Turbos and BlowThru Carbs

[TC]

Re: Turbos and BlowThru Carbs

I doubt any blow through is going to be a perfect bolt on and go. They will all need some fine tuning. Even a custom built one might need some work.

Unless they have made changes the BG blow through Demons tend to run on the lean side and need some jetting, air bleed changes and the pvcr drilled to get closer to a better tune. especially if you run over 10 psi. They are mass produced carbs after all.

I've seen plenty of Holley's run perfect right out of the box.

As for fine tuning the AFR's that nothing my LM2 can't handle.

The thing I like about the Demon BlowThru is the modifications have been done from you, which is going to get you in the ballpark faster, so all you'll have to do is the fine tuning. Put it this way I bet the Demon runs better out of the box, than that 850 they modified in the CC article. And remember they dumped 22lbs of boost through that carb.

[seanm]

Re: Turbos and BlowThru Carbs

Hey Eric, is a large portion of that cost the dual needle and seat bowls for the E85 carb? I wanted to try E85, but the cost of the bowls and limited availability here turned me away from E85 for now.

TC: Can't say on the Car Craft build, haven't seen the article. Let us know how the BG works for you. I'm interested in the results.

[Matt Cramer]

Re: Turbos and BlowThru Carbs

The eBay turbo kits are something to approach with caution. The turbos are kind of a grab bag on quality - they seem to be getting better since the notorious SS Autochrome epoxied-together compressor housing story, but I still hear about cases where one loses its oil seals after a month. I've also seen ones hold up for 20,000 miles and still going. The turbo headers are worse, I'd say - I've personally seen a set of headers that couldn't possibly fit the application they were advertised for without a rack and pinion conversion. And I know other people who had to weld their headers back together every month. A lot of them tend to use thin, cheap steel, or have welds that look good outside but have very little penetration.

I did try to prep a blow through turbo carb once - or more precisely, salvage someone else's epic fail at prepping a carb for boost. Previous owner of the carb knew how to use a milling machine, but didn't know that mechanical secondaries on a single pumper Holley make for a massive bog on acceleration. It had been pieced together from several different carbs, the choke horn was milled off, and I have no idea what other mods it may have had. Never did get it working right.

[Eric68]

Re: Turbos and BlowThru Carbs

Hey Eric, is a large portion of that cost the dual needle and seat bowls for the E85 carb? I wanted to try E85, but the cost of the bowls and limited availability here turned me away from E85 for now.

TC: Can't say on the Car Craft build, haven't seen the article. Let us know how the BG works for you. I'm interested in the results.

Yes, the bowls are fairly complex and are cast as a specialty piece. After I get them I have to do some fairly serious mods to them to make them do what I want them to do. Metering blocks also require a lot of work to make them move enough fuel, and I have to make my own boost reference power valves (lots of mill / machine time).

I have made an annular booster Demon work well to about 900 HP on gas without dual N&S bowls, but it was really pushing the limits.

[IRONHEAD]

Re: Turbos and BlowThru Carbs

powervalves, can't you have a cnc machine bolt out a lot of them

[seanm]

Re: Turbos and BlowThru Carbs

Hey Eric, is a large portion of that cost the dual needle and seat bowls for the E85 carb? I wanted to try E85, but the cost of the bowls and limited availability here turned me away from E85 for now.

TC: Can't say on the Car Craft build, haven't seen the article. Let us know how the BG works for you. I'm interested in the results.

Yes, the bowls are fairly complex and are cast as a specialty piece. After I get them I have to do some fairly serious mods to them to make them do what I want them to do. Metering blocks also require a lot of work to make them move enough fuel, and I have to make my own boost reference power valves (lots of mill / machine time).

I have made an annular booster Demon work well to about 900 HP on gas without dual N&S bowls, but it was really pushing the limits.

Cool. How do you reference your PV's? Through the vent tube or outside activation, or top secret?

[seanm]

Re: Turbos and BlowThru Carbs

powervalves, can't you have a cnc machine bolt out a lot of them

Have never seen the ones Eric builds, but most of them need some clearance and vacuum routing/blocking work on the carb body also.

[milner351]

Re: Turbos and BlowThru Carbs

Eric - thank you for sharing your knowledge and experience with us.

What power or boost level would you consider the limit on a gas carb that was home modified?

or - in other words - is it easier to tune a lower boost - say 10 - 15psi max set up than it is a higher boost level?

as I see it from a budget perspective - lower boost levels equals fun power without allot of money in uber custom forged pistons, rods, crank, block, head studs, head gaskets, yada yada yada - not to mention carb, intake, turbo, inner cooler, plumbing etc.

If I'm going to go to the trouble of building something - I'd like it to last long enough to say it was worth the effort.

[Eric68]

Re: Turbos and BlowThru Carbs

I'd say probably 10-15 psi of boost regardless of power level is pretty doable with a lightly modified carb. There seems to be a point where all of a sudden the carb hits a wall with fuel and things start to get lean. It really depends on the carb and honestly I haven't spent a lot of time trying find the limits -- it gets expensive replacing pistons you know ;)

There are a few ways to do boost referenced power valves. The term seems to mean completely different things to roots guys and blow through guys.

With a roots blower setup the goal is to use an "alternate" signal passage from the manifold (below the blower) to control a stock style power valve. That way when the manifold sees the vacuum set point it allows the spring to open the PV and enrichment occurs. It is common for a good sized roots blower to be able to pull a vacuum at WOT on the under side of a carburetor so a conventional power valve can possibly close at WOT. I do mine by milling a slot in the base plate to connect the power valve cavity in the main body to the spark advance port in the metering block. You also have to plug a couple holes to make this work.

To the blow through guys "boost reference" means a special power valve that will stay closed under all vacuum conditions and will not open until there is a positive pressure (boost) in the manifold. There are a couple ways I know of to do this. So far my most reliable way is to reverse the spring on the power valve so that it holds the valve closed instead of trying to pull it open. This requires a special mod to the PV and a bracket attached to the metering block. The other way that I am working on (read not yet perfected) is an adjustable valve I machine from brass stock to operate off the differential pressure between the bowl and main well.

Maybe one day I will have a big expensive CNC machine to turn out work fast for me. You need to be able to do a lot of high end work to be able to pay for one anyway. For now it's a tedious process of setting up a piece in my mill, spinnin' the old hand wheels, mic it, and cut some more.

[TC]

Re: Turbos and BlowThru Carbs

Ok been doing some research, it seems those Demon carbs are good to about 18lbs of boost. But the turbo kit I'm looking at is good to 30lbs of boost. So it seems that going the blow thru way is going to limit me to the amount of boost I can run. So my question is if I go with a Votech carb box, is it basically a bolt the carb on and go thing, or will I still have to do modification to the carb for it to work? And will it still be limited to the boost you can throw at it?

[Eric68]

Re: Turbos and BlowThru Carbs

So my question is if I go with a Votech carb box, is it basically a bolt the carb on and go thing, or will I still have to do modification to the carb for it to work?

NOOOOO!!!!!!

There is nothing magical about the Vortec box. If you make 20 or 30 pounds of boost you still have to move 20 pr 30 psi of fuel through the carb.

Fuel being a liquid cannot be compressed like air, so to do it right you absolutely must do the mods to the fuel passages and boosters to make it work. A blower squeezes the air through the carb but that solves only half of the problem! You have to get fuel through the carb and you CANNOT COMPRESS FUEL!

[IRONHEAD]

Re: Turbos and BlowThru Carbs

So my question is if I go with a Votech carb box, is it basically a bolt the carb on and go thing, or will I still have to do modification to the carb for it to work?

NOOOOO!!!!!!

There is nothing magical about the Vortec box. If you make 20 or 30 pounds of boost you still have to move 20 pr 30 psi of fuel through the carb.

Fuel being a liquid cannot be compressed like air, so to do it right you absolutely must do the mods to the fuel passages and boosters to make it work. A blower squeezes the air through the carb but that solves only half of the problem! You have to get fuel through the carb and you CANNOT COMPRESS FUEL!

even oxinated fuel?