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Original Message
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| Sonic testing & overboring an FE (long) |
| By Dan Davis - 12/12/2001 3:44:18 AM; IP 202.130.141.98 |
The attached pic shows the results from a sonic test of the RH bank of a 428 cylinder block that has been bored 0.40" over (4.17" bore). Sonic test measurements are taken at four locations around the bore, and at three different depths (approximately the top, the middle and the bottom of the bore).
First off, the measurements have to be examined carefully. Not every data point is a valid representation of the actual block thickness at that particular location. Sometimes a localized pit in the cylinder wall, or a flake of rust inside the water jacket can lead to a thickness measurement which is completely out of bed with the other measurements in the same vicinity. Such erroneous data points have to be filtered out in order to ascertain an average wall thickness in the region.
To get a truly accurate map of the wall thickness around each cylinder would require that many more data points be measured and probably that each point be measured two or three times and the results averaged out. But this would be prohibitively expensive. All we are really trying to do here is figure out what a safe bore size is for a rebuild.
It is important to remember how increasing the bore size will reduce the cylinder wall thickness. The meat between the cylinder bores will be reduced by an amount equal to the increase in bore size creating more localized heat. However, the rest of the cylinder wall will only be reduced by half of the increase in bore size.
It is also important to consider exactly where block thickness is critical and about how the thickness of the cylinder walls affects block integrity and overall performance. In the world of maximum performance (engines putting out 600 HP and up), the general consensus is that the minimum cylinder wall thickness should be above 0.180" - 0.200". In the world of street performance engines (350-450 HP) engines can remain happy with 0.140" - 0.160". I've seen a few "grocery-getter" rebuilds that have taken it down to 0.100" without any problems, but that seems extreme.
But where is the thickness most important? Certainly it is important in the region between the cylinder bores. If for no other reason than that there must be sufficient meat to absorb and transfer the heat generated by combustion.
Things like block flex are probably not an issue at the power levels (<500) most of us will achieve, but this does not mean that the cylinders can be made paper thin. The cylinders must still contain the pressures of combustion. Also, as the pistons move down on the expansion stroke they exert a force on the outside of the cylinder wall. This force is proportional to rod angularity, thus on big stroke engines the side loading on the cylinder walls is increased. So on a 3.98" stroke 428 engine one might want to play it conservatively with the thickness of the cylinders. 390s and 427s would be put less loading on this region.
Fortunately, in this regard the 428 block (this one anyway) appears to be thickest on the outside of the cylinder walls. Unfortunately, it is thinnest between the cylinders. This area therefore is the limiting situation in determining how far one can go when over-boring the block. Even going to 0.30" over would leave these regions at around 0.120" with an offset bore. This is at the extreme lower limit of what most engine builders find acceptable, and it is only over a small area of the total cylinder. Going too large in this region might lead to bore distortion during the expansion stroke. This can cause blow-by (and an associated loss in power) and it can also cause premature wear via piston scuffing due to localized hot spots.
Ultimately when deciding how big to go with an over-bore on an FE, you should sonic check the block and make a plan to offset bore the block to get the most uniform cylinder walls as possible. Offset boring seems to be a necessity on the FE as this 428 block was marginal at best before the overbore and scrap now.
Cheers,
Dan |
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