Hi.

Just interested in everybodies experience with maximum compression ratios.

I have bought a 1000 race car with a hot A15 which needs a rebuild. It is currently running a 55/85 cam with massive port job on head and twin 45's. The current c/r is very high and runs fine. Idles under 1000rpm when warm and goes like stink down the straights.

To make matters more interesting, the race fuel it has been running is being phased out, so I need to set it up for premium unleaded (98 octane). I need some definate figures for what works and what doesn't !! Can anyone help me ??

QIK1000

I would say 10:1 would be safe.
If you want to REALLY push it you can go to 11:1.
Most motorcycles run 11:1 comp ratio, BUT the newer motorcycles have knock sensors to prevent pinging.
Maybe, maybe even 10,5:1 would also work.

According to Ricardo's testing, for 98 ron 10.6 : 1 would be top CR

This is figure is available from a table in the book "Design and Tuning of competition engines" by Phil Smith.

I faced a similar problem a few years ago when I was no longer allowed to run Methol Benzene in my race engine. I rebuilt it to be 10.5:1 and it runs ok on 98 octane, just be careful about your advance curves because they can be critical.
But 10.5:1 should be what you have to aim for, any higher than that on 98 octane and you are going to have problems

I am facing a similar dilemma..... My car has been running AVGAS, but that is about to be phased too.....

I had an interesting conversation with someone yesterday who is pretty well connected in the racing world. He was telling me that in the past, Woolworths were importing their petrol from Singapore, and the premium was actually 102 octane, he has been running it in his race car. However, He also believed that Woolworths had done a deal with Caltex recently, and so this situation might have changed.

I have no idea of how accurate this is, but it is something I am going to investigate further.....I need the octane too!

We had a similar thing here too. When Challenge opened, they were getting petrol from singapore as well. But then shell bought them, and now they use the rubbish made at the marsden point refinery. Pump gas here is complete and utter sh!t. Some companys sell so called 98 octane, but it's not availble in the south Island yet.
The fine for using leaded gas on the street is ten grand, and it's not cheap either.

(sorry to go off topic, but this is a sore spot for me. When we lost leaded gas, cars that ran sweet turned into pigs overnight)

How did you get such high compression in an A15? I sruggled to get mine to just under 12:1, before the valve cutouts in the pistons were as thin as I wanted to go. What chamber shape head and what pistons do you have?
Mine runs sweet at 11.75:1 on avgas.

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.

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.

["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.