Dynamic Cranking Pressures

[Autoholic]

I've been diving into the complexities of dynamic compression ratios and I was wondering what some of you guys who have been around this green earth longer than I have know about the threshold for cylinder pressures. Since this forum, and my interests, revolve around the FE, lets base this knowledge off of FE pistons, including the SOHC, for N/A and FI. I'm looking at how much the DCP increases with the DCR and the increases aren't huge. I'm seeing 8.18:1 with 165.7 psi and 8.50:1 with 171.94 psi in a a 427 example (4.238" bore, 3.784" stroke and 11.067:1 SCR), at sea level with 0 boost. Please correct me if I'm wrong, but what makes the fuel mixture detonate is too high of a DCP and cylinder temp. I would think if you were around a detonation threshold at say 170 psi, you could decrease the lambda a little bit to give it just enough cooling from the fuel to offset it. This is all based on theory however, so I figured it's time to ask the wise men around here. On a related note, is it correct to set boost to 0 for a N/A engine? Thanks in advance

[My427stang]

First, it's good that you are looking into more than simple math to forecast performance.  Second, hope I fit in the wise man category, at least close enough for this discussion LOL

Using measured pressure is very difficult, even using DCR is difficult if you rely on it as a true measurement.  I am a DCR fan, so don't get me wrong, I use it to tweak cam timing a little, know when combos are extreme, etc.  However, measured pressures can be all over the place

For instance, you could get 30 psi difference based on starter speed, change the valve lift or port size significantly, it changes again, so I would be leery of trying to get to a cranking compression number to determine detonation

As for variables that affect it, its far more than heat and pressure, and you eluded to that with your bore size and mixture discussion

- Mixture
- Temp
- Chamber shape
- Bore size
- Piston design
- Compression
- Cam profile
- Load
- Timing
- Atmospheric pressure
- Air temp
- Fuel (both octane and chemical make up)

I am on my first cup of coffee so there are likely more to talk about.  However, FWIW, my EFI 10.7:1 489 inch wedge FE has a cranking compression of 195 psi and will run on almost anything, and if it gets fussy I can pull a little timing or fatten it up.  My carbed 9.98:1 445 wedge FE has a cranking compression of 185 and it's fussier on fuel.  However, by all counts, the truck 445 should be at least as octane tolerant if not more (tighter quench, smaller bore, better cooling, lower compression), however it's iron head in a 4300 lb vehicle and not EFI

As far as setting boost to zero, if you are saying in a calculator, yes, as long as you do the same thing all the time to allow comparison.  In theory, you could adjust for pressure above or below 29.92 (standard atmospheric pressure) we have to do that when calculating jet engine performance for a given day based on weather, but adjusting boost by that small amount is likely insignificant unless you were comparing Cocoa Beach performance to Denver.

[Autoholic]

Thanks for your reply! I know there are a crap ton of nuances to a running engine's DCR and that any calculations, unless based on actual measurements of every single parameter, are theoretical and should be treated as such. So you should err on the side of caution and go with a slightly lower DCR in order to have a happy engine. But in looking at the problem, there was little difference in DCP between a rather decent jump in DCR. I can easily understand why carb vs efi has to be taken into account when looking at DCR and DCP, efi is like a rifle and carb is lot a shot gun. You're going to have more precise control over the AFR with EFI in most cases (Webers is a whole different discussion).

The change in pressure based on starter speed is very interesting and helpful. 30 psi is a rather significant variable. The WallaceRacing Calculator is what I'm using for this. I'm sure I could find the formula for it but that requires looking at tables or other calculators to get the density of air at various elevation levels. I learned long ago that outsourcing the right things will result in good and proper results and I'd say DCR fits that.

I'm actually in the process of putting a document together that covers my engineering design parameters for a future engine. I'm addicted to cars, so this is a way of scratching my "I have to know more!" itch. The goal is to one day build a 427 SOHC, and one of the issues I plan to dive into is cam design. I believe that the DCR / intake closing angle, intake length, and desired head flow should give you a rather good idea of the cam specs. From there you can apply more universal cam knowledge to narrow it down.

[Autoholic]

Is dynamic cranking pressure the same as mean effective pressure?

[Barry_R]

DCR is a useful predictive tool.  It can tell you if you're way off track.  It can steer you in the right direction.  But please, please, please do not try and optimize a combination around some desired DCR number.  It'll only make you crazy and not necessarily give you any measureable gain between similar combinations of parts.

A running engine is a very different animal.  DCR does not take into account many of the key variables that make or break a real engine.  A modern chamber wedge with a spherical dish piston, fuel injection on a long runner intake, and a high flowing small cross section head will show the exact same DCR as an old school Hemi with a dome piston and huge slow ports with a tunnel ram and two Dominators.  Yet the two extreme examples are obviously going to behave very differently in reality.

[machoneman]

Great example Barry as to why DCR's are valuable but not an end-all.

[TorinoBP88]

DCP does give an idea of what might bend to go in an engine, but the actual pressure filling a cylinder is based on the real world VE (Volumetric Efficiency) which can range from 80% filled to 110% filled in a naturally aspirated engine.

So a high DCP with a terrible intake/head/exhaust combo wont make up the low VE.  Conversely a great VE on an engine with 8.5:1 can still make a ton of power if the cylinders are filling all the way.

[Autoholic]

All points are very true and something I am aware of. I was wondering, in my last post, if DCP and mean effective pressure are the same? Assuming a VE of roughly 100% for peak torque. MEP can be used to determine potential torque. If not, is there a way to determine MEP from VE and other engine parameters?