Rob84

Does hi-comp = high torque and low-comp = low torque? Obviously I understand its all relative.

The reason I ask is that a fair amount of jap turbo engines are reasonably hi-comp but seem to yield a lower torque figure then say an equivilant powered YB and also...i'm bored

cheers

Anonymous

I suspect you put that the wrong way round?

But on a turbo engine, less CR = more boost safely = more torque

Rob84

i deliberatly put it that way chip, shows how much i know

i remember "itsmeagain" mention that RB26GTT (thats the code isnt it?) is relatively high comp yet doesnt give massive torque figures.

Am I pissing in the wind again?

Mike Rainbird

iTrader: (3)

The reason the high comp engines give relatively low torque is as Chip explained - high comp engines can't run as much boost as low comp engines. Boost = torque.

Rob84

ah fuck.... just re-read it

YES, I should of put it the other way round.

"forum goon off the week" award goes to me!

But cheers for the answers

Disabled Account

Mike, I notice in the Cosworth Racing catalogue that they stock/list pistons that are 6.8:1 and 6.2:1 What sort of application are these for?

And how would a big power (say over 600 BHP) road car engine perform on such low comp pistons?

Anonymous

Mike i run a 35psi spike and a held 32psi!...On std comp

Also Mike you say lo comp can run higher boost...yet a rally car runs a 3bar boost spike do they not on like 9.5-1 engines and make about 500ft lb.....IF hi comp was so spaz why dont rally cars run lo comp and get more reliability...there must be an advantage to hi comp

Stavros

Boost = Torque

Torque+Revs= Power

So want to be able to safely run as much boost as humanly possible.

End of, thread over, thread locked.

Try telling the Jap lot that thats a good idea tho, over high comp and undrivable cams and exhaust housings.

Blinkers...

Stavros

RACE FUEL you fool.

If you ran their engine specs on pump fuel itd end in a big bang very quickly, and if you re-mapped and re-boosted their engines to run safe on pump fuel you will have fuck all worthwhile.

89xr2

Rally cars are relatively low power though (around the 300bhp mark) so wouldnt that make a big difference as they're not after all out power?

89xr2

Whats the formula for working out torque? I know it's all divided by rpm which is why f1 engines etc make so little torque as they rev to stupid numbers

Anonymous

Steve you FOOL...i am asking Mike why rally cars dont use lo comp as to him there is no reason to run high comp anywhere..thats how i am reading it...

Anonymous

IF your brief it so hunt for as much torque as possible (ie the thread subject) on an unrestricted (ie not a rally car which are limited by rules) engine then you want low cr.

If rally cars didnt have the restrictor, they would run lower CR, simple as that, the high CR is there because the restrictor is there.

Stavros

Low and High is relative purely to the fuel used tho IMO.
And as we talking about pump fueled road cars, Mike is right.
We not talking about restricted, race fueled, cars.
For the race fuel they run, WRC engines are quite low comp, but theyd go far lower if they didnt have the restrictor, they need to make the most power from the RESTRICTED airflow they got, with the fuel they got, hence the comp they use.
Simple maths that theyd do.

We havent a airflowe restriction.

Anonymous

...a high comp engine will make more bhp per psi over a lo comp though...i know we are limited by the fuel...

Anonymous

Its the same regardless of fuel really, all that the fuel does is change what constitutes high or low cr in the first place.

That sweeping statement is incorrect in some instances.


At lower RPM, the volumetric effiency of the engine is too high and you will have to retard the timing massively to stop det if you go for high cr on a big boost engine, and that loses you more power than the hi cr gains you.


Its the same for an N/A engine, 13:1 compression is PERFECT for power at high rpm, but it means no bottom end torque cause the ignition needs to be as retarded as the average skyline owners opinions on CR

Anonymous

Essentially Phil, its like this.


Take an example engine (yours if you like)

Take a point in the rev range, ie 4000rpm
and a particular boost figure ie 32psi of boost



run those numbers on the samge engine at every CR:

7.0
7.1
7.2
....
11.9
12.0


For a particular fuel, and then map it on the dyno for each CR


you will find an OPTIMUM CR for that particular circumstance.


Then do the same again 4500rpm, 5000rpm, 5500rpm


You will see that for each successive rpm increment you find that optimum figure moves higher and higher up the CR chart.


If you want you can then plot that as a graph

You will then have a curve of CR against Torque at each point in the rev range on the boost and fuel you are choosing to use.


You then decide from that curve which CR you want to maximise the power of the engine at the RPM you want it, it will be a compromise for every point in the rev range except one, by definition.




Ive slightly over simplified things there, ive ignored cams and their effects on VE through the range when ive said higher rpm = higher cr, for any given engien there will be some small parts of the rev range where that isnt true.

But with the level of detail its practical to go into in a post like this, what ive said holds water pretty well.

89xr2

What ron is the fuel used by WRC cars then?

Disabled Account

At least 102 RON

WORLD RALLY CARS
HIPERFLO CF500
UN-LEADED RACE FUEL
RON 102, MON 90

Primarily developed as a superior "green" fuel for use in World Rally Cars this product is used by many of the worlds leading rally teams.

Delivers top performance whilst minimising harm to the environment.

Low sulphur and benzene levels - well below those required by current legislation - make Hiperflo CF500 cleaner and safer to handle.

Far exceeds FIA "Ultimate Appendix J" requirements

Stavros

Heard upto 120 so far, depends who you believe.

foreigneRS

torque * revs = power

Stavros

didnt mean it in technical terms you coont, just meant the more torque and more revs you got, the more power it makes.

Anonymous

Thats how i read it TBH, tht both influence it, not that you add them together, im sure no one REALLY thought you mean 214lbft + 8000rpm = 8214bhp

Otherwise it would have had a /5252 on it, which nick forgot anyway

89xr2

So by your logic lets take say an f1 car with what 270ftlb and 19000 rpm. using your equation the power is 5130000 so are you telling me thats 513000bhp?

Think Itsmeagain should get out the 007 smily now.

Anonymous

Technically, he was correct, as he never mentioned the units.

YOU are the one who is implying the units that suit you.

and he would be correct with his formula if the units it refferred to where:
torque = lbft
revs = rpm of engine
power = bhp*5252

Mike Rainbird

iTrader: (3)

Phil,
run a rally car / WRC car on normal Super-unleaded and see how long it lasts . IF pump fuel was as a higher octane, I would gladly be running higher compression, as it would be a win-win situation. Unfortunately it isn't. It is much safer / better more power to run low compression high boost then having to run the retard you are running to allow the boost you are running. Perhaps you should stick an EGT gauge on your car to frighten yourself .

89xr2

bugger. for me then haha see this is why i ask lots of questions as i know fk all

Anonymous

And the only way to map it to control the EGT = more fuel

which = less power
and = worse economy



same as the retarding does




so basically you have to try and map around the high cr to make it safe.



Phil, Karl has performed more tricks coming up with your basemap than a box full of magicians at a kid party

You dont see any of the downsides yourself cause he has done such a good job of your chip that its hidden them from you in the main.



Thank fuck you went to someone who REALLY knows what they are doing!

89xr2

Chip, what is the magic 5252 number in that equation?

MikeR

From what i am led to beleive my Evo is relativly high comp and after seeing the MUT 11 machine on it the ignition advance alters hugly after you put your foot down and the boost comes in,

Just my input


Mike

Anonymous

The figure that converts between lbft and bhp against rpm mate.

Stavros

8.8:1 on later models IIRC.

Anonymous

High compared to a "normal" modified cossie yes.

But the ignition would be retarding massively anyway even if it was a lower CR, it would just be retarding from a different point (ie maybe from 4 to 20 instead of 14 to 34)

Anonymous

Well all i know is the car is soooo drivable and it has the ability to scare me shitless...i nearly crashed the other day by goin up behind a car that was doin 30mph and banging it down to second and flooring it as turning to overtake..YOU MUST NOT DO THIS WITH THIS BHP ...I took the 348 out last night and couldnt beleive how slow the thing was...whereas the Missus thought it was well quick (refuses to get in the Cossie!)..it really felt like i could get out and walk faster

Anonymous

Have you got decent suspension on it yet phil?

Helps, although will still be an animal with that sort of power and RWD.

Might be worth considering a Zoo beam though, by all accounts Rods is still pretty controllable despite the power and the RWD only delivery.

My car when i first got it (on the original rs500 shell) was fooking shocking, REALLY tail happy, since building it into the new shell and fitting Koni's + Ahmed springs and replacing all the bushes though its MUCH tamer on the back end, even though it now has more power.

89xr2

Ah, so it's just a number in an equation then and doesnt actually relate to 5252 rpm or anything like that.

Anonymous

Well, yes and no.

It IS just a number in an equation, but it implictly defines the cross over the point for torque and horsepower in lbft and BHP.

If you quote in NM and PS you get a different number, and hence a different crossover point.


5252 is the point in the RPM range when ALL engines will have the same power and torque if they are quoted in lbft and bhp

Anonymous

PS

If the original horse they used to compare an engien against was a smaller horse, then we would be using 4000 rpm instead (for instance) and a car that has 200bhp by our current definition would instead have 256 bhp or whatever.

MikeR

Why is that Chip i have heard it talked about and have seen it on a bunch of graphs i have had from my cars but never been told why ???

Mike

Anonymous

In order to manage peak cylinder pressure points within the cycle.

If you have more boost then it takes longer forthe flame front to propogate which means that you have to retard the timing to avoid detonation occuring.



Fuck, that doesnt read very easily, umm, lol


Attempt 2:


Basically when your car fires, its NOT an explosion of fuel and air, its a burn, as this burn happens gasses are given off (exaust) which pressurise the cylinder.

If the pressure in the cylinder goes past a certain number of PSI then the fuel/air mix can ignite on its own, before the burn gets to it, so the whole lot goes up at once (THIS is an explosion).

So what you have to do is manage the amount of pressure that builds from the burn to make sure it NEVER reaches that point, but you want to be JUST before it, or you are wasting power.

So the more boost you cram in, the more you have to start the burn happening while the piston is further back in the cycle, so that you dont end up with this happening.


Not an easy thing to explain in a post really TBH



Question me further if ive still not made that clear?!