What do you think about the 4/7 cam lobe swap some grinders are doing? Could it benefit the LT-1 cam? I am starting to see more articles printed in mags.
Chuck (sent you an e-mail too)
Chuck (sent you an e-mail too)
-
-
Re: Duke .. a question
You'll have to explain what "4/7 cam lobe swap" means.
Duke -
Re: Duke .. a question
Okay, the "4/7 cam lobe swap", is changing the firing order from the traditional SB to the firing order used on modern SBs beginning with the LS1.
My understanding is that GMPD changed the firing order because it yielded smoother idle characteristics. Firing order has a big impact on crankshaft torsional vibration modes, but I doubt if it creates a meaningful improvement in the torque curve on a street engine, so I don't think it's worth anything.
If you go this route you will have to buy an aftermarket cam and suffer all their deficiencies, like too much overlap for a given duration. Most aftermarket high performance cams are not designed for OE exhaust manifolds. They're designed for headers with open exhaust or a very low restriction exhaust systems and can make more torque and power with such an exhaust configuration, but the extra overlap will hurt the torque curve with OE manifolds. The OE cams optimize the torque curve with OE manifolds because that's the configuration used in their development. As far as I know, all the aftermarket manufacturers use headers in their testing and development.
The aftermarket is always looking for a marketing angle, so I guess the latest one is cams that alter the firing order to the modern LSx firing order. IMO it's just more marketing hype, but the hot rod mags always seem to find that the new stuff is wonderful. Of course, they are financially supported by the same aftermarket vendors and would disappear if not for all the advertising revenue from aftermaket suppliers.
Caveate Emptor!
DukeComment
-
look at cylinder heads
The hot rodding mags all hype the aftermarket heads. You would think they are far superior to the GM issue parts. About 10 years ago we ran side by side tests of similar big block heads ie: pocket ported stock oval port with bigger valves to D@rt oval ports with the same valve sizes, rectangle port to similar cc aftermarket rectangle port.... unlike the mags, we compared the EXACT SAME engine and cam, only changing the heads (they now compare stock head/cam to racing head/cam go figure, more power) and the difference was MINIMAL. Usually 10 to 15 HP at over 4500 RPM, and almost identical below that.
The factory spent LOTS of money developing heads. Many of the aftermarket designs started with a stock head and simply tweaked the casting a little (removed the bulge in the exaust port by using a longer head bolt, built up the port floor a little) so the heads aren't all that different when they started with an already good piece.
I know readers will argue the small block heads to death, but I hardly think comparing a stock 150cc port size to an aftermarket 200 cc size is a heads-up comparison. The bigger head has different performance characteristics and with it a different lower RPM character as well. It's a fact that the high RPM big port heads have less torque at RPM below their torque peak if all the rest of the engine including cam and compression ratio is the same.
Back to your question, the factory went to a different firing order with the LS series engines. Nascar was doing this swap for years, but in their situation, 10 horses means winning the race. On the street, you'ld likely never know except by the change in your wallet's fullness.Comment
-
Re: look at cylinder heads
Good info! If you go to a big port volume head on a carbueted engine, you're going to lose low end torque and driveability due to poorer air-fuel mixing because of the lower velocity.
I've always thought the OE SB heads, especially 461X versions are excellent.
If you want a real laugh, go over to the Corvette Forum C6 Z06 discussion. The new LS7 heads are fully CNC ported by GM and guys think that the aftermarket "tuners" are going to work wonders on them to make a lot more power. I tried to explain why this is not likely, but they don't listen.
DukeComment
-
I had a set of 461X heads
on a Monza in the early 80's with a 400 block, 350 crank. It was a real screamer - heads were ported by a grand national engine shop.Comment
Comment