Thread: How to know safe boost on pump gas?

Hey,

OK, I made a post about this a while ago but the thread for whatever reason took an offtopic turn.

I'm trying to figure out how you know what boost level is safe and why. How do you know the point where you need to increase the octane and run race gas?

As far as I can tell there's 2 main factors at play. The first one is efficiency range of a turbo. When a turbo runs too much boost and is out of its efficiency range, it will produce too much heat and will cause issues. My question with this is how do you know the efficiency range of a turbo? Compressor maps? And at which point do they produce too much heat for pump gas?

The second factor is air flow vs fuel. If boosting too much and even in the efficiency range of the turbo, the turbo could be flowing so much air that pump fuel octane(91/93) will burn too quick??? idk, that doesn't make sense to me. Maybe I'm off w/this one. It seems everything comes down to heat. The hotter the engine runs the higher octane it needs. Why would high boost cause so much more heat?

I can see a wideband helping. I guess if the car isn't burning enough fuel you'll see it in your A/F ratio. But the problem is that low octane burns fuel too quick?? So that doesn't make sense to me either. If low octane burns quicker and more fuel, would that even show up on a wideband if you were detonating or causing engine damage?

-G

Yes compressor maps.
You'll need to learn how to read them. It's not hard. Google is your friend though.

Not exactly. More dense air/fuel mixtures do burn faster than less dense mixtures, but a quick burn is not the essential problem.

This is the basics of it, yes. It's mostly about heat. Heat is a main source of detonation. And detonation is the real problem we're trying to avoid.

The laws of thermal dynamics. If you compress something, it heats up. The more you compress something, the hotter it gets. The turbocharger compresses the air and that heats it up, and then the piston compresses that air, heating it more.


Contrary to popular belief, different octanes burn at about the same speed. So that's a "no".

There are lots of things you can learn from looking very closely at wideband data if you have a high enough sample rate, however for this discussion lets' just assume you can't.



There are two basic ways you can know when you're being safe, or when you have exceeded the safety window of an engine.

Listen for detonation: This can be done with tuned knock sensors, or specialized stethoscopes, or other listening devices. If you hear or detect detonation you have gone too far and you need to back off or do something else to keep it under control.

Use historical data: You can use other people's mistakes to help you get a good idea of what is safe and what is not given a specific setup. Someone along the lines has damaged an engine by going too far, and you can use that to your advantage. Also someone along the lines has made a lot of power without damaging anything and they too can help you form a window of appropriate amount of boost, etc.

Detonation is the main engine killer and the main worry when it comes to finding out how much boost is "safe" and in what conditions it will be safe. Once you understand detonation and why it happens and what you can do to prevent it you'll be a lot better off.

You can find the flow charts from the manufacturer of the turbo. That will help you decided how fast the turbo will spool and how much power you can make on a given boost pressure.

However the second thing your referring to is detonation. This is when the fuel burns to quick you were correct. More heat in the cylinders causes the fuel to ignite faster and thus causing detonation. When you use a higher octane or race fuel the higher octane ensures that the fuel combusts completely at the time you want. I will be running methanol injection with a 50/50 water to meth ratio and the meth raises the octane to around 117 on 93 pump gas and the water turns to steam reducing the intake temps thus decreasing detonation allowing me to run high boost. The same thing goes for race fuel its just like methanol injection just double the price per gallon.

As far as heat soak goes if you run a t-25 at 18psi the tiny turbo has to work way harder to push that boost. However a larger gt35r would take less effort to make 18psi and in turn make less heat considering its in the efficiency range of that turbo is around 18psi.

You can also hear detonation it sounds like the cylinders are missfiring a little. Look up detonation on youtube...its a quick way to melt pistons and destroy rings lands (just ask me) that is why everyone is so crazy about fuel and not detonating because we want to save our motors.

I put Bens explanations in normal people terms just in case everyone here doesn't understand thermodynamics

Or just doesn't understand Ben, period.

So it seems to be the idea here is to up the boost until you hear detonation? lol, doesn't sound like a very safe method. Or in Ben's case, keep a trained Dr. and a stethoscope in the passenger seat

correct me if i'm wrong... from what i've been reading it's all about getting a proper tune. even when u turn up the boost the ecu will adjust the timing to reduce the knock. so as you increase in RPM's and the knocks increase, it'll compensate and reduce the timing to avoid knocking too much. or is that only with the more modern ECU's?

it's not super complicated, it's just tedious. All about tuning the motor properly first, then slowly creep up until you're satisfied with the fuel/timing maps with the boost goal. proper tuning is the best solution.

but if you don't have a laptop to tune with or a place to go to for a fine tune, that's a different story. someone else can chime in on how to tell when the boost is too much.

Wait I thought popular believe was, higher octane means more POWERz and MPGz.

It makes sense to why lower octane fuel will burn faster. Since they have less resistance to knock. But then again maybe once the mixture is ignited it might just burn the same, it's the leading up to the ignition of the mixture.

The limit is turn it up till it melts down or blows and thats your limit. haha. I tested the limits on a couple jdm de motors with different turbo setups.

Plain and simple the T25 should not be pushed any more than around 250-260whp or you will start having problems. i was running my very first setup with a jdm 9.5:1 de motor, S13 tubular manifold, s13 t25, s13 3" j-pipe, at 12psi for a while and it was good. I also had s4 cams. With the times i was running i was definetly in the 250-260whp range. Conservative timing, a/f spot on on the safe side and when i went to 17psi spike droping to 14psi by redline i ended up detonating.

I put a new motor in and even with 100 octane in the tank and running that same 17psi spike i still ended up melting a piston and cracking ringlands on that one. It was just too much heat.

I also went with an s15 bb t28 at 21 psi with 110 octane in the tank and still ended up detonating the lands. Nothing wrong with the tune, a/f rich side. Just too much for the turbo.

The last setup was the large .82 turbine 60 trim and ive gone to 30psi with 110 and C16 in the tank without problems and ran 20psi on 91 octane pump gas for a year and zero detonation. So it all depends. Like they said, learn to read compressor maps and it will tell you where the turbo maxes out and will start just making heat.

According to Garrett, a compressor map is not very useful without 3 pieces of information:

TurboByGarrett.com - Turbo Tech103

· Things you need to estimate:

· Engine Volumetric Efficiency. Typical numbers for peak Volumetric Efficiency (VE) range in the 95%-99% for modern 4-valve heads, to 88% - 95% for 2-valve designs. If you have a torque curve for your engine, you can use this to estimate VE at various engine speeds. On a well-tuned engine, the VE will peak at the torque peak, and this number can be used to scale the VE at other engine speeds. A 4-valve engine will typically have higher VE over more of its rev range than a two-valve engine.

· Intake Manifold Temperature. Compressors with higher efficiency give lower manifold temperatures. Manifold temperatures of intercooled setups are typically 100 - 130 degrees F, while non-intercooled values can reach from 175-300 degrees F.

· Brake Specific Fuel Consumption (BSFC). BSFC describes the fuel flow rate required to generate each horsepower.

Which makes sense..because looking at the compressor map of my turbo:



You can see that at 2 Pressure Ratio(14.7psi), the efficiency island in the middle is at 76%, but depending on how much load the engine is putting on the turbo(from the above measurements) you could be over spinning it and hitting the outside choke limit of the turbo at 14.7psi less than 60% efficiency.


Also, on that same page you can see a 5.0L engine vs a 2.0L engine and the boost pressure needed to make 400hp on a disco.



You can see the 5.0L engine needs a lot less boost but is way outside the efficiency range and the 2.0L engine according to Garrett needs 23psi of boost which is again way out of the efficiency range.

So even though the 5.0L engine is making only about 7psi of boost, it will kill the turbo pretty quick. So without knowing how much load our engines put on the turbos, we can't really tell where the efficiency range will be for a given boost pressure just by glancing at a compressor map.

Anyone have any idea what the #'s are for our engines for:

Engine Volumetric Efficiency
Intake Manifold Temperature
Brake Specific Fuel Consumption

Maybe I'm reading this wrong? If so, please correct me.

-G