Compression Work Up

Alex170984
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Compression Work Up

Postby Alex170984 » Wed Jul 29, 2015 8:38 am

I am somewhat confused of what compression I am now running as various websites have different piston dish volumes and it's all very confusing!

Can someone work out my static compression ratio based on the below please:

AW11 MR2 16v 4AGE
Big port head
81.5mm NPR oversize pistons
81.57 approx cylinder bore
82mm bore 1mm thick MLS HG
77mm stroke
Head has been skimmed so say 35.5cc
Piston deck clearance -.2mm

Thank you so much

Alex

yoshimitsuspeed
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Re: Compression Work Up

Postby yoshimitsuspeed » Wed Jul 29, 2015 9:51 am

Alex170984 wrote:I am somewhat confused of what compression I am now running as various websites have different piston dish volumes and it's all very confusing!

Can someone work out my static compression ratio based on the below please:

AW11 MR2 16v 4AGE
Big port head
81.5mm NPR oversize pistons
81.57 approx cylinder bore
82mm bore 1mm thick MLS HG
77mm stroke
Head has been skimmed so say 35.5cc
Piston deck clearance -.2mm

Thank you so much

Alex


Don't trust any OEM piston volumes you find online.
Unless you can get actual volume from NPR the best ghetto hack around this will be using the compression calculator to find the dome volume using the assumed 9.4:1 compression.
Now to be honest with you I don't know where the stated compression ratios for the 4AGE originally came from. I don't know if they were initially released by Toyota or if they were just calculated or guessed by someone. I assume the former but I have never seen it in official Toyota documentation as far as I can remember. It's still the best thing we have to work off of for stock numbers though.
The other question is how close NPR came to the original numbers. In my experience I have come to trust not to trust any piston design to be anywhere close to what it's supposed to be.

So yeah if it's real important try to get the piston specs from NPR. Or buy pistons that have those specs available.
Otherwise use 9.4:1 to calculate your dome volume then use that volume with the above spec changes to calculate your new compression ratio.
Theoretically they should change the dome volume to keep the speced compression ratio on oversized pistons but that takes us back to not knowing how much attention to detail is paid on a piston like this.
Whatever the case this should get you pretty close.

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oldeskewltoy
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Re: Compression Work Up

Postby oldeskewltoy » Wed Jul 29, 2015 2:47 pm

Alex170984 wrote:I am somewhat confused of what compression I am now running as various websites have different piston dish volumes and it's all very confusing!

Can someone work out my static compression ratio based on the below please:

AW11 MR2 16v 4AGE
Big port head
81.5mm NPR oversize pistons
81.57 approx cylinder bore
82mm bore 1mm thick MLS HG
77mm stroke
Head has been skimmed so say 35.5cc
Piston deck clearance -.2mm

Thank you so much

Alex


I come up with a static of 10.17, but that is making a few assumptions... Assumption #1 - the NPR piston has the same volume as OEM. A stock 9.4 to 1 compression ratio OEM piston has a volume of about 3.8cc. Assumption #2 when you record the deck @ -.2mm I ASSUMED the piston sticks above the deck by .2mm - **IF** the piston sticks .2mm below deck, the static would be about 9.85
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Alex170984
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Posts: 345
Joined: Thu Jun 20, 2013 4:08 am

Re: Compression Work Up

Postby Alex170984 » Thu Jul 30, 2015 1:16 pm

oldeskewltoy wrote:
Alex170984 wrote:I am somewhat confused of what compression I am now running as various websites have different piston dish volumes and it's all very confusing!

Can someone work out my static compression ratio based on the below please:

AW11 MR2 16v 4AGE
Big port head
81.5mm NPR oversize pistons
81.57 approx cylinder bore
82mm bore 1mm thick MLS HG
77mm stroke
Head has been skimmed so say 35.5cc
Piston deck clearance -.2mm

Thank you so much

Alex


I come up with a static of 10.17, but that is making a few assumptions... Assumption #1 - the NPR piston has the same volume as OEM. A stock 9.4 to 1 compression ratio OEM piston has a volume of about 3.8cc. Assumption #2 when you record the deck @ -.2mm I ASSUMED the piston sticks above the deck by .2mm - **IF** the piston sticks .2mm below deck, the static would be about 9.85


Hi mate thank you for coming up with a figure. When I measured the deck height the piston protruded 0.2mm into block