Camshaft Overlap@.050" vs Idle Quality

[pastry_chef]

I think I had seen Paul Carter, a very well respected pontiac builder, say that the 704 camshaft was very explosive when you step on the throttle in the 455-467 Pontiacs, and he actually seemed to think the Idle of the 704 wasn't all that radical at all, but I know that depends on the person, some people's "radical", isn't bad at all to someone else.

Agree with Paul there.
Yes, it is a subjective topic, different people have different feelings about.
Rhoads lifters are not needed but using the Rhoads V-Max would give you some adjustment there.

[Procharged 434]

I think I had seen Paul Carter, a very well respected pontiac builder, say that the 704 camshaft was very explosive when you step on the throttle in the 455-467 Pontiacs, and he actually seemed to think the Idle of the 704 wasn't all that radical at all, but I know that depends on the person, some people's "radical", isn't bad at all to someone else.

Agree with Paul there.
Yes, it is a subjective topic, different people have different feelings about.
Rhoads lifters are not needed but using the Rhoads V-Max would give you some adjustment there.

I know it depends on several factors, but what is a rough estimate for the amount of vacuum the 704 would produce in a 9.35-1 467 with standard lifters, at say 800-850 RPM?

[Old School]

I can't answer for the 704 but last week I was adjusting on a 402 bbc, 9-1, newly built. Cam was a comp hr with 230-236@.050, 110 lobe separation. Vacuum was in the 10-5 to 11 at 800 rpm idle. The engine also had a working pcv that may have cost a little vacuum.

To answer your original question about intensity and vacuum. Every time I have gone to a more radical cam while keeping the seat duration and lobe separation basically the same I have noticed that it was louder and not as smooth, idle was rougher. By smooth I mean it had more of a "cam sound". This would be changes of 5 to 8 degrees duration at .050 and around 10 or more at .200. I have never checked vacuum back to back so I cannot definitely say the vacuum would be less although I suspect it would be a little less.

If your car were mine I would go with the smoother cam. The 704 might make a little more power but at the rpm's a street car operates the most of the time it is not worth a few horsepower.

[Procharged 434]

I can't answer for the 704 but last week I was adjusting on a 402 bbc, 9-1, newly built. Cam was a comp hr with 230-236@.050, 110 lobe separation. Vacuum was in the 10-5 to 11 at 800 rpm idle. The engine also had a working pcv that may have cost a little vacuum.

To answer your original question about intensity and vacuum. Every time I have gone to a more radical cam while keeping the seat duration and lobe separation basically the same I have noticed that it was louder and not as smooth, idle was rougher. By smooth I mean it had more of a "cam sound". This would be changes of 5 to 8 degrees duration at .050 and around 10 or more at .200. I have never checked vacuum back to back so I cannot definitely say the vacuum would be less although I suspect it would be a little less.

If your car were mine I would go with the smoother cam. The 704 might make a little more power but at the rpm's a street car operates the most of the time it is not worth a few horsepower.

Thanks, after all of the research I've done between the two camshafts, I was under the assumption that the 704 camshaft produced more explosive low end power than the "smoother" Crower camshaft. Most agree that the Crower camshaft is an excellent choice for an application like this, but I have seen some say that is "feels" boring when you step on the gas compared to the 704 camshaft. One would think with results like that, the 704 would have better drivability than the Crower, even if it did have a little more sound at idle.

[Procharged 434]

Finally got the Stage 2 qjet from Sean Murphy for it yesterday, it looks fantastic. 76 Jet/46K Primary Rod/DA Secondary Rod..... hope it runs as good as it looks.....

[ClassKing]

In my building of pontiac street engines I would take the 112 cam for sure.Tom

heh, heh.. oh Tommy.

My King Street 455 91 octane daily driver high water mark is 626 lbs ft at 3000 rpm. (Stock stroke Stock rod length) And my lobe sep is.... Not 112.

[tjs44]

Thats why your the "king"!

[PRH]

My experience has been that cams with faster ramps, and the resulting additional area within the overlap triangle, idle “rougher”.

[Stan Weiss]

My experience has been that cams with faster ramps, and the resulting additional area within the overlap triangle, idle “rougher”.

This is why Blair looks at overlap area in 4 different segments intake area ivo to evc, intake area ivo to tdc, exhaust area ivo to evc, and exhaust tdc to evc

Stan

_______I__N__T__A__K__E
Rocker_Arm_Ratio_=_1.650_________Valve_Lash_=_0.02100

Intake_BTDC_(IVO_to_TDC)_=_4.559
Intake_Pumping_(TDC_to_BDC)_=_106.103
Intake_Ramming_(BDC_to_IVC)_=_15.992
Intake_Overlap_(IVO_to_EVC)_=_25.558

VALVE_____Lift______Opens___Closes__Duration
_________________Deg_BTDC__Deg_ABDC_____________Area
_________0.00000____47.69_|__84.37_|_312.05_|__61.01
_________0.00600____44.55_|__80.80_|_305.35_|__60.99
_________0.01000____42.73_|__78.67_|_301.39_|__60.98
_________0.02000____38.71_|__73.99_|_292.70_|__60.91
_________0.04000____32.32_|__66.84_|_279.16_|__60.69
_________0.05000____29.68_|__63.94_|_273.61_|__60.56
_________0.10000____18.96_|__52.59_|_251.56_|__59.70
_________0.15000____10.52_|__43.84_|_234.35_|__58.68
_________0.20000_____3.06_|__36.19_|_219.26_|__57.27
_________0.25000____-3.97_|__29.07_|_205.10_|__55.41
_________0.30000___-10.85_|__22.20_|_191.36_|__53.71
_________0.35000___-17.79_|__15.20_|_177.41_|__51.04
_________0.40000___-24.96_|___7.98_|_163.03_|__48.74
_________0.45000___-32.52_|___0.38_|_147.86_|__45.30
_________0.50000___-40.80_|__-7.88_|_131.31_|__41.46
_________0.55000___-50.19_|_-17.26_|_112.55_|__36.16
_________0.60000___-61.45_|_-28.50_|__90.05_|__29.76
_________0.65000___-76.72_|_-43.55_|__59.73_|__20.32
CAM
_________0.00600____56.01_|__92.95_|_328.96_|__39.04
_________0.01000____50.50_|__87.43_|_317.93_|__39.00
_________0.02000____41.86_|__77.65_|_299.51_|__38.83
_________0.04000____30.98_|__65.35_|_276.33_|__38.48
_________0.05000____26.89_|__60.95_|_267.84_|__38.29
_________0.10000____11.47_|__44.82_|_236.29_|__37.07
_________0.15000____-0.70_|__32.36_|_211.67_|__35.53
_________0.20000___-12.08_|__20.95_|_188.87_|__33.37
_________0.25000___-23.72_|___9.23_|_165.51_|__30.83
_________0.30000___-36.39_|__-3.48_|_140.13_|__27.21
_________0.35000___-51.53_|_-18.59_|_109.88_|__22.27
_________0.40000___-72.76_|_-39.65_|__67.59_|__14.31



_______E__X__H__A__U__S__T
Rocker_Arm_Ratio_=_1.650_________Valve_Lash_=_0.02100

Exhaust_Blow-Down_(EVO_to_BDC)_=_14.529
Exhaust_Pumping_(BDC_to_TDC)_=_82.863
Exhaust_ATDC_(TDC_to_EVC)_=_3.255
Exhaust_Overlap_(IVO_to_EVC)_=_16.929

VALVE_____Lift______Opens___Closes__Duration
_________________Deg_BBDC__Deg_ATDC_____________Area
_________0.00000____84.89_|__52.45_|_317.34_|__61.20
_________0.00600____81.92_|__48.12_|_310.04_|__61.19
_________0.01000____80.15_|__45.63_|_305.78_|__61.17
_________0.02000____76.28_|__40.48_|_296.77_|__61.08
_________0.04000____70.06_|__32.70_|_282.75_|__60.84
_________0.05000____67.42_|__29.60_|_277.03_|__60.75
_________0.10000____56.59_|__17.84_|_254.43_|__59.92
_________0.15000____47.85_|___8.73_|_236.59_|__58.63
_________0.20000____40.15_|___0.74_|_220.88_|__57.16
_________0.25000____32.89_|__-6.74_|_206.15_|__55.52
_________0.30000____25.81_|_-13.90_|_191.91_|__53.56
_________0.35000____18.78_|_-21.09_|_177.69_|__51.22
_________0.40000____11.49_|_-28.44_|_163.05_|__48.58
_________0.45000_____3.78_|_-36.17_|_147.61_|__45.14
_________0.50000____-4.64_|_-44.58_|_130.78_|__40.80
_________0.55000___-14.18_|_-54.15_|_111.67_|__36.02
_________0.60000___-25.51_|_-65.58_|__88.91_|__29.06
_________0.65000___-41.10_|_-81.03_|__57.87_|__19.59
CAM
_________0.00600____92.42_|__62.66_|_335.08_|__39.20
_________0.01000____87.53_|__56.19_|_323.71_|__39.15
_________0.02000____79.33_|__44.51_|_303.84_|__39.00
_________0.04000____68.70_|__31.10_|_279.80_|__38.62
_________0.05000____64.62_|__26.50_|_271.12_|__38.44
_________0.10000____48.83_|___9.75_|_238.58_|__37.24
_________0.15000____36.27_|__-3.28_|_212.99_|__35.45
_________0.20000____24.55_|_-15.19_|_189.36_|__33.47
_________0.25000____12.76_|_-27.17_|_165.58_|__30.73
_________0.30000____-0.15_|_-40.11_|_139.74_|__27.12
_________0.35000___-15.52_|_-55.51_|_108.97_|__21.84
_________0.40000___-37.00_|_-77.00_|__65.99_|__13.86

[Stan Weiss]

Thats why your the "king"!

Tom,
"King" of what?????

Stan

[ClassKing]

Thats why your the "king"!

Love ya man. Sure miss Lieby and Tyree something fierce.....

[BLSTIC]

I don’t know the answer to any of these questions, but I do have two thoughts or really further questions: First, for idle rpm (and throttle cracked cruise rpm) engine operating conditions, is it the seat overlap or the overlap at 0.05” that better predicts idle quality? Second, is the answer to the idle quality question very different with EFI ITB stacks vs. dual plane 180-degree carbureted manifold vs. short big runner single plane carb manifold?

Interesting thoughts. Mine are similar.

My best guess/estimation/mental model is that idle quality is largely affected by reversion, which goes up with manifold vacuum at overlap and overlap (*area*/time). ITBs have strong vacuum pulses on the intake stroke but that vacuum decays fast between IVC and IVO so there isn't as much happening at overlap to generate reversion. I'd wager comparing vacuum traces with dual plane vs single plane would show dual plane having similar variations to ITBs but weaker.

[hysteric]

My experience has been that cams with faster ramps, and the resulting additional area within the overlap triangle, idle “rougher”.

This is why Blair looks at overlap area in 4 different segments intake area ivo to evc, intake area ivo to tdc, exhaust area ivo to evc, and exhaust tdc to evc

As in Prof. Gordon Blair?

[Stan Weiss]

My experience has been that cams with faster ramps, and the resulting additional area within the overlap triangle, idle “rougher”.

This is why Blair looks at overlap area in 4 different segments intake area ivo to evc, intake area ivo to tdc, exhaust area ivo to evc, and exhaust tdc to evc

As in Prof. Gordon Blair?

Yes. There is a PDF by Blair and Associates "Back to Basics" which goes into this some.

Stan

[ptuomov]

I don’t know the answer to any of these questions, but I do have two thoughts or really further questions: First, for idle rpm (and throttle cracked cruise rpm) engine operating conditions, is it the seat overlap or the overlap at 0.05” that better predicts idle quality? Second, is the answer to the idle quality question very different with EFI ITB stacks vs. dual plane 180-degree carbureted manifold vs. short big runner single plane carb manifold?

Interesting thoughts. Mine are similar. My best guess/estimation/mental model is that idle quality is largely affected by reversion, which goes up with manifold vacuum at overlap and overlap (*area*/time). ITBs have strong vacuum pulses on the intake stroke but that vacuum decays fast between IVC and IVO so there isn't as much happening at overlap to generate reversion. I'd wager comparing vacuum traces with dual plane vs single plane would show dual plane having similar variations to ITBs but weaker.

I have ended up with the following beliefs on this:

What matters for idle quality is really the stability and consistency of cylinder fillings low rpms.

The IVC doesn’t really matter much for that as long as everything else is stable, at least not for fuel injected engines. The reversion right before IVC doesn’t hurt idle quality, as long as it is consistent.

What does hurt stability is the valve overlap period. If the previous cycle had good filling, the exhaust pulls well and the intake stroke fills well — even better than last time. If the previous cycle had poor filling, then the exhaust has less pull and the intake stroke fills poorly — even poorer than the last time. This sets up an unstable process with the engine either surging or stalling. This low rpm instability is greater the greater the valve overlap.

Importantly, with a plenum manifold with a large volume between the intake valve and the throttle plate, there’s no help coming from the intake side to keep the unstable idle in check. However, with ITBs that have a small volume between the intake valve and the (individual) throttle plate, the destabilizing impact of the overlap is much smaller.