Once I start up my car I get full boost, but after driving it for about half an hour my boost begins to leave. Could this be because my air filter is plugged (I have no cannister around it), since the hot is is more dense and it can't suck in as much?

Your right about the air temps causing probs. But if I remember correctly hot air is less dense, maybe I'm wrong? I think your problem is your intercooler getting hot (heatsoaked). Hot intake temps = less power. I am still learning myself so I could be wrong though. By the way do you have a boost guage, and if so do your boost reading change after warmup?

 

Yes I do have one and once the engine warms up (about 30 minutes of driving) I lose somewhere around 5 lbs of boost.

 

Do you have anything blocking the air filter from all the engine heat? The vfaq has instructions how to build a simple wall that seems to reduce heat alot.

 

heat isnt going to cause 5psi less, its something else

 

Yea, heat would only increase psi not decrease it.

 

Yea, heat would only increase psi not decrease it.

No, heat would decrease your boost because the charge air is warmer (less dense) therefore you are getting less air into the chamber. The only reason it would feel faster when the engine is hot is because when the engine is really cold it pulls timing.

All that said, heat soak would not cause 5psi loss now, but I have seen that much boost loss from my talon in the middle of summer here in AZ (130+ on the pavement)

 

Maybe once some hose gets warm, it begins to leak. Just a thought.

Have you tried to leak test the system?

Jake

 

"No, heat would decrease your boost because the charge air is warmer (less dense) therefore you are getting less air into the chamber. The only reason it would feel faster when the engine is hot is because when the engine is really cold it pulls timing."

This is incorrect. The mean pressure would not be affected at all - the reason that you get less VOLUME is that a given volume of hot air takes up more physical space than cold. Same pressure, fewer molecules per unit of measure. When the intercooler heatsoaks, it loses the ability to cool (and thus condense) the air that is being supplied to it. Again, this will not create a real pressure difference, but it will sightly reduce the normal pressure drop expected when the IC is working well. In the case of a stock sidemount, the difference would be too little to measure on the average boost gauge.

As for why you would be losing boost once it warms up, there are a few things. A bad O2 could be teling the engine to dump fuel. That would contribute to it. You could be seeing high knock counts, resulting in pulled timing. If ou have a lot of carbon built up, that's a real possibility once it warms up. You could also have a bad water temp sensor that's keeping the engine in "warm up" mode. Most likely it's a combination of some of the above with some other stuff thrown in. Little things like dirty plugs, "leaky" plug wires, old oil that's lost viscousity, any of the above could contribute in a very real way. Remember, turbos are more than pinwheels - they use exhasut pressure AND HEAT to spin up. If your exhaust temps are staying low - say, 1400 range - the turbo will not be very efficient. The fact that it runs good cold would lead me to look at the temp sensor, plugs and wires, and to check for an exhaust leak before the turbine. Just my $.02 - and probably worth about that much.

 

You're right. My Bad.

This is incorrect. The mean pressure would not be affected at all - the reason that you get less VOLUME is that a given volume of hot air takes up more physical space than cold. Same pressure, fewer molecules per unit of measure. When the intercooler heatsoaks, it loses the ability to cool (and thus condense) the air that is being supplied to it. Again, this will not create a real pressure difference, but it will sightly reduce the normal pressure drop expected when the IC is working well. In the case of a stock sidemount, the difference would be too little to measure on the average boost gauge.

As for why you would be losing boost once it warms up, there are a few things. A bad O2 could be teling the engine to dump fuel. That would contribute to it. You could be seeing high knock counts, resulting in pulled timing. If ou have a lot of carbon built up, that's a real possibility once it warms up. You could also have a bad water temp sensor that's keeping the engine in "warm up" mode. Most likely it's a combination of some of the above with some other stuff thrown in. Little things like dirty plugs, "leaky" plug wires, old oil that's lost viscousity, any of the above could contribute in a very real way. Remember, turbos are more than pinwheels - they use exhasut pressure AND HEAT to spin up. If your exhaust temps are staying low - say, 1400 range - the turbo will not be very efficient. The fact that it runs good cold would lead me to look at the temp sensor, plugs and wires, and to check for an exhaust leak before the turbine. Just my $.02 - and probably worth about that much. [/B]

 

You probably have a cracked exhaust manifold that is expanding under hi heat thus causing you to lose boost pressure or spoolup capability once the cracks open up when it's hot.

 

You probably have a cracked exhaust manifold that is expanding under hi heat thus causing you to lose boost pressure or spoolup capability once the cracks open up when it's hot.

I second that....

Stock manifold on a 91 should be cracked to Hell underneath...

 

I second that....

Stock manifold on a 91 should be cracked to Hell underneath...

Correction............Change should be to IS cracked to hell underneath.