Boost limit in 1,2,3

[Nick_Walczak]

I've heard mutterings that there is a boost limiter in 1st, 2nd and 3rd gear. I wondered why I had to work at making the wheels spin http://300zx.co.uk/200sx/ubb/smile.gif I also wondered why you don't get the same kick in the back that you do in other turbo cars.

This must be easy to 'fix'. Just speculating here, but expect there is a lead going to the EMS from the gearbox, this should be easy to 'hotwire' to make it always think it's in 5th. Anyone done this?

[Bean]

I though the boost limit was only in first gear, for obvious reasons.

You shouldn't need limits in second gear and beyond?

Does anybody know this for a fact?

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Red 1995/S14
K&N, Private Plate

Any views expressed in this message are based solely on personal opinion.

[Nick_Walczak]

See reply to my actuator thread in 'general'. Someone with a boost guage could confirm this. Under hard acceleration, it should be possible to reach max boost in second. If you can't then this must be true.

[Tom_S14]

Hi,
I first read some of this boost limit in lower gears stuff on an australian site. They were talking about a limit just in 1st.
I've checked that my car will definitely do 0.5 bar in all gears (don't have time to look down at the guage between 6k and 7k in 1st to see if it peaks at 0.7).
I've read the whole ECU chapter in the service manual. It talks about a different boost map used for 4th and 5th. As I understand it, this is just how much boost the ECU dials in for changes in throttle position. It's quite noticeable on the motorway in 4th/5th that a small amount of right foot sends the boost guage round a lot more readily than in lower gears.
The lack of turbo kick in the S14 is partly down to the way it's designed to come in at very low revs I think.
I agree that once in gear and rolling, trying to spin the rear wheels in a straight line (in the dry obviously) can't really be done. From a standstill, if the road is dry and warm, you can normally bring the clutch in pretty quick at around 4k revs - this seems to be the cut off point for mine anyway. 5k revs and it's all over the place.
Anything under 3K revs and it feels like there's a really bad over-fuelling issue, and the car doesn't take off half as well.

Tom

[Tom_S14]

... just thought. Some S14s (like mine) don't have boost (MAP) sensors fitted at all. I'm wondering if this makes a difference ? According to the service manual, the standard ECU (expecting a MAP sensor signal) will limit the duty cycle of the boost control solenoid to 20% if the sensor is missing. This seems to me like it would severly limit boost under all conditions. So whether cars without boost sensors fitted have a modified ECU program I don't know.

Only gear I haven't tried testing full boost in is reverse http://300zx.co.uk/200sx/ubb/smile.gif Keep on meaning to practise those J turn things - but never have the balls http://300zx.co.uk/200sx/ubb/smile.gif

Tom

[Nick_Walczak]

You're probably right. I find it wierd that the boost only reaches 0.5bar until 6k rpm. I thought that the turbo would spool up to max boost ASAP.

Does this mean that if you up the max boost to 15psi, you only get about 12-13 until 6k rpm?

[Tom_S14]

Hmm, this I don't know. I'd like to know how much of it is the ECU limiting boost under 6k, and how much of it is just down to the mechanics of the boost actuator. Assuming it's controlled by the ECU, I suppose it all depends how ND has specified his actuator.
Since he quotes the S14 as having a std. boost of 10psi (0.7bar) then I would guess he's staying consistent with that, and like you say, a 15psi actuator would probably only give 12-13psi at lower revs.
Fortunately the S14 has a knock sensor (re: fuel issue), although from reading about it, the software is set up with poor grade fuel in mind, and sets about a list of draconian measures, limitting boost, ignition advance etc.. assuming you've filled up with the wrong stuff and want to get home in one piece. Still - it will probabably save the engine if you wind up the boost too much by mistake http://300zx.co.uk/200sx/ubb/smile.gif

Tom

[Nick_Walczak]

Tom, You really don't want to rely on a knock sensor, they are only to avert catastrophy and you could still get minor damage to the engine if it pinks. Having said that, I'm sure Norris would not make such a fundamentally duff recommendation http://300zx.co.uk/200sx/ubb/eek.gif

[Tom_S14]

... hmm, you're probably right. Oh for a friendly air field with a copious supply of av-gas http://300zx.co.uk/200sx/ubb/biggrin.gif Dreaming...dreaming...dreaming...

tom

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1996 Blue/Green S14 (manual)
K&N cone filter
2.5" Tube-torque Cat Back Exhaust.
TechTom MDM-100
Race-Technology AP-22
http://tom.marshall.tripod.com

[Paul_S13]

I've sat in 2 S14's and the reason you don't get a kick in the back is because you have a relatively small T28 turbo being spun by a 2 litre engine, it also depends on the ecu map. I recently changed from a Norris Designs chip to a SE stage 2 and before the boost came in quite smoothly, now its an on/off switch, now I hit 1.1 bar at 3800rpm.

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Black S13
Tuned with Fords parts bin, coz its cheap! http://www.themelee.com/smilies/s/otn/violent/spectator5.gif