19 replies to this topic

[carlt]

Anyone on here clued up on the physics of supercharging ?
I could do with some feedback on my planned system

I'm planning to supercharge the 1172cc ford sidevalve/flathead engine in my Buckler trials car and am thinking of trying a duel throttle set up .

The aim is to bring the boost in very early in the rev range and then to limit boost and hence power nearer the top of the rev band [ essentially over rev the charger but restrict its total air intake ]

The objective being to increase torque from low engine revs and limit total boost/power at high rpm to prevent headgasket failure or worse !

The engine is naturally , being a sidevalve , a low rpm torquer rather than a revver ;
peak safe revs [all stock internals] about 5500-6000 rpm

I plan to boost to 6-8 psi but aim to reach this at as low rpm as I can , and then to maintain this boost right to max rpm by throttling/restricting the chargers intake .
It is going to be a blow through SU carb for fueling , a vacuum controlled recirculating bypass valve for off throttle and another carb body as the pre charger throttle/restrictor , both carbs linked to accelerator pedal,
a fail safe BOV also fitted

The best analogy I have for the idea of the restricted inlet is when you put your hand over the intake of a vacuum cleaner the motor keeps sucking but also revs up due to it moving next to no air and therefore being under less load ' in the charger application this should mean less load on the engine from the charger , ie more power for the wheels , and less heating up of the intake charge

If the theories are correct and if it works in practice I'm hoping the the engine will perform as if it were just a larger capacity NA sidevalve

Comments welcome

Carl

[gruntguru]

What type of supercharger - centrifugal, roots, screw, vane etc? I await your reply but regardless I recommend draw-thru for 90% of supercharger applications.

[TDIMeister]

Carl, you should be more concerned about detonation when supercharging a flathead than anything else... I don't know what you have exactly in mind for restricting the intake flow, but it would seem to achieve little but operate the charger out of an optimal part of the map and cost you mechanical efficiency (compressor work) and charge heating. You need to map the swallowing characteristics of your engine and select an appropriately matched charger and pulley ratio, beginning conservatively at first and working your way to a pulley configuration that is knock-free on the fuel you're using and worst-case ambient conditions

[carlt]

It is the Eaton 45 roots type -5th gen teflon coated rotors - apparently the teflon coating don't like fuel going through them

GG - Interested to know why you recommend draw through . I am using the HIF44 blow through SU from a Metro Turbo

TDI - there is an old Shorrocks set up info that gives the diameter for the sleeves to fit over the distributor advance bob weight stops , to reduce max advance for supercharging

The idea of restricting the inlet opening is to operate the charger in its optimum map from say 2000 rpm instead of 5000rpm , then at say 5000 rpm it reaches the point where it can't draw in any more volume of air through the restrictor ,- i'm thinking it will not cost any more mechanical efficiency or charge heating as it will not have any more air to compress - like my vacuum cleaner analogy it will just spin with less load , in fact there could be less compressor work after 5000rpm ?

This is a Trials engine - it needs to produce smooth torque from idle ,-- lots of top end power is not so necessary - just breaks traction

[gruntguru]

I agree with TDIM. The blower you have selected will favour a "constant torque" engine characteristic and your main limitation will be knock - at low rpm (although I think your engine will tolerate more boost than most might think). You wont need to worry about too much boost at high revs - especially once you find the boost level that works at low revs. The limited breathing of your engine will limit the top end. Retarding timing is a fairly limited band-aid. You are better off reducing CR if you need more boost to get the torque you need. If you do need a "top-end" restrictor, why not a fixed restriction like an upstream venturi?

I prefer draw thru for: blower cooling, part-throttle blower unloading, ease of carb adaption, accuracy of carb calibration, improved fuel distribution.

Indicators for blow-thru: fitment of intercooler or any large downstream volume, one-choke-per-cylinder carburation, blowers that don't tolerate fuel.

Provided the downstream volume is sensible, there is no perceptible advantage in throttle response as often claimed for blow-thru.

[carlt]

You need to map the swallowing characteristics of your engine and select an appropriately matched charger

I matched the charger to My intake of breath characteristics .................. It was free !

[carlt]

I agree with TDIM. The blower you have selected will favour a "constant torque" engine characteristic and your main limitation will be knock - at low rpm (although I think your engine will tolerate more boost than most might think). You wont need to worry about too much boost at high revs - especially once you find the boost level that works at low revs. The limited breathing of your engine will limit the top end. Retarding timing is a fairly limited band-aid. You are better off reducing CR if you need more boost to get the torque you need. If you do need a "top-end" restrictor, why not a fixed restriction like an upstream venturi?

Standard CR for this engine is 7:1

Back in the days when these engines were supercharged it was easy/common to blow head gaskets , in fact it was sort of the limiting factor so they breath well enough to break !

I am trying to achieve a generous boost level at low engine rpm without the head gasket problems that are associated with trying to get more power

My first plan was a fixed venturi as a top end restrictor , then I thought why not a variable venturi ? - I have a box full of old SU's for spares , ranging from 1", 1 1/8", 1 1/4" etc which might do the trick
Why not limit the intake venturi and make it tunable as well with butterfly and dashpot piston/spring ?


My basic/generalized laymans calculations go :- these are not actual real figures but just as an e.g.

Charger/Crank drive ratio 2:1 -Engine at 2000rpm charger at 4000rpm , at 3000rpm charger at 6000rpm , at 5000rpm charger at 10000rpm - lets assume this gives me my max safe boost of 8psi at 5000rpm
Charger/Crank drive ratio 3:1 - Engine at 2000rpm charger at 6000rpm , at 3000rpm charger at 9000rpm , at 5000rpm charger at 15000rpm BANG !

but introduce a restricted intake venturi pre charger , that restricts any more air volume intake at 9000 charger rpm , then it cannot produce more boost pressure and I get my 8psi from just over 3000rpm all the way to max 6000 engine rpm

[gruntguru]

I think you would only need to run the M45 at about 1.7 x engine speed to get the 8 psi at 3000 rpm (assuming your cam is stock or very mild). With no restriction, the boost at 6000 rpm would only be about 11 psi. If its much higher than that, its just the carb or cylinder head choking it off - the pressure is not actually getting through to the cylinders. The M45 mass-flow per revolution is only 10% higher at 10k (6k engine) than it is at 5k (3k engine).

Keep it simple. Get the thing making your 6-8 psi at 3000 rpm and see what the boost does at higher revs. If you can't live with it, add a restrictor.

Don't assume that peak power is the biggest risk to your head gasket. More likely to be detonation at lower revs.

By the way - restricting the intake of a roots will not unload it the way it does with a centrifugal (as in vacuum cleaner). The drive torque is directly proportional to the pressure difference (outlet - inlet).

[MatsNorway]

Don't assume that peak power is the biggest risk to your head gasket. More likely to be detonation at lower revs.

I think its often at VE or at peak Torque.. Same thing?

[gruntguru]

I think its often at VE or at peak Torque.. Same thing?

Often even lower revs than peak torque because detonation is time sensitive. Low revs = more time to heat the end-gas.

[cheapracer]

Don't you run methonal being an historic?

Use a VW rotor button, they cut out at 4,500 rpm from memory.

[carlt]

I think you would only need to run the M45 at about 1.7 x engine speed to get the 8 psi at 3000 rpm (assuming your cam is stock or very mild). With no restriction, the boost at 6000 rpm would only be about 11 psi. If its much higher than that, its just the carb or cylinder head choking it off - the pressure is not actually getting through to the cylinders. The M45 mass-flow per revolution is only 10% higher at 10k (6k engine) than it is at 5k (3k engine).

Keep it simple. Get the thing making your 6-8 psi at 3000 rpm and see what the boost does at higher revs. If you can't live with it, add a restrictor.

Don't assume that peak power is the biggest risk to your head gasket. More likely to be detonation at lower revs.

By the way - restricting the intake of a roots will not unload it the way it does with a centrifugal (as in vacuum cleaner). The drive torque is directly proportional to the pressure difference (outlet - inlet).

Interesting info , thanks .


By pure coincidence I just won a Cank pulley on ebay today which should fit without too much fettling - diam 114mm . the charger which I think is off Merc slk , pulley diam 69mm = 1.65 ratio , should do for starters

Standard cam to start with and I don't foresee the carb being a restriction 1 3/4" SU with plenum designed for Metro turbo

Larger valves and a lot of porting/gas flowing should aid the breathing

There is plenty of scope to lower the compression below the production 7:1 - ?

I follow the KISS principle with all my builds - the pre charger throttle was intended for the second faze of development

Pump gas - road driving and Classic Trials http://www.wheelspin.info/
http://www.actc.org.uk/

Edited by carlt, 04 March 2011 - 21:31.

[gruntguru]

Larger valves and a lot of porting/gas flowing should aid the breathing

Emphasis should be on the exhaust side. Increase exhaust valve diameter to 90 - 95% of inlet.

[TDIMeister]

My readings and recollections, backed-up by ZERO first-hand experience, suggest that SU carbs are really fickle to inlet pressure conditions. I think this was the rationale for the suggestion for a draw-through layout instead of having the carbs see boost pressure.

[gruntguru]

He did say he is using a "blow-thru" SU from a Metro turbo. I'd still prefer a standard carb in draw-thru location personally. There is still the question of whether the coating on the blower rotors will tolerate fuel.

[Fatgadget]

I matched the charger to My intake of breath characteristics .................. It was free !

[carlt]

Emphasis should be on the exhaust side. Increase exhaust valve diameter to 90 - 95% of inlet.

Cheers - already in hand - later Ford ohv exhaust valves

The SU HIF 44 Blow through as found on some of British Leylands Finest creations has a plenum specifically designed to give the carb the correct airflow/pressure signals [ I hope ]

[Kelpiecross]

[quote name='carlt' date='Mar 3 2011, 09:42' post='4874011']
Anyone on here clued up on the physics of supercharging ?

Carl - It might be best if you treat your problem as needing a constant manifold pressure - of plus 8Psi for example. This could be done by using a blower and drive speed etc. capable of more than this pressure and using a suck-through SU (like everybody else, I would avoid blow-throughs like the plague).
To get the constant manifold pressure you could put a pressure activated diaphragm in the throttle cable control to the carby. A sort of primitive "fly-by-wire" arrangement.
The idea being that even with your foot flat down the diaphragm etc. would modulate the SU's throttle plate and only allow a maximum of +8Psi.
I have seen this general idea done successfully on a very high CR engine that needed to have its manifold vacuum limited to a value quite a bit less than atmospheric to avoid detonation.
If you build a fair bit of variability into the system (like being able to change springs or vary spring tension) you could experiment with various constant manifold pressures etc.

Edited by Kelpiecross, 07 March 2011 - 02:50.

[carlt]

Carl - It might be best if you treat your problem as needing a constant manifold pressure - of plus 8Psi for example. This could be done by using a blower and drive speed etc. capable of more than this pressure and using a suck-through SU (like everybody else, I would avoid blow-throughs like the plague).
To get the constant manifold pressure you could put a pressure activated diaphragm in the throttle cable control to the carby. A sort of primitive "fly-by-wire" arrangement.
The idea being that even with your foot flat down the diaphragm etc. would modulate the SU's throttle plate and only allow a maximum of +8Psi.
I have seen this general idea done successfully on a very high CR engine that needed to have its manifold vacuum limited to a value quite a bit less than atmospheric to avoid detonation.
If you build a fair bit of variability into the system (like being able to change springs or vary spring tension) you could experiment with various constant manifold pressures etc.

Thanks for the input .
Very interesting idea , if the simple blow through system proves unmanageable then this gives me something else to try

[carlt]

Carl - It might be best if (like everybody else, I would avoid blow-throughs like the plague).

Why ?

I have never been good at 'like everyone else' , it makes me want to try it out even more !