My freind and I are assembling a vortec headed 406 for his 87' monte carlo. His dad told him something about seeing one of the powerblock shows, on Spike TV, showing that they(whatever TV show it was) had picked up 30 h.p. with a 4/7 lobe swap cam. Honestly I don't know much about these cams, but I find the 30 h.p. claim a little far fetched. What little I've been able to find on the internet, Lunati claims a 3-12 h/p increase. So, My question is: What are these cams really worth, horsepower wise? I assume they cost more,Are they worth it? Thanks, Mike  

at least 3 magazines have covered this issue. i think the TV show was overstating their gains. the benefits are usually reaped at a high sustained RPM level. a normal street/strip motor will see little or no gain at all, and in a few cases lose HP. i would assume the cost to benefit would not even come close to feasible for most guys  

i run one in my bbc 496. not sure if it really helps but, i am happy with it. cost about 50 more than a high end cam.  

I had submitted this for everyone's consideration over on the Super Chevy side of Primedia the other day. I made some attempt at explaining why you see a power gain. Also note that the gain against the cost for a street engine isn't all that great, nor is it consistent. But if you're a competitive racer, or want to grow into that camp, this can be helpful not only from a small power gain but mostly from a great improvement in reliability for engines that are hanging out on the edge of the envelope.

Yes it's often good for about 10, 15 horsepower, claims of 30 is telling me that the engine has other problems such as detonation that this is fixing. For the most part all this firing order re-sequencing does is recover otherwise lost power as it corrects to some extent number 7's problems and moves them someplace more tolerant.

The reasons for this improvement are several.
7 and 8 tend to run hot as they see coolant that's passed over 3 cylinders before getting to them, unless you install external plumbing to take coolant off the pump and put in into the block often at the soft plugs. However, some guys drill and tap the block for fittings. The flow from the back of the heads isn’t the best either, 7 and 8 tend to suffer from flow stacking or stalling above the combustion chamber where steam pockets form, more on that in a bit.

Number 7 has the additional problem of exhausting into number 5's slightly earlier event, as a result, has some pressure and temperature problems. It also, tends to get the short end of the mixture stick, especially with a 2 plane manifold and carb. Injection gets around this problem, as do ram designs with 2-4's typical of drag engines. But with a circle track configuration of one carb, there's lots of mixture issues, especially with 7. Attempts to solve this have included cross jetting to richen that side, twisting the carb to move the left rear barrel closer or further from the valve, or the Duntov/Holley "Z" that connects 7 and 8 with a balance pipe have been tried with varying successes. Two plane manifolds have problems on the lower plane, those cylinders see anywhere from 20 to 50 cfm less at WOT and full valve lift than their cousin's feeding off the upper plane. Of these on the lower plane number 7 is by far the worst. This is not a problem unique to Chevy, considering Ford's numbering scheme of 1 thru 4 on the right side and 5 thru 8 on the left, they've always had problems with 7 and 8 which are in the same place on the firing order as Chevy's 5 and 7. From my days at Ford when I was a smoker, I have this really neat collection of melted and busted #8 pistons I used as ash trays once upon a time.

Chevy's tried to deal with the exhaust interference problem a bit with a left side manifold that exits between 5 and 7. But this makes problems for 1 and 3. The Ram horn manifold probably does as good a job as any manifold at getting 5's exhaust out of the way, but all these factory designs allow too much interference as the blasting gases from 5 will happily go back toward 7 then surge back toward the head pipe as much as going promptly out. So 7 is always seeing some additional backpressure and heat that it has to work against. As a fix, headers tend to reduce the 5 to 7 exhaust issues but if you're stuck with factory manifolds you’ve got a problem that really doesn’t have any sort of a solution so far as exhaust interference goes.

Cooling is another large issue both at the cylinder and combustion chamber and getting some control on the back of the engine will result in improved reliability as well as some power not lost to preigniiton or detonation. Coolant flow off the back of the engine is poor having been preheated by the preceding cylinders causing 8 and especially 7 to run hot. A cheapo/weepo cooling solution for the average guy with a street/strip machine, who isn't running air conditioning but just has an old fashion heater, is to put a return fitting on both sides at the rear of the intake where the heater sources hot coolant. Remember to remove any plugs from the manifold to head gasket so the cooling passages are open. Then "Y" or "T" the two sides together and run them into the heater core and back to the pump or radiator connection, which ever return configuration your vehicle has. This provides an extra flow path off the back of the heads and helps eliminate "pressure stacking" and steam pockets within the back of the heads that lead to 7 and 8 running too hot. Lots of racers tap the head temp sender port for a return, while this works pretty well for the right side, on the left the head is flipped around and this tap ends up between 1 and 3 where it isn't needed. So use the heater hose trick off the intake. If your intake isn't set up for a fitting on both sides drill and tap it. You can also do this without the heater core and just run back to the intake side of the cooling system. The problem with air-conditioning is that these systems usually have a shut off valve for the heater, so unless you’re running the heater there’s no coolant flow present.

So the big advantage in changing 7 and 4 around is that 4 is running cooler being only 1 cylinder behind the pump's inlet to the block. But now you've got a 2 to 4 exhaust interference but it has less of an effect because of 4's lower operating temps.

Another trick used in years gone by was to "retard" 7's timing by filing off the leading edge of the contact inside the distributor cap, resulting in the rotor having to move a few degrees further before sparking that cylinder resulting in a little retarded timing right there. That seems self defeating, but if you're loosing power to pre-ignition or detonation, that technique can help squelch it thus increasing power.

I'm going to stop here a moment and say something about the differences between preigniiton and detonation. Somebody wrote in one of these boards I frequent and intimated that pre-ignition and detonation were the same thing. They are not; each has its own cause and very different result. When you have to replace the piston you can see the different failure modes. Pre-ignition is when the mixture commences burning before the spark initiates it. This is usually caused by a piece of carbon, an overheated spark plug, a metal burr or sharp edge that is glowing hot. The failure mode on the piston will be melt, usually down a side thru the ring pack. Detonation happens after the spark has occurred. It's like watching video of a room fire flash over where the temperature and pressure  are so high the gasses spontaniously ignite. In the cylinder temperature and pressure ahead of the flame from goes out of control and the unburnt mixture explodes. The failure mode on the piston is a jagged broken hole, like somebody hit it with a sledge hammer. Often you also get a rod bearing failure as the wallop is so heavy it blows the oil film out of the bearing. You may find you're having bearing problems on a cylinder or it may actually blow the rod out of the block, you might be seeing the effect of detonation before the piston has a chance to cave in. Both modes make a pinging sound and until a piston lets go it can be hard to tell which is going on. Both modes respond to getting the cylinder temperature down by whatever technique.  

I have to say that of all the books out there on performance engine building, not many say much about cooling. It's like a big racers secret. But the fact is that there are no competitive racers that depend upon water going into the block in front of 1 and 2 flowing to the back of the engine then into the head and returning thru a passage of the intake manifold. Certainly the LT-1 an LT-4 were something of a belated attempt by the factory to deal with temp problems if only those of the head. I'm left to scratch where ever you think my brains are at the Jack Evans law suit against GM regarding utilization of "his" reverse cooling process. Backwards cooling had been done by many race teams for many years by drilling and tapping the water pump arms for AN fittings. Then plugging the water intake to the block thus forcing coolant into the hoses which then fed coolant into the heads, then into the block to be returned from hoses attached to the soft plug locations. This requires a vent line back to the pump from the front and rear of the heads as is done on the LT-1/4. Or a similar set up fed the conventional flow direction by routing into the soft plug locations of the block, flowing internally up to the heads with return fittings drilled and tapped into the ends of the heads, or other options. These techniques establish that no cylinder is more than one away from a fresh coolant source. But they're messy looking and certainly don't fit into the modern look of hiding the engine under plastic covers, Yuk! One might consider that for a street engine that all these hoses reduce cooling system reliability, plus when you bring your drilled and tapped water pump to the parts store as a core for a replacement pump, they probably won't take it.

Bogie