Welcome to the North American Subaru Impreza Owners Club Wednesday April 16, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
NASIOC
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Normally Aspirated Powertrain

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 03-13-2001, 11:02 AM   #1
fellfrosch7
Scooby Specialist
 
Member#: 2389
Join Date: Sep 2000
Location: Las Vegas, NV
Vehicle:
'98 Impreza L Wagon
'90 Talon TSi AWD

Post Compression ratio question

OK, here's a theoretical question... two same exact RS's, 2.5L engine, same turbo kit, exhaust, etc. One has the stock pistons, with stock compression, the other has aftermarket (such as Rallispec's) pistons with 8.7:1 compression ratio. What difference does the compression ratio make? From what I've read, the lower compression ratio engine will be more reliable due to less strain on the engine. How much more reliable are we talking about, rough figures? Mileages? And with better pistons, is it possible to safely raise the rev limit as well? Thanks all.

EDIT: I found one post I missed before, and if I read it correctly, a high compression/low boost engine will make more low end power and less high end power, and low compression/high boost will make top end power at a cost to the low end, correct?

[This message has been edited by fellfrosch7 (edited March 13, 2001).]
* Registered users of the site do not see these ads.
fellfrosch7 is offline   Reply With Quote
Old 03-13-2001, 11:12 AM   #2
8Complex

Moderator
 
Member#: 922
Join Date: Feb 2000
Chapter/Region: MWSOC
Location: Schaumburg, IL
Vehicle:
04 FXT
Red

Post

You'll get less bottom-end power with lower compression. The lower compression really helps in preventing detonation, which blows up motors quick.

Better pistons will not do much for you as the limiting factor on our engines with the rev limiter is the valvetrain. Over 6700 stuff starts to float, and I'm sure even below that when you boost the engine since the springs wouldn't be strong enough to hold the valves shut with the pressure coming from behind all the time.
8Complex is offline   Reply With Quote
Old 03-13-2001, 11:23 AM   #3
ColinL
Scooby Guru
 
Member#: 114
Join Date: Jul 1999
Chapter/Region: MWSOC
Location: Wichita, KS and Whoring, OT
Vehicle:
'03 Evo, Rice White
'01 Erion CBR 929

Post

Valve float is closer to 7200 rpm, and intake manifold pressure over some 10 psi or so is NOT going to hold open an intake valve when the cylinder is seeing 175-200 psi.
ColinL is offline   Reply With Quote
Old 03-13-2001, 11:36 AM   #4
scoobiejosh
Scooby Specialist
 
Member#: 3139
Join Date: Dec 2000
Chapter/Region: RMIC
Location: SLC Utah
Vehicle:
2001 Dodge Pikumup
& 08 Triumph Speed Triple

Post

ColinL: Cobb said valve float does hapen somewhere around 7-10 PSI. Once the valve shuts all the way, then that pressure in the cylinder will keep it shut. But there isnt 175-200 PSI till the valve is closed, and the piston has gone a little past the bottom of the intake stroke and has begun the compression stroke. The boost on the back of the valve keeps the valve open for longer thus letting out some of the gasses in the chamber that would normaly be left in there to compress and combust.

Josh

[This message has been edited by scoobiejosh (edited March 13, 2001).]
scoobiejosh is offline   Reply With Quote
Old 03-13-2001, 11:53 AM   #5
svxtrem
Scooby Newbie
 
Member#: 1008
Join Date: Mar 2000
Location: Grand Forks, ND
Vehicle:
2006 STI
Steel Grey Metallic

Post

Compression ratio is an indicator of thermal efficiency. Given two identical engines, and proper fuel, the one with the higher compression ratio will have more power and better gas mileage. Higher compression ratios raise the cylinder pressure, so if you add a turbo, the cylinder pressure can be raised enough to cause detonation with a given fuel. It's not true that you can't run a high compression ratio with a turbo. You just need better fuel, or water injection to make it work.
svxtrem is offline   Reply With Quote
Old 03-13-2001, 04:06 PM   #6
Kevin Thomas
Street Racing Instructor
Moderator
 
Member#: 110
Join Date: Jul 1999
Location: 1997 OBS, 1996 SVX, 1988 RX
Vehicle:
1989 1989 XT6

Post

How much more reliable are we talking about, rough figures? Mileages?

There is no definite answer to this. Some engines are built better than others even if they come off the assembly line the same day. Tuning will play a part in this too. A poorly tuned low compression engine will not out last a properly tuned higher compressioned turbo engine and vice versa.

I think your point is that if two turbo'd engines were equally 'properly' tuned which one will outlast the other or be more reliable. The answer is whatever engine is less stressed will be the most reliable of the two. I take it the engine with the most compression of the two identical engines will be the least reliable. It probably won't have the same engine life as the lower compression one, even if they both run well over 200K.


No-one can tell you what an engines lifespan will be. If so, I'd like to know how long my poor 2.2L is going to last. Now that's stressed. Hehe!


98mpreziveRS,

While we are on the subject of lowering your compression, do you know of a manufacturer/company who makes lower compression head gaskets for Subarus? Just curious!

[This message has been edited by Kevin Thomas (edited March 13, 2001).]
Kevin Thomas is offline   Reply With Quote
Old 03-13-2001, 04:16 PM   #7
98mpreziveRS
Scooby Specialist
 
Member#: 972
Join Date: Feb 2000
Post

none so far i am aware of. I had some custom made for me..but they did cost a pretty penny
Try giving Cobb a call.. he may be able to make some for a lower cost I DO have mucho faith in Trey Cobb.. pity i sold my motor before his stage 2 cams came out..and pity he didn't have the DOHC cams either.. *srug*

anyhow..let me ask around to some more people and see if they know of any, either here or in Japan, or Aus... i know too many people
98mpreziveRS is offline   Reply With Quote
Old 03-13-2001, 07:13 PM   #8
fellfrosch7
Scooby Specialist
 
Member#: 2389
Join Date: Sep 2000
Location: Las Vegas, NV
Vehicle:
'98 Impreza L Wagon
'90 Talon TSi AWD

Post

Thanks for the great responses, everyone... Like I mentioned in my example, I may be using Rallispec's 8.7:1 pistons when I get a turbo. I definitely won't be running more than 10psi, if that. I was thinking more like 7-8 most of the time, occasionally cranking it to 9. Should 8.7:1 be good enough for those psi levels, or should I look elsewhere? The turbo kit I'm considering getting is (surprise, surprise) Rallispec's. If they ever finish it, that is. It looks like a very complete kit, but I'm researching the other additions that will make it better/safer/more reliable.
fellfrosch7 is offline   Reply With Quote
Old 03-14-2001, 12:09 AM   #9
ASR
Scooby Newbie
 
Member#: 1945
Join Date: Jul 2000
Location: CA
Post

I can supply you with stiffer valve springs, titanium retainers, etc so you can raise your rev limit, and be able to run high boost, safely without floating your valves. I also have stainless steel valves available. Contact me if you are interested.
949 412 4ASR.
ASR is offline   Reply With Quote
Old 03-14-2001, 12:27 AM   #10
Michael
Scooby Specialist
 
Member#: 989
Join Date: Feb 2000
Location: New York
Vehicle:
99 RS Turbo
Graphite Gray Pearl

Post

like everyone mention above..
lower compression you will loose torque on the low end.
the advantage of a lower compression turbo car is that there will be more space for the air to flow so your engine won't go ka-boom.
with lower compression you can run higher boost.
the ideal is to have a high compression turbo engine... but we all know that is impossible.
hope this helps
michael
Michael is offline   Reply With Quote
Old 03-14-2001, 12:31 AM   #11
scoobiejosh
Scooby Specialist
 
Member#: 3139
Join Date: Dec 2000
Chapter/Region: RMIC
Location: SLC Utah
Vehicle:
2001 Dodge Pikumup
& 08 Triumph Speed Triple

Post

It isnt impossible, just not praticle. High boost+high compression has been done, just on a race gas only car.

Josh
scoobiejosh is offline   Reply With Quote
Old 03-14-2001, 12:35 AM   #12
strap 1
Scooby Newbie
 
Member#: 3478
Join Date: Jan 2001
Location: Woodburn, Oregon
Vehicle:
2001 2.5RS
black diamond pearl

Post

ASR do you have a webpage because Iam going to start building a N/A monster and Iam kinda intrested in the springs and retainers.

thanks
strap
strap 1 is offline   Reply With Quote
Old 03-14-2001, 12:43 AM   #13
98mpreziveRS
Scooby Specialist
 
Member#: 972
Join Date: Feb 2000
Post

http://www.i-club.com/ubb-files/Foru...ML/001980.html
read that..it should help to answer it..on to your specific question though

If you run high boost levels on a high compression engine, a couple things are going to be happening and need to be done..
One is the chance for preigition or detonation increases dramatically due to the volatility of the new mixture entering the cylinders. One more is you will have to run a better fuel.. either 100 octane unleaded or build your motor to use some good old american leaded 110 octane race fuel
If you have octane questions..see this http://www.i-club.com/ubb-files/Foru...ML/001937.html

now..what you CAN do to help this high boost/high compression out is either run a cam that is custom ground to keep the valves open on the exhuast side open longer as well as close the intake valve at about the same rate. What this will do is allow a lower compression due to the increase in volume if you will of the cylinder..due to the valves bleeding off some of the mixture/pressure..OR you could have them close FASTER to give you a lower compression becuase of the fact that you will have MORE room in the cylinder when the valves are fully seated as the piston starts the compression stroke, allowing it to obtain a SLIGHTLY smaller compression ratio..not much smaller..just slightly smaller (fraction of a point)
Also you WILL need to retard the timing immensly just to prevent det. and pre ig. which will create a loss of power..albeit safer power..but a loss of power while being safer.

If you plan on high boost i would say a 7.5:1 or 8:1 CR would be great for the scooby motor. you would loose some low-end grunt, but it wouldn't be anything that you couldn't gain back with some other tinkering and custom parts (intake manifold, etc)

but then again..this will cost more $$$ but be a safer solution and allow you to run pump gas.. and i will shut up now..becuase i probably lost a good portion of ya and the main reason is because that is what is getting covered by me tonight.. cams, heads, and some into turbos

typed some up yesterday..got carried away, so it will be done tonight

and on that..i will shut up
98mpreziveRS is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
compression ratio question rpm_nick Built Motor Discussion 9 09-09-2007 01:14 AM
compression ratio question petawabit Built Motor Discussion 6 03-03-2006 10:26 PM
Compression ratio question. AlbanianImpreza Engine Management & Tuning 13 01-05-2005 02:32 PM
EJ22-T Swap - Compression Ratio Question Andursun0013 Subaru Conversions 4 03-03-2004 10:04 PM
Question about turbos and compression ratios. cRayZee Factory 2.0L Turbo Powertrain 5 08-04-2001 03:26 PM


All times are GMT -4. The time now is 11:04 PM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.