I prob asking something thats already been done to death. But what the benefits one against the other?? As i going to start a rebuild. thanks,

 

Turbo'd cars favour lower comp. Not sure of the specifics but higher comp leads to det. (a problem I am having at the moment which will be solved by piston skimming)
Less comp, more boost.

 

Isn't that from just dumping fuel into a low compression sure read somewhere that rally cars etc run high compression

 

higher compression is better for low boost and n/a applications, partcuarly at high rpm, low boost the opposite.

basically if the compression is high and you get a good fill of the cylinder (ie low rpm and high boost) then you end up risking detonation unless you retard the timing an awful lot, and then that can lose you power

rally cars break the rules seemingly, but only because they run very high quality fuel which means that detonation is less of an issue

I suspect you would get EGT problems if you filled a WRC car with 97 ron fuel and remapped it to work on it, especially if you then held it under load towing a caravan up a hill or similar

 

u no what i think and what mine is mate

 

Interesting as mine is 8.4:1 runs 2.9bar & makes 780 on pump & 850 on 106 explain please Chip.

 

As per what I said above mate.

high rpm -> high cr
low rpm -> low cr
low boost -> high cr
high boost -> low cr



yours is high rpm and high boost (doesnt even spool till over 4Krpm so certainly isnt low rpm) so is "middle" cr as a result, it doesnt need to be low compression because you arent making boost at low rpm, and it cant be high compression because you wouldnt be able to get any timing into it in the midrange.

If yours was low (ie under 8) you would lose loads of power on race fuel, if yours was high (ie over 9) you would lose out on road fuel.
Where Mark has put yours is a good compromise for the two fuels.


FYI (with regards to my comments)

Under 8:1 = low compression
8 - 9 = normal compression
Over 9:1 = high compression



There are other factors too of course, such as the flow characteristics of the head and the cams that are used which will also effect things, but unless he is doing anything very wild in those respects they dont really need to be covered by this thread particuarly.

 

Lol Wondered how long before you piped up stu lol.

 

Chip, what would you consider high boost?

 

As chip said rod 8.4:1 isn't really a high CR, modern FI engines are using over 10:1 in some applications, made possible with good combustion chamber design and direct injection.

The higher the compression ratio the more vital timing and fueling becomes, a low compression ratio is much more forgiving at highish boost than a high cr...

IMO an mfi car is better suited to a slightly lower CR at over 20psi due to less accurate fuelling/timing etc.

My car is a bit over 8:1, if i were to build it again i would use stock CR if not slightly higher.

Rob,

 

2 bar +


Bare in mind though that any generalisation is exactly that, and intended as a guide only, there is NOT a simple formula that works for all engines to accurately determine best CR, there are just too many variables.

 

i thinks a can of worms has just been opened lol

 

Ever thought of being a teacher Chip, that makes sense & i understand it.
I had assumed over standard commpression was high & below 7.2 was low.

 

11:1 is high, thats what my mates na golf is running ill tell you how it performs when i start to tune it!

 

All depends what you call high compression really. 7.2:1 or lower is just silly.

Standard compression on a Cossie is nice

 

For N/A 11:1 is about normal these days for a sporty multivalve engine.

Even a toyota aygo shopping car is 10.5:1 for example

 

yeah, its a 9a 16v with mad cams and bike bodies running MS efi, just got it running, will be tuning it shortly, should be fun never tuned na before.

 

So on a cvh what the bonous of running high compression over the normal route??

 

less lag more revs, but a cvh doesnt rev anyway, keep it abou 8.5:1 IMO

 

If the cams are really mad dont be surprised if it wants to be richer than you expect down the bottom end.
I mapped an over 300 degree cam engine last week that wanted to be low 13s in places where I would normally have prefered high 14s, but the difference in now it drove dictated what it ended up with.

And bear in mind you wont find peak ignition via det, you need a set of rollers.

 

cheers, i had a feeling it was going to have to be rich to make it run sweet (its not my fuel) also ill be using the arse dyno initially but a RR session will be on the cards eventually, what sort of timing figures will i be looking at at what rpms very roughly?

 

Disclaimer: VERY vague figures for fucks sake dont just enter them and drive it around


At full throttle you are probably going to be in the region of about 25 degrees at 5000 (or where you make peak torque on whatever cams it is you are running) raising to about 30 degrees at 8000 (or whatever your limiter is)

Idle at about 10 degrees

Part throttle low rpm at 30 ish

And at high rpm at low throttle angles it will be about 40 as anymore than that never seems to help even though it wont det no matter what you do to it at those points on the map.

 

Where abouts are you from chaffe?

 

my zetec turbo is 9-2-1

 

Interesting reading guys...
Chip - You really should be a teacher or an engine tuner (which im guessing you are?)

I always thought low comp was the way to go for Turbo engines, but i read that the lower the comp the more lag as less response you get. High comp boosts quicker and produces more power... I think i'll stick with standard comp on my engine after reading this - around 8.2:1?

 

Less lag should be a good thing sure if run lest boost engine will be less stressed

 

Any tuners have an opinion

 

I guess the above is talking about peak power... making peak power at high rpm = high comp etc...?

as i cant see the problem with having a high compression ratio and reducing advance at high load to eliminate det as most modern cars seem to do?

As far as i understand it, CR vs Advance is a bit of a trade off, its whether a high compression ratio with less advance makes the same power as a low compression ratio and more boost. There are many obvious advantages to having a higher compression ratio.

Is that correct Chip?

 

my mate had a turbod pinto engine and pissed over cossies[ stage 1 ones and 2] off the lights the cosies got left. but midrnge it started to die down.. hight comnp is good for response aswwell

 

surely the mapper can map out den on a good system

 

If you do that on big boost, you will end up with high EGT's as a result.


Yes, but if you end up retarding the timing too much the power you are losing is because the burn hasnt happened quickly enough in the cycle so the energy ends up as heat rather than used to push the piston down.

 

Only to a certain extent or the way you have to map it out starts to have other unwanted effects like increased heat, poor mpg, etc

 

agreed, you are tuning round a mechanical problem, when the best thing to do was build it to the correct spec to begin with, of and btw ill just put those figures in and see how i get on

 

Sorry, good point. I really meant retarding the timing to with in reason. CR and advance is still a bit of a compromise as far as i can see. As you have said above, lowering the compression ratio and advancing the timing will reduce egt's, but too much and you just loose power/drivability and make for a generally less efficient engine.

 

Its why you go to a recognised engine designer not just a builder to get an engine designed to what you as the driver require. It aint cheap it aint going to happen overnight but hell is it worth it.

 

....and they will say bin the cvh and use something way better, and you wouldnt have to look far for that now.

 

My road car is 21.8:1 compression and 2.5 bar.

 

Is that an attempt at some sort of basic humour or a typo?

 

Not always true, Karl certainly wont for example as he seems to enjoy the challenge of the CVH, just a case of finding the right person who specialises in the engine you want built.

 

I was running 12.5:1 at one point on my Fiesta. eeek!