6lb OR 4lb w/ 2.8

[00Bolt]

after I get my new exhaust installed... i figure with the loss of boost, ill be able to afford to add a little more... maybe kinda even it out to where I am now...

can someone please explain what the difference is between having a 6lb lower or a 4lb with a 2.8 upper...

i know the physical difference, but performance wise... is it about the same, or does one give more boost/power than another???

thanks

[bobbywade69]

Of course there is more power out of a 6lb than a 4lb, u are creating more boost, so more HP and Torque!! But I don't know about loss of boost with headers, never have heard that!!!:bs Nothing that u would notice!!


after I get my new exhaust installed... i figure with the loss of boost, ill be able to afford to add a little more... maybe kinda even it out to where I am now...

can someone please explain what the difference is between having a 6lb lower or a 4lb with a 2.8 upper...

i know the physical difference, but performance wise... is it about the same, or does one give more boost/power than another???

thanks

[PoorSvtman]

you might loose some boost maybe 1lb, hard to explain... something about pressure differences with the headers and the factory setup.... But you will gain some hp with the exhaust.. Just because you loose boost you arent gonna get slower... Less boost with more hp is nice and in my mind safer...you can never go wrong with more hp with less boost.

[00Bolt]

i realize that less boost does not mean less power. but i talked to jim jr at jdm and he said the full exhaust will lower my boost. and that I can move up to a 6lb and be about the same boost I am now with the 4lb and no full exhaust...


but my question was... which gives you more power... a 6lb lower or a 4lb w/ 2.8 upper.... i always thought the 2.8 upper was like adding 2lb to lower.... is that correct? if they add about the same amount of power... then which is the better route to go?

[StormShadow]

Get an interchangable upper. Then you can switch it whenever you want. Takes less thank 5 minutes.

[BLACKSUNSHINE]

i realize that less boost does not mean less power. but i talked to jim jr at jdm and he said the full exhaust will lower my boost. and that I can move up to a 6lb and be about the same boost I am now with the 4lb and no full exhaust...


but my question was... which gives you more power... a 6lb lower or a 4lb w/ 2.8 upper.... i always thought the 2.8 upper was like adding 2lb to lower.... is that correct? if they add about the same amount of power... then which is the better route to go?

its all the same, there is no better really. A 4# lower with a 2.8 or 2# upper is 6#'s. Same as just a 6# lower. But better? I have the interchangable lower...easier, ALSO the you have more coverage area on a lower, whereas when you change to a smaller upp you decrease the belt coverage area...which aids in slippage. So if you change the upper, I would go with a metco idler pulley if you dont already have one. ALSO I didnt notice any boost loss when I got my long tube headers on. Im hitting a sollid 14lbs (cool outside air) with a 4# lower and my other mods.

[BigBobsL]

The level of boost is not a messure of how safe you are, cylinder pressure is the issue. Your boost level may have gone down, but your cylinder pressure is at least same and likely higher since the engine is now more efficient. Cylinder pressure determines octane requirements (likelyhood of detonation) and load on rods, etc, not boost. Boost is a measure of back pressure in the intake.

[wesman]

can you explain to me how making the engine more efficient is going to increase cylinder pressure? :confused:

--wes

[PoorSvtman]

can you explain to me how making the engine more efficient is going to increase cylinder pressure? :confused:

--wes


Im with wes.

If your letting your engine exhale better by using a free flowing exhaust like headers, high flow cats,etc. your cylinder pressure is going to decrease. You may be pushing the same amount of boost but i believe the pressure will be less with the free flowing exhaust.

I may be wrong as im still learing about these motors, but i dont see how adding exhaust to let the motor exhale the exhaust faster and more efciantly (sp?) is going to increase cylinder pressure



Also i recently added a 2.8 upper with my lower 6lb. I think i only gained aout a 1lb of boost. Im also getting a little bit of belt slip in the upper rpms at times. The butt dyno sure did notice a difference :burnout: So untill i get a tighter belt and maybe ditch the 100mm idlers for a idler bracket i know i will continue to see belt slip

[BigBobsL]

OK, you asked for an explanation, here we go.

First exhaust backpressure and cylinder pressure are different things.

Total (average) and peak cylinder pressure is what generates horsepower (HP). Total cylinder pressure is the force pushing down on the piston from Top Dead Center (TDC) until the exhaust valve opens somewhere before bottom dead center (BDC). Any pressure generated before TDC costs HP because it increases pumping losses, the power consumed in rotating the motor.

An engine is an air pump. All things being equal, the more air/fuel it consumes in a given time (rpm), the greater the HP and thus efficiency. A supercharger pumps air to the motor faster than it can consume it, the excess air/fuel the motor can not ingest is seen as boost (backpressure in the blower intake). The amount of air/fuel the motor ingests and burns in a given time (rpm) determines its HP (efficiency). If we make the motor more efficient by better emptying the cylinders of burn fuel by improving the exhaust (reducing backpressure) so that it can ingest more air/fuel, the motor now makes more power. Boost is also reduced since the motor can now ingest more of the air that the blower is generating and an added bonus is the blower is now producing less heat since higher boost reduces the blower’s efficiency. We can also increase the engines efficiency by improving head flow in and out of the motor via bigger valves, porting, or better cams. An increased amount of air/fuel ingested and total cylinder pressure and less backpressure will be evident on the boost gauge for a given set of pulleys and blower).

From a volumetric efficiency standpoint, if we burn more air/fuel into the same space (cubic inches) in the same amount of time (rpm) to increase efficiency (HP), engine efficiency increases since it is generating higher total cylinder pressure, but also more heat.

From a tuning standpoint, timing, fuel quality, air/fuel ratio, and compression ratio have a big impact on total and peak cylinder pressure and thus horsepower. Horsepower is the power generated by total cylinder pressure after TDC on the combustion stroke and before the exhaust valve opens sometime before Bottom Dead Center (BBDC). Any cylinder pressure generated before TDC increases the engines pumping losses since it is working against the movement of the piston up the bore rather than forcing it down the well bore in the direction of rotation (HP).

There is only a brief finite time for combustion to occur with maximum efficiency and as rpm increases, that time gets shorter. If timing is set at 20° Before Top Dead Center (BTDC), we are starting the air/fuel burning 20° BTDC. This provides more time for the fuel to burn after TDC, but while the piston is still near TDC where it has the greatest mechanical advantage (slowest piston movement and smallest combustion space. Maximum efficiency is achieved when we achieve maximum total (average) pressure after, but near TDC. If we have too much advance, too much pressure is generated before the piston reaches TDC. The piston is being forced to compress the expanding combustion gases rather than being pushed down hole by the pressure as the gas expand. The expanding combustion gases pressure is then working against motor rotation and inducing extreme peak loads on the rods, pistons, etc. The resultant extreme peak pressure is what pushes a lot of broken parts out on the ground. It’s like a boxer taking a punch, if he is moving toward the punch and continues to move forward even after he is hit, it hurts a lot more (breaks rods) than if he is moving away and with the punch. Both situations lose power in the motor. We want the punch (peak pressure) to land just after TDC. If timing is too slow, the burn takes place too late and is lost when the exhaust valve opens. Much of the energy is also wasted because it is trying to accelerate an already fast moving piston rather than a very slow moving piston. The piston moves slowest while near TDC and BDC.

Air/fuel ratios and low octane fuels also have an impact upon optimum timing and maximum safe cylinder pressure. An overly rich mixture burns, slower, cooler and less completely, thus the engine generates less cylinder pressure and can tolerate more timing, but likely will make less power. Up to a point (near 13.5:1), the leaner the air/fuel ratio (AFR), the quicker and hotter it burns, the greater the total and peak cylinder pressure, thus AFR also affects optimum spark timing. Reality is optimum AFR will be limited to 13.0 – 13.5 for NA engines and 12-13 for SC engines. When the peak pressures generated by the spark plug generated flame front get too high for a given (octane) fuel, the remaining air/fuel will spontaneously combust. Extreme peak pressures are now generated by the two different flame fronts since the combustion process is now much shorter and maybe occur before the piston reaches TDC. High octane fuels can endure higher temperatures and pressure without spontaneous combustion and/or burn more slowly and thus reduce peak pressure. That is why sometimes you can see a slight power loss under normal conditions when you have more octane than a motor requires, it has reduced burn rate and timing may need to be increased to regain the efficiency.

Cold plugs do not make power, but the shorter insulator absorbs less heat and thus runs cooler so that it does not get hot enough to act as an ignition source before the plug can fire (pre-ignition). Too cold a plug fouls and causes power robbing mis-fires, no spark.

I will stop here since you asked for the time and I am likely telling you how to build a watch. While not short, it is not as detailed as it could be for complete accuracy, it hopefully will help.