I wonder if I'll get any opinions on my question about cranking compression here? Or...maybe need to quiz guys at SpeedTalk?

 

Most likely the limiting factor at some smaller chamber size becomes shrouding of the valves.

It isn't intended to account for that. It's still just a compression ratio, but gives a better idea of the actual amount of compression going on. That remains just one of several factors that determine cylinder pressure. Flow rates of the entire intake tract, including valve size and angles, air density on any given day, fuel qualities, ignition timing, and some other things I've surely forgotten also help determine actual cylinder pressures. Cylinder pressure is still just one of several factors that determine detonation characteristics.

 

Thanks! That was probably it.


I should have stated (more directly) that the slope/steepness of ramps factors into the accuracy of the calculation. If you get the valve shut "faster", DCR seems likely to be higher.

Flow rates...and length of intake tract is something I might revisit in the near future. Hopefully, you won't mind if I piggyback/resurrect an August thread you PM'd a couple of weeks ago?

Drawing clear conclusions about some of these building considerations might make good "stickies" to add to the top of the forum?

(Unless C4's are getting so old/cheap these days that builds will be fading into the sunset!?)

 

Ya know I've seen pretty complicated math formulas for DCR but those were years ago when I first looked at them and decided to stick with my simple thumb rule for aluminum and iron heads. With the small/short cams I use it was more than adequate. When moving to cams of 280 degrees or more I realize I need something more accurate to optimize the engine using the cam.
Nowadays I see some fairly simple DCR calcs requiring few parameter specifications. My advice is to try as many DCR calcs as you feeling comfortable with its complications. You can compare them also. I guess I have to admit I don't know enough about how the DCR calc works to suggest one for others. But I would recommend using a DCR calc that I can understand what the correct parameters are to use and has a math formula I can use.

I guess what I'm saying is you need to use and compare a DCR calc until you are proficient using it. It may take longer than you expected but it doesn't cost a dime to crunch numbers.

 

I don't know what to say. It is what it is. Now 155psi ain't all that bad and your engine will have a much longer life. As long as you have a high VE/good cylinder filling it will still make good power. And if you really have to have higher compression pressure then a smaller/shorter cam can get you there. Also advancing the intake C/L will help also.

Now I've been reading again from the professionals and some will advance the I/L as much as 12 degrees. They say the motor acts like it has a tighter LSA but not the overlap. Now with a mature motor like yours you might have fun toying with the cam.

Good luck.

 

Yes, but using the intake closing angle accounts for all of that with a single number. The valve closes whenever the valve closes, regardless of the ramp rates that got it closed.

 

I'm thinking a slower/milder ramp has more time between the .050" and fully open/closed event. During that interval, less air gets in -- to be compressed. Less air lowers compression. You can't compress a vacuum.

That said, I understand the DCR (by nature) is an approximation.

 

..... Intake valves close after bottom dead center on the compression stroke ... how far after BDC determines how far up the cylinder the piston is when the intake valve closes ... how far up the piston is determines the actual compression distance or dynamic stroke of the engine which is where the dynamic compression ratio comes from ..... In other words , if the piston has traveled 1" up into the cylinder when the intake valve closes that is where the actual or dynamic compression begins ... your 3.750" stroke engine now has a 2.750" stroke in which to compress the fuel air charge ... this obviously doesn't take VE into account ... this is just swept volume vs chamber volume .....

 

This is true, and we're in the territory of arguing semantics here. But as C409 points out, by definition any type of compression ratio is a simple calculation of the volume of uncompressed space divided by the volume of compressed space. DCR just acknowledges that compression can't actually start taking place until the intake valve is fully closed (as opposed to static CR, which ignores this important point).

What you're describing is really in the realm of VE, not CR. And it is certainly true that different ramp rates affect cylinder filling (VE). It is one of many things that affect VE, such as overlap, port size and shape, air density, rpm, exhaust scavenging, etc. VE and DCR both affect the power, efficiency, and knock resistance of an engine (as do still other variables).

So DCR is a useful data point, but it's important not to conflate it with VE. And it's important to know that neither DCR or VE are sufficient to predict everything about an engine's output or knock characteristics.

 

So this prompted me to do a small search on DCR formulas. What seems like a simple ratio/comparison of the swept piston volume it does require geometry and specifications for rod length and piston location relative to the intake valve closing location.

It also requires accurate data for piston crevice vol/top ring location, piston crown height, deck height, piston dish/dome/valve reliefs. Or just an accurate liquid volume instead. Regardless you need an accurate static volume to begin with.

I do recall formulas I saw decades ago that did include a VE parameter and those calcs seemed intimitating to use. I also read of many confused posters trying to use it. Well maybe time has allowed DCR calcs to progress enough for the hobbiest to use. I see now some fairly simple calcs but again I believe you need to use it enough to become proficient.

 

Gave this some thought today and without seeing the complicated models and it falls apart with rpm. Air flow is measured in CFM some time is an important input. In this case time is RPM as the faster the engines turns the less time for the so called leak to reduce cylinder pressure. Given airflow at lets say 0.002 lift and rpm a correction curve might be drawn up. The tip off was very simple, if memory serves me correctly 8.5 DCR and 155 psi cranking compression were mentioned. The DCR predicted under 125 psi and 155 recorded.

 

Isn't that what we do?!?!

An online calculator I found showed something around 200psi with a 10.3 SCR. It accurately showed percent swept volume and 8.5DCR. It even required my elevation.

I'm left wondering about the accuracy of the gauge, the barometric pressure on the day I tested, and (mostly) if I landed significantly lower in DCR than I planned.

Maybe I need to BOOST it now?!?!

It would be interesting to know where you came up with 125psi?

 

Simple 8.5 DCR X 14.5 PSI (1bar)=123.25 PSI

 

That seems clear and straight forward. Makes sense....except that my cranking pressure comes out correct IF I used static instead!

I'm trying to think of a reason why it would hit full static compression with a compression gauge. Must have to do with multiple strokes (since it takes SEVERAL to hit max PSI).

It'll come to me.... Don't RUSH me! LOL

So...I guess that means the other couple of 383s I found posted -- with pressures around 200psi are running SCR above 13:1? Apparently much bigger cams and more "bleed off".

 

Yes big overlap big bleed off. IF you hit it 50 times wouldn't max pressure be max pressure? It is not like pumping up a tire as it is an open system. And yes this is all conjecture as to what might mean as opposed to hard facts. Oh and the big chambers for an 18 degree head like an AFR that would go on a L98 is all about unshrouding the long side of the port that follows the combustion chamber.

 

I agree. Can't think of anyway it could "squeeze" all the way to static w/o being CLOSED 100% of the stroke. The compression tester I borrow from AutoZone had a schrader valve that held compression between "pumps". Plus, it didn't/doesn't hit full compression WITHOUT cycling several rotations. That alone, makes me wonder. Why not one stroke to get cranking compression?

I have to think that gauge was wrong. This calculator shows my SCR would have to be near 8:1 with a DCR below 7:1 to have a cranking compression near 150psi.

Compared to a stock 350, it's too improved/strong for my combo to be that far off. Compared to a 350, it pulls like a 400+ci motor....even at the lowest rpms.

Maybe it could only hold 150psi (due to the seal or something)? I'm not sure I can even say I [at least] proved the cylinders are the same pressure with that gauge.

FWIW, I was checking to make sure all my valves were seating/sealing correctly....in the presence of "tappet" noise....that's been there since day 1 of the build. Finally decided WTF...I should check, just to make sure tapping is "normal". Considering it's a short-duration/high-lift/steep-ramped cam, I've always assumed that was part of the reason for tick-tick-tick.

 

The length of the hose adds to the chamber volume and it takes several pumps to charge the whole system and then the little check valve in the system only flows so much per stroke.

 

Though I can't argue with the logic of this formula (and admit it sounds like something I would also post), something doesn't add up.

I found several stroker builds posting over 200psi as their cranking pressure. Specifically in the 210-220psi range.

210psi/14.5 = 14.5:1 DCR

With a DYNAMIC compression in the 14.5 range, static would be through the roof. What are we talking about here? 17:1 or more?

I even found posts saying cranking compression for a STOCK motor was in the 170psi range. That's 11.7:1 DCR. No way....

I can't believe "dynamic" pressures are THAT high. Even builds much over 9:1 DCR would be toying with the possibility of detonation.

Something seems off with this approach. Right?



Edit: BTW...Looking at record high/low barometric pressures in Kansas City, 1bar can ONLY vary from 14.2psi - 15.2psi

 

Plus, there's probably some volume loss as the compressed air rises in temperature. In short, the method of applying 14.5psi as a direct multiplier of "compression" doesn't seem to work.

I posted the question at SpeedTalk if you want to pop over there and read their feedback. They (of course) repudiated the accuracy of the cranking compression calculator I found.

They also agreed SCR and DCR were obviously too high if you divide measured compression by 14.5.

That said, I liked the line of thinking...which is why I posted it at SpeedTalk. I was actually hoping to learn a revised version of the calculation since it would seem fairly helpful.

 

What?? There is no volume loss as you heat air you have a gain. Either the volume goes up and pressure stays the same or pressure goes up if volume stays the same. What will happen is the density will go down as temperature goes up.