Slant six single, to slant six double.

[Cudafishboi]

Good day, all hope everyone's doing fantastic,

Long story short, I am trying to get a little more pep out of my slant six. I have a 1920 carburetor at the moment and am trying to upgrade that to at least a two barrel. I've pretty much decided to go aussie Speed for the intake but am confused for what carburetor to get. .I know one of the big issues is the kickdown. I'm gonna reach out to lokar to make sure and get the right one. But that hinges on what carburetor y'all recommend.

Just a few details about the car, its a 1969 barracuda 225 slant 6. Holley 1920 carburetor, stock exhaust, electric fuel pump.

I know this question gets asked a lot.I'm sure, but like everything in life, everybody's situation is just a little bit different, so hopefully y'all can help me out.Thank you in advance.

Attach is a picture of the dog in the car for tax.

 

[MidnightSwinger]

I’d go with an Offy intake with a 2bbl Holley 2300 in the 350 cfm variety.

Any other aftermarket intake you will either need headers or a block off plate for the stock exhaust manifold heat riser.

 

[jos51700]

A buddy installed a Thermoquad on a stock 225 in the '90's. All you need are carb adapters.

 

[Cudafishboi]

I’d go with an Offy intake with a 2bbl Holley 2300 in the 350 cfm variety.

Any other aftermarket intake you will either need headers or a block off plate for the stock exhaust manifold heat riser.

I intended on picking up a pair of aussie headers as well. They have the long tube ones. I know people swear by the durtas but not a fan of the look.

 

[TT5.9mag]

I vote leave it 100% stock, tune it up real good and drive it. When you’re ready for more power, toss it in the can and put a small block in it. Don’t waste a single dollar modifying or upgrading a slant as most I’ve talked to that have done it end up unsatisfied and swap em anyway.
Fire away slant6ophiles.

 

[Cudafishboi]

I vote leave it 100% stock, tune it up real good and drive it. When you’re ready for more power, toss it in the can and put a small block in it. Don’t waste a single dollar modifying or upgrading a slant as most I’ve talked to that have done it end up unsatisfied and swap em anyway.
Fire away slant6ophiles.

Of course I've thought about it. Just seems like throwing a 318 in it is alot more work. From the suspension to a different trans. Just alot haha

 

[Charrlie_S]

Fire away slant6ophiles.

OK, You're fired

 

[TT5.9mag]

Of course I've thought about it. Just seems like throwing a 318 in it is alot more work. From the suspension to a different trans. Just alot haha

You aren’t wrong. It’s a lot of work and most guys never finish em and end up selling the project half baked. But that’s a different topic. The good news is ITS WORTH IT!! If you’re after some power.

 

[Cudafishboi]

You aren’t wrong. It’s a lot of work and most guys never finish em and end up selling the project half baked. But that’s a different topic. The good news is ITS WORTH IT!! If you’re after some power.

Maybe when I retire in about ten years from the navy, I will.But for now, I just wanted to be a little more peppy, without major spending, you know.

 

[TT5.9mag]

Maybe when I retire in about ten years from the navy, I will.But for now, I just wanted to be a little more peppy, without major spending, you know.

Thanks for your service!

 

[Cudafishboi]

Thanks for your service!

Thank you for the support and the advice.

 

[TT5.9mag]

March 15 2025

Also March 15 2025

 

[Cudafishboi]

View attachment 1716473557
March 15 2025

View attachment 1716473558
Also March 15 2025

I have so my questions now

 

[TT5.9mag]

I have so my questions now

Sorry my bad.

 

[Cudafishboi]

Sorry my bad.

Shoot me a message if able

 

[Jim Kueneman]

You have to have your expectations set correctly when modifying a slant. If you look at the formula for torque (then calculated HP) it is _directly_ proportional to displacement then the rest is almost second order. You will still have less than a 250 cu-in engine with the slant so if you are expecting drag race fast then it is way cheaper to add cubic inches with a V8. If what you are after is a nice running engine that can easily keep up with today traffic, can create enough torque that you don't think twice about gassing it to pass and know it can it, be a lot of fun in the open road high speed corners with the low end torque (that is what you optimize a slant for not for high end HP) and want something that can still pull 20 MPG so you can use it for a cruiser and not break the bank then that it what you will get buy updating you slant. I have built 225, 318 and 360 in different A bodies and for just driving around and enjoying the day I will pick slant every time.

 

[Cudafishboi]

You have to have your expectations set correctly when modifying a slant. If you look at the formula for torque (then calculated HP) it is _directly_ proportional to displacement then the rest is almost second order. You will still have less than a 250 cu-in engine with the slant so if you are expecting drag race fast then it is way cheaper to add cubic inches with a V8. If what you are after is a nice running engine that can easily keep up with today traffic, can create enough torque that you don't think twice about gassing it to pass and know it can it, be a lot of fun in the open road high speed corners with the low end torque (that is what you optimize a slant for not for high end HP) and want something that can still pull 20 MPG so you can use it for a cruiser and not break the bank then that it what you will get buy updating you slant. I have built 225, 318 and 360 in different A bodies and for just driving around and enjoying the day I will pick slant every time.

Love that, which is kinda why I was leaning twords the upgrades vice a whole new engine. Im not racing the thing. Just wanna unlock that potential. Hence the move from 1 to 2 barrels cause I've read its a decent noticeable amount of pick up. Seems the hardest part is just finding the correct kickdown.

 

[Jim Kueneman]

Love that, which is kinda why I was leaning twords the upgrades vice a whole new engine. Im not racing the thing. Just wanna unlock that potential. Hence the move from 1 to 2 barrels cause I've read its a decent noticeable amount of pick up. Seems the hardest part is just finding the correct kickdown.

I have made most of my kick-down from modified odds and ends for the V8s. Steel bar and a welder. The Offy dual one manifold came with a linkage kit for the early ones with the rotating throttle rod.

 

[Jim Kueneman]

Love that, which is kinda why I was leaning twords the upgrades vice a whole new engine. Im not racing the thing. Just wanna unlock that potential. Hence the move from 1 to 2 barrels cause I've read its a decent noticeable amount of pick up. Seems the hardest part is just finding the correct kickdown.

Oh and the #1 thing that brings a 225 in to the 21 century is decking the block and/or head to get some compression out of it. The two I have pulled apart have measured out to a miserable 7.2:1 compression.... Take 0.100" off the mating surface (head/block or mixture of both) and you will get into the high 8:1 range and it will run MUCH MUCH better and be a good base for the 2 barrel to utilize later. My recommendation for a nice street torque engine in order of importance (in my opinion)

1) get the compression up
2) more carb (400-450cfm is plenty for a torque engine)
3) cam like the Oregon Cams 2106 which is mostly like the RV10 from Doug Dutra
4) Split the front 3 and rear 3 cylinders (like Dutra Duals) and put the "Y" where the 2 banks come back together at minimum behind the transmission crossmember. The further back to lower the torque peak goes in RPM. behind the crossmember is about 2300 RPM and back by the rear axle it can push it down to 1500 RPM'ish.

For higher RPM (over 4000) then add
- oversized valves
- little head porting clean up

 

[Cudafishboi]

Oh and the #1 thing that brings a 225 in to the 21 century is decking the block and/or head to get some compression out of it. The two I have pulled apart have measured out to a miserable 7.2:1 compression.... Take 0.100" off the mating surface (head/block or mixture of both) and you will get into the high 8:1 range and it will run MUCH MUCH better and be a good base for the 2 barrel to utilize later.

How does one get that done? I imagine a machine shop?

 

[Jim Kueneman]

Yes any machine shop should be able to cut the head or block.

 

[TT5.9mag]

How does one get that done? I imagine a machine shop?

Yessir. That’s getting in to complete rebuild territory.

 

[Cudafishboi]

So much to think about, but genuinely appreciate the assistance

 

[slantsixdan]

I am trying to get a little more pep out of my slant six.

That's a real nice car (and dog).

Picking parts should be a lot closer to the end of your process. If you pick/buy parts first, you are highly likely to wind up disappointed in the result (and how much money, effort, and time you spent getting it). It's possible for a 1bbl Slant-6 in good condition and proper tune to be plenty peppy around town…with something other than a 2.76:1 rear axle (standard equipment starting in '68) and big tires (evident on your car), for example.

Start out by reading this.

One other thing:

electric fuel pump

That's not necessarily good. If it's not done all the way correctly, it can be a giant safety hazard; read this.

 

[Bewy]

Spending a LOT on a /6 is not worth it in my opinion.....because at the end of the day it is still only a 6 cyl.....& still only 225 cubes.

I would put a 2bbl Weber carb on it. Racetep in Calif or Clifford Performance will have what you need & can probably supply the correct jets, emulsion tubes, etc to make it a bolt on deal.