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

SquareRoot · Oct 7, 2024

bucket said:
I'm not arguing, but can you explain this further?

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 · Oct 7, 2024

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.

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.

YakkoWarner · Oct 7, 2024

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.

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 · Oct 7, 2024

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.

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 · Oct 7, 2024

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?

Correct. TBI is sort of a hybrid. Like a mullet or an El Camino. Good at both but not great at either.

Catbox · Oct 7, 2024

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 · Oct 7, 2024

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".

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 · Oct 7, 2024

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.

Ricko1966 · Oct 7, 2024

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.

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.

SquareRoot · Oct 7, 2024

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.

Correct and I wonder why they don't have that issue with the diesels?

Nasty-LSX · Oct 8, 2024

Catbox said:
The current trucks big block runs a 750cfm and we can get between 7 and 12 mpg with it,

Catbox, 3 speed auto trans, or with overdrive trans? :cheers:

Ricko1966 · Oct 8, 2024

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.

If it was LS swapped it would get 30mpg

59840Surfer · Oct 8, 2024

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 · Oct 8, 2024

Nasty-LSX said:
Catbox, 3 speed auto trans, or with overdrive trans?

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 · Oct 8, 2024

Ricko1966 said:
If it was LS swapped it would get 30mpg

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:

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

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