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Old 09-28-2008, 11:31 PM   #81
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Originally Posted by ray_sir_6 View Post
Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Compression ratio that is too low will result in an engine that is a bit sluggish in off-boost operation. However, if it is too high this can lead to serious knock-related engine problems.

In bold so noone misses it. That is what this thread was asking, so that is the CORRECT answer.



I'm not gonna try to get into formulas trying to prove how it works, cause there are too many variables that have to be included that we are purposely removing in order to get to a simple answer for a simple question:

"With all other things being the same, which is faster, a HC low boost or a LC high boost engine, both making the same peak hp?"

Even the "fact" that Jason C is trying to say that he tested his engine after the LC pistons were installed and got almost the same power, yet had faster spool, only shows one thing...that his HC pistons were fucked, either his rings were shot, or a piston was cracked, etc.

I base this on several things:

Common Sense I understand how an engine works, and how a turbo works. I am not an EXPERT by any means, but it doesn't take one to figure out the answer to this question.

Experience I work with a man who has been making his living building turbo cars. He is an EXPERT, and I try to get as much info out of him as I can. He is the one who said the question is not answerable due to all the different variables that can play into it. But he knows that, making all variables the same, a LC motor will make less power under the curve, and thus be slower overall. Maybe only .1 sec slower in the 1/4 mi, but still slower.

Hmmm....another idea...Forced Performance...they should be a good source of info on this.

"Higher CR will make it more responsive, have faster spool, and more power out of boost."

Anyone have some "turbo" books they can quote from? They will all say the same thing.

HC works so long as you have adequate knock suppression, or you have to lower the CR till you have adequate suppression.

So if you want to make a 300whp Miata, you CAN do it with 11:1 pistons, but you'll be running 120octane gas to do it.

If you want to make a 300whp Miata and use 93 octane gas, 9.5:1 or lower CR.

This isn't including the options for running alcohol injection, or anything else like that. Again, trying to limit the variables to make for a simpler answer. The complex answer is, "If you have enough money, you can do whatever you want."
And again If you look at my first few posts pat, youll see that I agree with the idea that there are too many variables, and that there is no reason to ever lower CR.

Hes pretty much on the mark here with what he is saying, although I dont think itll take 120 octane to make 300HP on the miata, but that is just from personal experience with HC/LB motors, and my experience with the BP engine in general. But thats the only thing I disagree with.

So, you have lots of information here reguarding the reasons/physics behind it. I would suggest an open mind and perhaps a reread of it, to try and better understand it.
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Old 09-28-2008, 11:33 PM   #82
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Originally Posted by jc_rotor View Post
I agree that it appears this way on the surface, but there are things that you are leaving out. #1 The LC/HB application is moving more air volume, but once the piston hits TDC, the total pressure in the cylinder is identical. This is because of the dynamic CR directly( HC/LB vs LC/HB). So, if you have two tubes, with equal pressure and one has 30% more volume, the lower volume is going to travel through the tube faster than the higher volume. However, the higher volume will be spinning the turbine for a longer duration. This would be if the gases in the cylinders were identical in behaviour, but because of the differences in efficiency and burn cycle you cant exactly compare them. The reason the higher CR motor spools faster simply has nothing to do with that though. It has to do with making more power off boost, and revving faster than the LC motor. If you were to put them both in the same gear and stomp it, the HC motor is going to reach a higher RPM in the same timeframe, because of the increased power and efficiency, therefore having physically more exhaust volume than the LC motor, no matter which way you slice it. If after 2 seconds, the LC motor is at 2500 rpms, and the HC motor is at 3000, the HC is making more power, more boost, and overall is just much faster.

Does that make it clearer Pat?

Like I said before, there are other factors such as cam timing and ignition timing that can make the effect even more dramatic, but this is the basic principle behind the reason that HC/LB cars are faster.
Please. That's pathetic. Who the hell races from a 2K dig in 5th gear? I'll be in boost QUICK and be gone.

Go read all my post since your last visit and show me I'm wrong.
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Old 09-28-2008, 11:34 PM   #83
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Notice they dont explain this at all. It's mentioned casually in an article about something else entirely. I'm still dubious.

dont get me wrong though. I have a 10:1 comp turbo motor at low boost making good power. Maybe for ***** I'll put my current head back on the motor when I build it with the 8.5:1 pistons and see what happens. No other changes. As long as you agree that changing rods wont affect power measurably.
depends on the weight of the rods. If the weight of the rotating assembly changes, there will be a difference in power, and thats why its so hard to measure the differences. Ideally you would want two setups equal in weight, but thats nearly impossible. It will be interesting to see the results reguardless though.
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Old 09-29-2008, 12:02 AM   #84
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Your math is wrong. Increasing the compression ratio will increase power to the crank by a max of 12%. That means the change in power to the crank will increase by 12%. You can't just add 12% to 1/3. 1/3 is an arbitrary value anyways as it is a constant in both scenarios.

Assume P=Power to the crankshaft, Q = efficiency added in %, R = increase in air/fuel, * represents multiplication.

We will let my 8:1 motor be the datum for which yours is measured off of.

So for me, Q is 1.00. For an 11:1 motor under ideal conditions, Q=1.12.

For me, R=1.3 as I will move 30% more air. For an 11:1 motor, R=1.00.

P = (R)*(1/3)*(Q)

For an 8:1 motor:

P = (1.3)*(1/3)*(1)= .433

For an 11:1 motor:

P= (1.0)*(1/3)*(1.12)=.373

Again the 1/3 is a constant in either case and is therefore arbitrary. Delete it from the equation and you will still get the same ratios.

All I'm assuming is that everything is held constant and we only change compression ratios. Assume whatever makes you happy, as long as it is the same for both motors. Only reason for this is to calculate the change in power from changing compression ratios.
I cant argue the math, but I dont have to, because this isnt how it is going to be in the real world. The concept behind this math is one that is never going to take place. You are negating the fact that the 30% more air/fuel is going to be hotter than the lesser value(heat from compression in the turbine), and then the timing will have to be retarded more than the HC/LB intake charge at the same effective Dynamic CR. The retarded ignition timing will make less power, and the reduced efficiency of the burn cycle will make less power.

You said all things equal, but you have to talk in terms of values, or actual build, not both. Example, you cant say they have the same intake charge temp without saying they have different sized intercoolers, and if they have different sized intercoolers, the smaller intercooler will have a smaller pressure drop.


There are simply too many changing factors from changing a CR on the same boosted engine to do the math the way you are trying to do it.

Like I said the 4% power per point of CR increase applies to cars that are running at 14.7psi, or 1ATM.


Theoretically, the LC/HB motor should make more power if all other things are constant. But they are not, nor will they ever be constant. So it does not matter.
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Old 09-29-2008, 12:08 AM   #85
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Give me your opinion on post 70 too.
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Old 09-29-2008, 12:19 AM   #86
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Originally Posted by jc_rotor View Post
I cant argue the math, but I dont have to, because this isnt how it is going to be in the real world. The concept behind this math is one that is never going to take place. You are negating the fact that the 30% more air/fuel is going to be hotter than the lesser value(heat from compression in the turbine), and then the timing will have to be retarded more than the HC/LB intake charge at the same effective Dynamic CR. The retarded ignition timing will make less power, and the reduced efficiency of the burn cycle will make less power.

You said all things equal, but you have to talk in terms of values, or actual build, not both. Example, you cant say they have the same intake charge temp without saying they have different sized intercoolers, and if they have different sized intercoolers, the smaller intercooler will have a smaller pressure drop.


There are simply too many changing factors from changing a CR on the same boosted engine to do the math the way you are trying to do it.

Like I said the 4% power per point of CR increase applies to cars that are running at 14.7psi, or 1ATM.


Theoretically, the LC/HB motor should make more power if all other things are constant. But they are not, nor will they ever be constant. So it does not matter.
Well, the math says I'll make 16% more power. You say you can't argue with the math, yet it's wrong and a HC/LB motor would make more power...

Adding a point of compression raises the final temperature of the charge X degrees before combustion begins. That number DOES NOT change when adding boost. It's constant. So why would the 4% rule change? Care to explain? What does change?

It does matter. Somebody is right, and somebody is wrong. For simplicity I said holding all other things constant. I'm only trying to define the system and its boundaries so we can make a valid comparison.

My math suggest I'm right. I believe it. I find it hard to believe your HC motor is more than 43% efficient at turning chemical energy into mechanical energy.
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Old 09-29-2008, 10:38 AM   #87
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I'd like to see someone with a HC/LB motor, and someone else with a LC/HB motor compare. I mean. It would have to be a collaborative effort to get the right results, but you have to change things inside the motor to achieve either. So. What compression numbers? 8.5:1 and 10:1? Leave the head alone and leave the crank alone. Don't touch the intake manifold and give yourself complete control of engine tuning. Change two things. The pistons and rods. Then tune both for... Psh, I don't know. Something that either could easily hit. Say, 200-220. I'd be interested to see which motor could actually make more power with radical boost and tuning, per the setup.

So, only a couple things change on both motors, and since its the same change on both, you don't have to worry about, "Oh, he decked his head." this, or, "He used cams." that. Just two motors with similar mods, with similar boosting setups. That's the other key. Do both systems use the same turbo and mani? Or do you change the turbine to better suit the setup? I think they'd both have to be the same for the test to show the right results.
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Old 09-29-2008, 11:00 AM   #88
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Originally Posted by 944obscene View Post
I'd like to see someone with a HC/LB motor, and someone else with a LC/HB motor compare. I mean. It would have to be a collaborative effort to get the right results, but you have to change things inside the motor to achieve either. So. What compression numbers? 8.5:1 and 10:1? Leave the head alone and leave the crank alone. Don't touch the intake manifold and give yourself complete control of engine tuning. Change two things. The pistons and rods. Then tune both for... Psh, I don't know. Something that either could easily hit. Say, 200-220. I'd be interested to see which motor could actually make more power with radical boost and tuning, per the setup.

So, only a couple things change on both motors, and since its the same change on both, you don't have to worry about, "Oh, he decked his head." this, or, "He used cams." that. Just two motors with similar mods, with similar boosting setups. That's the other key. Do both systems use the same turbo and mani? Or do you change the turbine to better suit the setup? I think they'd both have to be the same for the test to show the right results.
Not easy to do and make the results valid. There's a million things that could be different that would skew the results. I'm not really looking to do an experiment either as the math tells the story anyways. Besides, I'm only giving jc rotor a hard time because he's wrong. In real life, if he was running 11:1 and 15PSI making X power, I wouldn't run 24PSI to maintain the same dynamic compression ratio and make 16% more. (though I could, more or less). I would run something between those two, say 20 PSI, and make the same power with less chance for knock. But he's so hung up on dynamic compression ratios and HC motors that are more than 43% efficient at turning chemical energy into mechanical that I have to question his reasoning. So far, it's "because".
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Old 09-29-2008, 11:16 AM   #89
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Just saw this thread so a bit late to the party.
Bloody interesting thread though.

I can't see how you can talk about CR's and efficiency without including knock?
Surely the ideal is too have a low enough CR to see MBT before knock, But high enough to be close to the knock threshold at MBT?

I can't see much point running a FI'd HC CR if the compression ratio is too high to reach MBT before knock occurs, as your not running the engine as efficiently as possible.

In this case the lower compression engine would be more efficient. no?

The way i see it is like everything in engineering it's a balance.
You have to balance boost level intake temps, with CR, knock threshold, timing advance, state tune and everything else.



Cheers
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Old 09-29-2008, 11:24 AM   #90
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Yeah. The math is there, and there are ways to get near-spot on results, given the right calculation. I was mainly pointing out that for some of us to really know the outcome for sure, you'd have to show test results (and even then, some people will cry wolf). I'm not about to sink the money, needed to prove someone wrong, and I agree with you. I know someone running low boost on 10:1 compression with AMAZING numbers (on a small V6), but he's always afraid of blowing the motor. He's running about 6 pounds right now with 280 at the wheels as of the last dyno run.

Now, if you wanted to go N/A and bumped your compression, then decided you want boost instead, you could be fine running low boost and make pretty good power. However, you don't have a single bit of give on the setup. You knock, you're blown. Go lean, you're blown. Look at your motor wrong, you're blown.

A low boost motor has a better buffer zone, allowing for small mistakes. And its not worth the risk, to go and build a motor that uses 15 psi to get the same power that a motor running 20 psi gets. Power and Efficiency don't care how weak your pistons are or how well you tuned. **** happens and when it does, you better hope your motor can take the results. A HC/LB motor is at its limits a lot sooner than the reciprocal.
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Old 09-29-2008, 12:40 PM   #91
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Surely the ideal is too have a low enough CR to see MBT before knock, But high enough to be close to the knock threshold at MBT?
Correct. BTW you left out "given the octane and expected air/coolant temperatures".

The point is that Mazda chose the compression ratio to be bullet proof with 87 octane. At 10 psi of boost, 91 CA craptane doesn't cut it without water injection, and any more boost will benefit from lower compression ratio. This is my experience on my '00. When CA dropped octane from 92 to 91 back in ~'02, I had to detune my timing which was close to MBT. When I lowered my compression ratio, I could run close to MBT again.

jc_rotor seems to suggest you can run n/a like compression ratios on boosted motors, and thinks that you get much more than 4% per point gains.
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Old 09-29-2008, 12:44 PM   #92
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jc_rotor,

Think of this.

The number 4% per point comes from thermodynamics theory of the Otto cycle.

If we lived on a planet with 30 psi sea level atmospheric pressure, would the 4% more output per point rule be any different? If not, you'd still only get 4% per point on an FI'ed motor.
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Old 09-29-2008, 03:28 PM   #93
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Originally Posted by JasonC SBB View Post
Correct. BTW you left out "given the octane and expected air/coolant temperatures".

The point is that Mazda chose the compression ratio to be bullet proof with 87 octane. At 10 psi of boost, 91 CA craptane doesn't cut it without water injection, and any more boost will benefit from lower compression ratio. This is my experience on my '00. When CA dropped octane from 92 to 91 back in ~'02, I had to detune my timing which was close to MBT. When I lowered my compression ratio, I could run close to MBT again.

jc_rotor seems to suggest you can run n/a like compression ratios on boosted motors, and thinks that you get much more than 4% per point gains.
Wait, I thought we weren't talking about knock? That would be the only reason to pull timing, is cause of the lower knock resistance of lower octane fuels. You could have left the timing where it was and just put a gallon of race gas in with every tank. It all goes back to what I posted, lower compression is to combat knock, and there are other ways to do it and keep the higher compression and make the same peak power, and have more power under the curve.

Yes, sometimes the math doesn't add up, but that's cause there are other variables that aren't being accounted for. If you want more examples of how HC is better, stroll over to the DSM boards...they are all going from 7.8:1 (4G63 6-bolt) STOCK CR to 8.5-8.8 or even 9:1CR. Alot just replace the pistons for higher compression on the stock motors and turbos. We had a guy who shaved the head on his 3SGTE in his MR2, he was running about 10:1 CR and it spooled INSTANTLY. 3k rpm he was at 12psi on the stock turbo with stock ECU. That was ALL he did, shave the head. It lasted about 2wks before it overheated and warped the head. These real world examples aren't wrong, cause they all, except for ONE (Jason C), have this same result. Contact ANYONE who tunes turbo cars, or develops turbos, and they all will say the same thing. None of them did the math on it, but their experience is better than all the formulas you could ever come up with.
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Old 09-29-2008, 08:34 PM   #94
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LOL a 3sgte? You mean with a STOCK ct26? You could spool those things instantly with a weed eater, man. Bad example. The head work probably helped a little, but those turbo's are pretty tiny.
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Old 09-29-2008, 08:38 PM   #95
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Actually, changing the ignition timing changes spool quite a bit and advancing it with good knock control (water/meth injection) you get more power and better spool than simply upping the boost. The low boost setup is popular, because it works. HC can work, but there are inherant risks involved that outweigh marginal increases in power. You have to want to do it, to make it worth the trouble.
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Old 10-02-2008, 01:02 PM   #96
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Ok. I'm a newb on here so I'll probably get crucified for even posting on this thread, but I'm gonna do it anyway.

You guys in all your mathematical and engineering terms are very smart and like to prove things with equations and calculators and there is a time and place for that. Maybe this is it and maybe not.

You want a real world example? Here it is, although not a BP. It's a Mitsu 4g63.

First build was with factory 8.6:1 static compression. Could run the car to around 20psi on 100 octane and 25psi would require 112 octane.

Decided that we wanted higher static compression for more low end grunt - it's an autocross car and we had the same theory as jc rotor. Put 9.2:1 pistons in it and decked the head for a static ratio of 10:1. Two things happened. You can guess at the first one. The same level of boost was not achievable with the same octane rating without excessive heat and knock at the same timing level. 112 octane would only support about 18psi. The results at this point were that the turbo DID light sooner. Now I'm use to discussing on a turbo diesel forum about my 650whp truck so I used the term "light" rather than spool, because spool is actually the time it takes the turbo to go from 1psi to full boost. My term "light" refers to the time from off idle to when the compressor actually starts to force air. So the "response" of our power increased. The turbo lights sooner AND there is more off boost power. We did lose upper rpm power, however. So peak horsepower went down, response increased, and I would say that the car was slightly slower overall. But remember we are now at 18psi due to octane limits. Next step? We got better fuel. With the ability to run more boost now, the car was WAY faster.

What did I learn that relates to this thread?

If you have two identical BPs other than static comp and boost - one of them being 8:1 and 20psi and the other being 11:1 and 15psi and assuming they can both be run with the same timing, and I want to win the race, I'm driving the HC/LB car. I will win. I don't care what the peak number is on the dyno, what matters is the shape of the curve. After all, you guys aren't building dyno queens here are you? No! You're building car that will be driven and raced. You really want to tune your car? Do it at the track where you can measure e.t. rather than on a dyno where you have a tendency to fix that peak number in your head.
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Old 10-02-2008, 02:24 PM   #97
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^^^Good post
Real world test data is always very valuable and in a lot of ways better than the equation or model predictions.

As an Engineer I realize there are a lot of variables at play here, some of which are difficult to predict. If all of the design details were completely understood and the models were perfectly correlated with reality, then competitive race teams would not need pre-season testing of their cars. They would just build the car or bike, skip testing, save their money, avoid the track spies, and show up at the first race.
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Old 10-02-2008, 03:07 PM   #98
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Good post, Omaharam.

That is what I have been saying, that REAL WORLD is what matters, and all the formulas they were giving were unable to account for all the variables.

So this is 2 threads that I posted info in that was called "incorrect" or "impossible", and then was proven to be correct.

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Old 10-02-2008, 04:28 PM   #99
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Well I don't know if anyone is going to accept my post as "proof" that you are right, but based on my experience I certainly believe you are right.
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Old 10-02-2008, 04:35 PM   #100
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Well I don't know if anyone is going to accept my post as "proof" that you are right, but based on my experience I certainly believe you are right.
well it's certainly interesting data and helpful.

but comparing the better fuel in the higher compression car at the end did somewhat skew the results. you dont have the LC/HB/better fuel combination to compare to.

You have before/after dyno plots to compare? I dont care if they have numbers on them as long as they are scaled the same.
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