adam Posted May 23, 2018 Share Posted May 23, 2018 Hi all, In short, can you run flat top pistons (whether it is standard 86mm bore or oversized at 87/87.5mm) in their N42 equipped head on their L28? No head work and a stock/mild cam? The general consensus is that no, you can't due to poor head design resulting in pre-ignition - however, a lot of these reports are based off American zed owner feedback; a location where fuel is lower in octane/grade in comparison to Australia. Their premium is 92/93 octane compared to our 98. I know some might say the following: - Why not run dished pistons, the extra points in compression to dial back timing is going backwards in power/response. - Why not run a P90 head as this is more suited to the flat top pistons. - You need to run a more aggressive cam to reduce chances of pre-ignition. - The higher compression is pointless with a stock/mild cam. But.. I just need the above question answered based on personal experience or known facts using 98 octane fuel. I have a spare F54/N42 that I am looking at doing a refresh on and it requires a minimum 1mm oversize due to a lip on the bore; thought I'd take the opportunity to go flat top pistons while I'm at it to increase compression. I've got a copy of the 'how to rebuild' and 'how to modify' your Datsun OHC Engine and didn't come across any details on the above. Again, this book is based on American quality fuel. Any advice or feedback on the above would be much appreciated. Thanks,Adam Quote Link to comment Share on other sites More sharing options...
1600dave Posted May 23, 2018 Share Posted May 23, 2018 Their premium is 92/93 octane compared to our 98. Octane rating in the US is different to Australia - their 92/93 is pretty close to our 98 OdinZ, adam and gav240z 3 Quote Link to comment Share on other sites More sharing options...
adam Posted May 23, 2018 Author Share Posted May 23, 2018 (edited) Ha! After a quick google search to verify (because you need to double check everything these days) you are absolutely right. 98 ron = 94 US (R+M)/2 , some average of RON and MON Was not aware of this. Thank you sir.. Edited May 23, 2018 by adam Quote Link to comment Share on other sites More sharing options...
Retro Z Posted May 23, 2018 Share Posted May 23, 2018 Adam i had detonation issues with my n42/n42 until i changed to the bigger cam, if i recall it was 280 degree cam with 88mm flat top pistons with Corolla conrods for 2.95ish litres. It was awhile ago, so memory is abit hazy. My motor is a diy build, so durability is unknown. adam 1 Quote Link to comment Share on other sites More sharing options...
Administrators gav240z Posted May 23, 2018 Administrators Share Posted May 23, 2018 Only way to know for sure is to cc the head and then calculate your compression ratio in my humble opinion of course. You could retard ignition timing to compensate and run premium fuel but you may find little wiggle room.. My L28 runs an early E88 head with flat tops, gets a bit of pinging under load, but plan to regraph distributor. In the interim octane booster in the tank seems to shut it up. Roady 1 Quote Link to comment Share on other sites More sharing options...
road rider Posted May 24, 2018 Share Posted May 24, 2018 Avgas will fix any detonation problems! I'd go with flat tops! You could upgrade the cam at the same time as the bottom end freshen up! Quote Link to comment Share on other sites More sharing options...
adam Posted May 24, 2018 Author Share Posted May 24, 2018 Thanks for all the input, fellas. I'd go with flat tops! You could upgrade the cam at the same time as the bottom end freshen up! I'm thinking I'll have to convince myself I need upgraded cams to minimise the detonation! Quote Link to comment Share on other sites More sharing options...
d3c0y Posted May 25, 2018 Share Posted May 25, 2018 Yes run flat tops, yes run a bigger cam. There done, next question. But seriously, as Gav said it depends what's been done to the head. Personally I would flat tops in regardless if you want any sort of performance going forward. adam 1 Quote Link to comment Share on other sites More sharing options...
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