HSC pistons and rods ???

[texacejr]

: Does anyone remember a previous post about using 2.3 HSC rods in a 200/ or 2.5 HSC rods in a 250 to raise the piston closer to the block deck.
I'm interested because I'd like to start a rebuild on a 250 I have for my 82 Futura. I have found a local machine shop to customize my oil pan and pump for a rear sump setup.
Scoped out several likely cams already and want an OZ head later, but for now I,m researching info on the block only.
Thanx

 

[MustangSix]

The 250 suffers from an extremely low (negative) deck height. The piston is over .120" down in the cylinder at TDC, so there is no quench effect and very low compression.

There are a couple of ways to remedy this. The first, which I employed on my own 250, was to use 255 V8 pistons. They are .085" taller than the 250 pistons, allowing you to cut the block .035 - .040" to get a zero deck height. Then by relieving the combustion chambers, you can get a reasonable compression ratio and decent quench area.

The second method would be to use a longer 2.5 HSC rod with the stock piston. This has the advantage of giving you a slightly better rod/stroke ratio. I have not tired this, but the numbers are right. The deck should still be surfaced and you would still have to relieve the chambers to get the CR down to a reasonable level.

Either way you have to have a cooperative machine shop that will allow you to assemble and measure assemblies along the way.

 

[Ronn]

wow, learning new stuff everyday here! So now I guess I should deck the block instead of the head. Seems right..if I can take off 70 thou, and shim the rocker towers 70 thou, I would have improved the deck height and quench, and raise my compression to the 9.2 realm...right?

 

[MustangSix]

With the stock chambers, your CR ends up in the 10:1 area. Way too high. You don't need to cut the head at all except to ensure it's straight.

Go to the Schedahl site at http://falconperformance.sundog.net/compcalculator.asp to check the calculations, but with a zero deckand stock dished pistons, you should be shooting for a chamber volume of about 70cc's on a 250.

 

[texacejr]

If I use the hsc 2.5 rods and I don't deck the block or shave the heads, would I need th do anything to the valvetrain at all??
I just want a decent running daily driver that's dependable with a little bit of ooomph !!
I alredy plan to us an AOD tranny for mileage requirements coupled with the 250 and a 3.55 rear (7.5).
Am I on the right track??

 

[MustangSix]

I think you need to shave the deck and the head just enough to get them flat again.

But the reason that there is such a low deck on a 250 is so that the CR would be reasonable when using the same head as a 200. If you reduce that deck to zero, the CR is way too high, even if you didn't shave the deck or head.

But if you simply do some reliefs on the chamber to open up the valve area you should gain enough volume to mitigate the reduction in deck height and get the CR up to around 9.0:1 at the same time. Doug had some pictures of his cylinder head work on his site. I would look at those as a guideline.

BTW, my 250 ended up at around 9.8:1 and would rattle like a can of marbles under load in hot weather. Just a half a point less would have made a big difference.