I just got my headers installed last night (mac mid- length & y-pipe), and today, driving around, it seems to have lost 1~2 psi of boost according to the gauge... I knew I'd lose a little boost, but going from 6~7 to 4~5 seems like a lot just from swapping from the factory manifolds. I haven't taken it all the way up to redline yet, but it seems like it was at least 1# lower throughout the whole RPM range from what it used to be.

 

Yup that is perfectly normal. You need a pulley change and a retune if you want to bring your boost back up.

BTW Boost is only a measure of restriction in the intake. You are still flowing the same cfm as before just with less restriction.

 

Yes there is less back pressure......less pressure on the guage.

 

So if I'm still flowing the same amount of cfm, then will I make any more HP? I'm supposed to get it dyno tuned again for the headers this friday, what kind of gains (if any?) should I expect??? Thanks for the info guys

 

You are going to see 40-50rwhp gain easy if you pulley back to 7psi. Enjoy it.

 

Thanks... Any speculation on what kind of gain I'll get with my current pulley (call it 5 psi) just from the headers alone?

 

You should gain anywhere between 20-30rwhp giving the same pulley.

 

Since I have a borla adjustable cat-back, and I am running the biggest insert (straight thru pipe pretty much) would it be to my benefit HP-wise to put a smaller insert in or completely block it off so that I could have some more back pressure since the exhaust would have to go thru the muffler and get maybe some of the boost back?

 

Swap that pulley now or push back your dyno appointment until you do. Trust me!!!

 

With a SC setup you want the least backpressure possible... So i would say no dont do it.

 

I would not do that. I would get the pulley rushed in before your next dyno appointment. That or postpone the appointment till it gets in

 

Yep ... I lost 1 or 2 PSI boost, but power was up.

 

Well, I don't have an extra pulley laying around lol. I'd have to find one and order it and I highly doubt it would be here before friday if I did. I don't want to push the appointment back as it took forever to coordinate the tuners schedule with the dyno shops schedule Guess I'll just have to find out what the headers alone will add

If I did swap the pulley to regain the boost, would it be harder on my engine? It seems to me that it wouldn't because it did fine with the 6~7# before, so 6~7# with headers should be equivalent on the block? I dunno, that just sounds like it makes sense to me lol. I don't want to grenade my engine either. Its still all stock internally...

 

With less restriction you should be making the same/slightly more HP just at a lower boost # than before. Changing the pulley to get your boost # back up will gain even more HP, but more HP will equal more stress on the engine too.

 

Well, I got on it and took it all the way to redline coming home from work getting on the highway and it spiked right at 6# before red-line. Before in good air it would go past 6 and almost to 7 like 6.8ish or so. So turns out I only lost less than 1#. Not worth it to upgrade the pulley at this time IMO. Thanks to Bob at EPP tho for your help on the phone

 

This is probably a dumb question. How necessary is a retune after going to a smaller pulley? What would/could be a result of more boost and not changing the tune appropriately? For example, if I went from a 2.87 to a 2.75 pulley on my (cough...vortech), would I need to get it retuned right away? I'm not all up & up on the tuning aspect, so i'm just curious. Thanks!

 

If you are running a MAF then you will most likely need a retune. The thing is with running a MAF is that most LS1 FI setups max it out between 3800-4500 RPM and then they add fuel manually via PE vs RPM table. So the extra air coming in from the 2.75 pulley is not accounted for by your MAF so the PCM doesn't have a way of adding fuel hence leaning out. The only way to do it is with a new retune.

Good luck

 

ANY re-pulley needs at LEAST a scan of performance/tuning parameters. Whether you re-tune (and you probably WILL), would be dependent upon you scan data.

 

so your saying this is NOT the case with SD?

 

I believe that you can get away with running 1-2psi with SD if you have the fuel system to support the increase air flow. SD reads of a 2/3 bar sensor which can recognize the increase in airflow and add fuel based on it. Now if you are running a 2 Bar SD and the increase of boost was from 14-15psi to 16-17psi then u might have to retune because the 2 bar doesn't read anything more than 15psi.