Does anyone know the factory comppresson ratio on an 1990 f-150 5.0.

 

Roughly 8.5:1

 

I am rebuilding a 5.0 mainly for power and boring it out 30 over, what is a good compression ratio for what im looking to do. 9.75:1 10.13:1 9.08:1

 

It depends to some degree on the heads and the cam. Swirl combustion chamber heads can take more compression and more timing. Aluminum heads pull more heat out, faster, so you can go about a point higher than an eqivilant cast iron head. If you use a large cam then you need about 10:1 to make up for a shorter effective compression stroke.

As general rule 9.2-9.8:1 is what you want, with mild cam and stock type iron heads, if you want to stay on pump gas, and run a healthy timing advance.

Flat top 5.0's with E7TE heads are listed as about 9:1, but I calculated it to slightly less once.

You want about 58cc combustion chambers to put you closer to 10:1 on a 302. Milling the E7 heads about .020 will usually drop the combustion chamber volume to about 58cc, if they are 61cc to start with.

 

I was just planning to rebuild but im going to put a mild cam in, pput new heads on, bore it out, new intake manifold and new carb, would the 10.13:1 be too much for it.
THX

 

As stated, it depends. The cam plays a big role, one with more overlap will "like" more static compression or you'll loose low end torque. If you build a torque motor with an RV cam then 10:1 will be a problem and require at least premium pump fuel. The EFI systems tend to run leaner than carbs over a large portion of the rpm range, so they can be more sensitive to detonation. But a carbed motor often has a lean spot somewhere in the power band, and making it rich enough there can often hurt performance in other areas. My vote says 10:1 will cause problems in a heavy truck.

 

thx for the info man

 

If you're using an EFI roller cam, 10.13 is way too much for 87 octane gas. The EFI cam grinds work great with carbs, but their LCA is wider and you end up with less overlap and this doesn't bleed off the pressure like a narrower LCA cam does. Shoot for a 9.0 to 1 ratio with an EFI cam. You'll also get better mileage with an EFI cam with a carb.

 

The important thing about cams and dynamic compression ratio, is the point were the intake valve closes after BDC on the compression stroke.

Increasing the intake duration means that the intake valve must be opened earlier and/or closed later. Usually both. If the intake valve opens earlier near TDC, it will be open longer while the exaust valve is still closing. This increased overlap will effect manifold vaccum, and may cause poor throttle responce at lower rpm, but it won't effect dynamic compression that much by it's self.

Increased Overlap may actually increase cylinder filling and tourqe production from the midrange rpm's and up, because of the scavanging effects. However, a tuned exaust system and a tuned intake system are really required to take advantage of this. It's possible to have high dynamic compression and higher overlap at the same time, if the cam has a narrower lobe seperation angle and more conservative duration.

If the intake valve closes later after BDC, that will reduce dynamic compression at lower rpm, because the compression stroke is effectively shorter. This is the main reason a bigger cam may reduce dynamic compression and low end tourqe. However, more duration may actually increase dynamic compression at higher rpm, through increased cylinder filling near BDC. The rod ratio comes into play in that case, with higher rod ratios further increasing dynamic compression.

On edit:The danger of a high static compression and mild cam together, is because the mild cam will close the intake valve relatively early after BDC, resulting in too much cylinder pressure near TDC with both valves closed.

In sum, it's best to keep static compression ratios below 10:1 with stock heads and a mild cam.