low comp pistons?

[mjkspyder_one]

It's time I start looking for some new pistons for my 2.3, anyone know of a good place to pick up a set of pistons?

*edit* and yes, it's turbo, so i'll need forged pistons.

[740Weapon]

alot of people on this board suggest that lower compression pistons arent neccisary and some even use the higher compression N/A pistons on there turbo set up. also there is an article that includes machining 1mm off the tops of your existing pistons to lower compression somewhere on this web site. that might be the easyest way to lower compression. as im sure speacialty low compression volvo pistons dont come cheap.

[mjkspyder_one]

You're kidding me, you mean to tell me that I can use high compression pistons with my turbo?! What kind of power does that create?

[the poi]

Originally Posted by mjkspyder_one

You're kidding me, you mean to tell me that I can use high compression pistons with my turbo?! What kind of power does that create?

The best kind: more. With no real expreince whatsover, I kinda like 9.0:1 as a good number. Search the forums, you'll be reading for days on compression ratios, but you'll be a better man once finished.

[dbarton]

Originally Posted by the poi

The best kind: more. With no real expreince whatsover, I kinda like 9.0:1 as a good number. Search the forums, you'll be reading for days on compression ratios, but you'll be a better man once finished.

I would hold on to a little of that caution. I have found higher compression in a Volvo turbo to be a mixed bag with a nasty downside. My stroker motor is unique, so I suppose some of my issues might not repeat themselves in a more normal motor. But my (approx) 9.4 to 1 static compression has not produced the power crazed monster I had hoped. The reason is simple... it requires too much boost retard on pump gas (91 octane) to avoid detonation.

If you think I'm exagerating, consider this... I have my ignition map set at 33 degrees total timing with retard set as follows.... 15 degees retard at 3 PSI, 28 degrees at 10 PSI, and 33 degrees at 15 PSI. And I still get a few ticks between 0 and 7 PSI on a hot day.

I'll be pulling my motor in a few months (after we get our Proturbo headers) and maybe another cause will be found, but until then I would consider caution.
Dave B.

[linuxman51]

with the gas the way it is out here, i can run the same timing on my low comp turbo motor as i can on my high comp "hybrid" setup (+- maaaby 3 degrees). same turbo and efi system, and the hi comp car had a wilder cam in it (k vs t). the high compression engine makes more torque but seems to fall off a little more on the top end(well at least thats what my dynochart says. roto's read a little better, but these were both done on bone stock engines with unported manifolds and turbos, stock intercooler, etc). now that we've got a local dyno i suspect that better testing and more firm numbers will be available, but to serve as a comparision:
my 87 740gle at 8psi, running turbo lh 2.2 and the stock M cam pulled away from my 87 760 turbo running 12-14psi, T cam (Also lh 2.2). both cars had performance air filters and mostly stock exhaust systems (deleted second muffler, hollow cat). the aforementioned 760 had previously trapped 92mph in the 1/4 a few weeks prior to this race, and was in the same state of tune as the 740.

argue what you will, *everyone* that turbos a hi comp motor instantly falls in love with them, I personally wont be investing in another stock turbo brick, at least not of the iron block vintage.

[RR]

Anybody remember the original question?
I bought my forged mahles from a place in Conn. called AmBoss. Mine were 2nd oversized for my b23ft because I needed them. 85.55 ea including rings. Shipping from Germany was around 50 bucks. Good luck.
RR

[Captain Bondo]

Something must be off, Dave, I've helped on a couple motors in that range without the effects you mention. What rev range are we talking about? Seems like your boost retard is super agressive- aggressive enough to make me question the ignition curve itself.

Amboss is awesome indeed as far as suppliers go. Just bought some pistons from them.

Cappy, Dave B's stroker motor is running a squish clearance of about 0.065in. We discussed this over a year ago in the "240TI Boost Restard" topic. If Dave has not changed the squish clearance since that discussion, then the detonation problem will not be easily resolved.

Static CR is not the main causer or preventer of detonation. The lack of squish clearance, squish area, and squish action is the main cause of the detonation problem. And yes, the questionable quality of the fuel available in california is also a factor.

Low static CR will not necessarily prevent detonation; nor will high static CR necessarily cause detonation. It is a squish issue.

Fuel management and ignition timing management do affect the detonation potential; but as often as not, fuel and ignition have to be played with to compensate for poor squish; regardless of the static CR. ie, if the squish is not right, then the fuel and ignition have to be fiddled with extensively to try to compensate for the deficiency in squish.

To get into the effective squish zone, the squish clearance has to be less than about 0.045in. 0.040in is better; and 0.035in is very good.

Sixty five thou ain't even in the ballpark. No offence intended, Dave, but that is the case. It is not a static CR problem. It is a 'no squish' problem.

****

as to the original posted question: what motor are you talking about that needs a set of pistons? B23FT or B230FT? the 23FTs have forged pistons; the 230FTs never did. The details of which has been covered extensively in the past on the forum.

Thomas Fritz
...the stealth FTi

[Captain Bondo]

Ah. That'd probably do it. Thanks stealthy. I had the squish issue in the back of my head but for some reason I thought he'd re-done his headgasket setup since then.

[the poi]

So stealth, to much squish clearnece will increase the amount of detonation? Is this because of the high squish's "cooling" effect?

poi,

tight squish promotes fastburn. Fast burn prevents detonation.
do a forum search for 'squish' and 'swirl' and 'fast burn' and 'fastburn'. you will find all the places where I have gotten into the discussion rather intently. a quick answer is YES; the boundary layer proximity effect improves heat transfer away from the pistons; and squish under 0.040in overlaps the boundary layer.....a very good thing.

my second tigher squish motor is done and on the road....a 90 765T with a 93 L block B230FT with ET pistons at 0.035 squish and an A cam. I did my 8 hour run in procedure, and 15A is enjoying it now. and I am slowly getting the pics edited and up at pbase.

After he gets a couple of thou more miles on it, and when finally on full syn motor oil [either mobil1 15w-50 or castrol syntec 20w-50], we will be doing some testing to see how well the motor performs on octane less than 92. one of my goals is to have ping free performance on 89 octane. but we will see. that wagon runs sweet. and there are bolt on upgrades to be done; ended up just porting the mitsu 13C. and the Acam overpowers that little thing very quickly. we will need to go Garrett .63/.60 or super 60.

TF

[dbarton]

Originally Posted by stealthfti

Sixty five thou ain't even in the ballpark. No offence intended, Dave, but that is the case. It is not a static CR problem. It is a 'no squish' problem.
Thomas Fritz
...the stealth FTi

No offense taken. I know the squish is excessive in that motor too. It's not really .065" though... my old calculations were based on an incorrect compressed gasket thickness. It's actually closer to .056", which is closer, but I know is still too much. I have some other questions on squish, but so this thread isn't cluttered, I'll post a new one HERE
Dave B.