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Old 29th May 2005, 13:05   #1
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Default Blow-thru turbo on a carb engine:How do you do it?

How do you turbo a carb engine? As far as i know one of the biggest problem in a blowthrough carb engine is the jetting. If you don't equalize the pressure i've read that jetting could never be set right. Those who have turboed their carb cars how did you do it, Did you build a pressure box to enclose the carb? Those who don't have a pressure box how do they equalize the boost and carb pressures?

Another method i've heard is equalizing only the carb float bowl pressure with the boost pressure, boxing is not done. This is done by connecting the float bowl with the turbo boost, am i right? How effective is this?

I had a turbo project in my mind. But after reading and hearing about the jetting issues and additional hardware needed to control fuelflow in a blow-through turbo system i'm getting double thoughts on it. Right now i'm considering a draw-through turbo system. I know it's a bit inefficient, it's neanderthal, i can't use intercooler or run higher boost and a bov would waste fuel if vented to the atmosphere. But on the plus side it's less expensive so i can easily afford and is okay to run mild boost.

This is an idea i have in mind, the would be turbo victim is an 800. I need suggestions & ideas on this one from our turbo experts on this forum.
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Old 29th May 2005, 13:43   #2
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psycho is the best person. has done it already and trust me it works!!!
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Old 30th May 2005, 12:46   #3
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Well these questions plagued me too for a while till I decided to take a plunge. I happen to be running a blow through with a max boost of 8 PSI, have compensated for the fuel pressure by adding an additional fuel pump, as for the BOV I havent even bothered to put in one have just left a leak in the plumbing and no intercooler either just use the fuel filter box as a plenum to expand the air before it goes into the carb. The setup is working beautifully (not as a performance vehicle as it happens to be a petrol contessa.) and by the way my jetting hasnt been changed from Naturally aspirated settings, however the carb (I wasnt using the conti carb anyway) I am using does have a pressure equaliser line for atmospheric pressure and this works for the boost too.
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Old 30th May 2005, 23:34   #4
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What carb are you using? If the carb doesn't already have a pressure equaliser then can one be added, like giving a line to the float chamber. If you're using an additional fuel pump (which is supposedly pumping at a higher pressure) then how does your carb float & needle hold up, woun't you get overflow and isn't a fuel pressure regulator needed? These things are worrying me and if these can be solved then blow through is the way to go.
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Old 31st May 2005, 00:07   #5
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Well you are welcome to come over and see the goodies yourself call me at 98440 69863
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Old 31st May 2005, 01:39   #6
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Of course, thank you I'll be in Bangalore after 19th of this month and i'm paying you a visit.
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Old 24th September 2010, 15:14   #7
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hi keshav,was hoping you would solve some of my doubts too

considering the fact normal carbs work on the principal of drawing fuel due to the creation of low pressure in the venturi , and the turbo converts this low pressure into high pressure leading to no or little fuel being drawn into the engine, i am guessing this pressure equaliser line you are talking about is the key to all the fueling problems

So could you please elaborate what is this pressure equaliser line and how it works?
And what carburetor were you using?

since i was thinking about turbocharging my car as well(and if i do it i will do it myself), either the conti or the 800, or both (depending on how well it goes), i need you to solve my doubts. so help me out here
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Old 24th September 2010, 19:13   #8
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Quote:
Originally Posted by mycarhasablower View Post
considering the fact normal carbs work on the principal of drawing fuel due to the creation of low pressure in the venturi
The velocity of the air passing the venturi creates the pressure difference creating a vacuum to draw in the fuel.

Quote:
Originally Posted by mycarhasablower View Post
and the turbo converts this low pressure into high pressure leading to no or little fuel being drawn into the engine, i am guessing this pressure equaliser line you are talking about is the key to all the fueling problems
No you got this wrong, even with the turbo there is a velocity at which the air passes the venturi however the volume of the air increases hence the need for more fuel to compensate, you can solve this with corrected jet sizes and getting the secondary butterfly to open quicker.

Quote:
Originally Posted by mycarhasablower View Post
So could you please elaborate what is this pressure equaliser line and how it works?
And what carburetor were you using?
If I remember right I was using a Solex and used a mechanical method to open the secondary carb butterfly as the vacuum line would keep it shut due to boost / blow the diaphragm.

Quote:
Originally Posted by mycarhasablower View Post
since i was thinking about turbocharging my car as well(and if i do it i will do it myself), either the conti or the 800, or both (depending on how well it goes), i need you to solve my doubts. so help me out here
Here is a trick that you can use to solve the pressure problem in a carb turbo, just mount the entire carburettor into a box and pressurize it with the turbo. I have given a link to an image that could be more self explanatory.

http://www.capa.com.au/pics/kits_paxton_mustang.jpg
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Old 24th September 2010, 22:59   #9
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Carb turbo set up options - draw through or blow through

Draw through - you may need carbon seals equipped turbo. Also it maybe difficult to setup an intercooler (fuel may get condensed in IC). But its supposed to run well for low boost applications

Blow through -
1. Unboxed setup - Where the carb outside just like how you have it in a normal car. In this setup, it maybe better to you boost friendly carb thats either designed for boost or something that can hold boost and not spit out fuel !!

2. Boxed carb- This setup may allow the use stock carb. In this setup the box/enclosure within which the carb sits air tight (even while on boost) helps to maintain a proportional pressure difference, in many ways similar to the NA pressure difference.
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Old 2nd October 2010, 18:05   #10
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sorry for the late reply, was really ill.
okay i thought about it and the boxed setup will only be of help over the unboxed setup if float chamber is vented ...right? other wise how would it be any different than the unboxed setup? the vent will lead to the pressure being exerted on the fuel in the bowl leading to the increase in amount of fuel delivered proportional to the boost.

but then we'll also have to use a high pressure fuel pump with a rising rate pressure regulator since the needle and float wont be able to handle the extra fuel pressure.

where did you get the dual venturi solex carb from? i have only seen the solex carbs from the padmini and they only had a single venturi arrangement. ive only seen, hitachi's , nippon kikaki's and mikuni's with the double venturi arrangement.
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Old 3rd October 2010, 11:39   #11
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There's another way to make most carburetors to work without the box - using an "air-seal". Its mentioned on page 61 of "turbochargers" by hugh mcinnes, and works well too. Give that a shot - its less hassle than the box.
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Old 3rd October 2010, 15:38   #12
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here's a pic of the air-seal as described in the book
Attached Images
 

Last edited by techn0l0gist : 3rd October 2010 at 15:41.
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