Automatic - no boost in park

[Kristian_Wraae]

I was wondering about one thing.

My car is a 7mGTE automatic and when in neutral or park the engine doesn't build turbo pressure at all when I press the accelerator. My friend's Supra which is a manual builds pressure in neutral and vents it through the BOV when you let go of the accelerator.

Why is that?

When I look at how the turbo controls the wastegate I can't figure out why mine doesn't build pressure in park and neutral. As far as I can see there should be no difference between a manual and an automatic. Besides my friend's Supra is an automatic converted to manual and he still has the A/T ECU.

Can someone explain? Is it supposed to be like that?

 

[mdr40z]

it is normal, i don't know why your friend's would do it unless his exhaust is restricted

 

[tbcmorris]

mine did not build any boost in neutral till i fond a boost leak

 

[Kristian_Wraae]

mine did not build any boost in neutral till i fond a boost leak

And your car is an A/T or M/T ?

 

[tbcmorris]

manual, 60-1 ct26 hardpipes tial bov, 4 in exhaust no cat coolezze dp.

 

[Kristian_Wraae]

What I don't understand is why my car doesn't build any boost.

It doesn't vent anything through the BOV and if I block the IAC nothing happens. So I can't see where the boost would go.

The only difference between my car and my friend's car (besides the transmission) is that he is running a 3 inch exhast without any cats and I have a stock system with both cats in place. Could that be the explanation?

 

[pogoism9]

As far as I know, the engine has to be under load to build pressure.

 

[tbcmorris]

it wont build alot....

your exhaust probalby has alot to do with it. boost leak test it just to be sure..i would at least. these cars are 20 years old

 

[jdub]

Your car should not build boost in neutral, and neither "should" your friend's car...yours is normal, your friend has some sort of problem if his car does.
The engine has to be under load to build boost.

Your auto tranny will build boost in gear, holding the brake...up to the stall speed of the torque converter. On a stock auto, that will be at about 2500 RPM. It's a great way to launch an auto tranny car

 

[Kristian_Wraae]

Ok, thanks.

What is it that prevents boost from building when in neutral? I know it doesn't vent through the BOV. So it must be through the wastegate. But how?

 

[tbcmorris]

jdub,

im confused my car has always built like 3 psi. and all of the forced induction cars at work. evos, stis, talon, mk4s, srt 4s and so on all act the same way

 

[NTRA08]

Same here. I have a a/t to m/t. t70 turbo, 3" mandrel exhaust, no cats, and a muffler. 32x12x3 IC, 3" piping. and my car vents at neutral. It doesn't show anything on the boost gauge but 0. but it still vents

 

[Justin727]

it will always vent because the system becomes pressurized. Now how strong the venting is depends on alot of variables.

You will not get a full blown hard vent just reving. Turbo is large so it actually requires load to build enough pressure to vent loudly if thats what you're trying to do.

 

[NTRA08]

I don't really care if it vents at neutral or not. Just as long as I get my boost under load. And building boost has a lot to do with your exhaust. When I put the t70 on all I had was the downpipe to begin with so I welded it to the stock system for temp. And I only got about 3-6lbs. Come to find out everything was clogged.

 

[Kristian_Wraae]

ok. What puzzles me is that we both have stock turbos. The only differences are that he has a 3 inch exhaust and we have different after market BOVs.

So I guess the real difference is that I have cats and thus much slower spool.

 

[jdub]

I guess I have to be less absolute in my statements...to get any boost in neutral, a little background is required:

Turbos start producing boost only above a certain exhaust mass flow rate (depending on the size of the turbo) which is determined by the engine displacement, rpm, and throttle opening. Without an appropriate exhaust gas flow for the turbine wheel, they cannot force air into the engine. The throttle point at which the exhaust flow is strong enough to force air into the engine is the boost threshold rpm....for a CT26, we are talking about ~2200 PRM.

When a engine is put under a load there is greater resistance to the expansion of combustion gases in the cylinder. This increases combustion pressure and temperature which, in turn, increases the pressure and temperature of the exhaust gases. An engine under a heavy load will drive its turbo at a greater speed than if the same engine is run at the same RPM under little or no load. This means that even though your CT26 may spool at 2200 RPM with the engine under load, it will take a lot higher RPM to get the exhaust gas flow for it to spool under no load.

You may see some boost pressure in neutral...how much is going to rely on gauge accuracy and how high an RPM you are willing to rev to. I typically do not rev a motor to 3000+ RPM and hold it there just to see if I will get boost.

 

[grimreaper]

Your car should not build boost in neutral, and neither "should" your friend's car...yours is normal, your friend has some sort of problem if his car does.
The engine has to be under load to build boost.

Your auto tranny will build boost in gear, holding the brake...up to the stall speed of the torque converter. On a stock auto, that will be at about 2500 RPM. It's a great way to launch an auto tranny car

It should be said that its a good way to heat the hell out of the tranny fluid if done for to long. Use common sense when doing this (i know jdub knows better, for those that dont...)

 

[RiyadYar]

also another way to build boost with no load is a two step.. which i personally dont believe in but many use

 

[Kristian_Wraae]

I guess I have to be less absolute in my statements...to get any boost in neutral, a little background is required:

Turbos start producing boost only above a certain exhaust mass flow rate (depending on the size of the turbo) which is determined by the engine displacement, rpm, and throttle opening. Without an appropriate exhaust gas flow for the turbine wheel, they cannot force air into the engine. The throttle point at which the exhaust flow is strong enough to force air into the engine is the boost threshold rpm....for a CT26, we are talking about ~2200 PRM.

When a engine is put under a load there is greater resistance to the expansion of combustion gases in the cylinder. This increases combustion pressure and temperature which, in turn, increases the pressure and temperature of the exhaust gases. An engine under a heavy load will drive its turbo at a greater speed than if the same engine is run at the same RPM under little or no load. This means that even though your CT26 may spool at 2200 RPM with the engine under load, it will take a lot higher RPM to get the exhaust gas flow for it to spool under no load.

You may see some boost pressure in neutral...how much is going to rely on gauge accuracy and how high an RPM you are willing to rev to. I typically do not rev a motor to 3000+ RPM and hold it there just to see if I will get boost.

Brilliant explanation.

Thanks jdub !!