Big Issues with Cylinder Pressure (DCR)

[travis]

If it makes the OP feel any better, using Pat Kelley’s DCR calculator my truck shows 8.4:1 DCR, and I’ve had absolutely no pinging issues on 91 octane pump gas. And...this is in Texas heat in a 4900 pound truck, with as much as 40* timing not counting the vacuum advance. My quench is a little tighter at .044”, but other than that I’ve got everything working against me and it hasn’t been an issue

[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

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).

We normally run the PAC 1218X
https://www.racingsprings.com/index.php ... -3903.html

[btwick]

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

Thank you Mike for the cam recommendation.

For fear of hijacking my own thread, your suggested cam brings up two questions. I got the ProMaxx 2169 heads with the car (uninstalled), and they had apparently been bench flowed by the previous owner's machine shop (can’t verify the shop or numbers). Heads are 185 cfm runners, with 2.02/1.64 valves.

The flow numbers in CFM are the following:
Valve
Lift Intake Exhaust
.200 132 86
.300 181 128
.400 227 153
.500 253 162
.520 254 162
.550 254 163

New to reading bench flow sheets, but thinking these numbers tell me two things:

1. The exhaust cfm numbers are below expectation and ProMaxx spec, so while your suggested cam is dual pattern on duration, lift is single pattern. Would this motor benefit from a dual pattern cam with more lift on exhaust to help it flow better?

2. These heads stop increasing cfms, both intake and exhaust, around .50 to .52 of valve lift. Would your cam with .576" lift (w/1.6) on both intake and exhaust be overkill for heads that stop increasing flow at .52 or so? Would this just be addtl. wear and tear on the valve train without much benefit?

Again, not sure I am reading this right, but your post really had me thinking. Thanks!

[skinny z]

If it makes the OP feel any better, using Pat Kelley’s DCR calculator my truck shows 8.4:1 DCR, and I’ve had absolutely no pinging issues on 91 octane pump gas. And...this is in Texas heat in a 4900 pound truck, with as much as 40* timing not counting the vacuum advance. My quench is a little tighter at .044”, but other than that I’ve got everything working against me and it hasn’t been an issue

8.4:1 and iron heads (aftermarket Vortec) were difficult to manage. Then again the majority of my issues were with lean cruise, maximum advance.
Whose cylinder heads are you running?

[CamKing]

1. The exhaust cfm numbers are below expectation and ProMaxx spec, so while your suggested cam is dual pattern on duration, lift is single pattern. Would this motor benefit from a dual pattern cam with more lift on exhaust to help it flow better?

2. These heads stop increasing cfms, both intake and exhaust, around .50 to .52 of valve lift. Would your cam with .576" lift (w/1.6) on both intake and exhaust be overkill for heads that stop increasing flow at .52 or so? Would this just be addtl. wear and tear on the valve train without much benefit?

Again, not sure I am reading this right, but your post really had me thinking. Thanks!

1: Most likely, the reason the exhaust flow numbers are so far below the advertised numbers, is that whoever flowed them, didn't use a pipe extension on the exhaust. Without an extension, the air flow isn't straightened out before the flange, and the flow path doesn't represent what's going on, when you have headers. This can hurt the exhaust flow cfm by 15%. Because the exhaust charge is under so much more pressure, then the intake charge, it vary rarely needs more exhaust lift, then intake lift.

2: As long as the port flow doesn't get turbulent and decrease flow at the higher lifts, going above the lift point where CFM flattens out, will increase power. If your port maxes out at 254cfm at .520" lift, and you put a .525" lift cam in, the engine is only seeing that 254cfm for a couple of degrees. Now, if you go with my .576" lift, the engine sees the 254cfm for 75 degrees.

[btwick]

Got it. Thanks Mike.

[F-BIRD'88]

If, after proper tuning you do happen to see some detonation you can simply remove the heads and open up the combustion chambers by 3-4 cc to bring the cr down a little to be under the autoignition point.
If the cr is high it will only be slightly high and only need a lil taming thru chambersize mods.
Tune it correctly first.. Often those twin elec fans on the
RADIATOR actually BLOCK air flow at speed.

All the cams in the world, at any price will not correct this sort of critical in vehicle install details and correct tuning.

[F-BIRD'88]

Another critical area is the function of the valve cover oil baffles and PCV breather etc.. The prettiest showy valve covers are often the worst offenders starting with the location of the vent holes... Make it efficient at stopping oil migration thru the PCV even if its visually ugly.

If wot power matters Cam Kings cam will likely out perform your comp cam.
Those heads are ripe for port flow/ chamber sweetening too. Finish the job of the as cast form.

[btwick]

Another critical area is the function of the valve cover oil baffles and PCV breather etc.. The prettiest showy valve covers are often the worst offenders starting with the location of the vent holes... Make it efficient at stopping oil migration thru the PCV even if its visually ugly.

If wot power matters Cam Kings cam will likely out perform your comp cam.
Those heads are ripe for port flow/ chamber sweetening too. Finish the job of the as cast form.

No confusion Mike's cam and head work would liven this motor up. Also perhaps bigger carb, 1.75" headers, and RPM air gap, and would be pushing closer to 500HP/500+TQ . But, this is a 71 corvette conv project, with a ton of other resto-mod work over 2 years and I'm in the home stretch of getting it on the road. Know I can always come back and pump it up after I've enjoyed the car a bit.

[F-BIRD'88]

On that vette the dual electric fans are probabily a big mistake... Blocked airflow at speed... The factory GM system is better. big mech clutch fan proper fan shroud.
Proper fan pulley speed. big rad with proper ducting to n front including the critical chin spoiler..

I see trouble. correct ducting of cool air to the carb is also critical chin on these cars.

[btwick]

On that vette the dual electric fans are probabily a big mistake... Blocked airflow at speed... The factory GM system is better. big mech clutch fan proper fan shroud.
Proper fan pulley speed. big rad with proper ducting to n front including the critical chin spoiler..

I see trouble. correct ducting of cool air to the carb is also critical chin on these cars.

Thanks for the heads up and will monitor. Cold air intake is tough on these cars, especially with the standard hood (not LT1/BB hood) which, btw, is why the shorter EPS manifold vs RPM AirGap. And, will need to get airflow through the three core rad, the Vintage Air condenser in front of that, and an auto trans fluid cooler blocking part of that as well. Will be running an 170 degree thermostat, a cooler plug, ensuring the chin spoiler is in tact, and also sealing the aluminum shroud as needed to ensure no issues with rad airflow. Yes, this could be a challenge on warm summer days.

[travis]

If it makes the OP feel any better, using Pat Kelley’s DCR calculator my truck shows 8.4:1 DCR, and I’ve had absolutely no pinging issues on 91 octane pump gas. And...this is in Texas heat in a 4900 pound truck, with as much as 40* timing not counting the vacuum advance. My quench is a little tighter at .044”, but other than that I’ve got everything working against me and it hasn’t been an issue

8.4:1 and iron heads (aftermarket Vortec) were difficult to manage. Then again the majority of my issues were with lean cruise, maximum advance.
Whose cylinder heads are you running?

It’s a 351w, with aluminum Promaxx 9175 heads. And, since I found the sweet spot on timing, I’m testing mid grade 89 octane. Again, no issues...but the weather is much cooler now.

[skinny z]

It's interesting to note, for me anyway, that one of the variables I neglected to consider was inlet air temperature. While I never experienced any overheating issues or even remotely had above average operating temps, with an open element filter case, I'd easily be drawing in air that may have been well beyond acceptable.
I may have had better luck with an aluminium cylinder head but that's not the case nor will it be moving forward.
Next go around is well engineered CAI.

[PackardV8]

For true, a cold air intake is crucial if an engine is on the edge of having too much DCR. Also blocking the exhaust heat riser ports to the intake manifold.

[skinny z]

Not to pull off another ST OP's thread tangent (although I'm sure they would find this all useful), I made all attempts to meet the needs of an edgy engine.
Blocked heat riser (or none in this case with an Air Gap intake), big radiator, big ass electric fan with good control. Even an open cowl hood although it's debatable whether there's a draw or a supply with that. Well developed timing curve considering this is old school with no electronics. AFR and vacuum gauges in the cabin. Etc.
But to the point, I missed the CAI importance and even though it's been part of my performance development plans, I never saw it as a contributor to my difficulties in tuning until the early stages of this thread.
It was Dave Redszus post,

The critical factor is inlet air temperature...

that created the "aha!" moment.
Wish I could go back and address that however, this particular project is mortally wounded and I need to start again.
Thanks to the OP for the loan.