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Revhead Kev Profile
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Date Registered: 10-2006
Location: Mona Vale, Sydney, AUSTRALIA
TOTAL POSTS: 6355
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Upgraded Compression - 11:1 Pistons


Hi all,

Just saw this while trolling around the net and wondered, especially from RedDragon, what this upgrade kit might do for our Xtrails.
11:1 Pistons QR25DE


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Kev X450(c) T30 Guru
03 Titanium Ti T30 Series 1 **MODIFIED**
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Eggins Profile
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Date Registered: 08-2004
Location: Brisbane
TOTAL POSTS: 184
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Re: Upgraded Compression - 11:1 Pistons


It'd be good if you want to run 110 octane fuel all the time, or the engine would be running with the timing so retarded there'd be no advantage. And in fact it'd probably drive you nuts trying to find fuel that works.
I know I have enough trouble now getting consistant fuel quality...

Definitly the way to go for racing though!

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Shawn and Belinda Eggins
2003 STR, K&N, Strut brace, Ground wires, Dunlop LM702's, Tint, Mud flaps, Tow, Nudge, Spots, Mats, Rear scuff plate
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30/Mar/2008, 8:43 am Link to this post Send Email PM   Send Private Message
 
basshead Profile
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Date Registered: 02-2004
Location: Panania, Sydney
TOTAL POSTS: 2852
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Re: Upgraded Compression - 11:1 Pistons


Yeah, you'd probably need to keep adding your own octane booster every tank-fill. I used Toluene before in my previous car (I am sure I've mentioned it on the forums before) and that made a noticeable and dyno-proven result, although the measurement/ratio was not very accurate emoticon

If needed, I could try and find an old email (or contact my brother) to find out where he got it from.


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Rich. X-013(c)
'04 S2 ST Auto. Nudge_roofracks_tow_tints_Lightforce_GME_YokoGeoA/T-S_TBS_bashplate_50mm-spacer-lift
Click here to see >300MB of photos and videos All For His Glory.
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Revhead Kev Profile
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Date Registered: 10-2006
Location: Mona Vale, Sydney, AUSTRALIA
TOTAL POSTS: 6355
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Re: Upgraded Compression - 11:1 Pistons


So, for a person like me who knows what increased compression does for an engine, but not the relationship between octane rating and compression, what would be the effects of running higher compression without increasing the octane ?

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Kev X450(c) T30 Guru
03 Titanium Ti T30 Series 1 **MODIFIED**
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30/Mar/2008, 7:49 pm Link to this post Send Email PM   Send Private Message Blog
 
basshead Profile
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Date Registered: 02-2004
Location: Panania, Sydney
TOTAL POSTS: 2852
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Re: Upgraded Compression - 11:1 Pistons


Kev, it would be likely that an aftermarket ECU would also be required as I don't imagine the stock ECU would be able to cope with the increase in compression. Our recently joined member (Kevin) from the US might be able to confirm this.

As Shawn mentioned, without upping the octane rating, the ECU would be trying to retard the timing (with input from the knock-sensor) to eliminate "pinging" or "pre-detonation", which if left to continue (say the factory ECU can't retard the timing enough) would damage the engine internals (the term "piston slap" comes to mind) - essentially when the explosion in the cylinder occurs, the piston is still on it's way up, or not quite ready for the downward stroke, thus it would be fighting against the explosion during the combustion process.

I also had a shaved head (which results in increased compression) in my previous car; the more you do to the engine, the more critical it is that it's tuned properly, and always requires higher octane fuel. I ran regular unleaded occasionally (when there was no premium available when I had to fill up) and only ever put $10 in unless I was on a long trip and didn't have a choice, and I noticed the difference in performance immediately. My aftermarket ECU was already programmed to run "rich", so it prevented damage occurring. In an ideal world, I'd have had it tuned to run more "lean" and performance would've increased, but with our petrol quality, it wasn't worth the risk of damaging the engine.


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Rich. X-013(c)
'04 S2 ST Auto. Nudge_roofracks_tow_tints_Lightforce_GME_YokoGeoA/T-S_TBS_bashplate_50mm-spacer-lift
Click here to see >300MB of photos and videos All For His Glory.
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Naff Profile
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Date Registered: 09-2005
Location: QLD
TOTAL POSTS: 786
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Re: Upgraded Compression - 11:1 Pistons


quote:

basshead wrote:

I also had a shaved head (which results in increased compression) in my previous car...



It might have gone faster if you shaved your legs as well!


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My Triton GLX-R

31/Mar/2008, 6:46 am Link to this post Send Email PM   Send Private Message MSN
 
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Date Registered: 06-2005
Location: Barossa Valley
TOTAL POSTS: 286
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Re: Upgraded Compression - 11:1 Pistons


High compression is OK for racing, but not suitable for 4WD driving where you mostly need torque at lower speeds.
11-1 compression would lead to burnt pistons without better fuel & timing changes

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Regards Dave, X162 Oct 04 Series 2 ST
Mods Strut bar_Fuel cap holder_earth wires_No door beep_Bonnet Struts_TBS_bash plate_16" Alloys_Rear sway bar_ F&R fogs
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31/Mar/2008, 11:28 pm Link to this post Send Email PM   Send Private Message
 
BlueMetal Profile
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Date Registered: 11-2008
Location: Sydney
TOTAL POSTS: 6
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Re: Upgraded Compression - 11:1 Pistons


quote:

Revhead Kev wrote:

... what this upgrade kit might do for our Xtrails.
11:1 Pistons QR25DE



Unless you want to undertake major surgey this mod would not be worthwhile by itself and would actually create more problems.

By increasing the compression ratio of an engine you are making the combustion mixture denser, so when it finally explodes, the explosion is hotter. This will lead to pinging.

Remember that the ECU has been programmed to deal with explosions within a certain (octane) range.

Doing this mod will puch this speed of explosion outside the normal operating range. the ECU will try to compensate by retarding the ignition timing, then riching up the mixture... expect the ECU to have a cow over this. This type of mod needs to be done with a stack of supporting mods to keep everything in balance.

However, did you know that by doing this sort of mod you run he risk of actually LOSING power? This compression may make the mixture explode so fast that the piston can outrun the explosion, a bit of an irony dont you think?

Last edited by BlueMetal, 24/Nov/2008, 11:26 pm


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