[Ophitoxaemia]

What a fantastic looking site.

Finally blew up the 302/AFR165 motor that's been in my car for 16 years (13 with the AFRs) and starting a new build based on a 331. Looking forward to a lot of reading on this site, thanks.

[cgrey8]

I relatively recently did a 331 build...mild build for my truck. And I have some amount of builder's remorse about that 331 decision vs 347.

A 331 is obviously more displacement than a 302 and will produce a tad more compression on the same heads assuming you go with flat-tops. But ditto for a 347 and even more so on both accounts. The real reason to choose a 331 over a 347 is that you want to rev the piss out of it and you want it to rev fast. A 331 using 5.4" rods has almost the same Rod-to-Stroke ratio as a stock 302 using 5.090" rods, has lighter pistons than a stock 302, and with I-beam rods doesn't require notching the block for clearance. If you are using H-beams, you probably can't get away with skipping the notching...and if you plan to rev it high, you really want the H-beams.

However if this is going to be on a daily driver or a mild/moderate street build that you don't plan to rev over 6000, then I'd say skip right to 347. The 347s do have a lower Rod-to-Stroke ratio as compared to the 302/331 siblings, but on lower RPM builds, that's not really an issue. If you decide on a 347, then do a compression analysis with the heads & cam you plan to use and decide whether you can still use flat-tops or if you'll need dished pistons to keep the DCR sane assuming you want to use pump gas. There are plenty of vendors with various piston options for 347s ranging from flat-tops with -6cc lashes all the way to -16cc dished pistons. And I think there's a dish increment between those.

When I was researching which direction to go, I remember reading over and over about how 347s aren't as durable as the 331 because of ring encroachment by the wrist pin and as a result, lack of good oiling. But those problems have been overcome as it relates to NEW 347 stroker pistons. The early solution to this issue was to use 5.315" rods instead of the 5.4" rod which dropped the wrist pin lower than the oil ring to avoid the encroachment. That worked but made the kit's rod-to-stroke ratio even worse than using 5.4" rods. Lower R-t-S ratios tends to side-load cylinder walls at high RPMs that some say produced more heat and thus led to overheating on engines that already had trouble keeping cool. The lower R-t-S ratio also makes the engine less efficient. With the oiling problem, this is what wore them out.

Modern day 347 piston designs (i.e. name brand 347 pistons made in the past 10 years) have improved the oiling while using the 5.4" rods. And so there are plenty of people running those 347 stroker kits with no premature wear issues making the 347 just as much of a viable option as a 331 for most any street stroker project where HIGH RPMs are not where the engine will live.

In my case of a mild truck build using ported GT40p heads & a high-lift/low RPM cam intended to be a daily driven power plant, I probably would've benefited more going the 347 route instead. But I got sucked into the 331-is-better argument and decided on 331 instead of 347. Now that it's built I can say if I had it to do over again, that's one thing I would've done differently is go the 347 route.

[Ophitoxaemia]

Thanks for all the info, that matches pretty much what I learned.

The 331 will be all the torque my 315 Hoosier A6's can handle, based on how quick it was with the 302. If I had a heavier car, I'd have done a 351 based 408.