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#1
10-25-2016, 06:18 AM
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Lumpy top Pistons vs low compression
Hi
I'm about to help a mate with a 400 build. Mild street build that won't see over 5000rpm. Assuming zero deck, .040 head gasket,100cc chamber size with 4.15 bore and 3.75 stroke what will be better, -6cc valve relief piston for 8.2:1 compression or 10cc dome for 9.4:1 comp? |
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#2
10-25-2016, 06:56 AM
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A 10cc dome is not that tall and since the plug in the head is at the very top of the chamber even in a iron Pontiac head there is no issue with flame travel.
You could go either way, but if the dome piston perserves the same quench area as the flat top , then the dome piston will provide the same or better mixing action to the fuel and air. Pontiac motors even with just the 067 Cam would rev right to 5500 if all was well , so you should atleast have that power range out of the motor !
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#3
10-26-2016, 01:57 AM
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Probably should have been more clear in my original post. What I'm asking is what's better, dome Pistons or flat with lower compression? Iv read dome tops aren't very good. Also, although the piston displaces 10cc of chamber, the dome itself is probably displacing closer to 16cc of chamber to offset the valve reliefs. Remember most Pistons are about -6cc for valve reliefs, so a +10cc piston is actually 16cc difference.
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#4
10-26-2016, 02:10 AM
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If you haven't ordered the pistons, you could go for a custom pin height and bring the flat top piston up out of the bore a little. Either that, or mill a little extra off the heads and leave the pin height alone.
__________________
Mick Batson 1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon. |
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#5
10-26-2016, 04:24 AM
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Raising the pin will only make small gains in compression due to the need of quench distance. With a .040 head gasket maybe the piston can be lifted out of the hole .005. Much more gain to be had milling a heap off the heads. The thing is, with a 100cc head and a 10cc dome it's the same compression ratio as a 84cc head and -6cc flat top piston. You just can't mill a 100cc head to 84cc. All things being equal, ie bore/stroke/quench/gasket thickness/cam, is a 9.5:1 engine with dome top piston more likely to experience detonation than a 9.5:1 engine with flat top pistons?
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#6
10-26-2016, 05:03 AM
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In my experience many years ago , the pop-up is more octane sensitive than a flat top. A wider LSA cam in the 112/114 range is better. The old Crane Fireball worked well and would get you about 375hp with stock low comp heads. I think the specs on the cam were 112lsa, 107icl, 220int, 226exh @ .050. Lift was around 470. I personally would not build a 400+ hp using pop-up pistons.
Last edited by Indianhead; 10-26-2016 at 05:25 AM. |
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#7
10-26-2016, 06:06 AM
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A dome piston still has valve releafs so you are talking some 13 CCs, not 16!
The valve notches on TRW or Sealed Power Pistons with 4 valve notches are listed as 6.7 CC's . Are we talking about a build here with a iron head that has a 1.66" Exh valve?
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! Last edited by steve25; 10-26-2016 at 06:13 AM. |
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#8
10-26-2016, 06:15 AM
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Yes. Iron heads, 1.66 valves.
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#9
10-26-2016, 07:10 AM
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If you install 1.77" Exh valves and get BB Chevy 2.19" intake valves and machine them down to 2.14" and convert them to a 30 degree seat you can get more power out of the head and also get back 3 CCs of chamber volume.
The best way to keep around town drivability good and make more power is with straight up air flow from the heads, not a bigger Cam if you can avoide it!
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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