600 v. 750 carb on my 350. Help me out! - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 11-13-2007, 12:32 PM
Registered User
 
Last photo:
Join Date: Jan 2007
Location: USA
Posts: 86
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
600 v. 750 carb on my 350. Help me out!

OK, I've searched the site and found some good stuff. But here's my situation:

I have a 350 running in a Porsche. I don't know much about the engine, other than it is a '70s 350, it looks like some care was done in building it, it has aluminum heads and a Performer intake, and tubular headers.

It was running an Edelbrock Q-Jet, which ran great until recently.

I took it to a well known shop for a dyno tune, since it seemed down on power. They ran it, and it was a flop. However, the reason, it turned out, was that the throttle bracket had shifted, and it was only getting maybe 1/2 throttle!

They said my carb was out of adjustment, too, the linkages were out of adjustment and the secondaries were only opening a tiny bit. (Although couldn't that be simply because the throttle wasn't opening? Wouldn't the secondaries not necessarily open all the way at essentially half throttle???).

They suggested rather than rebuilding the Q-jet, simply getting a new carb. I know that they didn't say that to try to make money, because they suggested getting the new carb from summit or jegs, rather than having them rebuild or work on my old one.

They suggested the basic Edelbrock squarebore 750.

I ordered that carb and put it on. Of course, now in reading more about carbs, I'm a bit concerned about the size. From what I'm seeing, 600 would be a better size.

The car runs well with the new carb, although I do feel like it isn't quite as "snappy" as my Q-jet. Cruising at 2000 rpm and nailing the throttle with the Q-Jet seemed to give really instant response (when the throttle cable was set correctly), I do seem to notice a very tiny bog or mushiness with the 750.

BUT, the car is set to go back on the dyno for dyno tuning of the carb. They insist that 750 isn't too big for the car.

So, my question is: What do you think? Is 750 a reasonable choice for a 350? Esp. if it is dyno tuned to the car? Should I expect that they are going to change the jet sizes? What other things should I expect they are going to do to the carb?

Thanks!

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 11-13-2007, 12:57 PM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Pittsburgh, PA
Age: 40
Posts: 5,128
Wiki Edits: 16

Thanks: 0
Thanked 2 Times in 2 Posts
It takes a pretty hairy 350 to actually need 750 cfms. 600 is more than enough for a 350 through 6500 rpms and more. A 750 will give you another 1-2 hp, but at the expense of the drivability problems you have noticed.

Give it a chance. Many guys run 750s on their 350s with no problems, but I'm like you... I can't stand knowing I'm giving up the snappy throttle and possibly low end torque.

As far as the Qjet is concerned, it was working perfectly. Qjets are flow-sensing. The secondaries only open as far as the engine demands flow. Some Qjets are cast with throttle limiters, or air door limiters, but the bottom line is, they flow up to either 750 or 800; the key word being up to. If you had a 750 Qjet on there, that is an excellent indicator that it wasn't using all 750 cfms. The secondaries were only opening as far as they needed.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 11-13-2007, 01:56 PM
Registered User
 
Join Date: Sep 2007
Location: SC
Posts: 42
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
CID*target rpm*V.E.\3456

In other words take the cubic inces of your motor, multiply that by the target rpm, take that number and multiply it by your volumetric effecency, then devide that number by 3456.

For a stock of reasonably built motor use a V.E. of 0.9, for a highly modified or racing morot V.E. of 1.1. Knocking off the V.E. will give you a bit more generalized end result and nothing wrong with doing that. But if you use the V.E. it makes an amazing difference.

In short unless you have an extremely well flowing 350 750 is more then you really need. Yes, it will give you more power at the top end but unless this is a strip car how often will you really be running that high?

My taste is to use a slightly smaller carb and get the better low end response and snappyness. For a 350 with pretty well flowing heads a 600 or 650 is more in line for a 350.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 11-13-2007, 04:01 PM
Registered User
 
Last photo:
Join Date: Jan 2007
Location: USA
Posts: 86
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks.

Since it is on the car already (I can't return it anymore!), and will be dyno tuned and rejetted/redone, I'm going to give it a shot.

Should be done tomorrow.

Any guesses on what HP my engine will put out?

'70's 350 block, some kind of aluminum heads, Performer intake, 750 Edelbrock, Jet Hot coated headers, HEI ignition, no smog, unknown cam specs (pulls about 17 lbs of vacuum at idle), non-roller/rockers, running through a Porsche manual 5 speed transaxle.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 11-14-2007, 04:56 PM
Registered User
 
Last photo:
Join Date: Jan 2007
Location: USA
Posts: 86
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
The car showed 270 hp and 305 lb/ft at the wheels on the dyno today with the 750 carb. They tweaked it a bit and it seems to run fine. They INSIST that the 750 is the right size. They say that chevy 350s came from the factory with carbs around that size.

At 15% drivetrain loss, that translates to about 310 crank hp.

(The shop says drivetrain loss should be 18%, at that conversion factor, crank hp would be 318).

My car weighs 2350 lbs, so it scoots pretty well!
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 11-15-2007, 09:55 PM
Member
 
Last photo:
Join Date: May 2003
Location: Shreveport LA
Age: 64
Posts: 5,103
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
350's generally came with Quadrajets, which are rated at 750cfm, not that they actually needed that much. The old 350hp 327 engines from '66-'67 came with a 585cfm carb, and they ran pretty well.

I can't believe they recommended that you change carbs in the first place, unless they just didn't know how to deal with the Q-jet. I wouldn't trade mine for anybody's aftermarket carb.

When you nailed the throttle on the Q-jet, you opened up a couple of fairly small primary throttles, each with a triple venturi, in other words you had the main venturi restriction, with a booster venturi, and a smaller booster within the first one. That results in excellent fuel atomization and throttle response. When you do the same on the AFB, you open up much larger primary throttles with only a single booster each, so you get a soggier response compared to the Q-jet. A 600cfm AFB will have smaller primaries than the 750, so it might give you back at least a portion of that lost response, without losing a whole lot of power overall, maybe a little at the highest rpm ranges.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 11-21-2007, 11:24 PM
Registered User
 
Last photo:
Join Date: Jan 2007
Location: USA
Posts: 86
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Well, because of my curiosity, I had my Edelbrock Q-Jet completely rebuilt, rejetted, flow tested, and matched to my engine by a place called "The Carb Shop."

Today I ran the car with the 750 square bore, then switched to the Q-Jet, then back to the square bore, then back to the Q-Jet (I can switch them in 10 minutes now).

To be honest, I can't tell much of a difference. I *think* the Q-jet is a bit quicker at part throttle and in the lower RPM range. But I'd have to get it onto the dyno again to be sure.

Both of them work very well, IMO.

I think I'm going to stick with the Q-jet. Mostly because (1) I now have more $$$ into my blueprinted Q-jet than I do in my new Edelbrock Performer 750, (2) the Q-jet has a simple choke, the 750 has none (I thought I wouldn't mind not having a choke, but I like having it on colder days), (3) I'm hoping the Q-jet will get better mileage than the 750, esp, if I keep out of the secondaries!

I'll probably put my 750 on e-Bay. If anyone is interested, let me know. This was only used for a few dyno runs, and maybe 60 miles of driving over the course of a week, so it is essentially new, will come with the box, the DVD, all the instructions, etc. Shoot me a PM if interested.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 11-22-2007, 01:26 AM
papa_clutch's Avatar
power hungry
 
Last photo:
Join Date: Oct 2007
Location: North Dakota
Age: 38
Posts: 64
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
the quadrajet should definately get better mileage. I modified a quadrajet for my 350 but I could not get small enough secondary metering rods, It would run lean on the top end, so I put a holley on it, that I can get parts for, but the quadrajet was snappier and of course got better mileage.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 11-22-2007, 02:57 AM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Pittsburgh, PA
Age: 40
Posts: 5,128
Wiki Edits: 16

Thanks: 0
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by Porschev
The car showed 270 hp and 305 lb/ft at the wheels on the dyno today with the 750 carb. They tweaked it a bit and it seems to run fine. They INSIST that the 750 is the right size. They say that chevy 350s came from the factory with carbs around that size.
A 350 making 270 at the wheels (320 or so at the flywheel) is nowhere NEAR the requirement of a 750 carb. You have a potent engine, don't get me wrong, but you don't need 750 cfms until you reach much higher numbers than that.

Your 350 even at 90% VE and 6500 rpms (which I doubt neither are seen) ingests 590 cfms.

Sure, GM put 750 cfm carbs on 350s. They also put them on 4.3 V6s, buick 455s, and also offered an 800 cfm Qjet on 305s in vans and trucks after 86. Qjets are flow-sensing carbs. They only open the secondary air doors as much as the engine demands. If your dyno techs are comparing a factory Qjet to the "requirements" of a 350, I would run as fast as I could to another dyno cell. I'm not busting chops, but these guys are a bit off their rocker if they think a 270-hp 350 "needs" 750 cfm.

You're on the right track... dyno tuning. You can get any carb to run right, its just what do you want?... do you want throttle response, torque, idle quality and MPG, or do you want 1-2 extra HP? That's really what it boils down to. Proper carb sizing might give up 1-2 hp on the street, but at such a huge benefit that it makes little sense to oversize.


its also what you want to do with the car. Give me two identical 350s. Both have a 234* cam with 11:1 compression. They are both capable of 6500 rpms. If I'm putting it on a strip-only car, I would use an 800 cfm carb. On the strip, HP is king. It doesn't care about idle quality or throttle response, it leaves the line at full throttle. The only quest is getting that last 1/2 hp. That same engine on the street, I'd use a 600-650 cfm carb. Sure, I'll give up that last 1 hp at the top, but I'll gain low end torque, MPG, idle quality, throttle response, and tunability.
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 11-22-2007, 08:02 AM
Registered User
 
Last photo:
Join Date: May 2006
Location: Maine
Posts: 152
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
just thought i'd throw this in... i've got a mild 350 Chevy in my 68 GMC, with a Holley 600cfm square-bore. The throttle response is so snappy, i'm pretty sure it would respond if you brushed a feather across the gas pedal. I'd much rather have the snappiness of a 600 than a 750. Save the 750 for a BBC or something.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 11-22-2007, 01:30 PM
Registered User
 
Last photo:
Join Date: Jan 2007
Location: USA
Posts: 86
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by LupinusPectus
CID*target rpm*V.E.\3456

In other words take the cubic inces of your motor, multiply that by the target rpm, take that number and multiply it by your volumetric effecency, then devide that number by 3456.

For a stock of reasonably built motor use a V.E. of 0.9, for a highly modified or racing morot V.E. of 1.1. Knocking off the V.E. will give you a bit more generalized end result and nothing wrong with doing that. But if you use the V.E. it makes an amazing difference.

In short unless you have an extremely well flowing 350 750 is more then you really need. Yes, it will give you more power at the top end but unless this is a strip car how often will you really be running that high?

My taste is to use a slightly smaller carb and get the better low end response and snappyness. For a 350 with pretty well flowing heads a 600 or 650 is more in line for a 350.

Here's what Edelbrock says about picking the right carb size:


"Carburetors are rated by CFM (cubic feet per minute) capacity. 4V carburetors are rated at 1.5 inches (Hg) of pressure drop (manifold vacuum) and 2V carburetors at 3 inches (Hg). Rule: For maximum performance, select a carburetor that is rated higher than the engine CFM requirement. Use 110% to 130% higher on single-plane manifolds. Example: If the engine needs 590 CFM, select a carburetor rated in the range of 650 to 770 CFM for a single-plane manifold. A 750 would be right. An 850 probably would cause driveability problems at lower RPM. A 1050 probably would cause actual loss of HP below 4500 RPM. For dual-plane manifolds use 120% to 150% higher."


So how much CFM does a 300 hp 350 flow?
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 11-22-2007, 02:58 PM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Pittsburgh, PA
Age: 40
Posts: 5,128
Wiki Edits: 16

Thanks: 0
Thanked 2 Times in 2 Posts
What's your redline?

At 5500 rpm (which is usually more than enough to generate 300 hp) and 85% VE, your 350 ingests 473 cfm

At 6000 rpm and 90% VE, it ingests 546 cfm.... so lets say an average of those two would suffice... 500 cfms.

First of all Edelbrock's rules are poorly worded. According to that phrasing, they want you to use 150% more flow which is more than double. They should have worded it 150% OF the flow. You certainly don't need 1250 cfms which is what their words lead you to believe.

At any rate, I still don't agree with their rules. According to that 120-150% rule, you should select a carb flow that is between 600-750 cfms. That doesn't narrow it down a bit. That just further complicates things and doesn't help at all.

Listen... trust me on this. I don't care about recommendations for carb sizes, the proof is in the pudding. On one of your dyno runs (or on the street for that matter) use a vacuum gauge to determine your manifold vacuum when you're at WOT right at redline (right before a shift). For an all out race engine, make sure the vacuum doesn't go above 0.5 inches. For a street performance engine like yours, keep it between 0.5 and 1 inch. For any other low-po stuff, more than an inch is fine and would only give up 5 hp anyway. I would much rather give up a couple ponies in favor of all the benefits that a smaller carb offers. Considering that the engine spends so little time at redline where its giving up those couple ponies, you'll never notice it on the street. In fact you might make it faster on the street with a smaller carb given the better torque and throttle response you get.

Oversizing a carb is stupid on the street. Its just right on a race car that only ever sees WOT.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
600 cfm Edelbrock jetting for 330hp GM 350 ctw Engine 1 12-15-2005 10:02 PM
350 carb? kreeep Engine 5 03-24-2004 07:12 PM
600 holley carb pops on acceleration bt101 Engine 2 02-28-2004 05:47 PM
Holley 600 vs. 750 jeff germano Engine 6 02-18-2003 09:06 AM
600 cfm holley vs 750 cfm holley 85 regal Engine 7 11-27-2002 06:25 PM


All times are GMT -6. The time now is 06:00 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.