LRP VS Unleaded petrol

I have a beach buggy with a twin port 1600 engine. It sounds like I have burnt a exhaust valve in number 1 cylinder. Some people have suggested that the cause might be that I use lead replacement petrol instead of unleaded fuel. Does anyboby else use unleaded petrol in these old engines? If you do use unleaded fuel, what do you use for a valve lubricate if anything at all?

I want to start this debate so that I can as many views on the subject as possible as I don't know the correct answer myself. The VW heads are alloy so they do have a inserted valve set but I don't know if set requires a lubricate or not.

Any views are welcome.

Steven

i use optimax and its great, but i have been told that LRP is rubbish and leaves a lot of carbon build up. if u want to run a upper cylinder lub go right ahead as it will not hurt

I quickly noticed heavy deposits from using LRP fuel when it came out that continued to grow (1500 beetle, 2L Kombi, R100 BMW bike), and can also lead to valve sticking. Most fuel companies recommended various changes to your engine to make them run hotter which removed the deposits. Fine on a watercooled engine, but deadly for a VW.

LRP replaces the lead with Phosphorus and potassium, who's oxides leaves a rusty brown deposite, as opposed to grey from lead oxide, and require a higher temp. for self cleaning.

I switched to unleaded, and now add an upper cylinder lube for peace of mind. I have done ~10-20 000km on each like this. As for the long term, who knows?

A note on Upper cylinder lubes, some brands advertise they also have cleaning adgents added. This is OK short term, but their long term use is not recommended, and best avoided. An upper cylinder lube also cannot prevent valve seat recession the same as leaded fuel and LRP do.
:alien

One additional note, I didn't notice any performance/economy improvements with Optimax and use regular unleaded.:bounce

Andy, do you put upp cyl lubricant in your R 100? How much per tank?
George ( R80RT)

I never use LRP - discussions here (and elsewhere) have basically convinced me that the stuff is crap. It will never go in one of my fuel tanks again.

I have noticed a difference between ULP and PULP. ULP works fine but can sometimes allow some pinging to occur in both my 75 kombi and 68 bug. Sure I could adjust the timing accordingly, but PULP fixes the problem and does seem to make both cars run nicer with a bit more 'pep' - and no pinging! 🙂

I have a 1776 with a CR of about 8.5:1
I use Ampol(?) Vortex. Not quite Optimax but better than ULP. (and not as dear)

Yep goes good. Have about 15,000 k's on this motor and did about 25,000 on the 1600 stocker before i changed over

yeah my car would run a bit hotter on ULP, now i just run it on PULP and everythings great. I used to run a lead substitute, although really when you think about it rationally i dont think there is a need for lead in a VW aircooled flat 4.

i know the valve seats have always been steel, the early ones 1200 & 1300 had sintered which is slightly less hard than the later ones.

either way they are hard, not like soft cast iron valve seats in an iron headed engine. also the stock rocker arms lubricate the valve stems.

and yeah i dont think anyone actually likes LRP !

Manxed 69 (and the group)

The short answer is - use unleaded fuel.

I've written a lengthy article about fuels and VW engines at
http://www.geocities.com/aussiebug1970/octane1.html which you might find useful.

Unleaded in Australia is 91RON - exactly what VW recommned for their 1500/1600 engines (7.5:1 compression ratio), and higher than the miminum needed for the 1200/1300 engines (between 6 and 7.2:1 compression ratios used), so it's just fine for all VW engines with original compression ratios.

Unleaded premium is 95 RON Octane (except for Shell which is 96).

Shell Optimax and Mobil Synergy 8000 are 98 RON Octane unleaded.

These higher octane fuels will work fine too but will NOT improve power over the normal unleaded, and are not needed unless you have a higher compression ratio than standard (up to 8.5:1 can use 95 octane; up to 9:1 needs 98 octane).

14yroldwith63beetle

As I just said - premium/optimax will work but is not "better" than unleaded. So long as the octane number is high enough to prevent detonation, there is no advantage in using higher octanes than that (read the article above).

VWrallycar and Andy

LRP originally used Phosporus and potassium compounds, but these caused rough running and starting problems, and now the active ingredient mostly used in Aus is (wait for it...) Methylcyclopentadienyl manganese tricarbonyl (MMT for short).

It can still cause some rough running problems and since the VW engine does NOT need protection from valve seat recession (VSR), you do NOT need to use LRP at all.

Just for info, LRP is 95 octane (96 for Shell LRP). It's needed in engines which have cast iron heads and no hardened valve seats (your old Holden 138-202, Morris Mini, older Ford 200-250, Valiant 225-265 etc - which have cast iron heads). MMT does not have "memory" like the lead oxide coating of Tetra Ethyl Lead (TEL)provided. With lead you may remember the "one tank in four" rule for some cars. When using LRP in cars that need it, you must use LRP all the time since it has no "memory" and will only protect against VSR whilst it's being used (source - BP Technical Advice service, and Info from Shell in the UK on unleaded fuels and LRP).


There is no need to add an upper cylinder lubricant to unleaded fuels either - the valves on VW engines run in bronze alloy valve guides (less lubrication needed) and the clever design which does not need rubber valve seals means they get plenty of lubrication from oil splash on the spring end (it migrates up the valve stem by capillary action). It won't hurt the internals to use an upper cylinder lube, but it won't help either - you'd get just as much "lubrication" from a cup full of diesel added to each tank full as you would from an expensive bottle of upper cylinder lube. But putting additives in the fuel CAN result in faster blockage of the heat riser - the more compounds in the exhaust the more likely it is that carbon will build up in the heat riser and block it.

And as you noted Andy - normal unleaded works as well as Optimax, unless like Blue 741 you have a high compression ratio, or unless like KruizinKombi you notice some pinging which adjusting the timing does not eliminate.


In the USA they have been using unleaded for about 15 yearts and they do NOT have LRP, yet the US guys report no problems with unleaded in their engines - even the pre65 heads with slightly less hard valve seats.

What they DO have a problem with intthe USA is the addition of oxygenates to the fuel - MTBE, TAME, ETBE (full manes supplied to those who want), and Gasohol are the main ones used.

Adding any oxygenate results in the fuel having less "fuel" in the fuel, because they contain "used" oxygen which is no longer available to burn in the engine. 11% MTBE in California results in the car running 2% lean.

We don't have MTBE in Aus, but we DO have alcohol (gasohol) added to some fuels, and 10% alcohol makes any carburetted engine run 3.9% lean (since ethyl alcohol is 39% "used" oxygen).

Lean means hot and hot means more detonation is likely, so the addition of alcohol in Aussie fuels means you either DO need a higher octane to counter the hotter running engine, or you DO need to increase the size of your main jet by 2 steps, for example from 125 to 130 or from 127.5 to 132.5. Each step up in Solex main jet size richens the mixture by about 2%.

So ask your servo - does THEIR fuel have alcohol in it? And if it does, avoid it if you can unless you intend changing the carby main jet.

Hope this helps.

I have been using ULP in my 1972 superbeetle for going on 3 years now. I have not noticed any valve seating or over heating problems in that time. The car still runs just as well as it did when I bought it

Cheers
Tnate

I would be interested in what people think about this article and if they know anything about this product. It claims to solve all problems regarding LRP and ULP.

http://www.doringindustries.com/fcat1.htm

i find that quality between servos in ULP varies a lot... another reason i like to use PULP.

mmm also static compression doesnt tell much about ur octane requirements if your setup is non stock i.e. hot cam

Che Castro wrote:i find that quality between servos in ULP varies a lot... another reason i like to use PULP.

That could be the addition of alcohol which is happening in the eastern states - sometimes without notifying the customer - no signs at the pump.

See my post above re the effects of alcohol in petrol on carburetted engines.

I believe that petrol supplied to the Petrol Plus (Woolies) outlets comes direct from overseas, rather than being made in local refineries - I don't know if that would make for uncertain quailty or not.

mmm also static compression doesnt tell much about ur octane requirements if your setup is non stock i.e. hot cam

Very true - dynamic compression ratio in an engine with a hot cam is less than the dynamic compression ratrio in an engine with a mild cam, even if the physical (static) compression ratio (ratio of head volume to cylinder swept volume) is the same in both.

As a result, an hot-cam engine can have a higher physical (static) compression ratio for the same octane fuel.

Hey Aussiebug,
Thanks for all the great info!
Andy:thumb

[Edited on 4-12-2002 by Andy]

First, I must say, as an engine-lover: DONT USE LRP!!! The petrol companies have to supply something under the new rules and they're doing it the cheapest way possible (the government approves, because they want to get old cars off the road.)

NO VW, MANUFACTERED AFTER 1967 (OR VW HEADS,) REQUIRES LEAD!!! Even earlier engines and components don't really suffer, as, with aluminium heads; they have ALWAYS had hardened valve seats (in '67 VW introduced chromed exhaust valve stems - eliminating all problems.) Really early original engines have rings (which are almost certainly stuffed by now, anyway,) which will suffer - but the time difference to the next rebuild would be negligible.

As for what unleaded fuel is suitable: Standard unleaded is fine for a fresh stock engine that sees regular use. I'm not sure how it goes in the cities these days, but the price difference to PULP here in the outback is negligible - and it can solve a few problems (particularly if your 1500 has magically become a 1600 - as most seem to have done {this will negate any arguments about the need for lead - all 1600 [TP] heads are unleaded, if stock}) If you don't have any hints of pinging, don't bother, higher octane doesn't provide any more power unless the engine needs it.

I promise I'll publish a full article on octane vs compression vs valve timing as soon as I can, but I'm moving (a mere 2000km,) so it may take a little while. But I will reitierate a couple of points:
1. The concept of "dynamic compression" dates from the dominance of two-stroke engines (when Australia was a legend,) but has little relevance until you start running all-out race cams in a four-stroke.

2. Increases in compression are of little value unless designed to increase "burn-rate" in order to match a new (higher) operational rev-range.

3. A higher-octane fuel will slow the burn-rate in order to bring it back into the effective range (the exception being exotic fuels - such as methanol and methanol-nitromethane mix.) I realise people are going to look at the, say V8 Supercars and the 100 octane/10.5CR they run - but remember; it took some major tweaking to make them run at the 40k (about 4000rpm) rev limit for 10 seconds at a time... Do you want that on the street?

4. I might as well be really controversial: We are talking air-cooled here - overly fast combustion rates create excessive head temperatures in liquid-cooled engines:- match your compression ratio and choice of fuel with how long you expect the engine to live.

If it's a stock engine - use ULP (or, of there's any hint of pinging, valve wear, or overheating; PULP.) If it's a non-turbo mild engine: PULP. If you, or your engine-builder, got carried away (and you live in the big city and don't intend to travel much🙂 use optimax, but you're engine has a rather short life expectancy (and, you're running the correct plugs.)

5. If you can't cope with this, or want more than 200HP; run methanol. You'll need a larger tank and, if you meet a canny boy-in-blue who notices the smelll you might have some explaining to do - but your engine will run really cool.

😀

70AutoStik wrote:First, I must say, as an engine-lover: DONT USE LRP!!!

As I said in my post above.

The petrol companies have to supply something under the new rules and they're doing it the cheapest way possible (the government approves, because they want to get old cars off the road.)

If that were true the oil compaines would be using dicyclopentadienyl iron and nickel carbonyl, or iron pentacarbonyl which are much cheaper than the Methylcyclopentadienyl Mangansese Tricarbonyl (MMT) they currently use; and those other compounds above would have the old engines off the road much faster - they cause spark plug shorts and increased engine wear due to the precipitation of iron oxides.

Do you REALLY think the fuel companies want to end up in court through causing excessive engine wear in engines???

NO VW, MANUFACTERED AFTER 1967 (OR VW HEADS,) REQUIRES LEAD!!!

Yep - as I said in my earlier post above. There was a change in the hardness of the valve seat material in about 65 too, but the earlier seats are still hard enough to resist VSR.

1. The concept of "dynamic compression" dates from the dominance of two-stroke engines (when Australia was a legend,) but has little relevance until you start running all-out race cams in a four-stroke.

So you don't think running an extra 20 degrees of valve overlap is going to affect the dynamic compression in a 4 stroke engine? I think it might!

2. Increases in compression are of little value unless designed to increase "burn-rate" in order to match a new (higher) operational rev-range.

3. A higher-octane fuel will slow the burn-rate

Increasing the compression ratio is NOT directly linked to increasing rpms. (Diesel engines rev slower than petrol engines yet have up to 20:1 compression ratios.)

Sorry, but the octane rating of the fuel has nothing to do with the "burn rate" - that's almost identical for most hydrocarbon gasoline blends.

High octane petrol does NOT burn slower than low octane stuff.

The octane rating is a measure of how resistant the fuel is to decomposition and auto-ignition and NOTHING ELSE.

As I suggested above, read this
http://www.geocities.com/aussiebug1970/octane1.html

Detonation occurs after the spark but before all the fuel is burnt - the increasing temp and pressure as the fuel starts to burn causes some components to decompose, and auto-ignite, causing an opposing uncontrolled pressure wave across the cylinder, and this causes the hammer blow to the piston and the characteristic harsh clicking sound of detonation.

in order to bring it back into the effective range (the exception being exotic fuels - such as methanol and methanol-nitromethane mix.)

These just have high natural octane numbers - about 129RON for ethanol and 133 for Methanol.

Adding Nitromethane to methanol has nothing to do with the octane rating - it's a form of chemical supercharging. As the methanol burns it creates heat which causes the nitromethane to decompose and release oxygen into the cylinder, which is then available for burning. So you can start with a rich mixture of meth/mitro mix and end up with more power from the chemical supercharger effect of the nitro.

4. I might as well be really controversial: We are talking air-cooled here - overly fast combustion rates create excessive head temperatures in liquid-cooled engines:- match your compression ratio and choice of fuel with how long you expect the engine to live.

The first part of the above makes no sense since it's not the RATE of burn but the resistance to decomposition which the octane number determines.

The second part makes perfect sense.

If it's a stock engine - use ULP (or, of there's any hint of pinging, valve wear, or overheating; PULP.)

Quite right.

If it's a non-turbo mild engine: PULP.

Only if it has a higher compression ratio.

5. If you can't cope with this, or want more than 200HP; run methanol.

Just running methanol won't give you any more hp, and it's unlikely you'd get anywhere near 200hp with a VW engine anyway, unless it's the 1/4 mile kind of engine with a life expectancy measured in a few hours.

You'll need a larger tank

Sort of true - Methanol requires jets about 2.5 times the area compared to petrol, as it contains 49.9% or used oxygen, so there is less "fuel" in the fuel.

but your engine will run really cool.

That much is true - Methanol has a latent heat of vapourisation of 1.15Mj/kg where petrol has 0.297Mj/kg, meaning that Methanol will absorb about 4 times the heat as it evapourates, compared to petrol. Of course if you are using Methanol you probably have a wild engine which will produce more heat overall, so it's not a straight case of "running cool" as some might think.

Some suggested "light" reading if you are interested:

The High SPeed Internal COmbustion Engine - Sir Harry Riccardo (about 1925 I think)

Modern Petroleum Technology - 5th edition GD Hobson ISBN 0 471 262498

Measurement of knock characteristics of gasoline, G edgar 1927

The chemical kinetics of engine knock, CK Westerbrook and WJ PItz Energey and Technology Review feb/mar 1991.

Secrets of Honda's Horsepower Heroics, C Csere Car and Driver may 1991 (getting huge hp out of a GP engine running on low octane fuels).

you can actually raise your compression and make more power and less heat. but thats another story 🙂

I have been told recently that SHELL [probably all others too] is Discontinuing LRP in the VERY near future. You will be advised to use unleaded fuel plus an upper cylinder lubricant.. Which ULP is anyone's guess... probably PULP ?? Here in the Country - [NSW in the Upper Hunter] Sales of LRP is virtually Nil :o:o

First I must say, the only thing missed so far is BP also makes a 98 octane fuel called BP ultimate. I have been running mainly BP ultimate, sometimes optimax, sometimes others for at least 3 years in a car with high compression and a decent dose of HP and have not had any problems.

Aussiebug knows what he's talking about.

Autostik doesn't.

My main gripe is with the dynamic compression comment. A big cam does effect dynamic compression. The dynamic compression ratio of a big cam motor compared to a smaller cam;
- at low rpm the dyn. comp. is lower
- at mid rpm they are getting closer
- at high rpm it will start to get more.

low rpm is where knocking will most likely occur, and as such you will notice a motor with a big cam will be able to run a higher compression than a small cam motor.

All fuels have the same calorific value, so they all make the same amount of power if burnt correctly (no knocking). Use whatever you need to use to prevent knocking.

Actually Baja, although you are looking in the right direction, you missed the point. The reason you can run higher compression without pinging at lower revs (below that where the cam comes in) is because the cylinders are not being filled efficiently. A well-designed engine with a wilder cam may actually fill the cylinders more efficiently than the stock did and cause an effectively higher compression. Simply fitting an efficient exhaust tends to produce this effect - this is why, I believe, our standard 91RON (the same fuel as the american 86MON+RON/2) seems a bit inadequate once we start warming our engines up.

Maybe claiming anyone who says something you don't like doesn't know what they are talking about is not the most intelligent response?

[Edited on 5-12-2002 by 70AutoStik]

Aussiebug - thanks for your reply. I'd just like to point out one thing - when I said, in point 4, "overly fast burn rates." I was referring to CRs that are too high for the combo - I chose the best word I could think of, we also need to consider the heat produced in the head when the engine is re-engineered to produce power in a much higher rev range.

I always thought ULP was 92RON......

The only Ethanol that SHOULD be in ANY fuel in Australia is a trial in Brisbane / QLD where BP is selling Ultimate 98 with 10% ethanol. This is marked and labelled at the pump, there is no confusion over what you are getting. Any other fuels that contain ethanol have most likely been blended by the service station owners, and will not conform the the australian specs for ULP / PULP. This is illegal I might add, as was the trend a few years back when servo owners were blending Toluene into the petrol to make it go further. At least Ethanol wont give you cancer......

fast burn chambers are better because there is a smaller opportunity for detonation to occur. Faster burning chambers also require less ignition advance which reduces the amount of negative torque.

dynamic compression is also not just determined by the cam and static compression. The shape of the intake manifold/ports, exhaust, temp of the air, RPM etc. it gets really complicated because it starts involving fluid mechanics which i know nothing about.

Doug Sweetman wrote:I always thought ULP was 92RON......

According to both the Shell and BP technical departments I've spoken too - it's 91.

The only Ethanol that SHOULD be in ANY fuel in Australia is a trial in Brisbane / QLD where BP is selling Ultimate 98 with 10% ethanol. This is marked and labelled at the pump, there is no confusion over what you are getting. Any other fuels that contain ethanol have most likely been blended by the service station owners, and will not conform the the australian specs for ULP / PULP. This is illegal I might add,

Yes - that's the problem - there have been reports of up to 18% (tested) alcohol from some unmarked sites in NSW when the maximum allowed anywhere in Aus is supposed to be 10%. For a fuel injected car with an ECU, this is not too much of a problem since the ECU will simply turn up the fuel use until the oxygen sensor tells it to stop - in other words, the engine will adjust itself to the fuel it's using.

Not so with carburetted cars. 18% alcohol means the engine is running about 7% lean, which means you'd be lucky to keep a VW engine running, let alone get any power from it. You CAN rejet the carby to use gasohol, but only if you can be sure the next tank full has the same percentage of ingredients!

as was the trend a few years back when servo owners were blending Toluene into the petrol to make it go further. At least Ethanol wont give you cancer......

Very true - Toluene (Methyl Benzine) was being imported at paint thinner (no fuel taxes) and added to the petrol to increase profit margins. It is carcenogenic (most of the aromatic Benzine compounds are), but at least it was 100% hydrocarbon, and it actually has a very high octane value (about 120) so it was a good FUEL for the car - in itself. Just not environmentally friendly, and cost the Govt some fuel excise moneys which got them a little annoyed.

As a teenager, I had a go-cart with an old two-stroke engine which I'd seriously shaved the head on - I had no idea what compression ratio it had. It ran lik c**p on two-stroke fuel (standard with oil added) but when I bought a 5gallon drum of Toluene from BP and added two-stroke oil to that - it was a screemer. Must have been detonating like a pig on the standard stuff, but there was too much other noise (no muffler) to tell.

Is anyone old enough to remember the Shell ads many years ago - "Over a pint of Methyl Benzine in every gallon of Super Shell" It was Good Thing back then. How times have changed.

Che Castro wrote:fast burn chambers are better because there is a smaller opportunity for detonation to occur. Faster burning chambers also require less ignition advance which reduces the amount of negative torque.

And what you you mean by a fast burning chamber Che?

Are you talking about higher compression ratios, or semi-hemi heads, or a big squish head, or what?

None of the above actually change the rate of burn much.

Various petrol blends have much the same burn rate.

But CONTAMINATION of the fuel/air mix has a marked effect.

Imagine a VW 1600 engine with 7.5:1 compression ratio - it has a head volume of about 53cc.

The same engine with 6:1 compression ratio would have a head volume of about 66cc.

You can't eliminate the burnt gases in the head space completely, so a low compression engine has a higher proportion of burnt gases mixed with the fresh stuff.

This slows the burn rate (flame speed), and is the reason that most 1200s have 10BTDC timing rather than the 7.5BTDC of the 1500/1600s.

So high compression require less advance, since there is less mixing of burnt and fresh gases to affect the flame speed.

Incidentally, unless you want a peaky racing engine, semi-hemi cutting a VW head is counter productive.

The Squish combustion chamber was developed by Sir Harry Ricardo in the UK in the early 20s when he was testing engine designs with the lousy fuel of the day (he designed the WW1 British tank engine which had a compression ratio of only 3.5:1 due to the "dregs" fuel they said it would have to run on - all the "good" fuel went to aircraft and road vehicles).

Anyway - he found that creating turbulence in the combustion chamber enabled him to increase the compression ratio a sygnificant amount for the same fuel. It also improved fuel economy quite a lot.

The squish head (as used in the VW) is the result of that research. It was so good that the London Bus Company was saving 100,000 Brit pounds a year on fuel costs in 1925 compared to the previous cylinder heads they used - think
how much that would be in today's money.

I believe the 1600 actually had, stock, a chamber volume of 51-52cc (sorry if I'm being pedantic,) the rest of the combustion chamber came from increased "deck height." You are quite correct in your description of the reasoning behind the introduction of the squish chamber. However, when we enlarge the engine, the necessary CC volume requires such large deck heights that they negate the value of the squish chamber's effect and the shrouding of the valves becomes an issue in providing the necessary flow - hence the popularity of the "semi-hemi" heads. It is not the ideal solution, but is a cost-effective solution in a large-capacity type 1: found in practice to be better than overly great deck-heights (and a lot cheaper than the R&D to redesign the heads.)

Taking the edges off a stock or modified head chamber is accepted and proven practice to reduce the chance of pre-ignition (any edges can build up heat faster than the rest of the chamber surface.)

Che Castro wrote:dynamic compression is also not just determined by the cam and static compression. The shape of the intake manifold/ports, exhaust, temp of the air, RPM etc. it gets really complicated because it starts involving fluid mechanics which i know nothing about.

I do, and it sounds like you're getting a feel for it. Yes, the dynamics of an Internal Combustion Engine are extremely complicated - a good all round knowledge and some common sense are what are needed on the road to building a better engine; that road, unfortunately, is called esperience. If you apply the kind of logic you seem to, however, that experience can be fun and rewarding (and think about engineering or physics - you may have the mind for it.)

Autostik, you seemed to have missed the point. I said why I didn't agree with you, and then you tried to justify yourself by saying

Actually Baja, although you are looking in the right direction, you missed the point. The reason you can run higher compression without pinging at lower revs (below that where the cam comes in) is because the cylinders are not being filled efficiently.

So what your saying is a big cam motor will have a lower dynamic compression ratio at low rpm.
didn't I say "The dynamic compression ratio of a big cam motor compared to a smaller cam;
- at low rpm the dyn. comp. is lower".
Wow, your disagreeing with me by saying the same thing in a different way.

A well-designed engine with a wilder cam may actually fill the cylinders more efficiently than the stock did and cause an effectively higher compression.

hmm, so at high rpm a big cam motor will have a higher dynamic compression ratio. Didn't I say "A big cam does effect dynamic compression. The dynamic compression ratio of a big cam motor compared to a smaller cam;
- at high rpm it will start to get more. "
Wow, again you have disagreed with me by saying the same thing in a different way.

And the funny part is I said what I said because you originally said

The concept of "dynamic compression" dates from the dominance of two-stroke engines (when Australia was a legend,) but has little relevance until you start running all-out race cams in a four-stroke.

So lets run through this again. You said something that was wrong. I said you were wrong and stated why. You then disagreed with me by saying the same thing as me but in a different way , thereby actually agreeing with me. And since you agree with me and I disagreed with you, you are actually disagreeing with yourself. And your upset because I said you don't know what your talking about!

No wonder people are confused.

btw - I think I know about fluid dynamics too, and probably more than you do. When posting info on a forum like this it is important to keep what your saying as simple as possible so people will understand it. I could explain dynamic compression in detail, but not many people would understand all of it, and wouldn't be bothered reading it, so what is the point.

Are the benefits of higher compression offset by the reduction in timing advance?

If you run a good enough quality fuel you don't have to retard the timing at all. You should be able to run a low CR motor and a high CR motor at the same 32 max advance (rule of thumb) timing if your fuel quality is good enough.

If you use low quality fuel then a high CR motor may start knocking, and then you may have to retard the timing to stop it from knocking.

Japanese spec cars will normally have higher CR motors than the same Australian spec motor because they don't use low quality fuel like we do in Australia.

Gotta love this place! Keep it up guys, very entertaining & informative.:thumb

well i kind of meant what people usually refer to as "fast burn" which are chambers that are typically have good squish and/or swirl, i.e. not semi hemi. The sort of chambers that usually are suited to high CR and less total advance. Kind of a more modern type of chamber.

sleepe3 wrote:Would someone care to enlighten me what the "resistance to decomposition" is?

Sure.

As the fuel is lit by the spark, the flame front races across the cylinder, increasing the them and pressure as it goes. In the few milliseconds that this process takes, the remaining (not yet burnt) fuel gets heated and pressurised and it starts to break down chemically - decompose. Some of these decomposition products are prone to auto-iginition - they will light up just from the temperature and pressure, so you get the reamining fuel burning before the flame front reaches them, resulting in an uncontrolled burn of the remaining fuel, which produces detonation (an opposing pressure wave in the cylinder and a "hammer blow" to the piston).

Resistance to decomposition depends both on the hydrocarbons in the fuel, and any additives in the fuel. The aromatic compounds like Benzine have very high natural octane numbers (they are chemically stable at high temps). But Benzine compounds are carcenogenic so fuel companies are trying to reduce the amount of these compounds in fuels, which tends to lower the natural octane number.

The additive Tetra Ethyl Lead interferes with the actual decomposition of the fuel - helping it remain "intact" as the flame front approaches, so it burns smoothly. In other words, the fuel does not decompose so readily and retains it's original chemical structure until the flame front reaches it. TEL operates mainly at the high temperature end of the fuel-burn cycle - above about 500c (the temp at the end of the burn cycle reaches about 2000c)

MTBE which is used in the USA, works differently. It reacts at lower temps (around 400-500c) with the fuel to create new molecules which as less prone to auto-ignition. The effect is the same - the fuel is "resistance to decomposition" in this case through changes to the chemical structrue of the fuel itself. This is the case with other oxygenates too - TAME, ETBE and alcohol.

To summarize - the lead used in the past helped prevent the fuel decomposing before the flame front reached it, and MTBE (and other oxygenates) change the chemical structure of the decomposing fuel so that fewer auto-ignition products are created.

The measurement of this resistance to decomposition is the octane number

The octane number is a comparison between the detonation resistance of Heptane and Iso-octane - heptane is zero and iso-octane is 100. So a 91 octane fuel has the same detonation resistance as 91% Iso-octane ad 9% heptane. These two compounds are use as they are far apart in detonation resistance but otherwise physically similar - similar boiling point, molecular weight and so on, very important when creating a "reference" fuel.

These days we also have octane ratings higher than 100 - they do this by extending the scale previously used - for example pure alcohol (ethanol) has an octane rating of over 120 when compared to the 100% iso octane.

A high octane fuel is more resistance to decomposition (into auto-ignition compounds) than a low octane fuel.

Nothing to do with burn speads, as some would argue.

Has anyone tried a PULP / nitromethane mix, if so what percentage of nitro did you use?