Volvo 850 boost control

[AW111]

I will be co-driving in a 1996 850 t5R in April.
We dynoed it tonight, and the boost curve has a pronounced hump, with a peak of about 11 psi around 3,500, drops down to around 10, then rises again with revs to around 11 at 5,000 rpm.
(figures from memory - it's late).

Am I correct in assuming this boost dip is due to the relatively crude factory wastegate control?
What replacement options are there?

Car has 3" exhaust, otherwise stock engine. Driver is looking into getting an MTE tune (stage 2), but I assume the stock wastegate would still be an issue?

We are not looking for more peak power, but flattening out that dip would be nice.
It is also very rich up top, but that's another matter.

Located in Australia, so recommending workshops is not a lot of help

[anonymous-user]

AW111 said:

I will be co-driving in a 1996 850 t5R in April.
We dynoed it tonight, and the boost curve has a pronounced hump, with a peak of about 11 psi around 3,500, drops down to around 10, then rises again with revs to around 11 at 5,000 rpm.
(figures from memory - it's late).

Am I correct in assuming this boost dip is due to the relatively crude factory wastegate control?
What replacement options are there?

Car has 3" exhaust, otherwise stock engine. Driver is looking into getting an MTE tune (stage 2), but I assume the stock wastegate would still be an issue?

We are not looking for more peak power, but flattening out that dip would be nice.
It is also very rich up top, but that's another matter.

Located in Australia, so recommending workshops is not a lot of help

If you are looking at boost curves from a chassis rolls, don't bother!

Go and run the car on the road, start at idle drive (or the lowest rpm the car will hold in that gear) go WOT, use the vehicle brakes to hold the speed, then start logger and let car accelerate to rev limiter. (this will of course rapidly get illegal in the higher gears, so usual caveats about quiet empty road, and taking sufficient care apply!)

This will tell you what the steady state boost control is like.

Ideally, also repeat with the boost control solenoid bypassed (ie plenum pressure directly to wastegate capsule) which will tell you your min boost line.

Ideally, but more dangerously (in terms of blowing up engine!) then leave wastegate capsule unconnected and log th emax boost line (CARE: you will probably want to back out the throttle if boost gets excessive, std cars usually will have an ECU boost cut safety in the ecu anyway)

Once you know your average, min and max boost characteristics then you can determine how good (or bad) your boost control is, and if you require a hardware change to better control boost.

[Evoluzione]

Dips in the curve after the first boost spike were discussed here:

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

Near the bottom of the page.

[AW111]

Max_Torque said:

If you are looking at boost curves from a chassis rolls, don't bother!

Go and run the car on the road, start at idle drive (or the lowest rpm the car will hold in that gear) go WOT, use the vehicle brakes to hold the speed, then start logger and let car accelerate to rev limiter. (this will of course rapidly get illegal in the higher gears, so usual caveats about quiet empty road, and taking sufficient care apply!)

This will tell you what the steady state boost control is like.

Ideally, also repeat with the boost control solenoid bypassed (ie plenum pressure directly to wastegate capsule) which will tell you your min boost line.

Ideally, but more dangerously (in terms of blowing up engine!) then leave wastegate capsule unconnected and log th emax boost line (CARE: you will probably want to back out the throttle if boost gets excessive, std cars usually will have an ECU boost cut safety in the ecu anyway)

Once you know your average, min and max boost characteristics then you can determine how good (or bad) your boost control is, and if you require a hardware change to better control boost.

I am aware of the limitations of chassis dynos, and I know exactly how the dyno control system responds to sudden increases in input torque.
The dip I am referring to is not a sudden short drop but a pronounced sag after an initial peak. I have seen similar boost curves published elsewhere for this engine.

Road tuning of any kind is basically impossible here : loads of cameras, big fines, loss of license... it's just not possible.

So, given the tool I do have, what is the best loading method to test wastegate perfornance?

[anonymous-user]

Slowest ramp speed possible on chassis dyno, in tallest gear that doesn't overspeed the rolls!

You need to be WOT and stable (no speed hunting) down around 1500rpm to get a truly accurate picture of the static system response. You also need a good dyno speed control tuning, so the ramp rate is maintained at a constant value as the engine comes on boost (where the torque increase is most sudden)


As an aside, a tiny 1psi "wobble" out of 12 is pretty irrelevant in the grand scheme of things because if you are chasing power:

1) you're going to need a LOT more than 11 or 12 psi

and

2) if your at 3500rpm you're in the WRONG gear! l (hint, peak power is up at above 6000rpm.... )


;-)

[stevieturbo]

1psi is hardly the end of the world...

And surely if he's looking to get it re-tuned...whoever is doing this will also adjust the boost in the new tune too ?

Of it's really important, fit an aftermarket boost controller.

[AW111]

Thanks for the replies. I am still learning what to look at on turboed engines.

The car was dynoed to establish a benchmark before getting it tuned/reflashed. The owner is not looking for a massive power hike, so boost increase will be conservative. I will run it again post tuning, and try some steady state and v. slow ramp runs.


ps I know the owner isn't using all the revs available, but I am not going to tell him how to drive before our first event together