Yeah...I didn't think about my first sentence/statement long enough. Things expand when heated....DUH.

 

Apparently they did not at Speed talk either from what you wrote or at least what I took away from their statements.

 

This was one reply....
"No different from any other online calculator........they're all wrong.

As to why this is wrong? (yes, I entered some numbers from my unmodified engines and it's wrong by quite a lot)
Varying heat loss with compression, ring leakage as well as an undefined IVC point."

This was the next...
"Did you see the part in your link that says: " temperature must be allowed to return to its prior value for Boyle's Law to hold true."?

Also, you should try another tester. I have encountered several whose Schrader valves have been replaced with normal tire valves; their much heavier springs will produce low readings in a compression gauge."

I mentioned Boyles law because it tends to support your theory of compression vs atmospheric pressure (multiples).

One guy posted that BDC pressure x DCR^1.33 was the formula for POTENTIAL cylinder pressure. 1.33 is the exponential ratio of air I guess. With this formula, it suggests I'd see 250psi during operation of the engine. He added that low cranking speeds and ring leakage were among the reasons you can't measure cranking "potential".

 

It's all trade-offs. Whether you think of it as volume loss, density loss, or an increase in pressure, the end result is the same: you get a higher measured pressure than the ratio of uncompressed:compressed volumes would indicate, due to the rise in temps as power is transferred to the air by the act of compression. If you used very little power to compress it and spent five minutes compressing the volume of air, it would barely rise in temps and that would not be a problem. But of course that doesn't work on an engine because the rings and such are not perfect seals: you'd have volume loss due to leakage.

So you can't really use just Boyle's Law to predict this. You need the Ideal Gas law: PV=nRT. That tells us that if all else stays equal, pressure is: 1) inversely proportional to volume, 2) proportional to the mass of air being compressed, 3) and proportional to temperature. Good luck predicting the temperature gain, though! It's significant, though. Just watching the heat waves off an air compressor head will show that, not to mention the usefulness of intercoolers after superchargers. The bottom line is that your cranking pressure should be higher than just PV=PV would predict.

Just to clarify, of course the actual cylinder pressures are much during operation because the air/fuel mix is ignited. Probably in the range of 1000-1500psi. In diesels it gets close to twice that. The only way to measure "unfired" cylinder - that which you referred to in that quote - would be to have an external source spin the engine at normal operating speeds without actually introducing any fuel into it.

 

Guys, to calculate cranking compression you need to take into account that the compression of the air causes heating of the air which is why 14.7 X DCR does not equal the number you expect to see. This is the best program I have seen that helps you determine cranking compression:

http://rbracing-rsr.com/comprAdvHD.htm

If you want to get a close approximation with just a calculator use this formula

14.7^1.11 X DCR = Cranking compression ratio "Use this when engines have 12 to 1 or less SCR"

14.7^1.15 X DCR = cranking compression ratio "Use this when engines have 12 to 1 or more compression)

The formula change is due to the amount of heating of the air, more compression = more heating which adds in addition pressure. It is an exponential increase so the more compression the larger the multiplier will be. (This is really related to dynamic compression but typically higher compression does = higher DRC but is is dependent on the cam specifications so this is just an estimate)

 

..... Why are y'all obsessing about cranking compression anyway ? .....

 

I am not, I am just answering a question that was asked.

 

Well I'm just cranky anyways.

 

If you read through the thread, you'll see I'm the one that brought it up. Due to an unexpectedly low cranking pressure, I was trying to figure out if all these internet calculators (e.g., Paul Kelley) guided me to an undershoot of my intended build. (Though several years have past).

In the future, I want to avoid suggesting (to anyone) said methods of determining components for "ideal" DCR.

At the very least, I probably need to rerun the cranking test on mine. And, figure out if 150psi is "bad" -- meaning I really undershot intended targets. If stroker builds often land north of 200psi...then it would SEEM I left a lot of efficiency on the table.

Of course, I'm drawing a correlation between DCR and cranking compression. If 210-220psi doesn't detonate, they I question how high DCR can really be? Articles I found suggested 7.5-8.5...which I shot for. Specifically, my build targeted 8.4DCR which would SEEM on the high side -- and ideal for compression/efficiency.

155psi cranking compression seems to imply differently -- IF stock builds are higher as are other stroker builds I've found (with CC listed) on the net).

Mostly, I don't like things that don't make sense...or add up!

 

..... How does your hot rod run ? ..... According to the Wallace calculator my junk has a DCR of 11.47:1 ... 13.4:1 static with cranking compression of 220-235# ... Remember , not all starters spin at the same rpm especially the hi-torque versions ... make your compression test with a battery charger on the battery , wait the same time interval between cylinders tested , and open the throttle blades some so there are no air flow restrictions ... compression tests are intended for cylinder to cylinder comparisons and are a good diagnostic tool ..... sometimes I begin a comp test at #2 cyl ... sometimes at #8 ... just to mix up the sequence a bit .....

 

When it was first launched, I compared directly to a friend's LT4. My response and torque was (is) night/day better than a stock 350. That's why I was so surprised the CC measured 155ish when a "google" of this forum shows stock compression higher than that.

Based on actual performance, I had to think it was wrong. OTOH, if it's right, maybe I should pull the motor, insert flat-tops and find out what stupid-high torque is like!

Seriously, I just never expected my compression to be lower than other stroker HP builds. They have "bigger" cams and bleed off MORE than my small-cammed/torque-build. Plus I never considered that it ran like something with low compression.

11.47 is (what I THOUGHT was) rediculously high compression. Why would sites advertise 7.5-8.5 ideal DCR targets, if you can land so much higher and be OK? I assume yours runs on gas?

I can see how much-faster cranking of a battery-charger might affect results. I also have forged pistons (compared to the stock cast), I tested it bone cold so maybe that has a bearing too. (Looser pistons?) Edit: I also have thinner rings. IIRC...they are a 1mm, 1mm, 3mm set.

 

You could pull a plug & injector plug and run the engine on 7 cyl w/a gauge in the down cyl.

It would be interesting to do that and get pressure readings at various RPM's. I'd expect TPI to have a noticeably higher reading at 3200 RPM than at idle.

 

..... I run E85 ... some of the current forged alloys run pretty tight piston to wall clearances ... my heap has a cam with 261/266 @ .050 duration , intake valve closes kinda early but there is a lot of area under the curve ... target power range is 7200-7500rpm .....

 

I have to ask how your measured your chamber and piston volumes. Did you use liquid or just the published numbers for what parts you have? Using liquid really kills any fudged or inaccurate numbers from the mfr. Now my fairly late model LT1 head chambers were right on but earlier stock casting specs were very conservative. I've found that with using liquid to measure volumes the only error likely places are the piston to wall crevice volume filled with grease and the irregular shape of the head gasket bore.

But if your performance is good enough I would be happy to have a strong running performance motor rather than the stock short block any-day. Its kinda spilled milk now and just something to learn by. And on a last note there could have been a math error in your calculation which happens to us all. Maybe disappointing to you now but wont affect the value of the car one dime.

Heck drive it like you stole it and have fun Gregg.

 

..... I discovered an error in my math when I was figuring my DCR regarding the intake closing number ... I don't have an advertised duration (custom cam) ... my DCR is closer to 10:1 based on an estimated IVC point ... it seems I used an .050" IVC degree number ... Sorry ! .....