Welcome to the Corvette Forums at the Corvette Action Center!

Another Carburetor Selection Error

Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
My neighbor stopped by yesterday to show me his new ZZ6 350" crate engine that he just installed in his '71 Chevrolet 1/2 ton pickup. It looked terrific with it's serpentine belt system, aluminum valve covers, and aluminum fast burn cylinder heads. Then he showed me his new 770 cfm vacuum secondary Holley and that's when I let out a BIG sigh. I told him a 350" engine can only swallow 608 cfm at 6000 rpm with a 100% volumetric efficiency and told him a street engine can only manage an 80-85% efficiency because of the mandatory exhaust system. Figuring an 85% efficiency his engine could only swallow 517 cfm at 6000 rpm BUT as he has a TH350 that forces the full throttle upshifts at only 4500 rpm that limits the air intake to a dismal 388 cfm; about half of what his new 770 cfm Holley is rated for. I told him his secondaries would most likely NEVER open because a 770 cfm Holley is way too big for his set up. Fortunately he chose a vacuum secondary Holley because a mechanical secondary (aka double pumper) would have been a total disaster as they aren't suitable for street use.

People make this same dreadful mistake all the time as they choose carburetors that are way too big for their application. Summit Racing recommends 1-1/2 cfm per cubic inch for street engines which would equal 525 cfm for a 350" engine. As my neighbor's new ZZ6 engine will never be twisted to 6000 rpm a little 500 cfm Edelbrock or 465 cfm Holley would have been the ideal choices.

I have posted my carburetor sizing chart in the past but here it is again so you folks can see just how much your engines can actually swallow at different volumetric efficiencies. Notice how my sizing chart calls for 517 cfm at an 85% efficiency which is almost identical to Summit Racing's 525 cfm need.
 

Attachments

  • Carburetor Sizing Chart .jpg
    Carburetor Sizing Chart .jpg
    244.4 KB · Views: 21
Last edited:

dougelam

Well-known member
Joined
Jul 19, 2011
Messages
453
Location
Michigan
Corvette
2002 Roadster
IF "he" was your friend and IF "he" valued your opinion "he" (if there is a he) would have known this before "he" went and made such a horrible mistake!

Sent from my SM-T530NU using Tapatalk
 
Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
Double Pumpers And Engine Dyno Tests

I have seen dozens of You Tube videos of engine dyno tests by the Engine Masters and others in which one thing is common. They all use either 750 or 850 cfm double pumpers. The smaller engines get the smaller 750 cfm double pumpers and the larger engines get the larger 850 cfm double pumpers. On an engine dyno the speed is raised SLOWLY so the effects of a mechanical secondary carburetor that is way too big isn't noticed but on a street engine a sudden full throttle opening at a low vehicle speed in a high gear will result in a massive bog because the venturi velocity drops to near zero. So just because an enormous mechanical secondary carburetor works well on a dyno it doesn't mean it will work well on a street engine operating at a low rpm/high gear condition. That's why AVS or vacuum secondary carburetors were used on almost every street engine; to prevent the low rpm/high gear bogging.

When it comes to selecting carburetors most people don't have a clue what they are doing and pick a carburetor based solely on the advice of other people who also don't have a clue what they're doing. Like the blind leading the blind or lemmings jumping off cliffs. For high rpm off road racing an extra large mechanical secondary carburetor isn't a problem but it's a HUGE problem when used on street engines because of the 15-20% lower volumetric efficiencies and engine speeds. Summit Racing's advice on carburetor selection is to multiply the engine's cubic inches by 1-1/2 to arrive at the cfm required. Which means a 350" engine would require 525 cfm, a 400" engine would require 600 cfm, and a 454" engine would require 681 cfm.

To verify the accuracy of Summit Racing's advice I have ran my 454" up a 6% grade under full power to 130 mph at 5500 rpm and found the secondaries of my 750 cfm Holley were still about 25% shy of opening all the way; proving a 750 CFM is more than adequate for a 454" street engine. If you're running a vacuum secondary Holley you can do the same test by clipping one of your wife's "bobbie pins" or a small paper clip to the secondary diaphragm lift rod and see how far the bobbie pin or paper clip gets moved. If you're running a 350" with a 750 cfm vacuum secondary Holley you'll quickly find your secondaries are BARELY (if any) opening.
 

Attachments

  • Carburetor Sizing Chart .jpg
    Carburetor Sizing Chart .jpg
    244.4 KB · Views: 16

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
When it comes to selecting carburetors most people don't have a clue what they are doing and pick a carburetor based solely on the advice of other people who also don't have a clue what they're doing.



Kind of like thinking cleaning paper air filters with gasoline, running ridiculously low air pressures in your tires, storing gasoline in non sealed containers, thinking engine oil pressure at 70 psi when the engine is hot is a good thing?

Funny how this seems to all come from the same person.
 

kpic

Well-known member
Joined
Mar 29, 2014
Messages
2,292
Location
Columbus, NC/NE Georgia
Corvette
1997 boosted silver coupe
To calculate required CFM; one needs more than a tiny chart.. One needs to know the VE or Volumetric Efficiency and there is no one answer for all engines. The answer is a range. Which explains why 2 identical stock cars, one was faster than the other. The one had a 65% VE and the other a 75% VE.

VE for a variety of engine types.


  • Factory engine: 65%-75%
  • Mild street performance, flat-tappet cam: 80% - 83%
  • Mild street performance, roller cam: 90% - 93%
  • All-out race engine, 95%-105%
  • Blown engines, typically over 100% or higher, depending on amount of boost.


Holley 800 cfm dual-feed carburetors were used on all 302s, <small>
dot_clear.gif
</small>although list numbers differed somewhat on 1967 models. Cars built at the Norwood, Ohio assembly plant <small>
dot_clear.gif
</small>used list number 3910 carburetors only, while cars built at the Los<small>
dot_clear.gif
</small> Angeles plant used list number 3910 and list number 3911, the latter being used<small>
dot_clear.gif
</small> on cars equipped with an AIR (Air Injection Reactor) emissions system. <small>
dot_clear.gif
</small>California laws mandated that any new car sold within the state must be <small>
dot_clear.gif
</small>equipped with an emissions control system, thus the difference in carburetors.


The 302 Engine

There was also a 2X 600 CFM dealer installed option which was known to stumble off the line. Or the VE% wasn't high enough.


According to TBTR's chart, a Z/28's 302 with a 100% VE was limited to 524 CFM. Yet, they ran quite well with an 800 CFM nor were they close to a 100% VE.

TBTR, one thing I know is you weren't at the drag strip or on the street racing scene back when those cars were new..





 
Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
I Feel Really Bad For Him But...................................

When my neighbor told me all about his new ZZ6 serpentine belt 350" crate engine I could tell he was really excited. I think he said he paid $7500 for it because he bought the most expensive "turn key" version. But right away I knew he had made several horrific mistakes. The engine he bought came with a G.M. aluminum single plane intake manifold made for 4000+ rpm operation and then he topped it with an enormous 750 cfm vacuum secondary Holley. And then rather than installing a reverse direction fan he decided to install dual 11" Spal fans so he'll certainly have overheating problems when the weather warms. I sure wish he would have talked to me first before shelling out $7500 but he didn't and now he'll have to live with what he bought. I really hate to see people make horrific and very expensive mistakes but I can't hold myself responsible for other people's lack of knowledge and subsequent screw ups. The engine he bought would be suitable in a Camaro with a 4 or 5 speed manual transmission and 3.73 gears but it's going into a 1/2 ton pickup with a TH350. He would have been a LOT better off buying the high torque 383" with a dual plane intake and 600 cfm Holley.

I have learned one thing. If I give advice to people without their asking for it I come under attack for being a "know it all". The ONLY way advice is accepted is when a person specifically ASKS for it and then they often don't like what they hear. So it's damned if I do and damned if I don't. And as far as my neighbor goes I'll let him work it out.
 

kpic

Well-known member
Joined
Mar 29, 2014
Messages
2,292
Location
Columbus, NC/NE Georgia
Corvette
1997 boosted silver coupe
Doug
The only thing TBTR knows is he is right and the rest of the world is wrong. Although they defined TBTR's opinion; for a 302", those little Z/28s screamed using the wrong carburetor

From Master Techs to seriously advanced hobbyists, to design professionals; everyone knows TBTR is in a world of his own. The only reason folks here waste time answering TBTR's empty spew is so newcomers and lurkers know TBTR's advice is not to be followed.

A legend in his own mind is putting it mildly.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
I have learned one thing. If I give advice to people without their asking for it I come under attack for being a "know it all". The ONLY way advice is accepted is when a person specifically ASKS for it and then they often don't like what they hear. So it's damned if I do and damned if I don't.



The very last thing you are is a "know it all".

You are indeed a "know nothing at all", and prove it every post.

Yes, you certainly could be damned. I cannot fault your logic there.
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,238
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Fortunately he chose a vacuum secondary Holley because a mechanical secondary (aka double pumper) would have been a total disaster as they aren't suitable for street use.

I suspected from your carb thread that you didn't know the difference between a mechanical secondary carb and a double pumper and that you were using the terms interchangeably. The above sentence confirms that. Let me help you out.

A double pumper is a mechanical secondary carb.

A mechanical secondary is not a double pumper unless it has dual accelerator pumps for additional fuel under initial acceleration. It has an accelerator pump shot delivered to both the primary and secondary side when you hit the pedal. A standard mechanical secondary carb only has a pump squirt on the primary side like a vacuum secondary carb.

If you need further clarification go to the Holley site and search both double pumper and mechanical secondary. They do a good job of explaining what they are.

Tom
 
Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
Getting Too Darned Old To Remember Things

Something JUST occurred to me. My neighbor also put a new set of HEADERS on it to make it even more powerful looking.
 
Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
Street Engines And Racing Engines

Street engines and racing engines are completely different as street engines operate over a very wide rpm range whereas racing engines operate at mostly at high rpm and full throttle. So the carburetors used are also very different as street engines require carburetors that can be "throttled" to suit the wider air requirement. Which means the secondary opening must be delayed until the engine is able to ingest the additional air. That's why the carburetors for street engines incorporate either an AVS (air valve secondary) or VS (vacuum secondary) to delay the secondary opening. A lot of people have been led to believe AVS or vacuum secondary carburetors are a government conspiracy to reduce power when nothing could be further from the truth. The AVS and vacuum secondaries are there to make the MOST power at the lower rpm range.

I have often heard people say "I want to determine when my secondaries "kick in"..............thinking they operate like an afterburner on a jet plane. When I was an ignorant kid and learning about carburetors I wired the AVS valve of my 396" Camaro's QuadraJet wide open and found out REAL quick what excessive air does to an engine at a low rpm as my engine literally fell on it's face. That's when I learned (the hard way of course) that secondaries on street engines MUST be delayed to prevent an "over throttle" condition that kills power.

When properly adjusted an AVS or vacuum secondary throttle opening should NEVER be noticed other than a steadily increasing air intake noise. In other words you should NEVER feel them "kick in" because if you do feel them "kick in" they are adjusted way too loose and the sudden noise difference you hear is actually a bogging condition until the engine gains enough rpm to accept the additional air.
 

dougelam

Well-known member
Joined
Jul 19, 2011
Messages
453
Location
Michigan
Corvette
2002 Roadster
"I have often heard people say"

Who are these people?
I believe he mutters it often to himself and he believes it is other people!




"I want to determine when my secondaries "kick in"

Again he assumes everyone is going to make the same mistakes he makes!

Back in my day it was called fine tuning

Sent from my SM-N920P using Tapatalk
 
Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
Neighbor Stopped By Today

My neighbor brought his '71 Blazer over this afternoon so I could look it over. His new dual Spal 11" fans look pretty slick AND the full shroud has 6 thin rubber flaps that allow air to blow through at cruising speeds. So it may work okay after all. The serpentine belt setup is awesome and we decided the lower part of the aluminum mount under the A/C compressor can be cut off so the lower hose can be routed easier. The lower portion of the aluminum mount is for the smog pump mounting and these old Blazers didn't use smog pumps so that part of the aluminum mount can be removed. The high rise single plane intake is VERY tall so I really doubt a ZZ6 could be used in a C3 with a standard hood even with a dropped base air cleaner. It has a very distinct exhaust leak right behind the #1 exhaust port and he's going to have to solve that issue. The HEI cap ALMOST touches the firewall but it can be turned enough to time it and the 750 cfm Holley will work okay even though it's just plain too big for a street 350" engine. At least it's a vacuum secondary Holley and not the mechanical secondary track racing style that has become so popular in recent years among the novices. Oddly enough the ZZ6 came with fuel hose rather than a formed steel tubing that connects to fuel hoses connected to the Holley carburetor and uses "spring ring" clamps. All in all the new ZZ6 really looks nice
 

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors

Dealers:

MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Parts/Accessories:

Vetteskins

Advertise with the Corvette Action Center!

Double Your Chances!

Partners

Top Bottom