Stock piston Rev limit

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derrangedgadgeteer

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Well, I've finally begun embarking on the motor project I've had sitting on the back burner for over a year now. And I have a question about stock 318 pistons.

I have a low mile 318 from a 1970 D200, And I plan to build a modest, budget-conscious twin turbo powerplant in the 550hp range. I'm hoping to install a cam kit, blow through carb, intake, 360 heads, and of course the turbos, probly shooting for 8-1/8.5-1 compression on no more than 6 lbs of boost.

To keep the costs down though, and since the block isn't worn to the point to require an overbore, I want to retain as much of the stock bottom end as possible. (save for balancing, studs and arp rod bolts) I know that running boost with cast pistons can be a recipe for disaster, but since this isn't going to be a racecar, I can afford to run it a little fat to keep the pistons cool. My issue though is with RPM.

Most of the chebby guys I hang out with recommend not running stock cast pistons above 5500-6200rpm. (depending on who I ask) The mopar people I know all own 2.2 and 2.5 cars and insist the stock slugs are good up to what the rest of the bottom end is good to. (their stuff breaks alot, btw)

So, asking those with experience, how high can I turn the motor with the cast pistions without them coming apart? Or would the rods give before the pistons do? I really need to know, 'cause my redline will determine my cam choice, head choice, boost, and my final overall power. And I can't seem to find much info on the subject...
 
The boost will kill the pistons before the RPM, IMO.
 
I've ran stock short blocks with 6 psi of Roots blower boost and upto a 150 shot of N2O, at the same time.
The piston skirts fail when exceeding 8000 rpm. LOL
Run full groove main bearings to get full time oiling to the rod bearings. With boost and RPMs you need the extra oil as those bearings will see a higher load.
 
Seen your work,in the mags,Miranda. Eye opening, to say the least.
 
MiradaMegaCab said:
I've ran stock short blocks with 6 psi of Roots blower boost and upto a 150 shot of N2O, at the same time.
The piston skirts fail when exceeding 8000 rpm. LOL
Run full groove main bearings to get full time oiling to the rod bearings. With boost and RPMs you need the extra oil as those bearings will see a higher load.
Click to expand...

so I should be fine with a 6200 redline (about max for a basic hydraulic cam) as long as I don't go lean and make piston soup. :headbang:

later on in the build I might get a 360 block and a stroker kit and go for 700+, but for now I just want something fun and practical. Something I can mess with people in.
 
Modest budget twin turbo ?? If your building a motor the foundation is important . What you're planning on building is not cheap , best do it right first time around . With turbos and superchargers I would not skimp on the pistons .
Good luck 550 hp and twin turbos is not modest budget territory , at least not for me . Hate to tell you what 412 hp out of a 340 cost me , but it's still running after 10,000 hard miles and many runs at the track
 
jerry6 said:
Modest budget twin turbo ?? If your building a motor the foundation is important . What you're planning on building is not cheap , best do it right first time around . With turbos and superchargers I would not skimp on the pistons .
Good luck 550 hp and twin turbos is not modest budget territory , at least not for me . Hate to tell you what 412 hp out of a 340 cost me , but it's still running after 10,000 hard miles and many runs at the track
Click to expand...

Well, yeah, the age old issue of cheap-fast-reliable pick two, but I've run into a connundrum. Quite simply there seems to be no consensus as to what the factory components are good for. on the one hand you have people who put thousands of dollars of forged lightweight components into a motor making only 30% more power than stock, but on the other you have people running nitrous on top of boost turning the motor 8000 rpm with factory parts.

There was a thread on here not too long ago asking about the power capacity of the stock block, and there were people recomending an aftermarket block at the 500hp level, and there were anecdotes of people making upwards of 700 horses trouble free on the original block.

And between these discussions here and talking with local racers, engine builders, and my machinist, it seems that stock components or aftermarket, an engine won't break just under it's own power. Engines break when the motor runs lean and detonates, or is starved of oil, or it's over revved, not from producing too much power. And the only thing that the expensive aftermarket components buy you is more tolerance of overrevving and detonation.

Admittedly this build isn't going to be bulletproof, but it's my understanding that boost (and turbochargers in particular) is easier, power for power, on bottom end components than high compression n/a builds and nitrous. (Imagine how much money I'd have tied up in the bottom end of this motor if I were trying to build a 550 hp 318 naturally aspirated.) And either way, if I break something in the bottom end of this motor I'll only be out a re-ring kit, and back to square one. (which will fasttrack me getting a 360 block, and a stroker kit)

It's a gamble, and it might be a timebomb, but if I can make my power goal with the parts I have, and have a little fun with it, it's mission accomplished. I don't need this to live for 200 passes, (hell, I'll get bored with it and build something else within two years I wager) But from the range of answers I've gotten, It's become clear that I won't know where I stand until I put it together and start running it.
 
I think you are being overly optimistic as to the ability of stock 318 internals surviving a boost + moderately high RPM situation.

I foresee a catastrophic failure well beyond "a re-ring kit". I foresee this failure occurring in under "two years".
 
I don't have any experience with this type of set up, but........I think since your planning on a dual turbo setup, as long as they are fairly small and build boost down low, you wont have to rely on a ton of rpm. Just size them to make torque in the 3000/5000 rpm range, along with a safe tune, and I believe you can get some life out of it for a time.
 
I have some experience with Boosting standard internaled engines. (Albeit not sbm) And what I've found is that on an otherwise stout bottom end, that cast pistons give up the ghost under any abuse. Broken plenty of ring lands, and burnt a piston or two. and one motor the slug completely disintergrated. (piston crown was snapped clean in two)
Things that help prolong engine life are, a nice big cam. More lift, and a bit more duration means more charge gets into the cylinder. less restriction means a lower boost figure for a given amount of charge. Lower boost=less heat. etc.
I've also found that having the crowns of the pistons thermal barrier coated worked wonders to stop them burning. I pound on my current supercharged motor every time I drive it. And it holds up.
I'd love to try what you're planning, but the cost (of even your low buck ambition) prohibits me. Most guys with properly built set ups start off on the cheap. And they break a bit of stuff. But that's half the fun isn't it? And maybe you'll get lucky?
 
If you have a machinist that says engines only fail because soemthing causes it - he hasn;t made the power at the level you're talking about. Cast pistons will deform - the ring lands can collapse, the dome surface can flex and break, the pins can move (and will under those loads)... It's not just detonation. It's using the wrong parts, or installing them improperly. Same is true for horsepower before a bloc klets go. I've seen 650+ naturally aspirated and I've read about 1000 with turbos. And yet I've seena 350hp 340 split the bores. So it's all relative. if it was me and I was going to buy a couple turbos, have a manifold or header made, and spend on the boost type car - I'd be damn sure the bottom end wasn't going to blow and send schrapnel through the oil system and intake tract. But thats' me.
 
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