Head Chamber Size and Compression Ratio
#2817093
09/04/20 06:18 PM
09/04/20 06:18 PM
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hp383
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How much does the head chamber cc affect the compression ratio. ?
I'm working on a 78 motorhome 440. It has 452 heads, research says about 90cc. I also have some 906 heads, research says 88cc.
Looking at aluminum heads, choices are 74cc, 80cc, 84cc.
Using the stock pistons, what sort of compression ratio will I be looking at with the various chamber cc sizes.
I'd like to see 9.5:1 if possible. Nice NA street and cruising pump gas motor.
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Re: Head Chamber Size and Compression Ratio
[Re: hp383]
#2817108
09/04/20 07:54 PM
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CSK
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FOR accurate #'s you would need to measure the piston to deck height.
Last edited by csk; 09/04/20 07:55 PM.
1968 Charger COLD A/C Hilborn EFI 512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim 2020 T/A 392 Stock 11.79 @ 114.5
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Re: Head Chamber Size and Compression Ratio
[Re: CSK]
#2817110
09/04/20 08:08 PM
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fast68plymouth
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Actual CR for a stock 78 440 out of a motor home will be in the 7.7-7.8 range.
You’ll have to work at it to get to 9.5:1.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Head Chamber Size and Compression Ratio
[Re: hp383]
#2817115
09/04/20 08:25 PM
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polyspheric
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How much does the head chamber cc affect the compression ratio?
There are only 2 dimensions: chamber volume (including in the casting, deck clearance, gasket thickness, valve reliefs, compression dome) and displacement. (CV + D) ÷ CV = CR
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Re: Head Chamber Size and Compression Ratio
[Re: fast68plymouth]
#2817127
09/04/20 08:59 PM
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hp383
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Actual CR for a stock 78 440 out of a motor home will be in the 7.7-7.8 range.
You’ll have to work at it to get to 9.5:1. Wow. I didn't realize they were that low. I may look into some pistons. Maybe something stock size from a late 60's. Simple cast piston if they can be had fairly cheap. I don't think this is going to need an overbore, probably just a good dingle ball hone.
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Re: Head Chamber Size and Compression Ratio
[Re: calrobb2000]
#2817263
09/05/20 03:18 AM
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hp383
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hi those pistons are prob .150+ down the hole .!
soooo no matter what head you use you will not have any quench ! = poor performance !
change those pistons !!!!
So browsing Summit they have an OEM style replacement piston fot the 1970 440. Would this be an acceptable choice? I haven't had the pleasure of specing pistons before. So a few of the specs they list are greek to me.
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Re: Head Chamber Size and Compression Ratio
[Re: hp383]
#2817695
09/06/20 07:31 AM
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BSB67
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hi those pistons are prob .150+ down the hole .!
soooo no matter what head you use you will not have any quench ! = poor performance !
change those pistons !!!!
So browsing Summit they have an OEM style replacement piston fot the 1970 440. Would this be an acceptable choice? I haven't had the pleasure of specing pistons before. So a few of the specs they list are greek to me. That piston will be 0.110" in the hole.
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Re: Head Chamber Size and Compression Ratio
[Re: BSB67]
#2817743
09/06/20 11:04 AM
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gzig5
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hi those pistons are prob .150+ down the hole .!
soooo no matter what head you use you will not have any quench ! = poor performance !
change those pistons !!!!
So browsing Summit they have an OEM style replacement piston fot the 1970 440. Would this be an acceptable choice? I haven't had the pleasure of specing pistons before. So a few of the specs they list are greek to me. That piston will be 0.110" in the hole. The compression height dimension is what you want to pay attention to when comparing pistons and looking get close to deck hieght. That is the distance from the center of the wrist pin to the top of the piston. A bigger number puts the top of the piston closer to the deck, but you need to know where you are starting from.
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Re: Head Chamber Size and Compression Ratio
[Re: hp383]
#2817746
09/06/20 11:14 AM
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polyspheric
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The 915 head is the last "closed" BR/B chamber, the 1965-67 440 piston has a quench compression distance w/o regard for saving the whales (what the 1968-* open chambers were designed to do).
Math: 440 deck height 10.725" RB rod 6.76" 50% of stroke 1.875" Compression distance for zero deck with a closed chamber (gasket thickness as the only quench distance) 2.09"
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Re: Head Chamber Size and Compression Ratio
[Re: polyspheric]
#2818113
09/07/20 10:53 AM
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gregsdart
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The 915 head is the last "closed" BR/B chamber, the 1965-67 440 piston has a quench compression distance w/o regard for saving the whales (what the 1968-* open chambers were designed to do).
Math: 440 deck height 10.725" RB rod 6.76" 50% of stroke 1.875" Compression distance for zero deck with a closed chamber (gasket thickness as the only quench distance) 2.09" To expand choices, Cometic has gaskets starting at .027 in small steps all the way to .120 thick. I use a .027 Cometic and pistons in the hole .018 to get .045 quench in my race motor. That also helps cut down on crevice volume where fuel gets jammed into the edges of the chamber at TDC.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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