93TurboSi

I would say that most production cars follow the low compression high boost rule. They just make the engine strong and beable to run easily on pump gas and then just use the turbo to make the power.

kpt4321

Evo 8 is like 8.8:1.

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by shermanyang &raquo;</TD></TR><TR><TD CLASS="quote">wow, this thread is interesting. i remember a thread about the same thing came up a while back and a majority of the people kept on saying high compression w/boost was better due to lag on low compression, etc, etc. on my old setup i ran 8.5:1 on my LS turbo and honestly even in low rpms it felt a lot stronger than my lil brother's mostly stock LS. </TD></TR></TABLE>

Yeah, in those threads I always got flamed for saying that lower compression and higher boost was better. Good to see people are finally turning around.

danl

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JDogg &raquo;</TD></TR><TR><TD CLASS="quote">

i wish people would go build some low compression and realize that its NOT laggy. hell we had an sc61 on the thing and it built full (20-25psi) boost by about 4300 rpms and if you were up in the rpms off boost and hit the gas the boost came in VERY fast.... ie. rolling at about 7000 in 3rd and punch it.. immediate tire smokage. the car got about 26mpg in normal city driving and made 500hp on pump gas. whats not to like about it? any loss in cylinder pressrure off boost from the lower compression was more than made up for with timing.</TD></TR></TABLE>

No kidding, its not like a FWD car can use 250 foot lbs of torque at 2000rpm's. I've got 22 psi on tap at 2600rpm's in my 7.8:1 compression car. Let me tell you something, boost makes torque and I have a ton on tap at low rpm's. If anybody on the board wants to meet up with their high CR low boost motor to race my car from a 40 mph roll with both of us in 5th gear I'm game.

shotty

Running a high c/r with low boost has a lower thermal limit as well as less volume for the air in the cylinder. Low c/r has a higher thermal limit and will also be able to put more air into the cylinder. More air means you can add more fuel and in turn get more power. Yes, lag is overrated on this site. You cannot compare two vehicles with same setup with different c/r and same boost. One guy is running lower c/r so he can boost more, and in turn put more air into the cylinder so he can add more fuel and make more power.

m R g S r

Yea, think about it. If car manufacturers ran high compression and moderate boost to achieve some of the numbers that the cars put out today, the tune on them would have to be dead on. They don't go and tune each car individually, but they produce a map that will be good for every car. With high compression, what would happen if someone put the wrong gas in their car?
I think the car companies do it as more of a safety precaution, as opposed to it being better for performance.

RTW DC2R

good info!

how much does displacement effect both of these configurations? say 81mm and 84mm. what effect would that have on a high boost/low CR vs a low boost/high CR set up? The larger displacement would allow more air in the chamber, thus making more power, but would it spool faster and reach full boost sooner?

thermal

Great topic!

My 2 psi on the matter:
I think that low compression and high boost will still be better just because of the tunability issues. I did make 307 whp and 208 tq on stock JDM ITR motor 11:1 compression @ 9 psi (peak) with a steady 8 psi. This is with Hondata S200b. The power band was still climbing but we stopped at about 7800 RPM on a T3/T04E 57T 63. It was starting to scare me. My next project will be different. I'm curious myself. I will be building a B18A/B with a target compression of 9.3:1 sleeved and boost it using the same WG. I'd like to see for myself.......

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by m R g S r &raquo;</TD></TR><TR><TD CLASS="quote">Yea, think about it. If car manufacturers ran high compression and moderate boost to achieve some of the numbers that the cars put out today, the tune on them would have to be dead on. They don't go and tune each car individually, but they produce a map that will be good for every car. With high compression, what would happen if someone put the wrong gas in their car?
I think the car companies do it as more of a safety precaution, as opposed to it being better for performance. </TD></TR></TABLE>

That's not true at all.

If two cars make 300 horsepower at a certain rpm, then you have the same end cylinder pressure in both of these motors. Thus you're not going to have much of a safety advantage with the lower compression car.

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RTW DC2 &raquo;</TD></TR><TR><TD CLASS="quote">how much does displacement effect both of these configurations? say 81mm and 84mm. what effect would that have on a high boost/low CR vs a low boost/high CR set up? The larger displacement would allow more air in the chamber, thus making more power, but would it spool faster and reach full boost sooner?</TD></TR></TABLE>

First of all, 81 and 84mm would refer to bore, not displacement.

Displacement is not really relevant here. A higher displacement motor will make more power at the same air density, and will also spool the same turbo faster because of the greater airflow.

That has absolutely nothing to do with compression ratio though....

kpt4321

Since I am getting a bit fed up with these threads, here is something I just typed up and saved on my computer:

Compression vs. Boost

When building a high performance turbo motor, there is always a question of what compression ratio to use. Higher compression ratios allow a motor to create more power at the same mass airflow, but they also create more heat and a greater cylinder pressure, which is harder on the motor. As such the question always arises: what is better, lower compression with a higher boost pressure, or higher compression with a lower boost pressure?

The degree to which you move the compression ratio, and even which side of the spectrum is best for your car, depends largely on the goals, and the other parts on the car. For example, if you plan on boosting a Honda Civic with a junkyard turbo, and you plan on making about 200 whp at 8 psi, then compression changes are not for you. To make that aforementioned situation more general, in any situation where your boost pressure is limited by turbo size (limited to a reasonably low level that will not achieve that horsepower necessary to push the limits of your motor on any plausible compression ratio), you cannot lower the compression too much. In this situation, higher compression is good because it allows you to make the most possible power with your limited airflow.

However, when building a motor for higher horsepower, the situation is entirely different. In this case we can assume that you have your choice of compression ratios, and that you will also be able to buy a turbo that suits the character of the motor. This is the sort of situation where people disagree about what is right for compression ratio, high compression and low boost or low compression and high boost.

First, a little bit of analysis is necessary. What is happening inside the motor in general, and how does that differ in each case? What is important to the performance of a street motor, and what can you sacrifice in the name of performance?

Something worthy of mention is displacement. A motor of constant displacement (displacement is defined by bore multiplied by stroke) will have a different total cylinder volume depending on the compression ratio. Changing the compression ratio from 10:1 to 9:1 on a small four cylinder motor will cause a displacement difference of about 1.2%. That certainly is a change, but it’s also not really enough to cause a difference worthy of changing the compression ratio alone.

Two of the most important things to watch for best performance in a turbo car are airflow, and charge air temperature. In both setups, we can compare both of these. First, let’s look at temperature.

Before we do a comparison such as this, we need to set some circumstances so that the comparisons will make sense. Since it is the most applicable, let’s say this is all geared towards a 2.0 liter 4 cylinder motor. Let’s also say that the compression ratios and boost pressure are chosen so that the “effective compression ratio” is the same for both motors; by this I mean that when you combine the compression ratio with the boost pressure, you will have a total pressure at the top of the compression stroke. In order to not choose totally arbitrary numbers for compression and boost, we will pretend that in all situations these two are chosen such that the mean pressure at the top of the compression stroke is the same. As you will see later, this actually will probably cause the lower compression motor to make more power, but I digress.

Heat energy is created by compression, in both the turbo and the motor. Now, the heat rise across the turbocharger is always going to be greater than as predicted by gas laws, because it is not 100% efficient. So, does that mean that compressing the gasses in the motor instead will be better, since it is more efficient? The answer is a resounding no, due to the fact that a good turbo system is intercooled.

The basic fact is, heat energy created by the turbo can be cooled by the intercooler, while heat energy created by the motor cannot be removed from the charge air in any easy manner. As a matter of fact, if you consider a good intercooler (~75% efficient) and a moderate boost level of around 15 psi gauge pressure, the system of the intercooler and the turbo has an “effective” efficiency of well over 200%! As a matter of fact, you can find an approximation of the system efficiency by dividing the compressor efficiency by 1 minus the intercooler efficiency. Now, in reality that’s not the best way to express what is happening, but the term “250% compressor efficiency” provides a good analogy to show you what is happening; it allows you to visualize how good the turbo and intercooler system is at dealing with heat rise as a whole.

This all just proves that it is better to create heat at the turbocharger, because you can deal with it so much better. In the quest to keep temperatures low for air density and knock resistance, the ability to intercooler the charge air is key.

With that said, we can add it to the discussion of airflow. With a constant displacement, airflow is dependant on density; pressure and temperature. In the situation of rising boost due to dropping compression ratio (to continue to hold our make-believe motor’s effective compression ratio constant), the density of the air in the intake manifold is increasing. The pressure and temperature both increase, but the temperature increases at a rate lower than the pressure does, and thus the total density increases.

While the airflow changes due to the greater displacement of the lower compression motor are negligible, the airflow changes due to this greater density are not. The higher airflow in this case will allow the pressure after combustion to be greater, even though the volume expansion ratio of the lower compression motor is smaller.

So, by the numbers, a comparatively lower compression will allow you a higher airflow, while keeping the effective end temperature lower. This, of course, does not mean that you want a ridiculously low compression, because at a point you begin to sacrifice too much combustion efficiency. However, it does mean that within the realm of streetable compressions there is reason to consider lower compression ratio choices.

One last topic which is often addressed incorrectly is the topic of boost lag and boost threshold. A lot of people think that compression ratio effects lag and onset times, but that’s not necessarily true. In order to spool the turbo you need exhaust gas energy, and there is no really good reason why one motor would have less exhaust gas energy than the other. As a matter of fact, the lower compression motor may have more exhaust gas energy because there is less of a volume ratio and therefore less of the energy goes into spinning the crank.

What does happen is the car will make more power out of boost; with the same air density and different compression ratios, the higher compression motor is going to make more power. This makes the car feel faster out of boost, which gives a lot of people the impression that it is spooling faster. Additionally, if both motor spool to full boost at 3750 rpm, then the motor with more power out of boost (higher compression) will get to that engine speed faster.

Regardless, the actual turbo lag is basically independent of compression ratio.

uncleben'sspanishrice

you are incorrect KPwhateverthephuck21

the sole contributor to any amount of turbo lag is a constupated fart can that can be fixed easily

walk down to the gas station and buy some exlax, now administer one tbspn down the intake while at wot, this will unclog said fartcan and allieviate all turbo lag

~creatively uncle ben's spanish rice

nigel

that was pretty gay. I bet you didn't even read the whole thing, or you werent smart enough to understand.

Dr Drew

That was pretty gay... I dont think he understood any of it either...

Has anyone used thermal coatings to offset cylinder temps enabling slightly higher CRs ??

BTW, nice post KPT. Thanx for the taking the time to write it...

XDEep

so if a low/high and high/low motor are both making say, 300whp at 7000rpm, are both motors producing the same cylinder pressures but at different heat levels? assuming both were tuned by the same person, similar EGT, timing/fuel- im not sure how this would work out. perhaps similar fuel, but the low/high boost would have more (or less?) timing pulled if its temps arent as high. both on pump gas.

is there a general way to calculate cylinder pressures and temps to determine the limits of internal components? it seems only whp is used, i.e. my understanding is 250-300whp for an ls's stock rods/pistons, 500-600whp for its stock sleeves. these would be on the lower end of reliability, so i would subtract at least 20-30% for a daily driver.

FATBOYeg6

good post. i have a question though. i was reading an article in scc a while ago about using math, and compressor maps to match a turbo to your engine. i don't remember the exact formula, but it used psi, bsfc, displacement, etc. to determine a good turbo. i was wondering if there is any set formula for determining a good compression ratio for an engine? or is choosing a c/r more theoretical based on how much boost/airflow you plan on having? thank you.

ps. when i get around to needing tuning, i think i will be talking to you kpt4321.

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by XDEep &raquo;</TD></TR><TR><TD CLASS="quote">so if a low/high and high/low motor are both making say, 300whp at 7000rpm, are both motors producing the same cylinder pressures but at different heat levels? assuming both were tuned by the same person, similar EGT, timing/fuel- im not sure how this would work out. perhaps similar fuel, but the low/high boost would have more (or less?) timing pulled if its temps arent as high. both on pump gas.</TD></TR></TABLE>

You are going to be talking about similar cylinder pressures AFTER the combustion event, yes. That means two things:

Likely less pre-combustion cylinder pressure in the lower compression motor, due to the greater air mass. However, the lower compression motor also has a lower volume ratio of expansion (basically, the volume of the chamber doesn't change as much with lower compression. duh) so it needs a tad bit more pressure in order to have the same force exerted on the crank, which really determines torque, which really determines power. On the other hand, it also has a little bit less energy going into the compression, so...

Yeah, similar post-combustion pressures in the end.

It doesn't really matter how they are tuned, in my opinion; the timing advance and such isn't really relevant. You just use whatever works for each setup, and they'll both make their optimum power.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">is there a general way to calculate cylinder pressures and temps to determine the limits of internal components? it seems only whp is used, i.e. my understanding is 250-300whp for an ls's stock rods/pistons, 500-600whp for its stock sleeves. these would be on the lower end of reliability, so i would subtract at least 20-30% for a daily driver.</TD></TR></TABLE>

While horsepower is a good way to define an engine's limitations, torque at the flywheel is much better. For example, 400 horsepower at 9000 rpm is much less stressful on a motor than 400 horsepower at 4000 rpm. Keep in mind that revving that high brings other things into consideration, but that is not what we are talking about here.

In terms of determining the limits of components through mathematics, I can't give you an answer. In theory it is possible, but the modeling required would be absolutely absurd. It's DEFINATELY not something you can work out on your TI calculator.

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ninefivehatch &raquo;</TD></TR><TR><TD CLASS="quote">good post. i have a question though. i was reading an article in scc a while ago about using math, and compressor maps to match a turbo to your engine. i don't remember the exact formula, but it used psi, bsfc, displacement, etc. to determine a good turbo. i was wondering if there is any set formula for determining a good compression ratio for an engine? or is choosing a c/r more theoretical based on how much boost/airflow you plan on having? thank you.
</TD></TR></TABLE>

Nope. My roomate (blackknight7) and I were just talking about this. The optimal compression ratio depends on a whole lot of conditional stuff, and is also very dependant on the goals you have for the car.

When you use calculations to do turbo sizing, what you are really doing is just making sure that the compressor wheel in question is the proper fit for what your motor will flow. That's really only one half of the turbo if you think about it, and the other half (turbine housing and wheel) cannot be determined as easily as the compressor side can. Compression ratio is similar. You can figure out theoretically what compression ratio will work well, but that doesn't necessarily mean it's going to feel right in your vehicle.

If I was building a nasty all-out street motor, I would probably rock it in the 8-9:1 range. Probably. Something like that.

FATBOYeg6

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kpt4321 &raquo;</TD></TR><TR><TD CLASS="quote">

Nope. My roomate (blackknight7) and I were just talking about this. The optimal compression ratio depends on a whole lot of conditional stuff, and is also very dependant on the goals you have for the car.

When you use calculations to do turbo sizing, what you are really doing is just making sure that the compressor wheel in question is the proper fit for what your motor will flow. That's really only one half of the turbo if you think about it, and the other half (turbine housing and wheel) cannot be determined as easily as the compressor side can. Compression ratio is similar. You can figure out theoretically what compression ratio will work well, but that doesn't necessarily mean it's going to feel right in your vehicle.

If I was building a nasty all-out street motor, I would probably rock it in the 8-9:1 range. Probably. Something like that.</TD></TR></TABLE>

now that you mentioned it, i remember the article saying it was geared towards the compressor side of the turbo. one of my friends is putting a turbo on his neon, and got 8.5:1 pistons. i thought he was crazy, but after reading this thread, it makes sense.

i have come to the decision to turbo, go with low compression, get all quality parts, and get it tuned right. now i just have to decided what engine i want to turbo, and get some cash flow!

thanks for the help

93TurboSi

All of this seems pretty silly. If you are talking about exactly what the title of the thread is High compression/lowboost VS Low compression high boost then the answer seems obvious to me.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by thermalfi'd16 &raquo;</TD></TR><TR><TD CLASS="quote"> I did make 307 whp and 208 tq on stock JDM ITR motor 11:1 compression @ 9 psi (peak) with a steady 8 psi.</TD></TR></TABLE>

so here we have a perfect example of high compression low boost making 307 hp. While i have seen 9.0:1 D16Z6s running 16-18 psi making the same power. And if you got that itr engine and drop the compression to 9.0 and ran 20psi it would make way way more then 307whp probably closer to 400. I have had a friend who was running 15psi on a built low CR GSR motor and was getting arounf 350 whp.

So if you actually stick with the topic it seem obvious that low compression high boost will make more power. Just do a search and look for comperable engine setups with the only difference being CR and boost and i guarantee you will see way more big numbers out of the low CR/ High boost crowd.

kevinoneill

t-t-t

b18cx

Stock JDM GSR 10.6:1, 8.5psi, 320whp.

JSMC

their's just something that mixe me up in this thread, the thing is that evryone seems to jump from race gas to pump gas in the same thread

I would have a more general question that would help me, if knocking/detonation can be lowered enough (by tuning, higher octane and some adder) which will make the more power, higher CR?

because as I understood from that thread is that the only problem seem to be that detonation is leading to a limit in boost for higher CR

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kpt4321 &raquo;</TD></TR><TR><TD CLASS="quote">While horsepower is a good way to define an engine's limitations, torque at the flywheel is much better. For example, 400 horsepower at 9000 rpm is much less stressful on a motor than 400 horsepower at 4000 rpm. Keep in mind that revving that high brings other things into consideration, but that is not what we are talking about here.</TD></TR></TABLE>
I disagree with that, if you are with the same stroke but in two different setups, I'm pretty sure that the stress will be much more on the 9000rpm one, because you have a BIG difference in mean piston speed, and huge difference on the acceleration your crank, rods, bearing, wrist pin they are taking which leads to a lot more stress since in the calculation of all this the velocity is squared

JSMC

ttt

pete98m3

high boost / low compression makes more power PEAK on a given fuel, its not a matter up to debate its a proven fact. Sure, at any given boost it'll make less power, but its not manifold pressure thats important its effective comrpession.

This stuff isn't new, its not unproven, and its not really up for debate. Go look at what the 1000+ hp supras are running for CR, or the big turbo porsches, any of the really really big hp pump gas setups you won't see any compression over 9.5:1.

bottom line is an intercooled turbocharger is a much more effective compressor then your engine, and therefore you can make more horsepower by using it to do your compressing then the actual engine.

JSMC its about torque not horsepower... 400 horsepower @ 4000 requires what 550 ft/lbs of torque? the torque is what effectively measures the peak force on your rods / crank... and the cylinder pressure required. Thats why its less stressful on the internals to have an engine with only 275 ft/lbs that makes 400 whp @ 9000.

p.s. 7:1 compression runs fine, in fact you can go as low as 6 - 6.5 : 1 and not have any problems with the car actually not "running" although i can't attest for your low end power. The early porsche turbos (70's) were 6.5:1 OEM.