Formula for calculating deck height on stnd 1600 Kent?

[dikko]

Can someone oblige, rusty brain, I may need to get my volumes right on a 1600 standard block & would make life easier if anyone has a thou-to-cc formula...thanks?

[ian_w]

The cross sectional area of the cylinder bore is given by:

Area = Bore * Bore * 3.14159 / 4 / 100 in cm2

Bore = 81mm?, therefore Area = 51.53 cm2

cc/thou = Area * 0.00254 = 0.131 cc/thou

[JohnD]

ian obliged but used 'bore', or diameter, when cross sectional area is given by Pi.radius^2, and for volume, Pi.r^2.height.
So 4.05cms^2 x Pi = 51.5cms^2 area
and then volume for one thou (0.00254cms) = 51.5 (area) x 0.00254 (height) = 0.1308cms^3/cc
Ah! Those are the figures that ian found!
His 'fudge factor' of dividing by 4 (2^2) gets around using diameter, and 100 around using mm, not cms, but Pi.r^2 is easier than Bore^2.Pi/4/100!

John

[drjim]

This any use??

http://www.johnmaherracing.co.uk/enginecalc.htm

[dikko]

Thanks very much you Guys I'm a bit short of nouse 'up noorth' so I've used drjim's recommended site and I think I've pinned it down to;
81mm bore @ 0.010" depth gives me 1cc
" .020 " 3cc
.030 4cc
.040 5cc
.050 7cc

Can't account for progressive anomolies - guess it's in the computer data.
Anybody feel like confirming above?

[JohnD]

dikko,
That's the problem with using someone else's ready reckoner. You have no idea of the rounding up used, or the errors included. Far better, IMHO, to research the problem and do the calculation yourself. After all, this is only multiplication and division and your calculator can do that arithmetic for you!

As I think you mean by progressive anomalies, I think the JMH site rounds down 0.131 to 0.1, but rounds up 2 (thou) x 0.131= 0.262 to 0.3

John

[dikko]

decisions, decisions, I have 4cc per pot too much, not a lot but could make me 14th instead of 15th (!!!!!!!!). Do I go for it I wonder.

[dikko]

I went for it, inspired by the comments above, simplified it for my small brain and I've worked it out to the best of my abilities. If I'm out somewhere and you use this as a formula then I apologise (you have been warned) but the engine is now in and goes!!! I'll find out how well come first race...

Firstly, the reason for 'Decking the Block'... The engine is for a Formula Ford (Historic Series) ie Kent 711M and as such the regs (5.7) state the minimum combustion volume in the pistons are 41cc presumably as a way of regulating the compression ratio.
So, using a dial guage to get TDC after putting some grease in the bores to make the piston water tight I filled the top of the piston with paraffin through a small hole in a flat piece of plastic covering the bore and found there to be 2.5cc too much. I did this to nos 1 & 4 to make sure the block face was level and both were the same.
Therefore, having 0.7mm of bore left above the piston this needed to be reduced by the equivalent of 2.5cc. to get my 41cc worth of maximum performance. Here we go......

Bore size is 80.8mm across so first the Area (in sq mm).... Pi R2.... ie 3.141 X 40.4 X 40.4 = 5126.6
I measured the BDC to TDC stroke at 77.8mm making the Swept Volume in cc (Area X Stroke) at 398.8cc
Therefore if I divided the Swept Volume in cc's by the stroke that would tell me how many cc's were in the bore PER MM...
ie 398.8 div by 77.8 = 5.1cc per 1mm.
Dividing 1mm by 5.1 gave me the answer to my question which equalled 0.196mm of bore depth PER CC.
So, multiplying that by 2.5 gave me the amount to shave off the block to lose the extra 2.5cc giving me the required minimum combustion volume. This comes out at 0.49mm and gave me a compression ratio (without calculating the head gasket thickness and any cc's in the flat head) of 9.7:1.
Enough to beat all the youngsters in Historic FF (if only I could use it properly...).

Hope I haven't forgotten anything and if anyone has any wisdom to add please feel free.

PS. I seem to have been won over by metric - haven't bothered to convert.
Will be writing about a fool-proof method of priming a new engine of oil for instant oil pressure at start. See seperate thread. Fire extinguishers at the ready...

[JohnD]

dikko,
I don't know your engine, so can only offer general advice.
1/ Paraffin in the bores is fine as a method, but you are dealing with rather small volumes. Are you sure that none was lost in the annulus around the piston? "Grease in the bore", sure. But did you grease the rings and gaps, that could have let some escape?
2/ In the arithmetic you quoted, you rounded the figures before doing the next stage. IMHO this is not a good idea, as after a lengthy calculation such as yours this can introduce error, if the rounding goes all one way. In this event it didn't and I agree with your result, even if you used six significant digits all the way!
3/ You don't give your method of calculating the new CR, but said you ignored head gasket thickness (Usually 30 thou or .76mm) and head volume. Again, risky short cut, IMHO!

Good luck!
John