Another 2.5ltr Header thread. Will it benefit me on the highway? -
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post #1 of 4 Old 03-08-2007, 12:28 PM Thread Starter
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1992 YJ Wrangler 
Join Date: Oct 2006
Location: Santa Rosa, CA
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Another 2.5ltr Header thread. Will it benefit me on the highway?

My 2.5 does pretty darn good IMO pushing the 33's around town and down the highway. As long as there's almost no positive slope on a highway I can cruise like a bat out of hell at 70-75 in 5th gear, and it holds those speeds damn good too. When I'm heading northbound for example, or if there's a slight slope in the freeway I can still cruise about 70 but I'm in 4th gear running right around 3 grand on the tac. I like the fact that with stock gears I can run down the freeway at 75 without winding out the engine, but would a header give me that extra little pull I'm looking for to hold 5th gear down the freeway 95% of the time, or am I just thinking silly thoughts? When I added the Magnaflow exhaust I got a fairly noticeable gain in power so I assume I'd see a slight gain with the addition of the header. Plus with the improved scavenging over the stock manifold it'd allow the Magnaflow to perform slightly better, correct?

I plan on regearing, but it'll be a ways down the road until I do. If the header will give me the advantage I'm looking for I'm going to do some shoppin'

Thanks in advance,

92 YJ. Lifted, rubbered, geared, and more
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post #2 of 4 Old 07-09-2007, 03:28 PM
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did you ever get any feed back from anyone on this? did you go with a header?
I am considering doing the exhaust on my 2.5L and would like to know how you did with power increases.
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post #3 of 4 Old 07-10-2007, 03:14 PM
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I would also like to know how the headed would work out.
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post #4 of 4 Old 07-15-2007, 09:05 PM
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1997 TJ Wrangler 
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I was on researching the heck out of this topic. this is a quote

"Yesterday a 4.0L TB from a '98 Cherokee arrived by UPS -- to be installed on a 2000 2.5L TJ. With the help of a neighbor who teaches auto mechanics at the local high school (absolutely cool woman who decided to make it a day project for some of her students), I dyno-ed everything in stages. Checked the air flow, too.

Results, with stock (2.5L TB & no spacer)as baseline:

(1) Poweraid spacer only -- +2 HP, 8% less airflow (Evidently, the helix bore, lips of which extends beyond intake & TB opening, actually cuts down on airflow. But the spacer does "cool" and extend air volume, hence horsepower increase.)

(2) 4.0L TB only -- + 6 HP, 16% more airflow (no surprise here)

(3) 4.0L TB & grind out intake opening to match lower TB opening -- + 13 HP, 29% more airflow (Wow! Grinding out the intake opening did make a difference!)

(4) 4.0L TB & Poweraid helix spacer & grinded-out intake opening -- +16 HP, 2% less airflow from #3 above(overlapping helix bore is still cutting off airflow, but spacer is still cooling & increasing air volume)

(5) 4.0L TB & grind out helix bore in Poweraid spacer to match lower TB opening & grinded-out intake opening (the Big Kahuna) -- + 24 HP, 39% more airflow (Wow again; the optimal configuration, obviously)

It should be noted that grinding out the helix in the Poweraid spacer still leaves a partial helix in the walls -- the main difference being that the helix now has rounded edges rather than sharp ones. My auto mechanics-teacher neighbor suggests that these rounded edges are actually more aerodynamic than the original sharp ones, and will still "spin air" -- so perhaps the "pulse-organizing" effect I mentioned in an earlier post will be retained, if in fact there is such an effect.

So, to reiterate, I ground out on the helix bore of the Poweraid spacer, and ground out the intake opening, both to match the lower opening of the 4.0L TB I swapped in. Dyno-ed & tested airflow, results: +24 HP, 39% more airflow.

Seat-of-the-pants impression: idles smoothly, more torque on the low end, smoother accleration through the midrange, and better performance above 2700 RPM. No hesitation, no engine codes.

One MAJOR suggestion: if you grind out your intake opening, be VERY CAREFUL about aluminum filings in the intake manifold. We stuffed everything with tack cloth -- sticky cloths which can be purchased at a hardware store -- rather than shop towels. The tack cloth caught about 95% of the filings, but there was still a small mess in the intake. We vacuumed a lot of the leftover out with a shopvac, but we still had to use tack cloth attached to a long, thin screwdriver to "mop out" the runners. On the TJ there are also four open plugs/hoses attaching to the intake below the TB -- you'll have to clean those, too. It took longer to clean out the intake than it did to bore out the opening, but it can and needs to be done.

Finally, should say that the TJ has a two-stage K&N filter system, an MSD ignition system & coil, high-gapped Champion truck plugs, a Flowmaster delta-40 catback, and a case-full of Mobil 1 synthetic oil. All or part of which may have a synergistic effect on my results, since mods usually affect each other."

Black 97 TJ SE 4bangin' away.
3" BDS lift
33" A/Ts
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