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Kawi J engine squish clearance? 17 Mar 2017 17:24 #756949

  • VTEC
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Hey,

Assembling a Kawi J engine with MTC 1075 kit. What's the best way to go on squish clearance? I've gotten answers ranging from .030" to .060".

Thank you for any help.
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Kawi J engine squish clearance? 17 Mar 2017 18:06 #756952

  • zed1015
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What are you aiming for ?
There is a maximum distance at which the squish effect ceases to be usefully effective and every engine has it's optimum figure.
The tighter the squish the better but you need to account for rod stretch, heat expansion, valve to piston etc etc.
Some serious race engines are run with tight squish that shows evidence of piston to head contact at max rpm.
If you are building for the road you don't need to go to extremes trying to extract every last bit of power and efficiency to the N'th degree which won't make a difference for day to day running.
around 40 thou is the norm but you probably wouldn't know the difference if it were 60 except on the psi figures and you may not even get to that if using the oem fibre head gasket and no other mods.
A stock Cometic fibre gasket is .043 so using that with zero deck will get you where you think you want to be but to get the deck height you may need to skim the block.
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Last edit: by zed1015.

Kawi J engine squish clearance? 18 Mar 2017 16:45 #757036

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Hey Zed,

That's what I was thinking. .040" being the best for combustion with a safe margin to avoid piston-to-head contact. As I understand it a tight squish pushes the air/fuel toward the center of the burn for better combustion, while a loose squish hurts combustion and leads to unwanted detonation.

I did some preliminary checks, and deck height is about .010". Plus .043" gives me about .053". But I'm planning on using OEM 1100 gaskets (I know the Cometic gaskets don't seal like OEM with the o-rings), and they are .004" or .005" thicker. That will put me closer to the .060" range.

I'm going to confirm squish on assembly at TDC using .060" solder. So the big question is should I shave the block deck .020" to get the .040" squish, or will upping the compression counteract the advantage?

Thanks for your help!
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Last edit: by VTEC.

Kawi J engine squish clearance? 19 Mar 2017 02:30 #757091

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Squish aims the fuel mix towards the centre of the cylinder ( and has more of an effect on two strokes) but detonation is caused by too high a compression ratio or too low octane fuel for the compression ratio used causing the fuel to spontaneously ignite.
If you are just fitting the 1075 kit as a general upgrade during the course of a normal top end overhaul and not as part of a complete performance engine build I wouldn't worry too much about squish, however unless otherwise stated a zero deck height is what the piston manufacturers base their static compression ratio on.
The decision is yours.
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Kawi J engine squish clearance? 19 Mar 2017 06:29 #757101

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Overview of the engine below. In your experience will this run without detonation on 93 octane? And without retarding timing to cover it up?

Right now there's about .006" off both the head and block decks to get them flat.

I do know that when the engine was stock it would detonate from carbon build up on the pistons. I would run a de-carb periodically to quiet it down.

1075 10.5:1
APE did the head with stock valves and street porting
APE .410 cams
FCR35 carbs w/pods
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Last edit: by VTEC.

Kawi J engine squish clearance? 19 Mar 2017 08:40 #757124

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The higher the compression the more risk of detonation.
Closing up the squish will increase compression.
No point in optimising squish if detonation is the result or you don't use a higher spec fuel.
I'd rather trade an unperceivable increase in efficiency against risking piston damage.
Over here in the UK stock unleaded is 95 octane and super is 97 and I never have any detonation with either when running 10.50 comp and also don't suffer excessive carbon build up either.
I read a lot on here regarding detonation issues on road tuned US motors but never gave it much thought that it was down to your fuel and not just a habit for pushing the compression figures.
I would suggest you use the highest octane pump fuel readily available .
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Kawi J engine squish clearance? 19 Mar 2017 09:52 #757131

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I always use super. But where I am in the U.S. that's 93 octane with 10% ethanol. Next up would be racing fuel at 98 octane for $10 a gallon.

Is there a good estimate on compression increase as per .001" off the deck?
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Kawi J engine squish clearance? 19 Mar 2017 17:04 #757171

  • TexasKZ
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Is the octane rating measured the same way in the UK as in the US? In the US, it is RON + MON / 2.
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Kawi J engine squish clearance? 19 Mar 2017 18:02 #757174

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TexasKZ wrote: Is the octane rating measured the same way in the UK as in the US? In the US, it is RON + MON / 2.


Good point.
I don't profess to be an international fuel expert, I just work with what we have at the pump over here and use VP in the race bikes.
Just had a quick google and your fuel is RON/MON whilst ours is just RON.
Your 93 octane is equivalent to our 97 so the detonation really shouldn't be an issue unless there is something else going on or other issues are being misread as detonation.
Our fuel is only 5% ethanol though which may or may not be a factor.
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Last edit: by zed1015.

Kawi J engine squish clearance? 19 Mar 2017 18:51 #757179

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I didn't realize the rating difference. I do know here the ethanol content gets closer to 15% in the winter-mix. And it often drags out to the hot months causing a hot-soak condition in some car engines.

Is there a KZ deck height to compression formula? I know APE makes block base spacers to reduce compression starting at .020".

I saw a compression to octane chart (not sure how valid it is) showing an optimum burn for 10:1 @ 100 octane,
and found a graph with similar compression/octane numbers at: garage.grumpysperformance.com/index.php?...pression-ratio.2718/

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Last edit: by VTEC.

Kawi J engine squish clearance? 19 Mar 2017 19:09 #757180

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Hey TexasKZ,

Where you at? I'm just outside of Dallas.

I see you're building an LTD just like mine. I've learned a lot, so let me know if I can help. Got a good story if you need rear shocks.
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Kawi J engine squish clearance? 20 Mar 2017 03:15 #757192

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VTEC wrote: Is there a KZ deck height to compression formula? I know APE makes block base spacers to reduce compression starting at .020".

You would need to cc the combustion chamber with the piston at TDC and then divided the volume of that cylinder at BDC by that figure to get the true compression ratio of your engine.
The thickness of a barrel spacer has no fixed drop figure. The same spacer on a 900 will reduce to compression by a different amount to that fitted to a 1075 etc.
To get the advertised compression of your pistons you need zero deck height (unless otherwise stated) and use the supplied head gasket which is commonly 0.040" (1mm) when compressed.
This also happens to give your safe .040 squish too.
I would get your deck height to zero and leave it at that.
Fitting slightly thicker gaskets will drop the compression slightly which would combat detonation but looking back at you first post I wouldn't say It was the fuel octane causing a problem but excessive carbon build up from worn cylinders burning oil.
Also I notice you have 35mm carbs.
I have never used those FCR's but 35mm seems a little too large for 1075cc and are probably more suited to a capacity over 1200cc.
I run 32 smoothies on a similar spec engine and they are more than enough.
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