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Boost vs. AFR Calculator

Hi,

I am currently running 14 psi (.965 bar) of boost in an '86 930 with a twin scroll GT35R turbo and twin wastegate header system (plus Garretson IC). The fuel system has been modified and I am getting AFR's in the mid to low 10's under full boost in the midrange (with boost enrichment line disconnected from the Leask WUR that is open to the atmosphere) when in second gear with my 8:41 R&P.

I have reached the limit of how lean I can run the warm pressures and would like to add some more air to increase the AFR under boost.

Does anyone know of any kind of calculator that will help me to estimate how much a 14% change in boost will affect the AFR?

My hope is that it will bring it up into the mid 11's which would correspond to the 14% change in boost but I don't know if they are linearly related.

I found a rough calculator here that calculates boost level and air required. To go from 0.965 to 1.1 bar I need to increase the amount of air from 49.5 to about 53 pounds of air while keeping the theoretical AFR of 12.0 constant.

https://www.rbracing-rsr.com/calcboost.html



Thanks very much!
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Old 08-06-2017, 01:19 PM
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You're going it at from the opposite of typical. Are you purposely wanting to run as much boost as you have fuel for or are you wanting to increase the AFR to be optimal for the 14psi you run now?
An AFR in the 10's in the midrange is pretty common but that figure will climb up as you reach redline. Need to know AFR at redline on full boost, you might be close to out of fuel now.
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Old 08-06-2017, 02:48 PM
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full boost in the midrange (with boost enrichment line disconnected from the Leask WUR that is open to the atmosphere

do you mean full boost around 4-5k rpm?
also, what line on the WUR is not connected? the top or the side? the top does not matter.

this is the problem with CIS, too rich too early.
the WUR should have a threshold adjustment to delay the boost enrichment.

also what brian said, you need to check it at redline on full boost. you do NOT want to be lean there.
my opinion, the gt35 reduces back pressure and leans out the top end even more, CIS cant compensate properly.
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Old 08-07-2017, 04:39 AM
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Quote:
Originally Posted by T77911S View Post
full boost in the midrange (with boost enrichment line disconnected from the Leask WUR that is open to the atmosphere

do you mean full boost around 4-5k rpm?
also, what line on the WUR is not connected? the top or the side? the top does not matter.

this is the problem with CIS, too rich too early.
the WUR should have a threshold adjustment to delay the boost enrichment.

also what brian said, you need to check it at redline on full boost. you do NOT want to be lean there.
my opinion, the gt35 reduces back pressure and leans out the top end even more, CIS cant compensate properly.
The one on the top of the WUR is just a vent line. The brass nipple off the LHS goes to one of the ports on the TB is the boost enrichment one. I don't understand why you'd have that disconnected. Wouldn't it be better to connect it and allow the enrichment feature to add the fuel as needed when on boost, after you've adjusted the off-boost base mixture?

To clarify, without the boost enrichment connected you are going to either be OK on boost and pig rich off boost, or ok off boost and lean on boost.
Old 08-07-2017, 10:32 AM
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i was trying to clarify from the OP what line he had vented to make sure it was the correct one, which I would think it was since he is so rich, but you never know. sometimes you just have to ask the stupid questions because I(we) are not there looking at the car.

correct.
its also a good idea to verify that the diaphragm is good. you can do that with a hand held vacuum pump. (not part of the OP's problem)
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Old 08-07-2017, 10:49 AM
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Your answer is found on the GT35R compressor map. Will take a look later.
Old 08-07-2017, 03:48 PM
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Hi All,

Thanks for your help. Here are some answers to your questions.

* At build completion, the car had been lean at redline when under full boost. It now has two 044 Bosch fuel pumps, a fully flow optimized fuel head by Larry Fletcher (opened all the way), euro fuel lines, Leask WUR, and euro injectors. Even with the adjustable WUR and an RPM switch set at 5200, it has been hard to reduce the warm control pressure to adequately control the amount of fuel delivered under full boost when in 2nd gear. The car gets so rich (AFR<10.5) that it stumbles and falls flat on it's face in 2nd. In 3rd gear there is less mechanical advantage and it seems to do ok. The dyno testing/tuning was in 3rd gear and I didn't discover the problem in 2nd until later. It might be because of the 8:41 R&P.

AFR Idle - 12.8
AFR Cruise - 14.0
AFR Redline under full boost <12.0
AFR 3000-4500 rpm under full boost - 11.0 in 3rd gear and <10.5 in 2nd (with side port of WUR open to atmosphere so the boost enrichment function was disabled)

* What resulted was so much more fuel that I can't lean out the warm control pressure enough to affect the midrange without leaning out the cruise AFR too much. In the present configuration the car runs rich with AFR < 12 at redline under full boost.

* Since I couldn't take out too much more fuel without making the cruise AFR too lean, my choices are to either send the fuel head back to CIS Flowtech to reduce the flow or add some more air (via increasing the boost from .965 to 1.1) to use some of the excess fuel that is being delivered under full boost. Since I've got enough fuel and the timing retard function works, I thought it should be safe to increase the boost.

* I disconnected the side port of the WUR to disable the boost enrichment. Even though I am running an RPM switch set at 5200 rpm, I wanted to see if the over-rich condition was occurring because of the boost enrichment signal. If it was getting too much fuel because of the boost enrichment signal, I could delay it with the RPM switch (making sure that it wasn't over rich because of a rpm switch malfunction - it wasn't). I talked to Larry Fletcher and he says that it is just the intrinsic behavior of the fully opened up fuel head.

* I have tried to adjust the threshold function of the WUR but it doesn't seem to make much of a change overall.

Many thanks!
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Old 08-07-2017, 07:15 PM
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I've read that dual 044s is too much for CIS cars. Most with CIS will run just one. I'm sure others will confirm this or tell me I'm an idiot.
If true, that would deliver too much fuel, I would think.

That, plus the euro heads opened up with 009 injectors seems too much to me. I have stock fuel head and 017 injectors with Lease wur and get enough fuel with K27HFS, TK IC, rarlyl8 exhaust and 1.0 bar spring all the way to redline.

Also, I think the rpm switch disables the boost threshold function on the wur as you don't need both... I think.
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Old 08-07-2017, 07:30 PM
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Quote:
AFR Idle - 12.8
AFR Cruise - 14.0
AFR Redline under full boost <12.0
AFR 3000-4500 rpm under full boost - 11.0 in 3rd gear and <10.5 in 2nd (with side port of WUR open to atmosphere so the boost enrichment function was disabled)
All this is good until you get here:
Quote:
<10.5 in 2nd (with side port of WUR open to atmosphere so the boost enrichment function was disabled
I don't know how that is even possible. Without a boost signal the WCP would remain unchanged at ~3.7bar which would be very lean. I'm tuning a RarlyL8 modded 930 in the shop right now and this picture below is what you should see under boost (pressure at 0.75bar)



This is what you should see at 0.26bar before enrichment:


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Old 08-08-2017, 04:48 AM
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no other engine mods other than turbo, IC and header? (ported, CR, pistons, cam?)
euro fuel head or aluminum?

running leaner on cruise wont hurt anything. I run in the low 15's. when I got my car it was in the 16's. (3k rpm and steady driving). you can go as lean a you want as long as you get the throttle response.

what is your cruising CP?
how much is the CP dropping when you go on boost?

I think I remember you having this issue for quite a while now.

an air leak will also make the mixture richer under boost.
the reason is the turbo is still providing enough air to satisfy the engine needs and reach max boost PLUS provide enough air to vent out the air leak. because of this more air is pulled IN thru the AFM thus the mixture goes richer.
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Old 08-08-2017, 07:22 AM
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Quote:
Originally Posted by RarlyL8 View Post
All this is good until you get here:

I don't know how that is even possible. Without a boost signal the WCP would remain unchanged at ~3.7bar which would be very lean. I'm tuning a RarlyL8 modded 930 in the shop right now and this picture below is what you should see under boost (pressure at 0.75bar)
Brian I totally thought the same thing. I was thinking that I was running rich because my RPM switch wasn't working correctly and the boost enrichment was happening anyway. That's why I disconnected the hose from the side of the WUR and verified that it wasn't the cause.

Quote:
Originally Posted by T77911S View Post
no other engine mods other than turbo, IC and header? (ported, CR, pistons, cam?)
euro fuel head or aluminum?

what is your cruising CP?
how much is the CP dropping when you go on boost?
I think I am running stock P,C, and heads with the exception of 993TT exhaust valves. It has a 964 cam and a aluminum fuel head that was modified by CIS Flowtech.

I don't know what my CP is under boost. That is a great question but the only CIS gauges I have need to be uninstalled when the car is moving.

Thanks
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Old 08-08-2017, 02:35 PM
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you can check it with the engine off and in the garage.
you are driving blind until you know what the CP is under boost.
here is how to do it.

regulate your air compressor down to about 10psi.
put air pressure into WUR side port and check pressure drop.

post your system pressure, WCP and boost CP.

like brian said, this is almost not possible and when I have an issue like this I back up and try to look at what "I" am doing wrong.

I take it larry modified the head to run WITHOUT a frequency valve or is it still connected?

is your BOV vented to atmosphere or recirculate?
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Old 08-09-2017, 03:59 AM
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Quote:
Originally Posted by T77911S View Post
regulate your air compressor down to about 10psi.
put air pressure into WUR side port and check pressure drop.
Or use a Mitivac vacuum/pressure pump to pressurize the wur. This might be a safer option.
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Old 08-09-2017, 06:06 AM
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Quote:
Originally Posted by 911nut View Post
Or use a Mitivac vacuum/pressure pump to pressurize the wur. This might be a safer option.
For anyone considering using the Mityvac. Make sure you buy one that does vacuum and pressure. I bought one for this very purpose and found that it only does vacuum and won't pressurize anything. So I ended up using a bike pump with a gauge on it.
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Old 08-09-2017, 06:50 AM
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i looked all over for one that did pressure. could not find one.

regulate the air down,.
I use an air blower on the hose. I start blowing the air before applying it to the WUR because the initial blast could be too hi.

it does not take much. you could even turn off the comp at about 30psi then reg it down to 5psi.


boost CP NEEDS to be checked.
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Old 08-09-2017, 09:22 AM
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Quote:
Originally Posted by T77911S View Post
you can check it with the engine off and in the garage.
you are driving blind until you know what the CP is under boost.
here is how to do it.

regulate your air compressor down to about 10psi.
put air pressure into WUR side port and check pressure drop.

post your system pressure, WCP and boost CP.

like brian said, this is almost not possible and when I have an issue like this I back up and try to look at what "I" am doing wrong.

I take it larry modified the head to run WITHOUT a frequency valve or is it still connected?

is your BOV vented to atmosphere or recirculate?

The fuel head is set up without the frequency valve and the bypass valve is set up in a recirculating circuit.

I changed out the wastegate springs to 1.1 bar (16 psi) tonight with the boost connection of the WUR still open to atmosphere. The result was a little better with the car stumbling from being over rich about 1000 rpm later when in 2nd gear. It pulls 3rd gear fine.

I'm going to lean out my warm control pressure a bit to see if that will help a little more. Then I'll hook up the boost enrichment port to my rpm switch and head back to the dyno.

I'll measure where the boost control pressure ends up after adjusting it on the dyno.

Thanks
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Old 08-09-2017, 07:26 PM
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is this dyno costing you money?

back to the boost enrichment and venting the WUR.
when you vent the WUR, is it truly vented or are you just disconnecting the switch?


I am with brian, I don't know how in the world you are getting good cruise and idle AFR's and yet so rich boost AFR if you are truly venting the WUR and just adjusting the WCP.

you want to take a trip to myrtle beach?
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Old 08-10-2017, 04:11 AM
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The 930 I am tuning in the pictures above is from Atlanta. Only 200 miles away so we get a bit of business from you all. I think we can solve this over the net if you can post some pictures that identify the areas we are talking about such as venting to atm and such. Could be a communication gap. The problem I always fight is not enough fuel, the problem you are having I WISH I had, ha!
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Old 08-10-2017, 04:46 AM
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I'm in Atlanta if you want another set of eyeballs. I'm not as smart or experienced as some of these other guys but I might could provide another point of view. My hangar is at Gwinnett County Airport.
Old 08-10-2017, 09:38 AM
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