Rb25/30

[rb20bmw]

Hey guys i know this has probably been asked before but couldnt really find anything

i am building a rb25/30 and am currently looking at a set of forgies to suit a rb30et with an advertised compression ratio of 8.5-1 when used with single cam head.

will this bring my compression down to about 8.2-1 with the rb25det head?

here is some specs on the pistons

Piston Type Forged Ring Set Part # C1686 Compression Height 32mm/1.261in Sizes Available: STD, 050, 1.00 Piston Crown Capacity 4.5cc Ring 1 Ring 2 Ring 3 Dish Type Steel Nitrided

Barrel plain

Reverse torsional taper face scraper Duraflex 3 piece oil ring Bowl Depth 0mm/in Width 1.5mm/0.059in 1.5mm/0.059in 2.8mm/0..11in Compression Ratio 8.5 Depth 3.4mm/0.133in 3.8mm/0.149in 3.9mm/0.153in Piston Mass 322 grams (0.50)

Pin Mass 84 grams Pin Diameter 21mm/0.827in Pin Locking Device

Spiral Lock Clearance 0.12mm/.005in Skirt Coated No

[N1GTR]

Are you trying to get 8.2 or 8.5?

[JonnoHR31]

would drop compression down to around 7.8 at a guess. way too low imo

[rb20bmw]

i want the compression to be above 8-1 with the rb25 head

i know spool do a 9-1 piston for the rb30det but they are away for another few weeks so cant contact them

[jangles]

CP pistons are what spool sell, im sure there will be a trader on here with them

[Gerg_R31]

I work in a machine shop and I've done alot of measurements with my rb30/26;

using JE +0.040" (+1mm) pistons (rb26 10cc domed piston with rb30 piston pin offset) the piston still sat 0.030" (0.76mm) down the bore

you will most likely still need to grind both the block and the deck to get a reasonable compression ratio

Below is the formula (the numbers used are my measurements as an example)

Bore x Stroke

bore x bore ÷ 4 =

87 x 87 ÷ 4 = 1892.25

Answer x stroke x π (pie) =

1892.25 x 85 x 3.142 = 505363.2075

answer ÷ 1000 =

505363.2075 ÷ 1000 = 505.363208 = SV

answer x number of cylinders = cc

cc ÷ 1000 = litre

Head Gasket

Same formula as above though this part is quite often left out

gasket bore squared ÷ 4

87.3 x 87.3 ÷ 4 =1905.3225

answer x gasket thickness x π (pie) ÷ 1000

1905.3225 x 1.4 x 3.142 = 8381.132613

8381.132613 ÷ 1000 = 8.381133 [ + ]

Head Chamber CC

Using a burette cc the head, when I ground my rb26 head I ground off 0.010" which took out 1cc, and same when I took another 0.010". But after grinding another 0.040" It had taken another 6.4cc's out. in total I ground 0.080" off the head and it took out 12cc's

Piston Dome/Dish /dome with valve releifs

They can vary a lot, most piston manufacturers will say what cc they have

mine say 10cc (Dome [ - ]) (Dish [ + ])

Deck height

This is the measurement from the deck to the top of the piston (not top on the dome) when the piston is at TDC

If the piston is below the deck (0.3mm is an example as mine is 0)

bore x bore ÷ 4 =

87 x 87 ÷ 4 = 1892.25

= x deck to piston x pie ÷ 1000

1892.25 x 0.3 x 3.142 =1783.63485

1783.63485 ÷ 1000 = 1.783635 [ + ]

0 Deck height is 0

Piston above the deck (0.017"/0.43mm for this set up)

top measurement of the piston squared ÷ 4

86 x 86 ÷ 4 = 1849

= x piston to deck x pie ÷ 1000

1849 x 0.4 x 3.142 = 2498.10994

2498.10994 ÷ 1000 = 2.49811 [ - ]

Adding it all together

Head CC + Gasket volume

then the following options

+ Piston Dish

+ Piston below deck

- Piston Dome

- Piston above deck

for me

Head cc + Gasket - Piston Dome - Piston above deck = CV

54.4 + 8.381133 - 10 0 = 52.781133

CV + SV =

52.781133 + 505.363208 = 558.144341

Answer ÷ CV = Compression ratio

558.144341 ÷ 52.781133 = 10.574694

=

10.57:1