Deciding on a particular compression ratio

[Buddafucco]

Not really looking for a "rule of thumb" on what compression ratio to use. I'm looking for information on how to make an educated decision on which compression ratio I should use based on other variables and such. I would think trial and error would be a route to take, but that isn't very practical. I understand most of how compression works and have read alot about high compression on naturally aspirated cars and why that isn't the best choice for forced induction, but I haven't found a whole lot about how the role a turbocharger plays is factored into everything. If it's more in depth than your time permits then maybe you have some recomended reading for it.

 

[Zumtizzle]

Take The Base Compression Ratio then factor in the boost on top.


For instance a cylinder with ~150 psi of compression, then add the 7-50 Psi of boost.

 

[Zumtizzle]

http://www.team-integra.net/sections/articles/showarticle.asp?ArticleID=233

This pretty much explains it.

 

[need new tires]

what is the cars intended use? that should be the focus of the build.
high power, street, fun to drive, longevity, DD, weekend warior?

higher CR is fun with a turbo but it yeilds some limits that a lower CR doesnt.

i have a 10:1 turbo and i just got done with the built 8.0:1 engine. they have 2 very different uses in mind when built.

 

[Adrian98]

I agree with need new tires, it all comes down to application. that will determine power, response, durability, longevity. but probably most important what fuel you want to run

 

[Buddafucco]

I want to build a motor and go bigger on everything for a daily driver / big power weekend dragster. Low boost + 93 octane daily & high boost + c16 for the track. I'm not looking for safe but if it blows then I don't want it to be because I did something I wasn't sure of.

Assuming all other conditions are ideal.
On a NA motor, a cylinder with 200psi would be ideal with 93 octane.
300psi would be doable for 116 octane. My understanding is that for every 1 octane number you can increase cylinder pressure by 5psi. I might be way off on that.
So then comparing the above on a turbo motor with a cylinder at 175psi + 25psi of boost (cylinder pressure of 200psi)= ideal on 93?? And 175psi + 125psi of boost= ideal on 116 octane???
My lack of knowledge is starting to show now. :slap:
Detonation is the determining factor in how much compression you can run. Cylinder temps are the link between compression and detonation.
Is there a cylinder temp ratio or equation or something that I'm missing?
I haven't even started on dynamic compression yet.

My brain spilled & random thoughts fell onto my keyboard.
I want to learn.:1zhelp:

 

[turbodriz]

I would agree to. I been in a integra with 10:1 and a t-66. Hella of spool early. Makes great power on low boost. During the summer it runs a little on the warm side. Yes u are forced to run race fuel at a lower boost level. The only part that I have'nt determined is how much boost u could run. They were still running 25-30 psi on fuel. Me personally I like 9:1. I am playing with this idea right now. If I can get like 8:7:1 I'll be happy. I just feel 8:4:1 is to low.

 

[IJ.]

Don't forget "volume" guys....

10 Psi on a CT26 isn't the same as 10 Psi on a T04z when the valve opens.

 

[selfinfliction]

Don't forget "volume" guys....

10 Psi on a CT26 isn't the same as 10 Psi on a T04z when the valve opens.

forgive me if i'm being stupid here, but...

the only way i see that being a factor is that if there wasn't enough charged air volume waiting when the valve opens. like the bigger turbo has more volume, so it can keep a constant 10 psi when the valve opens, just form the physiscs of the turbo, but when the valve opens on the ct26, it can't move the volume a t04z does, so it may drop down for a split second while the chamber fills up.

on the otherhand, couldn't that smaller turbos lack of volume be compensated for by using a larger charge plenum?

(i don't pretend nor claim to have anything other than elementary turbo knowledge)

 

[D-Dayve]

Don't forget "volume" guys....

10 Psi on a CT26 isn't the same as 10 Psi on a T04z when the valve opens.

I've had this argument with sooooo many people!!

10psi is 10psi, no matter what turbo you have AS LONG AS YOU SPECIFY AN RPM.

If I make 14 psi on my DSM, but the turbo runs out of poop and has me back at 8 psi by redline, that's one thing. If I put a larger turbo on that still runs 14 psi, but holds 14 psi all the way to redline, that's a faster car for sure.

If a larger turbo makes 14 psi, it's still 14 psi, but if it holds that 14 psi to redline, the car will be faster WHERE THE FIRST SMALLER TURBO RAN OUT OF AIR. If they both made 14 psi at 4500 RPM, they'd make the same horsepower at 4500 RPM.

So, you have to classify that if a turbo is going to hold 10 psi to redline, it doesn't matter what the CFM is, as long as you're comparing it to another turbo making 10 psi at redline.

If you have two turbos that hold 14 psi to redline, they're going to make the same horsepower at redline, it's just the spooling characteristics that are going to make one feel faster.

In theory, the best turbo you can choose would be the turbo that would run out of air at the boost you want to run, right at your redline. If you're not using the full potential of your turbo, then you're wasting time spooling a big turbo. If you run out of boost before redline, you have more horsepower available that you can't supply air to.

 

[Adjuster]

The funny thing about theory.

It's often trumped by the real world results.

IE: The same pressure, at the same RPM, but the higher flow turbo on the same engine makes more REAL power. (Not theory power mind you, but real power.)

I'm sure it has to do with all kinds of technical reasons, but the bottem line is, IJ is right.

18psi on a CT26 is nothing like 18psi on a T04Z. (As an example.)

The CT is mainly blowing very hot air at that pressure, flow is minimal over say 15psi... But the larger turbo is just starting to get busy, and it's flow rate at the same pressure blows the smaller turbo out of the water.

Think water hose.

You put your thumb over the outlet, and the pressure goes up... Water squirts out.. And you can hold "18psi" of water in there all day long, but less water is coming out...

Now, try that on a fire hydrant supplied 3" hose v/s the 3/4" garden hose... At 18psi, the fire hose will kick you on your ass, and can knock people down...

So, it's not just about pressure, but about flow.

 

[IJ.]

Boost is only made when there's a restriction to flow......

Open a valve and boost will drop, in this time available a larger Turbo can supply more Volume and the end result if you add the correct amount of fuel and manage to light it off at the correct time is more power.

So YES 10 psi is 10 psi but that's NOT the full story or we'd all still be on a CT26 as it can make 10 psi....

As I said earlier it's all about Volume/Mass supplied at 10 Psi.

 

[JimR]

I'd like to add a different angle here, which also demonstrates why 10psi on the ct-26 is not the same as 10psi on a larger turbo.

It is the mass of the air entering the engine that determines the amount of fuel that can be added, not the pressure of the air. The same volume of air at 50 degrees actually has far more mass/density than at 300 degrees. More mass = more oxygen atoms = more power.

The larger turbo can supply that 10psi while spinning at a much slower speed than the ct-26, which does not heat the air as much. That contributes to the higher air mass that is actually entering the engine, even though you still see 10psi on your pressure gauge.

Rebuttals/corrections welcome.

 

[Nick M]

I've had this argument with sooooo many people!!

10psi is 10psi, no matter what turbo you have AS LONG AS YOU SPECIFY AN RPM.

And if you argued with all those other people, you were wrong with everyone of them. PSI is not a measure of volume.

If you try and put more of something in a confined space, "pressure" will go up. That is because pressure at a basic level is resistance to flow. And it won't flow as well. That is where many think PSI is equal to flow, when it isn't.

 

[Buddafucco]

Compression ratio! My thread is being jacked!:3d_frown: I have DEfence! :2ar15:
With all this talk about turbo flow & pressure, how can I figure out how much I can compress those numbers in my cylinder volume?

 

[Clueless]

I've thought about 9.1 CR with a gt35r....

 

[F&H Performance]

I'm doing 9.5 with a to4z on a current project.

 

[Adjuster]

Sorry Budda, but the PSI argument reared up, and mass/flow/volume needed to be defended!

Now, as a practical matter, I have really enjoyed the 9:1CR 7M stroker engine. It has plenty of low end power around town, and still seems to take very well to high boost levels with no detonation or ping. (Keep in mind, everything in my engine is coated, smoothed and was assembled with an eye on removing any sharp edges that can foster detonation hot spots... Also the head is ported, and the combustion chamber smoothed out, so the "designed" ratio of 9:1 might be slightly reduced, but then again, we shaved the block twice to remove some scratches that accidently got put in the gasket surface... and the head has been shaved a few times... I don't have any numbers, but I've used the same 1.3mm Greddy metal head gasket now on 4 motors... So much for the idea that brake quiet does not work to seal them up eh? Just don't use brake quiet on "new" rubber coated gaskets, and you should be fine.)

I run 91 or 93 octane gas, depending on what's available. I also have added in up to 30% Toulene, which is 114 octane, and brings the whole mix up to about an octane rating of 98 for the tank. (This is only when I'm going to push higher boost levels, say 28 psi off my old bolt on T4, 60-1. Which BTW, runs out of flow just like a CT-26 does on a stock motor over 5500rpm on my built motor..)

The reality is that the engine around town was fine on pump high octane gas and 15 to 20 psi of boost. Any more than 20 anyway, and it's all just tire spin in the lower 3 gears... So this really is a pretty funny conversation.. And I run 295 wide rear tires, so I suppose, short of some 315's. traction limited is really a reality with this type of power, and a vehicle that is not AWD.

I have not run my larger turbo build up yet. LOL Bought a naked bike, and I've been spending my time riding and modifiying it.. (Nothing like sitting on about 150hp at the wheel of a 430lb bike... The power to weight ratio is hard to beat. But the Supra has been calling lately, and I've got everything back together, and the Maft Pro almost complete, so the T70 Masterpower and "Chinabay" manifold will be put to the test with some 780cc Delphi injectors very soon...)

Now, back to your question.. With the right fuel, or amounts of fuel, you can run high static CR, and have a very responsive engine at low speeds, off boost, and still get a great top end rush.. but the compromise is always going to be how much air and fuel you can ultimately stuff into the combustion chamber, and it's always going to be more in the lower static CR engine. Variable displacement engines are not reality yet, and the 7M is pretty old school on many things, so some conservative planning here will make for a motor that will last longer depending on your goals.

Again, personally, I think the best "Street" combination is a static CR of about 9:1, and a medium sized turbo that will supply up to 30psi of boost at high flow rates, yet still spool up early into the RPM range of the motor. (at say 3000rpm the turbo needs to be providing 10 to 15psi or more, otherwise, the motor will feel lazy, or peaky.. not my style.)

Then again, the bike I have makes most of the power on top, and while it's making more power than most Hardly's by 5k, it really wakes you up when 9k is on the tach... (Hold on... 12k is just a second or less away.. and so is 150mph. The dang bike goes over 90 in FIRST GEAR! LOL)

I still love my Supra however..

 

[suprahero]

^He took the words right out of my mouth..........he also did away with my silly accent and made it sound like a well written novel.............now I'm sure of it....I hate him.

 

[JZ_killa_t68]

I built my 7m with .060 over pistons, and 8:1 CR. at first I had a t04e 60 trim, and it was laggy-60ft times of 2.2 seconds +, with trap speeds of 115ish at 12.6 seconds. I replaced the turbo with a t68, dual ceramic bb, q-trim, .84 a/r. stock head. I had 1 bar boost at 3500 rpm (paid 2600 for the turbo), and it would pull all the way to 30 psi. the down side is I'd lose traction in at the high rpms (even in 4th). but was fun. Never had any dynos or 1/4's with this turbo though.

cliffnotes:

lower compression sucks without a kickass turbo
go low for high psi
look at your needs my choice now is faster spool vice high boost like in the past.
Get a kickass turbo, and it doesn't really matter anymore
sell your arm and left nut for said turbo