GerryXJR

Trying to decide what CR to run for an end boost PSI goal... of say 20# ?

this is more for figuring detonation danger zones over head lifting and things of other natures...

TIA

IronBlocked

On what fuel? gas 93:9.0 or less is where it's at.
e85 10.5 or greater, gotta love e85.

GerryXJR

lets say 91 thats the most commom octane around here.. but most of the time it will be on 93

IronBlocked

Then go 9.5 and below, for comparison, the 2jz comes with 8.5:1, unless you decide to go meth-water

GerryXJR

Looking to use an LQ4 or LQ9

The q4 is 9.4:1
the lq9 is 10.0:1

@20#s boost what would the CR get up to ? approx ?

i am going to install forged pistons as well, as i understand the #5 and #7 pistons seem to be prone to ring land failure due to detonation...

just wanna be more than safe on my short block build...

i also understand that the later 6.0's had better rods than the earlier blocks...

sodium filled rods ? sopposedly stronger but snap rather than bend..

if that is the case i may go with forged rods as well, instead of stock...

any advise would be great...

I would like this build to last a very long time, mostly street driven, but usualyy will be dialed twords the higher end of its range most of the time...

i would like to see rwh in the 8's, but having it spool on the earlier side for more bottom tourqe, as the car weighs in the low 4's I want my bottom to handle it with ease and not have to worry about pushing boundries all the time...

Any advise would be greatly appreciated TIA

soundengineer

iTrader: (16)

The formula you are looking for to figure out "total compression ratio after boost amount"

((Boost PSI/14.503)+1) x Engine Compression Ratio = total compression

GerryXJR

Compression Ratio - Boost 101

Is a low compression engine better for forced induction than high compression?

Depends on how much boost you're putting into the engine.

The big issue here is managing the amount of internal pressure within the cylinders; by making sure it's not going to damage the engine whilst still making the most power possible. Too much pressure can cause catastrophic failure where you literally blow the head off the engine. Hence why Top Fuel drag cars have big straps to hold the superchargers down in case they get blown off.

The higher the compression ratio, the more natural torque an engine produces. Adding forced induction increases the effective compression of an engine, because although you have the same compression ratio, air and fuel are entering the cylinder already at a higher pressure. This increase in pressure translates into a bigger bang at ignition, and a larger pressure from the expanding exhaust gases - resulting in more power.

Dropping the compression ratio allows a higher amount of induction pressure to be used, meaning a greater volume of fuel and air can be squeezed into the cylinder. This results in a big increase in torque and power - as long as that volume is being delivered.

When the turbocharger or supercharger is not delivering the full volume - when it's 'off boost' then the engine is relying on a lower amount (and pressure) of air coming in, which results in less power. This breathless lack of power is often mistakenly referred to as lag.

A low compression engine with big induction pressure will perform very poorly 'off boost' (i.e. when the turbo/supercharger is not delivering), and will very rapidly build power as it comes 'on boost'. In extreme cases this can literally be like flicking a switch from no power to instant full power - and a car that will be quite a handful to drive hard. Depending on the induction device, this 'boost threshold' can be quite high in the engine rev range.

A higher compression engine with low induction pressure will perform much better 'off boost' because it still has its own natural compression to generate power; it will generally not have a big jump in power, and as the induction device is generally smaller, its boost threshold will be much lower.

A low compression, big boost engine will make an insane amount of top end power, but be very wheezy and powerless down low, whereas the same sized engine with higher compression and lower boost will be very torquey low down, but won't make as much top end power.
"What's better, low compression and more boost or high compression and less boost?"

There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a stree tcar, or when using a very small displacement motor. But when talking purely about on-boost power potential, compression just doesn't make any sense.

People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break-even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder; it just squeezes it a bit more.

So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:

sqrt((boost+14.7)/14.7) * CR = ECR


Either way, i do not understand how to do the math.. UGHhhh

GerryXJR

this car will be street driven every day, and beaten on every day.. the goal is to have all the power this will make ever on hand all the time...

is this asking too much, or not feasible?

and also get the right turbo that will spool the fastest "possible" on the bottom that the CR will allow, and also be the most productive from the begining of spool to the top side end of spool...

all with being in an EFC that wont bring detonation ...

i cant really pick a turbo till i can figure this out i guess...

I'm a newb as you can tell, i totaly get the theory's of FI as i have had numerous factory FI cars, but this will be my first "bulid".

And i see these are the things you need to know to get this right on the first try..

thats where i'm at.... any guidance would be appreciated

soundengineer

iTrader: (16)

Its simple... you dont idle down the street at 14 psi of boost.....
and your actual compression is not static... its dynamic

with a big enough camshaft, a 15:1 compression motor has no issue on pump gas because its actual dynamic compression if far less
It certainly would not be smart to try....but if you put a Gi-Normous camshaft in it, it could lower the dynamic compression far enough to be pump gas compatible.


the formula I posted...is the calculated Boosted compression ratio
is the accepted formula from multiple sources...

After a quick search
the formula you posted with the square root, is also accepted.....but yields a very different answer
which is a more normalized ratio to help determine max boost based on your static compression ratio....
it makes it easier to think about when you compare to a naturally aspirated engine and you think about when the gasoline will start to detonate and you will need to switch to a race fuel with a higher octane.

I have attached an excel file (uploaded in a zip file) that is a calculator, with a nice big table as well for visual

I did not make this file, it was downloaded from a turbo website a few years ago....

GerryXJR

Thanks i opened the table, and see the schedule of CR and boost for the ECR....

at a 9.4 CR and 20#'s of boost it = 22.36 CR

guess my next question is how does the engine live on 91 octane pump gas, i know timing is involved but still not wrapping my head around all of this yet...

i am very new and green to this stuff...

I am a professional body, paint and restoration guy, concourse style work, BUT not a professional engine builder. LOL

This will be my first time, and i really wanna understand all of this and do this one myself, its about time i do

soundengineer

iTrader: (16)

coming from a guy (me) who tunes turbo cars all the time on the dyno and at the track.....
the simple way to tell what you can and cannot do... is to take the vehicle to a reputable tuner and test it out on the dyno

you can turn up the boost a little at a time...
and you can creep up on timing a little at a time as well...
when you notice that you basically make less power or no extra power by turning up the boost because you have to pull so much timing t okeep it from rattling, then you know your limits...

as an example(theoretical)
if at 14 psi you can use 12* of timing you make 600 RWHP, and 16psi you have to back it down to 8* to keep it from knocking, and you still only make 600 RWHP... then its time to switch to race gas as the extra boost didnt help make any more power because you had to remove more timing to keep it safe..


and realistically... somewhere in the 12-16 psi range will be where you have to switch from pump gas to a race fuel(or add some water/meth) depending on your motor compression

GerryXJR

K thank you for your help ..looks like i have alot of homework to do...

soundengineer

iTrader: (16)

dont worry so much about the homework....

build a motor in teh mid 9's for boost, and use pump gas up to about 14 psi, and then use race gas at teh track to turn it up to 20 psi...

you will find that anything more than 7 or 8 psi will roast the tires on the street anyways...LOL

GerryXJR

In the scenario you just layed out for me what would adding water/meth do for those pump gas numbers you listed above ? TIA

And BTW i am looking forward to doing alot of rolling tire annihilating LOL

soundengineer

iTrader: (16)

Water Meth adds octane and cooling
the meth will add octane as it burns slower than gas, and the water adds another level of slowing down the flame front

it has as similar result as switching to race gas

JoeMama's GTO

iTrader: (3)

Hmm, this is some really good info. Your range of 12-16psi before race gas would also be affected by compressor type. I would imagine a turbo or definitely a roots style blower that has all the boost in at low RPM would fall more towards the 12psi range for meth/race gas. However, a centri blower that doesn't make its max boost until redline you could probably push further towards 16psi before needing more octane than pump gas.

soundengineer

iTrader: (16)

you cant assume anything, and you cant guess at it....

you will only know when you go to the dyno and test it

I've done combos that didnt like more than 12 psi on any type of boost device...
and I've done combos that could take 16 psi all day long

it doesnt matter if its a Turbo, or a centrifugal, or a roots style.....

Pick a weapon.. and test it on the dyno... that is your only real answer.

JoeMama's GTO

iTrader: (3)

Agreed 100%, but for a rule of thumb it may help. My tuning is a mix of dyno and street with a lot of plug reading. I have had too many wonky wideband readings to do it any other way.