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Old 05-08-2017, 08:18 AM   #1 (permalink)
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Ok, so I'm trying to debunk some turbo info that is spread all over the internet from big block Chevy forums to German cars to Japanese imports. They all seem to have the same theories, but at some point they conflict. What's the truth behind them?

1: "Go for effective compression."
ECR = sqrt((boost+14.7)/14.7) * CR. Some people use the formula ECR = [ (Boost÷14.7) + 1 ] x CR, which I believe to be false because the numbers end up in the 15-25:1 range which isn't very realistic.

2: "1PSI from a big turbo is not the same as 1PSI from a small turbo."

3: "PSI means nothing, aim for a HP goal"

These rules of thumb conflict with each other.

Let's try rule #1. An engine with 8:1 compression and 20psi will have an effective compression of 12.3:1 (using formula 1). An engine with 10:1 compression and 7.5psi will have the same 12.3:1 effective compression.

Now rule #2. So let's use the high compression engine from above... We'll say it's using a GT3076R (I know, not efficient at 7.5psi). According to rule #2 if you use a smaller turbo you can run more boost. So now let's use a 16G and run 12psi. Your effective compression just went up from 12.3:1 to 13.5:1. So, going by rule #2 would contradict rule #1, because now your not at a "safe" compression using the same octane.

Lastly rule #3. Using the example​ engines let's aim for HP and just keep upping the boost until we get our desired HP. In this case I'm going to say both engines are using a 16G and we will aim for 250hp. The 8:1 compression engine would need 17.3psi to make 250hp while the 10:1 compression engine (assuming +4%hp per 1:0 increase in compression) would need 15.2psi to make the same 250hp. Now when we go back to rule #1 we get engines with effective compression ratios of 11.8:1 and the other, 14.26:1. This again breaks rule #1. Both engines are making the same power on the same turbo at similar boost, but will they both hold up?


So the question is... What's the real way to figure out if a turbo setup is safe, before you actually build it? Should we rely on effective compression, psi, horsepower, cylinder pressure, etc? I know this may seem dumb to most people, but I like figuring out how well something will work in theory, before dumping thousands of dollars into it.

Last edited by Animag771; 05-10-2017 at 05:52 AM.
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Old 05-08-2017, 12:07 PM   #2 (permalink)
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According to zeals calculator I'm running a 15:1 dynamic ratio at 10psi and loving it on pump gas.

That does't figure in valve timing events or actual air volume produced from the turbo.

Dynamic compression calculations that don't factor in camshaft variables are futile.

Google compression vs camshaft. There is enough info too make your head hurt.
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Old 05-08-2017, 12:46 PM   #3 (permalink)
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According to zeals calculator I'm running a 15:1 dynamic ratio at 10psi and loving it on pump gas.

That does't figure in valve timing events or actual air volume produced from the turbo.

Dynamic compression calculations that don't factor in camshaft variables are futile.

Google compression vs camshaft. There is enough info too make your head hurt.
Zeals calculator uses the second formula I listed, which doesn't make much sense. Depending on your static compression ratio (I'm guessing around 9:1) your actual effective compression is about 11:1, using the other formula. I have looked into how a camshaft change compression, but at the higher rpm range where most engines make their power I don't think there is enough time to bleed cylinder pressure enough to change the effective compression much. Regardless, the examples I posted would be comparing two engines with all things equal other than static compression.

Last edited by Animag771; 05-10-2017 at 05:47 AM.
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Old 05-09-2017, 02:40 PM   #4 (permalink)
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i dont know all the math and fancy stuff.

But those rules arent to go together in the way you said.

i.e.
Pick a hp number.
Next work towards that.
Each turbo will need different needs on different motors to output the same.

i.e. a small turbo on a low comp motor running high hp could be the same as a high comp motor running low psi on a bigger turbo.

And i think just IMO you build it to take what you give it.

There is no one set guide for all the builds.
you might want high comp, or a small fast spooling turbo, or drag build that doesnt see below 6k rpm etc....
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Old 05-11-2017, 08:43 AM   #5 (permalink)
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It's starting to seem like low compression allows you to pretty much pick whatever turbo and hp goal you want (as long as the turbo can keep up) and just crank up the boost until you get the number you're happy with. Plenty of room for error and inefficiency, probably why most people go that route. On the other hand high compression means you HAVE TO run low boost (probably around 12psi max on pump gas) and then you have to size your turbo based on your hp goal, while also not going over your boost pressure limit to achieve that number. Everything must work together with little room for error, but more efficient overall for a given hp goal. Effective compression probably comes into play more for the high comp than for the low comp, because anything south of 8.5:1 can handle 20psi on pump gas (13:1 ECR), assuming stronger pistons/rods.

i.e. A low comp could use a 16G at 17psi and hit 250hp while a high comp would start breaking things at that boost pressure so you would have to go with a bigger turbo to make the same power on less boost. The high comp would then be sacrificing a faster spool in order to keep detonation down.

Maybe the sacrifice in spool time is made up for, by not having to build as much boost for the same power and having better out of boost?


Edit: The reason I keep referring to a 16G is because I already have that compressor map plotted at a range of different boost pressures for a D16.

Last edited by Animag771; 05-11-2017 at 09:02 AM.
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Old 05-11-2017, 10:22 AM   #6 (permalink)
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Quote:
Originally Posted by Animag771 View Post
It's starting to seem like low compression allows you to pretty much pick whatever turbo and hp goal you want (as long as the turbo can keep up) and just crank up the boost until you get the number you're happy with. Plenty of room for error and inefficiency, probably why most people go that route. On the other hand high compression means you HAVE TO run low boost (probably around 12psi max on pump gas) and then you have to size your turbo based on your hp goal, while also not going over your boost pressure limit to achieve that number. Everything must work together with little room for error, but more efficient overall for a given hp goal. Effective compression probably comes into play more for the high comp than for the low comp, because anything south of 8.5:1 can handle 20psi on pump gas (13:1 ECR), assuming stronger pistons/rods.

i.e. A low comp could use a 16G at 17psi and hit 250hp while a high comp would start breaking things at that boost pressure so you would have to go with a bigger turbo to make the same power on less boost. The high comp would then be sacrificing a faster spool in order to keep detonation down.

Maybe the sacrifice in spool time is made up for, by not having to build as much boost for the same power and having better out of boost?


Edit: The reason I keep referring to a 16G is because I already have that compressor map plotted at a range of different boost pressures for a D16.
I think your missing the fact that the higher compression engine is going to spool a turbo faster then a lower compression engine(if it was the same turbo). The biggest advantage to a higher compression engine with boost is the fact that you are making more power when you are not in boost. A properly sized turbo on either engine is going to be more fun on the higher compression engine over the lower compression engine because it will be making more power sooner.
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Old 05-11-2017, 10:55 AM   #7 (permalink)
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I think your missing the fact that the higher compression engine is going to spool a turbo faster then a lower compression engine(if it was the same turbo). The biggest advantage to a higher compression engine with boost is the fact that you are making more power when you are not in boost. A properly sized turbo on either engine is going to be more fun on the higher compression engine over the lower compression engine because it will be making more power sooner.
I didn't include the assumption that a high compression turbo spools faster than low compression, because there seems to be a lot of debate about if that is true. I can't say if it's true or not, but I have yet to see any solid testing to support it, so I didn't factor it in.

Also I did already mention that the higher compression engine will have more power before boost starts building.

I agree that the high compression engine will be more fun at the same house as the low compression turbo with a properly sized turbo.

My questions have been around how to actually size a turbo "properly" to achieve the best turbo and engine combination. There seem to be contradicting opinions on which guidelines to use to figure out what the engine can actually take before things start breaking. i.e. boost pressure, turbo size, effective compression, horsepower, cylinder pressure, all of the above?

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Old 05-11-2017, 11:53 AM   #8 (permalink)
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The way to size a turbo properly depends on your application. Are you going to be hanging out mostly in the high RPMs with very small drops in engine speed between shifts? Do you need good transient response because you are modulating the throttle and moving all around the RPM range? Also the turbo itself can have variables beyond just the size, twin scroll, variable geometry, porting, wheel design, bearing style.

As far as what breaks an engine ... everything you listed is directly related to cylinder pressure ... that, ultimately, is where the power comes from. That, in turn, is related to every thing else how much boost pressure, from what turbo, what effective compression ratio you have. It all comes down to cylinder pressure at the moment of combustion because that will determine how fast and hard the piston is pushed away from the head which will determine how much power you make.
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Old 05-11-2017, 03:45 PM   #9 (permalink)
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This is all very interesting and quite hard for me to get my head round.
To use the same item numbers as the OP:-

1. I don't get the concept of "effective compression". The purpose of the turbo is to push more air in. Other clever stuff matches the fuel to that. Together, they increase the cylinder pressure - assuming that the ignition and valve timing both suit the new mixture and its velocity.
Using the word "compression" seems misleading to me. It sounds as if the turbo simply replicates what you'd get if you skimmed a head to the max (or more, with other mods). That is surely not the case?

2. PSI is less important than volume per time. Or have I missed something?

3. Aiming for a wheel hp goal is what we're after ... or torque ... or even mpg. These things are all achievable, given the right materials and tolerances within the engine. But you can't know what's safe unless you either
a) take on the massive exercise of calculating all the stresses inside - which is what Animag771 would have to do if he (she) wants to know if it will work in theory, or
b) do it empirically (ie: look at what's already been achieved.)

And then there's the issue of spool time.
Just for the hell of it, let's think of turbos as a kind of vtec. Lower revs give us more economy, the engine at higher revs allows more mixture in and gives us more power. The vtec doesn't have turbo lag, but it does have a step, which suggests it's not a perfect solution, either. I don't know if there's a vvt system that gives a smooth transition and is still worth having; in the old days, that's what we thought cam profiles were for! Neither does there seem to be a high-power single-turbo system that fits that bill.
For smooth and almost instantaneous spooling, a smaller, lower inertia turbo is needed; for lots of extra intake air, a larger one ... so fit both!

Lastly, there's the thing called 'efficiency'. The volumetric flow of exhaust gases does not rise in direct proportion to engine revs. Vane leakage in the turbo isn't linear with rpm either. The engine might just end up being safe even if all the calcs suggest not. All systems I know of become less efficient when they approach their limits.

Oh, look! We're back to empiricism again!
(It's what the car manufacturers do.)
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Old 05-11-2017, 07:51 PM   #10 (permalink)
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effective.
The way that works is a turbo is design to run within a range.
Think of a motor if it idle to low it woll stall and if idle to high it will not be idling.
Same for a turbo.
There is an effective effective range where it can run.

And a since a turbo os an air compressor it must take in X amount of air to output Y amount. And all this has to line up in its effective range.

So a small turbo on a car that is over flowing the turbo. Will cause it to not be effective. It will be outside that range on the max side. You wont net any more power it will start to choke the car.

Now if you have a turbo that is to big it will be under the effective range. Which can cause it to stall and stop making boost. Or just not outputting enough air to add any power.

So it breaks down into different parts.
First you have to make a power goal X amount of HP.
Then how will you use this power? Low rpms or drag strip high rpms.

Once those are know then you can size the turbo for your setup.
Turbo map will help here.
There a good guide on it on this site.

Also you could just drop the conpression and max out a turbo but would it be effectife to what your driving needs are?
And lower compression means less power off bosst and slower spools
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