Push through or Pull through MAFS on SR20det [Archive] - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

Jsquared

03-11-2003, 12:29 AM

the only boosted car i've driven (dad's modded Porsche 944 turbo) has it's stock BPV vented to atmosphere (long story... essentially the BPV return flange on the aftermarket intake for the new MAFS (draw-through, stock location, better MAFS) didn't line up so the BPV is venting to atmosphere with the flange blocked off) and it never stumbled between shifts... did make a nice lil' fireball out the exhaust though :D (test pipe + B&B Tri-Flo + vent to atmosphere = cool flames)

RedSuns

03-11-2003, 05:32 AM

you can still use the "draw through" set-up with a open atomsphere BOV...you just have to make sure that the BOV is shut at idle...if not you will run into problems.....

Foxcolt

03-11-2003, 08:15 AM

Originally posted by cIvIcKILLA240style
you can still use the "draw through" set-up with a open atomsphere BOV...you just have to make sure that the BOV is shut at idle...if not you will run into problems.....

Exactly...that whole stumbling at idle thing is a myth.

piston type BOV's usually have that problem. Like Jonathan said as long as your bov is shut at idle you won't have any problems.

Jeff240sx

03-11-2003, 10:28 AM

Originally posted by Foxcolt
Exactly...that whole stumbling at idle thing is a myth.

I was talking about the instant richness right after the shift...
-Jeff

Foxcolt

03-11-2003, 10:31 AM

Originally posted by Jeff240sx
I was talking about the instant richness right after the shift...
-Jeff

ok, does that acutally cause any problems that would warrent the effort and money to convert to a blow through system? or even a speed density system?

d240t2

03-11-2003, 11:12 AM

It isn't a myth. It happens. On some cars, it is worse than others. If it doesn't do it on your car, then you got lucky. There is a sound basis for why it happens, and there are also a ton of cars that had problems with it...so it isn't like it was just made up. My old black hatch used to have problems with it...that is why I went blow-through on my white hatch (I wanted to experiment...and was one of the first to do the blow through and share my findings).

Dennis

Foxcolt

03-11-2003, 11:54 AM

Well, here was my situation.

I had an older blitz piston type. stayed open at idle. Just a tad. Had to heel toe everytime I came to a stop to keep it from stalling. Bought an hks diaphram type. Haven't had a problem since.

Civickilla has a piston type. Turboxs. keeps it closed at idle. No problems.

the reason why your car stalls is because of the fact that idle air is being let out of your bov. of it not shutting all the way when you come to a stop or let off the gas from highrpm's. wouldn't it be obvious that the solution would be to stop the leak?

Not trying to insult you. I just wanted to show you that I've had experience in both situations and to let everyone know that it's easily corrected.

Almost all bov's on the market are piston type, most of em you can't adjust to the point where it'll stay closed at idle it was impossible for me to do it with my older blitz. I'm pretty sure if we took a poll of who had what bov compared to who sufferes from the idle problem we'd start to see a corelation.

RedSuns

03-11-2003, 12:06 PM

Jeff240sx

03-11-2003, 08:17 PM

Originally posted by Foxcolt
Well, here was my situation.

I had an older blitz piston type. stayed open at idle. Just a tad. Had to heel toe everytime I came to a stop to keep it from stalling. Bought an hks diaphram type. Haven't had a problem since.

Civickilla has a piston type. Turboxs. keeps it closed at idle. No problems.

the reason why your car stalls is because of the fact that idle air is being let out of your bov. of it not shutting all the way when you come to a stop or let off the gas from highrpm's. wouldn't it be obvious that the solution would be to stop the leak?

Not trying to insult you. I just wanted to show you that I've had experience in both situations and to let everyone know that it's easily corrected.

Almost all bov's on the market are piston type, most of em you can't adjust to the point where it'll stay closed at idle it was impossible for me to do it with my older blitz. I'm pretty sure if we took a poll of who had what bov compared to who sufferes from the idle problem we'd start to see a corelation.

Jed. It's not an idle problem. The stumbling problem both Dennis and I are talking about is right after a shift.
When the MAF is used as a draw-through maf, it meters the air before going into the turbo. Then the air travels through the piping and intercooler, it has already been metered. Once an open-atmosphere bov lets the metered air out of the piping, bad **** happens. The ecu expects a certain ammount of air to come through once the throttle opens, but it's not there. The BOV let it out. So you have a ton of gas ready for that metered air, and no air. You run very rich, and it stumbles.
This is the sound basis for why the stumbling happens after the shift.
-Jeff

Jsquared

03-11-2003, 09:49 PM

the vast majority of the time you won't be so rich too stumble, most people will just have a little "afterburner" thing going on when the extra gas hits a hot spot on the exhaust...

AceInHole

03-11-2003, 09:57 PM

Originally posted by Jsquared
the vast majority of the time you won't be so rich too stumble, most people will just have a little "afterburner" thing going on when the extra gas hits a hot spot on the exhaust...

to say "the vast majority of the time" really isn't correct, because anyone running a decent amount of boost (maybe over 7psi) WILL have a problem, because then we're talking about a LOT of metered air not actually seeing the combustion chamber, resulting in a LOT of fuel dumping into the chamber with no air to burn it.

transient

03-11-2003, 10:18 PM

How exactly does a recirculating BOV work? If it recirculates, how does it lessen the pressure and keep the turbo from stalling?

Jsquared

03-11-2003, 10:35 PM

to say "the vast majority of the time" really isn't correct, because anyone running a decent amount of boost (maybe over 7psi) WILL have a problem, because then we're talking about a LOT of metered air not actually seeing the combustion chamber, resulting in a LOT of fuel dumping into the chamber with no air to burn it.
do you consider 15psi "decent"? because we've been running that for 3 years and never had a stumbling problem.

Jeff240sx

03-11-2003, 10:37 PM

Originally posted by Jsquared
do you consider 15psi "decent"? because we've been running that for 3 years and never had a stumbling problem.

There are so many variables. What turbo do you run? Piping size? SMIC or FMIC? What BOV? Are you using a standalone EMS? Apexi AFC?
-Jeff

Jsquared

03-11-2003, 10:43 PM

Porsche 944 turbo: stock turbo (K26), stock everything except: AutoThority MAFS (stock location, electronic instead of flap-type), K&N cone filter, AutoThority Stage III chipset, Magnecor spark plug wires, B&B Tri-Flo test pipe and cat-back, stock BPV/BOV vented to atmosphere, LindseyRacing "boost enhancer" (progressive valve on wastegate actuator line, basically keeps wastegate totally shut until 4,000rpm instead of it constantly creeping open like stock... solely for quicker spool-up and better low-end torque, overall boost level is controlled by ECU). the only variables that should matter in this case are the test-pipe+catback, increased boost from chipset, and BPV vented to atmosphere. oh yeah, piping size is about 3", and stock small FMIC.

AceInHole

03-11-2003, 11:25 PM

Originally posted by Jsquared
Porsche 944 turbo: stock turbo (K26), stock everything except: AutoThority MAFS (stock location, electronic instead of flap-type), K&N cone filter, AutoThority Stage III chipset, Magnecor spark plug wires, B&B Tri-Flo test pipe and cat-back, stock BPV/BOV vented to atmosphere, LindseyRacing "boost enhancer" (progressive valve on wastegate actuator line, basically keeps wastegate totally shut until 4,000rpm instead of it constantly creeping open like stock... solely for quicker spool-up and better low-end torque, overall boost level is controlled by ECU). the only variables that should matter in this case are the test-pipe+catback, increased boost from chipset, and BPV vented to atmosphere. oh yeah, piping size is about 3", and stock small FMIC.

so, you're trying to compare a Porche to a Silvia or 240sx?

Jsquared

03-12-2003, 12:07 AM

whether Porsche, Silvia, RX-7, whatever, the same thing happens. BOV vents to atmosphere, computer sees a lot more air than is actually getting into the pistons, injectors spray too much fuel, get split-second lean condition, sometimes ignites in exhaust stream. same thing with any turbo car with any vented-to-atmosphere BOV with any cat-less exhaust system. RX-7s, Supras, Eclipses, whatever. same mechanical events occur with the same components.

Jeff240sx

03-12-2003, 12:15 AM

I'm pretty sure that if you don't have problems, it's because the car came stock with the bov, and the fuel delivery is probably controlled by the ecu during the shift.
In the sr20det, I really don't think that they come with a bov, and have stumbling problems. The KA-T.. well.. you know. Not even supposed to be turbo'd.
Also.. cars like the z31 turbo had pop-off valves, which are more like safety mechanisms than turbo saving bovs are. Pop-off valves suck.
-Jeff

Jsquared

03-12-2003, 12:41 AM

NO. the 944 turbo comes with a bypass valve stock, as does nearly every single turbo car made by anyone. the Silvia also comes stock with a bypass valve. a blowoff valve is just a bypass valve vented to atmosphere instead of rerouted back into the intake between the MAFS and turbo inlet. running a vented-to-atmosphere BOV on an S13 SR is no different than running the OEM BPV and venting it to atmosphere on a 944turbo (aka "951" in Porsche-speak) which is no different than a BOV venting to atmosphere on an Eclipse turbo which is no different than a BOV venting to atmosphere on an RX-7... get the drift? (no pun intended)

pop-off valves are completely different. a BPV/BOV prevents compressor surge to protect the turbo when the throttle closes under boost (as you know already)... a pop-off valve merely releases pressure when a car overboosts, mainly as a back-up for the wastegate in case the wastegate for some reason malfunctions and can't keep the boost down (which really doesn't make any sense because a wastegate is much more effective than a pop-off valve at regulating boost pressure). the only modern vehicle you should see a pop-off valve on is a CART Champ Car, and the governing body uses these to prevent teams from "turning up the knob" on the wastegate control and getting more boost, and thus more power, than other teams (the pop-offs are distributed by the governing body and are all set to the same psi).

Jeff240sx

03-12-2003, 12:48 AM

So now that we agree on the pop-off valve, and the bypass valve, and the difference in a bypass valve and a blow off valve, you mention that you have a bpv.
Is that why you don't stumble?
-Jeff

Foxcolt

03-12-2003, 08:19 AM

Originally posted by Jeff240sx
Jed. It's not an idle problem. The stumbling problem both Dennis and I are talking about is right after a shift.
When the MAF is used as a draw-through maf, it meters the air before going into the turbo. Then the air travels through the piping and intercooler, it has already been metered. Once an open-atmosphere bov lets the metered air out of the piping, bad **** happens. The ecu expects a certain ammount of air to come through once the throttle opens, but it's not there. The BOV let it out. So you have a ton of gas ready for that metered air, and no air. You run very rich, and it stumbles.
This is the sound basis for why the stumbling happens after the shift.
-Jeff

Your completly right, but as I asked in a post before. Is this a big enough problem to warrent the effort and money to go with a blow through setup?

Also what exactly is this "stumble" you guys are experiencing? When I shift at high rpm the worst that happens is my car shoots a flame. Never really had anything that I could equate to a stumble.

I've always heard people complain about stalling at idle due to mafs based cars w/ atmoshereic bov's but this is the first time I've heard of people having problems when they shift.

d240t2

03-12-2003, 08:26 AM

Just because you have had 2 cars that didn't have problems, it doesn't mean you have solved the issue and that everyone else is making stuff up. 2 cars = 2 data points, which is not enough to pull a firm conclusion.

My car now, I run a blow through MAFS, so I don't have problems. My last car did have problems. Some do, some don't. It depends on a lot of factors.

If you want to tell people that they won't have problems with a certain setup, go ahead...but won't you feel bad when somebody does have problems because you made a conclusion too soon?

Dennis

Foxcolt

03-12-2003, 08:28 AM

Originally posted by Jsquared
NO. the 944 turbo comes with a bypass valve stock, as does nearly every single turbo car made by anyone. the Silvia also comes stock with a bypass valve. a blowoff valve is just a bypass valve vented to atmosphere instead of rerouted back into the intake between the MAFS and turbo inlet. running a vented-to-atmosphere BOV on an S13 SR is no different than running the OEM BPV and venting it to atmosphere on a 944turbo (aka "951" in Porsche-speak) which is no different than a BOV venting to atmosphere on an Eclipse turbo which is no different than a BOV venting to atmosphere on an RX-7... get the drift? (no pun intended)

pop-off valves are completely different. a BPV/BOV prevents compressor surge to protect the turbo when the throttle closes under boost (as you know already)... a pop-off valve merely releases pressure when a car overboosts, mainly as a back-up for the wastegate in case the wastegate for some reason malfunctions and can't keep the boost down (which really doesn't make any sense because a wastegate is much more effective than a pop-off valve at regulating boost pressure). the only modern vehicle you should see a pop-off valve on is a CART Champ Car, and the governing body uses these to prevent teams from "turning up the knob" on the wastegate control and getting more boost, and thus more power, than other teams (the pop-offs are distributed by the governing body and are all set to the same psi).

I think what Jeff was trying to say is the reason you don't stumble or have any problems is due to the ECU.

All stock turbo euro cars be they crapwagons (Sorry! I hate em, no offense intended) or porshes are boost controlled from the ECU. Our cars whether they be ka-t or SR's the ECU has no idea there's a turbo in there. It only reads mafs voltage. In that sense i think it's safe to assume that your porshe ECU has sometype of algorithm to compansate for lost air during shifts.

Foxcolt

03-12-2003, 08:35 AM

Originally posted by d240t2
Just because you have had 2 cars that didn't have problems, it doesn't mean you have solved the issue and that everyone else is making stuff up. 2 cars = 2 data points, which is not enough to pull a firm conclusion.

My car now, I run a blow through MAFS, so I don't have problems. My last car did have problems. Some do, some don't. It depends on a lot of factors.

If you want to tell people that they won't have problems with a certain setup, go ahead...but won't you feel bad when somebody does have problems because you made a conclusion too soon?

Dennis

hmmm... I see your point.

How would you reccomend I test this theory further? I firmly believe that the conclusions I've drawn are correct, and I'd like to have it stated as fact as opposed to theory but unfortunatly at this point I only have frequent access to one turbo car (mine) so I can't test my BOV on other people's cars. Maybe during the summer when the turbo 240's around my area get done I can find the one's that experience this mafs problem and see if putting my BOV on their car corrects it.

litlespic

03-12-2003, 09:01 AM

I think a big reason that the 240's see a stumble with a BOV more htan the porshe is that the porshe most likely has a 1.1 afpr.. where as ace in the hole recomends a 10.1 which means A HELL OF A LOT MORE FUEL when the car is goin into a rich state...

And now I see why you'll hated me for comparing the 2.3 lima to a ka.. sorry guys

Jeff240sx

03-12-2003, 11:15 AM

Originally posted by Foxcolt
Is this a big enough problem to warrent the effort and money to go with a blow through setup?

Also what exactly is this "stumble" you guys are experiencing?

I've always heard people complain about stalling at idle due to mafs based cars w/ atmoshereic bov's but this is the first time I've heard of people having problems when they shift.

1) For you, no. For other people.. it depends on how bad it is. It was bad enough for Dennis to go with it. I'll let you know how mine is in a few weeks. Also, if you have the problem even the slightest bit, and are a drag racer (need to extract every ounce of speed), you'll want to switch to a blow-through setup.

2) From what I've heard about, its a flooding out problem, where the car makes no power just after the shift and hesitates. The fuel needs to be burnt off before power can be made.

3) Yea. The idle problem is the bov leaking at idle as you said before.

Oh yea. The s-afc has a setting for atmospheric bovs, and helps some.
-Jeff

Foxcolt

03-12-2003, 11:35 AM

ah I see.

that clears things up.

So in conclusion.

To stop stalling on idle, get a BOV that doesn't leak.

To stop hesitation on shifts get a recirculating BOV. Or convert to a blow through setup.

done

andrave

03-12-2003, 01:12 PM

well, not done.. that sounds like lotsa work, and I planned on running an S-AFc or an F con.. I would be able to get around those problems with one of those computers, correct?
if the S-AFC solves them with its atospheric BOV setting than that just made up my mind when I finally swap.

Jeff240sx

03-12-2003, 01:26 PM

Originally posted by andrave
well, not done.. that sounds like lotsa work, and I planned on running an S-AFc or an F con.. I would be able to get around those problems with one of those computers, correct?
if the S-AFC solves them with its atospheric BOV setting than that just made up my mind when I finally swap.

The s-afc does have a setting for atmospheric bovs. I don't know about the f-con though.
-Jeff

Foxcolt

03-12-2003, 01:38 PM

aish, this is what I didn't want to happen. Now people are gonna think that this stumbling issue is a huge problem. When in reality it's pretty minor.

Jsquared

03-12-2003, 10:10 PM

So now that we agree on the pop-off valve, and the bypass valve, and the difference in a bypass valve and a blow off valve, you mention that you have a bpv.
Is that why you don't stumble?
a "BPV" is the exact same thing as a "BOV," the BPV is routed back into the intake stream and the BOV is vented to atmosphere. it's the exact same part, mechanically. i turned the stock BPV into a BOV just by disconnecting the hose that would send the vented air back into the intake.

Just because you have had 2 cars that didn't have problems, it doesn't mean you have solved the issue and that everyone else is making stuff up. 2 cars = 2 data points, which is not enough to pull a firm conclusion.
1) Porsche 944 turbo (dad)
2) 2G Eclipse GS-T (friend)
3) SR S14 (friend)
4) SR S13 (friend)
5) FD3S (friend)
6) FC3S (friend)
7) S14 KA-T (friend)
8) MkII MR2 turbo (friend)
9) Porsche 930 turbo (PCA member, acquaintance)

as well as numerous people on numerous forums...

All stock turbo euro cars be they crapwagons (Sorry! I hate em, no offense intended) or porshes are boost controlled from the ECU. Our cars whether they be ka-t or SR's the ECU has no idea there's a turbo in there. It only reads mafs voltage. In that sense i think it's safe to assume that your porshe ECU has sometype of algorithm to compansate for lost air during shifts.
no. read what i've said. the stock setup has the BOV/BPV routed back into the intake behind the MAFS and before the turbo inlet, thus the ECU doesn't expect anything other than what it sees at the MAFS. and the ECU (fuel, timing) is a separate computer from the TCU (and don't forget, this is 1986 electronics we're talking about). there is no "algorithm to detect lost air."

I think a big reason that the 240's see a stumble with a BOV more htan the porshe is that the porshe most likely has a 1.1 afpr.. where as ace in the hole recomends a 10.1 which means A HELL OF A LOT MORE FUEL when the car is goin into a rich state...

there really isn't that much of a difference in fuel pressure and injector flow...

Jeff240sx

03-13-2003, 01:36 AM

Originally posted by Jsquared
no. read what i've said. the stock setup has the BOV/BPV routed back into the intake behind the MAFS and before the turbo inlet, thus the ECU doesn't expect anything other than what it sees at the MAFS. and the ECU (fuel, timing) is a separate computer from the TCU (and don't forget, this is 1986 electronics we're talking about). there is no "algorithm to detect lost air."

there really isn't that much of a difference in fuel pressure and injector flow...

Yea. You're right. Myself, Ace, Dennis, we're all in collusion to get people to stop and think about lost air during shifts. Apexi and other huge companies are just making it up as a marketing gimmick to sell to a niche market. Companies also make blow-through maf setups... and they, of course, are lying.
What the hell do you want to hear? There are numerous people here and on FA that have had stumbling issues during the shift. The larger the turbo, the more air you have in piping and the intercooler, the faster the bov lets the air out.. they all play a role in the severity of the stumble.

Also, there is a huge difference in fuel pressure and injector flow. Read up on the formulas.
Take the square root of the new pressure divided by old pressure. Then, multiply that number by the injector flow rate.
43.5psi to 50psi change gives a product of 1.0721. On 50# (525cc) injectors, a 6.5psi change gives 53.6# of flow. Thats per injector, giving a total of 15.4 additional pounds of fuel off 4 injectors. This doesn't seem so large (actually, it does), but when you are saying that a 1:1 fpr and 10:1 fpr are compatible.. you're severly incorrect. At 10psi, thats an additional 90 psi of pressure. That gives a product 1.752, changing 50# injectors to 87.6# injectors.
Please.. stop spreading stupidity.
-Jeff