Patrick sez:
> My 6 cylinder Jag engine came in 2 different
> compression ratios - 8:1 & 9:1. The only difference is the pistons -
> the 8:1 pistons have flat tops, and the 9:1 pistons have domed tops.
> The domes on the 9:1 pistons decrease the size of the combustion
> chambers. So you're stuffing the same amount of fuel/air mixture into
> a smaller space, which is by definition, higher compression.
Right.
> If someone just bores an engine, I don't see how the compression can do
> anything other than decrease, as they'll have increased the size of the
> chambers
Nope- you increase the size of the clyinder bore, which may have nothing to
do with the combustion chamber. While some engines have pistions that don't
make it to the top of the block at full up, most at least come close.
"Blueprinted" engines are often "decked", meaning the top surface is milled
so the pistons rise flush to the top of the block.
Some things you can do to increase power (or torque)-
-increase engine bore, to get more piston surface area to push against.
-increase compression ratio, which increases efficiency.
But, this Compression Ratio thing- lets do some numbers. Lets take a
cylinder with a volume of 90cc (clyinder volume= area of bore*stroke), and a
head with a combustion chamber of 10cc, and play with it. Assume a flat
piston to start, and ignore little factors like the gasket thickness, and
the space around the piston sides...
1) Cylinder Volume + Chamber volume = total uncompressed volume (piston at
the bottom of stroke) 90cc + 10cc = 100cc
The volume of the combustion chamber is the compressed volume (Piston at the
top of stroke) 10cc
The ratio of uncompressed volume to compressed volume is the Compression
Ratio, ie 100cc to 10cc, or simply 10:1.
2) First, lets keep the bore original, and decrease the combustion chamber by
1cc from 10 to 9, by either milling the head, or using domed pistons. We get:
(90cc +9cc)/9cc = 11:1 CR This yields an increase in compression ratio,
without changing the engine displacement.
3) Next bore it out, enough that the cylinder volume increases from 90cc to
100cc. but use the original combustion chamber, & flat pistons. The total
volume then increases to 110cc. (100cc cylinder + 10cc chamber = 110cc)
So, (100cc+10cc)/10cc= 11:1 CR
The bigger displacement and higher compression ratio both increase potential
output, but with the same compression ratio as in #2. (and you can use the
same petrol)
4) Say you want some power, but don't want to buy higher octane petrol than
in the original state? OK, lets increase the both bore by 10cc,*and* the
combustion chamber by 1cc (dished pistons). That gives us:
(100cc+11cc)/11cc= 10.09:1 CR -very close to the original, but with a 10%
increase in engine displacement.
5) Want more power, and you've got good petrol available? Then do both,
increase the cylinder volume from 90cc to 100 cc, and decrease the combustion
chamber from 10cc to 9cc:
(100cc+9cc)/9cc= 12.11:1 CR
Bigger displacement, Higher CR.
****************************************************************************
Now we can apply this to a real engine:
The OHC engine on the MG P-Types have the following specs:
PA 83 x 57 mm = 847.1cc
CR = 6.2:1
HP=36 @5500 RPM
PB 83 x 60 mm = 938.7cc
CR = 6.8:1
HP=43 @5500 RPM
These 2 engines use the same head & stroke, only increasing the bore for a
gain in volume, compression ratio, Horse Power (and torque as well).
If you don't know how big the total compressed volume is, you can calculate
it from this info, and our previous formula. A little algebra yeilds:
Combustion Chamber Volume (Vcc) = Cylinder Volume (Vcyl)/ (CR-1)
So, 847.1cc / (6.2-1) = 162.9cc
[Note: This volume is actually the total of combustion chamber+head gasket+
volume around the piston (+ or -) piston dish or dome volume. The actual
spec is 162cc, which means the stock CR is really 6.229:1]
Stock combustion chamber spec = 162cc
Stock depth of head = 3 41/64
I've measured the cylinder bore of my engine at 2.300" (58.52mm), which is
0.060" over stock. If the head has been milled, I'll have to measure the
combustion chamber volume some day when I pull the head. If it's still stock
thickness, then:
Cylinder volume= ((bore/2)^2)*pi * stroke = ((5.852cm/2)^2)* pi * 8.3cm =
223.242 cc
My Engine displacement= 223.242cc * 4 cylinders = 892.968 cc (call it 893 cc),
(which is 45.1 cc bigger than stock displacement)
My Compression Ratio = (893+162)/162 = 6.51:1 CR
(you can tell I don't need premium petrol!)
PB 938.7cc - PA 847.1cc = 91.6 cc (The volume difference between the two
stock engines)
This puts my engine halfway between the stock PA and PB. From this I can
guess that I have gained at least 1/2 the difference in HP as well (about
3.5 to 4HP) giving me an increase of about 10%, probably to around 40 HP.
Now I feel like going out and measuring the head, as the tuning manual
mentions a standard thickness to take off for a known increase in output...
Now you can go work this out for your own engines...
________
/___ _ \ Roger Garnett (Roger-Garnett@cornell.edu)
/| || \ \ Agricultural Economics | "The South Lansing Centre
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