Too much cranking compression

[pcnsd]

Back to my original questions, rephrased:

What is too much DCR?

Should a cam ever be selected with a certain DCR in mind?

In Don Terrills book, "The Horsepower Chain" He references a "critical compression" ratio for gas of 11.5:1, Alcohol 13.5:1. These are do not exceed numbers for compression and are based on the VE Effective CR. VE Eff CR is the DCR acted on by the VE. You need the VE Eff CR to equal swept area after valve closure plus chamber volume for the VE Eff CR to equal the DCR as most calculate it. He also provides a calculation for positioning your ICL based on the VE Eff CR. Very high CR's with high VE get retarded cam positioning to dump pressure and remain below the critical CR.

A book worthy of a place on the shelf IMO.

Paul

[MadBill]

My answer would start with a question: how did the customer arrive at these cams specs and CR? If he's been circling in on the optimum cams through many iterations and also finding gains with each increase in CR, then there is nothing to 'fix'; just keep on trucking. (but don't try to get by with 50% pump gas! )

If the 'tune' came out of thin air or unproven crossover assumptions from a different application, then "too much DCR" would be any number that provoked detonation within the operating rev range on the best available fuel and/or one that lost power Vs. a lower value. It would never be an arbitrary number on a compression gauge.

FWIW, many decades ago a friend of mine bolted a much-milled aluminum head for an 850 cc Mini Minor onto his stock-cammed Morris 1100 (cc). It was a real rocket on Sunoco 280 pump gas (~ 107 Octane, IIRC), but kept blowing head gaskets. After the second one went out, we checked the cranking compression. It pegged the 300# gauge...

BTW, I don't accept the logic of including VE in any discussion of DCR* re a high RPM race engine. (*itself a controversial topic as a stroll through the S/T 'Search' function will reveal) Detonation is a very time-related phenomenon and is far more prevalent at the bottom of a race engine's rev range, normally far below the peak VE. For example, a shop I frequent specializes in 572" to 678" Hemis. Some of his strongest engines run over 16.5:1 CR and produce V.E.s in the high 'one-teens' , but with huge cam durations, have cranking compression in the 165-175# range...

[My427stang]

Even if we had 15 psi atmospheric pressure, you'd need 20:1

because cranking pressure is obviously static CR × atmospheric pressure, right?

No, it's not.

I won't argue with you as I understand there is energy added to the system from the starter and negative exhaust pulses and inertia will cause additional cylinder fill, but it seems real high to me in an engine turning relatively slow and not firing. Adding approx 32% more cylinder pressure than atmospheric pressure alone seems high.

Admittedly I have not built anything with that much compression, and the other posts seem to indicate it's plausible or at least not crazy, so I don't want to take away from the OPs question, but 300 still seems high. Verifying with a second gauge would be easy.

As a devil's advocate, my own 11:1 street builds crank at 23% higher than compression x atmospheric, so arguing with myself, if cranking speed is high and the ports are efficient, I guess I can be easily swayed

[MadBill]

..As a devil's advocate, my own 11:1 street builds crank at 23% higher than compression x atmospheric, so arguing with myself, if cranking speed is high and the ports are efficient, I guess I can be easily swayed

It's way more complicated than that. What you say would be pretty close for isothermal compression, which would mean that the cylinder contents would remain at a constant temperature throughout the process, the heat of compression somehow being entirely 'bled off'. Real world compression in a cranked engine is closer (but not entirely) adiabatic, which means no heat is lost through the cylinder walls, etc. during compression.

Google the italicized terms to learn more than you really want to on the subject.

[My427stang]

Good stuff, thanks I will look.

I never thought that it was simply atmospheric x ratio, but the differences seemed too great, I will say though I didn't consider increasing air temp at all. I will look close at the terms. Thanks!

[teamrude]

Nice post, Most people forget temp increase. How do you think a diesel works anyway?????????

[MrBo]

Now-a-days I would feel very uncomfortable running a new combo without knowing the cranking compression or DCR. In about 2008 I had a setup I thought would crank about 195 psi…it cranked 235 psi.

I can understand people thinking it is a useless metric, and why it may be a very controversial topic.
They probably think people who use it, look at it like a performance meter.
Speaking for myself, I look at it like a DANGER meter.(or change gas meter)

In the 1970’s I popped a hole the size of a quarter in a cast Buick piston. A couple of years later I melted the side of a forged piston in a chev 327. I know what that stuff looks like first hand.

When I started racing legally at a track, my dad (a real tuner) gave me a spark plug magnifier/light.

[My427stang]

Nice post, Most people forget temp increase. How do you think a diesel works anyway?????????

Indeed, brain fart more than lack of knowledge. That's what happens when a diesel mechanic flies airplanes for 19 years, older brain cells die off, never to return! Probably why I can never find my keys or cell phone too

[plovett]

PV=nRT baby. I love that damn law.

JMO,

paulie

[DCal]

As a piston maker I see problems after 235-240 cranking. I know there will be some who have gotten away with more but for the average engine builder this is a good number.

[racear2865]

Im telling yall my gage will not go to 300 for a reason.
reed

[Stan Weiss]

As a piston maker I see problems after 235-240 cranking. I know there will be some who have gotten away with more but for the average engine builder this is a good number.

As a piston manufacturer, on the power stroke what is the max (peak) cylinder pressure that you would want to see?

Stan

[DCal]

Stan. I don't have Scientific data on my numbers but I do have 42 years of looking at damaged pistons and have contributed some of that data from my own motors unfortunately. It's my first question when looking at damaged pistons and the second is "how much timing?" Racers are finally backing that down with the lower valve angles and plug placements that are available today and going faster.I remember an F/ED customer with 280 Cranking and 15.2 CR and he murdered the pistons but he wouldn't change the combo. After a valve job (52cc to 55cc)his CR dropped, his Cranking dropped and his speed went up 3 mph setting a new record. Usually, when I see a car that's not running good MPH based on the ET it has too much cylinder pressure, Sorry that I don't have Scientific data for you but don't throw out years of observation that some of us old-timers have and I don't mean that sarcastically. We get to learn all the time from scientic data, dynos and practical usage.

[Stan Weiss]

Stan. I don't have Scientific data on my numbers but I do have 42 years of looking at damaged pistons and have contributed some of that data from my own motors unfortunately. It's my first question when looking at damaged pistons and the second is "how much timing?" Racers are finally backing that down with the lower valve angles and plug placements that are available today and going faster.I remember an F/ED customer with 280 Cranking and 15.2 CR and he murdered the pistons but he wouldn't change the combo. After a valve job (52cc to 55cc)his CR dropped, his Cranking dropped and his speed went up 3 mph setting a new record. Usually, when I see a car that's not running good MPH based on the ET it has too much cylinder pressure, Sorry that I don't have Scientific data for you but don't throw out years of observation that some of us old-timers have and I don't mean that sarcastically. We get to learn all the time from scientic data, dynos and practical usage.

First I have nothing against old-timers, I am one of them. Also I would never throw out or ignore empirical data. Since we are talking about cranking cylinder pressure at starter motor RPM and different (trapped) VE's at higher RPM can cause the same cranking compression to have greater or less cylinder pressure I was just hope you might a number.

Stan

[jeff swisher]

Cranking speed does change cranking pressure even on a little 355"
Buddy changed his starter to the newer smaller style and gained 20psi
I always checked with carb closed.. Then for giggles i checked my 210 psi cranking pressure engine and got 225 psi with the carb open.

I had a 12.02 compression 350 with 270H comp cam 224@.050.. cranking pressure with old style starter and carb closed
was 245psi.. ran great for 70,000+ miles with 41 total timing and 93 octane. 38 total timing with 91 octane.
54cc chambers. .028 piston to head clearance... this includes gasket.

Cast 345np pistons not Hypereutectic just regular 345NP.
Pulled trailers and even ran with 229 gears for a while... never hurt a piston with that tune.
Tried 2 different gauges.. same reading. Pick up screen broke off the pick up tube and it was shelved
I would think the bike engine spins quicker and at the compression you state i can understand the 300psi.