Alaska442

Im switching out my Holley sniper to a blow through carb and found a few good video on yu tube about building one and so will give it a try. Has anyone tried building a blow through carb?
I going to use a Holly 850 with 2 power valve. I bought a boost refrenced power valve and was going to use it on the primary side only. These are set to open with an adjustable spring from 3 to 9 pounds of boost pressure as a way to run leaner jets for idle and cruising but open up the flow of gas when boost opens the power valve spring.
I wanted to just use the one on the primary side and either block of or use a stock power valve for the secondary side - jets 68 in the front -primary and set power valve to open at 3psi and jets 86 on secondary.
What are your thoughts on using a power valve on the secondaries.

fleming442

I'm curious to see how this pans out. I thought power valves were for mid throttle transitions and were to be sized to half idle vacuum. As far as the secondaries go, my Brawler didn't come with one at all; it's plugged. I would think they're different for turbo vs supercharger, too, since the boost ramps differently.
In the end, carbs are dumb to a point, meaning that the air/fuel mixture is set mechanically. As long as you've got it jetted correctly, other adjustments are for drivability, in my opinion. You should be good with an AFR gauge.
What are the main features of a blow through carb over NA?

Alaska442

Im no expert but i have researched it online some and this will be my first attempt.
Have to have solid floats and jet extensions, good gaskets, and you plug the power valves port on the bottom, some people enlarge the ports but I'm going to use the boost reference power valve so it should open with 3 pounds of boost and provide extra fuel. There is a way to run tubing from the Bowl Vents toward the incoming boost signal hat helps pressurize thw bowl There are many you tubes but there is a series from ( user name ) Titan 1500 that i found most helpful.
You put smaller jets on the primary because the power valve will open when boost comes on and provide more fuel, Larger jets on the secondary. The torque storm my only give 7 pounds of boost so nothing crazy for me.

oddball

Tuning a carb is tough. Tuning a blow-through is extra tough.

Fleming, the power valve has a different purpose. The primary usually has smaller jets so cruise is tuned decently well. But the size of jet that's good for cruise isn't good for WOT (or anything over 50% throttle). So the power valve acts like an additional jet that comes in during low vacuum to supply extra fuel - so the circuit acts like it has a larger jet during high throttle (aka low vacuum) but smaller jet when low throttle (aka high vacuum). The "half idle vacuum" tidbit is just a good rule of thumb as that's where the engine is usually transitioning to needing more fuel than the cruise jets can provide and the PV spring will be strong enough to stop it from flooding the main circuit idle/cruise.

Lonnies Performance

As stated above... carbs are "dumb" as they cannot measure air density, only velocity through the venturis.
This is why re-jetting needs corrected for altitude changes etc.

A touchy subject is jetting as it needs to be somewhat balanced for equal distribution within the manifold.
You can hurt a cylinder due to unequal distribution... especially when you are asking the power valve circuit to provide a bunch of extra fuel.

At 7psi boost, you will need around a 50-60% supplemental fuel increase over normally aspirated, but your super charger increases boost progressively as rpm increases.
The power valve basically does it in 1 step.

This is where modified boosters come in handy to provide a non-linear increase fuel rate with air velocity so you get proportionately more fuel as rpm increases, in-line with how the boost curve increases.
Not sure what Holley you are starting with, but you will definitely want a carb with adjustable air bleeds as well to help tune this fuel curve across the rpm range.

I recommend buying a carb that is designed for blow through applications (or having yours modified by someone specializing in this type of work) as it can be cheaper than hurting the engine through trial & error modifications.
Personally, I'll stick with EFI when using forced induction as it, offers a lot more tuning capability.

fleming442

Thanks, oddball. I guess I got lucky tuning or the Brawlers are that good out of the box. Maybe both.

Went over to the Outlaw shop today. There's a dude putting a turbo on his race truck. He sent his carb to IRD (or bought one), and had the vent tubes extended up into the elbow. I see what you're saying.

How much power do you expect to make? I've been considering a Torq Storm, but the 7psi cap kind of turned me off.

Alaska442

Well at the time i bought mine TS they were affordable (they have gone up for the basic olds kit and original head unit). I had the unit running last summer with a Holley Sniper X with timing control and even went on 1 positive short road trip but the Sniper took a **** with some unknown problem and i will never spend money on another FI type throttle body. I believe the TS can add 150 or more HP to an average engine so we will see.
I have read now TS offers a new head unit that can make more boost but the cost is up there North of 4K for just the Head unit alone,so like they say Money equals HP.
I sent the Sniper in for warranty and the re-flashed the CPU and, with never an explanation on what was good or bad I have not and will not try it again. It cost me another summer of messing around with it and paying tuners, lost short summer cruising and $ - Sniper was $1500 and the fuel system another $600.
I will try try carb out this summer and report back.
Lonnie, good info about the air bleeds. Im using a proform holley 850 DP knock off mine has front and rear power valves and i don't think it has the air bleeds in the venturi like the Brawler does. I put an 850 Brawler on my 500CI convertible and i like it. It ran pretty well out of the box and the electric chokes helps with cold starting.
So learning experaince.

fleming442

Yeah, those TBI units are really starting to show the brown eye. I have 2 friends battling Snipers right now. On one, injectors randomly drop out, even after bench testing good. The other won't turn the fuel pump on once hot. I ditched my fiturd 800 last spring, and haven't looked back. I battled it for a solid 6 months, found a factory F-up in the VE table (jumped from 93 to 106 at 2700 rpm), and had injectors drop out, too. It took 2 weeks to dial in the carb, and picked up 5mph in the 1/4.

fleming442