Hi, I posted a picture of my dished pistons in the photo section!

Quote:

The reason why my engine has worked with this high cr is bacause the cylinderhead has bad flowfigures and the exhaust manifold is original, the volumetric efficiance suffers from this... maybe it will be about 80-85%, according to my experiance this allows you to use a higher cr without detonation but hp and driveability also suffers from low VE so it would be better with a lower cr and a engine that breathes better!!

About dished pistons, In my new turboed A15 (pictures under my photos) I am using dished pistons with the dish offset, the cr is 7,8:1 and I am running 12-20psi boost without much problems, I belive that well designed pistons are a huge help, the squish are also very important..... exactly like Feral said!! Feral has a huge fanclub here in Finland

Cheers Marcus

The L16 runs to 9000. Shifts are normally done at 8500. The 8.2 figure was for 98 RON pump premium unleaded.

I would be looking at around 0.8 mm until I learnt how far the rods stretch.

If using a desktop dyno to do comparisons, set it up so it gives report on the range you want to use. If you have the option report on average torque and average power for the range.

I will dig up the block etc figures tomorrow. I dont have them here with me. Chamber volumes vary a bit with head casting mumbers

Thats very interesting! I guess that L16 would have seen similar rpm to my engine will (ie 7500 normal to 8000rpm on mistakes).

Feral, would you say 0.6mm squish would be a smart aiming point for at 8000rpm a-series with cast pistons?

Also, you still haven't said what octane the fuel was that derived the 8.2 dcr figure. Is this a secret of yours ?

Does anyone know a good source of cheap forgies?

Also, I'm surprised to see that Desktop Dyno reports power gains for my engine by going with a smaller cam. It's currently got a 320degree (i&e) cam. It seems to peak around the 270-280 degree mark! But the racers I've spoken to so far run bigger cams than that! Ideas?

I've checked out the mopar link! Extremely good read! - Thanks. Though, I have seen the cam timing v's c/r article before.

What are thoughts then of making an offset dish in a piston. I think the original A15 pistons I through out had an offset dish. The idea being to provide a squish with maximum flat piston area, and still have a dish.

When compared to a flat-top piston and standard head chambers, I see the advantage of obtaining small squish clearance, and unshrouding the valves as much as possible, as the piston dish will allow more meat from the bottom of the head.

The disadvantage is obviously running a piston which unbalanced about the piston pin's axis. This will increase piston side wall loads and heance piston-bore wear.

Also, as I don't have a standard engine anymore, does anyone have actual figures for the following:
- Standard deck height of A14/A15?
- Standard combustion chamber volume?
- Standard piston to block clearance?

Pretty interesting thread and a very good link to the Cam Timing vs. Compression Analysis article. This was very readable.

One of the comments in the linked article hit it right on the head,
"To sum up - DCR a useful tool, but widely perceived to be of greater worth than can be supported by physics".

It was interesting to see how much variation there was between contributors for the idea of a safe? clearance between piston and head.

As mentioned previously in this sites thread, current technology allows the piston to kiss the head in race engines depending on head and piston design. The builders manage to keep them reliable by making the squish area larger and chamber smaller (within limits) to increase the shocker effect and increase burn speed.

The shock absorber effect means that the rate of resistance to movement of the gases (between piston and squish area) rises exponentially as the distance decreases between piston and head, if both suraces are parallel. It is this factor that allows the piston to kiss the carbon without doing damage.

I will watch that Mopar site in the future even though I dont know anything about the engines. Looks like some good tech stuff there occasionally.

On the topic of squish, heres a link to a discusion about it.Quench, how close is too close?

If you bother to read the whole thread, see if you can spot my post in there.
The gist of it as I understand, is that a motor with a tigher squish, will have more resistatnce to detonation. Even if the result is a higher CR. within reason. Obviously you couldn't run .015 of quench with a 15cc chamber on 91 octane.

On the L16 race engine I ran 0.020" squish clearance and when the engine was pulled down you could see the chamber shape in the carbon on piston.

In current V8 supercar and Porsche cup race engines they run down clearances to where the piston just about touches the head with rod stretch. The squish acts as a shock absorber on valve overlap when the rod receives maximum loading, and also gives maximum chamber turbulence on compressiion.

The 8.2 dynamic figure is based on the actual dynamic comp of an engine I ran. It had detonation at static 10.6, but ran fine at a static of 10.4. This translated to the 8.2 dynamic comp I gave you. Timing scatter and mixture control were the main reasons for the problem.

This dynamic figure will vary depending on the maximum RPM you intend to use. Higher Rpm allow you to use a higher dynamic comp due to increasing inlet inefficiencies at higher inlet velocities (100 m/sec and above) .

Feral,
What squish would you recommend? I'm guessing mine to be about 1mm. If I go much smaller, rod stretch could cause a meeting of the piston and head at high rpm's.
Daniel.

My A14gx is 10:1...has run on regular unleaded a couple of times, run fine. but i fill up with Optimax (98o) 99% of the time

My engine runs.....I've driven it round the carpark at work
It struggled to idle too.....till we put the right coil on it. I had a GT40 on it cause I forgot it was electronic.....oops. Now it has an electronic coil and happily idles at 800 rpm.
I haven't run the engine in yet though.....strill trying to find time and place.
I have some "stale" avgas in the tank at this point......still trying to work out what I'm going to do with fuel as I don't have a CAMS licence or fuel passbook yet. Nor have I transferred the cars log book over.

All to be done soon......and do some KM's in it......once I clearance the guards for the bigger tyres.