Its my understanding that adding lt headers will drop the boost some. Along that line of thinking , I would like to know if changing from a stock exhaust, si do, system to true duals , would have an affect on boost.

 

it all depends on how restrictive your current exhaust is and well the new setup flows...

 

I lost 2 psi with my long tubes. But the truck is faster. Boost is not a measure of power.

 

An interesting parallel question is:

If one drops boost by 2# when adding long tubes, is it safe to restore the previous boost level by changing pulleys, etc.? Even if the boost is decreased, does the combustion chamber pressure also drop or remain the same during the power stroke?

Lee

 

I have to agree with JMC,
I added ported heads with long tube headers(lost about 3PSI boost) but it added power to the setup like I added 4 or 5 PSI boost!
So if adding back the 3#boost on top of that ,the power level would be that much more increased, calling for an adjustment in my tune.

Phil

 

It makes more sense to think in #'s per minute of air flow, mass of air flow. More air flow will support more fuel making more power. Less exhaust restriction allows more flow and causes less back pressure which is measured as boost.

 

everyone is correct, boost is a measure of back pressure, cfm is the true measure of power

 

Boost guage is really like a engine efficiency gauge. The blower will make whatever boost that it is mechanically pullied too despite what a boost gauge reads. If you are pullied for 10psi but the gauge reads 5psi, adding another 5 psi would really be 15psi not 10psi. The same amount of air is being put into the cylinders and being pushed out. With headers it is just pushing the air out easier showing the motor is more efficient thus the boost is lower.

 

I don't think that you can measure cfm, at least not through our pcm's. The software and maf measure by weight (mass) of air flow, not by volume of air flow. You can see # per min with datalogging. I have not personally used the # per min PID, but it is available in the Live Link software for some strategies.

 

How about, if you are pullied for 10 (or 20) psi and the gauge says 5 psi, then ya got's 5 psi. 5 + 5 = 10 psi.
Boost showing on the gauge is only a measure of resistance to air flow.

 

Sure sounds good pullie till the motor shows the psi you want and see if it stays together

 

I think this is a pertinent question to ask, since many of us are still running stock bottom ends. If you're increasing the combustion chamber pressure by pulleying up to regain a certain boost level, then that could blow a motor that's been running close to the edge before the headers. So, are boost and combustion chamber pressure at detonation proportional even with differing exhaust flow characteristics?

Lee

 

More air in a cylinder more compression there will be in that cylinder, thats obvious. However if the exhaust is to restrictive, then some exhaust gas may not exit the cylinder and when more air is introduced, it will be even more gas then the previous compression stroke. This is all figureativly speaking, but in a sense you will be pushing boost pressure through the blower and the exhaust causes a backpressure making even more pressure.

 

Good discussion! So, you're stating that exhaust gas, unable to fully escape the cylinder, contributes to the measured level of boost. Therefore, scavenging the exhaust more completely leaves additional "room" for increased contribution from the blower. Is that about it?

So, you could re-pulley to achieve the original boost level and the only difference would be that the cylinder charge would be more "fresh".

Lee

 

i had long tubes before boost. so im not sure how much i had lost.. but i also have a ported intake manifold. i still manage 8-9psi on 3.70 stock pulley