High compression 2.9L?
 

I dont know how many of you guys have looked into this, but I found something interesting while doing some unrelated research. It looks like you could use pistons for a 2.8L V6 in a 2.9L V6 and pick up a lot of compression. See specs here:

http://webpages.charter.net/beckracing/slvpg43.htm

Essentially what you could do here, is replace the stock 2.9L piston, with it's 13cc dish, with a flat top from a 2.8L. I'm not sure what compression ratio this would give you, but I'd think it would be at least 10:1. Is this old news, or has this been done before?

Thoughts?

The 2.8 itself was a lower compression engine than the 2.9. I think all you would gain would be the stroke of the 2.9, and I doubt that would be anything signifigant. Blocks were basically the same.

Did you actually look at the piston specs?

You're missing the point. The 2.9L has a dished piston, whereas the 2.8L has flat top pistons. Replacing the stock 2.9L pistons with 2.8L pistons should give a sizable increase in compression.

Hard telling. Some sources list the 2.8L and 2.9L as the same bore size, the manufacturer of the two pistons I listed does not. See here:

2.8L piston: http://www.kb-silvolite.com/spistons...etails&S_id=88

2.9L piston: http://www.kb-silvolite.com/spistons...tails&S_id=580

Now THEY list the bore as the same size.

Also realize that you may not want to just swap pistons. The bores could have taper or a ridge that could break rings. I would suggest going to the nearest oversize of 2.8L pistons and bore the 2.9L block to fit, along with a complete balance of the bottom end.

In short, yes. There will be a ridge left on both ends of the ring travel, not just on the top. Plus you'll want to bore it anyway to get the proper piston to cylinder wall clearance with the new pistons.

Do you really think it would give a creditable increase in compression?

Yes I do, and it absolutely would increase compression. You're going from a piston with a sizable dish to a flat top. The compression height is actually a bit higher on the 2.8L piston, though IMO not enough to cause problems. That too will give you more compression.

I'll put it this way. The combustion chamber volume of my 4.0L heads was 60cc. The 2.9L heads I have out in the garage have visibly smaller combustion chambers. When you go from a 13cc dish, to a flat top with no dish, how much do you think this would increase compression?

Have you looked at the chamber on the 2.8 heads??? Its the same size if not smaller than the 2.9 chamber. The 2.8 was a 8:1 compression engine and the 2.9 was a 9:1 engine. For the 2.8 to have 8:1, it would have had to have a dramatically smaller compression height than the 2.9. The 2.8 has 96-7% the displacement of the 2.9 not 88% which would explain the rise in compression according to stroke. If all things were equal except for stroke, the 2.8 would have 96-7% of the compression ratio of the 2.9, or 8.7:1. Now the kicker is the dish in the 2.9 piston. That means that the compression height of the 2.8 piston is going to be even lower or the chamber cc's is going to be higher (or headgasket thickness, which I doubt is the case). Somebody has their numbers wrong. If you put a 2.8 piston in a 2.9, I doubt you would gain compression, in fact, you might lose it.

Well send an email to KB-Silvolite then.

For the record, I've seen two different heads with chambers that looked identical, and one had a much higher combustion chamber volume than the other. The only way to really tell is to CC the heads.

True, but Im talking about a significant difference, At least as much as the dish in the 2.9 piston. For example, You can most definately tell the difference between an early 302 (58cc) and a late 302 (69cc) head chamber size by looking. All I am saying is that you cant just assume a piston is going to have a higher CR just because it is a flat-top design.

True, but I can assume that if the compression height of the flat top is slightly higher than the dished piston! Like I've shown, it's listed in two different places. Like I said, an email to KB / Silvolite is about the only way to resolve this IMO.

Lol, if you wanna email those guys, be my guest. I dont care enough about the subject to put that much effort into it. Im just going by the factory CR's and displacements.

I turn tho this, http://www.fordracingparts.com/download/charts/217.pdf everytime I want to pull an engine measurement, I encourage everyone to bookmark it. It sure makes it handy when trying to scheme up a low budget JY part swap engine build lol.
The only incorrect info I have been able to find is either the 2.9L or 4.0L deck height, they are listed as the same when in fact they arent, the 2.9L deck height isnt correct.

To figure out if there will be a significant increase in compression use this calculator, or we have one in our calculator section (FTE) but I dont know how to get to them lol so here http://kb-silvolite.com/calc.php?action=comp I dont know the head CC volume otherwise I could figure it out.

Notice the FRPP catalog I posted also notes the 0.17" difference in compression height between the two pistons, with the 2.9 being shorter.