High Compression Pistons

[Skeeve]

... the fact remains that chasing higher CR's by putting a bigger dome on the piston of what is essentially a very primitive 2 valve motor is IMHO not a particularly good choice if you want to chase power and tractability. Unfortunately such designs invariably end up with incomplete scavenge and pockets of end gas that will pollute the incoming charge. This, along with the crap fuel we get nowadays, greatly increases the chance of detonation and all of it's associated problems.

 

There is also the fact that a 2 valve Hemi is not exactly cutting edge in terms of volumetric efficiency so a large part of the benefit of a higher CR is going to be lost, certainly in terms of outright power, at the higher end of the rev range.

 

I'm not saying you shouldn't fit higher CR pistons, (The FBF pistons are I believe Wisecos so they will be a good, light, forging but with a more limited lifespan than some.) although I can assure you that living where I do bumping up the CR is not something I'd be keen to do. All I'd say is that it's probably one of the LAST things I'd think about doing, and only then if I was desperate to chase ponies out of a Guzzi donk...

 

No news to anyone who's been hanging out in v11LM for any length of time, I'm a big fan of Mr. Roper's generous contributions & have yet to find anything I disagree with in any substantive degree in anything he's said. To lend further support to his point:

 

I got a nice email over on Wildguzzi a while back from someone who'd been inspired to look up Sir Harry Ricardo after I mentioned him; I decided maybe I should read his book myself, having only had read reference to it over the years. Great stuff, so good I'm thinking of shelling out the $80+ to get a copy of my own, since the Ricardo Institute has seen fit to finally reprint it for the first time in 20+ years, & who knows how long that will continue? (FWIW, 1st editions sell for over $600 & that's not even a signed copy!)

 

That led to a whole lot of further reading, with which I'm even now engaged; oddly enough, all of it seems to agree with what Sir Harry wrote 80-odd years ago: you want more power, you can squish it harder, spin it faster, or build it bigger, but they all have their tradeoffs. In the end, it comes down to basics, and one of those is that the smaller the combustion chamber, the more resistant to detonation, so the higher the compression can be. A hemi-head, "spectacle" combustion chamber like Guzzis w/ hi-comp pistons possess have a broader combustion chamber than the bore! [Due to "folding"] This is why Guzzi had to go to a twin-spark layout on the Breva & onward models. The alternative would have been to go to a bathtub chamber like H-Ds use (quite effectively!), but I guess Guzzi/Aprilia/Piaggio decided that it would be easier/cheaper to go the former route than to have to change their castings & machining programs to redesign the combustion chamber & pistons. Too bad, as the dual-plugging just keeps them even with smog cert. requirements, w/o necessarily bringing the higher combustion efficiency that a better chamber would. [Or were you thinking Harleys going from the

 

I hope that anyone with a Guzzi who wants to run the hi-compression pistons has a dependable source of hi-octane gasoline around, & I'm not talking about the p!ss that's sold as "premium" here in L.A.-la land! Either that, or carry around a can of Avgas 100LL (& ditch the stock silencers w/ cat converters, if your model is recent enough to have them; no sense ruining them by running leaded gas!)

 

I would dearly love to experiment with redesigned heads, try going to a 3-valve head, maybe a 2v wedge head, at the very least building up some of the combustion chamber to provide more squish band & taking some of the crown off the piston. Ah, rampant empiricism! What fun!..

[helicopterjim R.I.P.]

The interesting thing is that there is absolutely no extra peak horsepower- the extra urge that owners of FBF bikes notice is from an enhanced midrange not extra top end.

 

That is exactly what I found with my bike even though I did not dyno it. I found my bike to be much, much more responsive to throttle and to have a big edge in roll-ons with other Guzzi's - even though in an out and out race it did not seem to be significantly faster.

 

I like mine!

[polebridge]

Ever talk with guys who've logged dozens of cross-continent moto tours? The first thing many of 'em say when you introduce y'erself is, "WHAT?" According to many I've talked with about this, it's the long-term effects on the hearing of wind buffeting inside the helmet without ear protection. It didn't bother them at the time. Wind buffeting doesn't bother me either. I might start using ear plugs on trips myself, starting tomorrow.

 

 

Rachethack,

 

I have just started wearing earplugs when I ride. I got these nice, corded, reuseable ones by Howard Leight.

 

If your interested you can check them out here:

 

http://www.hearingportal.com/products/products2.asp?id=8#

[mdude]

As my hearing has been suffering absurdly loud rocknroll for years (saw The Cramps a couple of weeks ago; totally irrelevant but fun), I am starting to feel the consequences and feel the need to protect my ears. In crowded environments like bars/parties, I can no longer make out what people say to me, it all blends together to a high pitched din. So, time to plug'em up to save the little hearing thats left.

 

I always use plugs on trips lasting more than 30 mins. One thing is the possible damage to hearing done by wind buffeting and howling engines (I sometimes drive behing my wife, and her Laverda is just howling...) another thing is that the noise drains your energy after a couple of hours and kills your concentration with possible dangerous situations as a result.

 

I use those yellow sponge plugs (E.A.R) , but find them to be too big to be comfortable so I cut them in half. With half a plug in my head the engine and other important sounds (like ambulances) seeps through, just loud enough. This also works at concerts.

 

Sceptical about the plugs with small "handles" on them. Dont they touch the helmet paddings? They also look uncomfortably similar to rubber toilet pumps.... what happens if ya yank'em out of yer ear really fast??

 

EDIT: wow, why did this end up in hicomp piston land???

[helicopterjim R.I.P.]

As my hearing has been suffering absurdly loud rocknroll for years (saw The Cramps a couple of weeks ago; totally irrelevant but fun), I am starting to feel the consequences and feel the need to protect my ears.

 

 

 

EDIT: wow, why did this end up in hicomp piston land???

 

I have hearing that has been affected by years of farm equipment, motorcycles and turbine engines. I would like to say that I wish I had felt the need to protect my hearing before all the damage was done not after. I tell young aviators and bikers to use hearing protection now - not when your hearing is already going.

 

 

As to why the previous post is relative to hi-comp pistons? When you hear the difference between the sound of 11:1 vs 9.5:1 it means a lot. The hi-comp sound has a sharper boom to the exhaust note versus a regular V11's. I don't know really how to explain it but everyone who follows me grins when they try to describe it!

That is relevant to me!

[dlaing]

Well, I guess I learned something when Ratchet wanted to optimize his squish, and Pete Roper said it was a good idea, in the soft valve and plasticene guide thread

So, it seems to me that rather than high compression pistons, all you have to do is optimize your squish which should increase the compression ratio on a stock bike.

From my rough calculations to go from the 9.31:1 stock ratio to a 9.75:1 ratio you would only need to bring the head about 0.4mm closer to the piston.

On this thread

http://wildguzzi.com/forum/index.php?PHPSE...c=1714.msg21394

Pete estimates most Guzzi stock squish to be about 50 thousandths (~1.4mm), and reducing it to 40 thousandths (~1.0mm) is safe and beneficial.

 

So, how trivial is it to measure the squish then either deck the heads, skim the jugs, or just go to a thinner gasket???

[Guzzirider]

Just a brief update on my motor- I have now done 5500 miles since the FBFs were fitted in late April and the motor feels nicer every day.

 

There is a real growl in the midrange where the motor is pulling really hard (for a Guzzi).

 

The retune of my ECU by HTM has also added a bit of a top end rush, which was noticeable when I swapped bikes with a fellow FBF V11 owner last weekend.

 

Guy

[Alex-Corsa]

Just a brief update on my motor- I have now done 5500 miles since the FBFs were fitted in late April and the motor feels nicer every day.

 

There is a real growl in the midrange where the motor is pulling really hard (for a Guzzi).

 

The retune of my ECU by HTM has also added a bit of a top end rush, which was noticeable when I swapped bikes with a fellow FBF V11 owner last weekend.

 

Guy

 

What RPM exact you mean by midrange?

[Guzzirider]

What RPM exact you mean by midrange?

 

Hi Alex

 

About 3.5 to 6k rpm- its really sweet at that point which is ideal for road riding. I tried a new disclipline on my V11 today- enduro! I went to the Stormin' The Castle rally in County Durham and it absolutely poured with rain yesterday and turned the site in a huge deep slippery mud bath. I pleased to say that our V11s coped well this morning even without knobbly tyres, and actually looked happier in the mud than the huge top heavy KTM Adventures that were camped next to us!

 

Guy

[Alex-Corsa]

Hi Alex

 

About 3.5 to 6k rpm- its really sweet at that point which is ideal for road riding. I tried a new disclipline on my V11 today- enduro! I went to the Stormin' The Castle rally in County Durham and it absolutely poured with rain yesterday and turned the site in a huge deep slippery mud bath. I pleased to say that our V11s coped well this morning even without knobbly tyres, and actually looked happier in the mud than the huge top heavy KTM Adventures that were camped next to us!

 

Guy

 

Thanks , I just saw your dyno chart at the other page , is this tourgue ft-lbs? WOW that is enormous, what is it the translation to nm? or kgm?

Cheers

[Guzzirider]

Yes the torque is in ft lbs- not sure how that translates into metric.

[dlaing]

So, how trivial is it to measure the squish then either deck the heads, skim the jugs, or just go to a thinner gasket???

 

Unless I get an answer, here are the real world options.

Source of images: Al's posts

http://www.v11lemans.com/forums/index.php?...amp;#entry36540

[Pierre]

Unless I get an answer, here are the real world options.

Source of images: Al's posts

http://www.v11lemans.com/forums/index.php?...amp;#entry36540

Looks like the Mike Rich pistons have a flatter top, or less pronounced "dome.". That should yield a better flame path. Not sure how he gets compression bump - perhaps a steeper rise resulting in a flatter top. Would require pretty deep valve pockets going that route I'd wager.

[big J]

With the proviso that I haven't done any internal work on my Guz yet,first thing I'd do is get good quality non-compressible head gaskets and measure how thick they are.Next,heads off and measure the old gasket thickness.Check squish with solder.Add gap to thickness of gasket.Use burette to measure chamber volume and calculate compression ratio.I personally wouldn't take anything off the heads,but I'd machine the barrels to achieve the required squish gap.

Mind now,measure,measure it again,then check again before machining anything.

After this,it's dual plugs,a cam,porting work,big bore kit,custom exhaust...................

 

BTW,this is just how I'd do it,best check with an expert how to do it properly.

[Pierre]

With the proviso that I haven't done any internal work on my Guz yet,first thing I'd do is get good quality non-compressible head gaskets and measure how thick they are.Next,heads off and measure the old gasket thickness.Check squish with solder.Add gap to thickness of gasket.Use burette to measure chamber volume and calculate compression ratio.I personally wouldn't take anything off the heads,but I'd machine the barrels to achieve the required squish gap.

Mind now,measure,measure it again,then check again before machining anything.

After this,it's dual plugs,a cam,porting work,big bore kit,custom exhaust...................

 

BTW,this is just how I'd do it,best check with an expert how to do it properly.

 

Bernd at Stein Dinse AU told me (and he decked the heads for my Guzzi) that experts agree squish should be the width of a crushed (properly torqued) head gasket - I think .032 but I'm working from memory. Will require recessing valves - or deeper valve pockets in piston etc - but that's what I've been told squish should be.

 

In short, I'm blushingly pleased to report that according to the experts I'm blessed with perfect squish.