Boost referencing a Holley carb

[Stoneshrink]

Re: Boost referencing a Holley carb

Interestingly I had run my blower without the boost reference and it ran fine... but I'm seeing why it ran fine... I had a 1" spacer between the blower and the carb' I have a "small" 144 cfm B&M blower; plus I ran an 870 Holley on my 406. Thus I am taking advantage of a couple points made above. I didn't have any problem with burned pistons and I've two engines apart that I ran with this set up, but I'm a big believer in knowing why something works the way it does. (or doesn't and goes boom).

I also have the Tuning secrets book by Dave Emmanual and that is why I asked the question to begin it's a great primer but doesn't have half the info provided here.... thanks so far

[Stoneshrink]

Re: Boost referencing a Holley carb - UPDATE

Okay, now driven my buick with a 750 boost referenced carb.... you couldn't wipe the grin off my face for two days!

[wikd69]

Re: Boost referencing a Holley carb

-> Modified to add photos...

This is an interesting topic. I had to spend a fair amount of time researching this myself to understand what boost referencing provides in a blown application.

To start with, the term ‘boost referencing’, however cool it sounds, is somewhat of a misnomer when dealing with carburetors. It should really be called ‘vacuum referencing’ or something similar since boost really has nothing to do with power valves. ‘Boost referencing’ is appropriate however when talking about boost referenced fuel regulators or ignition timing, etc. My two cents <g>.

Most carburetor power valves sense vacuum levels in the intake manifold directly through a port on the bottom base plate of the carb. This is a problem with blower-mounted, suck-through carburetors, since the vacuum levels seen at the *top* of the blower do not reflect the actual manifold vacuum *below* the blower. We’ll see below why this is a problem and why you should really consider adopting boost referencing for your blower application.

Basically, power valves constitute part of the idle-to-WOT transition circuits within a carburetor, designed to ensure the air/fuel mix stays within acceptable limits. Too rich (excess fuel) and you'll foul plugs and over time, wash cylinder walls and rings causing scoring and premature wear, etc. Too lean (excess oxygen) and you run the risk of inducing abnormally high cylinder temperatures which can burn valves and pistons, trigger catastrophic detonation, etc. Power valves are rated in inches of vacuum (4.5, 6.5, 10.5, etc.), indicating at what point they will open to introduce more fuel into the air flow. A 6.5 power valve remains closed at vacuum levels *above* 6.5 inches but will begin opening when vacuum drops to 6.5 inches or lower. Your individual motor build (number of carbs, cam lift & duration, compression, etc.) will dictate what power valve rating you should use.

Boost referenced carburetors have a modification which supplies a vacuum signal, obtained from *beneath* the supercharger, directly to the power valve. The original base plate vacuum port is blocked and a new passage is drilled through the body of the carburetor. An external vacuum line is used to route the vacuum signal from a port on the intake manifold (below the blower) to the new direct port on the side of the carburetor. Once this is in place, the power valves in the carb can operate normally, smoothly bridging transition from initial idle through throttle opening on to main metering jet draw and WOT.

The original build on my Camaro had a pair of non-boost-referenced Dominators sitting on the blower. In addition to the overly rich eye-and-nose-searing carb settings (running 10.5 power valves on both primaries and secondaries on each Dominator), the lack of smooth transition under throttle made for some interesting stop and go traffic experiences. Think ‘pull the pin and wait for the big noise’. This is *not* the behaviour you want in a blown big block ride with 4.88 gears. Like I said, interesting :o

You can modify your existing carbs to provide a vacuum signal by blocking the original vacuum port and drilling through the side of the throttle body. I considered doing this to mine and had located a shop over in Carson City, NV who quoted $350 per Dominator to rebuild and convert the carbs. Instead, I opted for the newer Holley 4150 HPs because I wanted to change the profile and look of the engine / hardware. If not for the purely cosmetic considerations it would have been well worth the dollars to do the conversion on the original Dominators. So user experience (drive-ability, etc.) and risk mitigation (avoiding a fatal lean condition for your blower motor) are strong incentives to modifying your carbs.

Please feel free to contact me if I can help out with your application.

Here is a current view of my ride:



This is what the original build looked like with the 4500 Dominators - very 'in your face':



Here's a shot of the current build - much cleaner, and a lot better behaved (taken at the Good Guys event in Pleasanton, CA, Mar 2007):



I have one of the early Weiand blower intakes on this BBC, produced without a boost/vacuum port in the casting. I had to drill and tap a port, as seen below (in the newer manifolds there is a port available back near the distributor):



This is a view of the boost-referenced Holleys as installed. I'm certain you could duplicate this on a standard Holley carb pretty easily:

[Stoneshrink]

Re: Boost referencing a Holley carb

$350 per carb may be reasonable machine shop charge for a dominator. I did the boost (okay vacuum) referencing myself, following the instructions from the Holley Carb book mentioned in prior posts.
Here are the pictures of what I did. It was really easy... Just requires a bit of thought and some careful drilling (and tapping).

http://www.hotrodders.com/forum/journal.php?action=view&journalid=64741

[squirrel]

Re: Boost referencing a Holley carb

Interesting stuff. I don't have any of the problem symptoms on my blown motor, but I may set up a datalogging system to measure (and hopefully record) vacuum/pressure above and below the blower.

I've put a lot of miles on my motor, the rings wore out after 50k miles, is that normal for something making 650 or so hp at the flywheel?

and it has great throttle response.

[Jesse N.]

Re: Boost referencing a Holley carb

There is a real simple way to do this. If you are not using your ported vac connection on the side of the metering block you can use that passage to feed the power valve well by either milling a slot in the main body or you might be able to just trim the gasket to allow the pressure in. Of course you need to epoxy the stock power valve well feed hole and the ported vac passage in the main body though.

I know this sounds confusing but just look at the back of the metering block. Tha rest is kinda self explanitory.


Eric68,

The boost refed power valve is used alot on blow thrus too. The issue is the press above the carb is a little higher than below, since the bowls are vented to the higher pressure above it can actually push the power valve shut under boost. There are lots of ways to skin a cat though, on theturboforums.com you can find like half a dozen ways to do this ;)

[Stoneshrink]

Re: Boost referencing a Holley carb

I can only speak to what I know as this is my first blower, and a small blower at that (144). I went from an 870 HP with 80 jets front and rear Holley carb to a 750 DP carb with 76 front and 72 rear. On the 750 I did the boost reference that you see in the pictures. With the 870 the car ran okay, but ran rich especially at WOT and always smelled like unburned fuel.
When I did the boost reference the difference was un*blank*ing believable. There was always a flat accelleration from 2000 to 3200, and a bog from 2000 to immediate WOT (2500 stall torque converter so I would bring the rpm up to stall to really launch).
The car is a beast with either carb, but with the boost referenced carb the bog is gone and the usable RPM starts at 2k. when you get into the cam's RPM range (3500) the car will go sideways at 70... before it would not.
Again, I'm certainly not a pro, and the reason the car performs better may be for something I don't know... that all said, I found that the car ran fine without the BR. It simply runs better now.. all for maybe an hour of work.

[squirrel]

Re: Boost referencing a Holley carb

Thanks for the further information. Sounds like you had some other problems too, did you try the 750 before you did the BR trick? or have you tried just moving the BR hose from below to above the blower, to see how different it is?

[Stoneshrink]

Re: Boost referencing a Holley carb

Actually my plan with the 750* was to convert the entire carb for use with E85. I bought the rebuild kit, and a Proform body. For a laundry list of reasons I didn't convert, and got involved with a discussion about boost referencing. The 870 was on the car, and the 750 was all apart to rebuild for E85... thus it became the subject carb. The 870 had the problems mentioned above (I had an Edelbrock 750 carb prior had simiilar problems as well) from the get - go. The first thing I tried was to put a 1" spacer on the carb; that seemed to resolve 90% of the problems. I drove it that way for a summer and other then what I mentioned before it worked fine. But as I said, the difference was remarkable when I 750 on it.

*I got the 750 so that I wouldn't lean the motor out on E85, I read somewhere that the simplest way to keep from running lean was to lower the cfms and increase the jets... although with the Proform body I'd estimate the flow at 790.

[Eric68]

Re: Boost referencing a Holley carb

Eric68,

The boost refed power valve is used alot on blow thrus too. The issue is the press above the carb is a little higher than below, since the bowls are vented to the higher pressure above it can actually push the power valve shut under boost. There are lots of ways to skin a cat though, on theturboforums.com you can find like half a dozen ways to do this ;)

Interesting Jesse. Since we both know the stock configuration pulls the PV signal from below the carb I assume you are recommending referencing PV to the hat? (slightly higher pressure).

What is the point of this? Are you implying the PV will open quicker or somehow behave differently when referenced above the carb? Just thinking out loud here, but it would seem to me that how much pressure eth PV sees doesn't matter as long as it sees less vacuum than the opening point of the PV.

Please enlighten me Oh great boosted E85 guru! Lol

[Jesse N.]

Re: Boost referencing a Holley carb

Ok, I wouldn't call myself any sort of guru but anyways.

On some blow thrus guys are having problems with the pv shutting under boost. The carb is a bit of a restriction so the pressure above the venturi is a little higher than below. The higher pressure goes into the bowls and pushes on the back of the pv. If the differential pressure from the bowls (carb hat) to the intake is high enough it will overcome the spring and push the pv shut.

It doesn't effect all carbs. Mine seems to be fine with a normal pv. But C&S builds their blow thrus with a brpv both front and rear. The brpv uses spring pressure to hold it shut and is opened by boost pressure. If you go to www.theturboforums.com and do a search for brpv you will find alot of info along with some pics of how guys are doing this. Some are even using an external source of pressure with a solenoid valve to open the pv. Looks like there is alot of options.

Thats my take on it anyway, I don't need the brpv on my carb so I don't have any first hand experience with them but the idea seems logical and guys are claiming that it is working very well. ???

I'm still not 100% sold on the idea though. It would seem that the carb would have a nasty lean area under light to medium acceleration without the vac operated pv.

[Eric68]

Re: Boost referencing a Holley carb

Thanks for the info Jesse. I'll keep that one filed away for future use. Seems to make sense.

I wonder if that is needed moreso when guys reroute the bowl vents with brake line. That little mod seems to make a pretty big difference in jet size so you know there is a lot more pressure in the bowls . . .

[Jesse N.]

Re: Boost referencing a Holley carb

You would sure think so. But then again I have the vent extensions on mine and still run a standard pv. I needed the vent extensions to move enough fuel when I switched to e-85. The thing is I really don't know if my pv is really open or not. It seems to be opening but there is really no way to be sure.