Hi I’m currently building a b series block ready for my new kraftwerks supercharger kit. The current spec of the parts I have are below.

b18c darton sleeved block to 84.5 mm
manley rods 89mm stroke
cp pistons 10.2 cr
b20 crankshaft

my question is I’m starting to think the cr pistons I have chose is a little low cr as I have seen people run this charger on stock 11.1 cr to 12.1 cr builds.
i want to get good figures but also some slight reliability. I spoke to a Honda tuner here in the uk and they didn’t recommend running a charger on a sleeved 84mm b series block. Though I got the impression slightly that it was because I didn’t get my block sleeved from them. Before I start building my block I would appreciate some advice on piston choice?

Many thanks

 

There's no reason why getting aftermarket/iron sleeves in an engine would impact it's ability to make power or be reliable (whether that be turbo, supercharger, or all motor). Assuming the sleeves were installed correctly.

The fuel you're using will make a huge impact on what CR would be recommended. If you're using something equivalent to 91/93 RON, then I'd stay around 11:1 compression for the supercharger. If you have E85 available, I'd push closer to 11.5:1.

Don't forget that after a certain point, especially with forced induction (mostly with a turbo however), the higher your comrpession the more you'll end up having to pull back timing as boost increases. Rotrex chargers tend to do a good job of keeping IATs down, so you can normally get away with a bit more compression than you can with a turbo.

TL;DR - Personally, I wouldn't bother changing out your pistons at this point unless you had a very good opportunity to do so without losing time/money. 10.2:1 won't be a "slouch" on the street, and will give you plenty of wiggle room. 11:1 would give you a bit more gains, but likely negligible. What cams and fuel are you using?

 

Thank you so much for your response.

I’m going to be running 98 Ron that we have on pump here in the uk.

I haven’t assembled the engine yet so there’s plenty of time to get different pistons.

il be running my b18c head with my itr camshafts and supertech flat face valves with buddy club springs and retainers.

I have found a company here who can and do bench run sleeved blocks and test them for any faults so il do that first before I order another set of pistons.

so going on my fuel choice and the fact I have high compression valves would you still say 11.5.1 be the best pistons to aim for going on my block passing all tests?.

im just concerned my 10.2 will be too low to make good power as you said superchargers will handle well with higher compression to turbo chargers.

 

What Rotrex supercharger (model/size) do you plan to use ?

 

It’s the c30-94 kit I have ready

 

I would use a forged piston with a "moderate" height dome... like the ITR or P30/PR3 style OE piston. Getting you in that 11:1 compression is really where you need to be for that supercharger and the type of fuel you intend to use. I would also use a conventional valve instead of the flat faced valves... no sense in adding the extra weight. The extra compression isn't worth the increased risk of valve float from the weight.

 

Thank you. I’m glad everyone is telling me the same things so 11.1 is the range il start looking into,

 

If you allready have the 10:1 pistons id say keep em and invest on a better exhautst manifold...
On the cylinder pressure thing id say get the block o ringed on the 84.5 Just for the peace of mind...

 

Hi I already have a decent manifold thanks. Im hearing a lot of people running a rotorex on stock compression. Hence why I’m starting to think of aiming for 11.1 cr. I want to make good power and 10.2 may not cut it.