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Marc Haibeck · 12-24-2017

Some injector housings were machined with chamfers to compensate for core shift in the castings. The correct port center is at the top of the chamfer. This problem was solved in '93 when the chamfer was blended 1.5 inches into the port for a smooth transition. This was a part of the 405 hp upgrade.

The crankcase vent flow restrictor configurations are different on the '90 verses '91 and later injector housings. To make your '90 housings compatible with your '91 cylinder heads you need to drill the small holes in the '90 flow restrictors to at least 3/4" to remove the restriction. When you are shopping for the EVAP tube at Jerry's, look at the injector housing index 10D3 on the 5th page of the Intake section. It shows the unrestricted crankcase vent ports. If they are not drilled out the engine will be double restricted because the restrictions are also in the cylinder heads for '91 and up.

12-24-2017	#5
Marc Haibeck Join Date: May 2007 Location: Addison IL Posts: 252	Re: Plenum & Injector Housing Porting Some injector housings were machined with chamfers to compensate for core shift in the castings. The correct port center is at the top of the chamfer. This problem was solved in '93 when the chamfer was blended 1.5 inches into the port for a smooth transition. This was a part of the 405 hp upgrade. The crankcase vent flow restrictor configurations are different on the '90 verses '91 and later injector housings. To make your '90 housings compatible with your '91 cylinder heads you need to drill the small holes in the '90 flow restrictors to at least 3/4" to remove the restriction. When you are shopping for the EVAP tube at Jerry's, look at the injector housing index 10D3 on the 5th page of the Intake section. It shows the unrestricted crankcase vent ports. If they are not drilled out the engine will be double restricted because the restrictions are also in the cylinder heads for '91 and up.