Opinions on spreadbore carburetors

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dart_68

dart_68

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I've been reading threads about cam suggestions and whatnot and the effects that a too large a cam will have on an engine with too low a compression ratio and a thought crossed my mind (I know, dangerous).

Would going to a spreadbore type of carburetor with their much smaller primaries help an engine overcome. at least in part, the low static compression ratio at lower rpm? The idea also being that the large secondaries will help the higher rpm range that the cam will like.
 
Basically You're not overcoming anything, but You are crutching the fact that You're anticipating low intake vacuum & efficiency at relatively low RPMs, and yes small carb
primaries will help w/a stronger signal to the mains at part throttle. It won't help overall performance one bit, but it will enhance driveablity, a plus. What will help is an
appropriate torque converter that won't trap Your mill down in "Lazyland", as long as that's correct, it should rock. Also, depending on how "too big" it is, You should give
serious consideration to advancing the cam.
 
What's the combination? They canwork really well,it needs to be thought out.
 
The smaller primary idea you are thinking of will not really save the engines low compression woes

I do like a good working TQ! Big bore version if possible.

Not so much the Holley spreadbores
 
Abodybomber said:
What's the combination? They canwork really well,it needs to be thought out.
Click to expand...
I don't have a combination. It's what physicists call a "thought" experiment. I do agree that the small primaries would gain some drive-ability but in the end you still need the right converter and gears. No carb can make that up.
 
Killer6 said:
Basically You're not overcoming anything, but You are crutching the fact that You're anticipating low intake vacuum & efficiency at relatively low RPMs, and yes small carb
primaries will help w/a stronger signal to the mains at part throttle. It won't help overall performance one bit, but it will enhance driveablity, a plus. What will help is an
appropriate torque converter that won't trap Your mill down in "Lazyland", as long as that's correct, it should rock. Also, depending on how "too big" it is, You should give
serious consideration to advancing the cam.
Click to expand...
Agreed. And crutching is exactly what I was thinking. Getting the engine to not be as doggy on the bottom end until the rpm gets up into the cam range. The right converter and gears are a must still.
 
dart_68 said:
Agreed. And crutching is exactly what I was thinking. Getting the engine to not be as doggy on the bottom end until the rpm gets up into the cam range. The right converter and gears are a must still.
Click to expand...


There is not a carb on the planet that will fix the wrong cam. It can cover up some issues, but it still won't be correct.
 
And with the calculators we have today, There's almost no excuse for bad combos any more. You just keep throwing numbers at them til something sticks.
And we're enjoying a time in history where hi-compression piston availability is very good.(except maybe for our little brother, the mighty 273)
But I guess there will always be someone sliding a 268 cam into a 7.8Scr 318,for street-duty, who never heard of a calculator,lol. So ignorance is a forgivable excuse,lol. (yeah I did that too,many moons ago)
But on the right combo, a spreadbore, small-block,street motor, can be a formidable force, not as to absolute power, but in day to day operation, the way we actually drive our cars. It's a nice tuning tool, especially for 318s,streeters, and DDs. Oh heck, for every street engine.
And for a streeter, you shouldn't really be thinking of a cam that big. If you need more power than about a 235 cam, then you should be thinking of a bigger engine.
The [email protected] power peaks around 5400/5600 in a 360, give or take, and with good heads will continue making power for maybe 600 rpm or more.And with an automatic will want to shift the 1-2 at about 1000 rpm higher. So now you're getting into the mid to hi 6xxxs,and reliability issues with the factory oiling system.
So then enter the stroker
 
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AJ/FormS said:
And with the calculators we have today, There's almost no excuse for bad combos any more. You just keep throwing numbers at them til something sticks.
And we're enjoying a time in history where hi-compression piston availability is very good.(except maybe for our little brother, the mighty 273)
But I guess there will always be someone sliding a 268 cam into a 7.8Scr 318,for street-duty, who never heard of a calculator,lol. So ignorance is a forgivable excuse,lol. (yeah I did that too,many moons ago)
But on the right combo, a spreadbore, small-block,street motor, can be a formidable force, not as to absolute power, but in day to day operation, the way we actually drive our cars. It's a nice tuning tool, especially for 318s,streeters, and DDs. Oh heck, for every street engine.
And for a streeter, you shouldn't really be thinking of a cam that big. If you need more power than about a 235 cam, then you should be thinking of a bigger engine.
The [email protected] power peaks around 5400/5600 in a 360, give or take, and with good heads will continue making power for maybe 600 rpm or more.And with an automatic will want to shift the 1-2 at about 1000 rpm higher. So now you're getting into the mid to hi 6xxxs,and reliability issues with the factory oiling system.
So then enter the stroker
Click to expand...


IMO, this is dead nuts on. Though I myself, am willing to live with a little more cam IF it suites the person driving it.
(A side note to me and more cam, I drive a manual 4spd which tends to like a larger cam & gear.)

A true double duty hot rod car (point of view here) will need a cam (& the rest of the drivetrain as a combo) to work in the 2,000 - 6000 rpm range really effective. In (very) general terms, that a hyd. flat tapper of approximately [email protected]. You can get away with more cam in bigger engines and do with a little less on smaller mills.
 
But back to the topic.....

Selecting a smaller cam to gain back the lost dynamic compression is really the only route to go or rebuild and build into the engine the correct compression ratio needed to properly operate the cam effectively as designed.

Operating a big cam low compression engine produces terrible low torque with soggy/ muddy/crappy running low end. I guess that is OK if your converter is flashing way above normal driving rpm’s and the car is really 80% or better track. But on the street it’ll basically suck.

The things to help it are band aids and of little positive results. The best method would be to use Rhodes lifters at maximum pump up delay for maximum benefit (of making the cam smaller @.050 until the lifter oumps back up) with a spreadbore carb for higher velocity through the small primaries to help enhance the off set package. Also use a dual plane intake instead of a single plane.
 
It seems to me that if the higher static compression ratio that is recommended for bigger cams is to make up for the lower cylinder pressure at lower RPM due to "bleed off" - that in itself is a "crutch". The difference is that a higher compression ratio will make power throughout the entire RPM range while the carb "crutch" won't.

I agree that the smaller primaries may improve driveability at lower RPM, but it won't make power and if you drive it on the street, you've wasted your money on a big cam. You've GOT to get the engine up into the RPM that the cam was designed for and the only way to do that is to have a loose converter and steep gears, both of which may not be suited to street driving.

The idea for my original posit came from thinking about why heads with too big of ports can hurt performance, or why too big of a carb, or even a carb opening full up too soon all can hurt performance.
 
That would be;

To big of a head - low flow through ports, bad air fuel mixing.

Carb, same as above. If the engine can not use it at the time it is given.
 
And now for the monkey wrench......... What type of carb would I choose on this combo and why?
A 360 with nice fat torquey 223* street cam/balanced compression, a stick, 2.76s and overdrive.
So the first question is why the 2.76s. Answer ;cuz the 3.09 pulls them off the line real good. The starter is the same as a 2.66 x 3.23
Why the overdrive? Well cuz I can tune that 223 to make very high 20's mpg, at 65=1740rpm.
Yeah but won't the engine lay down in second gear? No, because it makes a peak of 440 ftlbs, and my car is only 3467 me in it; so wake up, I'm coming thru.
So what's the problem,put a 750DP on it and lay some rubber. Answer,Ok that works dandy on Saturday night, but what about the rest of the week?
The rest of the time is spent in second gear mostly, averaging 30 odd mph. This makes about 1965rpm. At this rpm the VP will be very high, meaning the engine is very willing to blast off.Same happens at the cruise rpm of 1740; lots of VP. This means that your 750DP is a very big 2bbl, just itching to go. The smallest throttle opening increases cause the car to surge ahead. This is very annoying while cruising, as you are constantly sawing on the gas pedal to find a particular mph.
So what's the solution? You guessed it a smaller throttle opening. But, wait that means like a 500, before the sawing stops.And really the engine wants a minimum of 650.
And that leads to the obvious answer; a spreadbore.

But wait, who runs a 2.76, lol? Answer; the factory.
But wait what happens with 3.23s? well that is a 17% in rpms so now the second gear Rs are 2300.....So the VP has dropped a bit, but the ftlbs are increasing, so sameold/sameold.
What about a smaller cam? Smaller than 223, are you kidding; why? The 223 makes mega torque in a 360 and gets great fuel mileage, and if you spring it just a bit heavy it'll rev to 7000 and with nice heads will make more horsepower than most chassis can handle.So no, smaller than 223 is .......insert your own adjective here.
What about bigger. Bigger is ok cuz the VP will drop, and so the jumpiness goes away. Plus she won't like 2.76s anymore. In fact with iron heads you'll end up with 3.91s before too much time goes by. So now you're in a whole new world; bolt on that 750 and go.
But a 223* cam is a sorta special place.And she likes a spreadbore especially with hiway gears.
I suspect if you extrapolate this to a stroker, which makes ultra-mega torque, this could become a serious issue.Remember, all examples are with a clutched car.
EDit: Added
V/P Index Calculation
 
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Comment in the quote
dart_68 said:
It seems to me that if the higher static compression ratio that is recommended for bigger cams is to make up for the lower cylinder pressure at lower RPM due to "bleed off" - that in itself is a "crutch". The difference is that a higher compression ratio will make power throughout the entire RPM range while the carb "crutch" won't.

I agree that the smaller primaries may improve driveability at lower RPM, but it won't make power and if you drive it on the street, you've wasted your money on a big cam. You've GOT to get the engine up into the RPM that the cam was designed for and the only way to do that is to have a loose converter and steep gears, both of which may not be suited to street driving.
This part looks logical, and I'm glad you put "bleed-off" in quotations.
But in paragraph 2 , sentence one, you're losing me, right after RPM. Bur after that I think we're on the page again.Or at least, I'm on your page lol.

The idea for my original posit came from thinking about why heads with too big of ports can hurt performance, or why too big of a carb, or even a carb opening full up too soon all can hurt performance.
Click to expand...
 
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A well sorted out TQ is going to be tough to beat, no matter what. Check out the power some of the stockers make running them, then consider Ma Mopar put them on low compression 340s and 360s back in the day. The whole concept is to cover a much wider airflow range, doing it well. I friend of mine messes around with two of them on a 605 cube Hemi with good results.
 
@AJ/FormS

“You liked me 3 times”
Is that like you kissed me 3 times in internet speak?!?! LMAO

Your last post on what carb to use with... was to long for my eyes to handle. Basically, that mentioned cam in a 360 with an OD and mentioned gears.

Spreadbore carb would be a TQ
Squarebore for pure street for me would be a AVS 750.
 
AJ/FormS said:
And now for the monkey wrench......... What type of carb would I choose on this combo and why?
A 360 with nice fat torquey 223* street cam/balanced compression, a stick, 2.76s and overdrive.
So the first question is why the 2.76s. Answer ;cuz the 3.09 pulls them off the line real good. The starter is the same as a 2.66 x 3.23
Why the overdrive? Well cuz I can tune that 223 to make very high 20's mpg, at 65=1740rpm.
Yeah but won't the engine lay down in second gear? No, because it makes a peak of 440 ftlbs, and my car is only 3467 me in it; so wake up, I'm coming thru.
So what's the problem,put a 750DP on it and lay some rubber. Answer,Ok that works dandy on Saturday night, but what about the rest of the week?
The rest of the time is spent in second gear mostly, averaging 30 odd mph. This makes about 1965rpm. At this rpm the VP will be very high, meaning the engine is very willing to blast off.Same happens at the cruise rpm of 1740; lots of VP. This means that your 750DP is a very big 2bbl, just itching to go. The smallest throttle opening increases cause the car to surge ahead. This is very annoying while cruising, as you are constantly sawing on the gas pedal to find a particular mph.
So what's the solution? You guessed it a smaller throttle opening. But, wait that means like a 500, before the sawing stops.And really the engine wants a minimum of 650.
And that leads to the obvious answer; a spreadbore.

But wait, who runs a 2.76, lol? Answer; the factory.
But wait what happens with 3.23s? well that is a 17% in rpms so now the second gear Rs are 2300.....So the VP has dropped a bit, but the ftlbs are increasing, so sameold/sameold.
What about a smaller cam? Smaller than 223, are you kidding; why? The 223 makes mega torque in a 360 and gets great fuel mileage, and if you spring it just a bit heavy it'll rev to 7000 and with nice heads will make more horsepower than most chassis can handle.So no, smaller than 223 is .......insert your own adjective here.
What about bigger. Bigger is ok cuz the VP will drop, and so the jumpiness goes away. Plus she won't like 2.76s anymore. In fact with iron heads you'll end up with 3.91s before too much time goes by. So now you're in a whole new world; bolt on that 750 and go.
But a 223* cam is a sorta special place.And she likes a spreadbore especially with hiway gears.
I suspect if you extrapolate this to a stroker, which makes ultra-mega torque, this could become a serious issue.Remember, all examples are with a clutched car.
Click to expand...
I'm drawing a blank on what VP is? Rest of this post is interesting reading, though.
 
rumblefish360 said:
@AJ/FormS

“You liked me 3 times”
Is that like you kissed me 3 times in internet speak?!?! LMAO

Your last post on what carb to use with... was to long for my eyes to handle. Basically, that mentioned cam in a 360 with an OD and mentioned gears.

Spreadbore carb would be a TQ
Squarebore for pure street for me would be a AVS 750.
Click to expand...


Maybe you two need to get a room or something????????????
 
Ok,Ok., maybe I worded that oddly
I just meant that I pressed the like button on each of his preceding 3 posts on account of I agreed and liked what he said in them. I never expected drivel and drool, comeon eh? Quitchersalivating guys
 
If spread bore carbs were something so fan'fukintastical... they'd be on gawd damn top fuel funny cars so help me Jesus hallelujah amen
 
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