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Pulley swap?


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I just had a set of long tube headers installed and wound up losing 1 pound of boost in the process. I can drop a pulley size and get it back which would put me at about 480 rwhp with a stock bottom end. I was planning on going forged when I bought the headers but the economy has me a little spooked about dropping that kind of $ right now and I might have to put that on hold for a while. I was wondering if I should just leave it as is since I might be pushing my luck now with a stock motor. I have read several opinions on what is considered the safe limits for the 4.6 3V and I have seen several guys go beyond that with no ill effects at all. I would like to hear from some of the guys that have been pushing what is considered the safe limits of a stock motor and actually drive their car hard at the strip, on the street etc... I realize that there are a lot of variables to consider such as the tune, longevity and so on, but I still wonder what the difference is between what is considered safe and what the stock motor can do without coming apart.

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Probably the people who choose to run their cars on the drag strip will have the best data as my experience is that the strip is where you will most quickly see the damage appear (sometimes all of it in an instant). I have the same set up you do and I have run it really hard only on a road coarse (most recently CS86). Have you checked with KB to see if they have a thought about your lost pound of boost? I do not see the correlation between long tube headers and boost. I understand you are trading some bottom end for top end when you go that route but have not heard of the boost issue. My first call would be to KB.

 

Your torque number is about the same as mine but the RWHP is different - probably just the air out here on the left coast.

 

Good luck.

 

Jim

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Probably the people who choose to run their cars on the drag strip will have the best data as my experience is that the strip is where you will most quickly see the damage appear (sometimes all of it in an instant). I have the same set up you do and I have run it really hard only on a road coarse (most recently CS86). Have you checked with KB to see if they have a thought about your lost pound of boost? I do not see the correlation between long tube headers and boost. I understand you are trading some bottom end for top end when you go that route but have not heard of the boost issue. My first call would be to KB.

 

Your torque number is about the same as mine but the RWHP is different - probably just the air out here on the left coast.

 

Good luck.

 

Jim

 

I did not realize I would lose boost either until after the install was complete and it was on the dyno. Since the KB tune will not adjust for headers and I had to go with a custom tune and only gained 18.5 rwhp and 20#s of tq.The boost loss was explianed to me as:

 

Boost or "psi" is NOT a measure of airflow.

It is a measure of system back pressure.

A more free flowing intake and exhaust path will flow more air before building

the same amount of back pressure, but move more air and thus make more power.

 

Seems to make sense, a reduction in back pressure = lower boost with the same pulley. I was expecting a gain of about 30rwhp with the 1 3/4" headers and HF cats plus a custom tune and that would be about right since a pound of boost equals about 12.5 hp. The car does seem to be quicker and one hell of a lot louder.

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...exactly, you lost boost but you actually gained rwHP by reducing the exhaust flow backpressure. You might also actually be a 'safer' now than before the change because the S/C isn't working as hard (even though you're making more HP), is generating less heat and, since a dyno measures HP at the wheels, the HP you gained mostly came from removing power demand from the S/C belt (KBs can be 'hungry') and improving intake-charge density. So you're making more power with less boost and running more efficiently overall ...exactly what you want when you're in the 'grey' zone of the 3V. You're probably already making 480rw (just from the restriction reduction unloading demand on the S/C a bit and a denser charge) which is maybe 565-570crank. At that level and with the shock-loads of running sticky tires at the track I think you're already well into the 'grey' zone for a stock-internals 3V, imo.

 

Using more pulley would really stress things disproportionately since every rwHP you make by driving the S/C harder will require the engine to work much harder since the S/C will eat much of it at the belt, put more heat in the intake, and only the remainder of the pulleyed HP increase will be seen at the wheels, but the engine is bearing the full stress and output, both what is seen at the wheels and the adder the S/C is eating and trying to do it with more heat ...stress.

 

If you want to be 'safer' (vs just making as much HP as possible), my suggestiojn would be to go for further boost reduction (efficiency) any way you can: race cats, straight thru muffs, less intake restriction (Fixed displacement S/C are hypersensitive to intake restriction) and maybe even going to a GT500 radiator and big HE. The intake flow improvements will make more power (more stress/danger) but further exhaust flow reduction help do it with the least additional stress possible and the better cooling will deliver the full 'dyno' power more reliably (under a broader set of varying conditions) without the tune pulling timing. That doesn't make you 'safer' in a HP sense, but makes you less reliant on the 'wisdom' of the tune ability to handle those exceptional conditions that can otherwise do progressive (or even catastrophic) damage.

 

You can then run a slightly larger pulley (less HP) such that you're still making more HP than when you started, but doing it with the least stress possible over the broadest set of real-use conditions. Of course you can also go to a built motor and pulley-away ;-) but the beauty of efficiency mods is that all will also benefit a built motor at higher boost levels too (both in terms of HP and reduced engine stress). Efficiency pays at all levels of output, either in 'safety' or HP, depending on your goals and what the engine can 'safely' handle ...so money invested in efficiency mods is never wasted money, imo. Just some thoughts...

 

Dan

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