What C/R should be use on a 2L turbo engine?

[TEAM78]

In the paddock people seem to run all sorts of various C/R's on their 2l turbo engines. These range from as low as 6.5:1 all the way up to 10:1.
what do people find works best here?

[graham bahr]

8:1 is a good starting point, but basically it depends on turbo size and boost levels, and what you want the engine to do,

good low speed power and lag free: use high compression 10:1 with low boost and smallish turbo, this is basically current WRC spec

for massive to top end power, big turbo and low compression 6.5:1 -7:1, this will result in lag, however big turbos will give more power when on boost with low compressions as they dont like pumping against the engines internal pressures, the low compression also gives the engine more internal combustion chamber volume which allows more mixture to be sqwashed in there, although you dont get as much force from burning a mixture not compressed as much the big turbo low c/r combo more than makes up for that by getting so much more mixture burnt in the first place.

unless you got bucket loads of cash and some very clever electronics and turbos the middle ground works best.

[TEAM78]

what is the limiting factor on increasing the C/R. by raising the C/R you can increase the volumetric efficiency surely. for circuit racing anything below 7:1 is surely too laggy?? who can speak with experience??

Thanks for your post Graham. very informative

[graham bahr]

yes increasing the c/r does increase v/e however, your reducing the amount of space within the engine that the mixture can occupy so with big turbos you loose more than you gain with a higher c/r.

turbos only lag if you let them drop off boost, the t34 on my car is lag free as long as you dont let the revs drop below 4,000, and thats without any form of anti lag gubbins.

drive them like a cammy engine and they should be lag free

[tzei]

Originally Posted by TEAM78

what is the limiting factor on increasing the C/R. by raising the C/R you can increase the volumetric efficiency surely. for circuit racing anything below 7:1 is surely too laggy?? who can speak with experience??

Thanks for your post Graham. very informative

Hi

I´m running about 7:1 C/R and max boost beeing 2.0 bar. About the lag... I think is´t also about gearing and how much abuse one is willing to give macinery. My diff is 4.45:1 and gearbox is sport-getrag. Spending time above say 3800 - 4000 rpm limit and there exists no lag. Grip problems more likely.

[Kev_205]

by running a lower compression ratio you can prevent pre detonation, but this will come at an expence of power. The higher the compression the more likely you will risk uncontrolled fuel detonation.
There are plenty of ways to combat this though, the WRC scoobies certainly used to run around 12:1 ratio! - but then the engines only had to last a few hundred miles.

Shows how the whole package needs to be considered before any parts are purchased

[phoenix]

Originally Posted by graham bahr

yes increasing the c/r does increase v/e

No it doesn't - but it does give better thermo dynamic efficiency.

[graham bahr]

Originally Posted by phoenix

No it doesn't - but it does give better thermo dynamic efficiency.

yes your right,
i was going along with what was ment rather than was actually written

[graham bahr]

Originally Posted by tzei

Hi

About the lag... I think is´t also about gearing .

agreed, its vital to get the gearing right, any pitfalls in the gearing will make any lag worse,

in my opinion its also important on a turbo race car to get the flywheel etc as light as possible, as the easier and faster the engine can rev the faster the boost will come on, my car certainly has a very light steel flywheel,
geoff kershaws ( mr turbo technics himself) awsome twin turbo sierra doesn't even have a flywheel as such just a flex plate similar to an auto to carry a ring gear for the starter

[graham bahr]

Originally Posted by Kev_205

by running a lower compression ratio you can prevent pre detonation, but this will come at an expence of power.

not always it all depends on the trade off between increased thermal efficency of higher compression compaired to the smaller amount of mixture in the cylinders and the fact that turbos do not like the pumping losses that high c/r can create

imadgine this

1000CC engine
1000cc combustion chamber

this engine would have 2:1 compression, and have 2000cc of mixture in the cylinders on the compression stroke.

lets raise the compression
with a 500cc chamber

the engine now has 3:1 compression, but the engine now only has room for 1500cc of mixture

if we now use 2 bar boost

the low compression engine could hold say 4000cc mixture
but the high compression engine would have a mere 3000cc of mixture in it.

in this instance the low compression engine would have so much more mixture in it it would make more power than the high c/r engine despite the increased thermal efficency of the high c/r engine.

add a restricter and it all changes, if you cant get much mixture in then you have got to sqwash it hard,

WRC cars may all have high compressions, but they have restrictors, and ultimatly still have lower power outputs despite the high c/rs than many a clubman turbo car

[TEAM78]

So the limiting factors on the C/R are how much air can you get into the engine and detonation.
would a 8v engine then run a higher C/R as the fewer valves act as a restrictor?

[graham bahr]

basically, and what sort of responce/ power delivery your looking for.

yes ultimatly an 8v would probably tolerate more compression if all else is equil, although it has to be said that when it comes to forced induction the difference between 8 and 16v engines is nothing like a large as in naturally atmo engines

[TEAM78]

Which ECU is best for catching detonation?
I have to admit that I have noticed the gap between 8v and 16v is smaller for turbo stuff as well with all things being equal

[graham bahr]

the make of ecu doesn't matter as long as its properly mapped, keeping turbo engines alive is about keeping intake and water temps down, making sure the fueling is spot on, and that you dont suffer from fuel surge staving the engine under load, and that the fuel you use is good enough in the first place.

we mapped my turbo on plain pump optimax, but i wont put anything else in it just in case.

[graham bahr]

Originally Posted by TEAM78

I have to admit that I have noticed the gap between 8v and 16v is smaller for turbo stuff as well with all things being equal

yep, compairing my 8v engine to a similar spec 16v cossy on the same turbo, i'm only a little down on power, although thats more than made up for by the fact i have considerably more torque

[TEAM78]

Whats your 8v engine? whats the C/R on that? how much power are you getting?

[graham bahr]

bmw 4 cyl 2.0 8v, 350bhp 350lbft torque with 7.8:1 c/r using hybrid t3/34 1.5bar boost

[TEAM78]

thats a lot of power from an 8v! most I ever heard was from a lancia 8v turbo.
The cossies though get 550hp reliabily though.

[graham bahr]

Originally Posted by TEAM78

thats a lot of power from an 8v! most I ever heard was from a lancia 8v turbo.
The cossies though get 550hp reliabily though.

it was my first attempt at a turbo engine, coosies may be able to get 550, but not with a similar turbo and engine spec to my 8v, a cossie in similar spec to my 8v engine will do around 375bhp and 320lbft,

i'm certain i could find more power from the 8v by using a bigger intercooler ( charge temp was getting rather high at high revs) and a more radical cam which would force the torque up the rev range a bit thus gaining power.

it has to be remembered i was looking for big torque and low lag, not silly power figures, my old n/a engine could often get the better of turbo cars through the twisty bits and out of the corners but then would loose out from halfway down the straight, so i figured the thing to do was build a turbo lump with the responce of my n/a engine and try and get the best of both worlds.

i now want more power,
but still dont want the wait for it, wait for it, wait for it, oh my god! big bang
that you get using a large T4 turbo, hense the 16v engine i'm now building,

that will have a similar 7.8 c/r, with a bigger t38 turbo, to try and avoid lag and spin the turbo up quicker the engines now 2415cc, the intercooler is now a massive Rs 500 job, which noramlly will increase lag, but i'm hopeing the big cc will hide that.

the m3 head i'm using is beautfull in terms of ports etc and is more than a match for a fully ported rs500 cossie head as the m3 head was designed to give over 100bhp per litre naturally aspriated

i'm never going to out horse power the best cossie lumps, for a start there are 1000's out there tuning and developing them, i'm a one man band, developing turbo engines that nobody else uses at home on my own,

target horsepower for the 16v is 450bhp with low lag and big torque, although i may not get that at the first attempt.

it has to be said that my 8v engine might be a bit low on power on paper, but this year serveral guys with VERY VERY fast cars have commenting how quick my car was in a straight line suggesting perhaps that they dont all have as much power as they think

[graham bahr]

i also dont need as much power as other turbo cars as mines quite light at 925kg, most are 1050-1200kg if i can get the weight down to 900kg with 450bhp i'd have 500bhp per ton thus matching a 550 1100kg car on paper, my real problems gonna be i've only got 2wd, so with the low weight tractions gonna be at a premium even in the dry, as for the wet, well i might just stay at home!

[TEAM78]

what ecu do you use?

[graham bahr]

Emerald M3dk