Silvia Club of NSW

question
i lowered comp after rebuilding to 7.95:1 with S13 motor and with 16psi with T28bb and after market ecu excessive fuel upgrades etc, full exhaust, cooler etc, max i could get was 180kwatw?? this seemed low as the only reason i was given for the low figure was the turbo running out of huff?? similar setups ive seen getting 200+kwatw???

the fuel system is good for 300+ and everything else is more than ample

it seems logical and may be a stupid question but just wondered if anyone had seen a similar situation
Cheers

lower compression = less power. that should be pretty obvious.

why did you rebuild the engine with a lower comp ratio? in this day and age it's a backward and time-wasting practice. you should have increased it from std,not lowered it.

higher compression when all things are right elsewhere to support it will give you both better off-boost response and better overall power.


Justin...

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STI - Made in Texas,not Japan.

yeah wasn't intentional to lower it, im pretty sure it was a miss calculation when ordering pistons! they are 20 thou over so prob didn't take that into consideration

If i were shave the head to get some of the comp back, should my power increase without the need of a re tune?? or my timing might then be out or too advanced and might detonate?

im not a mechanic so im just looking for some theories thats all
cheers

20thou oversized bores won't alter the compression ratio,it's the crown of the pistons that'll have altered it.

the timing will be out ever so slightly due to having dropped the head (and therefore the camgears) closer to the block,but 20thou would be so small an amount that I doubt you'd even be able to measure it with anything other than degree wheels.

skim the head and fit a thinner gasket. and I would advise a re-tune of the ecu,just to be sure and to take full advantage of the changes.


Justin...

_________________
STI - Made in Texas,not Japan.

Cheers mate

Will have to give it a go

20thou pistons will alter the comp ratio because the displacement will be increased. If all you do is add 86.5mm pistons to an otherwise stock motor the CR should go from 8.5:1 to 8.58:1.

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11.40 @ 128mph

so the comp ratio will go up not down is what your saying?

so its not a contributing factor to my ratio dropping its actually the opposite?

how does increasing the diameter of the piston alone affect the comp ratio?

surely it's the height of the crown and the design of the crown that makes the difference.
all a diameter increase will do if the piston design is the same everywhere else is slightly increase capacity. you'll go from 86x86 to 86.5x86.

I understand that you're enlarging the volume of the chamber slightly by overboring,but you've also increased the bore diameter by the same amount,so they should maintain a similar ratio to before.

Zac,that 0.08:1 increase you mentioned,were all other things kept the same bar the larger diameter of the pistons? same gasket size,head not skimmed,same deck height,etc?


Justin...

_________________
STI - Made in Texas,not Japan.

yeah that sounds right mate, you've increased the bore by the same amount so yeah the ration should be spot on.... so i guess the crown of my new pistons simple have less material i.e. less volume than the nissan originals??

cheers for the lesson in engineering....

Because adding 20 thou over pistons and boring it 20 thou to suit them will increase the volume of the chamber at BDC alot more then it will at TDC

Here's what i got for the two combo's

Stock turbo SR would be

Volume at BDC = displacement (86mm x 86mm =499.557 cc) + gasket volume (86mm x1.1mm = 6.389cc) + deck clearance (86mm x 0.05mm = 0.290cc) – piston volume (-15cc) + head volume (45cc). = 566.236cc

Volume at TDC = head volume (45cc) + gasket volume (6.389cc) + deck clearance (0.290cc) – piston volume (-15cc) = 66.679

566.236cc / 66.679cc
= 8.49:1

Then only adding 0.5mm to the bore

Volume at BDC = 505.382cc + 6.464 + 0.290 – (-15) + 45 = 572.136cc

Volume at TDC = 45 + 6.464 + 0.290 – (-15) = 66.754cc

572.136cc / 66.754cc
= 8.57:1

_________________
11.40 @ 128mph

Yeah I agree with Zac on this one. Im glad he did the maths cos i cbf

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Disclaimer: I actually DO know everything

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Quick anal note is that piston volume is 11cc, not 15cc.

yes, that is anal

but it does throw zac's calc's out a bit....

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meh

I figured that if we're looking at comp changes from a 20 thou overbore 4cc wouldn't be too extreme in analness

But 20 thou does still change it I just went off a bunch of numbers on the net that added upto the stock cr, they did the job.

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11.40 @ 128mph