I have been working on getting my Marina 1.3 engine ready for my Frogeye. I have been looking everywhere for information about compression ratio and finally after getting too frustrated at there not being any online help, I did it myself. So here it goes, this is the first public access to this spreadsheet. Please give me feedback.
http://www.4shared.c...sion_ratio.html
Sorry about the trouble getting through this free file sharing service.
Calculating Compression Ratio With A Pocketed Block
Started by
matt large
, Feb 06 2012 09:47 PM
16 replies to this topic
#16
racingenglishcars
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Mini Mad
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Posted 18 February 2012 - 11:53 PM
#17
Cooperman
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Uncle Cooperman, Voted Mr TMF 2011
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Posted 19 February 2012 - 01:21 PM
CR Calculation:
1. Measure the average distance from piston top to block deck level with the pistons at TDC. Take the average for all 4 pistons.
2. Calculate the capacity of this volume (Call it ‘A’)
For 0.010” piston top to block deck this capacity will be:
Standard bore at 1275 cc = 0.99 cc
At +0.020” overbore = 1.01 cc
At +0.030” overbore = 1.02 cc
At +0.0402 overbore = 1.025 cc
At +0.060” overbore = 1.04 cc
3. Measure the volume of the dish in the top of the piston using the 5-cc syringe with paraffin, meths, or even clean water. With Hepolite 20250, 20251 & 20253 pistons this is 8.3 cc.
Call this ‘B’
4. Set the head up to be absolutely level using a spirit level. Measure the combustion chamber volume in the head using the syringe and paraffin, meths, or clean water. Do this for a couple of chambers and take the average. Call this ‘C’
5. There is a volume in the head gasket area and we’ll make an assumption here that with the average gasket, say a BK450, it’s 4 cc. Call this ‘D’
6. There is the ‘ring land volume’. That’s the volume down the side of the piston to the top of the top ring. Allow 0.75 cc for this. Call it ‘E’.
7. Now divide the actual swept volume of your engine by 4 to get the swept volume per cylinder. Call this the swept volume ‘SV’
For a standard 1275 cc = 318.75 cc
At +0.020” overbore, i.e. 1293 cc = 323.25 cc
At +0.040” i.e. 1310 cc = 327.5 cc
At +0.060” i.e. 1330 cc = 332.5 cc
8. Add all the unswept volumes together i.e. A+B+C+D+E.
Call this the Total Unswept Volume ‘UV’
9. The Comp Ratio (CR) = (SV+UV) divided by UV
So, if you have a standard bore 1275 cc engine with the SV = 318,75
And if the total of the other volumes, the total unswept vol. UV =, say, 36 cc,
The CR = (318.75 + 36) divided by 36
= 354.75 / 36 = 9.85 to 1
However, if that is not the CR you want, you have to work backwards to calculate what the combustion chamber volume in the head should be.
Say you have the figures we just used, but you want the CR to be 10.4 to 1.
The formula to work out what the UV should be is:
SV divided by (CR-1) = UV
So, in the case we are using:
UV = 318.75/(10.4 – 1) = 318.75 divided by 9.4. = 33.9 cc
Add A+B+D+E, as established earlier and subtract this from the new figure for UV, in this case subtract it from 33.9. The resulting figure is the actual volume you need each combustion chamber to be.
Using the syringe again, after setting the head up to be absolutely level, put EXACTLY this amount of fluid (paraffin, meths or clean water) into a combustion chamber.
The distance from the head face to the surface of the fluid is the amount to be skimmed from the head. To measure this put a steel ruler across the head above the chamber and measure down to the fluid surface with a digital vernier, subtracting the thickness of the ruler afterwards. In practice, this is the most difficult dimension to measure as the fluid has a ‘meniscus’ which sort-of ‘reaches up’ to the bottom of the vernier calliper. You have to make a judgement on this, but don’t be put off, it is not that hard.
Once the head is skimmed as necessary you just fit it and your CR is correct. Remember if you are increasing the CR you will need to check and adjust the timing.
1. Measure the average distance from piston top to block deck level with the pistons at TDC. Take the average for all 4 pistons.
2. Calculate the capacity of this volume (Call it ‘A’)
For 0.010” piston top to block deck this capacity will be:
Standard bore at 1275 cc = 0.99 cc
At +0.020” overbore = 1.01 cc
At +0.030” overbore = 1.02 cc
At +0.0402 overbore = 1.025 cc
At +0.060” overbore = 1.04 cc
3. Measure the volume of the dish in the top of the piston using the 5-cc syringe with paraffin, meths, or even clean water. With Hepolite 20250, 20251 & 20253 pistons this is 8.3 cc.
Call this ‘B’
4. Set the head up to be absolutely level using a spirit level. Measure the combustion chamber volume in the head using the syringe and paraffin, meths, or clean water. Do this for a couple of chambers and take the average. Call this ‘C’
5. There is a volume in the head gasket area and we’ll make an assumption here that with the average gasket, say a BK450, it’s 4 cc. Call this ‘D’
6. There is the ‘ring land volume’. That’s the volume down the side of the piston to the top of the top ring. Allow 0.75 cc for this. Call it ‘E’.
7. Now divide the actual swept volume of your engine by 4 to get the swept volume per cylinder. Call this the swept volume ‘SV’
For a standard 1275 cc = 318.75 cc
At +0.020” overbore, i.e. 1293 cc = 323.25 cc
At +0.040” i.e. 1310 cc = 327.5 cc
At +0.060” i.e. 1330 cc = 332.5 cc
8. Add all the unswept volumes together i.e. A+B+C+D+E.
Call this the Total Unswept Volume ‘UV’
9. The Comp Ratio (CR) = (SV+UV) divided by UV
So, if you have a standard bore 1275 cc engine with the SV = 318,75
And if the total of the other volumes, the total unswept vol. UV =, say, 36 cc,
The CR = (318.75 + 36) divided by 36
= 354.75 / 36 = 9.85 to 1
However, if that is not the CR you want, you have to work backwards to calculate what the combustion chamber volume in the head should be.
Say you have the figures we just used, but you want the CR to be 10.4 to 1.
The formula to work out what the UV should be is:
SV divided by (CR-1) = UV
So, in the case we are using:
UV = 318.75/(10.4 – 1) = 318.75 divided by 9.4. = 33.9 cc
Add A+B+D+E, as established earlier and subtract this from the new figure for UV, in this case subtract it from 33.9. The resulting figure is the actual volume you need each combustion chamber to be.
Using the syringe again, after setting the head up to be absolutely level, put EXACTLY this amount of fluid (paraffin, meths or clean water) into a combustion chamber.
The distance from the head face to the surface of the fluid is the amount to be skimmed from the head. To measure this put a steel ruler across the head above the chamber and measure down to the fluid surface with a digital vernier, subtracting the thickness of the ruler afterwards. In practice, this is the most difficult dimension to measure as the fluid has a ‘meniscus’ which sort-of ‘reaches up’ to the bottom of the vernier calliper. You have to make a judgement on this, but don’t be put off, it is not that hard.
Once the head is skimmed as necessary you just fit it and your CR is correct. Remember if you are increasing the CR you will need to check and adjust the timing.
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