[DirtyLarry]

Originally Posted by 83GMCK2500

I dug around a little and I swear I've read most of DirtyLarry's 8.1 posts, have yet to find the answers I am seeking.

My DD, Vortec 7400 powered, love it, am looking into some upgrades for it. I get that heads/intake/block all have different bolt hole locations, bolt patterns, etc, nothing interchangeable there with other engines(excluding classic bellhousing pattern). One of the upgrades I'm researching is coil-over-plug ignition, another is headers...they are slim pickings for GMT-400 7.4 powered trucks.


Coil on plug did exist on a somewhat rare L21 7.4L used in GM medium duty trucks. I believe this engine was only used from 1998 to 2000. Many people get this engine confused with an 8.1L because the coils are on the valve cover. I’ve personally only seen 1 L21 7.4L in person. L21's are easy to spot once you look for the throttle body on the side of the intake instead of in the front like an 8.1L. The L21 also has a huge cam sensor where the distributor would be and were the ONLY 7.4L to have electronic throttle.





- Will 8.1 exhaust manifolds/aftermarket headers bolt to a Gen VI? If so, recommended brand/model, or is there a nice high flowing factory manifold?
All older BB (396, 366, 402, 454, 502) exhaust manifolds and headers will interchange with the 8.1L. I run a pair of 1980's motorhome 454 exhaust manifolds on my 8.1L, as I am not a believer in headers on a driver. In all reality, there is nothing special about 8.1L exhaust manifolds that would benefit an older BB. Below is a picture of the various BB heads I had laying around a few years ago where you can see the intake port differences and exhaust port similarities. Even though the exhaust port sizes are different the exhaust manifolds are the same size




- Do 8.1 valve covers bolt to Gen VI heads?
Yes and no. Yes, it is the same dimension but the 8.1L valve cover depth is very shallow due to the taller heads. The 8.1L valve covers would bolt on to older BB but there would be no room for valve springs and rocker arms.

- Does the 8.1 timing cover bolt to my Gen VI?

No the timing covers are entirely different