I am looking at the summit racing LT1-LT4 conversion kit, and I can not imagine a H/C/I/Valvetrain swap making a 150hp difference. I could be wrong.

What is the LT4 and why don't more people do it if it makes such a great difference?

 

mainly heads,cam,intake powerwise...other internal upgrades as well, rr's,timing chain,etc.etc.

 

Does this help?
http://www.malcams.com/legacy/misc/LT4.htm

 

The "over 425HP" claim that vendors use to advertise the LT4 conversion package is a little misleading. That claim is based on an engine dyno test done by MORE Performance in maybe 1998, and documented in a couple of the Chevy mags. In order to get that level of HP, they ran long tube headers, larger injectors and a larger throttle body.... maybe a few more things. Not sure if they even had the accessories on the front of the engine. They also failed to clearly document that they used a $3,000+ aftermarket ECU to tune the engine. I think the people here who have done the conversion and properly supported it with the required headers, injectors, etc feel that they are getting an honest 400+ flywheel HP. Many also feel that simply having your stock LT1 heads ported correctly and using a slightly more aggressive cam than the LT4 HOT cam would provide more bang for the buck.

 

Yes as Fred stated you are trying to compare flywheel ratings as installed in the car with manifolds, full exhaust, all accesories, quiet intake etc. to engine only HP numbers. Well tuned/setup b-body LT1 cars with stock engines exept for springs and rockers are putting over 300 to the wheels through 4L60Es so you are talking pretty easily over 350 at the flywheel and that is still with some sort of intake before the TB and probably more exhaust and accessories as well. Seems to me that if tested under the same conditions as that LT4 package 360-70 is not unrealistic, does that put it in better perspective? The b-body cam is smaller than the f-bodies too.