Pete Roper's thoughts on Mods

[Guest captain nemo]

I saw this thread on Wild Guzzi and it was so good and so pertinent to what we talk about that I thought it would be good here.

 

pete roper

Joined: 09 Jun 2003

Posts: 645

Location: Bungendore, NSW, Australia

Posted: Tue Dec 02, 2003 8:56 am    Post subject: Boring techie stuff. Ignore if not interested.

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Hrrrrrgh! I'm always suspicious of 'Drop in' magical bits, especially pistons. Why?

 

Well, most aftermarket pistons claim to enhance performance and assuming that they are of a similar strength they usually go about offering this in a couple of ways.

 

1. They up the compression.

 

2. They are constructed differently

 

3. They weigh less.

 

Lets look at what this actually achieves and benefits and pitfalls.

 

1. Upping the compression.

 

Lifting the compression ratio can have benefits. The higher the compression ratio the more efficiently the fuel available can be used. Thermal efficiency increases exponentially with an increase in CR. Unfortunately the higher the compression ratio the greater the likelyhood of detonation. This is a phenomenon where all the charge in the combustion chamber ignites spontaneously rather than being ignited by the spark and burning evenly in a flame front across the combustion chamber. Those of you who own guns or have been in the military will know that this phenomenon is not limited to internal combustion engines, the propellant charge in projectile weapons can, and sometimes does, detonate if the round has been incorrectly constructed, often with disastrous results.

 

In a petrol engine the results can be equally damaging. Certain engines are more prone to detonation than others and it is unfortunatelytrue that large, hemi-headed, two valve motors with an offset plug are particulrly prone to it. To up the CR you effectively have to put a bigger dome on the top of the piston. This makes the flame path longer and more tourturous and makes the likelyhood of pockets of end-gas remaining in the chamber to pollute the next incoming charge much greater. A poluted charge also is far more prone to detonation than a clean one so it tends to produce a self perpetuating cycle.

 

You *can* greatly improve things and get an obsolete hemi-head motor to run unfeasibly high CR's but this requires a LOT of work in adjusting the shapes of both the piston crowns and the actual shape of the combustion chamber itself as well as setting up the squish, (The clearance between the flat deck on the *outside* of the piston crown at the periphery of the dome.) and the head itself at the periphery of the combustion chamber. Setting the squish, (also known as the 'Quench'.) correctly is vital for clean running and avoiding detonation on high comp motors and can only be done correctly by using differentially thicknessed base gaskets or compression plates and/or machining the head and barrel. Scarecly what I'd call a 'Drop-in' modification and one that requires repeated disassembley and checking to get spot on. FBF build some of the fastest racing Ducatis in the world. They know this. Therefore they must know that any 'drop-in' is going to be a conservative compromise. That isn't to say that they won't work. Obviously they do, they have a lot of satisfied customers. But to get them to work properly you'd need to do more than simply install them or run the risk of detonation problems and engine damage.

 

2. Different construction.

 

Most performance pistons are forged rather than cast. While forgings are intrinsically stronger for any given weight it's not a one way street. It is impossible to forge something as complicated in shape as a casting meaning that sometimes some of the weight saving is lost by the need for a simpler design. They also tend to expand differently and go all sorts of queer shapes before they reach operating temperaure leading to less efficient sealing, blow-by problems, contamination of both the oil and the incoming charge, (See above re-detonation.). This also tends to make them both noisier and shorter lived than a similar cast piston.

 

Performance pistons also tend to be of a slipper or semi slipper design with very thin rings in the intersts of rducing frictional losses and increasing the mechanical efficiency. once again this will lmost always compromise the service life expectancy.

 

3. I've really covered the weight issue above. Lighter pistons obviously impose less stress on rods and bearings but with weight eduction also tends to come a reduction in service life. While this is a sacrifice many are willing to make it is something that should be acknowledged before you shell out big bickies on some new slugs.

 

I'm not trying to put you off the idea. As I've said FBF know their stuff. The only thing I'd say is think carefully before you embark down this route as there are going to be pitfalls and compromises. To do it right it won't be a 'fit and Forget' excercise.

 

One final thing, the '03 motor has the hydro lifters doesn't it? Power=Torque x Revs to get the best out of high comp pistons you have to couple them with a fairly aggresive cam that offers it's best cylinder fill at higher RPM where the laws of physics dictate that VE is going to be dropping off anyway. That is exactly the opposite of the softy-cam suitable for a machine with hydro lifters and designed to produce maximum torque at lower RPM. Given the problems that some '03 owners are having with cams and followers anyway the last thing I'd want to do is spin the wretched cam faster!!!!!!!

 

Pete the killjoy

[al_roethlisberger]

Yep, Mike Rich said much the same whilst describing his new pistons to me a couple months back.

 

Thanks for the info

 

I'm still not certain who's pistons I'll be getting, but I'm leaning towards Mike's just because I know his work is so good, and he seemed to consider(although not necessarily solve) the issues mentioned above whlie designing his.

 

al

[Lex]

Just a note to ask people to read what Pete has said carefully. You'll note it isn't saying such modifications are a bad thing, just that they are not (or should not be) "drop in", the parts can be garbage if you aren't careful and it may cost you in other areas (e.g. engine life).

 

I have no experience building up Guzzi motors but from working on other bikes I have to say a well built, mild engine can be a huge amount of fun and no less reliable than a stocker. However, it will not be cheap and, like the guy with the hopped up Harley in a following post on this subject, it could be a huge disappointment. Listen to experts regarding what would make a good motor for your needs. Poorly designed or made parts, lots of compression and radical cams sound good but will make you unhappy in the long run. People with the skills to select the right parts and build a strong, reliable engine (I'd guess an example in the US Moto Guzzi world would be Mike Rich) will charge you a lot of money. They aren't getting rich, good parts are expensive and it takes a lot of skilled labor to do this stuff correctly.

 

Cheers,

 

Lex, Thinking he might have a talk with Mr. Rich next winter...

[Guest callithrix]

Timely post considering I've been toying with the idea of getting the FBF piston Kit, especially after removing my intake boots and wondering if I needed new piston rings. I can draw from personal experience with my sportster 883/1200cc conversion that "drop in kits" with even slightly higher compression (9:1 vs 9.5:1) and stock cams resulted in pinging and demanded reconfiguring on my behalf.

[dlaing]

Yep, Mike Rich said much the same whilst describing his new pistons to me a couple months back.

 

Thanks for the info

 

I'm still not certain who's pistons I'll be getting, but I'm leaning towards Mike's just because I know his work is so good, and he seemed to consider(although not necessarily solve) the issues mentioned above whlie designing his.

 

al

So was Mike Rich knocking(pun intended) the drop in kits that raise compression, and suggesting that his will drop in and work better than stock, without problems because he already took everything into consideration, or was he suggesting that his too, would need carefull attention to squish, detonation, etc. when installed.

 

Also, I did not understand Pete Roper's comment,

"Lighter pistons obviously impose less stress on rods and bearings but with weight eduction also tends to come a reduction in service life."

I thought an increase in service life would be the result.

[Lex]

Also, I did not understand Pete Roper's comment,

"Lighter pistons obviously impose less stress on rods and bearings but with weight reduction also tends to come a reduction in service life."

I thought an increase in service life would be the result.

 

I think Pete's point is that light parts tend to have a shorter service life than heavier parts. Your rod bearings may last longer but your new, lighter pistons may not.

 

This often true but sometimes not. I've seen cases where OEM parts were heavy (and cheap to make) and aftermarket parts were lighter (and much more expensive) because of extra work and/ or better materials, not because they just removed most of the piston's skirt. I know this is often true with older (airhead) BMWs, I wouldn't be surprised if it is also true with Guzzis but I lack the experience to know for sure. Again, this points to the advantages of having someone who knows what parts work well and last on your side.

 

For better or worse, most Japanese parts are very hard to improve on, the stock pistons in late model Japanese super-sports look like the super high end racing parts I dreamed about as a kid, they are even forging some of the OEM pistons these days!

 

Cheers,

 

Lex