Need Advise On Engine Compression Ratio For A Forged Motor

[GeorgesR34]

Hi guys. I'm planning to forge my Rb25det next year and looking for some advise. It appears Pistons manufactures offers differenct compression ratioed pistons such as 8:1, 8.5:1, 9:1 and 9.3:1 for the RB25det.

Refer to what I've read the RB25det NEO engine runs 9:1 compression. What would be the advantage or dis-advantage of increasing or decreasing compression ratio? I’m looking to make around 350rwkws.

Thanks for the input.

[zebra]

What fuel? E85 or 98?

[JonnoHR31]

if you can get e85 easily go for the highest you can get

otherwise i'd go 9 or 9.3 depending on the use of the car, fuel used etc

[GeorgesR34]

Its going to be a daily driver. So will perfer to stick with 98 fuel

[.:: GimpS-R34 ::.]

you will not utilise 350rwkw on the street.. you also wouldnt require a forged engine..

[DR 32R]

you will not utilise 350rwkw on the street.. you also wouldnt require a forged engine..

In fact you could get by on the street in an old corolla hatch back, do wonderful touring in an r31 Pintara. No one requires a forged engine or a turbo, or a sports or sporty car! Perhaps we could change from a forced induction forum to a NA Lada group. Perfect for street use. The topic question was on compression ratios, not whether it's worth building an engine to a goal. Edited November 18, 2010 by WHITE gtt

[BezerkR32]

higher comp will give you more torque and mid range power, but will restrict the boost pressure & total ignition you can run.

[R31Nismoid]

9:1 is fine, its what everyone uses for a street application most of the time on PULP.

[GTScotT]

9:1 is fine, its what everyone uses for a street application most of the time on PULP.

+1 stick to this if its a fairly basic build.

[discopotato03]

For me the trade off is actual cylinder pressures vs power/torque output . The better you can make an engine inhale and exhale the lesser the boost pressure it generally needs to make a power target .

If you want a broad spread of power there has to be enough cylinder trapping ability to make torque at low revs but not choke things up at higher revs .

The old rule of thumb was that the hotter the cam profiles are the higher the static compression ratio can be . In NA land the only two things you can do to an engine to increase its output over the same rev range is to increase its capacity or its compression ratio .

What higher CRs do to a turbo engine off boost is increase the compression pressure and in theory the combustion's cylinder pressure which equates to more torque . It also tries to minimise what you lose low down with longer period cams at low revs .

My thoughts on hotted up RB25 road engines .

I'd personally go for the upper or 9.3 CR pistons because you don't go everywhere on full boost , I'd look at good mild cams even if it means changing springs so they don't coil bind .

A critical thing is TO MAKE SURE whoever builds you engine DOES NOT remove the heads quench zones and that the flat outer areas of the piston crowns are as close to the heads deck face as is safely possible . This is the way to minimise the end gasses volume in the fight against detonation . I'd also insist on having some sort of ceramic thermal barrier coating applied to the piston crowns to insulate them from combustion heat and give an extra level of detonation protection . If the funds are there I'd also ceramic coat the chamber sides of the exhaust and if possible the inlet valves . The main enemies of turbo engines are heat and detonation and heat drives detonation to a degree .

Pistons conduct their heat to the cylinder walls via their rings and valves to the guides and head via their stems . The less heat you lose the more is available to burn the fuel air charge and push the pistons down .

I also tend to think 350 Kw is a bit high and while certainly achievable the state of tune of the engine would be a bit high as in lack the every day round town poke . Its also debatable that you can get that sort of power to the ground in a RWD car and wheel spin never got anyone anywhere except broke and harassed .

Call we a wuzz but 260-280 well tuned usable Kw I think is more fun in a RWD Skyline . If your engines bottom end is in good nick I run it as it is , the right head mods cams and turbo should be able to make more than enough power on the street .

Your call , cheers A .