Fuel enrichment with a blow through setup

Hi all. Been lurking a while and have searched but have come up empty on an idea I have. I have read Linc's write ups and will be adding a turbo to my daily driver Fairmont and have a few concerns.

It seems that the only reasonable way to keep the engine alive with the stock carb is to fatten the mixture across the board by drilling jets etc, and this still seems like a lot of trial and error. My setup is as a daily driver that will spend most of its time commuting and will see boost rarely. My thought was to use a fuel injector or nitrous nozzle by the carb to enrich the mixture under boost but to leave the carb stock otherwise to keep mileage and driveability normal. I am doing this on a budget and am all about using junkyard parts and keeping the cost low.

Has anyone done this? I cant seem to find any info on this at all.

Thanks,
Sam

Fuel injectors require high pressure to work right, much higher than the fuel pump a carb uses to work. In order to run a fuel injector in parallel with a carb, you'd essentially need an entire, additional fuel circuit running at high pressure. I have to believe that someone somewhere has done the legwork for you, and can give you a very good idea about what jet to run for moderate boost pressures. Maybe not perfect, but certainly good enough to give you safe road time to tune it just right.

Hi, thanks for your response. The problem is that without a secondary enrichment system on the stock carb, I don't see any way of maintaining driveability and the mileage I need to keep this as my daily driver. Those things are important to me as I will rarely use the boost with my driving style.

I thought maybe a lower pressure injection idea would work for me and had thought about using a water/meth injection system but substituting fuel instead. Or by using only the fuel side of a nitrous fogger nozzle and the needed solenoids. I am assuming the nitrous nozzle idea could be made to work with fuel only being supplied by a Holley blue pump? I am unfamiliar either of these systems though so I am looking for information by anyone familiar with them.

Again thanks for the response.

Unfortunately, you don't get to control when you are using the turbo and when you're not... turbos respond to load, so even part throttle acceleration will provide some amount of boost. It's not like you can flip a switch and say "no turbo this time, thanks." (Well, you *could* flip a switch and use a solenoid or something to mechanically adjust wastegate tension, I suppose...)

Any type of injector is going to be rated for a some minimum operating pressure, and that is the pressure you would need to supply. Whatever those pressures are, it's going to be greater than the ~5psi or so the fuel pump for the carb is generating. Like I said before, you would need an entire, separate fuel circuit to run this enrichment device. What that pump is, what the size of that line is, and when/how that operates would be dictated by whatever the injection component requires. You would need to make choices about how you are going to provide boost enrichment before anyone can tell you if a specific pump can handle the job. The only thing I can tell you is that it isn't going to happen using the same pump the carb needs.

It sounds to me like you are creating a lot of unnecessary complexity. Any fueling done by the carb is going to be referenced (must be referenced) against manifold pressure anyway, so you will get progressive fueling based on boost that is actually being created at any moment in time. Carbs being what they are - and that is not great at managing extremes - there probably will be either waste at no/low boost or insufficient fuel at high boost, but you can't really fix that... without installing a whole separate fuel system to handle boost-dependent enrichment.

If you want to keep this simple, keep fuel economy up, and are just looking for a little extra oomph, I would strongly consider running a tiny little turbo - maybe like a GT15 - and running some tiny amount of boost. Small boost can overcome breathing inefficiencies of an engine, making the first few psi pretty dramatic compared to the diminishing returns you get with high boost. Point is, I think you'd be surprised what just a few pounds of boost can do for driveability - especially when it's available at very low RPM. Doing so would eliminate the need for an intercooler entirely and minimize increased fueling requirements. You'd still need to index pressure against boost (because blowing 4.5psi of fuel into a 5psi manifold is sorta problematic), but that and a little jet work wouldn't be too tough.

If 5 or 6psi isn't enough, you could probably bump it up further with water/meth injection and not change much else.

Thanks very much for your reply. I certainly don't mean to disagree with you, but this idea seems workable or at least worth a try. I am going to try to use the same pump for both systems but plumb the higher pressure injector to the pre regulated side of of fuel system which would have 14 psi. I know that is enough to operate a TBI injector and am assuming it will operate a nitrous nozzle which is what I would prefer to use for the adjustability of it. I am aware of boost referencing the system and plan to use a boost referenced adjustable regulator to the carb. I am just trying to find a reasonable way to avoid killing the mileage and driveability of the car by enriching the mixture only when I am really on it hard. I figure that part throttle or light loading could be compensated by the carb as is but want to keep it safe under a full throttle load by using this system with a hobbs switch instead of using a huge main jet or overly rich mixture all around. I know it is unconventional but it seems reasonable to me and I would like to give it a whirl.

Anyone try using just the meth injection as an enriching system?

Anyone familiar with the fogger nozzle I mentioned and whether it can be used independent of the nitrous?

how much boost are you talking? Linc was running a fair amount of boost on his setup.

I was running just a plain 350 cfm holley on my turbo setup with no real problems (I fought a partially blocked fuel line for a long time)

your best bet would be a hobbs switch (pressure activated switch) triggering a nitrous fuel solenoid spraying fuel. the problem is that it will dump alot of fuel in.

a better more costly solution would be a water injection setup spraying a methonal mixture.

I daily drove my setup for a few months in college and even drove it home with no problems (about 250 miles one way)

Hi and thanks for your response also. I would love to run 20 lbs like Linc did but will of course start a lot lower maybe 5-6 psi and work up in stages and will stop with whatever is workable and driveable.

How much boost did you run with your 350 holley? When you say no changes do you mean that the carb was out of the box with just a boost referenced pump/regulator?

I think I may pick up a nitrous solenoid and fogger nozzle and try a bench test and see how it works with my blue pump. Having never used these parts I don't know what to expect. If it is unworkable perhaps I will try the meth injection. Is is possible to use the meth/water as a sole enrichment device as opposed to fuel? Straight methanol for my setup maybe?

my only carb mods:

choke delete with JB weld in the old linkage hole (I mileld the horn but that was not needed.....only did it since I had access to a mill)
Solid float.

Thats it.

You can get into drilling out the power valve passages and changing the power valve rating (this is where alot of your under boost enrichment comes from)

I was running no boost gauge but had a stock SVO T3 turbo no the car (I want to say they are setup for 10 psi or so) This was on a rebuild 78 block with a 65 head. stock dished pistons were around .050" down in the bores. I was having a fuel problem that I later found out was a partial clogged fuel line. with that fixed it ran much better (it would starve for fuel under boost) Later I switched to an electric EFI style pump and return regulator and it helped clear up transition issues I had.

Awesome, that is pretty much out of the box. Thanks for mentioning the choke removal as I have read conflicting reports on this. Is this truly necessary? It would really suck living without a choke in northern Michigan. Is the choke removal a matter of airflow, or is it a mechanical problem under boost (choke blowing closed or something)?

Reviving another thread here for prosperity.

To date, the only fuel enrichment modification i have needed On this factory carb has been a very slight increase in main jet size, and the drilling of the power valve channel restriction. That's it. These mods seem to work pretty well and net very reasonable AF ratios at idle, cruise and under boost up till about 10 psi. I have blasted it to 15 a couple times but it ran pretty lean and I got out if it quickly.

I would like to run higher boost levels because let's face it, it is awesome, but in order to do that safely It will require more fuel. I am not ready to abandon the 1 barrel carb yet, as it provides terrific mileage and very good performance, so me thinks a homemade water/meth system will be installed. 8)

I will update again when I do if anyone is interested.

Your lean issues at higher boost may be a limitation in the power valve circuit. Because there are usually no jets in the power valve circuit, the total fuel flow is dictated by the size of the passage. Boost or no boost, the power valve will open based on a relative pressure differential in the carb venturi. But with the boost on, while the pressure differential may be the same, the mass of the air is much higher. The passage is sized for NA air requirements so it needs to be drilled out.

How much to open them is a matter of experimentation, usually.

You might be able to come up with an electronic power valve of sorts using an injector. But once you've gone that far, you may as well go all the way to EFI and full ignition control.

Glad you mentioned that about the power circuit mod. For those who have not done this, it is is definitely a one way deal when modifing these channel restrictions. Once they are enlarged too far and are over richening the mixture, it is tough to fix. It can be done by drilling and tapping the pvcr and I have done that on other carbs, but on these stock carbs, it is more difficult but it still can be done.

I love the simplicity low cost and reliability of carbs, and would likely never go efi on anything, but a separate enrichment injection system would be very inexpensive compared to efi, and I would get to keep my smelly old carburetor.

When I originally posted this question, I had very little knowledge of meth injection. Instead of a gasoline enrichment injector, I will give the meth a try. Simple hookup, no extra fuel pump and cheap.

Again guys, this idea was important to me because I commute 400 miles to work and I drive this thing every day. During that commute, I will not use any boost whatsoever, zero. Without boost i obviously dont need the extra fuel and my idea was to have this enrichment on demand to keep mpg up. If someone is building a street/strip setup this approach is unnecessary and the fueling requirements could be handled in a different way.

Thanks for the update. Could you (or is that would you) detail what your exact set up is? i.e. carb, turbo, pump mods(?), ignition timing, etc...

I may have not have caught part of this discussion on Boost and Fuel Enrichment , but that's what I'm working at with my current project.

As i understand it - most carbs are set up so that at - light load/light vacuum - the Power Valve does not allow for fuel enrichment and as Vacuum drops due to higher load (more throttle), the Power Valve enriches fuel in various vac. and mech. schemes until the engine Vacuum builds to draw additional fuel from main jet setups. ...,

... as I can figure a blow-through setup : the intake vacuum goes from idle-low cruise-No/Boost /light vac through to mid throttle Slight Boost basically matching engine vac then at WOT or close to it reverses and Pressurizes Carb Venturi(negative vacuum). So a Power Enrichment scheme must be very different than an NA setup to account for it to work for a turbocharger.

One Fuel Enrichment scheme I'm researching uses an isolated port for the carb's power valve referenced to the compressor housing boost.

have fun

.

The powervalve is a pretty simple device really, and is not that different at all with a boosted engine. When manifold vacuum drops below a predetermined level, the valve opens and allows additional fuel into the main well and subsequently a richer mixture is delivered through the boosters. On a full throttle pull, the vaccum never increases enough to close the power valve until the throttle is lifted, this applies to NA and BLOW THROUGH turbo engines. As such this circuit only enrichens under full throttle when it is needed at low vacuum and allows leaner mixtures for part throttle and cruise operations. Cool right? 8)

On a Drawthrough application like Powerband's, the powervalve circuit should be modified by changing its signal location to the compressor housing as the carb is on the "suction" side of the turbo and will thus have a strong vacuum signal even at WOT. Without this modification, the power valve would never open on this kind of system.

On a blowthough system, there is always positive pressure above the venturi and negative pressure below it. Under boost, manifold vacuum is obviously below the rating of the powervalve and it is activated and should be doing its job the same as it would NA. The carb knows no difference as far as the activation of the powervalve, it just "turns on" at anything under lets say 8" of vacuum, and that includes anything "below" 0 inches or what we would refer to as boost. The air DENSITY through the venturi is much higher under boost however, and it is for that reason that a richer mixture is required through the power circuit and why the PVCR needs to flow more fuel. The problem is the powervalve circuit can only flow a fixed amount of fuel and when it is open it will be essentially adding that amount of additional fuel. So if it is sized to flow the amount of fuel required at high levels of boost pressure, it will likely be pig rich when it is first open and boost is low or worse when the engine is still running NA and hasn't begun making boost yet. Or if it is sized to flow enough at transition boost to lets say 6 or 7 pounds, it will then continually lean out at anything above that level and your engine will let you know something is wrong if you dont catch it in time. 8)

There are also modifications that can be made to the air bleeds and emulsion circuits on some carbs and again, stock carbs like mine are harder to tune in those areas but it can be done.

vssman":193zbay7 said:

Thanks for the update. Could you (or is that would you) detail what your exact set up is? i.e. carb, turbo, pump mods(?), ignition timing, etc...

Click to expand...

Yeah I wouldn't mind doing a write up if there is some interest in it or it would be helpful to anyone. I take it a new thread would be appropriate for that? Is there a given section on the forum for that kind of info? I don't remember.

I've toyed around with the idea of trying to turbo mine and am looking to see what the rest of the boost guys are running. Currently I have a tri-carb set up and while it looks good it doesn't give the overall performance that I was hoping for. It's better than stock but I'm used to the snap of the turbo (when it kicks in) as my daily driver, which until recently was a turned up Cummins, and the wife's is a turbo Subie.

..' been looking at the H/W 5200 series progressive 2Bbl's again. Their Power Valve, vacuum signal port and restrictor bung are part of the top cover and passage bore plugs available for external porting for draw-thru setups. Typically OEM is much too small for draw-thru app' but versatile for mods' especially for blow-thru.

. . .

'also found another ratty turbo application Thermo-Quadrajet ( OEM Buick 4Bbl) application with the remote Power Valve fuel enrichment porting to play with ...

vssman:
this is really a separate thread but typical log tri-power type triple intake runner ports' mod' could add capability to a turbo conversion, a blow thru: TC -I/C - 2/4Bbl and intake ports for 1/2 , 3/4, 5/6 ... .

.


have fun

'Band, cool intake adapter. I was kicking around getting another head to play with the turbo idea and getting a Holset HX35 take off from a Cummins as I have a bit of experience altering them for more flow, etc...

It's tough to get a turbo blow-through to work on the street. I've tried.

After 3 different carbs and a multitude of jet changes I figured out that the problem is in the way the enrichment circuit works. Power valves and metering rods work on the pressure difference between the intake manifold and the air pressure above the carb. The problem is at part throttle you can have significant boost with enough pressure differential to force metering rods or power valves into the lean position when the engine needs enrichment. Smaller turbos make this even worse.

One thing I would have liked to try is to reference the power valves/metering rods with underhood air instead of the carb hat. It would have required some clever plumbing to do it though.

I ended up running the carbs fat to compensate, not good for economy. All my street turbos were EFI after that. Solves all the problems.