Advice on Compression Ratio needed

[lewtot184]

i was looking on an engine computer program I have, and yes I know they aren't 100%, but it shows 72% v.e.@ 2000rpm and 97% v.e. @ 5000rpm; so how is calculating a "dynamic" compression ratio at fixed points accurate thru the whole power curve with a 25% difference in v.e.?

what about closing ramp speeds? hydraulic vs solid; grapefruits to apples. I think there are just too many variables for the dynamic compression thing to be 100%.

 

[wyrmrider]

Mike Jones has 40 years plus his great dad Dick before he and brother Rick doing computer simulation and engine component/ camshaft selection
you can't do any better
go to his website and check out what input data he's looking for

 

[wsutard]

When using the UEM calculator, and the .050" timing, add the 15 degrees. Other calculators like the Wallace, use the advertised cam duration and the +15 degrees is not used as it is already figured into the calculation.

If you are using the UEM calculator and the timing @ .006 lift, do not ad the 15 degrees.

Dynamic compression is figured from when the intake valve closes. At .050" lift the valve is open .050" x the rocker ration, be it 1.5, 1.6 or whatever. The UEM calculator wants you to add 15 degrees to the timing and that estimates when the valve physically closes and compression begins to build.

Hopefully this makes some sense.

This settles it. I will use the UEM + timing @ .050" + 15 degrees. This puts me right where I wanted to be in the first place. 8.5 Dynamic.

This all started when my machine shop said that their calculator only uses timing @ .006 lift but we couldn't get that number from Hughes for the solid cam I have.

 

[66Satellite47]

This settles it. I will use the UEM + timing @ .050" + 15 degrees. This puts me right where I wanted to be in the first place. 8.5 Dynamic.

This all started when my machine shop said that their calculator only uses timing @ .006 lift but we couldn't get that number from Hughes for the solid cam I have.

Seems to me "dynamic compression" is just an indicator, a tool to be considered.

 

[451Mopar]

As mentioned, don't get too hung up on DCR numbers. The DCR is supposed to be calculated from the point of the intake valve "appearing" to be closed to airflow at very low engine RPM to estimate engine cranking cylinder pressures. In very general terms it can be used to check if the cam matches up well for the static compression ratio. Knowing some other information like quench distance, chamber design and spark plug location, vehicle application and such, then comparisons can be made to other known builds that either run on pump gas or need higher octane fuel.
Some overlooked factors in addition to Quench and DCR are:
1 - Temp of the incoming air/fuel charge. Compressing the air/fuel increases its temperature. If start with hot air, the engine is more likely to see pinging compared with starting with cold air.
2 - Engine operating RPM. Pinging usually occurs at lower RPMs when there is more time to create an additional flame front. At higher RPM there is more cylinder pressure, but less time to form an abnormal flame front, and there is much more air and fuel cycling through the cylinders.
3 - Engine loading/acceleration. Really the rate of change in RPM, and how the cylinder/heads build and release heat. A light car with a high stall converter (lightly loaded and fast accelerating engine) can run much higher DCR than something like a dump truck (heavy loaded and slow acceleration.)

Just saying that when someone posts their "pump gas" DCR or compression, you also need to know the rest of the story.

 

[Garys1969RR]

Well said, 451. Another factor that affects an engines tendency to ping, is altitude. There are so many variables that affect this, which makes every engine and application different. It may not ping under load in first, but it might when under load in third gear. Use tight quench, to help with combustion chamber burn characteristics.

 

[451Mopar]

Well said, 451. Another factor that affects an engines tendency to ping, is altitude. There are so many variables that affect this, which makes every engine and application different. It may not ping under load in first, but it might when under load in third gear. Use tight quench, to help with combustion chamber burn characteristics.

Your right, the altitude gets calculated into the compression pressure. I think something around 12 psi here vs. 14.7 at sea level.
Don't go too tight on quench, seen the results when the engine (440 with maybe 0.018" to 0.020") was over reved and the pistons hit the head, ouch!

 

[BSB67]

When using the UEM calculator, and the .050" timing, add the 15 degrees. Other calculators like the Wallace, use the advertised cam duration and the +15 degrees is not used as it is already figured into the calculation.

If you are using the UEM calculator and the timing @ .006 lift, do not ad the 15 degrees.

Dynamic compression is figured from when the intake valve closes. At .050" lift the valve is open .050" x the rocker ration, be it 1.5, 1.6 or whatever. The UEM calculator wants you to add 15 degrees to the timing and that estimates when the valve physically closes and compression begins to build.

Hopefully this makes some sense.

Agreed. The problem is that the calculator result you end up with when using advertised, or actual seat timing, or 0.006" lift, don't seem to align with the normally accepted convention for an acceptable DCR, and the dynamic cylinder pressure generated by Wallace is considerably different (lower) than what is actually measured. So after using the intake closing point, now you have the theoretically perfectly determined DCR using the Wallace, and the result is a DCR of 8.0 and dynamic cranking pressure is 160 psi at 1000 ft. Will will this aluminum head motor run on 93 pump gas?

 

[451Mopar]

So after using the intake closing point, now you have the theoretically perfectly determined DCR using the Wallace, and the result is a DCR of 8.0 and dynamic cranking pressure is 160 psi at 1000 ft. Will will this aluminum head motor run on 93 pump gas?

Yes, if those are the correct numbers.

 

[IQ52]

Well of course all these calculations are just to get you close to where you want to be and warn you if you are way off base. All my efforts anymore are with pump gas. If I start calculating and end up with 7.5 dynamic I know I'm leaving something on the table and i can change a few things to crank it up. If I'm at 9.5 I've got a potential problem and I need to reevaluate. I've got a hydraulic cam engine going together right now at 11:1 and 8.7 dynamic @ 2,800' elevation that I believe will be a safe combinati

Agreed. The problem is that the calculator result you end up with when using advertised, or actual seat timing, or 0.006" lift, don't seem to align with the normally accepted convention for an acceptable DCR, and the dynamic cylinder pressure generated by Wallace is considerably different (lower) than what is actually measured. So after using the intake closing point, now you have the theoretically perfectly determined DCR using the Wallace, and the result is a DCR of 8.0 and dynamic cranking pressure is 160 psi at 1000 ft. Will will this aluminum head motor run on 93 pump gas?

Will it run? Isn't that kind of vague? So here is my silly answer.

No it won't even start. You'll wear the battery down to nothing and burn up the starter before you can even drive it.

I've had a 451ci stroker make 554 RWHP at 7.7:1 dynamic with 91 octane.

Oh, and aluminum? It's got nothing to do with it. The advantage with aluminum heads in a MOPAR is the better combustion chamber shape over the factory iron chambers. The compression ratio has nothing to do with heat transfer and the aluminum does not require more compression to make equal horsepower.

 

[BSB67]

Oh, and aluminum? It's got nothing to do with it. The advantage with aluminum heads in a MOPAR is the better combustion chamber shape over the factory iron chambers. The compression ratio has nothing to do with heat transfer and the aluminum does not require more compression to make equal horsepower.

Interesting.

I think you are inferring that my post regarding an example DCR and detonation is so vague that it is silly. Okay. Maybe you're right. But it does support my larger point that DCR calculators do not, or cannot answer the question of detonation.

But it seems you have developed a very clear understanding and a response for things I did not say.

 

[Garys1969RR]

I would put it together right up to the edge. Then if it pings you can raise the octane, or add water/methanol injection. If an engine pings at 80 degrees air temp, 2000 RPM WOT in 3rd gear under load, it may not, or is less likely to ping at 60 degrees, 4000 RPM WOT in 2nd gear. Just experiment and see what happens. These ideas are all just guidelines.

 

[BSB67]

I've had a 451ci stroker make 554 RWHP at 7.7:1 dynamic with 91 octane.

What was the actual measured cranking cylinder pressure?

 

[IQ52]

Interesting.

I think you are inferring that my post regarding an example DCR and detonation is so vague that it is silly. Okay. Maybe you're right. But it does support my larger point that DCR calculators do not, or cannot answer the question of detonation.

But it seems you have developed a very clear understanding and a response for things I did not say.

Your question wasn't silly, it was vague. Run how?

My answer was silly, "It won't even start"? Of course it will start. Then what?

What was the actual measured cranking cylinder pressure?

That was 10 years ago and I didn't keep notes on the cranking compression. Best recollection? Less than 125psi.

I just went back and recalculated the dynamic on that engine and it was not 7.7:1. It was 6.7:1.

 

[IQ52]

I would put it together right up to the edge. Then if it pings you can raise the octane, or add water/methanol injection. If an engine pings at 80 degrees air temp, 2000 RPM WOT in 3rd gear under load, it may not, or is less likely to ping at 60 degrees, 4000 RPM WOT in 2nd gear. Just experiment and see what happens. These ideas are all just guidelines.

I can agree with most everything here but the first sentence. No way would I take it to the edge and then deal with the potential problems. It's just not necessary to take it to the edge to get sufficient power out of an engine.

 

[lewtot184]

I can agree with most everything here but the first sentence. No way would I take it to the edge and then deal with the potential problems. It's just not necessary to take it to the edge to get sufficient power out of an engine.

I agree. i wouldn't take an engine to the edge either. it's not easy to walk it back. building for street pump gas is not like building for race where there are multiple fuels. the pump gas thing is pretty much fixed where ever you go. I stay conservative on this stuff. if somebody makes more power than me I couldn't care less.

 

[451Mopar]

Hear is a link to a good source, description, and equations used in calculating DCR and cranking cylinder pressure:

http://victorylibrary.com/mopar/cam-tech-c.htm

 

[Garys1969RR]

I can agree with most everything here but the first sentence. No way would I take it to the edge and then deal with the potential problems. It's just not necessary to take it to the edge to get sufficient power out of an engine.

I suppose you're right on that. I tend to push it to the edge, just to see where it is. Lol. I ran a street/strip 451 with 13 to 1 C /Ratio, and a 509 cam. Tight quench, rich side on the jetting, 38 degrees total timing. The motor was super responsive. It would run great with a 50-50 mix of premium and 100 octane race fuel. Any lower octane would ping. That RR ran low 12s all day long. This is at 5500 feet MSL. I wouldn't try this at a lower altitude.

 

[BSB67]

Yes, if those are the correct numbers.

Looks like you managed through the vagueness alright.

The relationship between those numbers is correct on the Wallace calculator, i.e. the 8.0 DCR and 160psi. And if we are using actual seat timing, I don't think it will run on pump gas without detonation.

Here is the real life example.
Static compression: 10:8:1
Actual measured intake closing (0.006" valve off the seat w/ 0.023" lash) 77° ABDC
4.25" arm
7.1" rod
DCR 7.7:1 (From Wallace)
Dynamic cranking pressure 147psi (From Wallace)

This motor has an actual cranking cylinder pressure of 195 psi (0.023" lash) with a standard factory starter, and is on the edge of detonation with some winter blends of 92 octane fuels available locally. You'll have to assume that I don't have something stupid going on with the tune. Pretty hard to imagine that if I put a smaller cam in (10° less total seat timing w/ everything else the same = 8.0:1 DCR) it would run on pump fuel.

And to be clear, my point is simply this, aimlessly stuffing numbers into a calculator and thinking you can conclude definitively something about detonation without some personal basis to draw from in making your conclusions is flawed.

From the Victory Library:
"To sum up: DCR a useful tool, but widely perceived to be of greater worth than can be supported by physics"

 

[66Satellite47]

For what it's worth, here's my real world example. My 500" motor runs just fine on 92 pump no ethanol. With head off, no evidence of detonation on piston or head.
Just re-ran the numbers on my motor:
4.385 bore
4.15 stroke
6.6 cc valve relief
88 cc EDE Performer RPM's
.010 deck
.051 thick gasket
4.41 gasket bore
10.35 static CR
6.768 rod
63* ABDC intake @ .015 lift
1000 ft
8.37 dynamic CR