Big Bore and carb

496 BB

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Is a big bore block and a worked over s/a carb gonna work? Or is it too much engine and not enough carb? Also if a stock bore is used will a bigger valve head benefit with a s/a carb? Trying to see what route is gonna happen. S/a and no alky only rules.

Also who makes best s/a carbs in your opinion?
 
Big bore works with everything, and makes everything work more.

Example: If your motor is 196cc stock, then you have a 2.992" bore. 25% bigger engine will produce 25% more velocity forcing the SA carb to flow 25% more air per every 4cycles.

It's like flow testing your carb at @20" of H20 vs testing it @25" of H20.
 
So a s/a carb will flow enough to make this work? Is there no limit to "forcing"?

What about adding a bigger intake valve on 14cc or 18cc head?
 
So a s/a carb will flow enough to make this work? Is there no limit to "forcing"?

What about adding a bigger intake valve on 14cc or 18cc head?

Of course the motor will benefit more if your carb was bigger because it would breath easier. It's like a 12 year old kid breathing through a straw, vs a grown man breathing through a straw. Sure your still alive, but not at your full potential.

The big valves are Not needed
 
Of course the motor will benefit more if your carb was bigger because it would breath easier. It's like a 12 year old kid breathing through a straw, vs a grown man breathing through a straw. Sure your still alive, but not at your full potential.

The big valves are Not needed

Just curious on your thought on not needing big valves?
 
Of course the motor will benefit more if your carb was bigger because it would breath easier. It's like a 12 year old kid breathing through a straw, vs a grown man breathing through a straw. Sure your still alive, but not at your full potential.

The big valves are Not needed

What Im asking is the cost of the big bore worth the HP gains?
 
That's a matter of opinion. Are you doing well without the big bore?

If so, I'd put the money into my tire program for the upcoming season.
 
A nice ported stock valve head out flows a SA carb as is. Going with big valves is just a waste, unless your planning on going with a 13 hp carb, or something bigger than a SA carb in the future.

Oh i forgot to add, with all SA motors go with BIG duration cams. 260 or more duration @ .050" lift.
 
A nice ported stock valve head out flows a SA carb as is. Going with big valves is just a waste, unless your planning on going with a 13 hp carb, or something bigger than a SA carb in the future.

Oh i forgot to add, with all SA motors go with BIG duration cams. 260 or more duration @ .050" lift.


do you think they make more power with a stock sized valve vs a bigger valve?
 
Reading between the lines, what he is saying is the carb is a limiting factor.
Bigger valves won't help as much as more duration would.
Your pulling longer on each intake stroke.

Big carb changes the equation, now the valves are limiting your ability to draw in the big carb.
 
A nice ported stock valve head out flows a SA carb as is. Going with big valves is just a waste, unless your planning on going with a 13 hp carb, or something bigger than a SA carb in the future.

Oh i forgot to add, with all SA motors go with BIG duration cams. 260 or more duration @ .050" lift.
Ported as in just short side? Or as in actually removing port material instead of smoothing?

Also was thinking a 308 cam was going to be my cam of choice but now have me thinking. Im on a bullring type dirt oval. Small as can be with braking in corners. Im running BP2 with 1.3 on intake now. Need/want more. Dont see a 308 with 260+ duration without getting into a 320 lift and Im not wanting to get into all those clearance issues. Maybe an F275 with 1.3's??? Or will I still have to cut pockets with that? Not wanting to cut pockets if dont have to.
 
to me thats like saying the 3 inch bore won't matter because the head will only flow -xxx

would a bigger valve and port not have more of a draw on the limited carb?

i know this isnt apples to apples but even with restrictor plates on a nascar motor they don't run small valves by any means.
 
Im on a bullring type dirt oval. Small as can be with braking in corners. So my question is why do you wanna go big bore? there a track in ny that has the same type rules anything goes but you have to run a s/a carb.
 
to me thats like saying the 3 inch bore won't matter because the head will only flow -xxx

would a bigger valve and port not have more of a draw on the limited carb?

i know this isnt apples to apples but even with restrictor plates on a nascar motor they don't run small valves by any means.

A multi cylinder car isn't the same as a single cylinder engine

The 25% bigger motor forces the head, valves, intake manifold, carb, and even pulse more. All evens out

You can put bigger valves in just for ****s and giggles if you want. There will be no gain

I'm the originator of all versions of the 3 inch bore. Like I said, you can spend the extra money and try it. I prefer NO rules, but I can only tell you what I have done myself.
 
In nascar the heads flow 425 cfm
How much does the carb flow, 830cfm.
How much does it flow with a restrictor plate, 600cfm. = still alot of power, only when they use the 390cfm 2 barrel carb is where the power drops alot down to 550hp.

In this case you have a stock carb flowing 40cfm, a SA carb flowing 60cfm. The fully ported stock valve head should also flow 60-65cfm.

Now unless his head is flowing less, let's say 50cfm. And the carb is flowing 65cfm. Then yes he needs big valves because whoever did the head did a poor job.
 
Ported as in just short side? Or as in actually removing port material instead of smoothing?

Also was thinking a 308 cam was going to be my cam of choice but now have me thinking. Im on a bullring type dirt oval. Small as can be with braking in corners. Im running BP2 with 1.3 on intake now. Need/want more. Dont see a 308 with 260+ duration without getting into a 320 lift and Im not wanting to get into all those clearance issues. Maybe an F275 with 1.3's??? Or will I still have to cut pockets with that? Not wanting to cut pockets if dont have to.

There is a cam with 290 lift and 260 duration or a 315 lift and 260 duration etc...the BP2 is a good torquey cam with what you have listed...
Do not remove material...ie make the port bigger...unless you are going to use a Tilly carb or something similar...
Clean everything up, take all the sharp edges off, smooth the bowl out, and between the valves. Maybe if you know what you are doing shape around the guide a bit...all this will improve quite a bit over stock...but do not over do it...
What you might gain in flow you will loose in velocity.

As Minidragbike always says dynamic over static compression everytime...
 
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