IVC and cranking compression

[travis]

As long as the intake valve closing point remains the same, does the cranking compression and general fuel octane compatibility stay about the same? I’m referring to street performance builds, not really all out racing applications

[digger]

As long as the intake valve closing point remains the same, does the cranking compression and general fuel octane compatibility stay about the same? I’m referring to street performance builds, not really all out racing applications

Cranking compression will be fairly the similar.

Fuel octane No, there is a relationship based on cylinder fill, the other 3 events contribute to cylinder fill to though typically not as strongly

[Nut124]

Travis, what are you trying to do? Are you changing static CR?

Not much correlation between cranking compression and octane requirement. Hypothetically, consider two cams with the same (why??) IVC and the same static CR:
1. short duration, low lift: Low rpm, low VE, low octane requirement
2. Long duration, high lift: High rpm, high VE, high octane req.

[David Redszus]

As long as the intake valve closing point remains the same, does the cranking compression and general fuel octane compatibility stay about the same? I’m referring to street performance builds, not really all out racing applications

As long as chamber volume, displacement volume and IVC remain the same, the cranking compression ratio will remain unchanged. However, the engine does not care about ratios; it likes real numbers.

Other factors that produce real numbers are: inlet air temperature, inlet air pressure, throttle, rpm, etc.

The factors that effect octane requirements (beside fuel properties) are mainly temperature, not pressure.
Compression temperature, by itself, has little impact on detonation. But, elevated compression temperature
will increase combustion flame speed, and combustion temperature which do have an effect on octane requirement.

[PackardV8]

Travis, what are you trying to do? Are you changing static CR?

Not much correlation between cranking compression and octane requirement. Hypothetically, consider two cams with the same (why??) IVC and the same static CR:
1. short duration, low lift: Low rpm, low VE, low octane requirement
2. Long duration, high lift: High rpm, high VE, high octane req.

Help me understand why I had exactly an opposite experience? A short duration had a higher octane requirement.

A Studebaker V8 came in with pinging problems which could not be stopped by timing or fuel octane. The cranking compression tested at 220 PSI. This is far beyond the 190 it should have been. Examination showed the high performance 10.25 engine had been rebuilt, overbored and heads surfaced, which probably raised the compression further. This would have already been a problem, but compounded because they used a standard 252-degree camshaft instead of the correct 260-degree high performance camshaft. Replacing the camshaft with the 260-degree lowered the cranking compression sufficiently to manage the pinging.

[travis]

Wow...I didn't realize that so much of my original post didn't show up (phone issues). Let me try this again...

If you have a built and running engine and want to make a cam swap (no other changes), by keeping the IVC the same would the cranking compression/octane requirements remain about the same? I understand that the running octane requirements *could* change, but I'm not talking huge changes...typical under 6000 rpm street stuff.

In this particular instance I'm looking at...building a near identical engine to one I have now in one of my trucks for a neighbor, but with a comp 270 magnum HFT cam vs my Jones HROL 264/268 cam.

[turbo camino]

An 11:1 383 that was perfectly happy on pump 93 with a 230/230/110 cam suddenly ate all the ring lands after changing to a 206/212/110.

[Nut124]

Travis, what are you trying to do? Are you changing static CR?

Not much correlation between cranking compression and octane requirement. Hypothetically, consider two cams with the same (why??) IVC and the same static CR:
1. short duration, low lift: Low rpm, low VE, low octane requirement
2. Long duration, high lift: High rpm, high VE, high octane req.

Help me understand why I had exactly an opposite experience? A short duration had a higher octane requirement.

A Studebaker V8 came in with pinging problems which could not be stopped by timing or fuel octane. The cranking compression tested at 220 PSI. This is far beyond the 190 it should have been. Examination showed the high performance 10.25 engine had been rebuilt, overbored and heads surfaced, which probably raised the compression further. This would have already been a problem, but compounded because they used a standard 252-degree camshaft instead of the correct 260-degree high performance camshaft. Replacing the camshaft with the 260-degree lowered the cranking compression sufficiently to manage the pinging.

Jack, as you state, in your real life case, the static CR was too high. The longer duration cam delayed IVC and lowered cranking compression and cylinder fill percentage. This reduced cylinder pressures and pinging at the low RPMs you observed. If the Stud engine was race prepped and able to run higher rpms, the longer cam could have made detonation worse yet. Did you check the cam timing? IVC could have been delayed by more than the modest difference in spec duration would suggest.

Using a long duration cam to lower fill rate and reduce detonation is interesting, kind of the opposite of what long duration cams are normally used for. In your case, the longer cam reduced VE and dynamic compression.

In my hypothetical example, the IVC of the two cams was the same as the OP had also suggested, so cranking compression and octane req at low rpm would likely be unchanged.

[MadBill]

Jack, as you state, in your real life case, the static CR was too high. The longer duration cam delayed IVC and lowered cranking compression and cylinder fill percentage. This reduced cylinder pressures and pinging at the low RPMs you observed. If the Stud engine was race prepped and able to run higher rpms, the longer cam could have made detonation worse yet.

Detonation is a time-sensitive phenomenon. When a volume of unburned mixture in a remote corner of the combustion chamber reaches 100% of the conditions required to explode/detonate, there follows a short but well-defined ignition delay interval before it actually does. If the normal flame front arrives during that time, detonation will not occur. As the RPM increases the burn time diminishes, so detonation becomes less likely. Think of driving a sixty year old manual transmission car in high gear on level ground and then up a steepening hill. Pretty soon it requires full throttle to maintain speed and then as the revs drop down and down a light rattle begins under the hood and can quickly grow to engine-shattering proportions. Shift down a gear to pick up some revs and the rattle fades..

[MadBill]

Jack, as you state, in your real life case, the static CR was too high. The longer duration cam delayed IVC and lowered cranking compression and cylinder fill percentage. This reduced cylinder pressures and pinging at the low RPMs you observed. If the Stud engine was race prepped and able to run higher rpms, the longer cam could have made detonation worse yet.

Detonation is a time-sensitive phenomenon. When a volume of unburned mixture in a remote corner of the combustion chamber reaches 100% of the conditions required to explode/detonate, there follows a short but well-defined ignition delay interval before it actually does. If the normal flame front arrives during that time, detonation will not occur. As the RPM increases the burn time diminishes, so the flame front arrives sooner and detonation becomes less likely. Think of driving a sixty year old manual transmission car in high gear on level ground and then up a steepening hill. Pretty soon it requires full throttle to maintain speed and then as the engine lugs down and down a light rattle begins under the hood and can quickly grow to engine-shattering proportions. Shift down a gear to pick up some revs and the rattle fades...