Sittingmongoose

iTrader: (1)

I had an interesting conversation with a guy who is supposed to know his stuff so lets see if he knows some stuff we didnt know before.
-The ls4 is acually the LQ4, the ls4 doesnt exist.
-Heads will never decrease power from being too big.
-I am losing power from my 1.8 rockers because my cam's and heads geometry is not designed for 1.8's. If i put back my 1.7 rockers, i may gain power.
-We can not simply snip the dod wire off even if dod is disabled becasue it needs to be a closed circuit, if we do, the engine will go into limp mode. I asked if we could just use a ls2 vally cover and he said that you will still need to connect that male dod wiring harness so something.

tokay57

the lq4 is the iron block version of the 5.3, ours is all aluminum. yes, the ls4 is real designation. too large head ports will decrease average horsepower below the curb, will increase peak hp at higher rpms. don't know about the rockers. your heads are just a tad too large, its not like the boss cleveland heads on the boss 302, now those heads were big. stock, they were slugs but put in some 4.30's on the back and that mustang could outurn a hemi cuda.

neilownz

LQ4 is a 6.0L truck motor.

TiredGXP

iTrader: (2)

There's a bit of mis-information in what you were told.

Yes, heads that are too big can decrease performance. When it comes to efficiently filling a cylinder many factors come into play - RPM range; valve size, lift, timing and duration; port size; exhaust back pressure, intake manifold design...

Limiting the discussion to the intake side, it's important to note that the intake valve is open for a considerable time after BDC, perhaps 45* or more depending on the cam. In order to keep air flowing into the cylinder after BDC (or at least to keep it from being pumped back into the intake) you need both positive wave pressure (determined by the length/shape of the intake tract) and momentum of the air moving past the valve. Excessively large ports slow down the intake charge, making it more susceptible to reversion. The larger the cross-sectional area of the intake ports, the higher RPM required to maintain velocity.

The impact of excessively large ports is to decrease cylinder filling/power at low- and mid-range RPM's. Large port race heads are probably not what you want for a street driven engine that spends most of it's time below 4k RPM. The $2,500 question is what are the right size heads for the application?



Edit:

Rocker arm geometry is determined primarily by pushrod length, not the rocker arm ratio. Higher ratio rockers provide quicker valve opening, more lift and more total flow area.

Next thing you know, this guy is going to say that you need backpressure to develop torque.

DP 08 GXP

iTrader: (3)

you wouldnt believe how many times ive heard this. when i went to the track awhile back and my traps sucked a few guys on another site were saying the cutout was hurting me since there wasnt as much backpressure with it open.

Sittingmongoose

iTrader: (1)

Well engines do need SOME backpressure and with turbo set ups you do need backpressure. Or at least thats what i was always told

JDMC5

iTrader: (3)

from the sound of it this guy DOESN'T know his stuff.

TiredGXP

iTrader: (2)

You never need backpressure.

When it comes to generating torque, the name of the game is airflow. Backpressure is the result of restrictions to airflow and reduces volumetric efficiency.

Think about it - an above atmospheric pressure in the cylinder at the end of the exhaust stroke, just when the engine is trying to bring in a fresh charge to the cylinder, means that you end up with a smaller, more diluted charge of air and fuel-->therefore less fuel to burn and less pressure is produced on the next power stroke. Less pressure=less torque=less power.

Tuned exhaust systems work because, within their RPM limits, they create a slight vacuum in the cylinder at the end of the exhaust stroke. At other rpm's they may create additional backpressure. I haven't been through all the math, but the Tri-y design is apparantly effetive over a wider rpm band than long tubes, but are more effective at lower RPM's as well, which is probably why you don't see them too much on drag cars.

I've been doing a fair bit of research into turbo's for the Fiero project (3.2 turbo) - I'm generalizing a bit here, but while the turbo's turbine wheel and housing create backpressure, what the turbine needs to function is air velocity. Reducing the A/R ratio of the turbine housing speeds up the airflow across the turbine wheel, but at the cost of restricting overall airflow. Small A/R ratio's give boost early, but choke the exhaust at higher RPM's, limiting overall power production. Larger A/R housings create less velocity, less backpressure, but take longer to spool up, ultimately more power is available at high RPM. Similar effects exist with turbine wheel size - smaller turbines spool quicker, but have a limit to the amount of air they can flow.

Backpressure is not a good thing for overall power production with a turbo either, but there you go, more trade offs--> have the turbo give you a fun, quick boost at low RPM and then go flat or get maximum power coming on just when you need to shift. In reality you size the turbo to avoid either extreme.

and sorry for the lecture

06 SS

No. Backpressure is bad for engine output at all times. Higher backpressure will ALWAYS lead to lower net output.

nmp0098

There is a sticker somewhere, in my case it is in the trunk under the carpet, that lists all of the RPO's for your car and LS4 is definately there. Your VIN also references LS4 if you look at the official GM VIN decoder.

As to heads being too big, it all depends on the rest of your combo. There are no absolutes in hot rodding.

The whole point of changing to 1.8s is to alter the valvetrain geometry, and in a way that dyno tests repeatedly show to improve power.

But, here's why I posted: he is way off about the DOD connector. There are five wires, one for each solenoid and one common ground. The ECM needs to output some voltage to energize the solenoid to disable the lifters for that cylinder. There is a DTC for each cylinder, and they can be disabled in HP Tuners. Even if you didn't disable the DTCs, I doubt the ECM checks for the presence of a solenoid until it tries to actuate it. But let's say it did... you could still fool it with a resistor. Even if it ran the most exotic diagnostic check known to man, you could still fool it with an impedance. Just disable the diagnostic check when you get tuned up and forget about it.

AlabamaGuy

iTrader: (5)

Turbo cars run the best with no backpressure.

SidewaysV8

So do Top Fuel cars.

20SS07

I would have stopped listening the second he said the LS4 doesn't actually exist.

chevyman9306

iTrader: (9)

Sounds like someone needs to refer said "knowing guy" to ls1tech so he can come into the modern world

Pauls325

iTrader: (1)

from everything i have read there is such thing as too little backpressure if you are teying to optimize the scavenging effect of the exhaust but seeing our exhaust manifold and dp setup i would say go for it i remember hearing that our cat needs a cartain ammount of backpressure to operate though...

06 SS

Backpressure is a detrimental side effect of pipes that are too small for the volume of exhaust. It NEVER does anything but hurt net engine output, regardless if naturally aspirated, forced induction of any kind, N20, whatever. Pipe size can influence torque production location in the operating range of the engine, just like an intake or a cam. Smaller pipes increase velocity and help draw charge in the cylinder during overlap at lower engine speeds. That will tend to increase torque production down low (lower the smaller pipe you use). A side effect of a smaller pipe is backpressure which is a pumping loss, it hurts net power. The myth of backpressure helping low end torque is based on this. Bigger pipes mean less torque down low, when you lessen the pipe size, the low end torque goes up because of the velocity of the exhaust pulling more charge in during cam overlap, not because of increased backpressure. Backpressure is bad. Bigger pipes (in an extremely simplified description) move the torque curve higher in the RPM range. So, again highly simplified, smaller pipes means the torque curve is shifted to the left and bigger pipes shift it to the right. Backpressure is extra work the engine has to do and output to move the car is reduced by that much work. Smaller pipes run into backpressure with less exhaust than larger pipes. As an example; the backpressure of a smaller pipe might choke off the output of the engine substantially at 4000RPM, but if you had bigger pipes on there, less backpressure would mean more net output at that point and above, matter of fact, the bigger pipe would start to flow faster with the greater volume of exhaust flowing through it and the increased velocity would boost torque at that RPM range vs the smaller pipe. Pipes can be tuned for the operating range you want in an engine. Cam, intake, exhaust, all have optimum RPM ranges, the combo can make or break an engine's output. You can dog out a low end torque monster by putting in huge headers and exhaust, you can choke a high RPM horsepower screamer by using small pipes to try to boost low end torque. Everything is a compromise.

Sittingmongoose

iTrader: (1)

So your saying if i go and just snip the DOD wire on my car, that because DOD is disabled in the car, it wont do any harm. Meaning, when do the intake swap, i can use my existing valley cover and just chop off the female dod connecter.

TiredGXP

iTrader: (2)

You'd probably also want to disable the DOD circuit/open DTC's:
P3401, P3425, P3441, P3449