For those of you wondering what it feels like...go find some turbo Honda buddies. The ones on the stock shortblocks (uncracked sleeves!) will have around 11:1 SCR...and the ones with built engines typically are around 9:1 You can ask them what the difference felt like. I like the off-boost throttle response of the high compression and the extra power capabilities that more compression will give you on any given size turbocharger

I really wouldn't consider lag into the reason why I chose high compression. I think that's why you're getting answers that fall on either side of the coin.

You can use the energy of combustion to do work on the piston or on the turbine. If you raise the SCR you're putting more of the energy into the piston, and therefore you have less energy to give to the turbine. However... you'll be making more power because the higher scr engine is more efficient.

I simply calculated what my DCR (dynamic) was stock with stock cams...and then back calculated my SCR usign the same DCR. New stroke and New cams...calculated my static compression ratio needed to be 10:1. Of course I could've gone higher... but then we run into knock threshold issues on 92 octane. I think if I was more daring I would run 10.5:1 on my 2.2L with FP5R's...

You will gain some insight into what's happening to the combustion process by playing with the dynamic compression ratio.

 

where can I find a HOW-TO thread as to calculating dynamic/static compression. I'm upgrading my E7 to ported/coated head, HKS 274 278 2010! type cams with 11.1mm lift.

I would like to buy a set forged pistons and HKS/PE headgasket and END UP with 10.0 compression. Want stock dimension pistons, stock stroke, stock rods.

DIRECT PORT HFS6 and HKS 7460r turbo/fab manifold.

 

I just went form 8.8 cr 2.0 to 10.5 cr 2.0 to a 9.0cr 2.3
In a very short period of time with no other changes.

The compression increase from 8.8 to 10.5 did not spool turbo faster. But it made better power everywhere. Fell more linear and usable.

Switching to the 9.0cr 2.3 form the high compression 2.0 had me never wanting to build another 2.0. The power band just wakes up with torque. Turbo spools way faster with the extra CC.

I would recommend a 10/1 2.3. I went with 9/1 because i am impatient and there arent any off the shelf pistons like that. I have had bad experiences with custom piston making so I tend to stay away from that if possible.

Higher compression has many advantages but the biggest for me is turbo efficiency. Not having to spin turbo so high is big on my list. Its rarely posted info but turbo failure over 30 psi is huge. Turbos live longer lives at lower rpms. killing a 1500$ turbo that is not under warranty is painful at best.

 

I think there needs to be a little more qualification to this statement. The majority of turbos out there are journal bearing. They are only designed to handle a certain thrust load, what would be expected in its usual range of operation.

However, being car guys that like to push everything to the limit, the turbos are often overspun (to get more boost) resulting in higher thrust loads on the bearings than they were really designed for.

Ball bearing turbos generally have a significantly higher thrust load capability and are less prone to failure due to excessive loading. Not completely failure free of course, but much less likely.

 

This all comes down to piston design to me and what piston provides the best combustion chamber shape is the question to ask.

IMO, a flat top piston with minimal deck clearance and a thin head gasket is best. This has multiple benefits like the crown is a consistent thickness. This results in a piston can be made lighter while still being strong. It will distort more evenly under loads allowing tighter clearances. It loads the piston more evenly. It removes potential hot spots and allows a more uniform surface temperature. It maximizes “quench” area (minimal on the 4G63). Among other benefits.

This happens to be around 9.4:1 compression on the 2.0L 4G63 if using a Mits MLS gasket. This is also what shops call 10:1, but it really isn't 10:1. A 0.040" HG will give you 9.7:1 with a piston like this.

 

I calc'd out what the SCR would be on a 100mm stroke crank with FP5R's to achieve the same dynamic as stock and it was closer to 10.5:1.

 

a 2.3 will help with spool because its an additional 15% more motor, as for the higher compression, its my understanding that it helps lower end power

 

EDIT*** Static compression calc found here.
http://www.not2fast.com/turbo/compre...pression.shtml

Static is max volume/min volume.

 

I tend to think every engine family will be different as far as dynamic CR goes and it would be impossible to put a generic formula on it .

One things that's always surprised me is that no ones made up a quad throttle inlet manifold for the 4G based twin cam engines . Done properly this can easily raise the dynamic CR because they are a very low loss throttling system (which helps bring effective CR closer to measured CR) and also allows you to run hotter cams without the chaff cutter effects of a single plenum and throttle manifold on big cammed engines .
Individual throttle plates help block the reversion/pressure fluctuations you get with big overlap cams , together with the twin scroll turbo systems on these engines it would give you more oomph for the capacity .

I wonder if the Nissan Pulsar GTiRs throttles could be grafted onto a sawn of 4G63 inlet manifold . From memory that Nissan also has the turbo out front and the inlet facing the fire wall so its factory developed plenum may also be usable or modified to fit .

Thoughts ?

Cheers A .

 

So whats the disadvantage to a High CR example 10:1? I understand say 11:1 and over would be a pain on the street using 92 or lower octain.....but is 10:1 the way to go? i already have a set of 9:1 CP's that are ready to go in.....starting to second guess here....

 

10:1 and higher is more oriented to cars that arent octane limited and need to maximize a given combo. E85/E98 or Q16/Import would be the cars that can really benefit from the higher octane. Past 10.5:1 the octane limit of even e85 starts to show up and running more timing vs knock voltage gets "interesting". 11:1 motors at high boost (40+) dont run as much timing as a 9:1 or 10:1 counterpart on a given high octane fuel.

Pumpgas cars dont need to run past 9:1 even if they are running meth. To answer your question, the disadvantage is it starts following the path of diminishing returns on pump fuel and becomes unneeded if you cant run the timing or the boost to take advantage of it.

my 2 cents

Aaron

 

a guy here in jamaica fit BT4AGE 4throttles to a 4G93T

 

i was just about to say that. they are 45mm diameter.
GTiR are 42mm?

And they are really cheap to pick up 2nd hand (less than $150usd)

 

So sounds like higher compression ratio would give you more low end torque. Am I right?

 

throttle bodies have DICK to do with dynamic compression ratio.

all it has to do with is throttle response, which is more dictated by the turbine and the turbo manifold than the intake manifold. complete bs.