High compression or more cylinder volume ????????............

[Guest vf20/21bg5]

As some of you may already know im in the process of building a semi monster tune,twisted turbo,link setup finished by tuning & performance(hamilton)..etc etc ...ive built many hi-compression engines over the years but never a hi-perf turbo engine

My question to you experienced subaru performance engine builders out thr is this.....

.... i want low down torque for quik off the mark,quik gear changes,never going over redline,street/strip racing and the way i see it is if i do my build with more compression (by skimming heads as far as thy go,thin headgaskets,bigger than standard duration/ lift cams,biggest forged pistons you can resulting in hi-comp after passing the clay test i may add lol) then will that give me quiker take off's due to the fact that you are helping the (450hp rated Garrett) turbo wind up quiker due to more exhaust compression on the impeller winding it up quiker....??(im sure im right)??

....now i ask this because i also know that when it comes to turbo's the more the swept volume of the cylinders,the thicker the headgaskets etc etc etc... the more the compression at HI BOOST due to the fact that the turbo is pumping in its max volume at that point uping the compression ratio to aroud 11:0 instead of 8:1 -8:5 (the oposite to hi compression as the ratio stays the same unless its supercharged) .

..im thinking that more swept volume would be better at top end and higher comp(less cylinder volume)would be better off the mark and low down....

it seems to me like its a bit of give and take but please realise wot i am building this engine for...really quik off the mark and quik as hell gear changes.......so wot do you experienced engine builders think of this...all sensable comments welcome as i want to do this once and do it right as im putting over 12k into this build for reliability...

[pl0x]

interested

[Guest vf20/21bg5]

 pl0x said:

interested

same here,cant wait till the big boys come onto this one as its a good know how can do topic which is never really bought up

[Guest]

Im no engine builder.

When you say volume do you mean capacity ?

If so i would decide on how big 2.2 2.35 2.5 to go then worry about the compression ratio.

2.5 has proven to do all you require

[BC5RA]

As Qwerty has pointed out.

Go big.

[Joker]

I would say he's talking about volume of air vs PSI not capacity..

there's no replacement for displacement!

[pl0x]

a 2.0L will always be a 2.0L no matter what the compression

high compression will mean better spool/low end/off boost, but cant run as much boost at top end?

low compression is a dog to drive off boost/slower spool but can run a lot more boost/bigger turbo safely?

^thats what ive picked up from other discussions ^ correct me if im wrong

How do you determine the displacement? area under piston?

how do you determine compression? area above piston?

[GC8E2DD]

 pl0x said:

How do you determine the displacement? area under piston?

how do you determine compression? area above piston?

I was under the impression it was;

Displacement: the sum of the areas above the pistons at BDC

Compression: the ratio between the displacement and the equivalent measurement at TDC

[pl0x]

whats BDC & TDC

[GC8E2DD]

Each piston has a TDC (Top Dead Centre) meaning that it is closest it will get to the top of the cylinder, and BDC (Bottom Dead Centre) meaning the farthest away from the top of the cylinder it will get. These would be 180 degrees apart on the crankshaft I guess.

[Koom]

 GC8E2DD']

[quote name='pl0x said:

How do you determine the displacement? area under piston?

how do you determine compression? area above piston?

/quote]

I was under the impression it was;

Displacement: the sum of the areas above the pistons at BDC

Compression: the ratio between the displacement and the equivalent measurement at TDC

Displacement is the bore times the stroke. Swept volume that the piston displaces as it moves from BDC to TDC.

(Pl0x, BDC = Bottom dead centre, TDC = Top Dead Centre i.e. top and bottom of the stroke)

Compression ratio is the total volume at BDC divided by total volume at TDC.

Simplistic version of that is (bore x stroke plus cc in head)/(cc in head)

You can then change the interaction within the engine a few ways while keeping the same displacement. Easiest is compression ratio by H/G thickness or use pistons with different size crown.

Another way, is to use longer rods matched with pistons that have a shorter pin to crown height which gives more torque.

[Guest vf20/21bg5]

....Koom your on to it......i may have said things a little backwards in my original thread starter but i was meening what you said here.....

[Guest vf20/21bg5]

 pl0x said:

a 2.0L will always be a 2.0L no matter what the compression

high compression will mean better spool/low end/off boost, but cant run as much boost at top end?

low compression is a dog to drive off boost/slower spool but can run a lot more boost/bigger turbo safely?

^thats what ive picked up from other discussions ^ correct me if im wrong

How do you determine the displacement? area under piston?

how do you determine compression? area above piston?

....yes just what i wany to hear p10x........ so by going as hi-compression as i possibly can I WILL GET WAY FASTER SPOOLING BUT AT A COST OF LOOSING MY TOP END POWER....is this right every-one?

[pl0x]

i don't think its losing power, just not as safe to run high boost compared to low comp engines.

[Koom]

Higher compression will limit your det threshold which could limit the amount of peak torque you can make, not just top end power as its not exactly choking flow in the top end.

[Guest vf20/21bg5]

...and my build will be a 2.0ltr build just for the fact it has a slightly smaller stroke and displacement that it should rev and get up to speed ever so slightly quicker than a 2.5 build altho i do reaslise the 2.5 build wld gve me more power i do think that power would be better at top end rather thn wer i want it dwn low..

ps...i do also think you can use a 2.0ltr crank too for shorter stroke but the volume of the 2.5 cylinders would still be better suited at my top end right?????????

[ODB]

this si currently being discussed in the "AVCs worth it or not?" thread.

Go check it out.

[Koom]

2.5L will make the same power as a 2.0L down lower in the rev range. No replacement for displacement. It'll spool the same sized turbo much faster. The smaller engines need to rev to make the power.

[Guest vf20/21bg5]

 Koom said:

Higher compression will limit your det threshold which could limit the amount of peak torque you can make, not just top end power as its not exactly choking flow in the top end.

ok cool...now wer getting to all the good bits....but wot if i want everything in my engine build to happen well b4 redline like changing gear at 7-7500rpm thus limiting bottom end engine failure as these engines are renound for ?????.....and in this im meening for quik spooling well earlier than standard obviously sacrificing my top end fully

[Koom]

 vf20/21bg5']

[quote name='Koom said:

Higher compression will limit your det threshold which could limit the amount of peak torque you can make, not just top end power as its not exactly choking flow in the top end.

/quote]ok cool...now wer getting to all the good bits....but wot if i want everything in my engine build to happen well b4 redline like changing gear at 7-7500rpm thus limiting bottom end engine failure as these engines are renound for ?????.....and in this im meening for quik spooling well earlier than standard obviously sacrificing my top end fully

2.5L with a VF23 would spool early and make 200kw by about 5k rpm if done right.

[Guest vf20/21bg5]

 Koom said:

2.5L will make the same power as a 2.0L down lower in the rev range. No replacement for displacement. It'll spool the same sized turbo much faster. The smaller engines need to rev to make the power.

..................ok now we are getting thr thanx

[ODB]

Doesnt funkytown have that on his 2.0???????

[Koom]

 ODB said:

Doesnt funkytown have that on his 2.0???????

Possibly, flatmates 2.0L longrod motor made about 200kw by 4k rpm and ran out of puff by 5.5k rpm. His dad's 2.35L made +220kw at 4k but also choked by 5.5k rpm. Totally different to a street engine though as they ran restrictors and were close to freeboosting down low for max torque and the restrictor kills power in the top end.

Bigger displacement will always win the spooling race (all other things kept equal).

[Guest vf20/21bg5]

 Koom']

[quote name='Koom said:

Higher compression will limit your det threshold which could limit the amount of peak torque you can make, not just top end power as its not exactly choking flow in the top end.

/quote]ok cool...now wer getting to all the good bits....but wot if i want everything in my engine build to happen well b4 redline like changing gear at 7-7500rpm thus limiting bottom end engine failure as these engines are renound for ?????.....and in this im meening for quik spooling well earlier than standard obviously sacrificing my top end fully

2.5L with a VF23 would spool early and make 200kw by about 5k rpm if done right.

......as thr is another thread debating the same things here ill keep it simple for my final posts.....is it better for my 2.0ltr build (quik off the mark,spooling hi asap etc etc) to go hi-compression(maxed out skimmed heads,thin gaskets etc etc..)..or is it not going to make much difference at all really???..all im after is an answer like "its better than not doing it" or "you shouldnt do it cause"?

[Guest vf20/21bg5]

 Koom']

[quote name='ODB said:

Doesnt funkytown have that on his 2.0???????

/quote]

Possibly, flatmates 2.0L longrod motor made about 200kw by 4k rpm and ran out of puff by 5.5k rpm. His dad's 2.35L made +220kw at 4k but also choked by 5.5k rpm. Totally different to a street engine though as they ran restrictors and were close to freeboosting down low for max torque and the restrictor kills power in the top end.

Bigger displacement will always win the spooling race (all other things kept equal).

...holly shit really maxed out at 5.5rpm WOW!...ok now thats alot shorter than what t i want from a 2.0ltr but i bet his bottom end would last???