High compression A15's

[jeremiahnz]

Hi Guys,

My brother bought a couple of A15 heads a while ago that had been modified including skimming the heads. There were no actual details on the compression ratio that was produced. I am now considering using one of them for a supercharged engine (I was previously going to use a non-modified head). Can someone please let me know roughly what compression ratio A15's with skimmed heads often produce? I know this is a bit of a "how long is a piece of string" question, but I am just looking to understand the range of CR these typically produce when skimmed. I believe that standard CR to be 9.0:1

BTW I am building a fuel injection setup, and if I do this would look to use an after market ECU to provide better control of the timing and prob run a knock sensor where the standard fuel pump usually bolts on.

The information on the trademe auction for the heads is as follows:
This auction is for 2 Nissan Datsun A15 oval port Cylinders heads. One head is extensively modified, high compression, very large valves, ported & polished. New valve guides & double cut valve seats, work was done by Cook Motorsport, Reg Cook.
The second head has big valves & a minor port job, double valve springs. Will be sold together at a fraction of build cost.

Thanks in advance.

[classicdat]

Not too sure about this piece of string. You really need to measure the capacity of the combustion chamber and the dish in the piston and calculate.
One comment I'd like to make is that the higher the compression ratio the lower the boost you can run before knocking.

[Josh]

Buy Barts setup with B110 attached

[Bartman]

Truth is, you need to measure the combustion chamber volumes, as well as the piston bowl volume, and the piston deck height with your engine at top dead center. From these figures you can calculate your actual static compression ratio.
set up both of the heads upside down. Get them as level accross both plains as you can. Use vasoline (Or similar) to make sure the valves seal against their seats. Find a piece of 8-10mm thick perspex. Drill a small hole through it. Stick it over the chamber. (Again using vasoline(or similar)to seal it against the head face)
Then get a syringe, or a buret, or other accurrate measuring vessel. using something like ATF, (or similar) inject or pour the oil through the hole, untill the chamber is full. (The perspex will prevent the liquid used from forming a huge curved bubble, giving an erroronious reading) Then do the same to your piston. (Or if your shortblock is alread assembled, do the whole cylinder with the piston at top dead center) otherwise measure the piston volume, and then measure how far below deck height the piston is at TDC. Also measure both the diameter of the fire ring on the head gasket, and well as the (compressed) thickness of the gasket.

Then take chamber volume + piston volume + plus gasket volume (Pi X R squared X gasket thickness) + piston below deck height X (bore radiusXPi) This will give you the actual volume of the cylinder at TDC.

Then take your Bore radius X Pi X the engine stroke + the number from above.

Then divide the big number by the small one. This will give you your true compression ratio. I can't speak for datsun motors, but it's not uncommon for the advertised CR of an engine to be higher than the Actual CR.

With an engine assembled, it's very easy to measure the true CR.
Sit the engine on an engine stand, then rotoate the untill the spark plug hole flange is dead level. With the engine at TDC, fill the cylinder with ATF untill it reaches the top of the spark plug hole. (Measuring the amount of ATF you've put in) The wind the motor over to BDC, and fill the cylinder up to the top again. (Again measuring how much it takes)
The devide the big number by the small number. and BANG! You've got your actual CR. Too fuckin' easy!

The above methods give you your static Compression ratio. An engine with a larger camshaft (More duration) will be able to resist detonation better with a given compression ratio, than the same engine with less duration. Longer duration cams "bleed off" some of the "suck" that draws in air/fuel. So the dynamic compression ratio is lower than the static compression ratio.
With a supercharged engine, the forced induction means that you rely less on valve overlap to scavenge the charge at high rpm. This blower style cams have a wider lobe separation angle, which tends to bleed off less of your static compression ratio.