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Discussion Starter #1
I am building a carbed turbo setup on a fox body with a 302 based motor and T76 turbo. I'm using 1 5/8 BBK shorties with the flanges hacked off for headers(this is low budget :). What size should the crossover pipe from the driver side header to the Y be? Should the placement of the waste-gate be above where both exhaust meet into the Y or does it not matter. Also, can anyone recommend a waste-gate to use? HP levels are expected to be in the 550-600hp range. Any help is appreciated.
 

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Discussion Starter #2
Fuel System Also

Before I buy a fuel pump, I have heard that an EFI pump will work with a boost sensative regulator, but will the pressure stay @ 7-8psi @ idle. I assume it has to do with the regulator and not the pump. Again, any help appreciated.
 

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BlackLX said:
I am building a carbed turbo setup on a fox body with a 302 based motor and T76 turbo. I'm using 1 5/8 BBK shorties with the flanges hacked off for headers(this is low budget :). What size should the crossover pipe from the driver side header to the Y be? Should the placement of the waste-gate be above where both exhaust meet into the Y or does it not matter. Also, can anyone recommend a waste-gate to use? HP levels are expected to be in the 550-600hp range. Any help is appreciated.
Your piping requirements won't change due to fueling method. So, you'll be pretty safe with a 2 1/2" crossover. You should place the wastgate downstream of all the exhaust pulses in order to more precisely regulate boost pressure. We prefer the TiAL gates, but TurboSmart and Turbonetics have some nice pieces as well.
 

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Re: Fuel System Also

BlackLX said:
Before I buy a fuel pump, I have heard that an EFI pump will work with a boost sensative regulator, but will the pressure stay @ 7-8psi @ idle. I assume it has to do with the regulator and not the pump. Again, any help appreciated.
In a carb setup, the regulator is not manifold pressure sensative and you do not want it to be. You can use an EFI pump but will need a regualtor intended for that, Holley makes some. Fuel pressure is maintained by fuel flow on the outlet side to the float bowls. That is to say, if you set the regulator to 7psi, then it will maintain that output pressure until the pump flow can't keep up (not very likely). The way you keep the fuel flow to the float bowls in check is to enclose the float vent tubes so that they get the same boost pressure that the intake manifold gets. This allows the venturis to work properly (pressure differential across the venturis and various bleeds).
 

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Discussion Starter #5
One question about the regulator, most EFI regulators won't go below 20psi @ idle..correct? I'm not sure about that but I'll have to look into it.
Also, when you say mounting the wastegate further downstream is better...would mounting it where both exhausts come together into one be the best spot (the single exhaust inlet to the turbo)?
One more question...should a blow-off valve be placed on the inlet tube to the carb for when the motor see's full boost then the sudden drop-off.
Thanks for all your help so far. Greatly appreciated.
 

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BlackLX said:
One question about the regulator, most EFI regulators won't go below 20psi @ idle..correct? I'm not sure about that but I'll have to look into it.
Also, when you say mounting the wastegate further downstream is better...would mounting it where both exhausts come together into one be the best spot (the single exhaust inlet to the turbo)?
One more question...should a blow-off valve be placed on the inlet tube to the carb for when the motor see's full boost then the sudden drop-off.
Thanks for all your help so far. Greatly appreciated.
Not sure about the EFI regs. I have never tried to get one to go below 20psi. Actually, the best spot to mount the gate is right before the turbine inlet at a 45* angle in the direction of the flow. The blow off valve will work the same on the carb setup as the EFI setup. A bit of discipline with your foot can go a long way. That being said, it does make your on-off transitions smoother.
 

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blowthroughcarb said:
for all blowthru carb questions visit my board below
http://www.boards2go.com/boards/board.cgi?&user=huntracers
you will need to boost reference the regulator onto the manifold under the carb. i just use a mallory regulator to do this. not very expensive and is good up to 1100hp.
Actually, you raise an interesting point here which I forgot about. I didn't ask him how much boost he wanted to run. If one is going to run significantly more boost pressure than fuel pressure. Say he was going to run 8psi fuel pressure and 15psi boost, then he really would need a boost referenced regulator. Most of the street carb guys run 8psi fuel pressure and about 5-6psi boost. I realize this is not the case for the racers. It also makes a difference which carb he is going to use since they don't all respond the same to the exact same fuel pressure.
 

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Discussion Starter #10
I'll want to run between 12-18 pounds of boost but I don't want to rev it too high. I am starting with a holley 750 with the choke horn cut off. I have heard they work decently with a few changes. A buddy of mine spent a fortune on a blow through carb with his Procharger setup but could never get it right under load at low RPM. Anyway, this is going to be mostly a track car but will see the street a bit. A few ppl have told me as long as you stay out of boost, it should drive fine. Thanks for the info guys...keep it comin..
 

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i dont care if you are running 1psi or 18psi, you will need to boost reference your regulator. say you are running 7psi of fuel pressure at idel with 0 boost. when you start to see boost it will directly decrease you fuel pressure. 3psi of boost means you will only have 4psi of fuel pressure. 8psi of boost means no fuel pressure at the carb. the boost pressure will push the fuel backwards. so with a boost referenced regulator, for each pounds of boost it will raise the fuel pressure a pound. maitaining the 0 boost setting of 7psi.

carbs are definitly not easy to tune with a blowthru. they can work great. look at filpowskey with his nova. he is running a vortec blower and a carb on drag radials, he runs well into the 8 second zones with NO intercooler. the main advantage of a carb over efi is $$$$. i would prefer a FAST system but i cant afford it right now so a carb works fine.

chad
 

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blowthroughcarb said:
i dont care if you are running 1psi or 18psi, you will need to boost reference your regulator. say you are running 7psi of fuel pressure at idel with 0 boost. when you start to see boost it will directly decrease you fuel pressure. 3psi of boost means you will only have 4psi of fuel pressure. 8psi of boost means no fuel pressure at the carb. the boost pressure will push the fuel backwards. so with a boost referenced regulator, for each pounds of boost it will raise the fuel pressure a pound. maitaining the 0 boost setting of 7psi.
We have seen lots of low boost street setups that do not require boost reference to fuel pressure. Boost pressure does absolutely affect fuel pressure are the seat. If you exceed fuel pressure with boost pressure, then you absolutely will stop the fuel flow from the pump. Most regulators won't let the flow reverse. If you drop the fuel pressure a few of pounds but maintain linear fuel volume and have a strong signal at the booster and venturi your mixture won't change much. If you can't maintain strong signal or your booster performance is not very good under pressure, then more fuel pressure can help. I have seen several setups where the base fuel pressure is setup 1-2psi over normal to adjust for this. Also, they setup their air bleeds, power valves and accel pump cams and rods (when using those carbs) to compliment the setup.

As I said, it really depends on what the intended purpose is. Now that I better understand what he is doing, there's no question that he will have to reference the regulator to manifold pressure.

carbs are definitly not easy to tune with a blowthru. they can work great. look at filpowskey with his nova. he is running a vortec blower and a carb on drag radials, he runs well into the 8 second zones with NO intercooler. the main advantage of a carb over efi is $$$$. i would prefer a FAST system but i cant afford it right now so a carb works fine.

chad
I suppose the tuning issue depends on how well you know your carb. Most people don't fully enclose the carb either, which causes issues of it's own. But there again, it may not be the ideal solution but lots of people still get away with it. I'm not knocking carb setups at all and I would hope nothing I have said would suggest otherwise.
 

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Discussion Starter #13
blowthroughcarb said:
i dont care if you are running 1psi or 18psi, you will need to boost reference your regulator. say you are running 7psi of fuel pressure at idel with 0 boost. when you start to see boost it will directly decrease you fuel pressure. 3psi of boost means you will only have 4psi of fuel pressure. 8psi of boost means no fuel pressure at the carb. the boost pressure will push the fuel backwards. so with a boost referenced regulator, for each pounds of boost it will raise the fuel pressure a pound. maitaining the 0 boost setting of 7psi.


chad
I know I have to use a boost sensitive regulator, I have a Vortech on my injected hatch....but what I am asking is can I use an EFI regulator to get the pressure down to 7-8 psi @ idle. With a little more research I found that Aeromotive makes a regulator which you can adjust from 5-35psi and is boost sensative.
 

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Discussion Starter #14
Where can I find an airhat to fit over the Holley 4150...I suppose i can look on ebay, but does anyone recommend any particular one....I'm sure an ATI unit will work?
 

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i have a brand new bonnet for sale for a holley carb. its a gofastparts.com one. they sell for 160. ill let it go for 115 plus shipping. go to the website for pics. let me know.
 

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that is a good bonnet to use. stay away from the ati units. they just dont flow as well.

preston, we are talking about two different setups. you are talking about a carb that is in an enclosure like the vortec unit. i am talking about a blowthru, where only the center section is pressurized. when this occures you have to boost reference the regulator at all psi levels.
chad
 

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Discussion Starter #17
ICU2 said:
i have a brand new bonnet for sale for a holley carb. its a gofastparts.com one. they sell for 160. ill let it go for 115 plus shipping. go to the website for pics. let me know.
Hey, whats your email...just PM it to me. Thanks.
 

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blowthroughcarb said:
that is a good bonnet to use. stay away from the ati units. they just dont flow as well.

preston, we are talking about two different setups. you are talking about a carb that is in an enclosure like the vortec unit. i am talking about a blowthru, where only the center section is pressurized. when this occures you have to boost reference the regulator at all psi levels.
chad
A blowthrough setup is only differentiated from a drawthrough setup. If you put the carb before the turbo, it's drawthrough. If you put the carb after the turbo, it's blowthrough. You can run a blowthrough with a bonnet or an enclosure. I prefer an enclosure but it's more complicated and less readily available.
 

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Discussion Starter #19
Carb Setup

Can anyone give me a starting carb setup to go with? I have heard solid floats, and I know the jet sizes are quite higher....but can anyone with knowledge on blow-through carbs help? Thanks in advance.
 

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Cartech Preston said:
A blowthrough setup is only differentiated from a drawthrough setup. If you put the carb before the turbo, it's drawthrough. If you put the carb after the turbo, it's blowthrough. You can run a blowthrough with a bonnet or an enclosure. I prefer an enclosure but it's more complicated and less readily available.
preston (and blacklx),

preston, i would have to disagree here (not flaming or any harsh feelings just debating).

i know the difference between blowthru and drawthru.

there are two kinds of blowthru like you mentioned above. for a blowthru NOT using an enclosure the carb bowls are not pressurized like they are when using an enclosure. therefor fuel pressure is a much more touchy thing with a non-enclosure/bonnett setup. you dont have to boost reference the regulator in an encloser setup for low boost, you do with a bonnett. im not saying that you cant sucessfully run a high boost (20+psi) setup with an enclosure, but high boost setups that i have seen usually run better with a bonnett. plus a bonnett setup is easyer to make changes with than an enclosure. you dont have to take it all apart just to make a jet change. if you have a set of quick change bowls, a jet change is very easy in a blowthru setup.

blacklx, go to my blowthru carb board and ask about the carb setup. there are guys there that can answer that alot better than i.

chad hunt
 
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