compression ratios

Post by **JEBB4** » Sat Feb 27, 2016 6:23 am

ok so im sorry if ive started a quite basic and useless thread here but i just want to make sure im right on this...

for naturally aspirated engines higher compression is best and for forced induction lower compression is best?

from what ive already read on the tinternet this seems to be right but i just thought id ask as i trust the word of the guys on here more than some random yank

oh and if anyone knows of the highest comp K24 block id quite like to know

Post by **RattyMcClelland** » Sat Feb 27, 2016 9:23 am

On petrol engines that is the case. Forced induction increases cylinder pressure so a low compression ratio is ideal to reduce detonation and pure ignition.

On diesels that is not the case. My 2.0tdi engine in my vw t5 is 14.0:1 compression plus turbo. But obviously diesels ignite the fuel by compression not spark. Some compressions go up top 20:1 on diesel.

mercutio · Post by **mercutio** » Sat Feb 27, 2016 9:27 am

yeah low comp on petrol turbo's is to make the engine last longer/more reliable

Post by **NafemanNathan** » Sat Feb 27, 2016 11:33 am

But also if you go too high on an NA build you'll start to loose reliability as well. Plus you'll also need to start considering using race fuel (13.5:1 and higher really).

Post by **4thgenphil** » Sat Feb 27, 2016 2:31 pm

NafemanNathan wrote:But also if you go too high on an NA build you'll start to loose reliability as well. Plus you'll also need to start considering using race fuel (13.5:1 and higher really).

Awkward

Post by **NafemanNathan** » Sat Feb 27, 2016 2:34 pm

You're alright aren't you? You never answered me before. Have you worked out your ratio?

I could achieve 14.5:1 possibly more, but the idea of having to use race fuel put me off, so I'll probably stick to 13-13.5:1

mercutio · Post by **mercutio** » Sat Feb 27, 2016 3:16 pm

could use water injection to reduce the chance of detonation

njord · Post by **njord** » Sat Feb 27, 2016 3:49 pm

It also depends on boost levels as well if u running low boost (ie 5 psi) then running standard cpr like 10.5 to 1 is oķ

I believe that it can also help to spool up turbos quicker

Post by **NafemanNathan** » Sat Feb 27, 2016 4:08 pm

mercutio wrote:could use water injection to reduce the chance of detonation

How much higher a ratio will water injection let you go before you need to start using 102-105 Ron fuel though?

I've not really looked into it since I'm not doing anything on the lude at the moment, but it's one of those things you need to know before you commit to a spec.

Post by **RattyMcClelland** » Sat Feb 27, 2016 5:13 pm

50/50 water and meth will increase the octane to roughly 108ron. This is what im doing. I worked out with my meth injector size that my 5litre tank will last roughly 15mins of full boost at full injector duty cycle. More than enough for a track day and enough for 2-4 tanks or fuel depending on driving style. Full tank of water/meth is about £25 and you take petrol away if mapped properly when using it so the costs will even out.
Also on my build, my compression on ratio will be somewhere between 10.0:1 and 10.5:1. Thats 10.0:1 CP pistons and a skimmed head.
This is for low to medium boost. 15psi out of vtec and 12psi ish in vtec.