Welcome to the North American Subaru Impreza Owners Club Sunday September 14, 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 > Built Motor Discussion

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 03-07-2009, 03:26 PM   #1
EGY5
Scooby Specialist
 
Member#: 165206
Join Date: Nov 2007
Chapter/Region: MAIC
Location: Columbia, MD
Vehicle:
2006 WRB DPSti
w/ white shoes

Default n00b cams and piston questions

I've had these questions for a while, but I've never really gotten a definitive answer by lurking here or searching. I know some of these will be easy questions but wasn't sure where else to get the info

1) When people do drop ins, how often are they getting higher CR pistons? I know enough to know that theres a whole lot that goes into a static or dynamic CR, but I wasn't sure what the pros/cons/requirements to using higher CR pistons were.

2) Why are bigger cams considered more aggressive? because they sacrifice more bottom end for top end?

3) what the eff is loping?

4) what should you be looking for when trying to match cams for a certain set up?

Note that I am no where close to needed or wanting cams on my car, I am just curious about the stuff.
* Registered users of the site do not see these ads.
EGY5 is offline   Reply With Quote
Old 03-07-2009, 03:34 PM   #2
mystilexzero
Scooby Specialist
 
Member#: 154674
Join Date: Jul 2007
Chapter/Region: BAIC
Location: Nor cal
Vehicle:
2007 wrx
ugm

Default

im also looking the anwser to drop in pistons with change in CR, for example CP pistons and Wiseco pistons are good choices, however if you have a bigger turbo such as a fpgreen which would you choose? CP pistons have a CR of 8:2 and Wiseco has a CR of 8:9.
mystilexzero is offline   Reply With Quote
Old 03-07-2009, 10:27 PM   #3
SlickRick51586
Scooby Newbie
 
Member#: 66735
Join Date: Jul 2004
Chapter/Region: South East
Location: Parkland, Florida.
Vehicle:
2004 WRX
PSM

Default

Quote:
Originally Posted by EGY5 View Post
I've had these questions for a while, but I've never really gotten a definitive answer by lurking here or searching. I know some of these will be easy questions but wasn't sure where else to get the info

1) When people do drop ins, how often are they getting higher CR pistons? I know enough to know that theres a whole lot that goes into a static or dynamic CR, but I wasn't sure what the pros/cons/requirements to using higher CR pistons were.

2) Why are bigger cams considered more aggressive? because they sacrifice more bottom end for top end?

3) what the eff is loping?

4) what should you be looking for when trying to match cams for a certain set up?

Note that I am no where close to needed or wanting cams on my car, I am just curious about the stuff.
(I'm by no means an expert on this, just trying to point you in the right direction)

1) A higher Compression Ratio in reference to just the piston means it has more "height", or distance from the wrist pin to the piston surface. (taking into account piston dish). Therefore, there is less area between the piston face and Top Dead Center (TDC), which increases your CR. Which really dosen't do much with just a small bump that a forged replacement would provide. Pistons aren't really where you should be deciding your CR imo... If they're drop in, just go with the brand with a better reputation. If your building your whole engine, it depends on what your trying to do.


2/3) I wouldn't say big cam's are more aggressive, 272's won't do somone with a TD04 much good. People use the term aggressive to say that there are cams with a greater duration and lift that would make more power for your setup. The noise from Cam Lope is caused byValve overlap, which in turn is from aftermarket cams having a larger duration with both intake and exhaust valves open.

4) what rpm range your looking to make power in, how much air your trying to shove in there, and if its a DD or track car.
It's actually pretty complicated. You could do all kind's of calcuations to figure out in theory what cams you would need.

Last edited by SlickRick51586; 03-07-2009 at 10:59 PM.
SlickRick51586 is offline   Reply With Quote
Old 03-09-2009, 08:54 AM   #4
EGY5
Scooby Specialist
 
Member#: 165206
Join Date: Nov 2007
Chapter/Region: MAIC
Location: Columbia, MD
Vehicle:
2006 WRB DPSti
w/ white shoes

Default

Quote:
1) A higher Compression Ratio in reference to just the piston means it has more "height", or distance from the wrist pin to the piston surface. (taking into account piston dish). Therefore, there is less area between the piston face and Top Dead Center (TDC), which increases your CR. Which really dosen't do much with just a small bump that a forged replacement would provide. Pistons aren't really where you should be deciding your CR imo... If they're drop in, just go with the brand with a better reputation. If your building your whole engine, it depends on what your trying to do.
So if you are doing drop ins, theres absolutely no reason you should be entertaining the idea of higher CR pistons?

Quote:
4) what rpm range your looking to make power in, how much air your trying to shove in there, and if its a DD or track car.
It's actually pretty complicated. You could do all kind's of calcuations to figure out in theory what cams you would need.
Is there any way you could elaborate more? I'm not looking for the complete and total solution, but to be honest, I don't know what 260/260 will do to my powerband vs 272/272 or 278/274
EGY5 is offline   Reply With Quote
Old 03-09-2009, 01:09 PM   #5
SlickRick51586
Scooby Newbie
 
Member#: 66735
Join Date: Jul 2004
Chapter/Region: South East
Location: Parkland, Florida.
Vehicle:
2004 WRX
PSM

Default

Quote:
Originally Posted by EGY5 View Post
So if you are doing drop ins, theres absolutely no reason you should be entertaining the idea of higher CR pistons?
The simple anwser is a large increase in compression with a turbo car means you see peak boost sooner and better throttle response, but might not be able to make as much boost because your higher compression engine heats the air in the clyinder more than the low compression engine.

Forged replacement pistons provide only a nominal bump or drop in compression because the piston has to be within certain tolerances it can't be too heavy, tall, or short (skirt length) over stock. If you throw in different crankshaft, length of rods, etc... the tolerances change.


I will leave it at that, I could go on...

Quote:
Originally Posted by EGY5 View Post
Is there any way you could elaborate more? I'm not looking for the complete and total solution, but to be honest, I don't know what 260/260 will do to my powerband vs 272/272 or 278/274
I never really bothered to crunch the numbers, someone else might chime in... You can go by what other people put in their builds or with what your engine builder/tuner suggests for your goals.


head to a library/b&n/Borders etc etc and read some car/engine books. I'm sure they explain things a lot better than I.
SlickRick51586 is offline   Reply With Quote
Old 03-09-2009, 04:44 PM   #6
ScorpionT
Scooby Newbie
 
Member#: 195677
Join Date: Nov 2008
Location: Midwest
Default

Quote:
Originally Posted by SlickRick51586 View Post
The simple anwser is a large increase in compression with a turbo car means you see peak boost sooner and better throttle response, but might not be able to make as much boost because your higher compression engine heats the air in the clyinder more than the low compression engine.

Forged replacement pistons provide only a nominal bump or drop in compression because the piston has to be within certain tolerances it can't be too heavy, tall, or short (skirt length) over stock. If you throw in different crankshaft, length of rods, etc... the tolerances change.
Well said.
ScorpionT is offline   Reply With Quote
Old 03-09-2009, 05:25 PM   #7
flycaster
Scooby Specialist
 
Member#: 60142
Join Date: Apr 2004
Location: Bend, OR
Vehicle:
2011 WRX
Grey

Default

1) You want to run the OEM compression - higher compression does have benefits, but it also has diminishing returns as well as being harder to tune right.

2) Generally, the higher the lift and the longer the duration, the more it shifts the powerband to the right. "Aggressive" is just a term used to describe this shift.

3) Cam lope is another descriptive term used to explain how "lumpy" or irregular the idle is. The more aggressive the cams, the more irregular the idle. With very aggressive cams, the idle rpm has to be raised significantly otherwise the engine will die at idle.

3) Cam selection is just another part in an overall power package. In many [most?] cases, unless you are building up a motor for BIG power (500+), the OEM cams will do a very adequate job of fuel/air delivery. Moreover, good aftermarket cams are not cheap, and there are a lot of lower cost mods that can be done that will give you far more bang for the buck - the cheapest, of course, is to simply crank up the target boost and tune the motor appropriately for it. IMHO, changing cams is part of a BIG DOG project.
flycaster is offline   Reply With Quote
Old 03-09-2009, 05:31 PM   #8
kellygnsd
Scooby Guru
 
Member#: 32669
Join Date: Feb 2003
Chapter/Region: SCIC
Location: Rancho C
Vehicle:
2007 2.34LR, EFR7670
LINK G4 hybrid STi

Default

Quote:
Originally Posted by SlickRick51586 View Post
(I'm by no means an expert on this, just trying to point you in the right direction)

1) A higher Compression Ratio in reference to just the piston means it has more "height", or distance from the wrist pin to the piston surface. (taking into account piston dish). Therefore, there is less area between the piston face and Top Dead Center (TDC), which increases your CR. Which really dosen't do much with just a small bump that a forged replacement would provide. Pistons aren't really where you should be deciding your CR imo... If they're drop in, just go with the brand with a better reputation. If your building your whole engine, it depends on what your trying to do.
Horribly wrong info. Internals are the best and most dependable way to get you desired CR and by calling any piston manufacturer you can get them change the dish to achieve whatever ratio you want... The right way.

Also, a .5 increase in CR is a noticable change from a tuning standpoint.
kellygnsd is offline   Reply With Quote
Old 03-09-2009, 09:47 PM   #9
SlickRick51586
Scooby Newbie
 
Member#: 66735
Join Date: Jul 2004
Chapter/Region: South East
Location: Parkland, Florida.
Vehicle:
2004 WRX
PSM

Default

I was trying to say that its better to make up your CR with a combination of parts rather than just one part, so as to not make one part the weak link. I hope your not implying I'm a fan of the head gasket band-aid-fix.

Thanks for the reading comprehension skills and using your jump to conclusions mat tho.

Of course it could be that my statement was not clear... Psh... never...

Being a novice at romraider, I think I can go as for as to say a small bump in CR may make changes in tuning as far as your timing advance,target boost, and fueling, but you can still attain about the same max HP. It all depends on how far your push the turbo outside of its efficiency, and how hot the charge air gets because of this, cause we all hate pre ignition.

While I appreciate your constructive criticism and I hope you don't take my comments too harshly, but you didn't explain much further to justify your opinion, and possibly misinterpreted what I said. You may very well be right, I'm no expert, and maybe you are.

Last edited by SlickRick51586; 03-10-2009 at 12:22 AM.
SlickRick51586 is offline   Reply With Quote
Old 03-10-2009, 01:12 AM   #10
williaty
Scooby Guru
 
Member#: 71092
Join Date: Sep 2004
Chapter/Region: MWSOC
Location: Delaware County, Ohio
Vehicle:
2005 2.5RS Wagon
Regal Blue Pearl

Default

I disagree with several things that have been said so far.

Quote:
Originally Posted by EGY5 View Post
1) When people do drop ins, how often are they getting higher CR pistons? I know enough to know that theres a whole lot that goes into a static or dynamic CR, but I wasn't sure what the pros/cons/requirements to using higher CR pistons were.
The main places CR comes from is stroke (determined by crankshaft), head volume (not something we can cheaply change), and the shape of the top of the piston (which we can change readily). Pistons are the most effective place to change the compression ratio for mere mortals like you and me.

Increasing the compression ratio increases the thermal efficiency of the engine by moving it closer to the theoretical ideal of the Otto cycle. In other words, more of your fuel actually gets turned into speed as you raise the CR. However, increasing the CR also makes the engine more prone to knock, requiring less timing or less boost. So there's a bit of a trade off there. In general, within sane changes, you can pick up efficiency and torque (especially at less than full boost) with an increase in compression ratio so long as you can find high enough octane fuel to pull it off.

Now, here's something that ties into your second question. The dynamic compression ratio, which is what actually matters, is a product of your static compression ratio (the number we all quote) and when your intake valve closes. So when you change cams, you change your dynamic compression ratio. If the cam closes the intake valve later, and most do, your dynamic compression ratio has been lowered. This means that, just to get back to a stock dynamic compression ratio, you have to run higher-static-compression pistons.

Quote:
2) Why are bigger cams considered more aggressive? because they sacrifice more bottom end for top end?
They don't idle well, they don't pull off throttle well, they're harder to drive around town with, and they only pay off if you're wringing out the very top end of the rev range.

Quote:
3) what the eff is loping?
Ever heard a bigassed engine in a muscle car have problems idling? Sounds like not all the exhaust pulses are quite the same distance apart. Makes the guy have to prod the throttle to get a burst of revs to settle it down some. That's loping.

Quote:
4) what should you be looking for when trying to match cams for a certain set up?
Basically, you need to try to make a compromise where you get what you want on the top end but don't hurt the bottom end to the point that it's a PITA to drive.
williaty 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
Cam and block question dads_cruizzzer Built Motor Discussion 5 04-19-2010 05:46 PM
FS: Used Cp pistons, 04 sti stock cams, and perrin big maf short ram sti charles MWSOC Private Classifieds 22 07-07-2009 06:11 PM
fs: oem sti pistons, rods, cams and turbo dc_imprziv Engine/Power/Exhaust 31 12-09-2008 03:04 PM
ok final cam and sti question dlowman Factory 2.5L Turbo Powertrain 6 05-03-2005 12:35 PM
Cams and High CR pistons ? G.Subramaniam Normally Aspirated Powertrain 9 12-10-2004 05:13 PM


All times are GMT -4. The time now is 10:24 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.