At what outside air temp the highest boost is obtained?

I have a friend, with just the tune, exhaust and performance filter (stock turbos) who has a feeling that his car boosts a bit lower than the usual. He is wondering if boost leak is at play?

Today it was 10 degrees Celsius, (18F) and we went for a test drive...

His boost was 1.2 bar peak which when over 120mph dropped to 1.0 bar and then to 0.9 bar.

I told him I think this is ok. He reckons it should hold 1.0 bar all the way to top speed. I said unlikely, as even my car doesn’t hold 1.0 bar all the way to top speed on larger turbos. I think his boost is rather good for small turbos?

At what ambient temperature boost is at its highest. Is it higher or lower in the summer at temps of 30 degrees Celsius (54F)?

 

Just noticed error on temp conversion: 10C = 50F and 30C = 86F.

 

What is the model and MY?

I believe ambient temp (conditions) have the greatest effect on intercooler performance. So as ambient temperatures increase intercooler performance (cooling of the charged air) decreases...which affects performance.

If by peak he means spike, then it is really not the boost that is being created. For a stock (no ECU mod) turbo, the boost should be 0.7 +/- 0.1, for a stock X50 (or S), boost is 0.9 +/- 0.1. For a tuned car the boost depends on the tune. If you have non X50/S then you really don't get much and (probably shouldn't) boost over 1.0. For X50/S 1.1-1.2 would be OK.

 

Since he has a tune, I would think it would hold boost. DVs?
Boost can be/should be slightly higher,at cooler 50 then 86 warmer.

 

His car is 2003. Standard 996 turbo. We tested this in 6th gear from 3K rpm. He floored it and boost showed 1.2 bar. As speed was climbing (still in 6th) it went downwards 1.1, 1.0 and then settled on 0.9. This is now 120mph +. He is saying he used to hold 1.0 bar. The question is I know cooler air = higher pressure. But I wonder if really cold air means less pressure? He is basically saying he went from 1.3/1bar to 1.2/0.9bar. 0.1 bar loss. Could this be leak? Or would the drop be more dramatic?

 

with stock turbos and especially stock wastegates your boost goes down at high rpm....loss around 0,1 bar...with ore without a flash

if you are at blind calibration..depends on your tune... your boost is higher at colder air...blind maf signal at higher load,,,but only with 1 bar wastegates

if you are at stock calibration (no tune or low tune) your boost doesnt go higher at cold air....because you dme can calculate load along the maf voltage

this are two different possible situations

 

I found this quote which is interesting:

"Aperently 996TT takes as much boost as is necessary in the conditions to give you 420 bhp. Hence one day booting it in 3rd gear it takes .7 bar and the next day on the same road it will be .6.bar. I say "apparently" because there is a lot of hearsay on this topic".

Is this possible?

Would explain though: colder air less pressure for same bhp!

I haven't idea do I laugh or take this seriously?

 

All I can tell you is that...
When my car was bone stock it always peaked at .9 in the winter months and .8 in the hot summer months.
I had a GIAC tune when it had stock turbos and it held 1.2 to Rev Limit..But I did the wastegate adjustment,tightened them as per GIAC.
When I had my prior EPL tune and now with my Protomotive tune my boost stays at the same boost until redline but will sometimes also will spike in the cold winter months an extra.1bar over the summer month spikes..I am talking etremes here,,35 degrees verses 85+ degrees.Plus I have 1 bar wastegates ,so that why I get that boost creep.
Also I would say,,do you believe everything you read? LOL..

 

Essentially yes, it is possible, in fact it is fact.

When I looked into this boost issue what I found is the DME attempts to satisfy the torque demand of the driver by what the DME gets from the e-Gas.

We are talking a healthy engine now with some reasonable quality of pump gas in the tank.

Under some conditions -- cool dry air for one -- and at sea level -- the "max" boost may only reach 0.6 bar.

Under different conditions the boost may get to 0.7 bar.

And it can under yet different conditions -- based on my direct observation at higher elevations -- the boost can obtain -- steady not just some brief reading -- 0.8 bar even 0.9 bar.

These high boost levels I observed while driving my 03 Turbo on I-80 in the wide open and high elevations (8K+ feet above sea level) of Wyoming.

Also, the boost reaches its max, whatever that is, then tapers off/downward as rpms rise. It does not drop off to nothing but can drop to IIRC 0.4 bar even 0.3 bar at just before redline if one is able to keep his right foot planted.

The trouble is in a lower gear one can only see this for a brief time before he runs out of rpms.

In a higher gear while one has longer to observe the boost behavior the blasted car is going so fast one had better be paying attention to the road and where the car is pointed.

 

When I was talking about my boost levels in the above post,it was in 3rd/4th gear pulls.
Also I know it is different how the electronics control boost when you start talking elevation changes in which I have heard different stituations that I dont fully understand..

 

Well I think cold air = more boost. What happened to me albeit only few times (in my 5 years of ownership) is that on some really crisp cold winter days (when I really floored it) high speed - max load, I spiked 1.3 bar,. When this happened car's limiter kicked in and the car went into limp mode. I switched the engine off and turned it back on whilst driving (CPU reset) and all was back to normal.

This also happened in my 944 Turbo S. I think this is because the tune is set right on the limit not taking into consideration ambient temps. I guess backing off 0.1 bar would eliminate this but in relation to only few cold days in 5 years - no way - I am giving up 5bhp for that. Ha ha ha!

 

Hello Tertminator
1 bar (100 kPa) boost at say 10 C is the same as 1 Bar at 30 C. Less temperature does not mean less boost. Boost stays the same at high or low temperature. Die difference is the MASS (weight) of the air. Meaning.... 1 cubic meter of air at 30 degrees C will weigh much less that 1 cubic meter of at at 10 degrees C. (Sorry for using metric Units only for this example folks). What does this mean. It means that the cooler the air the more mass of air will you get into your car and the more HP you will get. Hence the need for an inter-cooler. I know the intercooler has other uses as well but lets keep is simple for this argument only. By using an inter-cooler you bring the inlet air temperature down and by doing that and keeping the boost pressure the same you increase the mass flow. I kept it brief and hope it helped. My 2'c

 

This is great info, but it does not deal with the question in hand. Boost pressures in our cars read diferently given diferent ambient temp. Some have more boost in hot weather some in cold. This means MAF and CPU, play a crucial role here adjusting the boost pressure. We know boost is rolled off in top gears/high revs unless you play with wastegates. The question is why the mixed reports? It really seems that cars boost what they needs in order to achieve given BHP set by the Map tuned or not.

 

How does he go about checking DV's?

 

Can he just tighten his wastegate? Will this affect tune? As I said he has unknown tune, exhaust, 1.2 peak, 0.9-1.0 bar sustained. Or does this sound ok.

My car peaks at 1.3 and holds 1.1 (X50) so it seems comparable to me? Or should I tighten wastegates as well?