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Forged pistons for EFI Turbo build

6K views 12 replies 6 participants last post by  jeffgfg 
#1 · (Edited)
So my buddy currently has a stock-block '96 EFI 460 running ~7 PSI of boost. Years back, he had grand plans to build the internals of the engine to handle something like 20 PSI of boost. We had put together a whole list of components - Scat h-beam rods, Probe forged dished pistons, etc.. Well the build got put on the back burner for ~3-4 years, and now that he's getting back into the swing of things, he's been looking to start buying parts.

The issue is, the probe pistons we originally priced out appear to no longer be available, and now I'm having a hard time finding a suitable piston. All of the pistons with the correct compression height (1.77" for the late EFI engines like this one) are flat-top or very slightly dished (3cc or so), which according to my calculations will push the compression into the 9.5-9.7:1 range, which seems a bit too high for a turbocharged engine?

I can get pistons with a lower compression height which would pull the compression down into the 9:1 range, which I'd be more comfortable with, but the problem with that, is that it pulls the piston down into the bore and makes quench much worse.

With 0.040" head-gaskets and 1.77" compression height and 3 cc dish piston, I'd be looking at a quench of around 0.055"-0.060" and a compression of ~9.7:1. With the same gaskets and a 1.756" CH 3cc dished piston, we're looking at ~9:1 compression, but quench is now around 0.074".

Is the extra quench that big of a deal? Should I maybe look into different pistons in different bore sizes than the standard 0.030" over (maybe .040" over 4.400" pistons?)

Also, we're debating on whether or not to keep the stock '96 truck cam. We're going for a "high-torque low RPM" type of build, so higher boost at low RPM. Expected RPM range is basically from idle to ~4500 RPM. Hoping to push enough boost for 500-600 HP. Any suggestions for a cam, or is the stock unit good enough? The cam must idle smooth and have good low-end power.
 
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#5 ·
As far as quench goes, there are many ways to get proper quench distance than just pin location--deck the block, for one.

Summit still shows Probe 2404's in stock for 0.060". With their 24cc dish, that should put you around 8.4:1 with block decking and work fine.

If you insist on a piston made of 2618, then the KB IC635 with 15cc step dish will still put around just 8.8:1, and with less block decking necessary than the above mentioned Probe.

One last thing: These pistons are fine for your noted, "enough boost for 500-600 HP," but if you guys also intend to go for "something like 20 PSI of boost" as noted and do that with blown gas, then you are looking at the wrong pistons altogether and seriously need to double-up on targeted piston quality, design, and price point.
 
#6 · (Edited)
I was trying to avoid going .060", as I've read not all blocks can be safely taken out that far? But if .060" is generally fine for a late model EFI 460 block, then we'll go with that. I'm just worried about potential durability issues with taking that much out of the cylinders. Most of the engine's I've built before had 0.040" as the absolute most you could safely take out, so .060" seems scary to me.

As for the "20 PSI of boost", like I said, those were the original "grand plans", which has now been changed to a 500-600 HP target. Also keep in mind that this will be a very low RPM (4500 RPM rev-limit) engine, with a very mild/stock cam and stock F3TE heads. So, it's going to need a lot of boost to hit 600 HP compared to a fully ported monster with giant valves and an aggressive cam. The idea is to essentially build a 460 that has turbo-diesel-like low-RPM power/torque. So starting with the EFI 460's anemic baseline of 240HP, my back-of-the-hand calculations suggest around 15 PSI just to get us in the 500 RWHP range.

If it helps any, the engine will be going into a '96 F-250 and be used for towing. Current engine is EFI and intercooled, and the built engine will be as well.
 
#8 ·
Diamond makes a nice piston for that combination. 25cc dish, should put you close to 8:1 compression. 2618 material too.

On the overbore, I wouldn't think twice about +.060, and rarely encounter a block that won't go at least +.080. Think about the difference in cylinder wall difference we're talking here. Pull out your dial calipers and set them to .010. That's the difference between +.040 and +.060. It's nothing, and it's not going to make any difference in durability or cooling.

On the cam, stock will probably work OK, but I'd probably want something with a wider lobe separation, and duration favoring the exhaust a bit more.


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#9 · (Edited)
If I'm looking at their catalog right, you're talking about part number 40201?

So what's the final recommendation? Probe 2404 and .060" over with block decking, or Diamond 40201s at .030" over and minimal/no decking? Which one is the better piston? I see the Diamond 40201s go for about $250-300 more per set than the Probe pistons. Would the Diamond pistons actually be any better for this application and power-targets, or would the Probes be fine?

Also, do you guys for-see any issue with the Scat H-Beam rods for this application?

Also, I really appreciate the help as I'm new to the BBF performance world. I spend most of my time around smaller 4-cylinder and motorcycle engines, so I'm unfamiliar about what's generally trusted for 460s.
 
#10 ·
I'm not really understanding your question about decking. Do what you need to do in order to set quench where you want it.

Either piston will work, but keep in mind that Probe no longer exists. If you burn a piston, you're not going to be able to call anyone for a replacement. If you're looking for the cheapest option, maybe take a look at the Speed Pro 460 forging. They're 2-ton slugs, but will probably work fine for what you're doing too.


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#11 ·
I didn't really have a question about the decking, I was more or less asking which setup would be "better" for this application. We're not trying to pinch-pennies, but at the same time we don't want to throw to waste money either.

Since Probe is out of business (did not know that!), I think we're going to go with the Diamond 40201 4.39" pistons.
 
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