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99 intake manifold swap . . . Meh

Old 10-09-2016, 05:39 PM
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Default 99 intake manifold swap . . . Meh

I have a 98 gc 4.0 and I did the 99 intake man swap but it seems not much changing in power and maybe even knocked a couple mpg off. Dissapointing. Anyway I was gonna do 703 injectors to see if that helped bring the gain's everyone talks about but is there something else I should be focusin on? Headers? Right now the only upgrade I'm running is magna flow cat and exhaust and k&n drop in filter
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Old 10-09-2016, 07:00 PM
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The steel tube header on the 98 is pretty free-flowing to start with, the tubes could be larger, but with space around the intake manifold, this isn't possible. You could spend a lot of money on a performance header with the same result. Not much difference. I went to Bosch 4-hole injectors in my 00 WJ and gained an average 1 mpg. I only did that because my original Siemans were cracking on the coil covers.

Some guys say relocating the IAT to the intake duct and out of the manifold helps, but I haven't done that because IDK how that will effect the mixture in sub-zero temps. I DO know my scanner shows an intake temperature of around 150*F at normal operating temp and outside air temp at 70*F.
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Old 10-09-2016, 07:22 PM
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I did the same swap on my XJ, I already had a larger throttle body, header, and 703 injectors. I initially lost power until I reset the computer and then I noticed the gain. My thought was the computer went from lean to stupid lean until I did the reset. I'm running OBDI though, not sure how OBDII would factor into that.
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Old 10-09-2016, 07:27 PM
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All you should have to do is disconnect the battery long enough for the memory to clear. Then the computer will relearn from the sensors once it goes into closed loop.
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Old 10-09-2016, 08:06 PM
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Thanks for the responses guys. Now that I think of it I may have forgotten to reset my computer, I'm still going to do the 703's and a local guy has a bank's header new for 250. I'll get those done too and see where that takes me. For now I just did the reset. I've seen a couple threads talking about the manifold upgrade not working so well on 96-98 zj's but Noone really talks about what exactly those years have against being 99 manifold friendly. I'll try and let you guys know what I think holds it back the most.
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Old 10-09-2016, 08:07 PM
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I hope you find good results!
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Old 10-10-2016, 01:41 AM
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I believe the problem is port matching on the intake and head. I think in intake ports are smaller on the 0630 head by about 1/4" at the bottom.

Last edited by dave1123; 10-10-2016 at 01:49 AM.
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Old 10-12-2016, 01:31 PM
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Well I guess I just learned the proper way to reset my ecu cuz it's running great and is getting better mpgs now Im not sure if there is more power because I've always had a light foot anyway so it's hard to tell but Def better mpg.
I thought disconnecting batt and turnng the key drained the comp but I read up on the touching positive to ground and lights on off way and that worked. I'm still gonna check the port match when I do the headers, thanks guys
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Old 10-12-2016, 10:00 PM
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I did some Dremmel work on my 99+ intake swap. I didn't notice any real power gain, but I did notice the engine can pull longer into the higher RPM range.
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