NA6C-Guy

iTrader: (1)

Just a question or thought I was pondering over. Scenario. Lets say 2 people have identical stock 1.8 NA's, same mileage and wear. One runs a 2554 at 15psi, and one runs a 2871 at 10psi, both make ~240whp and close to the same torque. Of course the power will come on at different rpm's, but given everything else is equal, shouldnt the guy with the larger turbo expect less engine wear and longer engine life since hese running lower boost for the same power? The main thought I had was how much of wear is related to the actual HP and how much is related to the boost pressure. How big of a difference will they see between their setups in terms of engine lifespan? Maybe this is apples to oranges, or I am leaving out something important.

May have been covered, but I had no idea what to search for, and Im not searching all night for a question kind of out of boredom.

Joe Perez

iTrader: (8)

What are we talking here, rings? Rod bearings?

Assuming that neither setup is causing detonation and they are both tuned to produce the same torque curve, I don't see a meaningful difference. Wear is a function of peak cylinder pressure during the combustion event, which can be relativistically estimated by measuring torque. So driving style will actually play a role here. If you typically gun it in the 3000-5000 range, the smaller turbo will be producing more power here and this the rate of engine wear will be faster with it. If you always keep the needle at or above six grand, the big turbo will still be happily chuffing out the air after the '54 has given up, making more power, etc.

But we're talking subtleties here. Changing your oil 100 miles sooner or later will probably have a larger impact.


And in a well-designed system, I don't think the difference is going to be anywhere near 10 PSI vs. 15 PSI. Remember that all this business about compressor efficiency and delta-T goes somewhat to a wash as intercooling is introduced. If you could design a 100% efficient intercooler, then the air mass represented by 1 cubic foot at 10 PSI would be the same for either turbo. If the little turbo is exhaling through an intercooler bathed in liquid helium and the big turbo ain't, then the little one will be producing more torque per PSI than the big one, regardless of RPM. (well, there's still the effect of restriction through the turbine on cylinder scavenging and thus VE, but you see where we're going here.)


From a practical, standpoint, I'm guessing that for two otherwise identical 1.8 Miatas, one equipped with a 2554 and the other with a 2871, the engine with the 2554 will probably last a lot longer. Why? The guy with the 2554 sounds like a practical fellow who knows his limits and just wanted some extra oomph merging onto the freeway, as he was tired of being passed by late '80s Chrysler minivans. The guy with the 2871 is a nutcase who enjoys embarrassing the owners of Corvette Z06s and AMG Mercedes, and he isn't going to leave it at 10 PSI for long. He'll keep turning up the boost until his rods decide they no longer enjoy being imprisoned inside the engine case.

hustler

iTrader: (6)

My engine builder had more to say for lowered static compression ratio than turbo size. Well, to maintain accuracy...he gave me the standard sit-down "tune the car before you blow it up speach" first and foremost. "There's a reason STIs and Evos run 8.2:1 or so."

I'm not an engineer, so I'll just leave it at that because I'm talking out of my *** with any observation or inference on this stuff which I do not really understand.

NA6C-Guy

iTrader: (1)

So overall wear (bearings, rings, holes in block) is more an issue of power and torque, not so much internal pressures? I always figured a larger turbo running lower pressure would be less wear. Since they both make a peak HP and torque of the same number, just in different places. So then does RPM of peak HP and TQ matter?

I do understand we are probably talking very minimal differences, but surely one setup is "easier" on the engine than the other.

Joe Perez

iTrader: (8)

It is all about internal pressure. Namely, the peak pressure in the cylinder during the combustion event. Problem is that cylinder pressure is, shall we say, somewhat difficult to measure directly, so instead we look at torque, which is proportionally related.


Manifold pressure is only one part of the equation, and not even a complete one. For any given engine, you need to figure in air density, which is a function of temperature, for MAP to have any meaning in this context. And then once you've got that out of the way there's VE to worry about, which is dependent cam profile and timing, port shape and size, pressure differential across the turbine, exhaust backpressure, etc.

If you increase exhaust backpressure or decrease turbine size, then scavenging efficiency during the exhaust cycle is decreased, more spent gas is left in the chamber at the beginning of the intake cycle, and VE goes down. Thus, even for in-cylinder pressure at the end of the intake cycle as a constant, peak pressure during the combustion cycle will be a variable owing to what percentage of that initial fill was "useful" (fresh air and fuel) vs. non-combustible exhaust gas.

If you increase intake / exhaust overlap a tad and increase intake lift and duration, then the cylinder fill will be more efficient during the intake cycle, and peak cylinder pressure will go up, even though MAP will go down, assuming the total mass of air being pumped by the compressor to be a constant.


Disclaimer: this is where I start talking out of my ***, based on speculation with roughly zero scientific evidence to back me up.

Let's say that the engine with turbo "A" peaks at 250 ft/lbs at 4,000 RPM and then drops off. And the one with turbo "B" produces 250 ft/lbs, but not until 6,000 RPM. We'll assume that the total area under the curve is identical. I'm also going to assume (fallaciously, I am sure) that the rate of wear is independent of RPM, that an engine will wear at the same rate turning at 4,000 RPM as at 6,000 RPM, assuming load as a constant.

Now, if you tend to operate at WOT between 3,000 and 5,000 RPM, and then shift, engine "A" will wear faster, because it's producing more power in this range.

On the other hand, if you tend to be WOT between 5,000 and 7,000, and then shift, then engine "B" will wear faster, as it is producing more power at this range.

Extremely minimal, to be sure.

NA6C-Guy

iTrader: (1)

Yes, Like I said, I was bored and got to thinking.

hustler

iTrader: (6)

so essentially, my 250wtq motor isn't going to last as long as everyone else on weak ****? How do we account for subaru and mitsu putting together turbo motors that last an eon?

johnhanson

Yes, the engine with the larger turbo will last longer. Substantially longer actually. A larger turbine will cause less back pressure, less residual gases that get mixed in with the new charge, and make more power per PSI, and thus require less boost to make the same power, lower peak cylinder pressures than the smaller turbo, higher average cylinder pressure than the small turbo. It's not just peak pressure anymore than it's just boost or just the color of the valve cover.

Huslter-your **** is fucked up and you need to fix it.

Highest peak pressure occurs at peak torque more or less, as this is when the highest VE is achieved. Things like rings and ring landings wear from high peak pressures. Where as rod bearings and pretty much anything that spins wears from higher RPMs AND higher loading. Higher loading from more torque input as well as higher loading from higher forces caused by spinning the engine faster which increase by the square of the speed.

hustler

iTrader: (6)

You're new around here, so let me clue you in on a couple things. Everyone that knows me in real life knows i am the **** and that i **** tons of hoes and that my dick is huge and that i get swole in the gym and that i am a COCKY ************ and i get away with it. As long as people that know me IRL know this, I am content. I dont give a **** what anybody on this site says or thinks.

johnhanson

Your ****'s so fucked up it's beyond your own comprehension. You cannot fathom how terrible your setup is. What the **** were you thinking when you bought low comp slugs and a large turbine? Response is what gets you down the track fool.

Joe Perez

iTrader: (8)

While I won't contest the assertion that BMEP is more closely related to measurable torque production than peak pressure, can you explain how swapping from one turbo to another will raise average pressure while lowering peak pressure?

johnhanson

The smaller turbo will place more of a restriction on the engine, you get more residual gases in the cylinder after the exhaust stroke, and it requires a higher pressure of boost to get some amount of air in such that the motor makes X hp, say 250hp.

Where as the larger turbo operates with less back pressure, and requires less boost pressure to get the same amount of air in the cylinder. This means it has a cooler, denser charge. Denser charges cause more PdV work later in the cycle, increasing the net torque input. IE-if you double HP, peak pressure doesn't double.

Where with the small turbo it's working its respective *** of to get all that air in the engine, and it's got a hot charge mixed with residual exhaust gases. It takes a lot more "stuff" in the cylinder (air and residual gases) to make the same power. And in doing so you get a higher peak pressure because there was more stuff expanding early, but then less pressure later because the residual gases don't expand and produce any PdV work and there's less new gases to burn and expand.

NA6C-Guy

iTrader: (1)

Ah ha, I knew I would get 2 sides on this!

DragonsMaw

iTrader: (3)

Edit: Skip to the TL;DR, this is mostly fluff.

This **** doesn't make sense. I'm paraphrasing a bit but basically you believe that there's more 'stuff' as you put it in the cylinder period because of backpressure?

First off I think we should forget about pressure, hopefully we can agree that corrected flow matters more since power (both in the sense of pink horsies and torque) comes from the amount of fuel/air mix being burned. The argument about smaller/high pressure and bigger/low pressure turbos at the same power level stems from an attempt at equalizing the charge flow going in. That same charge that goes in and burns has to come out. If it doesn't come out completely then we're comparing apples to oranges in the dynamics case. I can agree on this part but I don't think this will cause peak pressures to rise any more. It's more likely to cause peak power loss due to inefficiency.

So let's ignore that for a second, and heat too, just assume that the charges going into the pistons are the same temperature thanks to our intercoolers. The point then is that the gas flow turbo->engine->turbo is a closed system. Whatever we put in and burn has to come out at some point, otherwise we can't put more in. So peak pressure as related to the charges should be the same assuming that we're putting in the same amount of fuel/air mix regardless of setup.

Going back to backpressure and exhaust going backwards, it's not going to combust and thus won't expand. It has negligible if measurable effect on peak pressure. I don't think this is even the case however since the increase in backpressure would equate to lower absolute flow through the turbine and thus lower charge pressure. So again with apples and oranges.

TL;DR: We can't put the expandy stuff in if the explody stuff isn't out. The explody stuff doesn't expand. Not as much expanding.

Am I right with this? What am I missing out?

Edit: Reread your post, I'm basically refuting your reasoning for higher peak pressure due to backpressure.

sixshooter

iTrader: (12)

High cylinder pressures at lower engine speeds inflict more stress on the bearings.
For this very reason you are never supposed to lug an engine.

High cylinder pressures at lower engine speeds allows more blow-by past the piston rings.
High cylinder pressures at lower engine speeds allows more fuel and carbon into your oil.
High cylinder pressures at lower engine speeds allows more heat into your engine block and pistons.

Smaller, responsive turbos get you down the track faster? Mazworx in Orlando has a S15 with a GT4718R turbo on an SR22 that will say bigger will get you down the track faster. 7.42 @ 184 mph



I'm just saying that you have to be a dumbass not to downshift into that "responsive" RPM range in whatever you are driving. Even with an N/A car.

NA6C-Guy

iTrader: (1)

Thats a very good point. I catch myself lugging my car every now and then. I like to do 2nd gear starts and go wot once It sees ~1200rpm. Which probably isnt good for the engine.

gospeed81

iTrader: (51)

Basically, you want to impart those higher cylinder pressures on a piston that is already accelerating quickly (at higher rpms). This means all those huge, laggy turbos are actually easier on the motor in this respect.

In an NA motor, revs usually do the most damage the quickest to an engine. As sixshooter has pointed out, this isn't true with a turbo. You have already accepted the fact you are going to pound on the car, now do the smart thing and do it the way the car likes to take it. On a motor that likes to rev, put the powerband up top, and of course, don't lug it.

I bought my little IHI kit thinking I would love the off-idle boost for driveability. I haven't even bolted it on and I can't wait to do a compressor swap.

It really does come down to driveability preference. If you like to play around in low rpms and don't sing it to redline often, you will more likely lug your motor with a larger turbo, constantly waiting for boost. If you're an average driver that likes a little response, but a lot more power, then there is no reason not to put a bigger turbo on, performance or longevity wise.

A similar application of this is why Hondas last so freaking long. Most folks say "well, they're made to rev." Why do you think that is? They make low torque, but do so at high rpms, thus making decent power, as well as driving characteristics a lot of people enjoy. If the motor is properly designed and built, this is a relatively easy way to deliver a decent amount of power with minimal stress on the motor.

These stresses do increase drastically with engine speed, but if you've ever owned a Honda/Acura, you know their engineers do their homework. I know this is kind of off-topic, but if we are talking about performance vs. longevity, then the "rpms are bad" arguement has gone out the window, because you already know you are going to rev the bejeezes out of it.

NA6C-Guy

iTrader: (1)

I do like revs, but I also think my Prelude with H22 let go because of how I drove it (alot of redline). I dont typically lug, since cruising around the city in 2nd or 3rd Im usualy running at least 3500rpm, and on average driving I shift at or around 5000rpm. So I guess on this engine you could say I like driving it in the middle, which is probably the best, since contant high rpm is no better than low. I think I will be overall glad with going 2560 (or approx size or larger if chinacharger). I also dont think Im going to be overly ---- about spool. As long as its reasonable, thats good enough.

johnhanson

PV=mRT. What happens when you add more m and T (residual exhaust gases) and hold volume constant (piston/cylinder arangement)? Like you said, closed system and burning fuel makes power.

gospeed81

iTrader: (51)

Newb, you sound smart, but not smart enough to read the stickies.

1. Go to the meet and greet section and start an intro thread, preferably with pics of your ride.

2. Put your damn location in your profile so others can help you or vice versa.

3. Put you car model year and a description of mods in your sig line to also help others help you when you decide that you don't know damn nearly ******' everything and find yourself asking for help.

Oh, and welcome