Less is more???

[Black33]

hey all,

When accelerating I find that my car seems to go faster when only using, say 75% throttle, but this only really seems to happen on cold nights.

During the day when it isn't too cold, I can use 100% throttle and car feels really fast, but on a cold night, use 100% and it seems to get a flat spot over 4000rpm.

I think it may be due to a boost spike (bleed valve) which I have noticed on cold nights, but I thought a spike wouldn't really rob power cos it only happens on a cold night when there shouldn't be any knocking (cold air), will the ecu retard timing based on boost alone or on the knock sensor?

I don't want to reduce my boost because it goes fine during a warmer day, and pls don't just say "go buy an EBC"!

Has anyone experienced this? Can anyone shed some light?

[TomR33]

I think you are right, it is the boost spike. The stock ECU will retard timing on excessive air flow = boost spike. Have a look at some data logging I have done for stock ECUs. The modified Skyline had more air flow and more boost.

http://home.iprimus.com.au/tkolo/datalog.htm

[Black33]

Thinking about it a bit more, not sure if the cold air causes a boost spike which in turn means more air volume flow - it's prob more like the denser cold air meaning there is more air volume and that causes the boost spike.

Sort of the same thing, but you get what I mean.

so what can I do to fix?

[Guest INASNT]

cheap bleed valves will always run different in different weather conditions.

[Lepperfish]

Ebc !!

[Black33]

Ebc !!

shakes fist!

I think I might just turn down the boost a few clicks before driving away on a cold night and see how it goes.

[mickr33]

black33. I too have this very problem. After changing plugs, raping tape around the coils, and finally dyno tuning was told with the standard ecu it is only good for around 9.8psi before going into a default mode. After having it set at 11psi, I then finally set it to 9 and for sure didnt notice the pulling back at around 4000 rpm.

[Sydneykid]

Hi guys, everyone knows that R33 GTST's have over boost protection, right?

[Black33]

Hi guys, everyone knows that R33 GTST's have over boost protection, right?

but wouldn't this cut the car out completely, not just give you a flat spot like I feel???

[TomR33]

I don't think it’s the "over boost protection". There is one but like Black33 says it just cuts out the engine completely.

I think it’s the protective timing map programming. If you have a look at a stock R33 timing map the last column has a dramatic drop in timing from 23 to 8. So if with a higher boost you get close to that area you will definitely feel a flat spot.

Tom

[Black33]

sydneykid, can I bleed some air from the factory boost gauge line (or disconnect it) so that it thinks the boost is lower than it actually is, and it won't pull out so much timing?

[Sydneykid]

but wouldn't this cut the car out completely, not just give you a flat spot like I feel???

Nope it retards the igntion timing and whacks in plenty of extra fuel.

can I bleed some air from the factory boost gauge line (or disconnect it) so that it thinks the boost is lower than it actually is, and it won't pull out so much timing

"Over boost protection" is what it is know as, but it is really "excessive airflow protection". The ecu makes this decision based on what the AFM is telling it. So boost really has nothing to do with it.

The fix is an SAFC, voltage clamp, Boost Cut Defeater or better still replace the std ecu with a Power FC.

There is one but like Black33 says it just cuts out the engine completely.

Yep, full cut is the next step in engine protection programming in the ECU, but you get "retard" and "rich" first.

Hope that clarifies

[Black33]

sydneykid thanks for the info, helpful as always, how does a voltage clamp work??

[Sydneykid]

Hi Black33, I wrote this for someone else, but I think it answers your question....

The only way to solve voltage max out properly is to fit an AFM with resolution capacity at higher air flows. That means a Z32 or Q45 AFM. For example an Q45 AFM will only show 3 volts at the same airflow as a RB20 AFM will show 5.1 volts (the max). So as soon as you go over 5.1 volts of airflow the ecu doesn't know how much. Remember ~1.1 lb of airflow per minute = 10 bhp.

A RB220 AFM will go 5.1 volts at around 40 lbs, so 360 bhp. If you have an engine that makes say 400 bhp, the afm will show 5.1 volts from (40 lbs) 360 bhp to (44 lbs) 400 bhp. So the ecu won't know about the extra 4 lbs of airflow and it won't add fuel for those 4 lbs. This means lean A/F ratios = not so good.

A good tuner can get around this by using the rpm and throttle position maps to tell the ecu how much extra fuel to add. This is not perfectly accurate, so you make the A/F ratios a bit rich to be safe. Plus tuning the ignition timing is also pretty tricky, so it tends to be tuned a bit retarded to be safe as well. This looses some power and it takes a lot of programming (mapping time ) to get right. This cost usually exceeds the cost of buying an Z32 or Q45 afm.

All a voltage clamp does is stop the ecu seeing voltage over a set amount, usually 5 volts. This stops the ecu from using overboost (really excessive airflow) protection, ie; retard and rich. But it doesn't help the tuning, the effect is still the same as maxing out the afm airflow resolution as per above.

Hope that is of some hlep