About 91-Octane Updating for Vintage 2-Stroke Street Bikes
Preface – At the time of their release, most 1970’s 2-stroke street machines had a very good reputation for durability and reliability. This reputation made these affordable machines become the first (and often favorite) motorcycle for many owners. However as decades have passed, many owners of vintage 2-strokes have experienced random and unexplainable breakdowns from piston scoring or seizing events. For many of these owners, it seemed like the reliability of their beloved vintage bike had mysteriously disappeared. In many cases, owners simply parked the bikes indefinitely for lack of a solution to the problem.
The majority of our 2-stroke street bike clients tell us that are not looking for race bike power, but rather the best possible reliability … and that is a very doable task. With relatively minor modifications, most vintage 2-stroke street bikes can be stone reliable, and put the “seized piston” reputation behind them.
At Klemm Vintage, we have been building reliable high performance 2-strokes for 40+ years. In the last twenty years, more than ever, the strongest accent has been on building in “pump-gas reliability” as well as performance. While there are many details that make up a reliable 2-stroke, the two most important details are pressure testing for air leaks, and “detonation management”.
Testing 2-strokes for air leaks is a widely known process that has been around for many decades, however detonation-management is a much newer technology that very few have seriously applied to vintage 2-stroke street machines. Given that, Klemm Vintage undertook the development of serious detonation-management modifications for most popular vintage two-stroke street bikes. It is our contention that the detonation-prone head dome designs of 1970’s 2-strokes are responsible for the lion’s share of current day piston seizure events experienced by vintage owners. We further contend that the reliability of most of these vintage 2-strokes can be soundly restored by proper combustion chamber dome redesigning. The following is the technical background of that development.
A Bit of Fuel History – It’s important to remember that in 1970, premium pump gas was often 100+ octane, and this 100-octane was the fuel that virtually all vintage two-strokes were designed to run on. The combustion chamber designs employed in most street two-strokes of the 70’s worked great on 100-octane premium. However those exact same designs have a very high risk of detonation when run on today’s 91-octane brew of oxygenated, and ethanol supplemented, fuels. This detonation (aka “pinging”) dramatically increases combustion chamber and piston temperatures to a point where piston scoring becomes almost inevitable. As an illustration, if you pulled a brand new Kawasaki H1 500 triple from a time capsule today, and tried to ride it at freeway speeds from Los Angeles to Las Vegas (300miles) on today’s 91-octane … you wouldn’t make it without seizing a piston. The combination of today’s low (91) fuel octane, and a very poor “70s technology” cylinder-head dome design creates a guaranteed recipe for a piston scoring event.
About Octane Boosters – There are many fuel additives and ”octane-boosters” available. The stark technical truth is that NO additive or octane booster can make a gallon of low-octane gas into a gallon of “high-octane” gas. At the very best, these additives can make a gallon of bad gas “less likely” to damage your high performance engine. The majority of these additives contain flame retardant compounds that “can” reduce the detonation risks of very low octane (85-87) fuels. However these same compounds have a microscopic effect on the detonation risk of higher (91-93) octane fuels.
Exactly What is Detonation, and How Can it be Managed – We have a detailed tech document on our site that explains this subject. To read that document, CLICK HERE.
The Real Solution – There are many small design characteristics to an “ideal” detonation-resistant 2-stroke head dome. For purposes of explanation in this document, we will focus of two of the higher priority details…. Squish bands, and squish clearance thickness.
“Squish clearance” is the thickness of the air gap between the cylinder head dome and the piston crown at top dead center. This clearance is important because the igniting of the fuel mixture “end gases” in this squish-band is the culprit responsible for causing 90% of all detonation. In a perfect head design, the fuel mixture in this very thin squish-band area is greatly cooled by the (relatively cool) nearby surfaces of the piston crown and cylinder-head. If the squish clearance is too thick, the cooling of these end gases is not sufficient to keep them from exploding, and they will become the source of detonation.
The shape of the combustion chamber, and thickness of squish clearance, have a huge impact on a 2stroke engine’s detonation risk. Ideally, a squish band should be shaped to closely match the contour of the piston crown for a 10-20mm width (depending on bore diameter). Ironically, the big majority of air cooled 2strokes of the 1960s-70s era had dome designs that were nothing like this …..And the head dome designs they did have, ranged somewhere between “not so good, to totally horrible” … no one knows why the manufactures did that. The good news is that most of these dome shapes and squish clearances can be modified (by machining) into an excellent detonation resistant design. In a few rare cases, the stock dome design is so bad that the stock dome needs to be completely filled in with weld so a completely new dome design can be cut in.
|The head on the left is a stock Honda MT-250 head with minor pitting damage. On the right is our 91-Octane updated head (which started out with the same dome damage).Our 91-Octane update for this particular machine includes a .020″ copper head gasket to replace the stock .040″ asbestos gasket employed on the MT-250. The copper gasket has much better heat transfer properties, as well as excellent long term sealing properties.|
|The head on the right is a stock Kawasaki H1 dome, and the head on the left is our 91-Octane updated dome. The difference in dome shape is very obvious. The H1 and H2 both require cylinder decking as well as the head mod mod in order to have correct squish clearance.On our triples, we cut the center head dome with a slightly lower compression ratio to help equalize operating temperatures.|
Much has been written, in the computer era, about how to calculate “maximum-power head dome designs”, including computer programs that do the task. We respectfully submit that we have spent more time prototyping and testing domes and squish clearances on vintage 2-strokes than “anyone” else there is. In short, there are some good “general parameter” rules that will get you a functional and safe result that is within a few percent of what the “supposed ideal” design might be. For vintage 2-strokes street bikes, having maximum detonation resistance is far and away more important than having a so-called “optimum-performance” head-dome design.
To be sure, the actual compression ratio also has a significant impact on detonation risk. For this reason, we consider head dome volume, dome shape, and squish clearances, as a collective of specs that all need to be tailored for the operating RPMs and loads that any particular engine will be subjected to. In the case of the 91-Octane Updates we perform for 2-stroke street bikes, we generally stay very close to the stock compression ratios to get the best in reliability. For customers who prefer a slightly increased compression ratio, we can easily accommodate that.
About Cylinder Decking – It bears noting that re-designing the head dome alone, does not result in ideal squish clearance on 90% of vintage 2-strokes. Most of the cylinders on vintage 2-strokes are too tall to allow ideal squish clearances. To get the ideal squish clearance, the cylinder must be additionally decked on both the top and bottom gasket surfaces. The specs for this decking is different for each model of engine, but the end result for all is an ideal squish clearance.
There is also a beneficial side effect of the cylinder decking part of this process. On most of the cylinders we deck, we find that the base gasket surface is either damaged or not perfectly flat, which means they had a very high risk of air-leaks at the base gasket. In addition, about 50% of cylinders we cut have base gasket surfaces that are not perfectly square to the bore diameter. This means that the piston was traveling up and down a bore that was literally angled to one side. We have been stunned by how many cylinders we see with this problem. To say the least, these “crooked-base cylinders” can, by themselves, be the cause of many scored pistons.
After all the machine work, the cylinder and cylinder head are “lapped” on a large lapping plate to assure perfectly flat sealing surfaces. This “lapping is what gives the dull surface finish that is visible in the photos.
|The photo on the left shows a Honda MT-250 cylinder being bottom decked as part of our 91-Octane update for that model.The base gasket was painted with industrial dye before our first skim cut to help show that it had a very visible low spot on the base gasket surface below the exhaust port, as well as a slight left to right crooked-ness. This left to right crooked-ness is harmful to both performance and long term piston wear.|
ABOUT IGNITIONS – For sure, ignition setup can also have some effect on the issue of detonation … and 91-octane compatibility. All 2-stokes of the modern era (after 1990) have CDI ignitions that feature an “advance/retard” timing curve. These ignitions have a very retarded ignition timing value for easy starting. As the rpms rise from idle, these ignitions steadily advance the ignition timing until maximum advance is reached between 5000-6000 rpm. After that max advance point, the ignition timing is steadily retarded as rpms increase. This kind of ignition curve offers a very strong mid-range acceleration (a result of the advanced value), along with a greatly reduced detonation risk at higher revs (a result of the progressively retarding values).
Sadly, most vintage bikes are equipped with points ignitions (or early CDIs) that offer only one ignition timing value. For machines that are ridden recreationally with no sustained freeway speed riding, the stock ignitions and timing values will work well. However vintage owners that are planning to do sustained riding at higher-rpm freeway speeds, would do well to slightly retard the ignition timing on their machines in order to get an additional safety margin from detonation.
There are modern advance/retard curve aftermarket CDI ignitions available for some vintage models, but they can be costly. Despite the cost, these ignitions are a wise investment for any owner planning to do extended freeway speed riding with their vintage 2-stroke.
Blue-Print Cylinder Porting – For owners that are interested in getting the best possible power from their updated top ends, Klemm Vintage also offers cylinder boring and Blue-Print Porting services. Our blueprint porting is intended to primarily improve overall acceleration, not top end power. Klemm Vintage does offer higher levels of porting for more serious owners interested in high-rpm output. However it bears noting that “sustained” high rpm operation is not recommended on 91-octane fuel.
|The cylinder on the left is a Blueprint Ported RD350 cylinder, and the cylinder to the right is stock.Our Blueprint porting makes very nominal changes to port timing in an effort to offer the best possible power-band width as well as strong overall acceleration. We remove all casting imperfections, and sleeve mismatching. During the porting, we assure that all transfer port heights match (they are often uneven), and the transfer and inlet passages are rough finished to help with fuel atomization. The exhaust ports are also matched, and then smooth finished to help reduce future carbon build-up. After porting, all the port windows are chamfered, and those chamfers are then polished in a effort to get the best possible long-term ring wear.
The end result is a cylinder that offers the best in overall power, as well as excellent long term wear.
ABOUT PISTONS – There continues to be a plentiful supply of either OEM or aftermarket pistons and rings for most vintage machines. As the supply of OEM pistons have dwindled for some models, the aftermarket often has offerings to keep the bikes going. Sadly, some of these aftermarket pistons are not “exactly” identical to stock specification. A perfect case in point, are the Wossner pistons offered for the Kawasaki H1. The Wossners are a very high quality part, but the crown angle is slightly different from the stock pistons. This changed crown angle is no problem at all on an engine with stock heads, however it can be a significant complication for a top end that is 91-Octane modified for stock OEM pistons.
All this said, we generally recommend that you include the pistons you are currently running, when sending your parts in for a 91-Octane Update. This will allow us to confirm the specifications to be the same as stock, or custom suit your top end for the pistons you are running.
ABOUT DAMAGED DOMES AND PREVIOUSLY MACHINED CYLINDERS AND HEADS – We prefer to start out with un-machined, and un-modified cylinders and heads. That said, in most cases we can work with parts that have had “minor” material removal. However any head that has been milled more than .020″ will likely not be a good candidate for this modification.
Heads that have damaged domes can very often be good candidate parts for this modification. It may sometimes happen that our modification eliminates 95% of existing dome damage, but still has a few small remaining pits. We have run many heads with such minor pitting, and they represent no risks to either reliability or performance. In mot cases, the remaining pits fill with carbon in short order.
MODELS AND PRICING – We are constantly adding new models to our list of 91-Octane Update machines. If you have a special model or application, we invite you to contact us about it.
Sourtce: Klemm Vintage.