Lots of great info here on carb sizing. I've seen that for a roots blower we need 30% more carb that what the engine will need at desired RPM.

https://www.rbssuperchargers.com/The_Guide_to_Carburetion.cfm

Chart on RBS supercharger site.

What kind of got me thinking, was the chart multiplier is based on boost level.

But the engine airflow is dependant on the cam, and diff cams will limit flow and consequently have boost in the intake.

A well tuned cam and head system will minimize boost and flow more.

Thoughts?

So are we talking about carbs or boost?
I ran 1050s on my set ups.. about 1000hp.
The originals were 850s .. didn`t change boost.
Boost is a measure of restriction, better flowing set ups will have less boost but still the same cylinder pressure as a bad flowing set up with more boost .

I dropped 2-3# of boost with better heads and intake same pulleys.IMOP the cylinder pressure is the same between both set ups as is the detonation danger.
Now most people would say turn up the boost to get back what you lost . Nope, that`s just more cylinder pressure and more chance for detonation.


That being said theres other things going on that affect boost , I have the last set up with less E85 but Im not sure why the boost changes between these , I suspect temp but Ill have to watch that episode again

I agree but you did get the HP it took to make the 2-3 PSI in the crank instead of blower pulley

Im not sure what you`re asking ...

Great info Dan, and I agree with you on the good cyl pressure with the better heads and lower boost.

What I realized as I was going through some great older threads, was the carb sizing chart speaks more to the boost multiplying factor, without accounting for flow.

great heads will flow with ease, and challenge the blower to keep up so to speak. Good heads and well selected cam will reduce boost, but total flow and carb CFM needs will be up.

Poor flowing 088 heads like I have...lol...will not demand as much of the blower.

My 454s at 6000 and 6 psi boost only need around 1150 cfm. 30% added carb cfm buffer makes the x2 750s viable. I don't think I would benefit much from 850s.
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you guys are scaring me .......i have never herd anyone say its to big in our hp boating world

It's a brave new world! LOL

A roots blower loves unrestricted airflow. I'm not sure if it necessarily benefitted, but it sure didn't hurt! My Formula 242 ran a best of 83 with me driving - that was with a pair of Nickerson 850's (I was told by Nickerson that they flowed well over 900 cfm each). I want to say that was around 7 PSI, but honestly, I was too busy trying to control the boat to pay attention to the gauges - no bolsters and Merc's flimsy power steering. The guy that got the boat from me ran 87 last I heard. Stock-bore 454 Mag with Dart heads that were way too big and a 525 EFI cam.

We have a pair of 800 CFM HP 500 carbs on the Bullet's 468s with 500 EFI cams and they "accidentally" glory-pulled to 670 HP at about 5800 RPM with more to go. 5-6 PSI.

If cost is a concern, it's easier/cheaper to find workable 750s all day long versus 800 and above.

242 at 87mph will push your hair back for sure.

I have the 750s, and I'll do all the CFM flow mods, and maybe even fill epoxy on the top, and radius like a K&N stub stack.

If I get vacuum under the carbs at WOT, I can get the 850 butterfly plates. 750 body & 850 plates.

Just opened up the 750s.

They were boost referenced. Stealth mod style.

Using the Ported Vac line from the Pri metering block. Base plate port plugged, and a hole drilled from the metering block/main body hole, into the PV main body port.
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Seems to be a good mod style. Apparently it's how Holley suggested how to do it years back.

But you would need a mech vac adv on a distributor.

My Jets:

PRI 75
SEC 83
PV IN pri 6.5
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​​​​​​No PV on sec.

I figured a 6.5 PV was a little light. I like the cruise to be at 0psi or a bit of Vac. Essentially cruising out of PV @3000 RPM.