Big Issues with Cylinder Pressure (DCR)

[CamKing]

BTW, if I was going to design a cam for your engine, with a 2,600 stall, and a 6,000rpm peak, this is what I would recommend
Cam# HR72360-73360-110
276/284 @.006"
228/236 @.050"
.576"/.576" Valve Lift w/1.6 rockers
110 LSA
107 ICL

[David Redszus]

Hi,

Hoping for help with my engine project. 383 sbc engine (OEM roller) was assembled and run on a stand for about :10, and now sitting in the car waiting for full install before driving. Intent of the 383 was a fun, torquey motor that would never see a drag strip nor see 6K revs on any consistent basis.

Started with the static compression ratio (chose 10.4:1) and then picked the cam (Comp XR276HR). WRONG!! Recently learned about dynamic compression ratio and how I may have messed this build up pretty good.

Engine specs:
· Bore: 4.030
· Stroke: 3.75
· Rods: 5.7
· Heads: 64cc ProMaxx 2169 with hyd roller springs
· Pistons: 7 cc indents
· Head Gasket: Felpro with .041 compressed height and 4.166 bore
· Deck clearance: .015
· Quench: .056
· Cam: With 1.6 ratio rockers has 224/230 @.050 and .536/.544 lift. 110-degree lobe separation/106-degree intake center line. (pt # 8-423-8)
· Edelbrock EPS Performer intake
· 1 5/8” long tube headers
· HEI ignition
· Holley 700 DP
· Other:
· 3.36:1 rear gear
· 2600 stall (T400)
· 3-core alum radiator, two 12” fans

Started exploring dynamic compression ratio calculators and found them inconsistent and somewhat difficult. Comp cams shows intake valve closing using ABDC degrees at .006 lift, when the calculators want ABDC at 0” lift, or .050 lift, or .050 plus 15 degrees…

Data:
· From my cam card: 64 degrees ABDC at .006” lift
· At .050 using the Go Fast Math calculator: 38 degrees ABDC (has a handy conversion tool to go from a Comp Cams .006 number to a .050 number)
· At .050 plus 15 degrees: 53

Results:
· Go fast math: 9.04:1
· UEM pistons: 9.03
· Wallace Racing: 8.39 (using the .006 ABDC intake closing on the cam card)
· Omni Calculator: 8.4
· Pat Kelly DCR downloadable app: 8.40
· RSR dynamic calc: 9.7:1

Sensing my .006 number is likely wrong for the calculators that don’t specify a lift number like Wallace, Omni, and Pat Kelly’s calc app.

Realizing I had built a motor prone to detonation as the consensus is that while detonation has many variables, staying under 8.5:1 or so for a street motor is advisable. My feeling is even with high octane fuel (or boosters) and detuning the motor (retarded ign timing, running rich) I will still have an issue with detonation.

To get more data, I ran cranking pressure tests yesterday which confirmed the problem. On a motor with maybe :10 run time, and a squirt of oil in the cyls, I was running in a range starting at 205, couple around 220 and one cyl. way up at 236 psi. Was hoping to see it below 200.

The biggest problem now is finding a cam that won’t throw the rest of this build off. For example, Comp has a bigger duration cam to bleed off cylinder pressure with the XR 288 HR, but it shoves the rpm range up to 2500 or so, and upsets my stall number, gear selection, etc. Even then, my dynamic compression is 8.7. Next size larger Comp Cam is the XR 294hr, and it gets my cyl pressure down, but pushes me further up the rev range to 2800, and I get a lift number over .56 when my heads (according a bench flow) really stop increasing cfm flow at .52 lift or so. Everything needed to lower cyl pressure means less grunt down low and a motor that isn’t what I wanted.

Nothing on the Howard Cams site gets me below 9:1 DCR and didn’t find anything on Lunati site either. Even played with lobe sep, going down to 107/108 and up to 112, and not much help at all. Do want a dual pattern cam as exhaust cfm numbers were relatively low vs spec and need help with more lift and duration.

A second, and lower cost option would be a cam adjustment kit to retard the current cam. With 4 degrees retard my dynamic compression would be 8.82 (gofastmath calc) and 6 degrees back gives me 8.7:1. Still higher than I would want but livable with fuel and detuning. Has anyone played with cam timing and noticed a significant difference in motor behavior? Know this option also pushes my powerband higher and deviates greatly from what the cam mfg intended with the cam.

Does anyone have any experience or ideas about how to move forward? Feeling between a rock and hard place right now. Not sure if I should stick with finding a new cam, or perhaps going to 72cc heads.

One thought was to get a custom cam ground, but that doesn’t sound right. A mild-ish 383 street build with 10.4:1 static compression has got to be one of the most ubiquitous hot rod builds on the planet. Not sure how this got so difficult.

Lesson learned--start with cam selection. Then work the rest of the build, especially keeping an eye on quench, to meet a target dynamic compression number using piston indent options, head gasket thickness, deck clearance, etc.

Thanks in advance.

Using the engine specs provided above and assuming a SCR of 10-1, and IVC of 116 deg BTC, the chamber volume would
have to be 86cc. The DCR is 8.07 assuming a fuel vapor mixture (not just air).

But that does provide much useful information. Assuming an inlet pressure of 14.7 psi and IAT of 100F deg:
At TDC the cylinder compression pressure is 236 psia and chamber compression temperature is 768F deg.
That is hot enough to ignite pump gas without a spark. Evaporative fuel cooling will lower this temperature a bit.

But at an ignition angle of 30 deg BTC, the compression pressure would only be 109 psia and the compression
temperature 527F deg which would allow most engines to survive on high octane pump gas, depending
on other chamber and coolant factors.

The critical factor is inlet air temperature, and to some degree, inlet air pressure.

[piston guy]

Static is still static and the limiting factor. Adjusting cam timing events does change cranking pressure , but that's nowhere near combustion pressure which is what we are worried about.

[panic]

Selecting a cam to "bleed off compression" means the cam is wrong.

[NewbVetteGuy]

David Vizard says that 200 cranking psi is a good max target for a hot street engine on premium pump gas...

Thats where I like to run my street stuff as well,the only thing I would change is piston to head clearance to between 28- 40 thou max for NA.

Cold air, cool engine temps, and a tighter squish distance for better quench.

Felpro 1094 0.015" and the 4.1" bore gets you 30 thou; Cometic C5246-023 0.023" x 4.1" gets you 38 thou. Both would be better.


Adam

[skinny z]

All right, DCR and cranking compression notwithstanding, in an engine that has a static compression ratio and cam spec that is what all here would say is borderline pump gas (for the application) at what point is output limited because the timing curve has to be so castrated in order to limit detonation?
Not necessarily WOT either as it is a streetable engine in a chassis that is raced. How about the effects of cruise timing? Maybe with a lean tune to help maximize fuel economy?
FWIW, I've been down this road and have wanted to know which gives the least amount of lost output if not optimized. Compression or ignition timing?

[jeff swisher]

I always push the limits on compression with my own engines.
I want to see just what I can really get by with.

My most compression was a 350" that i still run today with a bit less compression.
I ran it in a 78 Nova and pulled heavy loads behind it quite often 5000+ lb loads and I would not change timing curve.
22-24 initial and 38-41 total depending on if i had 91 or 93 octane.

The pistons were flat top cast 5.5 cc reliefs and .015" gasket and pistons down the hole .013"
Heads were ported and milled 186's 54cc chambers.
Stock 4" bore and stock 3.48 stroke.
Compression was a wee bit more than 11.9 :1
Small cam in that one a comp cams 270H flat tappet 224@ .050 110 LSA 106 intake centerline.

24 5PSI cranking pressure.

I later stuck a thicker head gasket on it a .039" and it was less finicky on the tune when the weather hit 85-90 degrees outside.

Never gave me any issues.
I run that short block today and it has 57 cc heads on it and 280H cam.
I have ran the 268H on a 106 and 112LSA with same 102 intake centerline install.

225 psi cranking pressure.
These new heads are ported 601 casting with worked over chambers, likes 35 total and only 3 wheel HP more if I dial it up to 38 total.

Compression is just about 10.76.

I pull my heavy loads with it also and it is now in a 57 chevy car 4 door.

[My427stang]

BTW, if I was going to design a cam for your engine, with a 2,600 stall, and a 6,000rpm peak, this is what I would recommend
Cam# HR72360-73360-110
276/284 @.006"
228/236 @.050"
.576"/.576" Valve Lift w/1.6 rockers
110 LSA
107 ICL

Which gets it to 8.35, which typically works pretty well. Of course all the lobe specs are not there so no idea how close it is to the XR cam, too many other variables but it does put the guy at ease that he isn't that far off

Selecting a cam to "bleed off compression" means the cam is wrong.

Thinking that is what DCR does is wrong, that is a blanket statement that doesn't do anything for anyone. In fact, I would say that if you need to counter compression, your compression is too high. There is no ability to bleed off compression with an intake lobe.

I won't feed a DCR versus non DCR fight, because nobody converts either way, but this thread did a good job of showing some serious misgivings when using any tool.

Air temp alone wont tell you octane tolerance, also all alone...
- SCR won't, and never exists in operation
- Quench won't, and is different with heat and rotating assembly materials
- IVC won't, which isn't the same when running and not the same from one spring pressure to the next in operation
- and...DCR won't, especially when there are 8 different calculators giving 8 different answers.

But they all give information, if you don't need the information, or it doesn't help, don't use it.

I will say though, the trend within a specific group, can give information, and if too high, will talk to you on a street motor requiring oddball timing curves. Now if a somewhat normal build can act predictably on the street when exceeding any of those limits, DCR, SCR, quench, etc, then the tool is not useless

[David Redszus]

Static is still static and the limiting factor. Adjusting cam timing events does change cranking pressure , but that's nowhere near combustion pressure which is what we are worried about.

Static compression ratio is useless as a predictor of performance unless we are comparing two otherwise identical engines. Never happens.

Dynamic compression ratio also has no value. Engines do not listen to compression ratios. But a DCR is very
valuable since it allows us to calculate cylinder pressures and...more important, cylinder temperature.

Cranking pressure matters little...except that it produces heat prior to ignition which then causes a very
substantial rise in combustion temperature,,,and pressure. Temperature, not pressure causes detonation.

But excessive combustion pressure for the engine design limits, tends to break hard parts. And with vibration, it will break even very strong parts.

[piston guy]

David ,
How do you establish what combination of CR and octane is safe? Heat is obviously generated as fuel and air are compressed , no argument. Controlling air inlet and fuel temperature helps decrease probability of detonation along with chamber shape again no argument. We all know you can't run 87 with 13-1 even with direct injection. Maybe I'm too old in my way of thinking and need more education. This would probably be a four or five minute "in person " instead of this medium which is often "read wrong" . No aggression here!

[skinny z]

BTW, if I was going to design a cam for your engine, with a 2,600 stall, and a 6,000rpm peak, this is what I would recommend
Cam# HR72360-73360-110
276/284 @.006"
228/236 @.050"
.576"/.576" Valve Lift w/1.6 rockers
110 LSA
107 ICL

What spring and retainer would you recommend with that?
Is Comps 26918 and their tool steel retainer a suitable fit?
Spring Rate:372
Seat Pressure:125@1.8"
Coil Bind Height:1.14
Just to be clear this is 10.4:1 and iron heads (in the OP's spec application).

[btwick]

Thanks to everyone for the great input and information. This is an amazing forum.

Will run it as is, and after testing, tuning, and tweaking, if I need to do something more invasive such as heads, cam timing or cam, will manage that afterward. But lots of good education which is much appreciated.

[Nut124]

... But a DCR is very valuable since it allows us to calculate cylinder pressures and...more important, cylinder temperature.

David, could you explain whether/how does DCR predict cylinder pressure and temp at anything beyond slow idle speed?

Is it not the goal of a cam design to maximize VE and torque at the desired operating rpm? This would mean that a properly designed cam, at target RPM, would produce cylinder pressures similar to or in excess of what the same engine would produce in a theoretical cranking test with IVC at BDC?

[David Redszus]

We all know you can't run 87 with 13-1 even with direct injection.

Well why can't we run 87 octane with 13-1 SCR? Such a fuel would typically have a RON of 92 and a MON of 82.
Engines obey MON values, not RON nor index values. Interesting but not helpful.

What causes such a fuel to detonate in said engine? It has nothing to do with compression and everything to do with
temperature. Specifically, the key factor is the auto ignition temperature of the fuel in the end gas region of
the chamber. Studies by Bosch have indicated that when the unburned end gas exceeds 1800R deg, it is subject to
auto ignition. Actually, each fuel has its own auto ignition temperature which is determined by fuel components,
mixture and heat transfer due to charge motion.

Anything that increases inlet charge temperature will increase combustion temperature and end gas motion.

DCR when used with inlet air temperature will allow us to calculate chamber temperature at spark ignition.
From there we need to understand other temperature inputs and chamber charge motion.

In the end, it comes down to a well educated and well calculated...guess.

Of course, a knock sensor helps with the guessing game.

[David Redszus]

... But a DCR is very valuable since it allows us to calculate cylinder pressures and...more important, cylinder temperature.

David, could you explain whether/how does DCR predict cylinder pressure and temp at anything beyond slow idle speed?

Is it not the goal of a cam design to maximize VE and torque at the desired operating rpm? This would mean that a properly designed cam, at target RPM, would produce cylinder pressures similar to or in excess of what the same engine would produce in a theoretical cranking test with IVC at BDC?

DCR, when used with inlet temperature and pressure will predict compression temp and pressure.
VE is maximized at the torque peak which must include induction dynamic effects as well.

For a NA engine, we can use ambient temperature and pressure as realistic conditions of an engine.
For a boosted engine or an engine operating at elevated altitudes, we must adjust temp and inlet
pressure accordingly. My preferred method is to use an inlet air temperature sensor and MAP sensor data.

A running engine will produce variable inlet pressure and temperature readings depending on induction
tuning effects. But idle IAT and IAP readings are often very similar to those found at the torque peak.

An engine with IVC at BDC would indicate a DCR value the same as a SCR value.
In fact, SCR assumes an IVC at BDC. But an early IVC would not properly fill the chamber at target speeds.