Long tubes and 2.85 pulley combo, no tune?

Discussion in 'SRT Hellcat Performance' started by hemipower, Mar 15, 2016.

  1. hemipower

    hemipower Hellcat Member

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    Thinking about something...

    Long tube headers are showing what 1-2 lbs boost "loss" but gaining about 20 whp correct?

    What would the result be if you did the upper 2.85" pulley to pickup ~3 lbs of boost and "lost" 1.5 lbs lets say due to LT headers...would fuel be enough there with the more efficient system to show the same gains without even changing the tune? Since net wise only seeing 1.5 lbs boost gained could the computer make small adjustments to keep from going lean at WOT? Flawed thinking there/bad idea? Dangerous?
    I'm most likely to skip long tubes on this car as it sits as low as my lowered 392 did and I dragged long tubes on bumps sometimes on THAT one... :rolleyes: but just a curiosity. I'm thinking it would be way lean as MORE air in and MORE air out...more power...without more fuel...bad. The boost pressure isn't the only important variable in other words.
     
  2. Rick@Metco

    Rick@Metco Gold Member Hellcat Supporting Vendor

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    Boost is a measure of pressure in the engine. The 2.85" pulley increases airflow substantially, and the addition of headers improves the efficiency and adds to the engine's capability to move more air.

    Even though the as-measured (say on a gauge) boost increase resulting from these two additions will be modest, the change in overall airflow will be significant. The car would likely be very lean without the proper PCM revisions.

    Having sufficient airflow to support the fuel added in the tune is where the big power comes from.
     
    Last edited: Mar 19, 2016
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  3. Hemi Rocket

    Hemi Rocket Gold Member

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    Not sure how long tube headers cause boost loss on the Cat... wrong cam certainly will. The boost pressure is measured on the intake side between the blower and intake valves. The boost would have to get past both the intake and exhaust valves before the header even enters into the equation. Now there is a period where the intake and exhaust valves cycles overlap between the end of the exhaust stroke and the start of the intake stroke and blow thru may occur but that period (time) is unchanged. The difference is the pulley change would create higher boost once the engine transitions into the boost curve. Part throttle and cruising speeds the engine is not operating in boost still in vacuum.

    While the long tube headers will increase efficiency... the stock Cat exhaust system flows so well you won't see the big gains like you would in other applications. Now with the pulley change once you get into boost the flow will increase over a given RPM range... meaning pumping more air into the engine. A few PSI doesn't sound like much but it will change the flow of air thru the engine significantly. Without a tune the fuel system will not supply enough fuel over the boost curve and you're going to go lean which will surely lead to detonation. Going lean in any engine is bad... in a supercharged engine it can be catastrophic. Certainly not worth the risk. If you're going to go that route get the Pulley, PCM & Tune and save the money on the long tube headers. And yes... pin the crank.;)
     
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  4. Top Cat

    Top Cat Gold Member Hellcat Car Club Gold Supporting Member Fastest Cat/Record Holder HCC Charter Member

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    Save your money on the long tubes they will do nothing for you and may even slow you down at what your talking about. As for the no tune there have been a few cars that did the upper pulley and the cars ran and no codes, but they were on a fine edge for AFR's.
    TC
     
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  5. Rick@Metco

    Rick@Metco Gold Member Hellcat Supporting Vendor

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    Headers (particularly long tubes) have a known effect on boost pressure, especially in cars that have pulley upgrades and high-performance tunes.

    I will absolutely agree that headers do not yield significant gains in the HC application.
     
  6. Hemi Rocket

    Hemi Rocket Gold Member

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    Rick - OK good to know... but I didn't think that was true of a car that already has a high flow exhaust such as the HCat. If your going from a manifold or a poorer flowing exhaust that creates a lot of back pressure it will impact it... so has it actually been test on our Cats... to document the difference? If so, just curious what the results actually were.
     
  7. BAADCAT

    BAADCAT Senior Hellcat Member

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    I don't know if the same holds true on the cat ( I don't know why it wouldn't), but running a long tube on a FI car has some other benefits/considerations.

    On my whipple'd Camaro I went from 1 3/4" LT's to 2" LT's (no cats on either) and dropped about 1.8 lbs of peak boost. We put it on the dyno and I made almost exact same HP and TQ. So I now had two options. First, keep as is and have a car making the same power at a safer boost level. Second, pulley down and make more power at the previous boost level. I chose the second option and picked up another 40ish hp at 13lbs where I had been running before the bigger LTs.

    Would the cat not benefit in the same fashion?
    For those that did LT install what was the change or what made it not worth it?
    Did you make the same power as before? What about boost?

    I would think dropping a set of LT's on the cat and then running the 2.65 (or some combination of OD and pulley) would put you right at 15lbs and make more power than a car running 15lbs with out the LT's.
     
  8. Aarcuda

    Aarcuda Gold Member

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    headers can lower the perceived (i.e., measured) boost by allowing the cylinders to empty more on the exhaust stroke thereby allowing more room for the intake charge. so it looks like a decrease in boost PRESSURE. but You dont lose HP because you are still pumping in the same (or maybe more) air/fuel and since that is a mechanical pump (supercharger) it will pump X amount of air with each revolution. all you have done is decreased the amount of spent exhaust gas left in the cylinder and allowed more room for pumping in more air/fuel. so you waste less energy turning the screws. the more power it takes to pump air into a cylinder as in the case of a partially filled cylinder filled with exhaust gas, the less you power make off the crank bc you wasted some energy turning the blower and you had less room for fresh air and gas.
     
  9. BAADCAT

    BAADCAT Senior Hellcat Member

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    Exactly! Just a big air pump. So why all these posts about LTs being a waste of time and money? May not see a direct HP gain, but will make the air pump more efficient.
     
  10. Top Cat

    Top Cat Gold Member Hellcat Car Club Gold Supporting Member Fastest Cat/Record Holder HCC Charter Member

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    That's not entirely true, you gain no efficiency just by changing to LT's, you need to add more air/fuel to see any gains and that you might not see until you have gone over what the OEM headers give you.
     
  11. Aarcuda

    Aarcuda Gold Member

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    if your cylinder wasnt empty because of a restrive exhaust, and the headers made it scavenge better, there is a gain in efficiency. there has to be. because theres less work done to fill it. it takes less power to fill an empty cylinder vs a full one- like pumping up a tire. its easy going at first but the more you fill it the hard the pump has to work.
     
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  12. Aarcuda

    Aarcuda Gold Member

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    thats whole whole reason to use headers- to get the exhaust gas out easier and to hopefully scavenge the cylinders using the exhaust flow to help pull out as much as possible.

    or did I smoke some dope during my "how to build a race engine" class?
     
  13. BAADCAT

    BAADCAT Senior Hellcat Member

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    Agree with Aarcuda you do gain efficiency (the pump doesn't have to work as hard), but efficiency doesn't necessarily mean power. To make power you will have change some other variables like Top Cat said to capitalize on that new found efficiency. Its like your car vacuum. Shove the tube against the carpet and you will hear the motor start bogging and working harder. Lift it up and it goes back to normal. Didn't change the power of the pump on the vacuum, but did make it more efficient letting it flow freely.
     
  14. Top Cat

    Top Cat Gold Member Hellcat Car Club Gold Supporting Member Fastest Cat/Record Holder HCC Charter Member

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    Nope, if the flow of the OEM is equal to the LT's you will need to change something on the front end to take advantage of any efficiency's the LT's may have , proven on the dyno.....you can think what you will but this has been proven out on the Hellcat OEM vs LT's....
     
  15. BAADCAT

    BAADCAT Senior Hellcat Member

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    I am confused. I don't think there is any debate that OEMs flow pretty damn good. I also don't think anyone debates that putting LT's on will not make more power over the OEM's. As you said lots of post and dyno's show they don't make more power (at least not at the levels we are at now). What we are discussing here is simply efficiency (nothing to do with power) and the potential to make more power with out getting crazy on the boost.

    Like I said in a previous post. I went from 1 3/4" primaries to 2" primaries and did not see a HP gain or loss. Much like going from OEM to LT's on a HC will not see a HP gain. What I did see is a decrease in boost (essentially a measurement of back pressure). Which means I gained efficiency and lessened the internal pressure forces on the motor (like my vacuum example, keep that vaccum hose on the carpet and see how long before you burn it up). The air is able to move out of the motor with less restriction. Again no HP gains anywhere which we all agree upon, but if I can make 700 hp 15lbs boost or 700 hp at 13lbs boost why would I not choose the 13? Or even better in the case of my Camaro pulley down and make more power at the same boost level I was before?

    Real world numbers from my dyno's on the SS:

    1 3/4" + stock pulley = 13lbs and 613 RWHP
    1 3/4" + small pulley = 15lbs and ??? RWHP (could have done it, but would it be safe or how long would it last?)
    2" + stock pulley = 11lbs and 610 RWHP
    2" + small pulley = 13lbs and 658 RWHP

    I would take the 658 at 13 lbs all day long because I know it's safe and the longevity of the motor should be better. Could I have saved the money on the 2" and just slapped a smaller pulley on there and made more power at 15lbs, sure. I didn't look at it as spending $600 to get nothing, I looked at it as spending $600 to save (or at least maybe prolong) $10,000 in a motor.

    That being said $1200 for HC LT's, I dunno I might have to think hardder about that one. :D
     

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