In addition to dreaded milkshaked oil, reversion causes some other problems in any engine. First, the presence of residual exhaust gasses in combustion chamber will dilute the intake charge resulting in reduced forces being applied to the pistons. Next, the increased combustion chamber temperatures resulting from the hot residual exhaust gasses could aggravate a potential (or existing) detonation problem. Additionally, residual exhaust gasses will retard flame front propagation during the combustion cycle.
Looking at the flow numbers listed in
http://users.erols.com/srweiss/tablehdc.htm ; assuming that the port shape was not damaged, how much work was actually done, valve size, etc.; the flow numbers will fall somewhere between the following numbers. Please note that the following exhaust flow numbers will be reduced on your boat because of the log manifolds compared to the flowbench tests which were conducted using a 2-1/8” exhaust tube/pipe.
Intake valve size: 2.08
Exhaust valve size: 1.65
Ford 429 D0VE
Lift: Flow: Int/Exh Ratio:
.100 68/56 82.4%
.200 138/105 76.1%
.300 207/128 61.8%
.400 246/141 57.3%
.500 268/144 53.7%
.600 281/146 51.9
.700 N/A N/A
Intake valve size: 2.19
Exhaust valve size: 1.735
Ford 429 D0VE ported
Lift: Flow: Int/Exh Ratio:
.100 70/59 82.4%
.200 146/119 81.5%
.300 217/149 68.7%
.400 269/167 62.1%
.500 299/174 58.2%
.600 308/180 58.4%
.700 310/- N/A
It is generally accepted that an intake exhaust flow ratio of 75% is considered good for a naturally aspirated performance engine. The D0VE heads are slightly lacking in exhaust flow in both stock and large valve ported versions. The exhaust valve flow deficiency combined with lack of scavenging present in logs (compared to headers) will need to be made up in exhaust valve opening. High exhaust valve lift and rapid acceleration rates are necessary to get the port into a range of high-flow as quickly as possible. A cam lobe with greater intensity, i.e. faster ramps, possibly combined with high-ratio rockers will help to even out the flow deficiency. The exhaust port flow on these heads really starts to die off after .400 valve lift and is done around .600. I wish that the flow numbers for .700 had been included because I think it is important to see if the flow numbers actually start to drop rather than just stalling. If the flow numbers actually drop with increasing valve lift (or if the valve is completely removed), the airflow through the port is becoming too turbulent and quality airflow suffers because too much energy is put into turbulence rather than laminar flow. Airflow can actually reverse direction at this point, something we are trying to avoid. Either way, the result will be substantially reduced power output. My recommendation on total valve lift on the exhaust side would be definitely low .600s, which your desired cam meets that requirement, but not the intensity.
One thing you didn’t mention is the type of intake and carb size you will be using. If you are running a smallish carburetor or a restrictive intake, the restriction to flow on the intake side can result in intake pressures (greater manifold vacuum) that are lower than exhaust pressures, resulting in reversion. Combined with restrictive exhaust, one cylinder, during its blow-down period can actually blow suspended water into an adjacent open exhaust valve during overlap period.
David Vizard Cautions about overlap and low speed drivability. Da Viz primarily writes about engines for cars, however low speed drivability does not present the same issue in a performance street car as it does in a jet boat. What that means to us is that the same reversion that kills low speed drivability in a performance street car also leads to the dreaded milkshake in a jet boat. As valve overlap on a “racier” cam increases, exhaust flow reverses direction through the intake valve at low RPMs. Low RPM means low piston speed which produces low exhaust gas speeds. As the RPM increases, the engine starts to ‘come on the cam’ and the flow direction is as intended.
Overlap is the point where both intake and exhaust valves are open at the same time. When looking at overlap, you want to look at total seat-to-seat valve timing or advertised duration and not the @ .050 duration. The seat-to-seat valve timing is what the engine actually sees. Consider a typical 280 degree cam, the intake valve open in the neighborhood of 30-degrees BTDC and the exhaust valve closes somewhere around 70-degrees ATDC. That is approximately 100-crankshaft degrees that the valves are open at the same time. During this period, both valves could be open some .200. This is to initialize intake flow through scavenging from a header system. Mr. Vizard is adamant about taking advantage of this scavenging energy, claiming that the hardest pull on the intake charge occurs during this overlap period, even before the piston passes TDC and begins its decent towards BDC. He estimated that the scavenging affect with headers compared to manifolds could make a difference of 70 horsepower on a 700 horsepower BBC. If the scavenging affect does not exist, as in the case with log manifolds, overlap should be reduced.
To reduce the overlap period, the Lobe Centerline Angle (LCA) is increased. To bring this back to your original question, “how much duration can be run with log manifolds?” If you wish to increase duration but retain the same overlap: For any given LCA, if duration is increased, that duration will (theoretically) increase equally on both sides of the LCA. Therefore we are only concerned about the added duration on the closing of the exhaust valve and the opening of the intake valve. Thus, for every 2-degrees of advertised duration per lobe, the LSA must be increased 1-degree. That means that if you increase both the intake duration and exhaust duration 2-degrees each, the LCA must be increased 2-degrees, 1-degree for half of the increased duration on the intake and 1-degree for half the increased duration on the exhaust.
The @ .050 duration can be tailored to your desired RPM range. Once the desired lobe profile is selected, then the appropriate LCA can be calculated to arrive at the desired overlap. I have stated several times now that my personal database of cam profiles, shared with me by various NJBA jet boat engine builders, trends toward LCA in the 114 to 120-degree range. The majority of the truly quick jet boats have 118 to 120-degree LCAs. Granted, those cams are installed in engines with dry tuned headers. I also believe that those cams are intended to be more of a “nitrous” cam rather than one better suited to a naturally aspirated program. I do find it interesting that these engines spend most of the run NA rather than on the bottle.
The question NOW becomes, what is the most overlap that can be run with log manifolds?
Cheers,
Joe
