question: Low CR vs. High CR for forced induction?

[reecegze]

yer got a ems standalone

[JustenGT8]

High comp turbocharging can be completely acceptable and safe, but you have to realise the ceiling of achievable power will always be lower than using a low static comp setup because you are working with a smaller combustion space.

Couldn't agree more. Rule of thumb is to use the highest comp ratio you can that'll let you produce the hp you are chasing.....and trust me you can run a much much higher comp ratio that most would suggest. Area under the curve (both torque and hp) is what you want for just about every engine other than drags or dyno queens, so chasing the biggest bang at the expensive of response is a great way to stay off the podium

The original 13:1 quoted for the WRC WRX is entirely feasible...Porsche ran similar CR years ago and ECU control, not to mention metalurgy and turbo design has come light years since then.

I happily run up to 16psi with 10:1 comp ratio and the only reason for not running higher boost is the poor stock piston ring lands might say bye bye. Jamie ran up to 18psi on his 1UZ. The day i get some forgies in there i'll be aiming for 20psi on the same 10:1 (only took 13psi to lay down the 320rwkw though).

Does depend on your engine design of course but talking modern Toyota 10:1ish up to 20psi i say. Don't stress it though, if there's an off the shelf 8.5:1 then just go with that knowing it'll do the job and chasing a custom CR costing big bucks ain't worth the effort.

[kingmick]

OK, what fuel do they use in a wrc car? how much money do you think they spend on the combustion chamber {guess id say a few $100k easy in development}, how much do you think the tuning and software cost for the ecu?{would make my house look like a bargin.}. How long do the engines last?
Now back to australia cicuit racers that spend $80k-$120k or so run from 12.5 to 13 or so to 1 in the atmo race engines on avgas.
So none of the above really works with a pretty stock street engine on plup.
With low boost of 12psi and with good charge cooling and good tuning you will be able to get away with 9.5 maybe a fraction high depending on setup on pulp, but you will want to be able to dump the boost if you cant get pulp for some reason or you get a bad dose and you dont want to have to park the car on the side of the road and dump the fuel or get it towed.
Go to high with the comp and you have no saftey margin at all, which is important for a street driven car and very much so when its daily driver.
The 4ag's have a good standard chamber, but it still not perfect.
You can go higher but you will have no saftey margin, but its fine for limited use like most do.

On a turbo race engine{ciruit}, we run between 8-1 and 9-1 with very good, worked chambers, good tuning and very good charge cooling.
We run below 28psi to give 800hp from a 2.3 litre engine on an engine dyno on avgas. Be nice to go high with comp as it would make the engines much more competitive{drivable out of the corners} with the V8's, but at the moment with out spending mega dollars its cheaper to spend less{$80k} on a 650hp-700+hp chev 6litre {notice how much less the chevs hp is) and leave the turbo engine for other things.Three years of trying it is plenty of time, to tell at high levels you just cant do it with a turbo engine, YET{the time will come} in some national classes.
The chevs been back in.
Thought the above might be an intresting read about comps etc.

Sorry if its boring but my opinion is somewere in there !hahahahaah

[myne]

I think that they're likely to be running drive-by-wire.

Drive-by-wire, in my mind, means that you can map the WOT throughout the rev range.
You think about it. You have a 13:1 engine, you need a max static compression of say 10:1, what do you do?
You optimise the cams for the low end, and limit the cylinder filling in the small crossover area where the turbos are starting to work and the revs are low enough for decent cylinder filling and you restrict the DYNAMIC compression to 10:1. If you do it right, you could theoretically maintain fairly similar cylinder filling throughout the rev range and a nearly dead flat torque curve.

Engines are an air pump. They compress air and fuel. The amount of air and fuel entering the engine depends on many factors and changes throughout the rev range. This air and fuel can detonate if it reaches a certain pressure.
If the amount of air entering changes, then the dynamic compression changes right along with it. This changes the maximum pressure of the compressed mixture.
So obviously a static CR that works at one RPM might destroy the engine at another.
Since all engines are a compromise, traditionally the static CR was reduced and the timing adjusted to compensate.
What if you could dynamically optimise the compressed pressure?
As we've been progressing through the years, engines are becoming less compromising. I think it's possible to change the dynamic ratio with the throttle.

You get better spool-up, better performance off boost and below the curve, better economy, perfectly flat torque curve.

I could be wrong, but it makes sense in my head.

eg :

[JustenGT8]

LOL yeah not sure what your point is Mick but all i'm saying is you can run higher comp ratio than what is currently being toted as safe...shit, if my UZ isn't proof enough i dunno what is?

Leave 13:1 to the big buck brigade for sure but i know i will never build a FI engine with less than 9.5:1 again. If it comes stock with less i ain't gonna get all antsy about it but if a custom forgie goes in it'll be of the high comp variety.

[reecegze]

Has anyone actually put in forged higher compression pistons into there 4agze and ran 10psi + above ?
It will be interesting to see if its worth it to give it some top end.

[Sam_Q]

anyone figured out where to get some highish compression 4a pistons yet?

[SillyCarS]

ask

30psi 4agte

[30psi 4agte]

Just had a quick look around then and couldnt find any forgies offering comp around the 10 's

Im not sure if acl will make up pistons to suit the 10:1. They may / should already make pistons to suite this comp ratio.

You should beable to machine both the head and block to achieve desired comp ratio.

[Sam_Q]

already thought of shaving the head, the limit on a 20v is apparently 1mm, that combined with a TRD gasket makes the ratio go from 8.5:1 to 9.2:1 or something, and thats not even including the fact that I have a blacktop head with a more open chamber. I have to say it is tempting to use aftermarket NA pistons, if standard can take 7psi then surely aftermarket can take more again?

[30psi 4agte]

Just did some calcs then and running a 0 deck height ( pistons flush with deck) and a .8 mm head gasket you will have a comp of 10.2 :1

Thats for a 16v. You are using a 20v arent ya ?
Did you find out what the 20v combustion champer volume was ? Ill beable to do a calc for ya.

[30psi 4agte]

Whats the reason for only 1mm ????

[-GT-]

What about the arias pistons? I think in your thread Sam about the metal HG somebody mentioned arias' at 9.0:1. Combined with the 0.8mm MHG will that get you where you want to be?

[Sam_Q]

yeah I thought about it but they are something like $1000 for a set, unless someone knows where to get them for a sensable price, maybe I should buy them in the US

[SillyCarS]

Technically this is the low down, which in practice agrees 100% with justen:

sorry about the pixels


I havent read the entire thread so sorry if this has been said.
This is a T-S (temp-enthalpy) diagram, as you can see by increasing the comp ratio (red area) the amount of fuel (energy/enthalpy) used is dramatically reduced, it is more efficient. However what you will also notice is that the blue area, however less efficient is much larger, meaning a far greater power output.

So as justen said you want to look at the numbers your shooting for before choosing the comp ratio thats right for you. Yes you will achieve more power uping the comp but it has limits as there is less room for fuel. Bottom line more fuel more power, the trade off in this case, higher comp equals more power, more stress out of a smaller engine. Which costs a lot of $$$ as can be seen with 30psi's fine example.