Mon Jul 31, 2017 6:39 pm

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Hooked
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Location: Hamilton, Ontario
Mon Jul 31, 2017 6:39 pm linkquote
I had an SAE paper on MSV design and max was 30 with 25 about being ideal. Anything below 20 is a waste of spark. Most engines operate at 20-25msv (on paper).

I used 25 as a guideline for a long time until I started to read information posted by some very smart people who developed engines for Gp125/250 teams. It makes sense that MSV guideline is iffy since the crank pin flexes and connecting rod stretches at high rpm. This means that MSV is actually variable due to clearance oscillation. So we don't know specific MSV as it changes with Rpm and piston location.

Squish clearance standard is 1% of the stroke, or 0.57mm on a vespa with 57mm stroke. Clearance has to do with efficiently pushing gases to the middle and keeping the periphery cool. Squish band width has to do with MSV or turbulence. You set the clearance as tight as possible for given design and then set your band width and then bowl volume for whatever octane you'll use.

From what I see 40-50% band width is standard except for KTM where 65% is used with success on some dirt bikes. Lower range for lower octane and higher end of the range for slow burning octane.
Mon Jul 31, 2017 7:09 pm

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Mon Jul 31, 2017 7:09 pm linkquote
Agreed that when designing from scratch it would be best fixing the gap and volume and then adjusting the squish area to get the desired MSV, With that in hand it's easy enough to design the bowl to fit the volume.

But much of the time all we can change (easily) is the gap, or we are trying to figure out the best way to modify the existing design.

In trying to juggle the numbers to get whatever MSV you are aiming for:
1. Reducing the gap will increase MSV (obvious, I know...)
2. Increasing the squish area will increase MSV
3. Reducing the compression ratio (increasing the bowl volume) will decrease MSV
Tue Aug 01, 2017 2:24 am

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
 
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
Tue Aug 01, 2017 2:24 am linkquote
Ok, not quite sure where I'm going wrong.

I've been using a stock aluminum base gasket (0.25mm) and the piston reaches TDC at 1.7mm above the deck (so I entered -1.7 at x_p Piston to deck clearance). With a 1.53mm head gasket I get 1.05mm squish. However you can see the program is saying it's 0.08mm.

I measured the head with a syringe tonight and got 29cc.



Tue Aug 01, 2017 4:31 am

Hooked
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Posts: 169
Location: Hamilton, Ontario
 
Hooked
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Location: Hamilton, Ontario
Tue Aug 01, 2017 4:31 am linkquote
You are entering your xp as though it is 1.7mm below deck. It is not -1.7 but +1.7 or just simply 1.7mm over deck.

Positive deck when piston above the edge of cylinder (+)
Negative deck when piston below the cylinder (-)
Tue Aug 01, 2017 4:55 am

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Tue Aug 01, 2017 4:55 am linkquote
Ginch wrote:
Ok, not quite sure where I'm going wrong..
The negative value is most likely breaking the calculation because it's taking a square of that value and the calculator doesn't account for that. It's the same problem as when calculating port timing on the BGM 177 due to the recessed head.
Tue Aug 01, 2017 6:05 am

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Tue Aug 01, 2017 6:05 am linkquote
The piston to deck clearance is measured without any gaskets in place. This then allows you to select the base and head gaskets that give you the needed squish clearance. The one value I don't have in here, that I think you have is how far the squish surface is recessed into the head. anyhow, as you have measured the actual squish clearance we can work backward from that.

1.05mm = 1.53mm + 0.25mm + (-1.7mm + recessInHead)

recessInHead = .97mm

in other words if you took out all the gaskets and measured the squish it would be -1.7 + .97 = -.73mm

I added in the extra row for the head clearance and now you get good results.





Tue Aug 01, 2017 6:54 pm

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
 
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
Tue Aug 01, 2017 6:54 pm linkquote
That's great... means I don't have to do anything! Thanks for adding the line.

The compression is showing as 13.7 which doesn't seem quite what I'd expect... or is it? I've been considering opening up the bowl a bit, is there a way to calculate the bowl volume if was to aim for (say) 11:1?
Tue Aug 01, 2017 8:03 pm

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Tue Aug 01, 2017 8:03 pm linkquote
I just noticed the value for the piston crown as -15,464. from the helper calculation (at the far right) it should be -9072 based on a dome height of 4.89mm and bore of 68.5mm.

That drops the CR to 10.3 but the MSV goes up to 33.8
Wed Aug 02, 2017 2:23 am

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
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Location: Victoria, Australia
 
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
Wed Aug 02, 2017 2:23 am linkquote
That figure is the piston crown, plus the amount above the deck (1.7mm).
Hope that doesn't mean that something has been counted twice?



Wed Aug 02, 2017 5:23 am

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Wed Aug 02, 2017 5:23 am linkquote
The 1.7 is counted twice.
Wed Aug 02, 2017 2:05 pm

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
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Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
Wed Aug 02, 2017 2:05 pm linkquote
Ok, so the 1.7 should definitely be shown as -1.7, that's correct Patrick?
For some reason the head volume is now showing as 25300 instead of what I measured at 29000.

I see that as I increase the head volume, the MSV number rises. And as I increase the bowl diameter, the number drops. Now I'm seeing the point of all this a little clearer.
Wed Aug 02, 2017 4:16 pm

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Wed Aug 02, 2017 4:16 pm linkquote
the 25300 was me playing with something I think I missed. when you measured the head volume, did it include the 1.7 recessed section of the head?

If so, I need to back that out of the volume calculations.


What you probably measured for the head volume


to do the volume calculations we need the volume left in the chamber when there is zero squish. I get this by taking the head volume shown here - piston crown volume. But I need to also take into account the recess.

Fri Aug 04, 2017 12:09 am

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
 
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
Fri Aug 04, 2017 12:09 am linkquote
I thought I had replied to this earlier sorry!

I measured the head volume by setting it up level, and filling it with water up to the gasket face level.
The 1.7 I included with the piston volume because it projects into the head space above the gasket face.
Fri Aug 04, 2017 6:30 am

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Fri Aug 04, 2017 6:30 am linkquote
ok, corrected the sheet to account for the extra space in the head. didn't change the MSV much and brings the CR to a reasonable level (11.9:1).



Fri Aug 04, 2017 4:35 pm

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
 
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8080
Location: Victoria, Australia
Fri Aug 04, 2017 4:35 pm linkquote
Thanks a heap. Very interesting to play around with it and see what makes a difference.
Wed Jun 09, 2021 3:14 pm

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VSX, Stella 177, VNX1T, V9B1T
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Location: Staten Island, NY
 
Ossessionato
VSX, Stella 177, VNX1T, V9B1T
Joined: 20 Jul 2018
Posts: 3177
Location: Staten Island, NY
Wed Jun 09, 2021 3:14 pm linkquote
oopsclunkthud wrote:
The spreadsheet is done in Apple Numbers and does not translate well to excel.

I've placed a copy in icloud and made it editable (via the browser) so you can play with the values and see the effects. If you want to keep others from messing up your own settings then save off a copy (even if it's just to your own icloud account)

I've also added a small table called "Spherical Cap" that can be used to calculate the volumes of the piston crown, and sections of the head.

https://www.icloud.com/iw/#numbers/BAIpwdoPL5_t5wrNaeCBsRMsi7Vj4hVriG2F/Squish-2T
Is this spreadsheet available elsewhere or still up on icloud? This link does not seem to work for me. Trying to delve into this concept after reading your unstealth build thread!
Wed Jun 09, 2021 3:53 pm

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Wed Jun 09, 2021 3:53 pm linkquote
looks like I need to update Numbers and re-share it. should be back in a bit
Wed Jun 09, 2021 4:18 pm

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Wed Jun 09, 2021 4:21 pm

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VSX, Stella 177, VNX1T, V9B1T
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Wed Jun 09, 2021 4:21 pm linkquote
Much appreciated!
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