A14 piston differences

[josh_t]

I'm trying to find out what  the difference in the dish is between a regular 77 A14 piston and a 77 FU piston is. Pictures would be nice so i can better I.D. them using my inspection camera in a plug hole. Are the regular 77's the same as a regular 76 piston?

[ggzilla]

The FU engine has a quench combustion chamber. The most notable change is that the piston dish is offset, and is smaller but deeper to keep compression ratio the same.

 

Per Datsun1200.com

[josh_t]

Well the bottom end appears original on mine but someone put a FU head and intake on it. Its an 80hp auto sedan 4 door with a 728 head. I know that wouldnt be a factory combo. The exhaust looks like normal a14 stuff. Would you have any estimate on a CR with the head/piston combo i have? I'll settle with not knowing if it comes down to me pulling the head just to cc everything.

[ggzilla]

Dunno but I'd guess stock 80hp compression ratio. Most A-series heads are same CC and the compression ratio is determined by the piston dish CC (there are maybe 4 different A14 pistons). Exception is the 1981-1982 heads have larger chambers.

[ericsb210]

I am scratching my head on this one, per the pic ggzilla posted I would have to believe your compression would be much higher with the head piston combo that you have.

 

The FU head appears to have a much smaller combustion area and the piston you are running isn't compensating for that smaller area by being dished.

 

Does your car tend to "ping" under load? Is the timimg advanced to compensate for the higher compression ratio? I guess one more question would be, does the FU head have big enough valves to make use of the higher compression?

 

As far as the exact compression ratio, I would have no idea.

[ggzilla]

The FU head is same CC. It has smaller chamber but its deeper, thus same CCs.

 

By the way. High compression doesn't need larger valves. It is good all by itself. Large valves are extra cake. But all A14 valves are same diameter anyway.

[ericsb210]

Thanks for the clarification. Good to know on the valves

[josh_t]

i wouldnt know what it does under a load. the transmission fails to apply said load. but idling and revving it runs great. smooth and quiet.

[josh_t]

I have an update, guys. I looked in my cylinders with my camera, and i saw almost flat top pistons. Lets reference the above picture. Put the head on the left with the pistons on the right. It does ping some under load (above half throttle, unknown static timing) but I'm also running on the 6yo gas that was in it when i got it. It's orange. I'm sure that's real high octane. 0_o JK It has compression specs like this: 1-170 2-175 3-180 4-180. That's operating temp, WOT. It does need a valve adjustment so i'm sure those are closer together after that. It idles very smoothly with relatively snappy throttle response off-idle (even with the mismatched intake manifold).

[ggzilla]

Valve adjustment won't affect compression testing - not unless the valves are so tight it doesn't run right.

[josh_t]

well i wonder what would cause my numbers to look like a set of stairs then. maybe its nothing to be concerned with since it really does run just about perfect.

[ggzilla]

170 to 180 is very good according to the repair manual. Read the section on compression testing.

[josh_t]

i know that is good compression i was just hoping they'd be closer together.

[ggzilla]

The manual explains how close it should be.

[josh_t]

I saw it. No less than 80% of the best number. With 180 that would equal 144. I guess at 170 I'm in great shape. I'm just OCD and want them all at 180 spot on.

[ggzilla]

A14 are tough little motors. 35 years old and many still running great! Best engine Datsun ever made arguably.

[josh_t]

well i'd expect no less. this vehicle seems to only have 63k miles on it. the head was changed but i cant figure out why. the rest appears numbers/dirt matching.