S13 compression change

[Un4Scene]

My current Nissan is an early '89 240SX. While doing some basic checks on the car I found a shop manual that says the compression ratio was dropped from 9.0:1 to 8.5:1 part way through the '89 model year to reduce exhaust noise. Nissan claimed that the maximum power output of the KA24E was not affected. If nothing else on the engine was changed then I don't see how that can be true. At the very least I would think this would affect the power band, even if it reaches the same peak figures. I'd like to find out what was changed to lower the compression ratio. I'm also concerned about how a new performance cam might affect things. If only the early '89 models had the higher ratio then I would assume that any performance parts would be designed to work on the lower spec.

Anyone out there able to offer any insightful commentary on this? Thanks in advance!

[benemorius]

The change was in the piston. I would also be interested to know how Nissan can say that the power wasn't affected. Maybe there was another change that isn't as well known as the piston change...?

[Un4Scene]

Thanks for the response! Any chance you happen to know what was changed on the piston? Any chance someone out there has used the stock internals from these early models along with aftermarket cams or valves or headgaskets? I'm considering doing a rebuild for performance, maybe with a bump in compression along with a more aggresive cam, but I'd hate to go to all that work and money just to end up munching some valves or something. For that matter any reccomendations for these parts would be more than welcome!

[acrazyjohnny]

the earlier/rare/nismo pistons had a dome instead of a dish and will yeild more compressin but in the de iv heard u need! valve reliefs so u dont bend all valves