mike13

iTrader: (27)

Here's a question I don't have a clue how to answer. Let's say you have a stock 03 Z06 motor it makes 650crank horsepower on 10psi running a conservative afr/spark advance. You take the same motor install LQ9 heads which lower the compression 1.5points to 9.0 how much hp/tq did you just loose without any other changes?

Now take the same combination with LQ9 heads and how much more boost could you run with that same conservative style tune and be at the same level of stress on the motor or maybe better put the same risk level?

TurboAv

iTrader: (6)

Boost>Compression

That said if you go to low the motor will be real lazy out of boost. It also depends on how you are boosting. The new TVS blowers, people are saying keep it around 10:1. With a turbo setup 8.0-9.5 is a good range. Mine is going to be at 9.3:1 for turbos.

Johnv

Also interested to no which will make more power if any,

two identical engines at same boost, one std compression 10.2 : 1 @ 15psi
(same fuel) one with lower comp 9.2 :1 @ 15psi

Obviously the lower compession engine will take more timing at the same boost level, but will it make more power ?

TurboAv

iTrader: (6)

The reason to run lower compression is to run more boost. OEM manufacturers do it to let you run more boost on pump gas. I would think in the above example the higher compression motor would make more power. I'm no expert though. Just an internet warrior in the vast cyber battlefield.

For my build I went 9.3:1 so it's not to lazy and will try to get as much out of E85 as I can. I don't want to run methanol, so it's E85 or bust.

jason02Z

iTrader: (10)

you going to prob have to run 3-5psi more..which you'll be up to the same cylinder pressure with you 10psi now. 650 is a good number for a stock motor, lowering compression and more boost wont help the bottom end last. I run lq9 heads on a stock ls1 and run 11-12psi to get the power back up. it only makes a around 380whp at 5psi so now i need double the boost. a boost cam would help me keep the boost down and still make reliable power.

mike13

iTrader: (27)

Does anyone have actual numbers, math formulas, etc?

youngrigo

Suscribed to this tread..... Im also intrested. Mods are in my sig and have been thinking of 317 heads as well. But dont want to boost more as im not sure if my block will like that.

mike13

iTrader: (27)

jason02Z said it better, if you lower the compression from 10.5 to 9.0 how much more boost does it take to get the same cylinder pressures?

bad6as

iTrader: (17)

lets take boost out of the picture first, same engine specs but one is 10.0 and one is 9.0 i would guess 40 flywheel hp.
but you throw boost back in and per pound of boost on 10.0 you might make 26 hp per pound.
but on 9.0 you might only get 24 per pound.
i very well could be 100% wrong.

youngrigo

My thoughts are this... and i may be wayyyyy off! Lowering the compresion will lower your power, but the 317 heads are suppose to flow like ls6 heads thus giving you better flow will add power (Just like if you were to put ls6 heads) but the lower compresion will take power away. So will it even out in power but give you less compression??? hum????

mike13

iTrader: (27)

I don't know I'm hoping someone can explain it to us. I'm just waiting

gametech

iTrader: (1)

Extremely rough math is that each CR point =4% torque, and each boost psi=7%power. Lowering compression =multiple psi of boost. This means that for total power, less compression and more boost is always better. However, at some point you trade too much off-boost power and fuel economy for more boost power. Also, off-boost torque will spool your turbo quicker, so the equation quickly becomes a matter of personal preference.

CARFREAKS

iTrader: (14)

Depends. Not all of this is true. We have v-8's not 4 bangers... Lower compressions ratio = lag... You need to run depending on your turbo about 5 psi to make up for 1.5 points of loss in compression. Also your not pushing your turbo's as hard. Now yes if you want 1000+ hp you need to lower your comp ratio but if you want like 700-800 ish the higher compression ratio will spool quicker and get you there sooner with less boost...

99Z28LS1

iTrader: (4)

Or for people that do not necessarily need a lot of TQ down low, I will trade to lower compression and more boost for my high way pulls

Johnv

The low compression engine will also take more timeing at the same boost level, so may not be to far behind at same boost,
but will have the potential to push boost further without going to a higher octane fuel.

njc.corp

It seems to me that one engine will have to do more work then the other-

one engine will make more torque then the other (the hi comp one)

I will take the hi comp engine any day of the week if we are talking about a pump gas type engine-

the hi comp engine has a lot of ups and down

it will make very good torque at a certain boost level

it will make a decent size turbo spool up

the negatives is that timing is set at one point due to a increase of cylinder pressure-

The low comp engine has to work a bit harder which is not good for parts in the long run if you have to double the boost for argument sake-

ups-can make more peak torque and power at a higher boost level and take a bit more timing
-less cylinder pressure at the same boost level of the hi comp engine

downs-lost of torque down low which then can effect spooling also as it does not have dynamic volume to spool the turbo as the higher comp one-

mike13

iTrader: (27)

Interesting theories on hi compression engines but why does just about all of the built turbo LS motors have lowered compression? I'm specifically talking about built motors in the 9.0 range?

Not disagreeing with people who are in favor of high compression they stated a opinion and gave reasoning just trying to create some discussion from those that have made the choice to go with 9.0+ - when they could have choosen any compression.

camarols1

iTrader: (17)

The CR of the engine should be built around the desired fuel to be used and power level.
For a race gas application, high CR and boost always make more power, but for pump gas applications the CR becomes a compromise between power and safety margin.

CARFREAKS

iTrader: (14)

Very well put! You can also use meth. or e-85 but you will need even bigger injectors with e-85. You can run pump gas in a high comp depends on how much boost your running anything higher then 10psi you cant use pump gas.

Also I drive a camaro not supra. I like low end torque I dont just want to be a highway queen...
Supra's have to play catch up. High boost low comp smaller displacement.

mike13

iTrader: (27)

There's a couple of issues that need to be explained with high compression and boost on race applications. Is it that your talking high static compression with a longer duration cam so that higher compression compensates for the lower cyclinder pressures?