I can't wrap my head around this. I have an FR-S which I love and have no intention of fooling with, I just want to expand my knowledge.
Having come up in the '80s working on cars from the '60s and '70s adjusting points, setting dwell, messing with carbs, etc. I can't imagine how adding boost to an engine with 12:1 compression ratio doesn't just grenade. And yet all sorts of kits exist for the "slow" twins. How does this work?
Sensors and engine management. All those sensor keep the engine alive no matter what gets crammed in it. Things a carbed engine could only dream about.
In reply to parker :boost just puts more into what's there. The concern of preignition can be dealt with several ways
retard timing
increase octane,
cool intake charge.
It already runs 93 octane due to the high compression ratio. It just seems like cramming boost into such an engine without lowering the compression ratio and whatever other internal modifications are needed is a recipe for disaster.
I had a Cobalt SS turbo and increased the boost a little. From 18psi or so up to 23psi but that is an engine already designed for boost.
Combustion chamber shape and proper thermal management.
Look at the compression ratios of modern turbo engines. They're right on up there.
The earliest turbo engines in the 1960s and 1970s had 6-7:1 compression ratio. An equal amount of time before *that*, naturally aspirated engines were running 4-6:1 compression...
Well, static compression ratio isn't everything.
In general the limiting factor of turbo engines is detonation. Detonation occurs after ignition (the spark) when fuel in some other part of the combustion space ignites before the flame front spreading out from the spark plug gets there. When this happens, you will burn all of the fuel in the combustion chamber quicker than intended. The reason it occurs is that pressure and temperature are related and the pressure in your combustion space 'travels' quicker than the actual flame front originating at the spark plug. If you have ever seen aerial footage of bombs hitting the ground you can clearly see a pressure wave moving outward much quicker than any actual flame. When pressure rises, temperature also rises. If fuel in the rest of the chamber is already near its auto-ignition temperature, a pressure wave hitting it could cause it to ignite.
One of the fundamental aspects of ignition (and thus combustion) timing is that it must be changed constantly to suit RPM and other conditions. The speed at which the piston moves changes far more than the speed of combustion. So, as rpm goes up, ignition must 'lead the target' and fire earlier so that the timing of the building pressure from combustion lines up with the piston, rod, and crank being in the right position to take advantage of it. If you imagine a cylinder with its piston at top dead center (uppermost position), you will notice that the connecting rod and crank arm it is attached to are in a straight line underneath it. Regardless of how hard you push down on that piston, it will not be converted to crank rotation when the parts are in that alignment. Ignition should be timed so that pressure does not build to its maximum until after top dead center where the geometry allows it to convert into crank rotation. If ignition is timed properly but you experience detonation, all your fuel burns quicker than normal and results in max pressure when the engine parts are less well oriented to convert it to crank rotation. If you manage to build up too much pressure BEFORE top dead center, then not only is your pressure attempting to slow the engine or spin it backwards, but the massive momentum and power the rest of the engine has is now having a fight with the pressure in your cylinder. You also end up with higher temperatures in the cylinder for a longer time. Generally your max pressure and temp only exist for a tiny moment because as the piston goes down on the power stroke, the combustion space expands, which means pressures and temperatures go down. If you burn all the fuel too early, pressure and temp stay high longer. One of the main ways in which detonation breaks engines is by heating the top piston ring until it expands so much that the gap closes. A piston ring is typically 3-4" in diameter which X3.14 means it's essentially a 9-12" long piece of metal that can only grow by 0.020" (typical ring gap) until it tries to seize the piston in your cylinder. If the ring ends touch, any further ring growth will act like brake shoes expanding in a brake drum and try to 'brake' your piston, which then 'breaks' your piston around the ringland area.
Anywho, static compression isn't the main indicator of what actual cylinder pressures or temps will be, or how sensitive the engine is to detonation. 'Dynamic' or running compression is very affected by camshafts. Atkinson cycle engines are a good example of this. They use late intake valve closing to drop the high static compression ratio to a much lower running compression ratio. Also, if you can imagine a spark plug placed off center in a large bore cylinder (as they always were in the V8s of the 60s, 70s etc), detonation would be less likely on the 'short side' of the cylinder (from plug to wall) and more likely on the 'long side'. Detonation becomes less likely both by locating the plug near the center of the combustion space (which is true in basically all modern 4v combustion chambers) and less likely in a smaller bore size (also usually true when comparing new 4cyl to old v8s). Detonation is also much more likely around 'hot spots' in the chamber such as carbon deposits or exhaust valves. Carbon deposits are much less likely in newer engines with better AFR control and better ring sealing (and thus less oil burnt in the cylinder). Also, i think exhaust valves probably run cooler in 4v heads than 2v heads because exhaust valves shed heat primarily through the valve seat and valve guide surfaces and 4v heads will have more of those surfaces for the same size of cylinder.
And then there's the benefit of direct injection! Port injection sends fuel in on the intake stroke, which means the fuel heats up along with the air during the compression stroke. Injecting fuel directly into the cylinder when most of the intake stroke is past means the fuel didn't heat up from contact with the compressing air for as long. Take the benefits of direct injection a little further and imagine the case with diesel engines. They are all direct injection and unlike gas engines which have all the fuel present at the time of ignition, diesel engines inject the fuel 'gradually' into an ongoing combustion, so it is literally impossible for them to experience detonation in the way gas engines do. I think this is one of the main reasons that people who seem like complete idiots can sometimes build fast diesels. 
Anyway, there's a bunch of reasons why new cars can run turbocharging with high static compression ratios when older engines couldn't.
Vigo said:'Dynamic' or running compression is very affected by camshafts.
Which are basically infinitely adjustible on modern engines with variable cam gears on both intake and exhaust.
The one answer on any comp/boost engine: E85. Corn juice can do wonders.
**Dutifully waits for Alphadriver to show up and spread some know-how**
accordionfolder said:**Dutifully waits for Alphadriver to show up and spread some know-how**
Vigo covered plenty.
But one highly likely thing is that there isn't much boost added- it really only takes a few lb to add a considerable amount of power. And most people who add such a device would not likely end up in the most sensitive region for knock- low speed, high load.
One thing to add to frency's list- more fuel. That does a great job for knock mitigation at higher speeds. While LBT is normally best at 12.5:1 under ideal conditions, when knock is bad, you can actually add a lot more fuel, and make more power. Which is to say- the loss of power due to being too rich is more than offset by the knock mitigation.
Yep. Not much boost needed, DI helps with the cooling the charge, etc etc.
E85 and 10psi on a turbo will have you approaching 350whp on the FA20. It doesn't need much boost at all.
alfadriver said:One thing to add to frency's list- more fuel. That does a great job for knock mitigation at higher speeds. While LBT is normally best at 12.5:1 under ideal conditions, when knock is bad, you can actually add a lot more fuel, and make more power. Which is to say- the loss of power due to being too rich is more than offset by the knock mitigation.
This is definitely a big one. A lot of the newer factory turbo cars are just getting into visible exhaust smoke at WOT because they run so rich at times to keep potential ham-fisted owners from blowing them up.
alfadriver said:One thing to add to frency's list- more fuel. That does a great job for knock mitigation at higher speeds. While LBT is normally best at 12.5:1 under ideal conditions, when knock is bad, you can actually add a lot more fuel, and make more power. Which is to say- the loss of power due to being too rich is more than offset by the knock mitigation.
Interesting, this is relevant to tuning my 4AGE. I had the impression that running richer than 13:1 was just pissing fuel away, so 12.5:1 gives a decent safety margin to avoid running leaner than 13:1 in any cylinder due to cylinder variances without wasting too much fuel. I'm running around 12.5:1 AFRs at WOT and less advance than stock to keep knock at bay. Maybe if I run closer to 12:1 I can add some timing and increase power.
GameboyRMH said:alfadriver said:One thing to add to frency's list- more fuel. That does a great job for knock mitigation at higher speeds. While LBT is normally best at 12.5:1 under ideal conditions, when knock is bad, you can actually add a lot more fuel, and make more power. Which is to say- the loss of power due to being too rich is more than offset by the knock mitigation.
Interesting, this is relevant to tuning my 4AGE. I had the impression that running richer than 13:1 was just pissing fuel away, so 12.5:1 gives a decent safety margin to avoid running leaner than 13:1 in any cylinder due to cylinder variances without wasting too much fuel. I'm running around 12.5:1 AFRs at WOT and less advance than stock to keep knock at bay. Maybe if I run closer to 12:1 I can add some timing and increase power.
For NA we always tuned to 13:1 or a little leaner.
Many FI cars from the factory are 11:1-12:1 on the AFR to keep knock at bay and help cool the cylinders.
GameboyRMH said:alfadriver said:One thing to add to frency's list- more fuel. That does a great job for knock mitigation at higher speeds. While LBT is normally best at 12.5:1 under ideal conditions, when knock is bad, you can actually add a lot more fuel, and make more power. Which is to say- the loss of power due to being too rich is more than offset by the knock mitigation.
Interesting, this is relevant to tuning my 4AGE. I had the impression that running richer than 13:1 was just pissing fuel away, so 12.5:1 gives a decent safety margin to avoid running leaner than 13:1 in any cylinder due to cylinder variances without wasting too much fuel. I'm running around 12.5:1 AFRs at WOT and less advance than stock to keep knock at bay. Maybe if I run closer to 12:1 I can add some timing and increase power.
The trade off for NA cars isn't quite as high, because they don't get into the mega knock issues that turbos can.
But, yea- if you knock at WOT, instead of retarding spark, I'd add fuel first. *Most* of the time, the loss of power due to spart retard will be far greater than the loss of power being too rich. It's all a matter of balancing compromises.
In reply to Trackmouse :Well said Track mouse. The higher octane of alcohol ( 114 for ethanol 116 for methanol) plus the cooling charge of alcohol really increases the allowable cylinder pressure and thus the potential for power gains.
Yeh!! E85!!!!
In reply to Vigo : Nice explanation
thank you
In reply to Vigo :
That's like a mini bible there on knock/detonation. Thanks for the informative comment.
alfadriver said:While LBT is normally best at 12.5:1 under ideal conditions, when knock is bad, you can actually add a lot more fuel, and make more power. Which is to say- the loss of power due to being too rich is more than offset by the knock mitigation.
Tell that to the friggin' Audi guys who seem to home in on 12.5:1 as perfect and end up running timing so retarded that they get themselves into a "coffin corner" of high EGTs melting valves vs. detonation killing rods. One build, the guy said he had a 3 degree window of tune between the two extremes...
Knurled said:alfadriver said:While LBT is normally best at 12.5:1 under ideal conditions, when knock is bad, you can actually add a lot more fuel, and make more power. Which is to say- the loss of power due to being too rich is more than offset by the knock mitigation.Tell that to the friggin' Audi guys who seem to home in on 12.5:1 as perfect and end up running timing so retarded that they get themselves into a "coffin corner" of high EGTs melting valves vs. detonation killing rods. One build, the guy said he had a 3 degree window of tune between the two extremes...
It's hard to deal with people who hone in on one particular thing as the most of the most of the most important thing. Harder when you have to convince them that it's far from the largest knob to tune with. Even harder when that knob has some really amazing interactive properties opposite of the "conentional wisdom".
I have recently learned that you can gain more power by boosting more with retarded spark. Even with REALLY horrible combustion timing, the gain due to boost ends up being more than the loss in spark. I honestly never dreamed that. But it is true on many engines. Still, when you see the pressure trace when that is going on, it's pretty shockingly bad looking.
Vigo, frenchy, Alfa, hitting us with the big-brain stuff!
AngryCorvair said:Vigo, frenchy, Alfa, hitting us with the big-brain stuff!
And I do know that a quarter pounder is Paris is a royal with cheese, because of the metric system.
Knurled said:alfadriver said:While LBT is normally best at 12.5:1 under ideal conditions, when knock is bad, you can actually add a lot more fuel, and make more power. Which is to say- the loss of power due to being too rich is more than offset by the knock mitigation.Tell that to the friggin' Audi guys who seem to home in on 12.5:1 as perfect and end up running timing so retarded that they get themselves into a "coffin corner" of high EGTs melting valves vs. detonation killing rods. One build, the guy said he had a 3 degree window of tune between the two extremes...
Tell that to same FR-S/BRZ guys (like the OP) that are also 12.5:1
Along with all the technical stuff already mentioned, there's a high demand for more power in the FRS/BRZ. So kits are going to be put on the market along with whatever extra retarded/rich tune it may need to live, regardless of how good a candidate for boost the engine really is.
BrokenYugo said:Along with all the technical stuff already mentioned, there's a high demand for more power in the FRS/BRZ. So kits are going to be put on the market along with whatever extra retarded/rich tune it may need to live, regardless of how good a candidate for boost the engine really is.
Except the tunes aren't like that.
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