TheSamba.com :: View topic - Why does a turbo boost only when the engine is under load?

Wed May 12, 2010 7:37 am

Why does a turbo build boost only when the engine is under load?

Wed May 12, 2010 7:44 am

HEAT.

Wed May 12, 2010 8:19 am

Stripped66 wrote: HEAT.
what????

Wed May 12, 2010 8:19 am

Load= Heat (EGT)
Heat= Boost.

Wed May 12, 2010 8:25 am

I thought it was because the little propeller only spun fast enough when it was driven hard enough... ?

Wed May 12, 2010 8:33 am

http://en.wikipedia.org/wiki/Turbocharger

"Operating principle". I dont have a turbo, but even I know how it works.

Wed May 12, 2010 8:46 am

jfats808 wrote: http://en.wikipedia.org/wiki/Turbocharger

"Operating principle". I dont have a turbo, but even I know how it works.

I understand the fundamentals of what a turbo does / how it works.

I don't understand why there is no boost at 5K rpm while in neutral, but there is boost at 5K when the engine is pushing the car down the road.

Load vs. No Load and how it relates to building boost...thats the specific question I have.

Wed May 12, 2010 8:50 am

junior55 wrote: I thought it was because the little propeller only spun fast enough when it was driven hard enough... ?

I like my answer the best so far....

5000 rpm's at NO throttle to speak of "is not driving it hard" or LITTLE VOLUME being forced through the turbo.

Wed May 12, 2010 8:51 am

Jake Raby wrote: Load= Heat (EGT)
Heat= Boost.

Okay, I get that, load equals an increase in EGT.

And when EGT increases...then what?

Wed May 12, 2010 8:57 am

FireBug1966 wrote:
I understand the fundamentals of what a turbo does / how it works.

An engine under no load produces very little heat, and an engine under significant load produces a lot of heat; it doesn't really care what the RPM is.

Wed May 12, 2010 8:57 am

in this case, heat is a byproduct of work, so the engine is working harder if the egt is higher.

(pretty much)

Wed May 12, 2010 8:58 am

FireBug1966 wrote:
And when EGT increases...then what?

That heat energy is converted to kinetic energy at the turbine to drive the compressor.

Wed May 12, 2010 9:02 am

Stripped66 wrote: FireBug1966 wrote:
And when EGT increases...then what?

That heat energy is converted to kinetic energy at the turbine to drive the compressor.

So, all else being equal, higher EGT pushes the turbine 'harder / faster' than lower EGT?

Higher EGT results in greater pressure inside the exhaust pipes? And that greater pressure pushes the turbine 'harder'? And pushing the turbine 'harder' makes boost?

Wed May 12, 2010 9:09 am

it's not all about heat. rear mounted turbo's work great on front engine cars. it's about the mass and velocity of the gasses flowing. thermal expansion due to the heat does help, but heat alone will only make things hot. at 5k rpm no load, the throttle is barley open and very little fuel is being burned. at 5k rpm full load you have max air and max fuel combusting. much higher cylinder pressures, much higher mass and velocity of gasses flowing through the turbo.

really, it's all about pressure drop through the turbo. that's what does the work.

Wed May 12, 2010 9:14 am

That's exactly what I had typed, before my computer decided to crash. :x

You have to have enough exhaust flow to spool the turbine, the heat just means there's more energy there to extract to drive the turbine. After all, you can spool a turbo with a leaf blower if you wanted to. The heat energy is why you want the turbo as close to the engine as possible, since the farther away it is the more heat you will lose through the piping.

Wed May 12, 2010 9:23 am

miniman82 wrote: That's exactly what I had typed, before my computer decided to crash. :x

You have to have enough exhaust flow to spool the turbine, the heat just means there's more energy there to extract to drive the turbine. After all, you can spool a turbo with a leaf blower if you wanted to. The heat energy is why you want the turbo as close to the engine as possible, since the farther away it is the more heat you will lose through the piping.

I knew it!!! Where is that thread regarding electric blowers? Better yet, hooking the shroud ducts to the carb...

Wed May 12, 2010 9:51 am

veedubcrazy wrote: miniman82 wrote: That's exactly what I had typed, before my computer decided to crash. :x

You have to have enough exhaust flow to spool the turbine, the heat just means there's more energy there to extract to drive the turbine. After all, you can spool a turbo with a leaf blower if you wanted to. The heat energy is why you want the turbo as close to the engine as possible, since the farther away it is the more heat you will lose through the piping.

I knew it!!! Where is that thread regarding electric blowers? Better yet, hooking the shroud ducts to the carb...

so if you put a leaf blower blowing into the cooling fan intake then attached the shroud ducts to your carb you would have a twin turbo!

Wed May 12, 2010 10:00 am

IF it were heat then run it off a blow torch, bs, exhaust velocity is what spins turbos thats why u never spin bearings when u blow cleaner etc out after cleaning, u can damage and i've heard of them coming apart from over speed. Sitting still and rev up up to 5k don't produce enough to spool up a turbo.

Wed May 12, 2010 12:46 pm

If you had a larger engine or a very heavy flywheel you would see boost while reving in neutral. The turbo is making boost in the situation you described but it is so little that the gauge just doesn't really move much.

The same principles explain why you almost always see a few psi more in higher gears vs 1st gear

Wed May 12, 2010 1:34 pm

if its not heat, then why at 5000 rpm coasting it has no boost and 5000 rpm wide open it does.......................heat and load!