7 Liter Corvette Heads - can they be improved upon?

[MAP]

My mistake, folks - I sent two replies on the same topic, thinking that the first one didn't "take," so to say. Best - Mark

[DavidVizard-GFN]

I have never measured the LS 6 base circle but I estimate it to be at least a 1/4 inch bigger than the old SB Chevy. Also not haveing the two side by side I cannot tell you how much longer (if any) the LS6 lifter bores are - any one - any comments?
DV

[automotivebreath]

It's obvious GM engineers built a lot more strength into the LS camshaft.
The base circle measures 1.550" with a 2.165 camshaft journal. An
approximate depth of the lifter bore is 1.562"

[SStrokerAce]

Base circle is all relative to the journal size and the lift.... a SBC Journal is 1.868 while a LS is 2.165" (55mm)

Say on each cam you have .350" lobe lift, the lobe cannot be larger than the journal of the cam or it will not fit in a V8 block. The easy way to estimate the max base circle size is: ((Journal Size/2)-Lobe Lift)*2=Max Base Circle Size

LS = ((2.165/2)-.350)*2= 1.465"
SBC = ((1.868/2)-.350)*2 = 1.168"

So almost .300" different so David's guess of around a 1/4" is pretty dam close.

Bret

[DavidVizard-GFN]

Mark,

You are correct about being counter intuative. Take a look at the view down the port on the left hand pic - last but one in the article Here you see the dam angled across the port 45 degrees. It looks like the dam would cause fuel to flow off it's edge and up to the left and on into the cylinder wall. This is not the case. As the fuel reaches the edge of the dam it shears off perprndicular to the dam's edge. That means it is directing fuel toward the middle of the cylinder not the chamber wall or cylinder wall as it first might seem. What we see here is the reason why we need to have both wet and dry flow benches. if the air and fuel always did what you expected we would not need either of these pieces of equipment.

Hope that sorts your dilema.

DV

[oilman]

David,
Do you think the 'dam' in the LS7 inlet port would be of benifit in the L92 head, which is aparently very similar, but at less than $200 each for a bare head, MUCH more affordable. I have a new pair of castings ready for a bit of porting. I am hearing various estimates of 20-50 extra HP from porting these heads, and want to have a go myself.
I am building a L76 engine with a big ish cam 236/242 (.600 lift) 112 using standard size valves.

[DavidVizard-GFN]

Oilman,

As charge burn and fuel effciency becomes more and more a factor I am thinking thet ports done such as we see here are of greater interest. The issue at present is building experience and skill at producing them. The point here is we get good at a certain style of port and as a result end up in a comfort zone. These ports with a dam are not as easy by far to produce so a flow bench for both wet and dry testing starts to be a far bigger necessity.

The heads you mention look to be a real bargain. Just another reason why I should be building some of these later model Chevy's.

DV

[oilman]

Looking at the ports on this head, I have a couple of observations. The first thing that springs to mind is the very large intake port, it reminds me of the pinto days. However it looks like the chevy engineers have been able to make it flow a lot better as cast than that ford engine however, so the tuner has a bit less of an uphill battle. The large split in size between the inlet and exhaust valve is worth noting as well, favouring cams with a fair split between intake and exhaust duration. Why did they do that?

One other small observation it the exit of the exhaust port. the port in the head is quite a bit smaller than the exhaust manifold, by about 1/8" all arround, also the head port is D shaped, with the manifold being circular. Not sure what is goin on there either. (Could it be a mechanism to reduce reversion? Is reversion even a problem with the stock cam?)
with a performance cam, will it be better or worse with port matching here?
With so many LS engines in the hands of gearheads, and the low cost of these castings from GM, there is a lot of interest in getting the best from these new heads.