87 hatch

i also dont have any idea about compression. What does a stock 5.0 HO have? and what do the numbers mean? what is the difference between the higher and lower numbers?

bobby

compression ratios is the number of air to the parts of fuel. an engine with 9:1 compression ratiomeans it burns 9 parts of air to 1 part fuel. which is what a stock 5.0 has. 10-10.5:1 is about the max that can run on premiuim fuel(octane93-94). the higher the compression, the greater the pressure build up on the upward stroke of the piston before the power stroke. higher compression motors make more HO obviously due to more force when the mixture is lit. for a blower of some kind ( supercharger or turbo) a compression ratio of 8-8.5:1 is about perfect. because with this compression ratio, you are allowing more air AND fuel providide by the blower

angcobra

First of all compression is a measure of cylinder pressure. To measure compression pressure you use a gauge in the spark plug hole, turn the engine over and measure cylinder pressure that is created.

Compression ratio (CR) is simple math to compare the volume of the cylinder (above the piston ring) and combustion chamber when the piston is at bottom VS the volume of the cylinder (above the piston ring) and combustion chamber when the piston is at top dead center. A CR of 10 to 1, means the volume when the piston is down is 10 times greater than the volume when the piston is at top dead center. For the same engine, increasing CR will increase cylinder pressure. Typically higher cylinder pressures will create more power. Forced injection cars run lower CRs to make sure the cylinder pressure does not get too high which could cause predetonation just due to the cylinder pressure and temp.

87 hatch

sounds like both of you are right. right????

angcobra

All except the air fuel part. The air fuel ratio is controlled by other means. The air fuel ratio can be vaired regardless of the compression ratio. One simple way to vary air fuel ratio is by changing the fuel pressure, higher fuel pressure allows more fuel flow through the injectors making the mixture more rich. For SC cars the lower CR and correct Air fuel (~ 11.5 to 11.8) ratio are both important to prevent detonation.

bobby

DUH!!!, lol, it was early in the morning before school and i had just goten up, lol

C/Note

i got 10.7 to 1 with alum heads runing super 92 oct truck pings ..they dont make gas like they use to... now i got to retard my timing so much its doggy...im just going to start putting 2 gals of 117 oct with every fill up ...then i can get my timing where it should be...

fats429

Compression = power

the same engin combo will more power with higer compression . the stock engin runs about 8.5 :1 this is so it can run on 87 octain
By decking the head or useing a after markek hed with smaller chambers will inreace compression ratio . also pop up pistons or dished pistons will increace it also by making the combustion chamber smaller .

increased compresson means higher octain gas ( the higher the octain the the tighter you can squease it befor it light its self off or detanates ) the guy that runs 94 or 100 octatn with stock compression is just wasting money and lieing to him self that it makes the car run better .

I think a stock HO piston with 58cc chambers makes 9.5 compression . you start running into prolbems with pings and pre ingtion on pump gas around 11.5 or 12:1 compression but some tuners can pull it off

If you run a turbo or the juice you ant going to want more than 9:1 compression because you are useing more than atmospheirc pressure and it will burn the s#$t ouy of a set of slugs

87 hatch

this is complicated **** . Is there any books out there that all of you learn this stuff from or do you just read "5.0 mustangs and super fords" magazine?

fats429

Don't learn from a hot rod mag , the info changes with the weather get a smipple tune and modify book from borders or barns & noble
They all start out the same with compression , how cylnder head works , induction then power adders , use the mags for entermant