I notice that there are three different compression ratios quoted in the manuals for the various 2.5 8v na's. Wondering how did the factory achieved the different ratios? I always presumed that all the heads were the same for these models, so did they just run different pistons?

1985.5 - 1986: 9.5:1 US and 10.6:1 Europe
1987 - 1988: 10.2:1

-Mike

 

I've wondered that as well. If you can buy a eruo head on UK Ebay, would it boost the compression on our US engines?

 

the pistons are what determines the compression ratios. IIRC, the heads are the same for euro and US (do a search to confirm this though).

 

Thanks, did a search but didn't come up with much on this specific topic and didn't find any picture comparisons.

I always assumed that all the heads were the same capacity and even that the turbo head has the same capacity as the 2.5 na 8v heads. I know the turbo pistons are dished more than the na's to get the compression down, so the euro pistons must be somewhat flatter than the US equivalent. Anyone have any pictures of the tops of the 10.6:1 euro pistons for comparison?

 

From my knowledge, pistons boost the compression ratio

 

It's seems like it's the pistons that will need changing not the heads, which are all the same.

From the FSM the euro 1985.5/1986 pistons are the highest compression at 10.6:1. They will produce 163hp@5800 (US is 143@5500). Torque is 151@3000 (US is 137@3000). So a 20hp improvement higher up the rev range and more low end torque.

Would be something to consider if you were doing an engine rebuild. You would have to run premium fuel and maybe fit an adjustable timing gear to dial in the timing.

 

I would consider putting on a new head to last me until I get my LS2, but I'm not going to do pistons.

Oh well... It was worth asking. (Maybe I'll pull the head off and weld in the dish so they're flat tops.... still in the engine LOL)

 

NA 2.5 heads are the same, pistons are different

us 8v pistons have a small dish and valve reliefs, turbo has large dish (also different rods to compensate for piston difference), havent seen euro piston first hand but i imagine there is either no bowl or a much smaller bowl.

 

and just to clarify, the turbo heads are different than the NA's in that they have the ceramic lining.

 

The 88's NA only has the 10.2 pistons for US and ROW. The 87's and down US have the 9.5 in the n/a. The 89 2.7 have a 10.9 piston US and ROW. The euro 10.6 have a dish about the size of a quarter. The 89 2.7 have a dish the size of a nickle.

 

the combustion chamber i believe is the same but the lining is in the exhaust port, and the exhaust valves are sodium filled for cooling.

 

Yes, the differences between turbo and na are in the valves and exhaust ports. Not sure if the cam was different. I would expect it to be.

 

ok, I'm clear now about the head & compression differences/compatibility.

What about rods. I know turbo rods are dimensionally different to 2.5 8v na rods, but are any other rods in the series compatible with each other. E.G: Are S2 rods the same size as a 2.5 8v na?

 

no. s2 rods were designed for a crank with an 88mm stroke, not 78.9 like the 2.5

 

WRONG.

USA
1983-1987 = 9.5:1 (large dish piston) - 944 and 924S = 110KW = 147.5 hp
1988 = 10.2:1 (small dish piston) - 944 and 924S = 118KW = 158.2 hp


Euro ( RoW)
1982-1987 = 10.6:1 (Flat top piston) - 944 only = 120KW = 160.9 hp
1986-1987 = 9.5:1 (large dish piston) - 924S only = 110Kw = 147.5 hp
1988 - 10.2: 1(small dish piston) - 944 and 924S = 118 Kw = 158.2 hp

Also when comparing 163hp to 143hp remeber there is a hp change from Euro rating vs USA rating. Alos the Euro 163 was without a Cat. When you look at the power numbers in term of KW and then convert to HP the difference is much less than 20. What it has to do with is the way power is measued. SAE Net = 143, but the 163 hp in Euro was using differnet paramaters so the numbers are directly compareable. Power in KW then converted to hp seems the most consistant way to compare Euro power vs USA power.