Thread: How to tell how much compression with this pistons?

Hi, just wondering, if i was going to put this pistons:

eBay Motors: CP Pistons Nissan SR20VE SR20VET 90mm 9.0:1 SC7329V (item 310092931543 end time Oct-24-08 15:12:39 PDT)

into a sr20de engine... how much compression would i get at the end?... better yet, could somebody teach me how to aquire this data? so i can learn to do it by myself...

all i can get is that i'd have a 2188.43 cc engine...

don't know wich headgasket should i use...... any more data you need?

this is just a learning experience dont even know to my limited knowledge if this build is posible


thanx

just search for a static compression ratio calculator on Google.

then enter the relevant numbers and away you go.

you will in this case need to know what the dish volume of the pistons are and the deck height(should be standard?)

thanx i'll try to do a little bit more research on this subject, but i'd be great if some of the people that really know bout this shared some exp....

if all things are equal DE vs VE except combustion chamber volume, ~46cc vs 43.1cc then a 9.0:1 VE piston will yield approx 8.44:1 in a DE.

after a quick search of the forum i found these numbers posted by UK-SRi



by changing the dish volume in the comp ratio calc(using ve chamber vol) i found these pistons have aprox 11.7cc dish. i then changed the chamber vol to that of the DE to get the comp ratio.

EDIT: this was done assuming an 86mm bore so will not be correct with these pistons. But it gives you an idea

EDITAfter a bit more number plugging it seems that assuming a 1mm headgasket these pistons will yield more like 8.67:1 with a 90mm bore in a DE

Fernando, static compression ratio calculations are easy to do, you'll need the following data:

1. Cylinder volume: 547.10 cm3 (CYV)

Bore 9.0 cm
Stroke 8.6 cm

2. Head gasket volume: 6.99 cm3 (HGv)

The choice for a 90 mm bore head gasket would be Cosworth, it's a 1.1 mm thickness MLS gasket that will hold anything you can throw at it (assuming an adequate surface finish on the block and cylinder head). I remember that most head gaskets have a bore 1 o 2 mm wider than the cylinder bore, but for this case we'll use 90 mm.

Bore 9.0 cm
Thickness 0.11 cm

3. Combustion chamber volume: 46.1 cm3 (CCv)

A quick search over the Internet got me a Kojima's quoted value of 46.1 cm3. This value will hold true only if your cylinder head has not been milled. Otherwise you'll need to measure the volume of the chamber by using a graduated pipette.

4. Piston dome / dish volume: -15.0 cm3 (PDv)

Unfortunately CP doesn't states the dish volume, and this is a KEY value to calculate compression ratio. In this case we'll assume -15.0 cm3 (negative for dishes, positive for domes). The only certain way of knowing this value is by measuring the volume with a graduated burette.

5. Piston deck clearance: 3.18 cm3 (DCv)

This value is simply how much the piston protrudes from or sits in the cylinder bore. This value is supposed to be 0.5 mm below deck.

Bore 9.0
Height 0.05 cm

The following image clarifies piston deck clearance:


6. Compression ratio calculation

We just need to divide the cylinder TOTAL volume and the volume "over" the cylinder:

(CCv - PDv + HGv + DCv + CYv) / (CCv - PDv + HGv + DCv)

46.1 - (-15) + 6.99 + 3.18 + 547.10 / 46.1 - (-15) + 6.99 + 3.18

So, in this case (assuming a lot of things) your engine's static compression ratio will be 8.67:1, try to get everything on Excel so you can play with the values. You can also take a look at the thread I opened a wile ago on NP: Como calcular la relación de compresión de un motor... - www.NipponPower.com.mx