The pistons may hit the whole head, not just the vavles, at the very least the deck clearance will be about 0, when .030-.050 is where most engines want to be. You will need a new set of pistons with the wrist pin raised 1mm or a set of rods that are 1mm shorter. Have you priced a 348 crank? I'm just curious. If the 348 heads fit like kermit says, that would also be a nice addition to the conversion.
Hi Guys, Some time ago I bought a 328 crank to swap into my 308 motor. I purchased the crank from T. Rutlands. Before I purchased the crank, I spoke to Ted, and he refered me to another guy (Sorry, can't remember his name off hand) and I spoke at some length to him about stroking the motor. From what I remember, he said if I really wanted something special, I could swap in a 348 crank just as what is being suggested here as he has done it before with great success. It could be as simple as adding a pilot bearing as Atlantaman has mentioned, but from what I remembered from our discussion, it sounded like it was more involved (back then I didn't have much knowledge about machining to ask the right questions so I'm probably wrong) otherwise I probably would have purchased the 348 crank. Anyway, you might want to call Ted from T. Rutlands up to put you in contact with that gentlemen. The guy was very knowledgeable and has done the swap before. Plus, I'm sure Ted has a 348 crank or two to sell. Regards, Alan Ing
Sorry, missed this, I'm pretty sure your going to have to have custom pistons made. Concours Automotive had a custom set of 10:1 JE Pistons made up for my 308 motor with the 328 crank. The wrist pin location on the piston had to be moved up higher by the amount of the increased stroke. Regards, Alan Ing
I would tend to agree on some point of the discussion, however I don't think that the heads would easily work due to the different timing belt tensioner bearing arrangement being different. Unfortunate, due to the 348 has a better port, and larger valves to boot. My search of the manuals didn't turn up the deck height dimensions, so I would think that custom pistons would be in order, as previously stated due to stroke length. However, as most aftermarket pistons are pretty much made to order, that shouldn't be a problem. Unfortunately, I don't have the combustion chamber volumes for the 348 heads, so calculating the final compresion is difficult at best. An educated guess is to stay with the 348 piston crown design, valve reliefs, dish etc., and the increase in displacement should provide a health increase in compression, without concerns on valve to piston issues. HTH Kermit
A quick summery of this thread: 348 crank works, but needs slight modification custom pistons needed 348 heads may work, but mostlikly have a number of issues (belt placement?) Back on topic, what size design/pistons would be the best then if one is jumping up to the 348 crank? Another thing I wonder about, increasing one aspect of the car will show weakness in the design of other parts, are you guys sure that you do not need custom head work to make it run reliably?
JE makes very good pistons and will make anyhting you want. If it were me and I was going to stick with natural aspiration, I would buy the pistons with a CR of 10.5 or 11. You don't need head work to make it reliable. The reason you would do head work is to get enough air flow to take full advantage to the increased displacement. When I was younger and working on hotrodding a HD I had, I got it in my head that what I needed to do was stroke it, that would make it a monster. The very wise older man at the shop I dealt with told me this "I don't care if you're Jesus Christ himself, without air, you can't make hp and your problem is still the heads, just like it was the last time you asked". An engines torque depends largely on it displacement. So stroking an engine will increase the torque. HP=torque x rpm/5252. Because rpm is in there it is important at what rpm the torque is being made. An engine is an air pump, the faster you spin it, the more air it tries to pump. When the rate it is trying to pump exceeds the rate the head will flow, the cylinders end up only partly full and the torque begins to drop, that it why any dyno curve you've ever seen, has a torque curve the falls quite rapidly rear redline. If you increase displacement, you increase the engines demand for air at any rpm. If you leave the head unchanged, that will cause it to run out of air at a lower rpm than it did before you stroked it. And again because hp=torque x rpm, the torque went up, but the rpm went down and the net result is the hp is unchanged. HP depends on the rate of air flow, if ther rate doesn't change, the hp doesn't change. Larger cams will help, but will narrow the power band because the torque peak is a function of cam duration. So longer duration cams shift the torque peak up, but the hp peak moves up much slower than the torque peak. This is because hp is still a function of flow rate, which has not changed.
Just being curious: Wouldn´t it be easier to simply fit a 348 engine? I recently saw a new 348 engine for 10.500 EUR. I have the vague impression that changing the crank, pistons etc. etc. will easily sum up to that number.
well----- a block is a block--as long as the rotation is the same you can "adapt" a 348 or 355 to your transaxle relatively easy. Hell I got a 330 V-12 onto one of mine. Next time i am over at TRutlands--I'll try to put a 348 block on a 308 ta and see if they fit
I am bumping this thread because the topic still interests me. I am still interested in what the owner who did this did exactly. Did he simply swap the slightly modded crank in with upgraded gaskets and leave the head pistons etc the same?
