compression ratio? long post
Turbo Mini Forums

I know some that are using/have used very low c/r to prevent detonation under high boost.

I never reduced compression that much to use boost up to 15 psi/1bar, even the first one kept the std cr with 10 psi and sometimes 12 psi with 98 fuell (9.4c/r which is very high) for more than one year with severe abuse (ignition set at 7 wich is std, and genuine bdd needle set at 3.2 co)a lambda sensor showed than I were generally on the rich side.......
this engine after some mods (still high cr lightly moded head and mg cam) have given me an over year of abuse...it was really fast and pleasant to drive

then after a fresh up (bearings, rings, gearbox)have been sold, the fresh up showed than it never suffered from pinking

then we built an other with a mate (274i cam 15cc piston 22cc st3head) that make a tad under 9 again good intercooling 12 psi,sometimes more.... no pinks no trouble, fuelling was fine with bdd needle at 10/12 and bgw over this .it was really powerfull, this have been dismantled for inspection and supercharger conversion and have shown no sign of wear (nice colour everywere) pistons as new

then an other one: 9cc pistons phase 2 30cc head setted at 12 psi this guy have used up to 20psi(8.2 cr) to see..,
no pinks...... and his fuelling/ignition wasn't setted for that much boost.
this guy was bad lucky and his monster head rapidly revaled porrous in one inlet port, bad porting (too much porting leaved about 0.5mm thickness)... worryless and bored he swapped this head for a std head, again 12 psi, worked well and bloody hard
untill a hot july day: pistons destroyed but that was 12 psi hot day on 9.6c/r....

a last one 7 c/r(round chamber) 1310 20 psi special needle : I have driven his mini and it is really........slow, even my little de tomaso 998 3 cyl at 13 psi was giving him three car lengh on a pull in secound.

I'm making a new engine wich will be used daily ,for some tracks and hope some 1/4 with some of you
the main specs are defined and the parts are collected, it just miss me the compression ratio....tempted to build a new "high cr" one, about 8.9 but this time I'm a little more worry....

how far did you go with psi/cr (beak chamber, small deck heigh)???

cheers fab

well its a hard answer fab .. but this is mine,,, all the top motors run an average of 8.2:1 cr

one top turbo guru stated namley cork bell from the states that dont be in a hurry to turn your motor into a low compresion slug

you know the rest fab

fab , one thing to do is make sure on your low cr engine ,that you have enough ignition timing on it ,especially below max boost,

tickover timing and timing up to 2500 may need to be a lot more than you would expect .and having this too retarded can create an awfull flat feeling car ,which may make boost (due to the retarded spark increasing the temp of the ex gases and so bringing on the boost sooner) ,but its late firing dropping cyl pressure to the point it feels flat .
on a 7:1 engine , i feel youd need around 25 to 35 psi to make it feel worthwhile .
if the fuel octane to remove det worries is available ,as peter says ,then run higher cr than would be common ,but in the ''use it every day super unleaded world'' ,then ,if you want to run a good amount of boost ,then a good compromise is as ben remarks , in the 8.2 to 8.7 arena ,

i strongly believe in the beak helping the swirl in the chamber ,and creating a faster flame front,that will engulf the remaining mix in the chamber before the flame front can raise pressure and induce a counter shock wave from a over heated or pressurised segment of the unburnt charge .(as found in the localised sub space time matrix continumum!)
an alternative is a knock sensor induced water alcohol injection ,which is my present fave idea.
regards
robert

Edited by robert on 12th Dec, 2006.

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

well
sure I have to choice between high boost and high cr.....
my answer to the Q would have been mid boost and mid cr :) ,
so I'm thinking to use 8.9/1 and 12 psi , 1380 head and cam will help to make some nice numbers.

how many have tried this sort of cr/boost

Peter, about the 274, I've tried it with happyness,though it was the best, then I met a phase 2..........
about same botom as the phase2 without the big increase after 4500, and very little more top than the mg mety after 4500/5000 without the strong lown end

Incidentally fab, my cr is 8.27:1

Edited by BENROSS on 12th Dec, 2006.

well mine is a high cr med boost motor last year i ran 9.4 cr dizzy fixed to 27 deg 276 cam and could run 12 psi with no det at all or 14 psi with a hint of det at 4000 rpm.

i say pick your power you want to get then pick as high a cr to get it.

they certainly do make a better turbo motor with a better low down pick up.

always looking for them bigger bunches of bannanas

Mine engine has a cr of about 7.9:1 but again it is a 1380. It pulls without hesitation. A 1275 is most likely totally another issue. I have retained the vacuum advance. Disconnect it and the 1380 will feel lazy at low revs.

WHat total iginiton advance where you running FAB?

Alex

AlexF

dizzy was set at 7° , corretc if I'm wrong but the total dizzy adv was 18° so it were about 25° total