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Thread Like Summary
bon, TinkererToo
Total Likes: 7
Original Post (Thread Starter)
#853409 07/10/2021 12:22 AM
by Dale Hoover
Dale Hoover
Hey folks,

I have a technical question for the collective brain power here…

When looking at engine performance parts for our old bikes, I was struck by how high-performance pistons found in widely available big bore kits still look like an old soup can with the long piston skirts, when compared to modern pistons. I assume light-weight pistons means a rebalanced crank, but crank balancing doesn’t seem to be an insurmountable task. Would combustion heat management be a factor? Or the primitive oiling system?

Are there other factors inherent in our (80-year-old!!) engine design that makes this physically impossible? Or has the performance ceiling of this engine design basically been reached with “period correct” design and engineering? Or does the law of diminishing returns play into this?

Are there “modern” big bore kits out there that I simply haven’t seen yet?


Liked Replies
#853450 Jul 10th a 07:07 PM
by Dale Hoover
Dale Hoover
Originally Posted by triton thrasher
Originally Posted by bon
Just saying, no point over stressing old bikes with engine tuning you are not going to use.

True that!

Agreed! I'm interested in finding hard data that shows at what point in power output these engines are actually over stressed, and how to eliminate stress points. Seems like lower reciprocating mass would be less stressful than higher RM.

It's interesting JS Motorsport claims, "The combination of lighter pistons with our longer lightweight rods makes for a much smoother and reliable ride whether you are cruising down the highway or pushing it to redline on the racetrack."

Nothing wrong with smoother and more reliable, especially if there's also a little more thrust! Clearly engineering isn't my strong point...but curiosity is. :-)
1 member likes this
#853446 Jul 10th a 03:13 PM
by triton thrasher
triton thrasher
Originally Posted by bon
Just saying, no point over stressing old bikes with engine tuning you are not going to use.

True that!
1 member likes this
#853470 Jul 11th a 06:17 AM
by Dale Hoover
Dale Hoover
Originally Posted by bon
Originally Posted by triton thrasher
Originally Posted by bon
Just saying, no point over stressing old bikes with engine tuning you are not going to use.

True that!

Except if you are racing or hill climbing or something like that. The speed limit here on motorways is 75 mph, if you are enough of a masochist to want to subject a classic machine to motorway riding, then you don't need 120mph performance machine to ride them.

Personally i think a larger capacity engine in a lower state of tune is the most pleasant. But everyone has their own ideas.

Hi Bon,

Thanks for the input and your point of view on respecting and retaining the character of these old bikes. It is much appreciated.

But don’t misinterpret my initial inquiry as being a naïve attempt to turn a 1960s T120 into a 2020s Panigale V4. This is intended to be an academic discussion about the “why” associated with old-school piston design in the context of everything we’ve learned about engine dynamics in the last 50 years…and applying it to an engine architecture I really love.

At 67 years old, I’ve owned over a dozen pre-Hinkley Triumphs since the late 60s — including my current treasure, a wonderful bone-stock 9pt restoration 1967 Bonneville. (Okay, I cheated and installed a ’68 TLS front brake, and late ’67 concentrics.) Love, love, LOVE that bike to pieces, for what it is.

But, maybe like others on this forum, I’ve also owned, ridden, wrenched on, and road-raced very modern liter-class Hondas, Kawasakis, Ducatis, and KTMs. My motorcycling interests aren’t trapped inside a 1960s box.

20 years ago, in an attempt to answer the “what if” question, I “modernized” my long-suffering ’73 TR7RV by building a modern hot rod with $20k worth of upgraded suspension and modern tires, high output electrics/lighting, 320mm front disk brakes, Magura controls, one-off custom billet triple clamps and rear sets, custom 1 5/8” stainless steel exhaust, Mikuni flat-slides (Bonneville head), Mega-Cycle cams, light-weight valve train, dry clutch, Buchannan-laced Akront rims, etc., etc. But alas, only 8.0:1 (basically stock) soup can pistons.

Brian Slark liked it so much he awarded it “Best Technical Interest” at Steamboat in 1999. But more than the national awards, the enjoyment for me came from conceptualizing the bike, building it, and RIDING it!! It was incredibly FUN to thrash that thing on the backroads of Colorado! (TR7RV Don is right…there is something about the character of our short-rod engines that the long-stroke 650s/750s can’t duplicate!)

I’m not trying to convince anyone to adopt my interests. Just trying to learn as much as I can about the technical limitations of these old bikes and get thoughts on why some of the engineering hasn’t progressed very far.

For some of us, as Don said, “This is fun stuff!”
1 member likes this
#853477 Jul 11th a 11:38 AM
by Hillbilly bike
Hillbilly bike
Me and Kevin on this site have land speed racing 650 Triumphs running in a modified production naked frame class. My bike has been on a dyno several times for tuning..You cannot compare different dynos ...but...about 58 rear wheel HP on leaded race gas...That would be 66 or so crankshaft HP. The bike starts using the kicker and has a wide spread of torque...We are not pro mechanics , just hillbillys,but both of our bikes are currently the fastest two bikes in the class.
Triumph engine limitations? depends on the power level....but no one in land speed racing runs 60 year old connecting rods and some use billet crankshafts...Depending on class rules, aftermarket aluminum cylinders..
My street 79 T140D. upgrades include Honda 750 VFR 41mm forks,320mm Ducati front disc and Brembo caliper.The engine has revised cam timing , the piston to head squish reduced to .032, Mikuni 32mm flatslide carbs, Boyer Micro Power with a fast advance curve. With 9.2 static compression it runs without detonation on 90 octane US gas..The engine makes noticeably more power than stock 750 and pulls well at all speeds. All internal engine parts except for the pushrods are Triumph production parts...
Step up to new theories of combustion efficiency by improving port velocity and chamber turbulence or dual plugs....Or just stay old school and complain about modern fuel limitations...
Attached Images
1 member likes this
#853467 Jul 11th a 03:17 AM
by reverb
--I asked that question in the British general forum few months ago.

Hi Bon; I understand your point but I am not a masochist and use (not now, waiting for a mint crankshaft) later T140 as a transportation. Bike can handle 115-130km/h constant speed for 2-4 hours. The problem with old bikes and speed is mainly with suspensions and tyres . Mine have new Hagons; and all new internals on the forks; nothing changed.
Regarding those new 250cc that you mention; in fact they cannot beat a later T140 and the feel on them is not good. I did not checked all of course but many.
Today I rode one of the fastest around 400cc; 44HP (one of the most powerful on that niche). Instantly fast but I did not felt those 44HP. All those are not so incredible they are just somewhat fast and possible more reliable than an old bike no matter the brand; but not so many more..
The point is on the suspensions; most have good ones and a couple have great ones. All are better than the old forks models and that is where you find confidence to open more the throttle.
1 member likes this
#853474 Jul 11th a 08:15 AM
by TinkererToo
Just for the fun of it, I'm building an engine with Norton based 270 deg crank even shorter, Thunder Enginneering rods, the nicasil barrels and a head modified for bathtub combustion chambers. What will it be like, who knows, and may just end up on a race track if quick enough. Then again, I do more spannering than riding, sad, but there you go
1 member likes this
#853502 Jul 11th a 07:09 PM
by Mike Baker
Mike Baker
Next project up will be a 650 street tracker that I initially built in 2002. Last time out it made weird noises and rather than completely ruin a motor (been there done that) it came home on the truck. It's pretty much stock save for a 750 intake cam and 32mm VM's. It dynos at 41HP and would outrun most Nortons around here. I have my last race bottom end set aside that will get some 10.5 ish pistons probably set up much as Tony has done with a tight squish. A race motor I built like this made a solid 49 hp. This is an experiment, to see if it will run reliably on 93 pump gas. I think it'll be ok as long as I don't lug it. We'll see.
If It doesn't work, I'll put the 9-1 top end back on and save the high compression top for the next race motor.
1 member likes this
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