Turbo Question

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Fisho

Fisho

Joined
Jul 11, 2005
Threads
1
Messages
18
Location
South Australia
Hi fella's,

I have been wondering for a fair while now,why do you need a low compression engine with a turbo? :confused: .I know people put them on standard compression cars though.
I just cant figure the logic in my head seeing as how a turbo increases the compression anyway ........what side effects might it produce?.

regards
Darren
 
Check out the 'how stuff works' website, there's a good write up on how turbo's work there. That being said, I'm not sure there is such a thing as 'standard' compression.

Low compression is favored because more air is crammed into the cylinders during each intake stroke. This effectively raises the dynamic compression ratio and produces more power (more air=more power). That's the short version, the exact reason is on the how stuff works website, and too lengthy for me to type here.
 
ok cool ,ill check the how stuff works section....im quite familiar on how they actually work though--studied aircraft gas turbines during college,same principle.

thanks
Darren
 
Short answer, if you don't start with a low compression motor you will end up with a high enough compression ratio at full boost that it will be almost impossible to keep the motor from predetonating on pump gas..
 
Mace said:
Short answer, if you don't start with a low compression motor you will end up with a high enough compression ratio at full boost that it will be almost impossible to keep the motor from predetonating on pump gas..
Click to expand...


ahhhh ,i see.

So is it possible to reduce the boost of an 'aftermarket turbo-stock engine ', to still function within the desired compression range?.
 
Yes, a wastegate is designed to reduce the psi built up by the turbo. But honestly, why would you want to limit the boost potential??
 
quote" why would you want to limit the boost potential??
"

To bring it to the same level as a low compression engine!.I was having a comnversation with a friend on a fishing trip over the weekend and this topic came up.Im trying to make sure i have the answers next time i see him.

Im not doing it personally,just trying to understand the theories behing it...My cruiser is already boosted.
 
A wastegate is a must have on a turbo setup. Otherwise a turbo will produce insane amounts of heat operating past its efficency range. Past that point the heat negates much of the boost. Also when you get into bigger turbos the amount of boost possible is often alot more than the motor can handle.
 
Anyone ever fitted a BOV for a little extra pzazz????. Make a pretty mean cruiser if it was "ppscchhh" on every gear change!
 
Fisho said:
Anyone ever fitted a BOV for a little extra pzazz????. Make a pretty mean cruiser if it was "ppscchhh" on every gear change!
Click to expand...
No need,

Actually, if you can keep up the boost you are a lot better off ;)
 
well, its same as wastegate but sounds better! It actually does keep the boost up though by virtual of the fact that it keeps the turbo spinning freely during the gear change.

oh well--- it's just the R34 skyline in me !!!! ;)
 
there are "low pressure" turbos too. look at most of the turbo gasoline motors coming from the likes of mercedes and audi/vw. static compression is still in the 9-10:1 range. it requires more control systems than were available in the infancy of turbo charging in the 70s and 80s
 
Hi guys, i'm new, i desided to join since it's nice to have people to talk to about the same thing.

Anyway, since i have a lil experience on this subject here it goes.

First, there are two major types of turbos:

1- The compressors that run of the exhaust, this kind is the mostly used on diesel trucks and on all out power cars. They have 2 compressores connected to each other, one is used to spin by the exhaust gasses and it spins the shaft, that is connected to the other side of the compressor and this supercharger takes the job of sucking air and compressing it. This ones usually requires intercoolers. (These are the real TURBOS)

2- The superchargers, this run from the crank by either a belt or some kind of gears (the FZJ80 use this kind). They are good for 4x4 and desert cars since these cars use the low RPM band more. There are two kinds, one that replace the Intake manifold or the one that just use tubing to push air to the carb or IM. The second can use intercoolers.

now, for the first you have to have two things that are VERY important, these are BOV (blow of valve) and Wastegate.
The blowoff valve works for when you are driving at WOT and the release the accelerator (to shift for example) and all that pressure has nowhere to go, so it lets the pressure go back to outside. This ones are the ones that make the PSSSHHHH sound.
The Wastegate goes between the head and the turbo, when the pressure at the exhaust manifolds past a certain pressure it let the exhaust gasses bypass the turbo so you won't overspin it (if you overspin a turbo it doesn't last as long)


To the main question.
Is not about the compression ratio is about the pressure it builds by the end. An atmospheric car at sea level works at 14.7 psi, now a turbo car boosting, lets say 5 psi, its going at 19.7 psi, now, lets say the atmospheric car at 14.7 psi creates a total pressure of 1000 psi at combustion, the turbo one will create something like 1500 psi at combustion (disclaimer, this are not actual numbers, just made up). See what compression ratio plays a big part?
Also, don't think you can't run a high comp engine with turbo, because you can if you have lots of tuning sessions, but that's another thing. It's nice to have low compression for turbo aplications for peace of mind.
 
My 12H-T Turbo diesel doesn't have an intercooler or a BOV. I'm glad that it doesn't have a BOV (probably not really needed on a low reving engine, anyway). Every second turboed rice burner around has a BOV and the novelty certainly has worn off for me. I think that it's more of an age thing as a BOV doesn't interest me at all.
I have seen that intercoolers are getting cheaper (on ebay) as more 2nd hand ones become available. I may fit one down the track for a bit more power and economy, but with 498,000 km's on the clock I don't wont to do anything that might effect reliability (increasing engine power). My 12H-T has plenty of torque and power for now.

I remember looking at turbos in my teens and there were different boost turbos for different results. High boost for heaps of power, mid range for general use, and low boost turbos that didn't add much to power but increased fuel economy.
 
Thanks GZERO for your last paragraph.I definetly understand how turbo's work but they way you worded your last paragraph on pressures makes a lot of sense.

Rodd- i can understand that bov's arent for everyone!...dunno why but im my 30's and i still love the sound!.I also have a 12H-T (422000kms) and it certainly does have enough power for the average man.

cheers
Darren
 
The exhaust side of a turbo is a turbine not a compressor. A supercharger is not a turbo because it is not driver by a turbine. It is forced induction but not classified as a turbo.

With a low boost setup BOV's are not nessesary, after that a by-pass valve is a better way to go. Only at high amounts of boost, where compressor surge is a problem, do you need a BOV. And most of the after market BOV's are tuned for sound not performance.

The lower compression you have, the more boost you can run. This equals more torque and HP
 
""The exhaust side of a turbo is a turbine not a compressor""---- that is correct but we knew what he was getting at.........

a supercharger manually drives a compressor.So in effect it does the same as a turbo but mechanically driven and provides instant power without turbo "lag".
 
Turbo lag is not a inherent thing.

It is all how you set up the turbo...

Superchargers can have a lag too.


Additionally, I prefer the turbo on a gas motor off road. It is load dependant not motor speed dependant. So when I am in low range I do not see the turbo. When you get on it however, you do ;)
 
turbo lag is not inherent eh? how can it not be????....being how it takes time for the turbo to spool up,exactly the same on an aircraft turbine,you ALWAYS will have SOME lag to a degree...Although most of this talk is irrelevent on 4X4's ,on a drag car it applies.

anyhow i think my questions been answered now,interesting response though.

thanks to all
 
If you have a sufficiently small turbo or it is designed well then it can essentially be "spooled up" right off idle.
 
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