Fiesta_Jed

Why does the yb engine seem to need to be low comp when running big power?

As far as i can figure out it must have something to do with the bore being so big, as the rod stroke ratio is only 1.66 so the piston is not at tdc for a very long time.

IIRC the rs500 touring cars had a comp ration of 6.8:1. Was this so they could run 30psi+ of boost and loads of ignition timing and still be reliable?

Or did the ecu`s at the time have a poor resolution meaning the low comp was essential?

also If there were to engines of indetical spec and boost pressure, The only difference is one has a comp ratio of 7.5:1 and loads of ignition timeing. The other engine has a comp ratio of 8.5:1 and less ignition timeing what would make more power?

There is alot of other variables that i would like to discuss but i thought that this would make a good starting point.

Please dont turn this into a high comp vs low comp thread as clearly the ideal comp ratio will be different for every engine.

saph4be

think most are high comp these days mate!

Fiesta_Jed

On a turbo car i considor 8.5:1 or higher to be high comp and on a na car anything above 9.0:1 i consider to be normal.

I dont think there is many YB engines with 8.5:1 comp ratio.

xr2wishy

iTrader: (2)

i know of you from another forum so you should already know the answer since you have the answers on everything else
but for those interested it's all about power/det, basically a lower compression ratio engine can run more boost and advance to get a certain amount of power out, the same power can be achieved with less boost in a higher compression ratio engine but the tolerances are so much tighter for ignition and timing, so lower comp is favoured.
if you want big power and don't mind pushing as hard as possible to achieve this including loads of time mapping getting every last bhp out of an engine, then high cr is the way to go.
also materials made for engine componants and cooling play a big part too.
that's it in a nutshell, there are lots of factors though that do finally decide the cr targetted.

Rod-Tarry

It doesnt my M.A.Developments engine is 8.5:1 runs 2.9bar & made 838bhp. The engine Tuner decides the CR When designing the whole package. Its moved on a billion miles since the Touring car days.

105epwr

Didnt the wrc motors run like 10:1 cr with race fuel?

Fiesta_Jed

What specifically has moved on from the touring car days?

If thats your static CR what Is your dynamic CR

Does your engine have longer rods?

Mark Shead

Being able to adjust the timing correctly/intercoolers/turbos have all had a effect on being able to raise the comp ratio and make it work.

Mark

J1mbo

iTrader: (1)

Culd be up to 10.5:1 cr with over 40psi boost at 3000rpm

Fiesta_Jed

I would also like to know what there dynamic CR was ?

How did they make so much boost so quickly was it the turbo its self ?

or did the compression ratio have an effect?

Rod-Tarry

Yes my engine does have longer rods & knowing the cam timing I could calculate the Dynamic CR for you or was i supposed not to know how to do that. My engine details are on a need to know basis & im afraid you dont need to know.

chrisa3

iTrader: (1)

Fiesta_Jed

I was pretty sure you would be capable of calculating your dynamic compression ratio and although i dont need to know your engine details it would have been interesting .

Dont suppose your going to tell me your rod stroke ratio ?

J1mbo

iTrader: (1)

Cr cams and turbo makes it pull so low down.

To run this they run a high quality fuel and a very careful ignition map.

I don't know the dynamic CR, sorry.

Fiesta_Jed

So does the static compression ratio have any noticeable effect on the boost threshold / low down engine response and power?

I have read several threads and I have not found a definitive answer.

I am now now thinking that for a big power road engine low comp (7.5:1 or lower) with lots of boost and ignition is best.

discuss

xr2wishy

iTrader: (2)

i run a CR of 8.2:1, only 17psi on a zetec turbo, plenty of power, no need to go lower really for my road car.
as said varying CR has an effect as do all the other components really, you can run the CR as high as you like to get the most power out of an engine, but you need to be more accurate on fuel and ignition, could make it awful on fuel and smoothness though. also consider the chance to hydrolock the engine if you go too high like on drag racers.

moonstoneblue 350

interesting read . my 2wd runs 7 6 1 with 15 psi for 300 bhp and 28 psi for 375 bhp built by harvey gibbs and is fast as f...

scoooby slayer

iTrader: (4)

it is strange there doesnt seem to be the low comp set ups on skylines and supras ?

Ad4m RST

More displacement & higher revvers. No need to lower the comp & ride the low end for all it's worth, they have much more scope to make the power.

Chip

iTrader: (3)

Also they have combustion chamber designs which are better at avoiding detonation as well.

The key thing in deciding how much CR you can run is what sort of timing the engine is taking, personally I would only ever want to drop the CR on an engine if I was getting down to under about 10 degrees of ignition timing for a significant part of the map.

YB engines dont seem to take timing the way that skyline engines etc do.

Rod's is different internal geometry which helps it and also has very well developed cam profiles that are designed to help it rev, so its quite different to a normal YB.

Fiesta_Jed

If there were two evo engines one with a 7.5:cr and the other with 8.5:1
would there be any difference to the boost threshold, lag and power/torque after the low comp engine had its map adjusted?

(both engines have identical specs turbo, boost, cams, rod ratio ect)

Chip

iTrader: (3)

Its actually very hard to generalise as to which would be better as obviously the cams are going to suit one CR better than the other, as is the rod ratio etc.
Engines are very much a package, and there isnt one answer that works on all specs.

With regards to boost threshold, not likely to be effected greatly either way as its more about the volume of air going through the turbo than what CR the engine it came from was.

With regards to power and torque, anywhere that the engine was taking lots of timing on 7.5:1 its likely to make more power on 8.5:1, and vice versa

Ad4m RST

Whats the deal w/ their combustion chambers? didn't know that...

Contrived to ask, but the fact Cossies have to run lairy cam profiles as a rule to match Skyline power horse for horse, would it be an exponential upping in risk or strain... say both around 450hp?

Guessing that makes timing parameters that much sharper on a Cos, in this comparison anyway?

Must be easier to

Fiesta_Jed

So the high comp engine will make more power where the low comp engine has loads of ignition advance?

I always thought that the more boost an engine had, the more ignition advance it "needed" and failure to do this would result in a loss of power. Especialy when running 30psi+

Chip

iTrader: (3)

Yes because if you are running too much ignition advance you make peak power too soon, so to avoid doing that you have to put less advance in than the fuel can take, which makes it lazier.
So if you have run out of gains from ignition advance before you hit det, your cr is too low.

The more boost you have, the more you fill the cylinder, so the more risk of detonation you run (especially on high comp where there is less space so the mixture is more compressed) so the less advance you can handle.

I suspect you are confusing an engine needing more ignition retard when it comes on boost. More ignition retard = less ignition advance
Confusing how some people misuse the terms which doesnt help when you are using.

Chip

iTrader: (3)

Subtle differences in the angle of the pent roof, the shape of the transition from pent roof to the squish parts, valve angle, and also not just in the chamber but the way that the YB has a nasty angle in the exhaust port which hinders flow and results in more heat.

Massively complex subject, and not one Im really qualified to comment on beyond on the basics, I certainly wouldnt be capable of designing a head like the honda engineers etc do, takes years of experience and trial and error mixed with some massively sophisticated computer hardware and software.

You have to remember the YB was designed by a couple of blokes who didnt have anything like the data available today, and had to fit on an existing engine which constrained where oil and water ways and head bolts were etc which has an impact on design too.

Also the YB head was developed for about 20 quid as an internal project at cosworth, would have been a different story with proper ford money behind it right from day one.



skyline is 6 cylinders, that changes everything, as its got more valve area as a result of having the extra valves, so the same cam results in less opportunity for airflow on the YB

Fiesta_Jed

when you say "make peak power to soon" do you mean the mixture will explode befour, 15 degrees after tdc?
and det being a second explosion after the spark plug has fired.

I was always under the impression that because you were filling the cylinder more it would take longer to burn and theoretically would need more ignition agvance.

Ad4m RST

I mean I'm aware what the engineering gap will mean between the two, something only the Japs can do is intricately refine on a budget, the German alternatives have always paid the price so to speak. Honda show up every brand in the industry IMO, so glad this green revolution hasn't sacrificed vtec too. Even they CR-V hybrids look trick... & massively tuneable.

TBH I'm glad mainstream Ford didn't get hold of the YB in the era the Cossie dropped, the fact they bolted so much to a boggo Sierra shell, if they'd gone into huge internal development the rest of the setup may have suffered, or keeping it to standard & pricing them well out the market, a la RS200... & what would Cossie owners have as fodder for the pound for pound brilliance that is the YB



600cc's & 3000 revs make all the difference then, fair enough... again ignoring the 5 year engineering gap. If only they brought out new incarnations of the YB, that's where I'd want major Ford involvement! look how far the RB engine came before the R35. Ford could still hold real clout amongst the "EVO" elite. Far reach to keep the comparison going that long I know.

Issue got rinsed before the last FRS dropped, but still...

Chip

iTrader: (3)

Det is an explosion, no "second" bit about it, there should not be an explosion in normal operation of a petrol engine, it should be a burn only, and yes around 15 ATDC is about the optimum I believe.

With a dense mix (ie a over 100% ve filled cylinder) the risk is great of exceeding the maximum peak cylinder pressure that the fuel can withstand before detonation, so you have to fire later so that the piston is well an truely on its way back down again before that peak is reached.

If you fire too late though, you lose efficiency, in which case your timing is too high for that scenario.

scoooby slayer

iTrader: (4)

my 2jz engine is a 3.4 stroker and its on the thickest head gasket they do giving the minimum comp ratio of 8 -1, thinnest i think being 8.8 - 1, 8 - 1 on a yb is generally considered high is it not ?

Fiesta_Jed

Yes on a yb 8.8:1 would be considered high comp.