1999 454 carb swap. best street carb, 600? 650? 750? CFM

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

SquareRoot

SquareRoot

Full Access Member
Joined
Mar 18, 2017
Posts
4,187
Reaction score
8,002
Location
Arizona
First Name
Mike
Truck Year
85
Truck Model
K20
Engine Size
350
bucket said:
I'm not arguing, but can you explain this further?
Click to expand...
A carb mixes air and fuel as it enters the manifold. When that mass of air hits the bottom of the manifold it has to turn 90 degrees and make its way thru the runners until it compresses against the back of the intake valve until it opens. Every change in direction makes the air/fuel mixture want to separate. EFI doesn't mix the two until it's ready to enter the combustion chamber. That's a dry manifold. To your point, dry air is lighter and responds faster than heavy air. That directly relates to throttle response. It's similar to sucking water through a straw versus a milkshake. Viscosity. A lot of people mistake that faster response as more hp. An EFI doesn't make more power than a carb, it just feels like it.
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,404
Reaction score
28,192
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
SquareRoot said:
A carb mixes air and fuel as it enters the manifold. When that mass of air hits the bottom of the manifold it has to turn 90 degrees and make its way thru the runners until it compresses against the back of the intake valve until it opens. Every change in direction makes the air/fuel mixture want to separate. EFI doesn't mix the two until it's ready to enter the combustion chamber. That's a dry manifold. To your point, dry air is lighter and responds faster than heavy air. That directly relates to throttle response. It's similar to sucking water through a straw versus a milkshake. Viscosity. A lot of people mistake that faster response as more hp. An EFI doesn't make more power than a carb, it just feels like it.
Click to expand...

The dry air vs wet air makes sense, but isn't that kind of splitting hairs? I'm betting port design and velocity are far more of an impact when it comes to throttle response. All things being equal, the wet/dry difference probably could not be felt on any typical engine, I'm assuming.
 
Y

YakkoWarner

Full Access Member
Joined
May 29, 2024
Posts
158
Reaction score
181
Location
Central Texas
First Name
Wolf
Truck Year
1989
Truck Model
R2500 Suburban
Engine Size
454
SquareRoot said:
A carb mixes air and fuel as it enters the manifold. When that mass of air hits the bottom of the manifold it has to turn 90 degrees and make its way thru the runners until it compresses against the back of the intake valve until it opens. Every change in direction makes the air/fuel mixture want to separate. EFI doesn't mix the two until it's ready to enter the combustion chamber. That's a dry manifold. To your point, dry air is lighter and responds faster than heavy air. That directly relates to throttle response. It's similar to sucking water through a straw versus a milkshake. Viscosity. A lot of people mistake that faster response as more hp. An EFI doesn't make more power than a carb, it just feels like it.
Click to expand...

Nice clear explaination - makes sense too. Based on that, I'm assuming that a TBI (which is still technically electronically controlled so would still in my mind be an EFI system) would have the same drawback as a carb in that its a wet manifold setup? Whereas a newer style injection with an injector for each cylinder would be the dry manifold setup?
 
SquareRoot

SquareRoot

Full Access Member
Joined
Mar 18, 2017
Posts
4,187
Reaction score
8,002
Location
Arizona
First Name
Mike
Truck Year
85
Truck Model
K20
Engine Size
350
bucket said:
The dry air vs wet air makes sense, but isn't that kind of splitting hairs? I'm betting port design and velocity are far more of an impact when it comes to throttle response. All things being equal, the wet/dry difference probably could not be felt on any typical engine, I'm assuming.
Click to expand...
I see your point. There's no way to test one versus the other without changing a bunch of variables. Comparing an electric fans performance with different air density is an easy way for me to understand the concept. The denser the air, the more amps is drawn to move it. I'll settle on "we're both correct".
 
SquareRoot

SquareRoot

Full Access Member
Joined
Mar 18, 2017
Posts
4,187
Reaction score
8,002
Location
Arizona
First Name
Mike
Truck Year
85
Truck Model
K20
Engine Size
350
YakkoWarner said:
Nice clear explaination - makes sense too. Based on that, I'm assuming that a TBI (which is still technically electronically controlled so would still in my mind be an EFI system) would have the same drawback as a carb in that its a wet manifold setup? Whereas a newer style injection with an injector for each cylinder would be the dry manifold setup?
Click to expand...
Correct. TBI is sort of a hybrid. Like a mullet or an El Camino. Good at both but not great at either.
 
Catbox

Catbox

Full Access Member
Joined
Jul 12, 2018
Posts
4,445
Reaction score
16,637
Location
Just Outside of Portland Oregon
First Name
Peter
Truck Year
1979
Truck Model
C20 Silverado Camper Special
Engine Size
461
I will be running a 750cfm on my 1997 L-29 Vortec Big Block when the time comes.

The current trucks big block runs a 750cfm and we can get between 7 and 12 mpg with it, similar to the worn out Q-jet it replaced.
Mine will be a Holley as we have several at the house and all the tuning parts for them.
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,404
Reaction score
28,192
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
SquareRoot said:
I see your point. There's no way to test one versus the other without changing a bunch of variables. Comparing an electric fans performance with different air density is an easy way for me to understand the concept. The denser the air, the more amps is drawn to move it. I'll settle on "we're both correct".
Click to expand...

I definitely think you are correct, no argument there. I just think that if all other things are equal, nobody could possibly tell the difference in throttle response, even though the dry intake would technically show a better result.

I'm just basing my opinion off of all the carb/tbi engines out there that often seem to have perfect throttle response. It's hard to improve upon perfection.
 
75gmck25

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
2,251
Reaction score
2,188
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
On some newer engines they have gone one step further and used Gasoline Direct Injection (GDI) to inject the gas directly into the cylinder - which means only the air stream is moving past the valves.

However, one downside of GDI is that the gas/air mixture is no longer passing around/through the valves, so you don't get any natural cleansing action from the gasoline and its additives.
 
R

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
5,382
Reaction score
8,577
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
Nasty-LSX said:
Thanks Bucket. I dont want to over or under carb it, Thats why I ask. Just was not sure what a stock 454
should have.
Click to expand...
A qjet you can't over or under carbureat, it the secondary air doors only allow as much air as your engine can use. That's why they successfully ran Quadrajets on 6 cyl OHCs to 301s to 500 Cadillacs. There are some quality qjet rebuilders out there,that sell rebuilt Qjets.
 
Last edited:
SquareRoot

SquareRoot

Full Access Member
Joined
Mar 18, 2017
Posts
4,187
Reaction score
8,002
Location
Arizona
First Name
Mike
Truck Year
85
Truck Model
K20
Engine Size
350
75gmck25 said:
On some newer engines they have gone one step further and used Gasoline Direct Injection (GDI) to inject the gas directly into the cylinder - which means only the air stream is moving past the valves.

However, one downside of GDI is that the gas/air mixture is no longer passing around/through the valves, so you don't get any natural cleansing action from the gasoline and its additives.
Click to expand...
Correct and I wonder why they don't have that issue with the diesels?
 
Nasty-LSX

Nasty-LSX

TogetherforeverCovid19
Joined
Nov 22, 2014
Posts
1,395
Reaction score
1,735
Location
HOUSTON TX
First Name
Mi Hung Lo
Truck Year
85/86/87
Truck Model
c10/k10/k20
Engine Size
LSX

Catbox said:
The current trucks big block runs a 750cfm and we can get between 7 and 12 mpg with it,
Click to expand...
Catbox, 3 speed auto trans, or with overdrive trans? :cheers:
 
R

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
5,382
Reaction score
8,577
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
Catbox said:
I will be running a 750cfm on my 1997 L-29 Vortec Big Block when the time comes.

The current trucks big block runs a 750cfm and we can get between 7 and 12 mpg with it, similar to the worn out Q-jet it replaced.
Mine will be a Holley as we have several at the house and all the tuning parts for them.
Click to expand...
If it was LS swapped it would get 30mpg
 
59840Surfer

59840Surfer

Full Access Member
Joined
Oct 8, 2020
Posts
206
Reaction score
335
Location
Montana
First Name
Joe
Truck Year
1986
Truck Model
K5
Engine Size
350 Stroker to 383 with a 400 crank, Crane Cam, Q-Jet, single 3.5" exhaust..
Never liked Holleys -- every time the sun went behind a cloud you had to re-jet. Power valves blow out if you get a backfire and they are just a lot of maintenance.

Been running Q-Jets as an old retired street racer and drags in dual-quad setups --- even the Edelbrock versions -- flawless.

I keep a new one, hermetically sealed in case I might need it --- but in over 20 years I haven't had to break the seal.

Kinda miss the old 1406 Pontiac version AFBs and the 9636SA ---- both are 600/525 CFM. They were a lot of fun.
 
Catbox

Catbox

Full Access Member
Joined
Jul 12, 2018
Posts
4,445
Reaction score
16,637
Location
Just Outside of Portland Oregon
First Name
Peter
Truck Year
1979
Truck Model
C20 Silverado Camper Special
Engine Size
461
Nasty-LSX said:
Catbox, 3 speed auto trans, or with overdrive trans? :cheers:
Click to expand...
With the extra fast speed racing SM465 manual dump truck box.
It is such a not speed racery kind of transmission, but it is what is in there currently.
When the L-29 goes in, I will be replacing the SM465 with a 4L80 and be just that much cooler.
I might even get 13 MPG then...
 
Catbox

Catbox

Full Access Member
Joined
Jul 12, 2018
Posts
4,445
Reaction score
16,637
Location
Just Outside of Portland Oregon
First Name
Peter
Truck Year
1979
Truck Model
C20 Silverado Camper Special
Engine Size
461
Ricko1966 said:
If it was LS swapped it would get 30mpg
Click to expand...
But then I would have to invest in $4000 of computers and training and plugs and learning.
The swap is easy, getting it to run correct is the part that will drive you nuts.

I can buy an awful lot of fuel for that $4k investment and the current big block. :happy107:
 
You must log in or register to reply here.

Similar threads

Nasty-LSX
1986 c10 with 1999 vortec 454 carb swap. degree Timing??? Help
Replies
2
Views
959
Ricko1966
R
C
1973 k20 tranny swap
Replies
2
Views
430
chevykid71
C
68dhayes
Building a new motor for my 79 C10 - would love some advice!
Replies
14
Views
1K
75BEASTK20
7
Urbanburban22
The Overlander: Gen3 LS, 6L90, MP1125 AND BODY SWAP
Replies
3
Views
372
Urbanburban22
Urbanburban22
squaredeal91
1995 454 has 2 timing tabs?
Replies
3
Views
982
bucket
bucket

Latest posts

Forum statistics

Threads
44,075
Posts
948,866
Members
36,146
Latest member
Nuggets40
Top