420A Engine Specifications - Calculating Compression Ratio

[BLACK'98DSM]

Hey guys.

I know there aren't many people still owning the 420a Eclipses, and even fewer building them. But I have questions about very specific engine measurements that I can't seem to find.

So what I'm trying to do right now is calculate my compression ratio. I built the hell out of my 420a back in November-January. And I know it's a fairly high compression engine, just wanting to know exactly how high.

All I need from you guys is some values, if anybody knows them.

What I already know:
- 88.5mm bore size (0.020" over, 3.465")
- 0.010" off block, 0.030" off the head
- 10.5:1 pistons
- Headgasket thickness 0.040"

What I need to know:
- 420a stroke (stock crank)
- Combustion chamber volume
- Piston volume
- Deck clearance ( if you know stock it helps, I can subtract what I've milled)
- Gasket bore (I'm guessing they mean headgasket?)

Any and all help us appreciated. Hopefully now when somebody asks what my compression ratio is, I can tell them at least a good approximation, instead of saying "somewhere between 11:1 and 14:1"

 

[DSMPT]

Have you removed .010" and .030" material from head and block? Not .001" and .003"?

Specs by referring manufacturer's catalogs are below. (Manufacturers that you mention in your car profile)
Stroke : 83.00mm
JE 10.5CR 88.50mm, Piston Top Volume : 7.6cc
Head Combustion Chamber Volume : 52.0cc
Deck Clearance : .004" (0.1mm)
Fel-pro Head Gasket Bore : 3.545" (90.043mm)

 

[BLACK'98DSM]

Have you removed .010" and .030" material from head and block? Not .001" and .003"?

Yes, ten thousandths of an inch milled off of the block, and thirty thousandths milled off of the head. Nice catch.

 

[BLACK'98DSM]

Would the cylinder head volume not become smaller once the head has been milled?

 

[DSMPT]

Would the cylinder head volume not become smaller once the head has been milled?

Yes of course it would become smaller. To know the exact volume, you have to measure it.

 

[Mech Addict]

Not sure why this level of precision is necessary, but you can estimate the "lost" volume on the head chamber by measuring the perimeter of the chamber and multiplying by the thickness removed. If you really want to be more accurate, you could estimate the "wedge-shaped ring" of volume lost by accounting for the slope of the chamber roof where it meets the flat head sealing surface. Direct measurement involves clamping a flat plexi plate over the chamber and carefully filling it with liquid until exactly full (known as "gravimetric volume"). Books on blueprinting engines explain this better. Unless you were racing in a class with very regulated engine specs, this seems like super-overkill, but I like to fix my own car, and plenty of people (wife and son included) think that is way-overkill, so who am I to tell you. You could measure the cylinder pressure via a normal compression check, divide by the local ambient/atmospheric pressure, and that will be pretty close, too. If ambient pressure is 14.7 psi, and compression is 147psi, then 10:1. Make sure your throttle is open when you check, for best cylinder filling during cranking.

 

[BLACK'98DSM]

Never hurts to be precise, especially not when building engines.

Compression is average 255 psi.

 

[Mech Addict]

Sure, I agree, when you are setting clearances. It's critical. The precision on Cr doesn't seem to have much significance, though, unless you were trying to hit some very specific ratio, and had to very carefully remove just enough material to do it.
255psi seems really high to me. That would be over 17:1 using a standard sea-level atmosphere of 14.7psi. That's more like a diesel engine. Hmmmm.
Given the information that others have provided, what would your calculated compression ratio be?

 

[BLACK'98DSM]

Showing 12.2:1 according to Summit's calculator

 

[BLACK'98DSM]

And cylinder #4 was ready 270psi before the tester blew off the hose and flew across the driveway

 

[DSMPT]

255psi seems really high to me. That would be over 17:1 using a standard sea-level atmosphere of 14.7psi. That's more like a diesel engine. Hmmmm.
Given the information that others have provided, what would your calculated compression ratio be?

His actual static CR should be about 12.2-12.3.

 

[Mech Addict]

Cars with direct injection and motorcycles sometime run ratios that high without to much trouble on pump gas, but I thought it relied on very sophisticated combustion monitoring and fuel control. Cam timing plays a big role, too, and can make a big difference between static Cr and effective dynamic compression ratio. Not sure how your static compression is so high. What is the range of the gauge? hard to calibrate a gauge at pressure that high, since an air compressor or bike pump will often max at 150. I can't think off-hand an easy way to verify it.

 

[DSMPT]

Almost 1mm material has been removed, this would cause a different valve timing and the different timing and many factors would affect a lot the dynamic compression.

 

[BLACK'98DSM]

I've got adjustable cam gears, just haven't found the time or place to get everything dialed in yet.

The day I finally gather all the time and materials I need to properly tune the car, it will be a beast. For now I don't really even drive it besides the weekends when I go to the shows to show off the engine bay.