What does it do on the Plenum?
Discussion
Firstly what is this block with the 2 water pipes actually for? heating the plenum or part of the cooling (doubtful with the air passing through it!)
Second the connection tubes look a little rusted up so are new blocks available or is it actually required?
Just thought I might as well put it right whilst I am playing around cleaning the and servicing my Plenum in the throws of completing a n/side rocker cover gasket change and polish/paint job. Just another day in life of wedge ownership.
Second the connection tubes look a little rusted up so are new blocks available or is it actually required?
Just thought I might as well put it right whilst I am playing around cleaning the and servicing my Plenum in the throws of completing a n/side rocker cover gasket change and polish/paint job. Just another day in life of wedge ownership.
Edited by stevoj on Thursday 4th December 16:21
The confusing thing about this then is that:
a) If it is using the heat to assist starting then there would be no heat there until the engine was running and warm anyway
b) We all know that cooler dense air supposedly helps engines run better (maybe that's the 2bhp Glenn), so once running why design something that so called warms the inlet air, which is doubtful the velocity that air would be at through the Plenum anyway.
Everything would seem to point that it shouldn't be there no matter what the engine size.
a) If it is using the heat to assist starting then there would be no heat there until the engine was running and warm anyway
b) We all know that cooler dense air supposedly helps engines run better (maybe that's the 2bhp Glenn), so once running why design something that so called warms the inlet air, which is doubtful the velocity that air would be at through the Plenum anyway.
Everything would seem to point that it shouldn't be there no matter what the engine size.
stevoj said:
The confusing thing about this then is that:
a) If it is using the heat to assist starting then there would be no heat there until the engine was running and warm anyway
b) We all know that cooler dense air supposedly helps engines run better (maybe that's the 2bhp Glenn), so once running why design something that so called warms the inlet air, which is doubtful the velocity that air would be at through the Plenum anyway.
Everything would seem to point that it shouldn't be there no matter what the engine size.
From memory when you unbolt it, you only have to re block up 2 holes that are open into the air way. Not all 4 bolts open up you seea) If it is using the heat to assist starting then there would be no heat there until the engine was running and warm anyway
b) We all know that cooler dense air supposedly helps engines run better (maybe that's the 2bhp Glenn), so once running why design something that so called warms the inlet air, which is doubtful the velocity that air would be at through the Plenum anyway.
Everything would seem to point that it shouldn't be there no matter what the engine size.
Just cut a bolt down with a bit of sealant and bobs your uncle
stevoj said:
The confusing thing about this then is that:
a) If it is using the heat to assist starting then there would be no heat there until the engine was running and warm anyway
Siberia is very cold..If you urinate at a certain time of day into the nothingness it will freeze all the way back to your balls...a) If it is using the heat to assist starting then there would be no heat there until the engine was running and warm anyway
..Bit like a baked Alaska...
stevoj said:
b) We all know that cooler dense air supposedly helps engines run better
And once its warmed up it goes like a nympho on a kilo of viagra....From memory its there to prevent accretion of ice in cold, damp conditions. Easy to remove and as mentioned, close the holes up with a couple of bolts and threadlock/sealer.
Powerful engine in plastic car means engine bay warmth is abundant in a TVR, hotspot not needed. As for the power gain? I don't have a dyno and have never attempted to experiment with it anyway, but it COULD give a couple of HP in the right conditions I suppose... That said, if you're a cold air fiend, you want to feel how hot the whole plenum casting gets when the engine is warm anyway. Coolant flows through the lower reaches of the inlet manifold anyhow, and the whole shooting match gets too hot to lay your hand on.
On mine i fitted a 1/16 ply (stop laughing at the back, you'd do very well to spot it because I did a proper job of it - inlet manifold off, rough cut the insulator, sandwiched and bolted and then shaped with a dremel using the casting as the template) insulator and that DID noticeably reduce the temperature of the trumpet base and plenum top.
There's a trade-off between cold air in and warm air in. Cold air is of course more dense and the volumetric efficiency of the engine is improved by cold air (more molecules of oxygen means more fuel to burn stoichiometrically, more fuel equals more energy released), however we're also trying to dissolve petrol into that air as a vapour in the inlet runner. Enthalpy of vapourisation of the fuel and the adiabatic decompression in the intake runner will cause the manifold casting to cool a lot, and if it gets too cold the fuel vapour will crash out on the walls of the manifold, starving the engine of a combustible mixture. A little heat in the intake runners stops this from happening by keeping the manifold walls above the dew point, but too much heat in the intake will just reduce the VE and cause a loss of power.
Short answer: you dont need the hotspot. I suspect it is there to prevent throttle plate icing, and in reality TVR engine bays are warm enough places that throttle plate icing is not a significant risk. If in doubt, hoon it!! Throttle plate ice will only accrete at low power settings.
Powerful engine in plastic car means engine bay warmth is abundant in a TVR, hotspot not needed. As for the power gain? I don't have a dyno and have never attempted to experiment with it anyway, but it COULD give a couple of HP in the right conditions I suppose... That said, if you're a cold air fiend, you want to feel how hot the whole plenum casting gets when the engine is warm anyway. Coolant flows through the lower reaches of the inlet manifold anyhow, and the whole shooting match gets too hot to lay your hand on.
On mine i fitted a 1/16 ply (stop laughing at the back, you'd do very well to spot it because I did a proper job of it - inlet manifold off, rough cut the insulator, sandwiched and bolted and then shaped with a dremel using the casting as the template) insulator and that DID noticeably reduce the temperature of the trumpet base and plenum top.
There's a trade-off between cold air in and warm air in. Cold air is of course more dense and the volumetric efficiency of the engine is improved by cold air (more molecules of oxygen means more fuel to burn stoichiometrically, more fuel equals more energy released), however we're also trying to dissolve petrol into that air as a vapour in the inlet runner. Enthalpy of vapourisation of the fuel and the adiabatic decompression in the intake runner will cause the manifold casting to cool a lot, and if it gets too cold the fuel vapour will crash out on the walls of the manifold, starving the engine of a combustible mixture. A little heat in the intake runners stops this from happening by keeping the manifold walls above the dew point, but too much heat in the intake will just reduce the VE and cause a loss of power.
Short answer: you dont need the hotspot. I suspect it is there to prevent throttle plate icing, and in reality TVR engine bays are warm enough places that throttle plate icing is not a significant risk. If in doubt, hoon it!! Throttle plate ice will only accrete at low power settings.
Well seeing as i don't intend using the car either in freezing weather or at an altitude any higher than the Lake District - i have removed it, now things are a little more clear, it really doesn't heat much up at all does it. You need to block 3 bolt holes up however, 2 that go through to the air inlet and one that goes through to the air pipe on the side.
Just realised that car should be faster now anyway as it will be lighter
Just realised that car should be faster now anyway as it will be lighter
t off a stick!!
OK - the boring science geek is here with what I reckon is the explanation....
If you remember many carb engines had a 'hot spot' set near the carbs in the inlet manifold, heated either by water, or by exhaust gas (Cologne V6 carb has an exhaust gas channel in inlet mfold for example)
The reason is that when air is sucked through a narrow venturi (throttle plate, carb, or similar narrow tube) its temperature DROPS at the point where the venturi ends and a low pressure area exists (like an inlet manifold on light throttle), and this drop can be quite a bit, like 5 degrees C. When it's cold and damp outside, this drop can be enough to start ice building up inside the carb and/or inlet manifold. Depending upon how well heated and/or remote from engine block the area(s) are, this can even happen in a hot engine. Long runners in inlet manifold are more prone to this, for example.
So the heater is there for cold climate conditions really, but is made worse by a cold engine as well. I think injection ingines are less likely to suffer, as the carb has mixed the fuel in already and so problem can cause problems with air/fuel vapourisation, whereas injection engines have fuel injected close to inlet valves (except for those big thottle body injectors), but it's going to depend on the specific setup, which defines how much cooling occurs.
I hope that all makes sense ....
If you remember many carb engines had a 'hot spot' set near the carbs in the inlet manifold, heated either by water, or by exhaust gas (Cologne V6 carb has an exhaust gas channel in inlet mfold for example)
The reason is that when air is sucked through a narrow venturi (throttle plate, carb, or similar narrow tube) its temperature DROPS at the point where the venturi ends and a low pressure area exists (like an inlet manifold on light throttle), and this drop can be quite a bit, like 5 degrees C. When it's cold and damp outside, this drop can be enough to start ice building up inside the carb and/or inlet manifold. Depending upon how well heated and/or remote from engine block the area(s) are, this can even happen in a hot engine. Long runners in inlet manifold are more prone to this, for example.
So the heater is there for cold climate conditions really, but is made worse by a cold engine as well. I think injection ingines are less likely to suffer, as the carb has mixed the fuel in already and so problem can cause problems with air/fuel vapourisation, whereas injection engines have fuel injected close to inlet valves (except for those big thottle body injectors), but it's going to depend on the specific setup, which defines how much cooling occurs.
I hope that all makes sense ....
What is the Rover SD1 Efi "Hot Spot"?
On the Rover SD1 Single Plenum Fuel Injection System, just below the intake tunnel is a heater unit plumbed directly to the primary circuit of the cooling system. Unsurprisingly, the Twin Plenum engine has two hot spots.
By 'primary circuit', I mean the circulatory galleries that are in play before the cooling system thermostat opens.
These include all the engine block and inlet manifold water jackets, the cabin heater matrix and the mentioned Efi "Hot Spot".
Because these galleries are independent of the thermostat they heat up rapidly as the water pump circulates fluid using its bypass output hose.
The Mythology.
I have never seen any documentation why the "Hot Spot" was introduced but if the mythology is to be believed, it's there to prevent icing inside the inlet tunnel. Personally I cant reconcile the science involved and in the absence of a genuine Rover document as to why, I postulate my own theory.
It's suggested: With engine idling above it's normal idle speed, say 1200 rpm or thereabouts, sucking in a lot of very, very cold air, maybe the butterfly freezes up and sticks shut.
I think the bit about very cold air is just part of the story - maybe - but the bit about frozen butterflies - I think not?
Challenging the Mythology.
Hopefully though, someone has come across a documented explanation, in which case I would love to discover the original reasoning why Rover fitted the plenum "Hot Spot" to it's SD1 Efi cars.
In the meantime, my advice to RV8 Efi enthusiasts planning to ditch their "Hot Spot" is, DONT! And to those without a "Hot Spot", time to re-think, methinks!
On the Rover SD1 Single Plenum Fuel Injection System, just below the intake tunnel is a heater unit plumbed directly to the primary circuit of the cooling system. Unsurprisingly, the Twin Plenum engine has two hot spots.
By 'primary circuit', I mean the circulatory galleries that are in play before the cooling system thermostat opens.
These include all the engine block and inlet manifold water jackets, the cabin heater matrix and the mentioned Efi "Hot Spot".
Because these galleries are independent of the thermostat they heat up rapidly as the water pump circulates fluid using its bypass output hose.
The Mythology.
I have never seen any documentation why the "Hot Spot" was introduced but if the mythology is to be believed, it's there to prevent icing inside the inlet tunnel. Personally I cant reconcile the science involved and in the absence of a genuine Rover document as to why, I postulate my own theory.
It's suggested: With engine idling above it's normal idle speed, say 1200 rpm or thereabouts, sucking in a lot of very, very cold air, maybe the butterfly freezes up and sticks shut.
I think the bit about very cold air is just part of the story - maybe - but the bit about frozen butterflies - I think not?
Challenging the Mythology.
- Any cold air being sucked in is not exactly laden with water so it won't cause icing - as is - it's just cold air, nothing unusual about that!
- However, because of the suction induced depression, the temperature of the air inside the tunnel does fall (opposite effect to a bicycle pump getting hot when the pressure is increased).
- At the same time - any moisture present in the air may condense out onto the cold walls of the tunnel, and from there it might form ice.
- As to whether that can freeze the butterfly is where I lose the plot, for the following reason:
- I've measured approximately how long it takes for a perceptible rise in coolant temperature to reach the "hot spot" and its something over thirty seconds and definitely less than one minute?
- In addition, because the only air entering the engine during idling comes via the idle air gallery and the Extra Air Valve, the flow of air thro' the tunnel(s) via the throttle disc(s) is virtually nil.
- Yet whilst the engine has been sucking during that period at it's raised idle speed, and if ice has somehow formed in the tunnel, suppose the driver now decides to move off after less time than it takes for the "Hot Spot" to get any warmed coolant, (say) only ten to twenty seconds, and at the same time the throttle butterfly had becomes jammed with ice, then two things can happen.
- 1) The throttle spindle might bend or break due to the very un-subtle forces normally applied to the accelerator pedal, and/or 2) the throttle jams in the open position when the pedal is released.
- Both outcomes are potentially bad, so, yes - in cold/humid conditions, the possibility of icing exists but the "Hot Spot" process is not fast enough to prevent a serious mechanical problem.
- To add to the unlikelihood of icing being a reason for the "Hot Spot", all the surfaces inside the tunnel(s) are coated in oily residues from the crankcase breather system making it virtually impossible for any formed ice to jam the throttle disc(s) anyway.
- Given that icing may not the reason for the Hot Spot, what remains?
- On the other hand, if it had nothing to do with potential icing up of the mechanicals, because for the above reasons such an unlikely phenomenon was to cause collateral damage at early launch.
- Perhaps more likely due to petrol being unable to atomise effectively in the presence of very cold air, and if true, the "Icing Theory" really is a myth perpetuated on Forums and in Pubs up down the country.
- So, based upon the generation of depression induced super cold air (ie: the natural laws of physics) leads to the "Hot Spot" being needed as soon as possible after starting to promote improved fuel atomizing.
- In turn, this facilitates more efficient combustion at launch and the first few minutes of driving in very cold conditions. An explanation which not only appeals to the needs of the engine but holds up under scrutiny!
- So at the simplest level, its an aid to efficient acceleration without hesitation when very cold, directly from idle. Think about it this way? Its a normal cold winter morning - maybe even freezing!
- Start the car and the cold start injector is normally needed but ceases after cranking.
- The Efi system adjusts mixture for coolant temperature and continues to do so until it gets hot.
- At this time the fuel/air mixture is rich and some pretty inefficient fuel atomisation is going on.
- The Extra Air Valve raises idle speed and gradually reduces it until it get hot.
- The coolant in the engine block galleries warms up straight away to be immediately driven round the plenum heater block by the water pump, bypassing the coolant system thermostat.
- The inlet tunnel(s) warm up, including the small amount of static air trapped therein.
- Little or no air passes by the disc(s), of course, so the tunnel air flow is virtually zero.
- The only air entering the engine comes via the idle air gallery and the Extra Air Valve.
- Now, at the point of launch the throttle is suddenly open, perhaps momentarily, even wide open..
- Instantly, the warmed air inside the inlet tunnels is ingested to be immediately met by the injectors firing in loads of fresh fuel.
- The warmed air aids fuel atomisation and the first few combustions are much more efficient.
- The car launches without hesitation, there is no flat spot and once the car is off and running - who cares?
- Some owners misguidedly disconnect the heater pipes to the plenum hot spots, with the result that the plenum intake tunnel walls now stay cold during the first few minutes of engine warm-up,
- The resident air also remains very cold and when the throttle discs do open there is only very cold air, poor fuel atomisation, inefficient combustion and a flat spot at launch.
- So there you have it, an alternative logical reason why Bosch/Rover/Lucas thought it necessary to add a plenum intake tunnel heater to the Rover SD1 Efi system.
- Just my personal theory of course, but it seems illogical that a car which is supposed to impress with its "get up and go" would be without a facility that helps to achieve that goal in all conditions.
- Imagine for a moment if you will, the posh owner/drivers of a souped-up road-going executive saloon (Vitesse or VDP Efi) being thoroughly disgusted with any hesitation at launch on a very cold morning and guaranteed to complain bitterly to the franchised seller. I should CoCo?
Hopefully though, someone has come across a documented explanation, in which case I would love to discover the original reasoning why Rover fitted the plenum "Hot Spot" to it's SD1 Efi cars.
In the meantime, my advice to RV8 Efi enthusiasts planning to ditch their "Hot Spot" is, DONT! And to those without a "Hot Spot", time to re-think, methinks!
Certain Kawasaki inline-4 motorcycles had to have their carburettors replaced with modified versions that included a conduit for the engine's coolant and the reason was, wait for it... to cure carb icing. Nothing to do with jamming spindles or fuel atomisation, the ice built-up on the venturi walls downstream of the carbs, closing-up the tracts and bu99ering the fuelling.
Were the same thing to happen to the Rover EFI system, the open-loop injection would not be able to compensate for it.
The closed-loop version on the US-market emission-controlled cars, on the other hand, might be able to - as long as it had a way to defrost the throttle body.
Were the same thing to happen to the Rover EFI system, the open-loop injection would not be able to compensate for it.
The closed-loop version on the US-market emission-controlled cars, on the other hand, might be able to - as long as it had a way to defrost the throttle body.
Wedg1e said:
Certain Kawasaki inline-4 motorcycles had to have their carburettors replaced with modified versions that included a conduit for the engine's coolant and the reason was, wait for it... to cure carb icing. Nothing to do with jamming spindles or fuel atomisation,
Is this saying 'keep the hot spot'?I just followed the science - same conclusion.
I've heard that one TVR owner has 'hot spot' removed and the engine bogs down at launch when cold, let's be hearing other experiences? The current cold mornings are ideal for a test hoon!
That would be me - it's not installed on the remote throttle which is in front of the blower rather than on the plenum.
If you think about it. the smooth walls of the plenum don't give much opportunity for wide-open-throttle air rushing past to warm up. But when the car is idling on the EAV, the air is pooling in that plenum inlet pipe waiting for the throttle to be opened - that's when it will warm up the most.
From what I have seen of a lot of wedge owners they tend to let the engine warm up a bit before driving it (not me though I don't like leaving the camshaft on low revs from the get go), in which case the hot spot is pointless as the engine has already gone past that phase and the plenum and trumpets are getting warm anyway.
If you think about it. the smooth walls of the plenum don't give much opportunity for wide-open-throttle air rushing past to warm up. But when the car is idling on the EAV, the air is pooling in that plenum inlet pipe waiting for the throttle to be opened - that's when it will warm up the most.
From what I have seen of a lot of wedge owners they tend to let the engine warm up a bit before driving it (not me though I don't like leaving the camshaft on low revs from the get go), in which case the hot spot is pointless as the engine has already gone past that phase and the plenum and trumpets are getting warm anyway.
Again, from the science point of view.....
None of what's expressed above is wrong, but I still reckon some of it is missing the vital point.
The cooling happens in accordance with one of the physics 'gas laws' of pressure, density, and temperature, because (as above) when air goes through a carb or throttle body, it will be significantly cooled. If it's a wet or humid day, there's plenty enough water vapour to form ice, EVEN IF IT'S A HOT DAY. In other words - ICING CAN HAPPEN ANY TIME, EVEN WITH A HOT ENGINE. That's why road engines nearly all have water heated or exhaust heated inlet manifolds, or very short manifolds. YES - cooling effect is much worse at idle and light throttle, as pressure in manifold is at its lowest. Wide open throttle won't cause much cooling at all.
When I was a lot younger and still learning, I fitted an SU carb to my Cortina Mk1 (1500 engine) with an adapter which I found at the scrappers. It was a DIY job, but well made. I discovered it would nearly ALWAYS ice up, this happened even on the outside of the adaptor. Eventually engine would start to misfire. Wrapping heater hose around it helped, but didn't solve it completely. Then I realised why the original manifold had a hot spot under the carb. I ditched that and went to a 28/36 GT carb instead. The adaptor was just too remote from a heat source.
Fuel doesn't freeze until some very low temperature, but it can start to form bigger droplets, so the atomisation gets worse. In injection engines, this is less likely, but not impossible....
SO if hotspot/heater is there on your RV8, it's there for a reason....
None of what's expressed above is wrong, but I still reckon some of it is missing the vital point.
The cooling happens in accordance with one of the physics 'gas laws' of pressure, density, and temperature, because (as above) when air goes through a carb or throttle body, it will be significantly cooled. If it's a wet or humid day, there's plenty enough water vapour to form ice, EVEN IF IT'S A HOT DAY. In other words - ICING CAN HAPPEN ANY TIME, EVEN WITH A HOT ENGINE. That's why road engines nearly all have water heated or exhaust heated inlet manifolds, or very short manifolds. YES - cooling effect is much worse at idle and light throttle, as pressure in manifold is at its lowest. Wide open throttle won't cause much cooling at all.
When I was a lot younger and still learning, I fitted an SU carb to my Cortina Mk1 (1500 engine) with an adapter which I found at the scrappers. It was a DIY job, but well made. I discovered it would nearly ALWAYS ice up, this happened even on the outside of the adaptor. Eventually engine would start to misfire. Wrapping heater hose around it helped, but didn't solve it completely. Then I realised why the original manifold had a hot spot under the carb. I ditched that and went to a 28/36 GT carb instead. The adaptor was just too remote from a heat source.
Fuel doesn't freeze until some very low temperature, but it can start to form bigger droplets, so the atomisation gets worse. In injection engines, this is less likely, but not impossible....
SO if hotspot/heater is there on your RV8, it's there for a reason....
Edited by RCK974X on Saturday 6th December 20:00
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