Big Issues with Cylinder Pressure (DCR)

[btwick]

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.

[F-BIRD'88]

You are all worried about a bunch of hoooeie.

Set the timing at 34 deg.. jet the carb right, use correct plugs and the best avail pump gas and drive it.

10.4:1 cr is not excessive.. Over camming it will not fix it.
Ditch the dcr calc...

[rustbucket79]

Engine will be fine, at a minimum the cylinder with the 236 # had lifter bleed down.

If you’re paranoid set total timing to 32 degrees total, slap in the stiffest advance springs you can find, and drive it.

I never wasted my time learning about DCR, just built them with enough static as I deemed safe for the application.

You’re build actually looks well matched.

[mt-engines]

DCR is just a bs formula for guys that have too much time to waste time plugging numbers into. Your build seems good. run it

[Stan Weiss]

Hi,
· 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)

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.

Thanks in advance.

Did you degree the cam in at 106 ICL?

Why did you use a squirt of oil for the cranking compression test?

Stan

[rebelrouser]

I would agree with the group, run good pump gas, and play with the timing curve. If you are using calculators, programs are out there that would calculate a timing curve to start with. If you are really worried, water injection, might be something to look into. I have built similar engines, and no issues running them on the street. As mentioned, tune the carbs, adjust the curve, all will be good, if you get it right.

[MadBill]

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

[steve cowan]

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.

[btwick]

Hi,
· 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)

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.

Thanks in advance.

Did you degree the cam in at 106 ICL?

Why did you use a squirt of oil for the cranking compression test?

Stan

Hi Stan,

I had a shop assemble the short block for me and degree the cam. I will check with them, but imagine they installed it dot to dot and checked the 6* built in advance with their wheel. Oh, and the oil in the cylinders.. The motor was run on the stand about a month ago, and so I primed the oil pump for a minute or so to get oil in the valve train and oil to prevent dry scraping in the new, non-broken in nor sealed rings. Might have been overkill, but knew it wouldn't affect anything in the data.

[Geoff2]

A good cooling system will help tame detonation tendency.

[Belgian1979]

Seems good to go with alu heads.

[PSA]

Give the engine cold air and keep the engine temp at a reasonable level, adjust the rest with the ignition to keep it safe.

[bigmike]

Are we taking 91 or 93 octane pump gas? I guess I'll be the lone wolf here but the flat top 383 SBC with 64cc heads around here never seem to last long. The combonation of west coast 91 octane pump gas and near sea level 200' - 600' and highway gears gets to them even with the timming cranked down. Scored pistons seems like a common problem but I also seen one that broke a main cap and also had scored pistons. I would rather have a little lower compression 9.5 - 9.7 to 1 Static Compression Ratio with good timming that can run in a real world 190 - 210 deg F without having a fit. Sure running a SBC at 180 deg helps but how many cars and trucks can do this in traffic or at a cruise in.

[My427stang]

So, easy to jab at DCR, which isn't dynamic, it's just valve event adjusted mechanical CR, but SCR never "happens" either.

Now, live and die by DCR, build by DCR, arm wrestle over a single DCR, silly too. However, "Will 10:1 run on the street on pump gas" is as unanswerable as will 8.4 DCR live on the street. Also, can't really say that it's perfectly accurate at the valve when measuring from the lobe. End result... It depends

Let me give you some of my experience, which I use it on every build, usually as a check figure on a common set of engines when I tweak something

1 - See the difference in all the programs, all you can do if you want to use it, is pick one and learn trends, unusable without trending
2 - I use Pkelleys and you are on the highest limit of OK for the combos I use. Have used it for many years
3 - My combos are bigger bore, usually a crappier chamber, and generally need 38 degrees total, even with a .041 quench on pump premium (all FE and Poncho stuff)

If I could extend it to you, I would say that your chamber and bore are smaller and likely better shape (more octane tolerant), even though the quench is on the high side of OK, my guess would be that the timing curve will be flatter than my common iron builds, you have alum heads (which I didn't want to believe mattered, but seems to a smidge) and as long as you don't start with too much initial or too fast of a rate, or too lean, you will be fine.

I cannot comment on any of the numbers other than the PKelley, but I trust that 8.4, if you actually measured everything, to give me some info along with the other things you did. If I were to tweak it, I would only retard 2 degrees, you are that close. More and the cam may not behave the way you want

Now, here is how I pick based on your comment for a given tight quench short block (minus pistons).

1 - Intake, head and headers to meet operating range
2 - Cam to support those pieces exactly where I want it
3 - Compression to match cam and chamber design. (Not DCR only, a logical SCR, DCR, quench distance, dome/dish design, etc)

Parts get a vote, so there is some wiggle room, and it really isn't that linear, but makes a point. On a street machine, I tend to focus on compression being "enough and not too much" not one of the bragging points of the build

I'd say run it

[CamKing]

You will be fine with 91 octane gas.
DCR is useless, and based on the false assumption, that the cylinder stops filling at BDC. That is far from the case.
DCR has no relationship to what's going on, in a running engine.