Importance of piston to head clearance, quench area?

[Thuppu]

So what suitable piston to head clearance(quench, squish) in these 5cyl engines? Have heard 0.8mm(0.032inch) is optimal. How big is the difference in knock sensitivity if you compare for example 0.8mm vs. 1.5mm(0.059) quench? Piston CR 8,5:1.


Seems that my pistons came 0.3-0.4mm above the deck, and if the torqued headgasket is 1.6mm, I'll get 1.2-1.3mm quench. Is it too much, engine really knock limited, or can I live with it, using high boost?

[Thuppu]

Tested to torque the headgasket to spec and measured the clearance. The quench exact 1.3mm(0.051"). It's a bit much...

[Billman]

where did you get the info on the first place about 0.8mm??? I really never heard that before.
I have been told that piston diameter is most common cause of knock than quench meaning that a bigger bore with be more prone to knock than a smaller one.

Thinking about it something like this would be as a minimum for a high compression engine or something like this and not on the opposite (like you suggest) but I would like to know more about it.

[JP]

Lol people worry too much I run a lot more than that. Small clearances are important on rubbish injectors as it helps mix the air and fuel in the combustion chamber.

[DJones]

Mines a lot more than that as i had the top of the pistons machined off to drop the compression to 8.5:1.... More importantly are they all the same...

[quattro84]

Squish area is mainly helping (pre)mixture events on low fueled injection quantities (lean burn) and needed to keep the flame front velocity on large bore cylinders at high revs. (which isn't needed that much for scavenging effects compared to single valve heads)
Don't think that this is having a great influence on pre ignition, better raise fuel pressure to get better atomization.
Ideal distance? Impossible to determine without proper combustion analysis tools, let alone the area where you want it to work.
The OEM needs it to reduce emissions and raise fuel economy, both of these issues aren't your main concern.

I've seen some Swedish Savar projects where they ruin the OEM crown design of the piston and still run insane power levels...

[Thuppu]

Ok great, thanks guys. Yea, I'm propably just worry too much.

I've heard that it should be under 1mm or over 3mm, the measure between those is worst, but of course the whole setup has effect on how knock sensitive or not your setup is. I'll forget about this and carry on with the build...

[JariP]

Do you also realize that there can be too much squish from too tight clearances. If the squish velocity is too high it can also cause detonation as too little squish velocity. And then there are boosted engines and they can be very different story again. I would really like to take a peek at some of the late model WRC engine cylinder heads and pistons to see how they are doing it. I don't think you could find something like parallel 0.6 to 1.0mm squish clearace from those engines...

I think I've also seen some of those Savar projects, I think they are just doing the same things that top level racing engine builders have been doing for years now.

[Billman]

I don't think you could find something like parallel 0.6 to 1.0mm squish clearace from those engines...

I think I've also seen some of those Savar projects, I think they are just doing the same things that top level racing engine builders have been doing for years now.

Care to explain a bit more?

[quattro84]

I would really like to take a peek at some of the late model WRC engine cylinder heads and pistons to see how they are doing it. I don't think you could find something like parallel 0.6 to 1.0mm squish clearace from those engines...

I know what it looked like on a '02 WRC Mitsu engine, high level of squish (like stock) with flat surfaced HKS pistons with small valve relief pockets. Ratio is high (12:1) and from the top of my head they were level to top deck so i guess clearance ~1,2-1,4mm. But completely different ballpark, running up to 50PSI to reach max compressor flow & torque with tapering in regards to the 32mm restictor choking it down. It ran with 'umluft' and needed the CR for transient response. Best co-drive i've ever had

[JariP]

Care to explain a bit more?

Not really, but if something like running tight squish was the hottest racing secret 20, 30 or even 40 years ago, do you really think they stopped developing things further at that point? Different engines want different things to work, there is no one -works for everything - answer. NA is different to turbo, or any other power adder and so on.

I remember reading WRC cars were running CR's up to 14:1 before the regulations were changed to keep the power levels down. They were starting to reach 400 hp even with the "300 hp" restrictors.

[radom]

Hi, sorry for hijacking your thread but I wanted to ask similar question
should the pistons be level with the deck in stock 3b engine?
32,8+144+(86,4/2) = 220mm right?
so if I'm using aftermarket pistons with 33,4mm of compression height (wossner 8,5:1) then they should come 0,6mm over the deck height?

[Error404]

Has anyone got an idea of a figure for minimum squish clearance? Been doing some digging on this as I'm concerned mine will be too tight - 0.60mm assuming HG compressed thickness of 1.55mm which I believe is the figure for the MLS gasket. Keen to hear any informed opinions.

Alex