I'm building a 72x94 turbo motor with a T03 pulling from a 45DCOE question:
is leanness a problem in a pull through engine? IMO the mixture of air and fuel is compressed so i can never go lean, is this true?
Also how much boost is considered "safe" for street driving- 15psi??
Rune
Avoiding leanness
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Muffler Mike
- Posts: 3186
- Joined: Tue Feb 20, 2001 12:01 am
Avoiding leanness
I am far from being a turbo guru, but I do my best to make my self aware, especially since I have changed my set up.
And my turbo experience has only been what I have played with my friend Jim?s stuff and my one test session of my stuff. But here it goes. Take it with a grain of salt and definitely not the bible.
On Jim?s stuff we used a little 40dcoe and had no noticeable signs of lean out. I think a lot of some lean out problems that may occur from using a dcoe will be from fuel supply. Since the webers will only take 3 ish lbs of fuel pressure. On Jim?s, I did the little trick of let it idle, and slowly crank up the fuel pressure until it started to bobble and then back off just a bit. What this did was allow max fuel pressure when running. But the second you turn off the motor, you look down the carb and eventually it would overflow. We got it up to 4.5 lbs. Doing this, was good for aprox. 200 hp out of his motor and exceeded what we wanted out of it. What I have seen on others that are trying to push it a little more is the have plumbed in a line from the boost signal to the top of the regulator. So as the motor comes on boost, the regulator gets pushed wide open to compensate for a draining float bowl. If you cant keep the bowls full, definite lean out will occur.
Then I start thinking too much about things like this and have a good theory, ok maybe a bad theory but still it may be something. If you look at the way a air filter is on a dcoe or any carb for that matter, the filter also covers the air bleed of the jets. In the event that the motor really starts to suck hard and the filter starts to restrict, this robs air and air pressure from the jets, thus allowing only fuel to be pulled through the jets creating its own richening system. So if I think this right, you can actually adjust the amount of richness by creating different amounts of restriction on the air cleaner, such as tape up part of it. But the only other problem with my theory is that the bowls also become under a vacuum and there is a lot of surface area of fuel that will see this condition and unless you can keep fuel pressure moving fast enough to overcome this little difference then its all worthless. But then if you can overcome this, will it be too much fuel and overflow the bowls. The thing is, I never had a chance to play with this on Jim?s motor, and now the car has been sold. Then I just thought of this, close off the vent to the bows in its original place and relocate a vent outside the filter. That would solve that issue. But as usual I am talking out my a__. But this may give you some ideas if you run into this problem. And if we never things, then we will never be able to move ahead.
Another option, just put a big holley on top and call it a day. And if you want extra richening, just plumb in the power valve to the boost pressure.
Is 15 lbs. of boost safe for street driving, I think no boost is safe, it just gets you in trouble hehhehe
But if your talking about for pump gas usage? Well from what I can see, there is a lot of factors on this. But I think 15 on any configuration will out do pump gas. But what I mean from configuration is difference in port restrictions, head stud limitations, where you may take the boost signal from, how much advance you run, etc. We ran Jim?s stuff at 15 lbs on 8mm head studs, but his motor also had small intake ports which are a restriction. Its like this, trying to push 15 through his head would not be the same as pushing that much through my superflows. The actual volume that reaches the valves and into the combustion chamber will probably be a lot different and volume will add to compression, and we know were it goes from there. But I may be wrong, this will depend on the evacuation ability of the chamber as well. if it cant evacuate on the exhaust stroke, then volume may be the same. It?s really hard to tell. As I said, I?m no expert. Because there is the theory of the way connecting rods ware is always nice on turbos because there is always pressure in the chamber through all the storkes and doesn?t let the rod unload and beat the back side like a aspirated motor. So what we need is some one that has a lot more experience then me. So disregard every thing that I have just said. Good luck
And my turbo experience has only been what I have played with my friend Jim?s stuff and my one test session of my stuff. But here it goes. Take it with a grain of salt and definitely not the bible.
On Jim?s stuff we used a little 40dcoe and had no noticeable signs of lean out. I think a lot of some lean out problems that may occur from using a dcoe will be from fuel supply. Since the webers will only take 3 ish lbs of fuel pressure. On Jim?s, I did the little trick of let it idle, and slowly crank up the fuel pressure until it started to bobble and then back off just a bit. What this did was allow max fuel pressure when running. But the second you turn off the motor, you look down the carb and eventually it would overflow. We got it up to 4.5 lbs. Doing this, was good for aprox. 200 hp out of his motor and exceeded what we wanted out of it. What I have seen on others that are trying to push it a little more is the have plumbed in a line from the boost signal to the top of the regulator. So as the motor comes on boost, the regulator gets pushed wide open to compensate for a draining float bowl. If you cant keep the bowls full, definite lean out will occur.
Then I start thinking too much about things like this and have a good theory, ok maybe a bad theory but still it may be something. If you look at the way a air filter is on a dcoe or any carb for that matter, the filter also covers the air bleed of the jets. In the event that the motor really starts to suck hard and the filter starts to restrict, this robs air and air pressure from the jets, thus allowing only fuel to be pulled through the jets creating its own richening system. So if I think this right, you can actually adjust the amount of richness by creating different amounts of restriction on the air cleaner, such as tape up part of it. But the only other problem with my theory is that the bowls also become under a vacuum and there is a lot of surface area of fuel that will see this condition and unless you can keep fuel pressure moving fast enough to overcome this little difference then its all worthless. But then if you can overcome this, will it be too much fuel and overflow the bowls. The thing is, I never had a chance to play with this on Jim?s motor, and now the car has been sold. Then I just thought of this, close off the vent to the bows in its original place and relocate a vent outside the filter. That would solve that issue. But as usual I am talking out my a__. But this may give you some ideas if you run into this problem. And if we never things, then we will never be able to move ahead.
Another option, just put a big holley on top and call it a day. And if you want extra richening, just plumb in the power valve to the boost pressure.
Is 15 lbs. of boost safe for street driving, I think no boost is safe, it just gets you in trouble hehhehe
But if your talking about for pump gas usage? Well from what I can see, there is a lot of factors on this. But I think 15 on any configuration will out do pump gas. But what I mean from configuration is difference in port restrictions, head stud limitations, where you may take the boost signal from, how much advance you run, etc. We ran Jim?s stuff at 15 lbs on 8mm head studs, but his motor also had small intake ports which are a restriction. Its like this, trying to push 15 through his head would not be the same as pushing that much through my superflows. The actual volume that reaches the valves and into the combustion chamber will probably be a lot different and volume will add to compression, and we know were it goes from there. But I may be wrong, this will depend on the evacuation ability of the chamber as well. if it cant evacuate on the exhaust stroke, then volume may be the same. It?s really hard to tell. As I said, I?m no expert. Because there is the theory of the way connecting rods ware is always nice on turbos because there is always pressure in the chamber through all the storkes and doesn?t let the rod unload and beat the back side like a aspirated motor. So what we need is some one that has a lot more experience then me. So disregard every thing that I have just said. Good luck
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Chuck Schneider
- Posts: 345
- Joined: Wed Mar 21, 2001 12:01 am
Avoiding leanness
I just put a turbo on a 1776 and I run 12 psi on 94 octane pump gas. It is easily making 180-190 hp. I have never heard the thing ping and it seems to get enough fuel. The main dictator is the jetting. Obviously if your jets are way off it can lean out. Another thing is the emullsion tubes the emullsion tubes dictate where in the RPM range the carb will richen up or lean out. In the dellorto tech manual it has a list of all the emullsion tubes and when they richen up or lean out. I think that F11's are used with a turbo. It automatically richens up the top end to keep the turbo happy. Also as Mike said you need to keep the float full but I haven't had any problems with that at all. I was using a junk Facet pump and it still supplied enough fuel. I would recommend a rotatary fuel pump, a Carter pump is the ones that CB sells and you can probably find one at your local parts place for 20 bucks. I would reccommend a Air fuel gage also, then you will know exactly what is going on, no guessing.
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12.70 @ 104
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12.70 @ 104
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Rune
- Posts: 7
- Joined: Sun Mar 11, 2001 12:01 am
Avoiding leanness
Thanks alot guys, i think if i use the trick with the regulator i won't have any problems for my use.
BTW dang Mike you sure do give some advice.
Rune
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BTW dang Mike you sure do give some advice.
Rune
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volkaholic
Avoiding leanness
One thing to keep in mind is that with a draw thru system, what you really want is fuel VOLUME not fuel PRESSURE. I run a 45DCOE side draft on a 1915cc type 1 with 3.5 psi fuel pressure at the carb. I put a Holly blue pump near the bottom of the gas tank & ran 3/8" fuel line all the way to the carb. Then put the biggest fuel valve in that I could find that would not flood out the carb. I ended up with a 225.
Carb jetting & emulsion tube choice will make a huge difference! This summer, I will be trying setting each carb throat up differently, one set for low end & one set for top end,,,, we will see how it runs.
I completely agree with the last guy, investing in an O2 meter is cheap insurance compared to running an engine lean for even a few seconds! Reading plugs is best left to guys with lots of years of experience, I'll take actual A/F readings.
Finally, putting the engine under boost does not change the A/F ratio. The A/f ratio at the carb is the same as the A/F in the cylinder. You just were able to put more air & fuel in than a normally aspired engine would.
Carb jetting & emulsion tube choice will make a huge difference! This summer, I will be trying setting each carb throat up differently, one set for low end & one set for top end,,,, we will see how it runs.
I completely agree with the last guy, investing in an O2 meter is cheap insurance compared to running an engine lean for even a few seconds! Reading plugs is best left to guys with lots of years of experience, I'll take actual A/F readings.
Finally, putting the engine under boost does not change the A/F ratio. The A/f ratio at the carb is the same as the A/F in the cylinder. You just were able to put more air & fuel in than a normally aspired engine would.
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Chuck Schneider
- Posts: 345
- Joined: Wed Mar 21, 2001 12:01 am
Avoiding leanness
I would like to add to volksaholic. He said putting an engine under boost doesn't change the air fuel ratio. Don't take that as you don't have to drive it to set the air fuel ratio. You have to load the engine out with the weight of the car under acceleration to actually get the air fuel ratio the motor is making under load. You set the idle as close to 14.7 as you can get and then you want the motor to see between 12.5 and 13 to 1 under hard acceleration. The combustion chamber shape of a VW will not allow 14.7 to 1 under acceleration. There are motors out there that are on the leading edge of combution chamber technology that can run 17 to 1 A/R under full acceleration and still not detonate. Some good and informative reading is www.theoldone.com. Go to the 1999 soft head article. These guys run like 12 to 1 compression ratios and 20psi boost with their combustion chamber designs. Very interesting reading, the question is can someone adapt it to an aircooled VW.