Compression Ratio

[RLJ]

What would the compression be on an L20b with flat top pistons and a U67 open chamber cylinder head?  
What would it be with a closed chamber 219 head?

[datzenmike]

If the flattops* are the same 85mm bore then the compression would be 9.94 with an open chamber U67 head.

 

Same as above but with closed chamber head like the 219 the compression would be 10.68

 

 

* Z20S or Z20E pistons. If these are L28 flattop pistons then slightly higher.

[RLJ]

Don’t know what the pistons are from.
Stuck a camera in the spark plug hole and see their flat tops.
Engine was rebuilt before I got it and don’t know exact build specs. Could be a rebuild oversize bore, but wouldn’t think much. First or second oversize. 
 

Been chasing pinging. Modified distributor mechanical advance to 10* at the distributor, 20* at the crank. Want to set initial timing at 12-14* but get pinging and have to retard timing 4-6*. 
Running SU’s and runs great as far as the carbs go at 12*. Losing a bit with the retarded timing. 
 

Been reading about the U67 head being prone to pinging on many different engine combinations. So wondering if it’s just the U67 head. 

[RLJ]

When you say if L28 pistons then slightly lower compression, why if it’s a flat top piston?
Is the pin height different and it sits below the deck at TDC?

[datzenmike]

I did say lower but it slightly higher.

 

If these are Z20S and E pistons you will see valve clearance reliefs on the tops.

Third over from the left is a Z20E piston.

 

ALL L series pistons have the same pin height.

 

Some L28 pistons are true flattop and the compression will be very slightly higher because they are 86mm bore. 10.1 open and 10.87 closed.

 

 At 9.9 compression you should be running high(er) octane gas.

 

[RLJ]

These were completely flat with no valve reliefs. Would they be L28 pistons?
 

When you say 9.9 needs higher octane do you mean pump premium, which here in California is 91, or higher?

[datzenmike]

Could be custom pistons but likely 280zx flattops.

 

Higher compression increases efficiency but it can get close to, or go past gasoline's auto ignition point and it will ping under load. There are several things you can do or try. It should be noted here that compression concentrates the heat in the air and the spark plug might be a hot spot and that's enough to auto ignite. Running a cooler heat range plug might fix the pinging. Ducting cooler air from in front of the radiator rather than hot air beside the engine will also help. Increasing the octane will increase the resistance to self ignite and allow you to move the timing up for more power. The increase in mileage might offset the added cost of a higher octane gas.

[Stoffregen Motorsports]

10:1 is my target for the average street motor. You don't need much of a cam to use up that cylinder pressure.

 

Any more than 10:1, you need more cam, possibly less timing, and obviously premium fuel.

 

I ran a 12:1 2200 in my 510 for years, on pump gas. It had a massive cam which allowed such a high CR. But I did have to back the timing down a bit. I think I remember running it at 28 degrees or thereabouts.