I use 8.5:1 as a maximum dynamic CR and only if the ignition advance is programable.
For a dissy engine 8.2 is about the maximum .

Ceramic coating works. Be careful about the quality of any coating you get done though. Cheaper is not always the same quality. I spend around $400 on each four cylinder engine I get coated, and a further $160 on the exhaust headers inside and out.

Its interesting to see that most V8 supercar teams and top doorslammer and rail drag teams use ceramic insulation on chambers, valves and pistons. Some have been using it for 15 years.

HRT replace the bottom end on thier cars after 5000 race Km's and the pistons mostly show no appreciable wear after this distance.

Cooler engines and lower thermal stress on rings (allowing lower ring tensions) are the biggest benefits to performance and reliability.

Engine Analyser Pro v 3.3 is the only one I have seen worth putting any effort into. It takes account of bearing friction ring type and number and lots more. It takes a while to setup but produces good base data to compare likely effects. It is not going to give hard output nmbers but makes a good comparative tool.

Thanks a million feral !!
My pc's on the blink and i've just read it now on my parents pc. I can't get enough of that tech stuff. Just as a point of interest, I am doing mech engineering at uni and I have passed thermodynamics - the stuff you said really makes sence but would have taken me for ever to think of myself.

So then, the fact that some people run 12+ and others only 10 (as noted by current posts on this thread) i guess is due to cams, squish, heat factors, and numerous smaller factors that tally a reasonable difference.

Do you have any figures for dynamic c/r that would be a good aiming point for 98 ron??

I would love to hear about the ceramic coating as well if you wouldn't mind !! haha!

Have you any experience with cam picking and desktop dyno or similar simulators ??

Daniel

It runs......but I haven't raced it yet so I'll let you know if it goes bang.
It is ready now however and new pics will be up shortly after the weekend.

I currently have avgas in the tank
I have a lead on some racefuel now thats supposedly about 102 MO and under $1.50 litre.......will let you know if it comes to pass.

No it is not a quote! It took me from about 6:30 am tilll about 8:00 am to get it the way I wanted and then I had an edit after that

Its hard to make it brief and keep on the plot. When there are so many variables, its ard to keep it from being a piece of techno_ speak that is not easily understandable. Its a great subject and I am always looking for ways to improve efficiency and performance.

I was going to put some info on ceramic coating of chambers and pistons in there as well as this has an effect on the compression ratio you can run. Ceramic coating reduces heat soak into the chamber and piston. This reduces potential for detonation at high engine speeds. Maybe that will come into another thread.

At least on this site, people are more interested in technical stuff and how to do things. Not the grand standing (mines better or bigger than yours) you get on other sites

Any one doing engineering, if you get to do thermodynamics: do it! This is one of the most useful "real world" sciences there is in my opinion.

Cheers Feral. Is that an excerpt from the book you mentioned earlier in this thread?

awesome read feral.
cheers

Thanks Feral, I've always wanted to know exactly what caused detonation.
My current race motor use to suffer a bit of pinging at 2500-2800 rpm before I put the fuel injection on and lowered the comp. ratio down to 10:1.
Thanks for the explanation

["QUOTE FERAL"]

Yep spot on!
This is why when someone asks the simple question of "what compression ratio can I go to" becomes a bit difficult to give an exact ratio. It is very dependant on the design and configeration of the completed engine. Then of course you have all the tuning issues, it can be pretty hard to get all this right based on theory alone. My view is that the theory can be used as a guide but thats about all. In reality the control of points 1, 2, and 3 that Feral mentions can be hard to work around especially considering one big variable can be "pump fuels". My experience over the last few years is that pump fuel quality can vary for a million reasons.
So be a little conservative when designing for pump fuels even when your goal is to push everything to it's limits.

The commodores also run lean burn at cruise. To maximise fuel economy the knock sensor does work flat out all the time during cruise and at full throttle. The ecu keeps spark timing just below the level where knocking occurs.

With normal combustion, the charge burn speed varies directly with charge density and the degree of charge turbulence. Higher compression increases charge density. Good port design and chamber shape helps determine the degree of chamber turbulence the fuel charge experiences. More charge turbulence exposes the mixture to the flame front more rapidly allowing it to burn in less time. Another factor varying burn speed is the amount of chamber squish. The squish area forces gases to be pushed through the already turbulent mixture. More squish usually means more turbulence and higher burn speed. One result of the faster burn speed is that less timing advance is required, reducing the chance of detonation under load.

For a given chamber, the Fuel burn speed remains nearly constant if there is a consistent quality fuel charge. This is in spite of engine speed variations. If the fuel charge burn takes an average of 4 milliseconds duration in the chamber- at 6000 rpm the burn duration available for 140 degrees of crank rotation in the burn cycle before the exhaust opens is only 3.888 milliseconds. This means the mixture is nearly completly burnt before the exhaust valve opens. - At 8000 rpm the burn duration available is only 2.916 millisecs, so only 2.916/4 * 100= 72.9% of the mixture is burnt by the time the exhaust valve is opened if the mixture was ignited at the same point as at 6000 rpm. This is known as burn lag. The burn lag is directly related to the burn speed. It is the relationship of the burn speed and ignition timing that determines where peak cylinder pressure occurs. It is where the peak cylinder pressure occurs in relation to the crank angle that determines how well an engine performs.

The wild card with choice of compression ratio is where the dynamic comp ratio is so high the fuel mixture is heated to a point it will spontaneously ignite. When this happens, there is a rapid combustion pressure rise before top dead centre is reached. Pressure increases further as TDC is reached. Chamber pressure becomes so high that now combustion is no longer controlled and the fuel charge explodes (detonates).

Instead of a constant push on top of the piston over a few milliseconds its like the piston being hit by a sledgehammer, with all combustion energy being released in less than a millisecond.

At higher engine speeds more heat is retained in the chamber and this makes detonation more likely with higher dynamic comp ratios.

The end result is the comp ratio you use will depend on -
1. Maintaining consistent fuel mixture quality
2. Having a good chamber design and fast burn speed through development of the squish area and chamber walls.
3. Accurate control of Ignition timing.

Thanks for the replies so far !!

I would like to further open the discussion with some a few points of interest.

1. New commodore v6 engines are 10.2:1, and v8's are 10.1:1 and both run perfect on regular 87 octane fuel. I know they have knock sensors, but I can't imagine the knock sensors working flat out all the time to prevent detonation.

2. datoman_1000 has an a15 setup at 12.9:1. Have you run this engine yet datoman ?? What fuel are you limited too?

3. Datsun100ATurbo runs 12.5:1 using octane booster (100 octane total) - This isn't far from 98 octance.

4. I've read numerous times that higher rpm's can tolerate more compression without knocking.

This is why some people use the "Dynamic" compression ratio instead of a static one. Now this explains why some people have achieved very high static c/r (12:1 up to 15:1) through the use of very large cams.

For those people who are new to dynamic compression, it is the c/r achieved in real life while turning the engine over very slowly. The difference is simple - static c/r assumes the inlet valve closes at BDC. Dynamic c/r uses actual engine geometry and cam figures to compute a more accurate figure.

I've written an excel file to calculate dynamic c/r , static c/r and cranking pressure if anyone is interested.

Does anyone have any dynamic c/r figures that work ??

nzdatman: the compression was achieved mainly thru shaving the head, which the a15's original gx head. Mazda UC pistons, turned down and then valve reliefs cut in them.