Hyper_pak
Old School Chrysler Fan
Why do 273 2-bbl engines have double eyebrow valve reliefs?
It's the pistons on the rods. The rods aren't switched to the other bank.Seems like you loose compression doing that. And rod oil holes locate the piston so you could make a mistake there right? It just seems weird to me.
Actually You theoretically could, but You would need a 3/4 inch head gasket and 1 1/2 inch intake gasket !So they can be used on either bank. The HP pistons are bank specific. (they can't be put in upside down because of the dome)
Spot on. You need to cc them. the 273 heads are what, 57 cc according to the book? I shaved .030 off mine to get there. the 302's and the magnums are closed chamber but larger than the closed chamber 273/318 headsDo you know what it would be with a 302 head? CR? The 318 magnum head? I hear 59 I hear 63. I know the only way is to CC them to be sure.
273 2 barrel pistons are about 9:1, 8.8:1, compression ratio. 273's were all done by 69. 68 and 69 273's had .020 higher pistons to keep compression up with open chambered 318 heads. They were never SMOG engines.
Removing 2 of those 4 eyebrows will drop 1.5 cc's from the equation... not a lot. The real compression drop vs the high CR is the lack of the dome, not the eyebrows.
And the 57 cc chamber is for NHRA minimum chamber spec 273 closed chambers..... production closed chambers are in the low 60's. So, no, in stock unmilled form, the 273 closed chambers are within 0-2 cc's of the 302's and Magnums.
The later open chamber 273 heads are around 68 cc's. The later ones still had decent CR in the high 8's with the 4 eyebrow 273 pistons and thin head gasket due to the much higher compression height versus the 318 (around .070" taller).
ok rusty, your mug was bad enough, but 7 liter ford?
Just to point out that the 2 extra eyebrows on a 4 eyebrow flat top like this is not much effect on CR; they are less than 1 cc each.[/QUOTE
I got the point but it's a moot one since they never had a two eyebrow flat top. I'd have to measure that though, because generally, two valve reliefs are "around" 5cc's. You can verify that looking up most any two eyebrow flat top. Even the 2.2 pistons I have for my slant are two eyebrow and they are 6cc's and only 3.445" bore size. So I think your 1cc each is a bit small.
Just to point out that the 2 extra eyebrows on a 4 eyebrow flat top like this is not much effect on CR; they are less than 1 cc each.
Yes, I measured the original ones out of a '68 273. Around 3 cc's total for the 4 eyebrows, so 1.5 cc's in 2 of these eyebrows. Pishta measured 2-3 cc's total IIRC.Did you measure one? That looks about right for a factory piston.
Yes, I measured the original ones out of a '68 273. Around 3 cc's total for the 4 eyebrows, so 1.5 cc's in 2 of these eyebrows. Pishta measured 2-3 cc's total IIRC.
Yup, Pishta told me I was off... so I HAD to measure one! I was using a larger number based on your same thinking; the 'norm' would be the deeper 2.5-3 cc per eyebrow rather than this small .75 cc per eyebrow. A big cam would have piston-to-valve clearance issues with these small eyebrows.Wow that's a good bit smaller than I would have guessed. I bet the "norm" I was talking about is for modern pistons with extra room machined in for more valve clearance probably.