall.
RPM 1600 to 5800 12-242-2 XE268H 268 280 224 230 .477 .480 110°

How is only a 2K power band and not a 4K possible? According to Comp cams Dyno sheet on a 350. It builds HP from 2K to 5K. This is the info you provided me. And I am building a 383 I understand it is only 33 cubes bigger but is should produce more torque and around the same to a little more HP

http://www.compcams.com/Technical/Dy...68H-10_001.asp

Looks like this thread opened up a new can of worms.

It looks like I will probably go with the street avenger 670cfm with vacuumed secs

I can't find a 650 square bore carb with electric choke and vac secs.

Any one knows where to look for a 650 vacuumed secs and electric choke?

Thanks

[piecesparts]

Quote{Double-pumper carbs are for competition only, IMHO. The point of the mechanical secondaries is to avoid even the slight delay of progressive opening of the secondaries. The point here is there is not "more power" in a DP carb....just no delay moving 1000 - 7000 RPM As you note, this KILLS milage on a street engine. Properly calibrated vacuum secondaries work FINE on a street engine.

We'll just have to agree to disagree [/QUOTe

Everyone has a definite feeling about the mechanical to vacuum actuated secondaries on most carbs. Keeping in mind that the vaxcuum secondaries will actuated as the vacuum slowly goes away and the mechanicals will actuate at a set point of the primaries opening, I would say that both are an impact ot mileage. I haven't had a vacuum secondary on my 383 since I took the GM garbage off and with the horsepower and gearing that I have, I can't say that the 15 MPG is a bad thing. As a matter of fact it is as good as the original 305 GM carb that was in it upon purchase. The only real impact that I have seen is the moment of opening of the secondaries is a major event because they are positive and there at that very moment, not fettered in at a slow change. If you are set up to drive under that point then the primaries are your carburetion not the secondaries and the primaries. However, if you are geared or adjusted to run into the secondaries all of the time, then the gas mileage is going to be down.

As for a choke, I have better things to do, than sit and hold my idle until the vehicle is warm enough, such as clean the snow and ice off of my windshield. In areas of warm weather, even a moderately cool day will actuate a properly adjusted elecric choke and then it comes off quickly. It keeps the motor rich at a point where it is needed and helps it to warm it up. The carbs without chokes are usually destined for a track or drag strip, there is no need there. The performance is well above the idle point and the driver has nothing else to do but get it ready to run.

[Schralper]

"We'll just have to agree to disagree"
I'm not arguing w/ ya. Yes, peeps put racy parts on and then go huh?!
kenm asked what works. These carbs have worked for me. From near sea level to 9K'. Not like FI fer sure but.... Between my Burban and 68, which is similar and has been driven to Bonneville a couple time and multiple times Central NV w/o any carb issues, like 1700 miles roundtrip, each.
Yes, I romp on it in town!

"The electric choke on my Holley 3310 in my '72 was last set in 1998"

You mean you haven't gone thur and cleaned the carb in 8 yrs?
You'll probably get better response from your engine, LT1, w/ 600 vac sec rather than the 750 you've got on now. It's to big.


I use a modded 650 dp in my 68 w/ 355, 13 mpg, and modded 3310, 13 mpg, in my Burban w/ 388.
The mileage diff between to 2 boils down to OD.
Pushing a brick thru the air sucks gas. It's just one brick sits higher than the other.

[Billla]

Originally Posted by Schralper

You mean you haven't gone thur and cleaned the carb in 8 yrs? You'll probably get better response from your engine, LT1, w/ 600 vac sec rather than the 750 you've got on now. It's to big.

No; I've overhauled it once but restored the stock choke settings

that the 750 is way too big - originally I had "plans" for this truck and the stock LT-1 manifold with the factory-correct LT-1 Holley 3310-3 was part of it. Life has intruded (read: teenage drivers) and I'm shifting my performance focus to my 1995 Impala SS (383 LT1 on the stand) and my 1969 'Vette (LSx block just bought) so I'm calming down my '72 quite a bit.

[Billla]

I think the powerband statement was more about where the peaks are and the shape of the HP curve. IMHO that's a great cam as long as you have gears beyond 3.55 and a converter to match; it's a bit much for my tastes for a daily driver, but if you have a heavy right foot it's all good

Not a can of worms, but there are differing opinions, experience and expertise. Spirited discussions are a great thing about the board

I think the Street Avenger is a great pick. The Truck Avenger are more for the 4x4 set than our 2WD trucks. The Edelbrock carbs jump from 600 CFM to 750 CFM...which sucks

[Schralper]

Originally Posted by kenm78

This statement confuses me!

"You'll NEVER have a 4K rpm wide power band, more like 2K max.
Look at these and the engine specs –"

Here is what Comp Cams is advertising.
HYDRAULIC-Great for street machines, largest Hyd cam for stock converter, better with 2000+ stall.
RPM 1600 to 5800 12-242-2 XE268H 268 280 224 230 .477 .480 110°

How is only a 2K power band and not a 4K possible? According to Comp cams Dyno sheet on a 350. It builds HP from 2K to 5K. This is the info you provided me. And I am building a 383 I understand it is only 33 cubes bigger but is should produce more torque and around the same to a little more HP

Thanks

Just cuz it dynos from 1600-5800 doesn't mean that's the powerband.
Look at the torque curve, while it's pretty flat it makes near peak from 2500-4500 rpm, 2000 rpm range. That cam is dead after 5K rpm. In a 383 it'll appear even smaller, to the engine. Figure 2000-4000 rpm, dead @ 4500rpm.
Look at the carb that Comp is using on the dyno.
It you want a wider powerband go roller cam.
It really helps to build an engine around the cam then cyl heads.

[kenm78]

Do you mean I should keep the 3.42 rear gears or should I go lower 3.55 or 3.73? Yeah I was just trying to enlighting the conversation seemed kinda heated didn't it but your right that is what makes this site great all this knowlege in one place can't go wrong there!!!!!!

[Billla]

3.42s are fine...something like 2.73's wouldn't be great for that cam - it's a bit more of a "top end" cam than a low-end torque cam.

The discussions get direct sometimes...but I've been on for a little bit and I've never seen anyone attack anyone (not and get it past Liz or the mods and the different experience and expertise is pretty awesome. I learn something from the board every day.

Originally Posted by kenm78

Do you mean I should keep the 3.42 rear gears or should I go lower 3.55 or 3.73? Yeah I was just trying to enlighting the conversation seemed kinda heated didn't it but your right that is what makes this site great all this knowlege in one place can't go wrong there!!!!!!