Bored Engine results to larger squish area

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bmwmaster
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Bored Engine results to larger squish area

Post by bmwmaster » Sat Feb 09, 2019 7:19 am

Hi All,

as in my bmw m20 engine im Using an Head with flat squish area, when block is bored larger I will get much more squish band around.

Please see the attached pic. Red circle is oem bore (80mm) green is how squish will increase (86mm).
So Im asking me If i should grind the chamber larger to reduce quish? or leave it like it is?

The installed pistons will be simple flat. Squish height will be about 1mm.

Whats your opinion to optimize the mod?
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Re: Bored Engine results to larger squish area

Post by BOOT » Sat Feb 09, 2019 7:31 am

You'll reduce compression, so depending what your comp goal is, I'd prob just deshroud the valves. Is the red line the actual bore or the gasket?
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Re: Bored Engine results to larger squish area

Post by Dan Timberlake » Sat Feb 09, 2019 12:02 pm

The end gasses furthest from the plug are where detonation likely starts, so quench effects there are pretty important.
By inspection, and armchair internet observation of modern cylinder 2 valve cylinder heads, the width of you squish band is not excessive, if there even is such a thing.


As far as combustion enhancing turbulence from a tight squish, versus any tumble or swirl provided by the OEM port and valve layout, I have no idea.

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Re: Bored Engine results to larger squish area

Post by PackardV8 » Sat Feb 09, 2019 2:35 pm

FWIW, as long as it doesn't increase valve shrouding, more squish area is usually beneficial. Has anyone seen it become a negative?

On obsolete engines where we mill cylinder heads .050"-.125", the effect slightly unshrouds the valves, increases compression and increases squish area; all good results.

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Re: Bored Engine results to larger squish area

Post by bmwmaster » Sat Feb 09, 2019 4:04 pm

OK, thanks for your opinions. as long as my compression will fit I wont't touch the squish.

Can you please explain valve shrouding or unshroud more :wink:

Does it mean the aera where valve seats are near to the edge (squish)?
And that grinding more free area helps flow in the chamber?

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Re: Bored Engine results to larger squish area

Post by Dave Koehler » Sat Feb 09, 2019 5:18 pm

I am with Boot. What is the red line? The actual bore or the head gasket?
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Re: Bored Engine results to larger squish area

Post by bmwmaster » Sat Feb 09, 2019 5:29 pm

The red line is the bore and head gasket where the head is normally used for( it's an 2litrr inline 6)

But I'm using the head on a 2.5 liter inline 6....which is stroked and bored to 2.9l.

So particular for this head the 2l bore is 80mm and the 2.9l will have 86mm.

The head gasket is 85mm. My idea now at the moment is to grind a slightly chamfer to the piston to avoiding interference with the gasket.

So yes there is pretty much meat from the red line(80mm) until you reach the gasket or the bore diameter.

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Re: Bored Engine results to larger squish area

Post by digger » Sat Feb 09, 2019 6:39 pm

just get a 86.5mm bore MLS gasket

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Re: Bored Engine results to larger squish area

Post by MadBill » Sat Feb 09, 2019 10:48 pm

bmwmaster wrote:
Sat Feb 09, 2019 4:04 pm
OK, thanks for your opinions. as long as my compression will fit I wont't touch the squish.

Can you please explain valve shrouding or unshroud more :wink:

Does it mean the aera where valve seats are near to the edge (squish)?
And that grinding more free area helps flow in the chamber?
Yes. If you scribe an arc centered on the valve stem, approaching the new cylinder wall diameter, grinding away material in the resulting 'new moon' shaped area usually improves port flow. Google "Unshrouding valves" and research before you cut.
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Re: Bored Engine results to larger squish area

Post by naukkis79 » Sun Feb 10, 2019 4:41 am

The more squish the better. M20-combustion chamber can be made really good.

It's pretty good idea to look similar good heads, let's take example from 70's as Nissan LY(which probably is copy from Cosworth, at least their really good 4-valve head from that time are)

Image

Image

Rounded squish pads are for domed pistons, for flat top pistons flat squish is preferred unlike BMW latest M20-heads that's very outright stupid design with combined dome-reverse dome pistons(wonder why they didn't just add lead as piston ballast)

For that M20 head to be real good shrunk valves, exhaust much more than intake to get it below intake level to get less blow through, better chamber flush and ability to use more overlap. Shaving head gives little bit more squish to start and makes it easier to reshape chamber.

For 2.9 litres you want to install as large intake valve as could, which also gives plety of meat to reshape chamber.

That Nissan LY-head makes wellover 400hp from 2.9 litres....

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Re: Bored Engine results to larger squish area

Post by Momus » Sun Feb 10, 2019 1:19 pm

I play with small 2 valve XR100 Honda bike engines with about a 50 degree valve angle. These have a straight valve arrangement rather than the cant of the BMW. Some of these are using a 62 mm bore under heads that started life over 47 mm bores which subsequently were enlarged in production engines to 53 mm. The resulting 9 mm annulus, a 4.5 mm ring, is all squish and met by a piston that is either flat topped or very slightly domed and set to deliver 13.6:1 CR. Usually the squish clearance is 25 thou. The inlet valve uses 45 degree seats with a pronounced concave top blend into the chamber. The diameter of this recovery blend is about 4 mm larger than the intake valve. The head gasket is a .25 mm steel shim that is 62.5 diameter.

These engines run satisfactorily on Australian pump 98.

The best versions with heads, produced by Takegawa are over 150 hp per litre (2.5 hp per cubic inch) with inlet valves well under 50% of the bore at 28.5 mm.

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Re: Bored Engine results to larger squish area

Post by bigblockmopar » Sun Feb 10, 2019 3:14 pm

bmwmaster wrote:
Sat Feb 09, 2019 5:29 pm
...
The head gasket is 85mm. My idea now at the moment is to grind a slightly chamfer to the piston to avoiding interference with the gasket.
...
I'm no pro-engine builder but I thought you don't want the headgasket to overhang the cylinder or the gasket will fail and/or blow out.
Not sure how much of this still stands if the head chamber is even smaller still.

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Re: Bored Engine results to larger squish area

Post by rustbucket79 » Sun Feb 10, 2019 11:50 pm

An uncompressed fire ring and s a nonfunctional fire ring. In other words, there must be material on both sides of a gasket for it to perform it's function. (Block AND head)

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Re: Bored Engine results to larger squish area

Post by Erland Cox » Thu Feb 14, 2019 5:43 pm

naukkis79 wrote:
Sun Feb 10, 2019 4:41 am
The more squish the better. M20-combustion chamber can be made really good.

It's pretty good idea to look similar good heads, let's take example from 70's as Nissan LY(which probably is copy from Cosworth, at least their really good 4-valve head from that time are)

Image

Image

Rounded squish pads are for domed pistons, for flat top pistons flat squish is preferred unlike BMW latest M20-heads that's very outright stupid design with combined dome-reverse dome pistons(wonder why they didn't just add lead as piston ballast)

For that M20 head to be real good shrunk valves, exhaust much more than intake to get it below intake level to get less blow through, better chamber flush and ability to use more overlap. Shaving head gives little bit more squish to start and makes it easier to reshape chamber.

For 2.9 litres you want to install as large intake valve as could, which also gives plety of meat to reshape chamber.

That Nissan LY-head makes wellover 400hp from 2.9 litres....

Really impressive build!

Erland

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