Leaning on your collective experience.... I have am going to go through this 3.0 this fall. I would like to get a bit more umph out of it when done. I noticed in the shop manual that the limit is .039 oversize on the cylinders? I have never hear of an odd size like this. Is this unique to the Mitsu 3.0? Does it require new pistons? Is the 8.9 stock compression the best I can do? Looks like very tight clearance on the head/block so decking isnt possible? I guess in the end just a fresh rebuild .030 over and a little head work is it? Let me know, Thanks <img src="/forums/images/graemlins/cheers.gif" alt="" />

Mitsubishi engines are measured and spec'd in metric terms, so .039" = 1.0mm. Not that you can't convert to inches. The FSM makes this a little easier, since it gives specs in mm and inch measurements.

Standard overbores (rounded off conversions) for the 3.0L engine are:

.25mm = .010"
.50mm = .020"
.75mm = .030"
1.0mm = .039"

Any overbore to the next size (or greater) will require new pistons.

You'll get a slight CR increase as a result of an overbore.

The service limit is .008" milled from deck and head combined. You can go more, but your cam timing will change and you may turn a non-interference engine into an interference engine. You can reset the cam timing by installing eccentric bushings in the cam sprockets.

Another issue with severe decking is that the pistons protrude ~.015" above the OE deck at TDC. If you take too much off the deck, the piston-to-deck clearance/quench height with a .050" compressed height gaskets becomes less than .035" - not a bad thing on an engine that turns 6000rpms max since it helps with detonation resistance.... but I probably wouldn't want to go below .030".

You can change to SOHC Diamante pistons and get 10:1 static CR, but you'll have an interference engine that's running on an 8.9:1 ECU which with the Montero cams will yield a Dynamic CR possibly too high to survive. The answer is to swap to the 7? retarded SOHC Diamante cams and install them with no advance. I don't know how that combination will perform - maybe ok, maybe not so good on the Montero ECU. Your torque peak will be 500rpms later and higher.... and perhaps not so well matched to the truck gearing - particularly with large tires.

There's not much to gain from the SOHC 3.0L without serious work, but a fresh stock 3.0L is a nice engine with adequate power.

Oh....... I don't know how MMC got 8.9:1 as the static CR. I haven't found it to be that high. Perhaps they use a number that reflects the maximum rework to service limits on decking and overbore to arrive at that number.

Good luck,
Frank

I don't know if I'll ever start doing a 3.0 rebuild, but I just have to THANK YOU! Frank for all the terrific info you put out on it. I'm sure you've given a lot of confidence to us tyros.

Thanks Frank. I'm sure I'll be sending questions along to you when I get started. It sounds like a fresh rebuild with few frills. Ever port or polish the heads or other head work?

Yes - I gasket-matched the lower intake and blower discharge plenum.... you could do the same with the upper and lower intake flanges. I also cleaned up any casting seams in the intake and exhaust ports. I first gasket-matched the exhaust manifolds to the gasket - then cut an anti-reversion step into the exhaust manifolds and polished the exhaust manifold inlets as far as I could reach with a die grinder.... the idea was to improve exhaust gas scavenging. Your guess is as good as mine if there's any benefit, but it only takes some time and effort to try it.

The exhaust ports are already larger than the exhaust valves, so I doubt there's much benefit in doing much more than a light polish job. You could try opening up the exhaust ports in the heads to match the gasket, but if you do, you'd better do the same (at least) to the manifolds or you'll have a restriction.

AFAIK, there are no larger valves available for the SOHC heads. You should examine the valve guides to see if any have dropped - fairly common with exhaust guides in the 3.0L. The new style replacement guides have a retainer ring.

There another cam/piston combination I didn't mention - using the 8.9:1 piston with the Diamante cam, but rolling the cam forward to regain some low end. That's similar to what I've done in my engine (but with 9:1 custom pistons), but spun 3 main bearings on the break-in when the oil pump relief valve stuck open. In the few miles before the engine went south, my butt dyno told me there was a definite improvement in the low end.... but that's all I know until I build another engine and get it on the dyno.

The Diamante cams have 4? more duration, so if they're rolled forward to recover DCR and low end performance with the 8.9:1 pistons, there's a good chance they may also extend the top end a little.

Frank