Hi all, new to the board. I am a bit different than most, I am looking for respectable power but am more concerned with fuel economy, when approaching 50 you start slowing down just a tad

My question is this, I have a 305 MPI out of a 98 Silverado, putting it in a 71 GTO, how much HP gain will there be with 1 5/8 headers with a 2.5 inch X pipe Exhaust running through duel Flowmasters? any idea?

230HP stock, is 275 HP too much to ask for?

Thanks

 

L30 is a wierd one to swap in when L31s are so much more common (350s)

The CPI isnt the greatest induction system GM created either, but the heads are pretty good for being stock

275hp is a bit of a stretch for headers only L30. Heck, the L31 would have trouble getting that. A mild cam and exhaust port work may get you there though

 

Sorry, I was looking at adding a Volant Cold air intake, these are supposed to be good for 18-23 HP, I thought Headers would be around a 10% gain which has it knocking on the door of 275 and the biggest problem with the 305 in the Silverado was it was seriously choked off by the small exhaust.

I figure 230 stock, 245 with the intake, 265-270 with headers and 275-285 with duels all of which should help fuel economy as well?

And I know everyone builds 350's, I had one but the economy is horrible!

 

Mileage on modded 305s is comparable to 350s

Dont get me wrong, the swap will be more efficient in the GTO than the truck, but power...

Carb to EFI swaps usually show a massive efficiency gain along with modern engine codes to speed troubleshooting the minor stuff