Technical > Matt Shaff's Engine Shop

The Nostalgic 265 Engine Build

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Van:
The top improvement needed is something around a 6.2 rod and a lot shorter compression height piston. On that 56 block, I cut a small groove in the #5 cam bore to connect the oil holes so it will oil the lifters without the cam having a flat. Do this before installing the bearing.

bfalfa55:
Thanks Van. The oil groove issue I know.  Before finding this engine with domed pistons, I considered using after market domed 305 pistons and using GM 5.94 in rods. I figured this would be a more cost effective way to get after market pistons that wold be similar to the bore of a 265.Why is a longer rod setup a big improvement ?

Roger:
A longer connecting rod will reduce the rod angle to the cylinder centerline. Less rod angle will reduce piston side loading so there will be less friction and less cylinder wear. The long connecting rod also allows the piston to stay at top dead center longer as the mixture ignites. That gives the combustion pressure more time to build before it starts to shove the piston down the cylinder. All this would result in additional power being produced along with a decrease in cylinder wall wear.

bfalfa55:
Thanks Roger, that makes sense. Is there any formula or way to figure out the percentage of power increase you would get from one rod length to the next ? For example, if using 5.7 rods made 400HP, how much more power would 5.94 rods or 6.2 rods make as was suggested earlier ? Or is it something that would just have to be measured on the dyno ?

Roger:
The conventional wisdom says that a rod-to-stroke ratio of 1.6 to 1.8 is a real good for a high performance engine. With a 5.7” rod and the 3” stroke in your 265 the rod-to-stroke ratio is 1.9; 5.7/3=1.9. With the 5.94” rod the ratio goes up to 1.98 and will probably do little good in producing more power as you are already over the sweet spot for rod length. The new engine I'm building for my altered has a 3.625” stroke with 6” rods resulting in a rod-to-stroke ratio of 1.65 and I’m very satisfied with it. Just my 2 cents.

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