frenchyd
frenchyd SuperDork
9/6/18 4:41 a.m.

Winters approaching and it’s time to get busy on the cars.  

Here’s my conundrum there are all sorts of formulas for  carb to engine sizes but few to none for carbs on top of superchargers. 

The engine in question uses 800 CFM carbs ( variable Venturi )  But 1200 CFM fuel injection.  In the stock configuration.  

Question sitting on top of a Roots type blower would a 1200 Holley double pumper be sufficient? Or is that too small?  

Second,  another engine  comes with a  more modern helical rotor type supercharger that’s oversized for the motor but uses only a 1&1/2 inch SU which I believe only flows 125 CFM  

would I be foolish to use a 1&3/4 SU if I intend to use E85? 

Finally if using a turbo charger on a road racing application with fuel injection is there power to be had by significantly increasing the throttle bore size? 

I realize this group may not have the required depth of knowledge but if not can you suggest a site that would be better?  

¯\_(ツ)_/¯
¯\_(ツ)_/¯ UltraDork
9/6/18 5:36 a.m.

I found a chart!

frenchyd
frenchyd SuperDork
9/6/18 7:45 a.m.

That’s a great chart,

I wish it jived with my numbers.  According to that the engine should have a carb of 611 CFM  when stock it has 800 CFM.  That would make a Holley 1050 double  pumper just fine.  

Or should I look up the multiplier for a 800CFM?   Which would be 2.088 for a size of 1674CFM ?  ( two 850’s ) 

 

¯\_(ツ)_/¯
¯\_(ツ)_/¯ UltraDork
9/6/18 7:50 a.m.

In reply to frenchyd :

If the engine requires at least 611CFM, it makes sense to me that the carb would be a bit oversized to 800CFM- you wouldn't want it to be operating at 100% capacity, right?

GTXVette
GTXVette SuperDork
9/6/18 8:10 a.m.

They did not teach ' Flow through a cylinder' at Bodine's School of Cypherin', I picked it up down the Road.

when your done with this Math, see how much H P that air can support, likely, you will cut back on numbers.

In the case of the S U you are adding Fuel to increase B T U'S NOT AIR. This will also apply to the other combo.

when flow testing a carb there is no pressure above the opening other than atmosphere, whether you blow or draw air through you won't change carb size as much as you may think . if there is a big differance between intake pressure and manifold pressure you might see help in a larger carb.

A Roots type supercharger may need bigger carbs as they do not really suck air in but require atmosphere to make it 'Drop' in, they make pressure below the rotors. newer design rotors can draw air in.

frenchyd
frenchyd SuperDork
9/6/18 8:23 a.m.
¯\_(ツ)_/¯ said:

In reply to frenchyd :

If the engine requires at least 611CFM, it makes sense to me that the carb would be a bit oversized to 800CFM- you wouldn't want it to be operating at 100% capacity, right?

I agree with you, Isn’t that really the principle that allows much bigger fuel injection than carburetor size?  Get in as much air as the engine can use and then inject the required amount of fuel at the right time.  

Carbs on the other hand have to choke down their sizing to generate a Venturi to draw fuel in and the Accelerator  pump  system acts as an artificial assist to richen the mixture up in order to accelerate. 

On the other hand, 611 CFM likely would not yield the nearly 300 horsepower this engine puts out in stock highly smogged condition. *net horsepower using mufflers-and choked down intake

rslifkin
rslifkin UltraDork
9/6/18 8:53 a.m.

As you mentioned, over-sizing a carb too much will cause fuel mixture problems.  But you can definitely get away with some over-sizing (how much will depend on carb design and whether low rpm / light throttle manners matter much).  

Don't forget, 4bbl carbs and fuel injection throttle bodies are typically measured in cfm at 1.5" Hg pressure drop across it.  So going bigger will yield less pressure drop and a bit more power potential (as you'll have more pressure in the intake manifold or blower inlet).  

frenchyd
frenchyd SuperDork
9/6/18 9:58 a.m.
rslifkin said:

As you mentioned, over-sizing a carb too much will cause fuel mixture problems.  But you can definitely get away with some over-sizing (how much will depend on carb design and whether low rpm / light throttle manners matter much).  

Don't forget, 4bbl carbs and fuel injection throttle bodies are typically measured in cfm at 1.5" Hg pressure drop across it.  So going bigger will yield less pressure drop and a bit more power potential (as you'll have more pressure in the intake manifold or blower inlet).  

I think you’re hitting the nail pretty well smack on it’s head. Bigger carb makes more power at the cost of driveability and idle quality.  

As a racer at heart it’s going to be hard to make that trade. The reality is this will mostly be a street toy.  Gotta try to remember that.  

The race car on the other hand,  that’s gonna be all about power, power pulling the competition out of a corner and shoven’ air outta the way on the straight. Gonna use my old, 

Somes good

More’s better

Too much is just getten’ fun!  

barefootskater
barefootskater HalfDork
9/6/18 10:16 a.m.

I seem to recall something like 190 hp on a late 70s 350 and they had a q-jet. I could be wrong but weren't all q-jets north of 725cfm? If that's not too big I don't know what is.

Frenchy, If you are looking at new carbs, I'd just call the manufacturer and tell them what you have, what you want out of it, and how you plan to use it. Chances are they'll be able to get you the right set up pretty easily and it should be really close right out of the box. If you are looking to find the right stuff at a swap meet.... good luck, I really don't know.

rslifkin
rslifkin UltraDork
9/6/18 10:50 a.m.

In reply to barefootskater :

Q-jets are all either 750 or 800 cfm depending on which version.  But they've got tiny primaries and vacuum secondaries, so it's not going to end up with all 4 barrels wide open and run like garbage if you stab the throttle at low rpm.  A design like that works pretty well for being able to over-size it without penalty.  I've always kinda wondered what 2 q-jets on a tunnel ram would do...  Of course, anything that could do could be done as well or better with EFI.  

NickD
NickD UberDork
9/6/18 11:24 a.m.

I know that with Roots blowers, they say that even a minor lean-out is a death sentence. So those guys tend to run a slightly larger carburetor to avoid that risk, with the supercharger smoothing over any low-speed driveability issues. They also hate any sort of inlet restriction, so running an oversized carburetor will pick up a good bit of power on tip. That being said, you can still over-carburete, and that may even result in a lean condition. Hot Rod Mag, back in '04, threw a Weiand 8-71 on a 426 Hemi crate motor, and thinking they were clever, ran 2 1150cfm Dominators on top. This resulted in a wicked lean condition because the carburetors were designed more for big-inch Chevy Rat motors with 10-71 to 12-71 blowers. Those pull on the carb harder and draw more fuel than the stock, small-cam 426, so they had to jump up to massive alcohol jets just to make it happy.

frenchyd
frenchyd SuperDork
9/6/18 11:34 a.m.
barefootskater said:

I seem to recall something like 190 hp on a late 70s 350 and they had a q-jet. I could be wrong but weren't all q-jets north of 725cfm? If that's not too big I don't know what is.

Frenchy, If you are looking at new carbs, I'd just call the manufacturer and tell them what you have, what you want out of it, and how you plan to use it. Chances are they'll be able to get you the right set up pretty easily and it should be really close right out of the box. If you are looking to find the right stuff at a swap meet.... good luck, I really don't know.

I’ll buy new when Holley has their midwinter sale.  They sell carbs that are already set up for supercharging which will save me a great deal of effort. I can guess what jets and accelerator pump sizes I’ll want and that’s included at the price. 

 Regarding Chevy, yes you are correct they made that sort of power or less while Jaguar makes 284 DIN horsepower and 294 lb ft. Torque 

That is 49 state versions while California is a bit less. On the other hand European versions are at 309 horsepower with some at 314. The prime difference is in the smog tuning. 

On the other hand Mercruiser using the same 350 motor Chevy rated it at 300 hp in the 1980’s 90’s ( the 454 made only 310! ) my 1979 Chevy 350 Mercruiser  is rated at 260 hp. I don’t know when they switched ratings, likely when they went to roller lifters on the camshaft. 

Regarding Holley’s help line,  well they dont know!  While there are a few V12’s running with 2 four barrels most are using swap meet carbs or whatever.  No one thus far has shared tuning details with Holley. 

frenchyd
frenchyd SuperDork
9/6/18 12:25 p.m.
NickD said:

I know that with Roots blowers, they say that even a minor lean-out is a death sentence. So those guys tend to run a slightly larger carburetor to avoid that risk, with the supercharger smoothing over any low-speed driveability issues. They also hate any sort of inlet restriction, so running an oversized carburetor will pick up a good bit of power on tip. That being said, you can still over-carburete, and that may even result in a lean condition. Hot Rod Mag, back in '04, threw a Weiand 8-71 on a 426 Hemi crate motor, and thinking they were clever, ran 2 1150cfm Dominators on top. This resulted in a wicked lean condition because the carburetors were designed more for big-inch Chevy Rat motors with 10-71 to 12-71 blowers. Those pull on the carb harder and draw more fuel than the stock, small-cam 426, so they had to jump up to massive alcohol jets just to make it happy.

I want to run E85 but Holley won’t go there with me. I’m forced to work things out on my own.  That’s the real reason for these questions.  

I’m using  Chevy 350 numbers and trying to adjust for the greater power of a Jaguar V12  which actually gets me up in Big Block Chevy territory. ( on only 326 cu in) 

 

frenchyd
frenchyd SuperDork
9/6/18 12:49 p.m.
GTXVette said:

They did not teach ' Flow through a cylinder' at Bodine's School of Cypherin', I picked it up down the Road.

when your done with this Math, see how much H P that air can support, likely, you will cut back on numbers.

In the case of the S U you are adding Fuel to increase B T U'S NOT AIR. This will also apply to the other combo.

when flow testing a carb there is no pressure above the opening other than atmosphere, whether you blow or draw air through you won't change carb size as much as you may think . if there is a big differance between intake pressure and manifold pressure you might see help in a larger carb.

A Roots type supercharger may need bigger carbs as they do not really suck air in but require atmosphere to make it 'Drop' in, they make pressure below the rotors. newer design rotors can draw air in.

The Jaguar XKE uses 3x2 inch SU’s that flow over 900 CFM !! That’s for a little 258 cu in six cylinder!   

When they went to the V12 they used 4 Strombergs that flowed only 800 CFM  on a really awful manifold. 

Later they went to fuel injection that flowed 1200 CFM 

 

wspohn
wspohn Dork
9/6/18 3:16 p.m.

SUs are somewhat more forgiving as they are a constant depression type of carb and will not open up more than the engine can take, but if you add too large SUs you sure can screw up the running of the engine on anything other than full throttle as there isn't sufficient flow to raise the pistons.

God knows why the MG factory homologated 2" SUs for the MGB and SCCA allowed them, but not the 1 3/4" SUs that gave just as good power but far better progression.

American 4 bbl carbs are not the same thing and over carbbing can result in big drivability issues unless the throttles are controlled so that they don't open prematurely, nor too much.

I ran a big block Chrysler in my Jensen, with 1200 cfm of Holley sixpack on them and they ran very well indeed even when you floored it, because the end 2 bbl carbs were secondaries and were opened with vacuum.  I recall several idiots back in the day that decided they knew better than the factory and coonverted to mechanical carb progressions and found that when you floored it, the engine gave a great gasp and never quite caught up again.  The only odd thing about the sixpack was that if you were cruising at 100 mph on the flat and came to a slight rise, the secondaries would cut in and it felt like you'd just donshifted, but you got used to that.

While I like the look of carbs (and hated adjusting the six Weber DCOEs on my old Lamborghini) I am pretty much a fuel injection advocate now.  Remember, the single biggest reason old carbbed engines do about half as many miles between rebuilds as injected is that they wash the cylinder walls when on choke and wear out more quickly.

Question for Frenchy - the triple SU Jags are seriously thirsty, and the twin 2" SU set ups have lousy manifolding and distribution.  I've always wondered what triple 1 3/4" SUs would be like on a 3.4 or 3,8 - probably get a lot more economy and maybe not give up much performance?

This is a triple carb set up I created for my MGC that gave another 15 mph on top end and ran like a train.

 

frenchyd
frenchyd SuperDork
9/6/18 3:58 p.m.

In reply to wspohn :

The JagXKE seemed to get away with the big SU’s nicely because of the long stroke (4.17)  but the best manifold design for the Weber’s gave only about 15 more horsepower than the 2 inch SU’s at a cost of fuel mileage.  Not to mention a very expensive carburetor and thousands of dollars worth of jets etc. 

Lucas mechanical fuel ⛽️ injection  was a night mare to tune.  But produced almost 30 more horsepower.   Hilborn fuel injection lost some mileage to the Lucas but was massively easier to tune. Later came out with dial a jet and it was simple as turning a knob to adjust to changing weather conditions.  

My thoughts are to put a supercharger on my MG TD like Moss Motors sells but instead of  the 1&1/2 inch SU use the 1&3/4  SU using E 85 

Im pretty familiar with the variousneedles and have a set up to turn my own needles based on rough tuning using an e haystack has analyzer to get close and final tuning on a chassis dyno  

 

Our Preferred Partners
5yHaORXteMfMl1J3Fm6uGeN9M8HrKDwjpUjzDsJwUASRZ6kgK4kFsi1EYpyICEj3