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Turbo Carb - Blow Through 101


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#1 MacDaddy

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Posted 05 September 2009 - 04:42 PM

Petrol - Blow through Carburettor. I've seen a few posts of people asking what's required here. I was going to put an addendum in the Newbie post

http://www.xfalcon.c...read.php?t=8441

But it started to get large. I'm happy for Mods to put it in there if deemed appropriate. I wrote it with Xflows in mind, though of course, the same principles probably apply to anyone running V8 as well.

This is not as much of a headache as many think, but you're probably going to need some help. As a given, before doing anything like this on a daily, talk to an engineer about what he/she will require to pass it.

Carbs combine the process of throttle-body and fuel delivery at one point. The fuel is metered by the air being pulled into the engine. The process of forced induction is exactly that - you're now forcing air in rather than just suppling fuel by the vacuum of the engine... which as far as a carb is concerned, is backwards.

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The carby will have to be setup for boost. Some manufacturers make complete, sealed boxes for carbs, but this is an imperfect solution as you still have to get fuel lines in there. Given that blow-through carb has had a little bit of a resurgence, you can often buy prepped Holleys from suppliers such as Hume or Rocket ind. Many of the carbs from these shops are going to be designed to support V8s though... If you've got a V8 and you're thinking of this, that's awesome, you bastard. For Xflows, the stock Weber can be boost referenced, as can a 350 Holley. In fact, more conservative carbs are generally better for these types of modifications. General modifications may include annular discharge boosters, power valve channel restriction modifications, accelerator pump pullover enrichment mods, air bleed sizing, large needle and seat assemblies and several other tricks to provide a flat fuel curve under boost and still provide acceptable non-boosted drivability. Another necessary modification is solid nitrophyl floats as the brass floats can collapse under boost pressure.

Heres an older page that goes through the setting up of a Holley DP. Obviously, if you're going for a Holley, a 600DP is a huge carb to run on the 6 on the street, particularly if you have to pass emissions. But you might be crazy or running methanol or both.
http://www.hangar18f...m/blowthru.html

If you're new to the idea and don't know carbs, you might want to have someone else do this for you. The cost of boost referencing if the carb is in good condition could be a couple of hundred dollars - maybe less. If the carb needs a rebuild at the same time, well obviously you're looking at the same again, depending on what condition it's in. Get the carb set up with mechanical secondaries, so the complete operation is derived from your right foot. It may also be worth considering having it setup for E10, which has an effective ron better than premium fuel...

With a Boost referenced carb, you'll need a way of getting the air to it. Hats or Plenums for the ADM weber don't really exist, or if they do, they're new. K&N make a small unit for ADFs and you could fabricate an appropriate base. I don't think the inlet will be a good size though, as most of us run 3" intercooler piping. This is a situation where people using Holleys have it a little easier as aftermarket hats are more widely available to suit Holley 4bbls.

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Of course, the same principles apply to supercharged applications as well.

You can always make you're own hat, or at least have a crack at designing one and having a metal shop put it together for you. The advice I received was don't just shoot the pipe down the carb throats - the plenum doesn't need to be large, but you need to think about having a pressurised chamber sitting where the airbox was. You can see how the hat flattens and fans out on the Procharger install above. Essentially you're designing a box with a pipe inlet. If you've got one of the rectangular Redline/Hardimans filter boxes your dimensions are pretty much there already... just think of where the pipe's going to go. If it's a setup without an intercooler, you can put inlet to the plenum facing over the rocker cover in the same direction as the EFI cars to give a really short intake path from the turbo, or if you're using a front-mount, then have the pipe facing forward. Remember to check your design against the space you have under the hood.

For the weber, the base of your current air filter box has the template for the base so far the shape of the hole and where the four bolts are located. The four bolts will go right through the top of the hat.

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You're going to need to get fuel to the carb to match the pressure you're going add. This means running an EFI style pump rather than the factory carb mechanical pump. Ordinarily carburettors might see 4-7psi. Actaully a stock weber may only see 3psi, so there's a significant difference to the kind of pressure you see in an EFI system, which will be running at least four to five times that. Start with your baseline N/A psi fuel requirement, add the boost (psi) you're expecting to run, and then add 20-30% for headroom. That's what the pump will need to be capable of.

If you've started with an EFI car, then the pump in the tank will support 200+rwkw and all of the return lines will be in place. If you didn't start with an EFI car, there are a range of fuel pumps out there that aren't expensive - Bosch units such as those used on the VL turbos (and heaps of other cars) will work. The tricky bit is that you will need to have a return line to the tank, which i don't think factory carb cars have. Gravelrash has a neat and inexpensive solution with the mech pump too (see pages 2 and 3 of this thread). Alternatively you could run a custom setup with a fabricated tank/cell, or you could also try to find an intact EFI tank and lines from a wrecker, or just have someone run lines. If you're thinking of the latter, you may want to have a breif chat with an engineer, because generally when you're running an efi pump the lines have to be metal, all the way into the engine bay, but this may differ state by state. Obviously you wouldn't want to run rubber lines that weren't designed to withstand the pressure or flow of a more aggressive pump - that would be a ticket to a fiery plastic death.

Now that you've got the fuel in the engine bay, you've got to bring the pressure down so that the carb only sees what it needs including under boost. Fuel regulators with return that can be referenced with the intake manifold are perfect for this. Malpassi and Aeromotive are common brands. These will siphon off the right amount and then circulate everything else back to the tank. These can be up to a couple of hundred dollars new, but they come up on ebay every now and then.

After the carburettor and the fuel supply are sorted, its really the same as any other setup - pick you're turbo to suit your application, find a manifold, exhaust and intercooler as required.

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To set a tune, use an electronic distributor with vac advance and boost retard, such as a scorcher or an ICE. You can also use an MSD BTM or equivalent Crane ignition controller and coil with adjustable retard as well, although an aftermarket dizzy will work quite well and is a little cheaper. Boost retard is useful as it pulls the timing out with reference to more pressure going in and help avoid detonation. I wouldn't really consider running a setup without any type of compensation. The advantage of the units such as the MSD and Crane is that if you get a batch of bad fuel, then you can pull it right back, or if you run race fuel you can keep a more aggressive curve and back it off for daily duties.

Ultimately,this setup should require very little tuning once it's set. Your peak power and torque figures will be comparable with an EFI setup, although efi is better often able to compensate for more variable conditions and will make maybe 3-5hp more through the revs to the similar peaks. By putting the top of the carb in a closed box you're removing a lot of the variables that carbs run into anyway. People have said that efi is better for idle control, but this generally refers to compensating for big cams on N/A. I'd have to say that if your carb is set up well for this type of setup, idle shouldn't be a problem and your cam won't be massive - just don't snap the solenoid off your Weber when you're working in the bay... that can make make you look like a bit of a tool with hunting revs and fuel surge under trailing throttle. I speak from experience.

Getting this on the road is, in part, a story for another time - suffice to say, you'll need an engineering certificate, but that's just part of the expense and if your work's good, it won't even be that much. Some states require an IM240 emissions test, but you should find that if correctly set up, this won't be a problem for any ADR27 cars running a standard style exhaust for testing. It's more challenging for ADR37 plated cars (check your build plate) because the tolerances are tighter, but it's still possible.

Costs, like any aftermarket setup are going to depend on the quality of the gear you have or have access to at the start, and how much you can do yourself. If you have an EFI car to start with, then all you really need are the reg, manifold, referenced carby, a way to control ignition retard... So it's kind of like buying all of the stuff for a Turbo-LPG setup, but not having to buy the LPG tank, carb/TB, mixer, etc. The LPG setup has a cash-back attached for the time being, and gas is cheaper than petrol, I'll admit, but the initial outlay is probably lower for the carb setup. Furthermore, correctly setup for E10 or 98ron, I would expect you'll get better average mileage and power considering that our LPG has a legislated minimum ron of 93, even if straight propane is 110.

Hopefully that answers a few questions that pop up time to time... everything else like exhaust and oil/water supply, gates and BOVs is exactly the same as any other setup.

Edited by MacDaddy, 17 January 2012 - 08:39 PM.
Update + Link to Gravelrash fuel setup.


#2 KLR250

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Posted 05 September 2009 - 06:02 PM

Great writeup there mate, Thanks for posting it up
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#3 KLR250

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Posted 05 September 2009 - 06:03 PM

Ive made it a sticky, could be very handy for other guys wanting to do a similar thing
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#4 ltd351t

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Posted 05 September 2009 - 07:45 PM

TOP stuff mate, the hanger 18 link is the best place to start, very good write up.
www.TURBO351.com

www.danspec.net

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#5 STATUS TUNING

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Posted 06 January 2010 - 10:41 PM

heres how holley does it in house, nice and easy DIY

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#6 THORNSPAWN

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Posted 06 January 2010 - 11:32 PM

Looks like a handy mod.
Another trick is adding a seperate fuel enrichment system, it's a bit old school but still works and can also be used for water/methanol injection.
When I did my Gemini I used 2 hobbs switches switching 2 gas fuel lock offs then spraying through 2 1mm jets mounted on the hat in right angle adaptors with metering needles through them. One switched at 5psi the second at 14psi. This worked well up to 22psi through the standard carb. The whole setup cost less than $500 and made just under 200hp from a 1.6L

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#7 82xefalcon

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Posted 29 March 2010 - 05:45 PM

just found a webber hat.

http://cgi.ebay.com....=item3ca0aa40db
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#8 AVATAR

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Posted 31 August 2010 - 03:09 PM

a few videos are whats needed now on how to tune...


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#9 kiwi xf

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Posted 08 December 2010 - 11:16 AM

gday im interested in how u modify a weber to blow thru is there a standard manifold under that set up

#10 MacDaddy

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Posted 08 December 2010 - 11:44 AM

Yeah, that was a standard manifold. Solid float and boring out of the chokes is what I think was done, but i can't remember. I paid for a local specialist to do this work.

Relatively speaking, the weber and manifold were probably the limiting factor on that car, as it can obviously only flow so much. That said, I'm pretty sure the combination of the tune and the weber was the key to passing emissions. Were I to do it all again and persist with the blow-through setup, I would probably be looking at the mech-sec 390cfm holley 4bbl on an AussieSpeed... but to mod the weber was $100, and to get that other stuff was $1000.

Good luck if you choose to go through with it.

Edited by MacDaddy, 08 December 2010 - 02:49 PM.


#11 kiwi xf

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Posted 08 December 2010 - 02:46 PM

no emmision rules or cert stuff here in nz yeah i was gonna go with a 450 mech secondary and aussiespeed manifold thanx for the reply u guys are lucky having so many experienced guys in the trade doing old ford stuff i cant even source a set of pistons ill have to come over for a HOLIDAY and fill my suitcase lol

#12 kiwi xf

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Posted 07 February 2011 - 03:54 PM

im hoping some1 can tell me in the imformation on modifying a holley for blowthru they mention 50cc accelerator pump kits if im using the carb on a 6cylinder will it still require this upgrade also has any1 actually used the imfo with sucess and can they provide anymore good advice im doing a 600 dp on my 4.1 i no its to big in theory but im gunna give it a try hoping that jetting will make it work as being under pressure the size of the throats shouldnt matter to much and if any1 knows what psi the standard floats wiil take before crushing as im gonna run low boost 7-10 max

#13 gravelrash

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Posted 08 February 2011 - 07:32 AM

my blowthru setup.
600dp Holley modded by me.

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#14 bluov1

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Posted 18 March 2011 - 10:25 PM

I am using a 600 d/p on my blow thru 4.0 i have tried a 500 2 barrel with no success just not enough fuel
and that was with 86 jets a 8.5 power valve and drilled a 2.5 mill hole behind the power valve which is as big as
i could go and still not enough fuel as soon as the motor would get close to boost it would lean out,put the 600 on
with similar primary set up but also put big jets in the rear and it loves it and this is a stock e series motor.
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Edited by bluov1, 18 March 2011 - 10:28 PM.


#15 gravelrash

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Posted 19 March 2011 - 09:09 AM

had my XP on the dyno a couple of weeks ago for some initial tuning. The jets we ended up with were 85 & 93. 50cc acc pumps and drilled out the pump shots a couple of thou. started to have a problem at about 10psi where there was a spray of fuel out of the throttle shafts.
At the end of the day we made 304hp at the tyres. we still had a fuelling problem coming on to the secondarys, but some playing with acc pump cams would solve this.

Since then i have ordered a Quick Fuel blow through carb from the states. Biggest advantage with this is the throttle shafts are already sealed and the power valve is boost referenced, this means i dont have to go so big on the jetting and can remain easier to drive around town.

btw - i love your blow through 4.0l

cheers :hy:

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Edited by gravelrash, 19 March 2011 - 09:43 AM.


#16 gravelrash

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Posted 19 March 2011 - 09:12 AM

im hoping some1 can tell me in the imformation on modifying a holley for blowthru they mention 50cc accelerator pump kits if im using the carb on a 6cylinder will it still require this upgrade also has any1 actually used the imfo with sucess and can they provide anymore good advice im doing a 600 dp on my 4.1 i no its to big in theory but im gunna give it a try hoping that jetting will make it work as being under pressure the size of the throats shouldnt matter to much and if any1 knows what psi the standard floats wiil take before crushing as im gonna run low boost 7-10 max


standard brass floats will handle about 2psi. imagine scrunching up a coke can. that is what air pressure does to them

#17 bluov1

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Posted 19 March 2011 - 10:21 PM

just ask for gas floats? (solid plastic) they are not expensive. Once i have drilled out behind the power valve i wont need to run
such big primary jets at the moment mine is running 8psi mine is not a street car so driveability is low on the agenda
once i stop procrastinating about what car i am going to have this
motor in now that its running okay i dont want to pull it out and put it into my xf .i have another turbo,gate ,modified
intake manifold ,and GRA lpg setup which is why i am contemplating using the x flow thats in it just need another c4 and
a exhaust manifold

#18 MacDaddy

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Posted 20 March 2011 - 03:12 PM

as the motor would get close to boost it would lean out


Interesting. What fuel setup are you using? Does it regulate with boost-reference? The reason I ask is that when I had my blow through setup, the exhaust gas analysis always came heavy with CO, not NOx; it was rich, even at 10-12 psi. The restriction in my choice of a smaller carb was because of not enough air getting in.

#19 bluov1

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Posted 20 March 2011 - 08:13 PM

i am running a walbro intank to a swirl pot then a bosch 044 to an aeromotive fuel reg set to 6psi and rises 1psi per boost psi
i am running a autometer air fuel gauge that just has the l.e.ds that go from side to side it would go to the red then
backfire through the carb and stall the more fuel i put into it the better it got

#20 slydog

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Posted 20 March 2011 - 08:24 PM

What fuel regs are you guys running,like what model number or part number?Say If using about 12PSI what do I need is there a preferred brand or is it just between Malpasi and Aeromotive?

Slydog




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