pistons and throttle body

[TRKK11]

I am currently building an engine for my lumina, and i have bored the 3.1 .030" over, but i am looking for pistons that are not dished like the stock ones. Does anyone know where i could get some flat topped pistons for this engine? Or, if not flat, less dished? I want to raise the compression ratio with the pistons, and not having to mill the heads and intake. Second question. There is a 62 mm throttle body on the 3.1 pushrod performance page, and it doesnt appear to say where i can get me one of these. Does someone sell them, or is there a website or a company? Thanks

[geoffinbc]

Well you cannot go completly flat or you will be running race gas and/or alcohol

But you can get less dished. Depending on how high you are looking for you can get some RWD pistons.

[smilinguy99]

TRKK11:

Just as a little exercise for yourself, work out the compression ratio with flat top pistons:

89.5mm x (1cm/10mm) = 8.95cm (this is the bore diameter in cm)

84mm x (1cm/10mm) = 8.4cm (this is the stroke in cm)

((8.95/2)^2) x pi) x 8.4 = 528cc (single cylinder displacement in cc)

Now the head gasket. Let't assume a pretty thick gasket, say .060inches or roughly 1.5mm (.15cm.)

((((8.93/2)^2) x pi ) x .15) = 9.37cc (gasket volume)

I'm sure you've cc'd your heads, but I don't know the result, however, the catalog combustion chamber volume of the Gen3 head is 28cc, so assuming your heads are roughly the same, we finish the CR calculation with that number:

(528 + 9.37 + 28 ) / (9.37 +28 ) = 15.1:1 CR (yipes!)

You see, the combustion chambers on these engines are quite small, and therefore even a higher compression ratio like say, 12:1 needs some dish. This is actually good news. It makes the piston lighter and it helps with piston-valve clearance in some situations.)

Now, let's add just 10cc of dish:

(528 + 9.37 + 28 + 10) / (9.37 +28 +10) = 12.1:1 CR (still pretty high, but more realistic)

sg99

[TRKK11]

Hey, thanks smilin guy. I really appreciate the insight. Now, i am looking for about 10 to 10.5 :1 compression, not much more than if i go with gen 3 3100 pistons, but its something. Does anyone know of any pistons that i can get for a .030" overbore 3.1 that would put me at about 10 or 10.5:1? The cc of the combustion chamber will pretty much stay the same, and i am just going to use a stock head gasket. And also, anyone have an answer on where i can get that throttle body? Thanks

[SappySE107]

I have 2 TBs for the 3100/3400, and will hopefully have more made along with some 2.8/3.1 TBs once I get the cad files.

[Doubt_Incarnate]

if you use the 3100 pistons with iron head gaskets your probably pretty close to 10 if not in between. its 9.5 to start, + the slight increase from the boring, and the iron head gaskets are thinner, so it bumps up a little more, so that would be close 10.

[RacerX11]

if you use the 3100 pistons with iron head gaskets

You CANNOT use 3100 pistons with the iron head gaskets. The 3100 pistons have their top above the deck of the block, and when combined with the shorter gasket, the piston to head clearance is too tight.

Marty

[Froggx]

I took some rough measurements (pulling all this off the top of my head might have misrecalled) on my 3400 and found my quench height to be .060" so going to a .040" HG would make quench height .040" and the minimum clearance is said to be .035" Of course I could have misremembered everything

[RacerX11]

According to my math, with a gasket compressed thickness of 0.060" for the 3X00's, and a piston sitting 0.006" above the block deck, there is only 0.054" quench distance. Swapping to the 0.040" iron-head gasket reduces this distance to only 0.034". That's a little too close for me.

Marty

[Froggx]

As I said, a little fuzzy math thrown in for good measure, this time I forgot the piston coming slightly out of bore. .040" would be ok with me, .034" would have me worried too. Is there not someone on this forum running a .040" HG on aluminum heads, I thought someone had mentioned that once?

[RacerX11]

It has been mentioned numerous times, and there are a bunch of people running the 0.040" head gasket with the aluminum heads. To do this, you must use the 3.1 pistons (gen1 or gen2), not the 3100. The 3100 pistons have 0.020" extra compression height, which puts the piston above the deck surface, and causes the problem.

Marty

[Froggx]

That's what it was, the 3.1 not 3100, sorry I still get motor features confused.

[slowbird]

Why not mill the heads? I'd say...use 3.1 Pistons. Shave the heads...and use the Iron head gaskets.

It won't get you very high CR...but you'll feel a difference...and high octane won't be needed.

[TRKK11]

2 Questions in response to responses:

I have 2 TBs for the 3100/3400, and will hopefully have more made along with some 2.8/3.1 TBs once I get the cad files.

How much for the 3x00 one?

and Why not mill the heads? I'd say...use 3.1 Pistons. Shave the heads...and use the Iron head gaskets.

It won't get you very high CR...but you'll feel a difference...and high octane won't be needed.

I do not want to get into milling the heads because that would mean milling the intake as well, and i can just see that becoming a nightmare. Doesnt anyone know of a piston with a shallower dish?

[Froggx]

You can get yourself some custom forged pistons for, I've heard, $ 600ish.