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Planning turbo build - how do i choose my ideal compression ratio when ordering pistons?

Practical Engine Building

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This isn't about calculating a compression ratio - its how to choose a new compression ratio when you go to change from a high compression N/A engine to become a lower compression turbo engine?

I don't want to slap on a turbo and have to run negative timing numbers under boost as thats just melting things out the exhaust ports with retarded timing :\

So how do i choose the compression ratio so that

a) I wont need to run negative timing anywhere on the timing map - or run so little timing that the exhaust ports are open significantly enough to affect things

b) i wont be promoting knock by running "too much" compression

The goal is 400whp from a k20a with a turbo sized for a quick response in a road car that will be thrashed.... with boost by gear.

The base engine is a jdm 2005 k20a type r engine with 11.5:1 compression so unusable for boost but with type r cams and head and crank - its a good start (this is already in the car).

My comparison for compression ratios is a jdm nissan gtir sr20det that I previously owned with 8.3:1 and that was "fun and acceptable".

So - how do I figure out what compression ratio range I should discuss with my engine builder when I go to order custom pistons?

Is this something you would calculate somehow - or is it just a "this range worked historically" sort of thing?

Not that familiar with the engine, but a few thoughts.

First thing is going to be fuel - I don't know if you're in the SI, but some have complained about getting anything better than 96 there, whereas many NI outlets have 98 or even E85 on pump.

Second thing is how cool you can get the intake charge.

You could run low and safe, but that could be giving away potential off-boost torque that could be quite useful. Possibly the best idea would be to check the Honda forums and see what they're running, so you're not re-inventing the mousetrap, or checking the piston CR ranges from the manufacturers. I believe there are folks who have been successful running that CR on E85, but I don't know how tolerant that engine is going to be to high-ish boost and high-ish compression, as some engines are better than others - or whether they're running the late timing you wisely wish to avoid.

You may want to start with picture attached to get general idea of CR anb boost relationship. But as Gord already said the maim thing will be fuel octane rating - this is the number one factor that will dictate your CR unless you want to have two tunes- one for daily driving and the second once for race gas. In that case you can go with higher CR rating for big power and at the same time use lower octane fuel tune just to get you moving around. 400 whp from 2 liters motor (Mustang dyno) should take about 2.2-2.4 Bar of boost so you're gonna need at least 98 octane fuel.

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@gord Yup - I'm up in auckland - I only ever use Mobil 98 in the car as thats what it was tuned on - i wish we had your south island NPT petrol stations with 100 octane but thats in the pipeline and apparently coming "sometime soon" whenever that may be. All the e85 is long gone now :\ gull stopped selling it - all the ethanol they blended with is now used for hand cleaner due to covid. They do sell e10 - but its a base of 91 octane with 10% ethanol blended in. Yup - for overseas people - you cant get e85 on the pump in NZ anymore :\

@georg1970 - thanks for that graph - i'll be aiming for a lower compression piston then - as the graph shows the percentage change is only a little bit and the risk of adding heat / causing detonation etc is not worth it. looks like 8.5ish range is what i'll be talking to the engine guy about when i go to order them.

With the graph showing only a few percent difference between compression ratios there isnt any point trying too crank up the compression when you can just use a few more percent throttle to get the same result with less stress / heat for the engine. Long term reliability is what i want. Im only wanting a "basic" build - pistons, rods, valve springs and retainers will be the main mods (+ the usual consumables you use when putting things together) - the rest of the parts are good enough from factory to do what im after. At the end of the day - itll be a road car - and has to stay street legal, certified and fully insured otherwise there is no point in starting this path :)

Cheers for the input guys - and if anyone else has anything they want to add in that would be awesome thanks.

With 98 octane fuel you can go with 8.8-9.0 CR- you will get little bit better throttle response at low RPM and a bit quicker turbo spooling up.

Ah, seems GULL not only stopped the supply of E85, but have no intentions of bringing it back - a big 2 finger salute to their customers who built their engines for it and who will, hopefully, remember it in the future!

Back in the day, I used Mobil 98, but that might still limit you. It's a hassle, and I don't know if it's still available like this, but one of the companies (Pacific?) would sell 17 litres in 20 litre drums that one would add 3 litres of petrol for E85.

This company may be an option, but for a daily driver it's still a hassle - especially if you have to dispose of the drums - https://www.nzchemicalsuppliers.co.nz/list/search?search=autostore

Some years back there was a company importing 100 octane, lead free from Japan, don't know if it's still available but I think it was on Dominion Rd. I also recall claims about the service station just south of Kumeu selling an extra high octane fuel for the off road bikes and race cars that raced over that way, but I haven't any confirmation either way.

Yup - that would have been Segedins (bad spelling maybe) on dominion road - and the Kumeu place had avgas on pump - but thats been pulled down and is being rebuilt and probably wont get that service again as it seems to be now a company owned station rather than a franchise with a workshop.

The previous tuner i used for setting up my haltech said he had done back to back testing on the pump fuels in auckland - gull e10 was too variable, bp 98 had weird additives and he said the mobil 98 seemed to have toluene already added to it and was more consistent and gave a "couple" more kw at the hubs (take with a grain of salt - he didnt work for a gas station and things are variable) but it sorta matches up with what ive experienced with seat of pants stuff.

and i dont really want to run ethanol fuel until i have a content sensor / new fuel pump - the injectors are already sorted when i did the ecu so they wont be a problem - but theres no point with it not being easily available so *shrug*

I'll live with the limits of 98ron pump gas and the engine will end up producing whatever power it produces - i'll go for the lower compression pistons to allow me to throw boost into the system and still run positive timing and build it to take enough boost to allow the power level i want so long as its in the ballpark range of what im after i'm sure i'll be more than happy with the results. I want entertaining not brain bending - it is a fwd at the end of the day so putting down stupid power levels wont be easy at the best of times :) Just sucks that its over $2.50 per litre at the moment :\ )$(*&%@$ gas companies and their price gouging.

The K20 is a massively tuned motor all over the world! These engines have been boosted 1000bhp and above. The engine will do 400bhp on average with basic bolt on mods and not even opened up. A quick google search will have you in the know, sign yourself up to some k20 turbo specific forums and have a look around and ask some questions. Myself & 5 of my friends have boosted k20s, never changed the compression ratio. These are no SR20 and not even comparable dude. They are way stronger and take a lot more than the SR like for like.

If you need any help give me a shout

Martin, which version(s) of the K20 - it's made in several different versions of the base engine, with CR from 9.8 to 11.7. With decent fuelling (octane) the former may live, but the latter may not.

I certainly agree it is a darn strong engine, with a lot of potential - IIRC, it's when you get around the 550-600hp mark it gets risky, exactly what were you able to get out of them?


the 2 that I personally worked on/Built were the K20A2 and a K20B my best mates was the k20 from a 06 JDM teg not sure what engine code it was so that's a few different ratios there but all made over 300bhp not massive numbers these days but info was thin on the ground back then. It was a few year back now but non of them were split and just case of bolt it all on and go but kept safe for DD with weekend fun. I'll have a rummage through the old google pictures and see what I can dig up.

We get 99 octane shell Vpower at the pumps here (not sure what you get) but that's all my cars have ever ran on. Tust get a turbo whacked on and enjoy it, remember higher the CR the more torque = more fun.

If I was to do 1 again I'd split the head from the block bang some ARP studs in with a cometic HG and a spacer plate on top to lower the CR, drop the sump and ARP the rods and be done, shortcut way but beats building a whole motor, can be done in the car and you know the HG is safe once boosted. Boost it till it brakes and fix that. Build, Tune, Brake, build repeat.

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