Roots 14/1671 supercharger bypass/blow off valve

[CcanDo]

Application:Offshore race/pleasure boat,BBC,800-1300HP @6-7000 RPM,engines are well built w/details available.
Problem:As these engines are used for pleasure, 92-93 octane is used. Due to lack of preferred inner cooler design,no inner cooler is used and engines are over engineered in other areas to control detination.However,throttle settings are constantly changed which is beleived to add heat,flex rotors and encourage detination.
Solution:Find method of installing pressure relief as throttle plates are closing/closed, w/o destroying distribution. Method considered is either going through the burst panel or finding best location in plenum tent, plumbing from tent to bored hole next to distributor to bypass valving to throttle body base plate, using 1.25" tubing.
Suggestions for the Newbee ?

 

[patprimmer]

Where are the throttles, upstream or downstream of the supercharger?

Detonation will not occur by changing the throttle quickly or by closing the throttle.

Detonation will occur if you have to much advance, a slightly lean mixture or at wide throttle openings with high compression ratio.

Regards
Pat
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[CcanDo]

Pat, Dual 4150's or dual 4500's set upstream, on top of a 1"+/- base plate which sets on top of the supercharger case. The carbs are turned 90* to the blower case for closer spacing. CL between the two carbs is adjusted from front to back, subject to case design and size, to enhance distribution.

While constant throttle plate changes don't create heat, there is concern the billet rotors are flexing. The gears also see back lash. The combination is thought to create some degree of heat soak, effecting A/F ratio change. Further, we are concerned, as heat soak builds, puddling increases and distribution is effected. The dyno A/F target is 12.4. Also,hydraulic,is a factor.

The safest method seems to be, oversized case, under driven 8 to 25 %. The boost curve flattens just after peak torque.

 

[patprimmer]

CcanDo

You really do not seem to have any understanding of what is going on.

Blower rotors are normally rigid enough to only have flex problems at WOT with high levels of boost. The longer the rotors the greater the problem.

Heat will have little impact on rotor clearance if you have an aluminium case and rotors as they expand at the same rate for the same temperature change. The rotors may get a little hotter as they have no cooling to one surface like the housing does, but that will be minimal.

You will only get major mixture strength problems if either yor carbys are out to lunch or the heat soak is boiling fuel in the float chambers which is unlikely unless you have cramped the system into a tight enclosed place.

Hydraulic lock is highly unlikely on petrol and carbies unlessyou have a sunken float or bad needle and seat and an electric fuel pump keeps running.



Regards
Pat
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[CcanDo]

Pat,

This thread started by explaining the application and hoping for suggestions that may provide insight. We are concerned that there should be a manageable pressure valve in the system, presently there is none.

The engines are $50-80,000 each, are they breaking, no. However, we need as much safe power as possible. Therefore, the mission is to make it a little better.($20-25,000 to freshen at 200 Hrs. or less)

The cases are magnesium, rotors are stage 3 billet, initial compression is 8.4, boost is 5-7.5 on 93 octane, (C16 allows greater) vac is 10". (when vac hits 8 it's time to freshen)

The engines are run just as cold as possible with maximum water flow. Iron blocks-aluminum heads.

Hydraulic may have been mis-used, puddling is real and hydraulic comes after puddling. The carbs are dialed in and thought to be spot on.

These engines may be run for miles at 5,000+/-/+/-, that's when density altitude in the blower case climbs. WHY ???

 

[IceStationZebra]

Are you running headers or water cooled log type? I was just wondering if after sustained use the exhaust heats up enough to change the scavenging? My other though is something is changing the timing due to heat expansion or lifter pump-up.

ISZ

 

[patprimmer]

Every roots blower manifold I have seen has a blow of valve or burst panel venting to atmosphere if there is excessive pressure from a backfire. No bypass is required nor even desirable with throttle plates on the blower inlet side.

For a moment I forgot this is a boat application and is probably always at sea level in relatively dense humid air.

The use of magnesium blower, aluminium head and iron block in this case is quite alarming re galvanic corrosion.

If I was sticking with roots blowers I would use aluminium cases and billet rotors.

8.4:1 and 7.5# boost from a roots blower and no intercooler on 93 octane is also a bit scary on carbies and I presume a mechanical ignition system timing device.

I Have never seen a 16:71. If I was going bigger than a 14, I would be inclined to try to source the twin 6s that John Force developed. It is a long blower with a centre bulkhead and takes two sets of 6:71 rotors. That fixes reotor flex problems in the blink of an eye.

Are you running salt water through the engine for cooling.

Are you running an intercooler.

When does the inlet valve close

$20,000 sounds like daylight robbery for a freshen up and casts serious doubts on the suitability of the parts being used.

For off shore I would be more inclined to run turbos and salt water to air intercoolers. I would think more like 7:1 CR and 9# boost if intercooled.

I would be using Iron heads and raw water cooling or all aluminium motor and fresh water cooling via a salt to fresh water heat exchanger.

I would ensure extremely good ventilation of the carbys and inlet air ducts and probably the blower housing. If the blower gets hot, I would consider running a thick insulating plate and heat shields under the carby. Make absolutely sure the fuel in the float chamber never boils and starts to perculate through the bowel vents.

If heat shields and insulation plates are still inadequate you could make a water cooled spacer plate and run dead cold raw water through it. Make absolutely sure it cannot possibly leak water into the inlet air tract and will be made of corrosion resistant materials.

I would not run as cold as possible, quite to the contrary, I would try to tune a raw water cooling system so the discharge water is about 80 deg C at sustained high load.

This can be tuned by scoop position, design and placement and water pump size and drive speed. I normally run the pump to keep engine at 60 to 70 deg C discharge temp and install a pump bypass with a non return valve in the bypass.

At the power levels you are looking at I would be running roller cams with over 0.700 lift and 270 to 290deg duration and something like 400# on the seat.

I would be using something like Carrilo rods and real good steel cranks.

I would be using forged pistons.

I would be using something like Ferrara extreme duty valves.

I would be inclined to use a Vertex or Supermag magneto tuned to continuous duty usage. I can put you in contact with a man to do the Maggys. A marine grade MSD is another possibility.

I have no idea what you mean by density altitude in the manifold. Do you mean boost pressure or do you mean inlet manifold air temperature or something extrapolated from both.



Regards
Pat
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[CcanDo]

IceStationZebra, The system includes polished S/S headers, double wall, water jacket, 2.25" primaries to 4.50" collector to 5" double wall 4'0" polished S/S dry pipes. The dry pipes have an adjustable orifice for volume and pressure control. The orifice also regulates water level within the walls of the inner/outer pipes and balance. The tail pipes see no water in the exhaust stream, safegaurding against reversion.

Regards

 

[CcanDo]

Pat, I'm working on a response to your suggestions.

Meanwhile, your comment RE; John Force sparks this, roots rotors at one case end and a screw at the other. As engine CU. In. consumes available roots boost the screw is charging and coming "up" to help the roots. Theory: Design creates reduction in lag,less heat and less parasitic loss at the low final boost pressure ?? And I get my bypass,maybe! Patentable ??

 

[patprimmer]

I don't see what advantage a roots has over a screw except durability and cost. If you are willing to pay the outlay and maintenance of a screw, why not just run a suitably sized screw.

The efficiency difference is minimal at low boost levels and in my opinion, really does not justify the cost difference in this case.

The real issue I see is long term durability in a sat water environment.

The large dry straight out exhaust sounds good so long as you have no noise problems. I presume they are over the deck and straight back over the transom. If noise is a problem it can be substantially reduced with minimal back pressure by turning it down under the deck and discharging through the transom with a bend deflecting it down into the turbulent surface of the prop wash.

What sort of drive system do you run.

Regards
Pat
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[CcanDo]

Interesting, We'll call it the "Force System", some application may use a combination of screw and roots better than others. However,as you know, people have been tinkering with a hybrid combo for some time. Detroit diesel with their 92 series, turbo on top of the roots, VW and etc. Somewhere in your "Force System" may be some rock to turn.

The dry pipes are a little loud, but safe and tolerated in some areas. We may have a muffler design that will be tough and swell, both. One needs to be careful when going deep, through the transom.

The drives are Mercury #6, dry sump, swept skag.

 

[patprimmer]

If yu search this site you will find several interesting and a few dangerous threads on twin charging. That is a turbo feeding into a Roots for the combination of instant response and high efficiency at high boost.

A screw type blower is definitely not required. For boats, the instant response is also generally not so critical as continuous high power output. The correct turbo combination may work well.

Regards
Pat
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[CcanDo]

IMHO, the bigger horsepower engines are an extreme duty application. While lazy throttle response may be considered acceptable, it is not ideal. Beyond all is durability.

Factors we consider relative include initial compression, crank to rod ratio, flywheel weight vs. horsepower, water flow (thermal shock), oil temp and control, vacume, oiling (piston sprayers,etc.)and now better induction.

Best Regards

 

[patprimmer]

A well matched turbo can get very quick response with greater durability at the same sustained power output because you don't have to generate the extra cylinder pressure to produce all the power lost in driving a supercharger.

In my experience boats rarely require full throttle from a standing idle. they are normally at least at a fast idle when you nail it.

Most of my experience comes from circuit racing and ski racing.

With fully submerged propellers, I normally put the water scoop just behind the propeller and that way protect it to some degree from debris If you run legs that is probably not an option and a presume your pick up is built into the leg.

I run all water through several less critical to thermal shck devices before it goes into the block so as to preheat it somewhat.

I size the system to run at a maximum of 80 deg C under the most severe stable conditions.

I size the discharge side slightly smaller than the inlet so as to build some pressure and thereby avoid possible localised boiling ans build up of salt deposits.

I check water pressure in the head and never allow it to go over about 100 psi as it is then to inclined to leak past the gaskets and enter cylinders or the sump.

I would not pull to much vacuum on the crankcase on an endurance engine as it lowers the boiling point of the engine oil and can damage gudgeon pins. Enough to stop leaks and to eat up blowby, but not enough to suck in seals or gaskets and let the oil on the bottom side of the piston crown and the pin boil or lose it's lubrication capacity. It does help clean fuel and water contamination from the oil. Maybe 5 psi vacuum.

A dry sump might be value at the cost/benefit range you seem to explore. Even a single stage scavenge and the OEM pressure pump system can be a good gain. A 5 stage system costs a truckload more and gains only a little extra over a 3 stage.

I think best value is one pick up from the back corner (that is back of the boat) of the sump on the side the oil throws toward and one from the valley with all other returns from the valley blocked off is the best value.

With a positive displacement blower like a roots, you do not need high compression for response as the instantaneous boost builds effective compression in an instant.

With a centrifugal pump like a turbo, you do need reasonable compression to get response before boost builds.

With a twin charger, you can run low compression, moderate boost from a slow turning blower then a truckload of boost from a real big turbo that would otherwise be to laggy, but you do increase cost and weight and complexity.

In a boat, you have the option of a really efficient inter cooler due to the infinite supply of cold water.

Rod to stroke does not have any significant impact so long as it is not out to lunch. The difference between 1.7 or 1.8:1 will not matter, but 1.4 to 1.5 certainly will matter.

A good oil system that cools the oil well and controls surge and windage is critical. Oil squirters to cool pistons certainly helps high compression, high boost motors so long as you can keep the oil cool.

Except for a stable slow idle and somewhere to hang the ring gear, a high speed power boat does not need a flywheel. I use aluminium or a flex plate. The flex plates tend to bend from bendix drive engagement if the ring gear gets rusty, so that depends on amount of use, conditions of storage and maintenance.

For this sort of application, I would use a blower drive hub with inbuilt torsional vibration damper (most commonly called a harmonic balancer). It would be steel, a tight interference fit on the crank and have two 1/4" keys or one 5/16" key, carefully hand fitted to a gentle tap fit in the crank and the hub. It would also be as long as possible to maximise bearing area in the crank and hub.

I have seen marine grade intercoolers, but I can't remember where






Regards
Pat
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[CcanDo]

Pat, IMHO, "Baseline" starts with the flywheel. The flywheel becomes a fuse link between the crank and tip of the prop. We have stumbled onto effects that support the opinion. Such as why lighter outdrives break. The light drive is forced to absorb inertia from not only the crank but also reverse loading from wt. of the hull. I'm not sure the flywheel shouldn't range up to 40-50#.

It looks like you know a lot about boats and probably like them.

More later!

Regards.

 

[patprimmer]

A flywheel will help absorb speed fluctuations within one turn of the crank due to power impulses from each power stroke, but in an 8 cylinder engine these power strokes overlap so that reduces the impulses when compared to a lesser number of cylinders. A TVD and a blower both also help smooth the pulses a bit.

I have never run outboard legs, only V drive or direct drive of the snout in centre mount ski boats.

I would think the load fluctuations from a semi surfacing prop would have a lot more impact on pulse loadings on the leg gears than the crank, but crank speed pulses could not be completely discounted.

I would think the speed loss from substantially submerging the prop would be greater than the acceleration loss from a 40# flywheel, but a flywheel that heavy will hurt acceleration.

The prospect of a flywheel coming adrift or blowing apart concerns me more in most boats than in most cars for several reasons. These are to do with the consequence of a flywheel coming adrift and the likelyhood of very rapid changes in rpm and the effects of corrosion and typical cruise rpm.

Any boat sustaining over 5000 rpm should in my opinion have a steel flywheel rather than cast iron.

Regards
Pat
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[CcanDo]

Spot On, Mate ! Alloy for the flywheel should end in 30 or 40 billet. Sizing is subject to Cu.In. and torque/horsepower. Blower or not is a consideration but not critical. Your correct about acceleration. However, the props are S/S surfacing 4, 5 or 6 blade lab finished Cleavers. Hard acceleration will cause the props to cavitate, build heat, unbalance and transmit harmonics everywhere. The marginal capacity drive gets caught in the middle. Then, when the boat is flown (cat)the drive (big or small)appreciates any cushion the flywheel offers. Bear in mind the throttle man "powers" across chop with feel for the transom or is it "BOOT". The ultimate sin is to "stuff" the nose.

Therefore, the heavier flywheel does slow accel/decel while smoothing. The Harmonic balancer should also be billet, w/elastometer rings, not lquid filled. Fluid filled balancers drive everthing nuts while the fluid is trying to find "Home".

There are boats like this in Australia, Dubai and etc. Some have big budgets and get serious.