Contour Enthusiasts Group Archives Forums Modifications Forced Induction Compression ratio Q's

Well it is a sad story but I will make it short my diamond forged pistons were stolen out of my car I was in the process of moving and they were just sitting in my back seat.

This whole thing really set me back because of my lack of funds now, and the time it takes to make another set of pistons, so I decided to wait, not cut any corners and build a really nasty 3L that will hold a lot of boost.

Well obviously to make the power I want to make I need some pistons with good rings to hold 12-15 psi. I could buy another set of Diamond 9:1 comp ratio pistons for around $700 or... I could get a set from Duratecperomance.com for $500 (Only if I buy the forged rods too, which I am seriously considering) And these pistons are 8.5:1 comp ratio.

Here is where it gets a little smokey for me. I bought the heads for my 3L from Warmonger and he had severly ported these heads, which I belive will drop my comp ratio a little.

Please give me some useful input on the pros. and cons of my setup with the different comp ratios. And if you don't know anything about my setup, click the link in my sig. Thank you guys

-Jeff-

Tom's ported 2.5L heads on a 3L engine with pistons designed for a 8.5CR would be:

~8.85 to 1

That would be a good compression ratio for a project running high boost and the appropriate compressor map.

For some reason I was under the impresion that those heads would lower my comp ratio... (For people who don't know they are 2.5L heads fitted with the 3L valves ported on the intake and exhaust sides)

Demon: What you saying is that I should go with the 8.5:1 CR pistons, and with that compresion ratio how much boost do you estimate it would take to put down 330-350whp and 320-330whtq? And with that low of CR I could run more boost due to less chance of detontation, correct? Thanks

Anyone else have any opions? Any info is welcomed

-Jeff-

The lower the compression the more boost it takes to make the same power. You have 2.5 heads on a 3.0. It should have been 11.x compression but was dropped to 10.1 or .2 IIRC. So you will actually added a couple of tenths to the final compression of the pistons.

You don't need to forged everything with the goals you have set. It would just be a safety precaution. You could just use a straight 3.0 and run less boost.

I would like to go all forged, and make upwards around 400whp that would be great!. But also I want to keep some traction for the street, and keep the boost down a little. I would just like to build it bulletproof incase I do want to boost that high. Does that make sense? How much power do you think I could handle with the stock 3L block, Forged pistons and rods with the stock crankshaft? Just out of curiosity.

-Jeff-

I'm no compression guru, but I know these engines perform well because they have a relatively high CR.
If you're going to have a "street" and "track" tune (i.e. less and more boost respectively) I would think something around the 9.0 CR would keep "street" drivability at a comfortable level.

13-15psi on a 60-1 trim.

Torque should be quite good!


You would want a two stage boost controller.

Quote:

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You don't need to forged everything with the goals you have set. It would just be a safety precaution. You could just use a straight 3.0 and run less boost.

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You make a very good point, But then my question is what is the benifit to runnig a lower compresion ratio? If I chose to go that route I could just throw in those sealed power pistons that tom gave me and, what 8psi? would prolly be the max boost I would run. And I would make simaliar HP/TQ numbers as I stated above?

If I go all forged and run 13psi with the lower compresion ratio, what do I stand to gain from that?

By the way Demon, I have got the Turbo XS dual satge boost controller sittin right next to me... lol... I can't wait!

Thanks guys,
-Jeff-