what will be an optimum CR on a CA18DET

[liquidsmoke]

I would recommend removing the squish areas as Danny said, or running a 1.9mm MHG for up to 400bhp. If your flooring it, then you will be in boost all the time anyway. If your on the track you'll just have to make sure your in the right gear

[ByReaL]

for certain it will be used for both driveing and racing


to post some more details:
the car has a laptop in it, a 8" TFT with touch screen, and the PC will be conected to the ECU (whatever ECU i'll chose it will be chosen to give me the posibility to load whatever map whener i want), EGT , AFR,oil pres oil temp,,, monitoring and logging - already bought all that stuff


for street use i'll have for certain a low boost map 0.5 bar or as low as it will be posible to achive with the GT2871R and another map for 1 bar of boost.
anyway on the road i do not need more then 250-300HP, no room to use that sort of power anyway, it also should be safe to drive good fuel economy (anyway), and afther all in my country the maximum speed limit on the highway is 130km/h and town speed limit is 50km/h, no need for nitrous no need for water injection (anyway you got the picture, no need of sky hi power on the road just a nice ride)

on the other hand on the track the car must (should) be a winer, for that higher boost will be run, nitrous will be added, (water injection also if necesary), afther all i have read on SXOC i beliefe it will not be imposible with 5000$ invested only in head cams, pistons, rods and other engine mechanical parts to obtain an engine that will be able to handle ~500HP (400HP only with boost and beyond that to use 100HP nitro shots) (lets give figure will be ~30 races per month no longer then 15 sec, and when raced the car will must give all the best, i usualy service my car before a race day and afther and have a good care of her)


anyway i'm opened to any sugestion regarding the CR

[ByReaL]

I would recommend removing the squish areas as Danny said

please can someone provide a picture or give more details, i'm not so familiar with english and i do not understand at what part this word refers to.


what exactly do this ?

will remove a hot spot, will change the CR or what ??, any down side in doing this ?

[Chris B]

Squish is the flat area either side in the combustion chamber of the standard cylinder head, it is supposed to squezze the mixture/charge away from the cylinder walls as the piston gets closer to the combustion chamber ceiling.

Removing it will increase combustion chamber volume and also lower CR, it will most likely reduce the chance of getting pre-ignition too, that should mean that you can use even more advanced ignition timing for that high boost

[George]

What you need is one of these
http://www.saab.co.uk/main/GLOBAL/en/vepsilon/
I like this idea.

[Chris B]

SAAB have been developing that for decades IIRC

Such a simple concept, why didn't I think of that?

Optimum torque at any revs

[cliffb75]

Squish is the flat area either side in the combustion chamber of the standard cylinder head, it is supposed to squezze the mixture/charge away from the cylinder walls as the piston gets closer to the combustion chamber ceiling.

But the purpose is to provide greater surface area to gas ratio to cool the end gasses, thus reducing the chance of detonation. The problem is that to be effective for this the clearance needs to be quite tight - probably around 1 to 1.5mm for a CA (don't quote me, its dependant on bore size and I haven't got my book to hand). The standard production tolerances mean you rarely get anywhere near this, but skimming the head to correct the squish also raises the CR, which is fine for an NA but not good for us. Conversely, As soon as you start dropping the CR by putting in a thicker head gasket you also make the squish band wider, reducing its effectiveness.

So in theory the 'right' way to do it is to either re-profile the combustion chamber or put a bowl in the piston (which in turn must have the right deck height at the edges still - its no use machining the piston down flat) to getgood squish and the correct CR.

IN THEORY this would then mean you could run a higher CR than would otherwise be the case, but I have no direct experience of it on a CA yet so I wouldn't listen to me........

[Chris B]

But the purpose is to provide greater surface area to gas ratio to cool the end gasses, thus reducing the chance of detonation.

Really I wouldn't have thought it makes all that much difference to surface area

The problem is that to be effective for this the clearance needs to be quite tight - probably around 1 to 1.5mm for a CA The standard production tolerances mean you rarely get anywhere near this. As soon as you start dropping the CR by putting in a thicker head gasket you also make the squish band wider, reducing its effectiveness.

The standard CA18 squish band seems to be pretty ineffective anyway would help if it was tapered too

So in theory the 'right' way to do it is to either re-profile the combustion chamber or put a bowl in the piston (which in turn must have the right deck height at the edges still - its no use machining the piston down flat) to getgood squish and the correct CR.

Or both so the piston is a sort-of reflection of the combustion chamber

IN THEORY this would then mean you could run a higher CR than would otherwise be the case, but I have no direct experience of it on a CA yet so I wouldn't listen to me........

I would say that's pretty much true with the right engine build

[ByReaL]

i gues i got an idee but i'm not shure i understood right

if someone has a picture will be a verry much help

[ks13]

yes picy please, not familiar with this squishy area.

[cliffb75]

Really I wouldn't have thought it makes all that much difference to surface area

Not surface area, but surface area to gas ratio.

Take a look at the pic


You can see that if you have a thin squish area there is not much actual gas volume, but quite a bit of head and piston surface to cool the gas that is there. The pic is taken from this link
http://www.muller.net/mullermachine/docs/squish1.html
which is talking about 2-strokes, which have even more dependency on squish to reduce knock due to them having to pass oil through with the air/fuel charge which lowers the effective RON, and increases the chances of knock.

For even more info check out:
http://www.motorcycle.com/mo/mcrob/rt-fuel2.html, This is heavier going, but includes this response to a question:
"It is important to realize the two important functions of reducing the squish band clearance: (a) to enhance turbulence due to rapid ingestion of gas into the combustion chamber, hence increasing the burning rate of the mixture and (b) to reduce the volume of the unburned gas in the boundary layer of cool gas near the piston top and cylinder head surfaces. Typically, gas trapped in the squish area doesn't burn, even if the squish band clearance is relatively large. The cooling effects of the large surface-area-to-volume ratio of this region will prevent any ignition of the fuel-air mix therein, even if the squish band clearance is rather large. Hence any gas caught in the squish band will not be burned near TDC when it does the most good, but later during the combustion process when one cannot extract as much work from the late-burning gases. The amount of gas trapped in the squish band can actually be a substantially greater amount than just the relative volume of the squish band because the pressure wave from the ignition process literally crams a lot of the unburned gas into crevice areas like the squish band. Reducing the squish band clearance will decrease the amount of unburned gas substantially, leading to more complete and faster combustion, lower emissions and improved power. It is one of the few "all gain with no pain" modifications one can carry out on racing or even street motorcycles."

So what we're getting to is that if you are going to use a squish band, you want it to be a thin as practicable. The other alternative is to avoid the squish band and reduce the compression ratio to an acceptable level to avoid knock. The advantage of using a squish band (if done correctly) is the ability to run a higher CR, and hence get better off boost performance and thermal efficiency. But if peak power is all that is required then the no squish, low CR, (i.e. thick head gasket) big boost method is entirely reasonable, and much easier to achieve, especially is oversize pistons are being fitted (e.g. Brens Wiseco's)

As with everything in life, you need to decide what you want, because its all a compromise

[Chris B]

Not surface area, but surface area to gas ratio.

Ah, to cool the gases being squished
Now I see what you meant
I guess it would cool the exhaust gases left in chamber, but it would heat the charge mixture I suppose
The cooling effect would be greater than the heating effect though so I agree that it could well be there to reduce in-cylinder temps

Good quote there Cliff

Isn't there a squish band built into the pistons though?

[cliffb75]

I guess it would cool the exhaust gases left in chamber, but it would heat the charge mixture I suppose

You're on the right track, but you need to think in terms of what causes knock and when it occurs.

First off, you're not interested in exhaust gasses. They're done with (exhausted if you will ). So they are out of the picture.

You're right that initially the combustion chamber walls will be at a higher temp than in incoming charge, but at that point the piston is on its way down the bore, drawing the charge in. Somewhere around Bottom Dead Centre (actually it can be quite a bit after depending on cam timing but lets not go into that here) the inlet valves close and the piston moves up and starts compressing whatever got trapped in the cylinder. This causes the charge to heat up - a lot! Then before the piston gets to the top the spark fires and the charge around the plug begins to burn, and now things really start to heat up.

What you have to remember is that combustion is not instant. Very quick, yes, but it still takes a finite amount of time for the actual burning flame front to move from the plug tip to the edge of the cylinder. However, the heat being generated from compression and the now burning charge travels to the end gasses more quickly, whilst all the time the piston is still actually rising and compressing the charge (which is now trying to expand as it burns) even more. This is the point at which knock can occur - after the burn has started but before it has had a chance to get to the edge of the combustion chamber. At this stage in the game, the charge is significantly hotter than the piston crown, cylinder walls and fireface, so they exert a cooling effect on the end gasses. Look at the picture again - the squish band only really works near TDC - before that the difference in the volume in the centre of the chamber and the edges is not that great as a percentage. Its only when you are nearly at the top that the squish band becomes significantly smaller in volume (relatively) than the main part of the chamber.

So in this case if the squish band is small, the volume of gas in that band is small, and therefore the surface can draw the heat away before it gets hot enough to self ignite, or at least delays the point at which this happens.

Isn't there a squish band built into the pistons though?

It depends on the engine. In some engines the cylinder head is flat and the combustion chamber is a bowl in the piston crown. In this case the flat area around the edge (if there is one) determines the squish band. On other engines, such as the one in the pic, and the CA, the piston is flat or dome topped, and the squish band is defined by the shape of the combustion chamber profiled into the head. Ultimately the squish band is defined by the interaction of the head and piston - its basically how close the two get to each other and over what 'squish band width', so both bits count.

[Rudi]

I would not raise or lower the CR at all with that turbo, not needed when using 98octane fuel. I would be worried about having a proper engine management with this sort of power, not the cylinder head.

You can polish the combustion chamber though, this will smooth out ridges and keep heat just a little bit more in the combustion chamber. The more heat gets transfered through the head upon combustion, the less power you'll have. Heat is what pushes the piston down, not explosions

It would be rather special if the squish could cool down the upcoming mixture when the squish is being exposed to the massive heat from the combustion. The squish ridge is actually a hotspot, not a coolspot. It is far away from the coolant.

If you wish to remove the squish, you could add compression again by skimming the head afterwards. You can obtain the correct value by doing a cc test of the combustion chamber before removing anything and then after removal once again, and skim it until it's within the original value once again.

Got a pic of my head, still has the squish but has been polished;




You can use water or methanol injection for cooling of the mixture. But this is only good for cooling and therefore avoids excessive fuel dumping. It is not an octane bandaid, more of a power adder due the denser mixture.

I can also highly disrecomend using thick head gaskets. The name says it; it's a gasket, not ment to add material. It will also increase the distance between the piston and the head, creating an oversized area underneath the squish.

Just my view on the situation

[ByReaL]

searched over the net (google) for compresion ratio, but i was not able to use the compresion ratio calculator .

i understood that fiting pistons with 1 mm oversized (84mm more) will raise the CR to 8.66:1

i need some formulas or estimation, about how much a 1.6mm HG will lower the compresion (on 84mm bore pistons) and what about 2mm HG,

what change and how much will be afected the Cr because of the squish area removal


anyway some info so i can spec my engine CR

thanks

[liquidsmoke]

to help you out, my combustion chambers are 57ml with the squish removed. See Oddins thread for pics of the head.

[ByReaL]

to help you out, my combustion chambers are 57ml with the squish removed. See Oddins thread for pics of the head.

thanks, already had a good look at the pictures

[saqmaster]

As Cliff correctly states, the purpose of the squish area is to remove the chance of det. I honestly, from my experience with other engines, see how removing it is going to benefit you in any other way other than reducing the compression ratio. What Cliff says is right, the correct way to do it is to get your deck protrusion correct (I run anything down to 0.006" clearance!! depending on application) and using the correct piston design to obtain the ideal compression ratio. This will give you the best running, least det prone, best emissions and best driveability package you can have. Believe it or not, the manufacturers do know what they're doing - but as always, costs are always the major constraint.

With regards to compression ratio. This highly depends on the engine. I don't know where the CA18DET dets on a particular given fuel, i've not had any dyno time with it. However, to compare to the ford zetec engine in turbocharged form, I run these at 7.5:1 for pretty much everything under 360bhp dropping down to 7.2:1 for up to 500bhp. This leaves you around 4 degrees away from det at every load site on the top line, using 98RON fuel (assuming a good 12-14 degrees advance). This is pretty safe.

And if you're worried about it being soggy, or laggy, a 300bhp engine with that compression ratio is producing 320 lb/ft @ 2700rpm, with over 200 lb/ft @ 2000rpm.. Ironically this is with a useless squish area due to the 2mm deck height on these pistons..

Personally on the CA, i'd go with 8:1 for > 300bhp engines, dropping linearly beyond that to say 7.5:1 at 500bhp. It's definately a good starting point.

[Thomasx]

I run on 8.9:1 as opposed to what most guys do here. I find this setup very driveable, I also got very impressive readings @ stage 1 - torque peaking (around 310 nm) at 4000 rpm.

I am at stage 3 right now, will soon be on dyno to see what I really got

The downside is that one can only run on custom maps, not a big issue though

[black smoke]

Mobnes 700bhp ca18det has them removed so thats a giant prove that it works with so many videos for u guys to enjoy

At 30 plus psi of boost and lots of power very unreal torque levels

The only ca18det seen in action enjoying the power