Hyper_pak
Old School Chrysler Fan
Why do 273 2-bbl engines have double eyebrow valve reliefs?
273 Piston Question
- Thread starter Hyper_pak
- Start date
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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)
Hyper_pak
Old School Chrysler Fan
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.
It's the pistons on the rods. The rods aren't switched to the other bank.
Wyrmrider
Well-Known Member
good question
correct answers
correct answers
Actually You theoretically could, but You would need a 3/4 inch head gasket and 1 1/2 inch intake gasket !
The valve reliefs are figured into the compression ratio, just like the dome is on the Commando pistons. Either way, the point is that the 273 has a very tight little combustion chamber.
Hyper_pak
Old School Chrysler Fan
Do 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.
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 heads
THe 4 eyebrow pistons do lose compression. That's the point. They are low compression 273 pistons.
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.
nm9stheham
Well-Known Member
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).
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).
Hyper_pak
Old School Chrysler Fan
66fs, I just saw you are from Greenwood SC! My folks, both sides were from Ninety Six, what a small world. I spent many summers down there and I had such great memories visiting all my aunts and uncles and cousins. Then my next door neighbors daughter decided to go to Lander when I lived in Yorktown VA. She told us and said, it's a small school in a small town, you might not know anything about it. We said, oh yeah we have.
That is Wild, my wife volunteers at Ninety Six middle school and Lander. I love it here.
Hyper_pak
Old School Chrysler Fan
Small world, for sure.
All that's true, but Chrysler never made a 2 eyebrow flat top, so why even bring it up? If it was 2 eyebrow, it was a domed piston.
nm9stheham
Well-Known Member
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.
Hyper_pak
Old School Chrysler Fan
ok rusty, your mug was bad enough, but 7 liter ford?
Dang skippy and don't you forget it. lol
Did you measure one? That looks about right for a factory piston.
nm9stheham
Well-Known Member
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.
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.
nm9stheham
Well-Known Member
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.
FWIW.... The great part for the later 273 flat-tops is that the CH is so tall that they are near zero deck..... works out something like .002" below nominal deck height. That's why the SCR did not drop into the toilet on these later 273 engines even with the larger, open chambered 675 head. Took me a while to learn all of this; sounds like 66fs has been down that road long ago LOL.
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