We’ve all heard of CC’ing an engine to get the compression ratio exactly right, but how many of us have done it? I’m just going through it myself for the first time.
First of all, why? It’s a fairly slow and not inexpensive process, so why bother? In my case I had two goals:
- I have to pass
- I want a nice smooth engine. All straight-6 engines inherently run smoothly as they are well balanced by design, but having a big stroke engine (going up from 69.8mm to 85mm) will work against that and I might end up with a shaker, which will be a real disappointment. I’ve been very careful to balance all the engine components as close as I can (within 0.4g), but the other big factor is the sizing of the combustion chamber. If the chambers are all of varying sizes I’m going to get varying amounts of charge in them and that will create uneven running. Equalling out the chambers will give me a nice smooth engine.
So what do you need to do? First things first, you need to measure some volumes. The two that can’t be calculated are the size of the combustion chamber in the head (it’s a very uneven shape so there’s no hope of an accurate calculation without just measuring it), and likewise you need to measure the volume of the piston crown – typically you have valve reliefs cut into the piston and it may be dished too. Dan Amaral, the very skilled chap who does the machining work on the engines for me, measured these volumes while working on the bottom end as he has a burette and I don’t!
The volumes I got were these:
Head volume: 51.00cc
Piston crown volume: 12.50cc
Now for an accurate number (why calculate if it’s not accurate?) we need to think about the other spaces in the combustion chamber that will affect the volume. Here are the ones we used:
Gasket volume – The volume of the compressed head gasket
Top ring volume – Pistons aren’t the same size as the bore, so there’s a gap around the edge of the piston, between the top ring and the top of the piston that you need to take into account
Deck height volume – The distance from the top of the piston to the top of the block. In some cases (like mine) the piston may actually protrude out of the top of the block and so the deck height volume will be a negative number as it actually reduces the size of the combustion chamber.
Swept volume – This is the big one. It’s the volume that the piston actually sweeps and is the volume that’s usually quoted for the engine.
First some measured numbers that we’ll use to calculate the volumes:
Bore: 95.00mm
Stroke: 85.00mm
Gasket compressed size: 0.75mm (an average of the numbers I’d read about for this)
Gasket diameter: 97.00mm (measure the gasket and you’ll be surprised!)
Piston crown diameter: 94.20mm
Piston crown to top ring depth: 6.00mm
Deck height: 0.05mm above block
And now the maths (ever wonder what maths was useful for?).
First the basic one: volume of cylinder
Volume = PI x radius squared x height
So for the swept volume that’s:
PI x 4.75 x 4.75 x 8.50 = 602.50cc
(notice I converted the mm measurements into cm because we wanted the answer in cubic centimetres, not cubic millimeters)
The other volumes are all calculated the same. The only interesting one is the top ring volume as that’s the difference between two volumes. The actual space we’re interested is the cylinder volume between the top ring and the top of the block MINUS the volume of the piston in that area.
Here are my calculated volumes:
Volume of top ring area: 42.53cc
Volume of piston in top ring area: 41.82cc
Top ring volume = 42.53 – 41.82 = 0.71cc
Gasket volume = 5.54cc
Deck height volume = -0.35cc (negative because the piston is above the top of the block)
So now we can calculate the volume of the effective combustion chamber:
Combustion chamber volume = Piston crown volume 12.50cc
+ Top ring volume 0.71cc
+ Head volume 51.00cc
+ Gasket volume 5.54cc
- Deck volume -0.35cc
= 69.40cc
Now for the compression ratios. Compression ratio is the ratio between the volumes with the piston at the top of the bore and the bottom of the bore (the difference in volume being the swept volume, right?). So, if we use ccv for “combustion chamber volume” and sv for “swept volume”, it’s:
(ccv + sv) / ccv
or
(69.40 + 602.50) / 69.40 = 9.68 : 1
If I didn’t have emissions issues I’d be happy with that. I’d need to run on a fairly high octane fuel but it’d give me good power. Unfortunately, I’m looking for 9.4:1 (the standard compression ratio of the 3 litre engine I started with) so we need to drop the compression ratio, and the way to do it is to increase the combustion chamber volume. We have a lot of components that make up the combustion chamber, so we can change quite a few things to do it.
The variables we can play with are:
- - Gasket thickness (you can get varying thicknesses of gasket for some engines)
- - Piston crown volume (grind metal from the piston crown)
- - Head volume (enlarge the combustion chamber in the head)
The usual way to do it is to enlarge the combustion chamber in the head, but if you need to take a lot of metal out, you might need to look at the piston too. Anyway, the obvious next question is, how much metal do I need to remove? Back to our compression ratio calculation…
cr = (ccv + sv) / ccv
If we rearrange it slightly we get:
ccv = sv / (cr – 1)
Which means we can now calculate our ideal combustion chamber volume…
ccv = 602.5 / (9.4 – 1) = 71.73cc
Our calculated current volume is 69.40 so we need to enlarge it by 71.73 – 69.40 = 2.33cc
That’s quite a lot, but in reality anything between 1.75 (gives us 9.47:1) and 2.33 (a perfect 9.4:1) would be fine in my book.
Dan’s now got a head gasket and some die grinders and he’s going to try to take the metal out of the combustion chamber in the head. We’ve put larger valves in the head (45mm inlet, 40mm exhaust vs the standard 42/37 size) so the first order of business is to remove metal around the valve openings to allow air to flow past the valves as efficiently as possible.
More on the head later…
