Did you remove the rear o2's? I just did an 80e swap and had to remove the drivers side rear 02 because it was hitting the shift cable bracket...arh longtubes with catted downpipes...noticed the truck didnt run near as well as before also noticed on my wideband it would go to 16 afr for a couple minutes then to 13 afr...wtf?, after racking my brain on my tune and doing some research on hpt it turns out that SOME not all operating systems will do this when the rear 02's are missing, even though you turned them off in the tune. Short story long you could def feel it when the 16 afr kicked in, maybe something to look at I,ve since reinstalled that 02 and no problems since

 

IMO. You need to retune. The reason i say that is because with long tubes your moving more air. More air means your on a different spot in your timing table. That different spot more than likely has less timing. Also since you basically just altered the airflow throughout the entire operating range your trims are probably off now. Since your trims are off it means the air you computer is interpreting the amount of air ingested is off which once again means your timing is off and your either rich or lean.

Hopefully all that made sense.

 

I think you have your info backwards man. LTs allow for higher exhaust gas speed which results in lower backpressure. Since TQ is affected by backpressure, you SHOULD see a slight drop in the lower rpms. The higher gas exit speed does facilitate higher peak HP #s because higer in the RPMs the gas needs to get out of the way to make room for the next exhaust stroke.

Shorties have a different effect (if any at all on these motors). They don't allow for exhaust gas speeds much higher than the stock manifolds, therefore they don't generate much (if any)loss in backpressure. Tq is not noticably affected. On that note, they don't allow for much gain in peak HP for the same reason.

 

Guys have been reporting a loss in low end when installing long tube headers with bigger primaries, collectors, y-pipe etc. forever and a day. As mentioned, the air out is moving faster and has different scavenging characteristics. Good tuners can help this some like Corey mentioned, but it is the nature of the beast.

The sluggish when cold though may be an issue with the front O2's not being up to temp though, as well as tuning. I'd definitely get some tuning software on there and see what can be done first.

 

Long tubes have always made better low end torque on NA applications.

Did it change recently? I am certainly not an expert but any vehicle i have had always makes better low end with long tubes. Both my carbed and FI vehicles.

I am not sure about upper rpm HP though.

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This may prove true on a old school carbed NA set up. These vortec EFI engines like a little bit of back pressure.. I certainly lost or had that feeling after I installed LT's on this truck when it was new, on a stock tune.. And since when have shorties benied over a long tube design? It is obvious that there are 1000 threads on this same issue after doing a LT install on a vehicle that was not ruined for them afterwards.

 

Actually sluggishness when cold is probably not due to O2 sensors.. It has to due with a porrly tuned VE/MAF table... Since Closed Loop fueling does not occur until a certain ECT, below that temp the computer is running in Open Loop, meaning no O2 sensor feedback....

 

Oh yeah, that's right...the O2 sensors don't wake up until 137* or something like that...

 

you are correct young grasshopper

 

yeah, seems a bit of misinformation floating in this thread...


This is one reason why headers with slightly smaller 1 5/8in primaries are preffered for 4.8 and 5.3 motors with little to mild modifications.
I bet the headers you just installed had 1 3/4in primaries.

 

Agreed. People seem to be drawing the wrong conclusion from the results. Long tube header design should still shift the rpm band for the most efficient scavenging effect lower vs a shorter header design.

The thing is, comparing them to the 'shorties' available for our trucks isn't a fair comparison as I wouldn't even really call them headers. They aren't equal length or even close to it and don't seem to have much of a collector to speak of. They're basically manifolds made out of tubing. Minimal scavenging.

The loss of low end when putting headers on our trucks seems to have more to do with larger primaries and collectors than what is optimal for the engine size. The manufacturers aren't stupid, they don't make headers with 1.5" primaries and 2.25" collectors because nobody would buy them, even though they would probably be awesome for low end on a 4.8.

People tend to be unrealistic about their goals and so do the people who give them advice. Half of the guys with 4.8s think they're limiting their potential with a 1-3/4" primary header becuase man, you never know, I might decide to build that forged 408 for boost next month after I win the lottery. I have a 5.3 and may do a 6.0 swap at some point, but even then I know that Thorleys would have been better for my application than the LPP's I ended up with. But I couldn't get Thorley's when I was ready to buy headers so I went with what I thought was the next best option.

All things being equal, a longer primary or collector and/or a smaller diameter should shift the performance lower in the powerband and vice versa.

Don't forget too that most of the offroad Y pipes are way bigger than stock. I think my LPP's are 3" into a 3" Y which is quite a bit larger than the stock stuff. I think the stocker is like 2-1/2" into 2-3/4" and really its smaller than 2-1/2" in the crappy non mandrel bends.

Big diameter = lower average gas velocity = where'd my low end torque go?