Originally Posted by smote
It depends on the application. a N/A D series will probably see gains of around 7-10whp (built engines may see more). a Boosted D series may see up to 15whp gain. B series engines (depending on model) may see as much as 15whp gain on a n/a engine, and about 20-25whp on a boosted engine. I have never seen a street driven 4 cyl honda make 25-30whp with just a intake manifold swap. I'm not saying it hasn't happened or cant be done, 30whp is just not a very realistic goal for just a intake manifold, especially if you don't have larger cams, higher compression pistons, Forced induction or anything else to warrant the need for much more air flow. Unless you are flowing about 25-30% more air than stock, you really don't need a larger intake manifold, unless your engine has a dual stage intake manifold (GSR, H series engines, some F series engines...)
a B18C1 will see a larger increase in hp over stock than a B16A or B18C5, because the GSR engine has a 2 stage intake manifold. B16A and B18C5 have a single stage manifold (which is what you are putting on a B18C1 when you do the swap.) A GSR may see a 15-20whp gain depending on the other supporting mods.
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