[OT] 99 Octane petrol

[Sylvain VAN DER WALDE]

Hmm! Let's try.

With the piston at B.D.C. (bottom dead centre), at the start of the compression stroke; the cylinder and combustion chamber will be filled with an optimum amount of fuel/air mixture. As the piston rises to T.D.C. (top dead centre), that fuel/air mixture will be compressed to an extent determined by the stroke/bore ratio of that particular engine and the capacity of the combustion chamber. Yes, using a thicker cylinder head gasket (if available) will reduce the compression ratio; and using a different suitable piston (if available) will raise that compression ratio.

Comments: The length of the stroke is controlled by the combined design of the cranckshaft and connecting rod. The bore size is the diameter of the piston.

Is that enough? I always stand to be corrected.

Sylvain.

[Sylvain VAN DER WALDE]

Static timing wasn't everything. Ignition timing "retarding" devices (distributor vacuum units) often became blocked, or just failed; and the automatic advance centrifugal weights' spindle could seize up. The weight control springs could also weaken, or even be the wrong ones fitted (exchange distributor).

Sylvain.

[Sylvain VAN DER WALDE]

However, tuning the

I believe that 2 stroke (racing?) engines were greatly improved (over a narrow revolutions band) by tuning the exhaust.

Sylvain.

[Dave Plowman (News)]

Absolutely. The advance curve should be checked if pre-detonation is experienced with the correct static timing. But only if sure the engine doesn't need a de-coke and the mixture is correct. Also check the operation of the vacuum unit.

[Dave Plowman (News)]

You can tune an exhaust to resonate at certain frequencies. This then produces an extractor effect. Useful on racing engines with a narrow power band, but not so much so on a road engine. The same effect is also used on inlet tracts.

[Chris Bacon]

You're sort of getting there, in that you've added the bore as well as the stroke, which was missing from the quote I included above.

[Chris Bacon]

Why the *combined* design of the crackshaft and the contorting rod? Surely just the throw of the crank?

[Chris Bacon]

Absolutely! However, tuning the induction side of things, which with many 2-st. engines is very interesting, is still the place where the bigger gains are made. Extreme tuning has the unfortunate effect of producing a narrow "band" of power, so lots of ratios are needed to keep things "on the boil", e.g. 0-5600 RPM=3BHP, 5601-X RPM=50BHP! Tuning the exhaust pipe itself is somewhat of a black art IMO.

[Dave Plowman (News)]

I just didn't think it necessary to give chapter and verse - I assumed since the only way a combustion chamber can me measured is by volume, and a ratio must be between like measurements, stroke of the *piston* would be understood to be the volume moved by it. However, what would this group be without pedants - including me. ;-)

[Rob Morley]

The crank can be offset from the bore, in which case the length of the con rod makes a difference.

[Bob Martin]

Not just racing engines - in the 1950s I had a 125cc BSA Bantam. One day the nut holding the exhaust end-cap on came off and the entire contents of the exhaust were shot out backwards. This affected the engine power output to the extent that I could barely ride back to pick up the pieces.

[Christian McArdle]

Well, for a start, n-heptane with a RON of 0 has more energy content than iso-octane with a RON100, so it fits into the general pattern. However, there are genuine counter-examples out there, although most just have a lower octane than you would expect for the energy. Finding a high octane, high energy fuel is more difficult!

The common octane improvers, such as benzene and toluene, which have octanes in excess of 100, have lower energy content. Toluene, for example, has

40.5MJ/kg, compared to 44.4MJ/kg for iso-octane.

When you start adding oxygen (i.e. alcohols, rather than pure hydrocarbons), you get much lower energy content for a massive increase in octane.

Christian.

[The Natural Philosopher]

Not at all. Certainly in my tuning days, the SINGLE most effective way to increase power in MOST stock engines was to hit the exhaust first.

On my old Triumph Spitfire, the gains were well known. about 10bhp increase from a free flow exhaust, then about 7-8 from fitting bigger carbs, and a hotter cam, and then another 5 from fitting gas flowed head, manifold and webers rather than SU's. I went as far as a fast road cam, better carbs, and better exhaust. Still go that car. Must get it back on the road some day..

Similar results on BMC A series engines.

The situation was almost reversed on B series BMC engines - that engine had a ghastly cylinder head and no amount of anything made much difference until that was re-ported and gas flowed with better valving.

Gas flow is all about removing the major bottle necks first. If you think exhaust is irrelevant stuff a potato in the exhaust pipe and see how the power drops off;-)

[The Natural Philosopher]

Mine din't pink on 4 star. They juts ran better on five thats all.

[The Natural Philosopher]

That's more a case of having wild cam timing on a 4-stroke.

VVT has more or less sorted that..but anyone who fitted a race cam to a road going engine knows how ghastly they were on idling, and part throttle fuel consumption - most of which went straight down the exhaust pipe.

[adder1969]

It would, but BMW say it's normal and won't do anything.

[adder1969]

Generally increasing the inlet flow means you get more charge in the cylinder and more power. Increasing the exhaust flow reduces backpressure and helps the inlet charge too but to a lesser extent. Becasue there's less backpressure you get better mpg. Increased inlet flow increases power. Do both and you're onto a winner.

[Sylvain VAN DER WALDE]

"Belt and braces", I suppose. I've forgotten a lot, and am playing safe.

Sylvain.

[Sylvain VAN DER WALDE]

I mentioned that in my answer to Chris, but then decided that it might not be relevent, and deleted that entry. I believe that the reason for this offset is to reduce the pressure on the thrust "face" of the piston, and thereby reduce wear.

Sylvain.

[Chris Bacon]

That's certainly an interesting idea - do you happen to know which engines this is used in?