High compression pistons - pro's and cons ??

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budgie955
Mariana
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High compression pistons - pro's and cons ??

Post by budgie955 » Sun Dec 12, 2010 1:16 am

Hi All
I am rebuilding an 860 GTS engine which has the standard pistons in it. however in amongst the boxes of bits that came with the bike is a set of high compression pistons with rings in great nick.

what would be the benefit in fitting these instead of the standards ? And whats the downside? Can the crank take the extra oomph?

thanks

Dave Burn
sheffield UK

jockduck
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Post by jockduck » Mon Dec 13, 2010 2:01 am

I'm afraid I hate high comp pistons in a bevel twin, When I rebuilt my GT in 1985 I put the original 86mm AE in with new liners, has run great since even runs on 91 Octane without complaining.
I rebuilt my GTS with the High Comp wiseco pistons the engine came with(87mm) the pistons had quite a high dome and I believe they were around 10:1. After a few hundred miles of taking it easy I found I could not even use half throttle without it pinging like a bastard, I retarded the timing abit and put in 1mm thick base gaskets and with 98 octane I could just about do a full throttle run without pinging.
The cure has been to machine almost 3mm off the crowns and blend the shape C:R is now about 9.5:1 and the bike runs very nicely on 95 octane, I haven't tried it on 91 yet.
900SS's left the factory with 9.5:1 pistons, I think Ducati knew what was a practical street piston.
Jock

Ray O'Donohue
Parallel Twin
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Post by Ray O'Donohue » Mon Dec 13, 2010 10:41 am

The compression ratio as such is not the entire issue. WHATEVER ratio one winds up with,it should incorporate a reduced squish band clearance,which is easily achieved by removing the liners and machining the base gasket surface. If you have squish band clearances greater than about .080,they are non-functioning,and you will have pinging or ignition advance requirements that keep you from using higher compression ratios.This set-up will also allow you to stick with a stock or "Ducati spec/dimensions" piston of low,medium or high compresssion,without the extra height/length above the wrist pin,which causes so many of those those rocking/ring sealing issues.I ran 750 Sport pistons with a squish clearance of about .025,and never had detonation issues,even with a very lean mixture. THat is too tight,they say,but this was a racing engine,and I got away with it. Reno L. said NOT to go so tight with an 860! What was the ratio with that set-up? Who knows.

Ray O'Donohue
Parallel Twin
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Post by Ray O'Donohue » Mon Dec 13, 2010 10:49 am

Also: I believe the standard Sport pisto was a nominal 9.5 to 1 ratio,so my set up was considerably higher c.r. (How much did we take off the cylinder base gasket surface? Can't remember.) Agan,without a snug squish band you will have combustion problems of one kind or another with any compression ratio.

budgie955
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Post by budgie955 » Mon Dec 13, 2010 11:17 am

thanks for the info guys.Reading that I think i will stick with the standard set up.

For sale 1 set high comp pistons and rings. !!


cheers
Dave

Ray O'Donohue
Parallel Twin
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Post by Ray O'Donohue » Mon Dec 13, 2010 12:06 pm

I'd suggest getting after that squish band with ANY compression ratio.Easy and cheap,with no downside.

llewdaert
Cucciolo - the Lil Pup
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Post by llewdaert » Tue Dec 21, 2010 7:07 pm

Hello Ray ,
I read with interest the comment on the Ducati Bevel forum, thanks for that. I am reasonably technical though no wizard when it comes to these matters, so I like to defer to others who have the experience. When you quoted the squish figures were they in metric?and if the base is machined off the barrel wont that increse the CR? I am fitting Wiseco pistons to my 860[10.25:1] and I am concerned for the pinging it will perhaps[probably] cause. To adjust the squish will help if I understand the forum post right, can you offer any other advise please? Should I machine off the crowns as well. I dont want to have a high performance motor just nice running and reliable.
Thanks for your time and Merry Christmas

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Post by Ray O'Donohue » Wed Dec 22, 2010 5:15 am

I know nothing about those Wiseco's,but if you have them on hand you can easily check that squish band clearance with some wax or clay.The compression ratio as such is neither here nor there,in the sense that if you have too much squish band clearance,you will have combustion problems.Yes,when you trim the base gasket surface,the c.r. will increase,but your likelihood of combustion problems will be nil.Of course you must check for any valve clearance problems as well. I would trim that base gasket surface for sure if you use any c.r. Ducati supplied pistons . You have nothing to lose and much to gain. When I learned about this in the bad old days,the older Ducati guys all knew about it,and I was sort of astonished. It seemed that the factory either didn't understand squish bands (not unlikely),or that they assumed the Ducati customer was himself a tuner/machinist .(often the case).Your stock pistons with a squish band clearance of about .035(?) will be a good option.

Ray O'Donohue
Parallel Twin
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Joined: Tue Oct 13, 2009 7:11 am

Post by Ray O'Donohue » Wed Dec 22, 2010 8:22 am

I'm speaking in inches,.035. To Mr.Jockduck: by adding a base gasket,your problem probably got just a bit worse. With all this in mind,and assuming the original equipment dimension pistons are available somewhere,I would also probably avoid aftermarket pistons when searching for more compression.

jockduck
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Post by jockduck » Thu Dec 23, 2010 1:53 am

Ray, yes I know the squish clearance is important, I can't remember the exact numbers I got but after I machined the piston crowns I went back to the normal base gasket. I think I measured the squish somewhere between 20- 40thou. When I built the engine with the standard Wisecos I did a compression test and got 205 and 210 psi and the thing was impossible to kickstart when hot thank goodness the electric foot still coped. Now with the CR at about 9.5:1 the engine is sweet, easily started hot or cold and still goes like a bat out of hell.
Jockduck

Ray O'Donohue
Parallel Twin
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Joined: Tue Oct 13, 2009 7:11 am

Post by Ray O'Donohue » Wed Dec 29, 2010 5:56 am

Sounds like you have things running well.I'll be curious to hear how it runs on lower or low octane fuel. How thick must a piston crown be? (I know that,like top rings,can sometimes be as much an issue in lower RPM's as in higher RPM's,depending on all the other dimensions,etc.)

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 7:11 am

Post by Ray O'Donohue » Wed Dec 29, 2010 5:57 am

Sounds like you have things running well.I'll be curious to hear how it runs on lower or low octane fuel. How thick must a piston crown be? (I know that,like top rings,can sometimes be as much an issue in lower RPM's as in higher RPM's,depending on all the other dimensions,etc.)

jockduck
Mariana
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Joined: Sun Jan 17, 2010 3:21 am
Location: South Australia

Post by jockduck » Wed Jan 19, 2011 2:22 am

I can't remember exactly the crown thicknesses but I think standard AE pistons were 10-11mm I found that the high comp ones were 13mm so I reckoned it was safe to remove 3mm, The high comp pistons were also heavier than the standards so removing some metal helped get the weight down, in theory if the piston is heavier than standard then the crank balance factor needs to be changed to keep the motor smooth( I bet very few people bother)
I found an easy way to check the crown thickness is to use a bench drill
Put a rod in the chuck and wind it down onto the table and that is your zero, put the piston upside down on the table and wind the rod down to the underside and read off the thickness on the scale pointer.

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