OK guys, so Flemming, and olds307 and 403, brought up good points when making a cylinder head decision... so I did a little research on compression ratios, and it's correlation with cam duration.
It is important that the engine's compression ratio matches the duration of the cam you're selecting. Too little compression ratio (or too much duration) will cause the cylinder pressure to drop. This will lower the power output of the engine. Below is the recommended compression ratio for the cam's INTAKE duration @ .050 . You can also see the effective power range of the engine, in respect to the duration of your cam.
I'll use my $25 Edelbrock Performer Plus cam & lifter package, you guys saw me recondition to "like new", in another thread. It's specs are as follows-
- 403ci
- Lift: .449''/.473''
- Duration @ .050: 204 INTAKE / 214° DURATION
- Advertised Duration: 280°/290°
- Idle-5500 RPM Range
Static Compression Ratio
(SCR)Intake Valve Duration
(degrees @ .050" lift)Power Range
(RPM)
8.00:1 185º (Idle-4,000) - 8.25:1 189º (Idle-4300) 8.50:1 194º (800-4,500)
9.00:1
204º (1,000-4,600) - 9.25:1 208º (1,200-5,200) - 9.50:1 212º (1,600-5,400) - 9.75:1 216º (1,800-5,600) - 10.00:1 221º (2,000-5,800) - 10.25:1 227º (2,400-6,200) - 10.50:1 233º (2,800-6,400) - 10.75:1 236º (3,000-6,800) - 11.00:1 (240º3,200-7,000) - 11.50:1 244º (3,400-7,200) - 12.00:1 248º (3,600-7,400)
As you can see, my target compression ratio is exactly 9.00:1. With that being said, here are the stock specs for my Olds 403 -
1977-79 403 are: 185hp @3600 RPM, 320ft/lbs @2200 RPM, 4.351 x 3.385 bore/stroke, 8.0:1 compression ratio, with 80cc combustion chambers. So you can see, I NEED more compression ( a full point ). Easiest way to accomplish that... reduce chamber volume. Enter the SBO 350 heads ( a direct bolt on & common swap ). In my case, 1969 #5 heads with 64cc chambers. A reduction in chamber volume by 16cc. I will mill/deck/surface these heads, as needed, to achieve the desired result, of 9.00:1. On a typical Olds 350/403, removing .005 of the head surface, Is equal to a reduction of 1cc. Each cc is equal to 0.1 addition to the compression ratio.
I won't be able to calculate this exactly until I rebuild the short block, decide on pistons, measure deck height, and determine which head gasket to use.
The rule of thumb for the compression ratios run in MOST street engines is: for every (1) point + change in the compression ratio your power output will increase by 2% ( debatable, as some say up to 4% ). Using this rule of thumb on an engine that produces 400 hp... every (1) + point change in compression ratio will result in approx. a 8hp increase in output. This is just food for thought, everyone... Final decisions, and outcome, will be documented in my build thread. Comments, testimony, and members who are more knowledgeable on this subject, than I am, please respond.
