Originally Posted by gofastz
iv talked about this b4, i want to build a 300hp tbi 350 for my 4x4 truck, the cam im looking at is comp ex249h to get close to the ole 300hp cam,and i dont want to use a stall converter unless i have to, i want to run reg gas, but i want to raise the compression up to 9.1-9.5, will the stock heads get me there, and what pistons do i need to use for this, this is my daily driver... thanx guys........
Maybe, these are Swirl Port heads die pretty early because of the swirl vane in the intake port pocket.
Bigger valves and higher lifts are of marginal value to these heads because the vane limits maximum flow, so big lifts and valves are the secondary order limit following the first order limit which is flow past the vane. So if you can't or don't fix the vane there's not much else that helps except for a longer duration lobe. This allows the limited flow port to flow for more time to make up on its limited capacity. Problem with TBI is that it is very sensitive to manifold vacuum which needs to be high with lots of signal separation between levels of power output and throttle position. Longer duration cams reduce overall vacuum and close up the distance between signal changes and power output, something TBI just doesn't deal with.
The cam you selected is good for TBI engines as it keeps the vacuum and the power change to signal ratio high, this is about as far as it's practical to go with these engines.
The TBI itself is too small to feed 300 horsepower. The injector sizing is just not large enough to run on an 80% duty cycle and feed that much power. You can run up the pressure and increase the duty cycle but this really eats into injector life pretty fast. The 454 TBI or the Holley 670 CFM unit with the biggest injectors they sell can get there and still not become a maintenance headache. All of this take a matching chip program.
The heads are the limiting factor, with the swirl vane they just will not deliver the air needed to get to 300 hp. The vane can be mostly removed which will provide a significant improvement but this is tedious work and requires the purchase of a die grinder and carbide cutters. If you have the time to trade for dollars this is the old fashion way, it still works. But by the time you do this, shoot new seats, refinish or replace the valves, renew the guides and stem seals you're a long way into the cost of better heads. While the Vortec head is highly recommended and it's a good choice there are plenty of aftermarket heads that are equally as good as the Votec. Here's a link to an article that pretty well covers the Vortec and why it is such a good selection. http://www.onedirt.com/tech-stories/...k-chevy-heads/
A huge advantage of these heads is the 40 hp gain that comes regardless of camshaft. The cam you're proposing to use can get you about 40 horses over what you have, but that comes at the cost of moving more air and fuel through the engine which hits back on the feeding limits of the TBI and the Swirl Port heads. The Vortec heads gain half or better of the 40 horses they offer through improved combustion, they just get more out of the air and fuel going through them, the other half comes with higher port flow so that part shows up in the higher RPM ranges. The cam you're proposing to use is compatible with the Vortec as it comes out of the box, there would be no need to modify the upper guides for retainer clearance. This cam really doesn't need anything special in valve springs the new OEMs on the heads would be just fine. Even your existing rocker covers will work. the combination of this cam and Vortec heads would get you with in shooting distance of 300 horses even with your present 450 CFM TBI you're looking at 260-280 hp without having to get serious about the TBI change.