Is my Boost "normal"

[DanaT]

I just installed installed a boost gauge in the 2G GSX I bought. I took the restrictor out of the BCS and installed a 1G BOV. Other than that stock.

The boost spikes to around 15-16 and then drops off. As it approaches redline, it only is making 9-10lbs of boost. Many turbo cars have boost roll-off because small turbos can't supply the air required at high RPM.

I have a MBC in the mail. With a MBC will the turbo output enough to hold 14-15psi?

TIA
-D

 

[brainlessher0]

Stock turbos will lose boost up top... I personally use to shift at 5500 rpms

 

[kris_stewart10]

that my friend is one HELL of a boost spike i know what you mean though my car did the same thing
but i just ignored it i mean i am pretty sure it is accurate if you have it hooked up right which i am not doubting you

but if it is holding a steady 10 lbs you need to adjusts your wastegate but i think the MBC should help

 

[viperlp01]

It is not a boost spike. It is simply that the t-25 cant hold good boost all the way to redline. I remember that adding full 3in would allow me to hold like 13 at redline. You might have some boost leaks for it to go down to 10 but it is pretty normal on those.

 

[Illmat1c]

Is everything else stock on the car? And I have seen somewhat of the same problems so they shifted at 55 too.

 

[DanaT]

That would have to be one hell of a boost leak to lose 2-3psi. I mean think of the volume of air through a leak vs the volume of 2 revolutions of the engine.

To lose 2-3 psi at 12psi boost, that would roughly be a 20% leak so 0.4L of air per revolution. That means is in about 10ms that a leak would be 0.4L. That is a lot of air leaking.

What I could see is the wastegate not holding shut and dumping boost that way.

I will maybe look a little tomorrow and see if I can find any leaks, but it just seems more like something other than a small leak.

-Dana

 

[forcefed4]

Sounds about normal for a T-2small.

 

[xioca]

Time for a 14b at least.

 

[Defiant]

What I could see is the wastegate not holding shut and dumping boost that way.

A wastegate doesn't "dump" boost. It opens and lets exhaust energy (not pressure) escape directly into the exhaust without going through and driving the turbine.

You're just outrunning your T25. If you do enough high-rev driving to make it worthwhile, upgrade the turbo. If you're just driving on the street, shift sooner. There's no payoff in running to redline. And, if you've not removed your balance shafts, bear in mind they're turning at twice the crank speed, on plain bearings.

 

[DanaT]

A wastegate doesn't "dump" boost. It opens and lets exhaust energy (not pressure) escape directly into the exhaust without going through and driving the turbine.

You're just outrunning your T25. If you do enough high-rev driving to make it worthwhile, upgrade the turbo. If you're just driving on the street, shift sooner. There's no payoff in running to redline. And, if you've not removed your balance shafts, bear in mind they're turning at twice the crank speed, on plain bearings.

This seems like a wastegate to me. Yes, I know that a wastegate doesn't open the pressure side, it slows the turbine by controlling exhaust gas flow.

I think I should describe why I beleive it is a wastegate and not just running out of turbo. As I said, the boost spikes to around 15-16psi and then drops to around 9-10psi and hold there until redline. What this seems like to me (based upon my other car doing this with the stock wastegate) is that wastegae holds until it gets to its "set pressure" (by this I mean the boost level where the wastegate is opened). At this point all presssure on the wastegate valve by the exhaust gas is holding it shut. Now the wastegate is opened and the valve has gas pressure on two sides. The signal to the wastegate actuator is cut off (or reduced) and a spring inside the actator must return it the valve to the closed position. This spring is now working against exahsut flow. IF the spring has lost its force due to many cycles and being close to a heat source, then it may not be able to return the wategate valve to full shut position. This causes boost spikes and then lower boost.

-D

Oh yes, and 14000rmp on plain bearings is not an issue. You do realize that motorcycle engines run well past 14000rpm on plain bearings dont you? There were very few roller bearing motorcycle engines made. All of the current 16000+ rpm 600cc engines use plain bearings with higher loads than a balance shaft.

Add to that that my motorcycle uses a balance shaft and runs to 11500rpm. That put a balance shaft spinning at 22000rpm on plain bearings. Why would 14000rpm be an issue for bearings?

-Dana

OK. I figured out what is going on. There is no boost leak.

I disconnected the line to the BCS. I then clamped the line shut allowing all pressure to go to the wastegate. The boost spiked to about 11psi and then dropped to 7psi.

Next up I just let the line dump to air so no pressure was going to the wastegate. I had to use the right pedi control for the boost. I could hit 20psi but I could also hold 15psi at part throttle.

What this tells me:
1) Removing the BCS restrictor allows a little extra air to be dumped by the BCS because of the bigger opening. This is what is allowing a little more boost than stock.
2) The BCS is a normally open selenoid. Once the computer see what it believes is the "correct" boost, it shuts the BCS and the pulses it to try and keep the boost where it wants to be.
3) The internal wastegate is a total POS! My other car does not have any boost spike what-so-ever. Even going straight off the spring (clamping the line to the BCS shut) the wastegate gives about a 3psi spike. This type of a boost spike is simply cheap-a$$ engineering and design.
4) I have no boost leaks (see earlier what it takes to loose boost)
5) The manual boost controller will allow me to run around 14-15psi of boost (I hope it comes soon..its in the mail).
6) I may need to adapt my proprietary pnuematic logic control to this car to speed up wastegate repsonse.
7) To make reliable power, an external wastegate is a must.

-D

 

[Defiant]

Oh yes, and 14000rmp on plain bearings is not an issue.

Not if they're engineered for it.

You do realize that motorcycle engines run well past 14000rpm on plain bearings dont you?

And are meant to.

There were very few roller bearing motorcycle engines made.

Not for a few decades, now. My SR500 is a roller bottom-end.

All of the current 16000+ rpm 600cc engines use plain bearings with higher loads than a balance shaft.

And a much different lubrication scheme.

Add to that that my motorcycle uses a balance shaft and runs to 11500rpm.

Again, chalk and cheese.

That put a balance shaft spinning at 22000rpm on plain bearings. Why would 14000rpm be an issue for bearings?

Because of how the DSM engine feeds the oil to the bearing, using the shaft itself for the oil gallery. Apparently -speculatively- the centrifugal force of the spinning shaft will starve the center main. Whether that's what causes it or not, that starvation and bearing failure is far too common.

Trying to compare engines -especially across hugely different implementations and performance parameters- seldom has much real-world relevance. Automotive and motorcycle motors are orders of magnitude in their differences.

 

[DanaT]

We agree then. It is an issue of plain bearings that was earlier stated. It is poor design of the balance shaft that causes this. There are other cars with balance shafts that rev to 7K with no issues and no bearing failure.

What we must keep in mind is that all engineering is a compromise. Many times a simple engineering change to save $5 is made that hurts performance and reliability. $5 may not seem like much but take that across a million cars and that is a $5 million savings.

DSMs were a mid level consumer car. We must keep that in mind. The sticker on my GSX in 1997 was $27200. That was not a cheap car, but also not expensive. In 1997 an entry level Boxster was touching $50K+. For double the money they can do more. Move up to a 993 Turbo and that was around $100K plus or minus. So we must keep in perspective what the DSM acomplishes at a reasonable price.

-Dana

 

[VETTE_50_TH]

One of the biggest reasons for boost spikes are the inadequacies of the turbine housing by way of its small wastegate flapper, the wastegate preload, and wastegate spring pressure.

And defiant, there is not such thing as centrifugal force.

James

 

[DanaT]

And defiant, there is not such thing as centrifugal force.

You are splitting hairs there. What he is saying is the mass of oil is being acclerated via the following equation:

a=w^2/r towards the outisde while their is lack of a cohesive force (or "centripetal force") on the oil to counteract the accleration away from the crankshaft. In essence he understands what is going on and using what is commonly used as the decription of the process.

There are many terms that are used incorrectly (i.e. the true differences between and engine and a motor) but people understand what it means because it is common usage.

-Dana

 

[VETTE_50_TH]

I know, i was being an ass with that statement.

But, my previous statement is what was more important.

James

 

[Defiant]

I was there when centrifugal force was invented, whippersnapper.

 

[DSMopar]

Ok you guys are Nazi's about spelling and grammar, now's my turn. They're called JOURNAL bearings not 'plain bearings'....

 

[johnathan30]

I have recently bought a 97 gst and im getting cut off around 5500 rpm it only does it when im shifting in between gears can anyone help?

 

[xxking9]

T25 is crappy. Pick up a 14b or hunt for a decent 16g with the wastegate. And u shouldn't have the issue anymore I had that issue on my 1g with a 14a stock Turbo I then changed to a 14b with a new wastegate. It stopped then. Found a good deal on a 16g . I may have a few 14bs with wastegates. Shipping should be reasonable in a flat rate box, I don't want much at all for them.