04-14-2022, 10:25 AM | #1 |
Senior Member
Join Date: May 2010
Posts: 218
|
compression
What does the compression usually run on a high compression head?
|
04-14-2022, 12:15 PM | #2 |
Senior Member
Join Date: Jul 2011
Location: Redondo Beach, CA
Posts: 5,849
|
Re: compression
|
Sponsored Links (Register now to hide all advertisements) |
|
04-14-2022, 03:26 PM | #3 |
Senior Member
Join Date: Jan 2012
Location: Western North Carolina
Posts: 3,897
|
Re: compression
About 6 to 1 unless you get a really high one that were made in the 60's. A racing engine may be 12 to 1 or higher, maybe 16 to 1. That requires a special fuel.
There is more to head design than the compression ratio. It requires clearance so that the valves can breathe and so that the cylinder is not choked off. The squish area is also important to provide turbulence for better combustion.
__________________
A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past. Last edited by nkaminar; 04-15-2022 at 07:10 AM. Reason: correct typo |
04-14-2022, 06:01 PM | #4 |
Senior Member
Join Date: Aug 2020
Location: Sagle, Idaho
Posts: 359
|
Re: compression
I'd like to see the figures on this also. Someone must know the compression readings for the 5.5 and 6.1 heads.
|
04-14-2022, 06:27 PM | #5 |
Senior Member
Join Date: May 2010
Location: Stuart Florida
Posts: 148
|
Re: compression
6.1 Xs 14.7 = 90 lbs compression reading.
5.5 Xs 14.7 = 80 lbs compression reading 4.2 Xs 14.7 = 60 lbs compression reading Or divide the compression reading by 14.7 for the compression ratio The above is at sea level, good engine, accurate gauge and is a rough guide |
04-15-2022, 03:48 AM | #6 |
Senior Member
Join Date: Jan 2016
Posts: 2,097
|
Re: compression
Sponsored Links (Register now to hide all advertisements)
|
04-15-2022, 06:38 AM | #7 |
Senior Member
Join Date: Jun 2014
Location: Melbourne, Australia
Posts: 7,496
|
Re: compression
That way of calculating the CR assumes standard bore. On one of my cars fitted with a 5.5 head, I get 115 or there abouts on each cylinder. By Flame's method, that is 7.8:1. Not bad from a 5.5:1 head on a motor 0.080" oversized bores.
__________________
I'm part of the only ever generation with an analogue childhood and a digital adulthood. |
04-15-2022, 05:33 PM | #8 |
Senior Member
Join Date: Jul 2010
Location: Auckland, New Zealand
Posts: 514
|
Re: compression
Remember that compression causes the air to heat up, increasing the reading. The simple calculation from atmospheric pressure assumes isothermal (fixed temperature) conditions whereas the compression test has adiabatic conditions (temperature rises with compression)
This has been discussed before. I think Jim Brierly quoted an approximate conversion factor to get a closer calculated result. My 5.5 to1 head on a 125 thou overbore reads 100 psi on my test gauge, Wide open throttle and hot engine. Closed throttle reads about 10 psi less from memory. Some say WOT makes no difference but that is not my experience on any engine I have tested SAJ in NZ |
04-15-2022, 05:39 PM | #9 |
Senior Member
Join Date: Dec 2014
Location: Shawnee, Ok
Posts: 3,471
|
Re: compression
I'm running 95 psi on all tubes with a 6:1 head
__________________
Keith Shawnee OK '31 SW 160-B |
04-15-2022, 06:27 PM | #10 |
Senior Member
Join Date: Jun 2010
Location: 34.22 N 118.36 W
Posts: 1,055
|
Re: compression
Thats why a compression test is only relative to all four holes, the absolute number doesn't matter. Too many contributing factors to influence it.
What does matter is that all holes should be relatively the same. As a rule of thumb, 10% difference is acceptable between all four. Last year at Bonneville, I knew there was a problem with one, the pumping compression was about 125 psi when it should have been about 230psi. Removed intake and exhaust and looked into ports, one valve guide broken off, that was the end for me. And in case your thinking to ask, that pass was over 170 mph. J
__________________
As Carroll Smith wrote; All Failures are Human in Origin. |
04-15-2022, 08:03 PM | #11 |
Senior Member
Join Date: Dec 2014
Location: Shawnee, Ok
Posts: 3,471
|
Re: compression
170 mph? And that's in a Modal A?
You must have that new Saturn V triple booster motor...Heck ya, it's after 10 AM, I'm going to start the rumor.
__________________
Keith Shawnee OK '31 SW 160-B |
04-15-2022, 08:42 PM | #12 |
Senior Member
Join Date: Jun 2010
Location: 34.22 N 118.36 W
Posts: 1,055
|
Re: compression
Keith,
Ya! start the rumor!! No, the car is a belly tank style lakester, wheels exposed. But, the motor is a "B" block, stk deck height, cam and crank in stk locations. John
__________________
As Carroll Smith wrote; All Failures are Human in Origin. |
04-15-2022, 10:13 PM | #13 |
Senior Member
Join Date: Jun 2014
Location: Melbourne, Australia
Posts: 7,496
|
Re: compression
For a smooth running engine that will pull, what johnneilson says about even compressions being more important than what figure you read is right. Lots of miles were driven on original motors with readings that were in the 65 psi range. That's very low by today's standards but very forgiving of inexperienced drivers and we've all been that.
I can scare myself with 40 hp. I don't need an even bigger scare!!
__________________
I'm part of the only ever generation with an analogue childhood and a digital adulthood. |
04-16-2022, 10:14 AM | #14 |
Senior Member
Join Date: Dec 2014
Location: Shawnee, Ok
Posts: 3,471
|
Re: compression
Driving anything at 170mph is insane, you are a much braver man then I, I would pay to see it, but I would never do it... I tip my hat to you Sir.
__________________
Keith Shawnee OK '31 SW 160-B |
04-16-2022, 02:49 PM | #15 | |
Senior Member
Join Date: Jun 2010
Location: 34.22 N 118.36 W
Posts: 1,055
|
Re: compression
Quote:
every job is easy with the proper tool, I sure as hell would not take a real Model "A" to the salt though some do. Stay safe, Buddy, J
__________________
As Carroll Smith wrote; All Failures are Human in Origin. |
|
04-16-2022, 06:01 PM | #16 |
Senior Member
Join Date: May 2010
Location: Johnstown, PA
Posts: 342
|
Re: compression
I have a Brumfield 5.9 and my readings range from 102 to 105.
|
04-17-2022, 01:43 PM | #17 |
Senior Member
Join Date: May 2010
Location: Southern Maine
Posts: 1,498
|
Re: compression
[QUOTE=johnneilson;2121575]Keith,
Ya! start the rumor!! No, the car is a belly tank style lakester, wheels exposed. But, the motor is a "B" block, stk deck height, cam and crank in stk locations. What location are the valves in? |
04-17-2022, 02:27 PM | #18 |
Senior Member
Join Date: Jun 2010
Location: 34.22 N 118.36 W
Posts: 1,055
|
Re: compression
Valves are NOT in the block, there is a limit of what can be done...
__________________
As Carroll Smith wrote; All Failures are Human in Origin. |
04-17-2022, 03:01 PM | #19 |
Senior Member
Join Date: Dec 2014
Location: Germantown,TN
Posts: 516
|
Re: compression
|
04-17-2022, 05:51 PM | #20 |
Senior Member
Join Date: Jun 2010
Location: 34.22 N 118.36 W
Posts: 1,055
|
Re: compression
this has been posted elsewhere before.
__________________
As Carroll Smith wrote; All Failures are Human in Origin. |
|
|
Sponsored Links (Register now to hide all advertisements) |
|