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Old 02-13-2013, 06:35 PM
oldbogie oldbogie is online now
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Originally Posted by streetrodderbn View Post
Putting together a mild street small block Chevy. I started with a 1991 L98 350. I overbored the block .030. had the block professionally baked and shotpeened, new cam bearings and brass soft plugs installed. I bought new weight matched hypereutectic speed pro flat top pistons/rings, had the connecting rods reconditioned, and the crankshaft polished (std/std). I'm using a ZZ4 camshaft-Hydraulic roller, and had the heads upgraded with all Lt4 springs,retainers,locks, and shims, and installed new swirl polished stainless valves with positive seals. The heads were resurfaced and treated to a 3 angle valve job. I'm running a Weiand dual plane intake with an Edelbrock performer carb. I'm thinking it will be @ 365-375 Hp. what do you think??
I'd say about 340 maybe a bit less, not the best head selection; even ported the L98 has problems getting out the power, mostly a function of a pretty good but obsolete combustion chamber technology pretty much a rehash of the old fuelie head redone in aluminum, compared to the Vortec type chamber they leave about 20 to 40 horses on the table depending on the cam. The ZZ4 cam is pretty meek. My dyno room experience with the ZZ4 engine shows it to be overrated by about 20 horses. You'll note the ZZ4 cam really adds a lot of timing and lift to the exhaust which says a lot about the L98 head's exhaust port capability. This cam is a crutch for the L98's exhaust port not a performance item as such.

For 375 horses (on the dyno) you need Vortec style heads, at least a Comp XE268H or equivalent cam, Edlebrock Performer RPM intake or equivalent, a Holley 750, long 1-5/8ths tube headers.

I don't recommend spending money on reconditioning old rods when you're done you have an old rod somewhere in its fatigue life cycle with restored dimensions and new bolts. A shiny, polished bomb but a bomb none-the-less. The connecting rods are the hardest working part in the engine, if anything is likely to let go, it's the rods. I don't use rebuilt rods in anything but grocery getters with the cheapest of rebulds.

Don't get me wrong I've built a lot of L98 headed engines, they're pretty decent, but they're pretty much done even when ported by 340 horses unless you run a really long duration and fast lifting cam which the ZZ4 isn't it uses some pretty long ramps and long ramps on the intake side cost power by bleeding off your hard won compression.

The problem in not using a head with the complete features of the Vortec style head is that because of its combustion characteristics you leave 20 to 40 horses on the table that then don't have to be made up with a super radical cam and tons of excessive compression. They're like a freebe source of power. The improved porting not withstanding the key that unlocks the power is the efficiency of the combustion chamber. The changes are small and will go un-noticed by the untrained eye but they are significant.
- The spark plug is pushed toward the center of the bore, this reduces burn time which makes it easier to place max cylinder pressure on the piston after it passed TDC but isn't lost down the bore. rod angle on the crank is everything, the best power transfer from the piston to the crank is in the first 110-120 degrees of rotation from TDC peaking just before and after the 90 degree point.
- The squish/quench deck has a beak between the valves this helps keep mixture from flowing out the exhaust unused during overlap.
- There is a relief between the intake valve and spark plug with sharp edges, this allows easier flow of the incoming mixture and the sharp edged features promote the mixing of fuel and air. Keep in mind that port fuel injection puddles fuel on the backside of the valve so this shape of the chamber insures it gets mixed. This is also a problem of carbureted and TBI engines so this head really wakes them up. This is really good if you're running a carb on an unheated intake as much of the fuel is flowing as liquid and needs to be remixed with the air. The other nice thing about keeping the fuel in a liquid state within the manifold is that it occupies much less space than when evaporated so there's more space for air to be injested thus more power at the crankshaft if you can get the fuel and air mixed in the combustion space. But you can't do that with any old head and get horsepower from this trick, the head has to have the features that remix the wet flow with the air flow to get at the power in the fuel into the crankshaft.

You'll see that the L98 head has some of these features but not all. So while they're an improvement over what came before in large open chambered smog heads, they just isn't quite up to what the heart/kidney shaped chambers of the Vortec will do for power and efficiency.

Bogie
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