Need your advise guys.

We did a comp test on my engine and cylinder #1 is pushing only @ 120psi. The rest are somewhere between 180-200. THe plugs were soaked in engine oil. Exhuasts are now belching out bluish-gray smoke.

I'm planning to rebuild the engine using oem parts but I was just thinking if this option is worth it: how about I install 96.00mm 11.1 pistons?

Has anybody done this and actually gained considerable amount of extra HP?
Do i need aftermarket fuel pump/injectors for this setup?

A friend suggested to go for low-comps and go FI but I decided to stay out of it bcoz I don't wanna go super-broke.

thanks in advance !

 

If you were planning on just sticking new pistons in the block, then going oversize adds machining work and other stuff that considerably ups the cost of the project. You won't gain a ton of power from a small bump in compressions, maybe in the 10-15hp range compared to the same setup on stock compression. Higher compression is just more efficent combustion, so your fuel demands shouldn't increase much at all.

 

i say do it if your not planning on going FI. and had better studs and gasket and call it done for now.

 

thanks for the advice!.

I decided to go this route:

Use the more reliable HR pistons. It's got 10.8:1 as opposed to the DE that has 10.3:1
Then i'm gonna throw in forged conrods.

Reason is i MIGHT go FI in the future but not soon, who knows. This gives me the advantage of making the car driveable for now, with improved realibility as N/A and FI.

 

I'm almost sure the factory rods are forged. As for the compression bump, its mainly for the addition of cams (cams with higher lift and duration generally lower the effective compression ratio because they bleed more air out through the cycles). Alone, you probably will not see any increase in base HP through the compression bump. I suggest pulling it apart and making a decision based on what you see. You may just need to clean up the bores and new rings. But regardless of what you decide I would suggest a proper balancing job. Maybe, a set of cams and springs...

 

You are not going to gain any meaningful power by merely installing slightly higher compression, larger pistons (read that to say you'll probably get nearly the exact power you have now). You will need to spend ALOT more to make any power

 

good call Z1 and Zazz!!! Your input will definitely save me a lot of $

I'll stay with stock internals then .

 

The above statement is completely wrong.

While higher lift & duration cams will loose some compression at idle and lower revs, the sole purpose of performance cams is to get a bigger cylinder fill in there designed operating range.

With a bigger cylinder fill there is a larger effective compression, not lower

This bigger cylinder fill results in a bigger explosion, bigger explosion = more power.

Same thing with higher static compression.

Nearly all newly designed performance motors (and soon every motor) are using direct injection.

Direct injection allows the motor to run a lot higher compression without detonation problems & we are seeing typically 10 to 15% increases in power from the same capacity motors using HC/DI technology

 

As you are going to considerable cost & time to remove, strip & rebuild your motor, IMO you should fit HC pistons providing you have a cheap & easy access to high octane fuel. (I would presume this would be the case in Dubai)

A set of HC pistons & Eagle rods are quite inexpensive today & with the addition of some form of additional engine management & HO fuel your motor should see a close to 10% increase in power, maybe more.

Add a set of 270 to 280/ 11+ cams & less restrictive exhaust and well tuned, your motor will get close to a 20% increase in peak power.

Search the japanese "OPTION" magazine tests done on the Z some years back to confirm.

 

To keep from confusing everyone I may have over simplified things, but to back up what I said this article along with many others should clairfy things. Let me know what I am misunderstanding, what your saying completely contradicts the article. Correct me if I'm wrong, but it sounds like you're saying the more aggresive cam will raise the dynamic C.R. which can't be.

Dynamic Compression Ratio

 

I do however agree with this assessment, I didn't do the cams the first time around and it ate at me. This time around the Eagle rods and HC pistons will have some aggro cams to go with them.

 

I have not used the word Dynamic Compression Ratio (DCR) and it seems to me to be a meaningless term.

As explained in the article it stands to reason that with the inlet or exhaust valve open open part way through the compression stroke the DCR will be less than the static CR.

Static CR is purely a mathematical calculation: swept volume + unswept volume divided by unswept volume. Valves & cam timing play no part in this calculation.

However early & late valve timing + inlet tract length are used to achieve the maximum cylinder fill.

At some stages of the rev range (usually @ max Tq rpm) most modern 4 valve NA motors are able to achieve a close to 100% cylinder fill.
Hard as it is to believe some of the more exotic ones with variable timing & variable inlet length get greater than 100% fill, that is why some NA motors can achieve the similar power outputs with the same capacities as FI motors.

You have to understand that it is the controlled explosion in the cylinder that determines the power exerted on the piston & then on to the crank.

All camshafts,headers,inlet manifolds, superchargers & turbos do is get the max amount of air & fuel in before it starts to compress. If there was any other way to get more in & then control the explosion then the motor will make more power.

Diesel motors are good example, they run a lot more compression and put out a lot more tq power for a given capacity.
Because the components needed to control this much larger self combusted explosion, are much heavier & the fact that it is self combusted it is hard to control at high rpm's, but even these restrictions are being somewhat overcome.(think 24 hr LE MANS)

 

Sorry to imply you were referring to dynamic C.R.

 

Static compression and what's often referred to as "dynamic compression" aren't entirely useful terms. Static compression can correlate with higher peak cylinder pressures, but there are other ways to increase the latter value without altering static compression.

Somewhat simplified, peak cylinder pressure happens a little bit after the piston passes top-dead-center. This peak value doesn't occur at all RPMs though. It has a kind of window based on the setup, and one can alter where it lands based on when and how long the valves open and close (cams).

Using the term "dynamic compression" adding more aggressive cams could possibly decrease that value over the same range, but will increase the value elsewhere, typically resulting in less power at the original range, but more power over the new range. All this is possible without ever changing the actual value of the peak pressure.

 

One thing to add for the OP:

Keep in mind that the OEM pistons and block are cast. Forged pistons while innately stronger experience differing thermal expansion than cast pistons. Cast block and cast pistons would generally experience closer thermal expansion properties.

It's still possible to fit a forged slug into a cast block effectively while minimizing any long term wear issues, but it's generally a step that OEMs take. I'd be surprised if any of the mainstream aftermarket piston manufacturers would go through the additional testing required for long term usage.

The cast slug would fit better over a wider range of operating temperatures, whereas the forged might be a little "loose" when cold, but fit properly when at operating temps. This is assuming that theres enough material left in the bore after machining to allow usage of the OEM pistons.

It's part of the reason why most engines use cast pistons and not forgings, minus the prohibitively(to me) expensive cars.