Pistons for a rebuild

[The_88_lariat]

Hey yall im rebuilding a 94 idit engine, i am punching the motor .030 over and need a few things answered about pistons. I can get stock Mahle turbo pistons for 500 ish, while on Mahle's website they offered reduced compression pistons. Does anyone have any experience with them and how much compression do they drop and how much boost should i run to compensate for the loss. if not the stock ones will do just fine.

thanks in advance.

 

[Thewespaul]

Is there anything wrong with your stock pistons? If you want reduced compression then just have them shaved, if it was me I would shave the stoc pistons then have the tops coated. What are your plans for this engine?

 

[The_88_lariat]

well yea the stock pistons are stock and the block will be .030 over. my plans are to just drive my truck tow a few things and have the pump turned up just a bit nothing crazy if the lower compressions ones are near the same money and they were better and i could run more boost they i would get thoes

 

[MTKirk]

Hey yall im rebuilding a 94 idit engine, i am punching the motor .030 over and need a few things answered about pistons. I can get stock Mahle turbo pistons for 500 ish, while on Mahle's website they offered reduced compression pistons. Does anyone have any experience with them and how much compression do they drop and how much boost should i run to compensate for the loss. if not the stock ones will do just fine.

thanks in advance.

If I was rebuilding a 7.3 I would sleeve every cylinder and install stock diameter pistons. rebuilding these engines is far too expensive to risk losing all to cavitation.

 

[The_88_lariat]

i cant afford sleeves i wish i could but the engine seems to be in great shape no rust or sludge anywhere i believe it should be fine

 

[Thewespaul]

i cant afford sleeves i wish i could but the engine seems to be in great shape no rust or sludge anywhere i believe it should be fine

But why are you going .030 over?

 

[The_88_lariat]

its going to be .020 or .030 i measured on cylinder and it was almost .020 over so im can assume .020 or .030 over depending on the other cylinders

 

[icanfixall]

Those cylinders have lots of wear. Without being condescending. Can you trust your measuring technique. I very seldom see a block that is that far worn.
As to using the Mahle low comp pistons I say use them. Remember you are INCREASING the bore so more air is pulled in and compressed. On my build I used the Mahle low comp and removed I recall 14 thousands off the new pistons. then CBX coated them for heat dispersion. You might be awhare that Mahle was the oem piston choice. Also know that you can purchase a set of non turbo pistons and rods and they will fit any size bore just as the turbo pistons and rods do now. Non turbo pistons and a 28mm pin and the turbo pistons have a 33mm pin bore. Same with the rods. Even a set of 6.9 non turbo rods are the same as any 7.3 idi non turbo rod.

 

[The_88_lariat]

I used an inside micrometer than a caliper to measure at bottom middle and top (just before the ridge) @90° and @180°. Now what is the ration between thousands off a piston to compression ratio

 

[Thewespaul]

I used an inside micrometer than a caliper to measure at bottom middle and top (just before the ridge) @90° and @180°. Now what is the ration between thousands off a piston to compression ratio

That would be a few too many decades of calculus for me the math is possible I'm sure but I don't think anyone here has that magic ratio

 

[typ4]

Get the shorter pistons, then if the block surface needs decked you won't lose valve clearance issues. That is what the pistons are made for, not to significantly lower comp. They are only 10 shorter.

 

[Knuckledragger]

You are courting disaster by boring out the 7.3 block. They are already thin wall bores, unless the turbo blocks had some magic last breath changes made. The very problem these engines have with block worm is exacerbated by the extra compression from turbo boost. By boring it out more you are really treading on eggshells. Consider installing sleeves and keeping your stock pistons, if the ring grooves are not worn out.

How many miles on the engine? Were the bores tapered, or just worn large all the way through the piston travel? Maybe hard turbo driving does that.

 

[david85]

Get the shorter pistons, then if the block surface needs decked you won't lose valve clearance issues. That is what the pistons are made for, not to significantly lower comp. They are only 10 shorter.

^^This! did the same one my 6.9. You're also running a turbo, so a barely measurable drop in CR will not hurt your performance

 

[yARIC008]

Don't bore it out. Walls are too thin, if anything you need to sleeve it so you have thicker walls. If the engine is stock and never had SCAs in the coolant, as soon as you bore it out it'll probably be toast right there.

 

[icanfixall]

These low comp pistons have the wrist pin bore machined 10 thousands higher in the piston. That drops the piston 10 thousands at top dead center.