What Cr? (Special Rb25/26 Hybrid)

[Turbo_Knuff]

Im building an rb25 with rb26 crank, Eagle rods and some pistons that says 8.5:1 compression in rb26.

But does someone know how much the cr would be with rb25det head? What I understand is the chambers a little bit smaller in the rb25det head then in the rb26 head? I want about 9.0:1 CR So what type of gasket should I use to reach it?

[DVS JEZ]

What cr are u after? It all depends on wht fuel u are using and what the engine is being built for? Street circuit drag etx

[Turbo_Knuff]

What cr are u after? It all depends on wht fuel u are using and what the engine is being built for? Street circuit drag etx

I want 9.0:1 and will drive on e85. Will use the car as an allround some street,some drag and som track/drift. Nothing serious just for fun.

[R31Nismoid]

9:1 is normal and what just about everyone uses

[Turbo_Knuff]

9:1 is normal and what just about everyone uses

Yes, Do you know if I will get that with this? What size of head gasket would I need to use to get it?

[tm_r33]

Yes from memory rb25 has smaller chambers in head. I think thats why they are 9:1 factorry and 26 are 8.5:1.

Most engine builder have a program that does a calculation to work out comp ratio.

they input...

head chamber size after any resurfacing has happened.

piston top size (in cc. Flat top is usually at 0cc. My dome tops were 13 cc I think.)

Head gasket thickness

and posibly gudgen pin height, rod length and stroke length.

They then select the block its for and the program does the calculation.

You can also use bore size inthere to get an overall displacement to I think.

Ask ur builder the question. As far as I know they regularly need to do these calcs

[Turbo_Knuff]

Thanks for the answers, Im gonna build the engine myself so I don´t have any tuner that helps me.

[Dose Pipe Sutututu]

You should blog it!

I'm planning to build a motor myself as well and have been doing research on CR, head/block combos as well.

[discopotato03]

Everything I've seen suggests that standard RB25 and RB26 heads have the same config and volume in their chambers .

Usually manufacturers take the easiest read cheapest changes to get what they want in production engines . By this I mean pistons are I'd reckon cheaper to change from suppliers than cylinder head castings so it makes sense to me anyway .

My own opinion ? If possible change the static CR with pistons . I think different thickness headgaskets is a half assed way of changing things when its not intended to pull the engine out and do it properly . Thats fair enough for those on a budget but if is out and apart I reckon do it the way a good engine builder would and that means the appropriate pistons and an OE thickness headgasket .

The fact that its an RB25 head and generally a single throttle inlet system means IMO that 8.5 is too low . If it was running a 26 head/inlet manifold/cams then 8.5 is probably Ok .

The big difference with multiple throttles is you have far less resistance to inlet flow and the dynamic or effective compression ratio tends to be closer to the measure compression ratio . Nissan raised the static CR on RB25's because they knew the dynamic CR would be lower off boost and they wanted a bit more part throttle torque and no doubt improved throttle response and fuel consumption . The variable inlet cam would have had a say in this too .

Your call but I think 8.5 is too low in an RB25 and especially when using E85 , I would be thinking more along the lines of 9.5 and if you never use ULP again maybe a tad higher . I think what would happen with a low compression 25 headed engine is a low static compression ratio dragged to an even lower effective compression ratio by its inlet manifold system .

A .