compression, how high is too high on 98fuel.

[DenZel]

hey guys, thinking of buying some pistons, and wondering what is the safest limit i can go to on 98 fuel, and be durable and last for years and years.

my 1uz is already pretty high, at 10:1, with the later models going to 10.5:1

i was thinking 11.5:1, but then some say i should stick to 11:1.

any experiences/advice would be great.

im sure i could run a 11.5:1, but im worried for how long till it goes bang.?

i want something that will last aswell.

cheers guys.

[oldcorollas]

cam duration/overlap combined with tuning will be other factors you should be considering in the equation.

[big_zop]

I know this is a stupid question, but this is meant for NA?

There is a chart that lists out what CR is possible depending on what fuel is being run - if i find it ill put it up.

One thing you can also look at is what turbo engines are being run at as there is a limit to what cylinder pressures are being generated. But if someone know this then they probably arent going to share.

And one tip, look at dynamic compression ratios rather than static. EDIT: what oldcorrollas is saying regarding cam duration/overlap

i know a certain motorbike with (i think) 10:1 std compression running up to 2bar boost because of this neat little trick (and the standard fuel was 98 too).

[DenZel]

yep na build, tuning and such is a thing im taking in consideration, hence asking whats a safe bet of how high i can go.

i was going to get these later down the track, but there is a deal on at a very good price, so im going to buy them now, and hold on to them for when i get my bigger cams.

big zop, that be great if could find that chart, and does it say how durable it will be?

[af300e]

So many factors. Fuel mixture, ignition timing and aggressive cams are just a few. 1uz is modern so, depending on chamber shape/squish, other factors, 11.5:1 could be fine, even conservative.

There are heaps of bikes getting around with 12:1, even 13.5:1 and all on 98 octane, some without knock sensors too.

You might have to suck it and see.........

[big_zop]

Not too sure about that - but i do think it was reffering to what would be 'safe to run'. What that means and how it translates to a milage, i have no idea.

One place to have a look at for good info is www.v-eight.com. ed_jza80 is the admin there and it is based towards the 1UZ.

[DenZel]

i was told 12.5:1 should be when avgas and such has to be used.

cams im planning will be agressive for sure.

[af300e]

I've mentioned this before in another thread, but my yz250f runs 12.5:1 and doesn't ping when I run bush fuel (95 octane). It is carburetted and has no knock sensor. The new YZ250f runs 13.5:1 again, carby, no knock sensor.

Obviously the cams are lumpy enough, the ignition timing slow enough and I run it ever so slightly on the rich side of things to keep all in check.

Oh by the way, V8 with pissed off cams = yummy

[JustenGT8]

I have run 25 psi in my 10:1 UZ on 101 no probs (stock cams), had to be right at the limit but does suggest 11.5:1 n/a would be fine.

I have run 11.5:1 with 98 on an 18RG so again a UZ would be fine

big caveat is a programmable ECU though so what are you planning to run?

[af300e]

I have run 25 psi in my 10:1 UZ on 101 no probs (stock cams)

Dynamic compression ratio on that bitch would be mind blowing!

[big_zop]

Oh, and have a look here too.

How to Build an NA Engine

[fixeruperer]

if the car is a car youd like to drive most of them time you have to keep in mind the 30+ degree months we have becuase it would ping its tits off.

11.5 is getting up there but there are some standard cars that come with that compression ratio anyway but its all in the tune, good tuners are awesome

[DenZel]

adaptronic ecu, thanks help so far guys, looks like 11.5:1 will be the go, just wondering how long it will last :/

[oldcorollas]

depends on your tuna

[DenZel]

ok....so IF my tuner is not the best, then what is safe 11:1>? or is this just a type of question like how long is a piece of string..

on 98 fuel what have people used? and for how long?

if the 18rg can do it, surely the 1uz can haha