courtesy of Jens Lyck


Benzin - kan en Moto Guzzi køre blyfri?

Opdateret, 12. maj, 2008

Den oprindeligee tekst til dette kapitel blev skrevet for flere år siden da bly-benzin blev udfaset i Europa og alle diskuterede emnet. Sjovt nok er der ikke nogen der taler om dette længere - hvad der indikerer at der ikke er nogen problemer med blyfri benzin.

En tysk ingeniør der arbejder for Hyundai biler, har venligt påpeget at kapitlet ikke var helt up to date, og har venligt tilbudt at dele hans erfaringer på området.

Peter Birtel er en virkelig kapacitet på området, så hvis du ønsker at vide mere om blyfri benzin, bør du læse hans artikel i boksen nedenfor.

Annotations to the valve seat recession problem

Perhaps you allow that i add briefly sth. to the "unleaded gasoline" chapter, not to teach or correct anyone, just to share my, occasionally business related experience:

A) some facts about leaded/unleaded fuel

  1. Bleitetraäthyl (lead tetraethyl) was mixed to the fuel only to fight combustion knock; even the "valve train pope" W.D.Bensinger denied any effect on valve seat recession - in 1954 !!.
  2. Stellite facing exhaust valves as protection against valve burning became mandatory due to leaded fuel; on unleaded fuel a lot of plain valves can survive without Stellite. This chain of clauses is very often put the other way round.
  3. Side step: In depth investigations at a famous OEM (at a time when they were used do things thoroughly, i.e., before power point spoiled engineers life) revealed a 25% - 50% reduction in wear ALL OVER the engine when running on unleaded fuel.
  4. The T4 i am going to buy, once was converted to unleaded fuel, even before the leaded fuel was phased out, by the following simple procedure:
    • don't touch the engine
    • stop filling leaded fuel
    • start filling unleaded fuel 95 RON
    • operate the bike as usual

    Result of this "conversion":

    • after 5000 km on unleaded fuel the low speed/high load knocking during hot summer days had disappeared, because the nasty deposits on the piston, valves and the combustion chamber had been scavenged by the unleaded engine operation
    • valve clearance check did not indicate any recession even during the next 40000 km on unleaded fuel
  5. On my beloved Tremezza i made the same conversion - with the same result.
  6. During my service at FMC i have spent tons of fuel on the recession problem and came, amongst others, to the conclusion: lead is poison to the engine !

Yes, this is a black and white statement, and i am also aware of some exceptions (seemingly!) - but the whole story would end up in a novel.

What is the practical recommendation for the milk maid in the neighbourhood, who might seek for some safety margin ?

Use RON 98 unleaded but not from white pump stations - that's the best for the engine, and it's a must for tuned (big barrel) engines.

WARNING: the following sections are pure neurotoxin to arm chair strategists – reading it is on your own risk

B) some facts about valve seat recession:

What are the parameters which are most likely to influence or provoque valve seat recession ?

The following charts are to give a generic impression; the given values have been dug out of my memory – magnitude and correlation however are accurate enough to demonstrate the generic response of recession to the respective parameter. The unit "nm/h" (= nanometer per hour) is a typicel unit when applying the RNT (Radio Nuclid Tracer) method on wear measurements. This technology allows to measure valve seat wear propagation (or any other metallic wear) during engine operation on line.

In order of priority the influencing parameters are:

  1. A/F ratio:

    Comment: the closer the AFR comes to stoichiometric, the higher will be the recession rate; at or even over stoich the increase is often dramatically due to hotter combustion, higher ex-gas temperature AND, if over stoich, the presence of free oxygen.

  2. Fuel Quality

    Comment: the additives (whatever they are) contained in SUPER PLUS (RON 98) are evidently decreasing the recession rate drastically. Recession can virtually be switched on/off by switching the fuel. This is an absolutely reproducable effect. Applying E25 / 85 (Gasohol/Ethanol) can be seen in the recession behaviour , too. These additives actually do today what the lead was believed to do some decades ago.

  3. Valve clearance

    Comment: the closer the valve clearance comes to a critical minimum (due to wear or due to deliberate setting) the more recession will be registered. Sometimes this is almost like a jerk function. The reason is the elongated phase of creeping aperture just releasing gradually a very narrow gap to the hot gases. This heats up the components (seat and valve) dramatically. The rhs branch of the curve is simply representing the purely mechanical effect of the increasing valve closing velocity when leaving the ramp or overreving the engine (valve bouncing).

  4. Load and speed

    Comment: this chart just depicts how the wear rate can be distributed all over the load/speed map. The peak around high speed/50 % load is predominantly determined by the point(s) where closed loop is abandoned and enrichment becomes effective.

  5. Other factors

    The impact of spark advance , compression ratio, coolant temperature level and gradient etc was almost neglectable.

    However the impact of the material combination valve/insert is most evident. It should be sought to get a 50:50 wear distribution between the 2 components. Making a bullet proof insert is quite easy but it will virtually eat away the valve (especially Stellite) and the other way round.

  6. ?

    The above annotations are neither complete nor are they differentiating towards all possible directions, so i really wouldn't be too astonished if someone had a different view on this subject

C) Question without final answer

Why does the legend of the wear reducing effect of leaded fuel still exist ? Elder engines like GMS/FMC once were using grey cast iron cylinder heads with parent material valve seats (i.e. without inserts), occasionally induction hardened at the exhaust side. It was reported that especially these engine types were suffering heavy valve seat recession when changing from leaded to unleaded fuel. BUT, the unleaded fuel became mandatory with the introduction of catalysts. Introducing catalysts however, to allow for their proper functionality, necessitated also all new fuel calibration – with the clear tendency towards enleanment with AFR target 14.7 -> effect see B1. Furthermore the formulation of unleaded but still not knocking fuel has for sure undergone massive alterations to achieve the target(s), i.e., the additivation of the fuel became a different one -> effect see B2. If, on top of B1 and B2, the initial material matching was second best the service intervall does not save the engine from reaching the extremes as revealed in B3. Frequent re-adjustment of decreasing valve gap means reducing the pre-load of the valve spring hence reducing the speed of bounce occurrence.

The above is all based on my personal experience, i.e., on real data, some of which were produced by quite scientifically means, others are based on controlled experiments. I am fully aware that this old fashioned way to assess evidences, fight problems and create solutions cannot compete to the supernatural capabilities of the arm chair strategists nor will it be the remedy to the verbal diarrhoea being continuously spread by the web – but i have to and will live with this my deficiency.

Original tekst

Det er ved at være svært at finde benzin med bly eller bly-erstatning, så det er jo interessant om den elskede Moto Guzzi kan køre blyfri når instruktionsbogen foreskriver benzin med bly.

Jeg har ikke et entydigt svar, men vil i det følgende fortælle hvad jeg ved om emnet og hvad jeg selv gør. Enhver kan så danne sig sin egen mening og handle herefter.

I gamle dage var metallegeringerne til ventiler og ventilsæder ikke så gode og stærke som de er i dag, så man var nødt til at beskytte ventiler og sæder med et smøremiddel der kunne forhindre at disse blev slået itu af de mange åbninger og lukninger. Man løste problemet ved at tilføje en blyforbindelse til brændstoffet, der klarede problemet på udmærket måde.

Men som bekendt bryder miljøet sig ikke om bly, så efterhånden som verden blev opmærksom på dette forhold blev der udviklet bedre legeringer til ventiler og sæder. Det betød så at i løbet af en årrække blev behovet for blybenzin mindre og mindre, og nu er der ikke ret mange steder det sælges mere.

Det skal bemærkes at der i dag ikke er bly i "bly"-benzin, men der imod et erstatningsmiddel der er bedre for miljøet. Det er vist nok en Kaliumforbindelse der bruges, og jeg mener at have læst at dens smørende egenskaber er på højde med bly, men den er noget dyrere så derfor brugte man bly.

Moto Guzzi'er holder jo længe og når ens cykel kun er 20 år gammel er der jo ingen grund til at skifte den ud, men kan den så køre på den moderne benzin uden bly eller blyerstatning ?

Her er en række betragtninger, så kan du selv danne dig en mening om emnet:

  • I Californien har blybenzin været forbudt siden starten af 80'erne, og de andre stater fulgte efter nogle år senere.
  • Der er stort set aldrig nogen der skriver om ventiler der er sunket op i ventilsæderne på Amerikanske forummer og sites.
  • Emnet diskuteres ofte på Moto Guzzi relaterede forummer verden over, og jeg har set mange der skriver at det virker, mange der tvivler, men meget få der har dårlige erfaringer med det.
  • Jeg tillader mig at tvivle på at Guzzi fabrikken nogensinde har haft så meget styr på tingene at de kunne producere specielle udgaver (med panserventiler og pansersæder) til USA.
  • Der er heller ikke separate reservedelsnumre på ventiler og topstykker til det amerikanske marked. Ventilsæder fås ikke som originalt reservedelsnummer (men de kan sagtens skiftes alligevel)
  • Men, amerikanerne bruger heller ikke materiellet så hårdt som vi gør i Europa. De kører traditionelt langsommere på vejene end vi gør, og motorcyklen er i langt højere grad et fritidskøretøj der ikke bruges til længere ture.

Som du ser har jeg ikke et klart svar, men mere noget i retning af "hvad synes du selv?".

Min egen fremgangsmåde er at tanke med blyerstatning når jeg kan komme til det, og så have en flaske blyerstatning i lommen til brug når jeg tanker andre steder. Og en gang imellem tanker jeg bare blyfri uden at komme blyerstatning i, det har aldrig skabt problemer.

Så personligt gider jeg ikke skille den ad for at sætte nye sæder i topstykket før den alligevel skal renoveres næste gang, større er irritationsmomentet ikke for mig.

Hvis du er i tvivl (og er nysgerrig nok), kan du jo lave forsøget selv:

  • Hvis din motor er sund vil du ikke opleve de store ændringer i ventiltolerencerne når du kontrollerer spillerum efter 6000km. Normalt er indsugningsventilerne lidt løse og udstødningsventilerne lidt stramme.
  • Prøv så at skifte til blyfri benzin efter du har justeret ventiler.
  • Kontroller ventilerne efter 1000-2000 km. Hvis de stadig holder mål kan du køre videre uden de store bekymringer, men hvis spillerummet er blevet mindre kan det være tegn på at ventilen er ved at synke dybere op i ventilsædet, som så ikke er velegnet til blyfri benzin.