Since fitting one to the Golf, I`m frequenty asked what the Accusump actually does and how it works. There are several explanations already online but I`ll try and break it down into a simple yet understandable post. It has ended up being quite lengthy but I felt it necessary to answer all the questions I had and I know others will have when talking about Accusumps.
What is an Accusump
An Accusump is a device that maintains oil pressure in your engine if you are experiencing an oil starvation situation. In a performance car thats either under heavy cornering or acceleration/braking, depending on engine configuration.
The first thing most people do is fit a baffled sump, it’s a simply yet effective way to reduce oil sloshing around in the sump and starving the oil pump pickup. The ultimate solution is a full dry-sump setup but for many people these are unaffordable.
The Accusump sits somewhere between the two. It helps even with a baffled sump, but isn’t as good as a full dry-sump setup.
I’ll try and explain how an Accusump works using some drawings which are simplified to show a very basic sump, oil pump, filter, cooler and return to engine. I’ve added a couple of boxes showing typical oil pressure and pump outlet and engine inlet.
An Accusump is actually a trade name for an oil Accumulator. The common accumulator we see is made by Canton in America and is marketed as Acusump. Moroso make almost the same thing in Australia. Both work in exactly the same way and from what I’ve been able to find are as reliable as each other and the deciding factor between the 2 is cost / size and availability.
Standard engine Oil flow
During normal engine running, the sump contains sufficient oil, the pump sucks it up, pumps it through the filter then the cooler and finally returns to the engine where it lubricates the bearings, cam journals etc. On a VW engine, the typical oil pressure out of the pump is between 4.5 and 5.0 Bar. On some Japanese engines this could be anywhere up to 6 or 7 Bar. The actual pressures aren`t particularly important, what we are looking at is the difference between the pump outlet and pressure going to the bearings.
During heavy cornering the oil sloshes to the side and in this case the oil pickup is uncovered meaning the pump starts sucking up air and not oil. This drops the pump outlet pressure to 0.5Bar (in this example) which is what we call ‘oil starvation’ and allows the bearings to run without the required oil pressure, often resulting in the crankshaft coming into contact with the conrod bearing directly, overheating the bearing and in extreme cases seizing the engine.
How does an Accusump work
Imagine a syringe containing a couple of litres of oil with a big spring pushing the plunger all the time connected to the oil feed line into the engine.
If the oil pressure drops that spring extends and the syringe instantly squirts oil into the galleries maintaining pressure in the engine until it is emptied.
That’s basically all an Accusump is, it’s an aluminium Syringe. It consists of a cylinder that has a piston in the middle, oil on one side and air on the other which acts as a spring.
This photo of a disassembled Accusump shows the design is pretty simple indeed. The piston has 2 ‘O’ rings to seal against the outer wall and keep the oil and air separate and is wide enough to ensure it slides smoothly along the bore without twisting and jamming in place.
As the oil is fed into the cylinder due to pressure from the oil pump, it moves the piston towards the opposite end of the cylinder. As the cylinder is sealed the air at the other side of the piston compresses as the piston moves. This is the ‘spring’ as the pressure is always trying to push the piston the other way and as soon as the oil pressure is less than the air pressure that is what happens, the air pressure forces the oil in the cylinder and back into the engine
The actual installation of the Accusump into a car lubrication system will vary depending on your setup. In mine I plumb it into the AN10 line from the oil Cooler to the sandwich plate on the engine with an AN10 Tee.
The check valve is there to ensure that the Accusump discharges into the engine and doesn`t allow any oil to go the other way, through the cooler and filter and back into the sump.
The actual Accusump is mounted behind the passenger seat but I’ve seen many installed in the engine bay if you have space. The distance doesn’t appear to affect the response time if the hose is large enough. Under 3m and AN10 is sufficient, over that and Canton recommended AN12.
With the Accusump installed and engine off there is no oil pressure anywhere in the system. The air side of the Accusump is pressurised with air to give it a ‘precharge’ This ensures it always has pressure to fully empty the cylinder. Depending on your setup the precharge may vary. After speaking to Canton I settled on 0.75Bar.
When the engine is initially started the oil flows round the system as normal. The feed into the accusump allows oil to flow into the cylinder, forcing the piston towards the air side which compresses the air as the oil side fills.
During normal engine running when there is sufficient oil pressure in the system, the oil in the accusump is not used. It is charged and ready for when it’s needed.
This animation from Canton shows the same thing
During heavy G loads when the oil sloshes in the sump and the pickup is uncovered, the pressure out of the oil pump drops too low. In many instances this can drop to zero. The oil is now discharged from the Accusump into the engine. The check valve ensures it can`t flow the wrong way.
Even though this is less than the normal running pressure, it is still sufficient to lubricate the engine until the oil pump starts producing pressure again.
The Canton GIF showing the Accusump emptying.
Once the pump is pumping again, the Accusump refills and waits to be used as before.
A big benefit of an Accusump is the ability to pre-charge the oil system with oil before starting the engine. We all know engine wear occurs during engine initial startup as there is delay between the engine starting and the oil pump supplying suffucient flow to build up oil pressure and that all important film between the moving parts and stationary bearings..
What the Accusump enables you to do is pressurise the oil system before you turn the key. If the Accusump is charged before switching the engine off, the oil is trapped in it under pressure. That oil can now be used to fill the oil system. All you need to do is open the valve on the Accusump and the air pressure will immediately start discharging the Accusump and producing oil pressure in the engine. This only lasts for a few seconds before it is discharged but it is plenty time to fire up the engine and then the oil pump takes over and maintains pressure as usual whilst also refilling the Accusump allowing the startup with all the bearings fully lubricated before the first revolution.
Which valve type should I fit ?
Electric Solenoid or Manual ?
The unit requires a valve between the oil fed to the engine and the Accusump itself. If you didn`t have a valve, the Accusump wold still work but when you stopped the engine the Accusump would empty itself. This would mean you would overfill the sump as it would contain the normal volume + the contents of the Accusump shown in the image below.
The valve is closed before you stop the engine which traps the oil already in the Accusump inside it. This serves 2 purposes. The first is to allow the checking of the oil level in the sump and set to the desired value. If you normally run on the MAX line on a track car, that’s without the Accusump contents being added to it as you want the capacity of the entire system to include the Accusump which is usually full and waiting to be discharged when needed.
It is easy to check the oil level with the Accusump empty. This would mean you are running with reduced capacity in the sump and the Accusump would be unable to do it’s job properly.
A manual valve is the cheapest and simplest way to operate the Accusump but whist it has the benefit of being cheaper and fewer things to go wrong, it does have it’s own particular issues.
Before stopping the car the revs are raised until the oil pump is producing the usual pressure and as that’s much lower at idle so the Accusump isn’t fully charged. You can either do this on your cooldown lap or if you forget, wait until parked up then raise the revs for a few seconds and then close the valve once the Accusump is pressurised. The oil level can now be topped up in the sump if needed.
The valve needs to be somewhere you can reach to operate by hand whilst harnessed into the car which limits mounting locations. If you can’t reach it a remove activation cable can be connected to allow the valve to be pulled open from you seat.
The biggest issue by far is that the valve means the Accusump can always discharge when the pump outlet presuure is less than the Accusump contents. What that means is if you pull high G and the oil pressure drops from the usual 4.5Bar to say 2.5Bar, the Accusump will discharge until it reaches the same pressure as the rest of the system, ie 2.5 Bar. If you then pull even more G and the pump outlet drops to 0.2Bar, the Accusump is already low and only has a small capacity left to try and maintain pressure to protect the bearings. Basically, you are using the Accusump when you don`t need it and then when it’s really necessary, the capacity is alreadypartially depleted.
The valve is full bore so when the oil pump starts pumping again, the flow is split equally between the engine which desperately needs it and the Accusump which is almost or fully empty
The electric solenoid valve is more expensive and has more potential to fail. The early valves were particularly unreliable and gained a bad reputation but the new ones are more reliable.
The benefits of the electric valve far outweight the negatives in my view. The valve is controlled by a pressure switch which screws into the side of the valve, this is wired in series with the solenoid valve so when the pressure drops below the setpoint, power is applied to the solenoid and the valve opens allowing oil to flow out of the Accusump and into the engine.
The Electric valve has an added bonus. It only allows oil to flow one way at a time, so when it isn’t energised oil can flow from the engine into the Accusump. When it is triggered and open to discharge oil into the lubrication system, oil can’t flow back into the Accusump which means any flow the oil pump is producing has to go through the engine instead of filling the Accusump.
A manual switch is also wired in series to enable / disable the system. This is done for a couple of reasons. The main one is so the valve can be closed when the engine is stopped to ensure the Accusump remains full and doesn`t drain into the sump as described above.
The second is so the Accusump can be emptied, usually for an oil change but I also do it after the sighting laps on track. The engine oil is up to temp and at 90 Degrees but the Accusump is full of oil that has been sat for days and could be 10degrees. The last thing I want to do is feed that directly into the oil galleries whilst on track but by emptying it into the sump when the engine is stopped, it is then refilled with hot oil and can then be left to run automatically all day. Obviously this shouldn`t be done before checking the oil level as the sump will read high when in effect it could be a couple of litres too low.
I have a light wired into the system which flickers everytime the Accusump activates. it’s not particularly intrusive and I am aware it’s there but what it does it let me know if there is a problem developing. If the engine oil is too low, the Accusump will activate more frequently and I can see this from the light. This prompts me to check the oil after the session.
The downside to the manual and electric valves is they need ot be manually opened as you go on circuit and closed before stopping the engine. The best way is to have the ECU control the solenoid instead. My DTA Ecu has spare outputs and I have set it up so the Solenoid is closed unless the RPM is over 2,000 AND the oil pressure is below 2.5 Bar. This ensure when I return to the pits and the engine is ticking over at 2.0Bar, the Accusump doesn’t discharge itself as it would if just controlled by the switch. Whilst not essential I find that since having the ECU controlling the operation of the valve I never forget to enable the system when on track and the trigger point can be tweaked if necessary.
I`ve also connected a pressure transducer to the air side of the Accusump using a Tee to retain the original analogue gauge and add the 0-10Bar transducer
Once installed, this transducer is connected to the ECU using a spare analogue input.
What this let me do is monitor the remaining contents of the Accusump. I have a warning set to pop up on my dash when there is only 25% remaining. That way, if I`ve just driven a sequence of corners which have required the Accusump and I’m approaching another, I don’t get halfway through them and find the Accusump is empty. The warning is sufficient to prompt me to either lift off and let it refill or make that snap decision tif I think the corner won’t cause oil starvation.
The best example of this is Pouhon at Spa. It’s a long high G left hander and the Accusump is discharging all the way round the corner. The 25% warning usually flickers on just as I’m starting to straighten the wheel after the corner which is just about perfect.
Another benefit of reading the air pressure is I can check the critical pre-charge pressure. When I empty the Accusump after the sighting laps the air pressure should be between 0.7 and 0.8Bar. If it drops under 0.5Bar the dash displays a warning telling me the precharge has been lost. Without the precharge the Accusump is useless and it’s vital that it is checked regularly.
Which size Accusump do I need ?
My personal suggestion would be to fit a 2 or 3qt version. If you are going to the expense of fitting one just for pre-starting lubrication, why not have it available for low oil pressure protection at the same time. ?
Canton have a helpful guide on size selection including part numbers.
Once you have chosen the size of Accusump, valve choice is next. As I explained above, I initially used a manual valve and if budget is limited they are a reliable choice but for improved protection the electric valve would be the one I’d recommend.
As I mentioned at the start of this post, the actual installation varies from car to car and I only have experience in plumbing in to the cooler > engine block feed line. For details of other setups I’d email Canton or your local stockist. I used and recommend ThinkAutomotive as they were extremely helpful and competitive on cost, they were able to supply the AN10 Tee, fittings and non return valve in addition to the Accusump and electric valve.
Does it actually save the Engine ?
Since installing after oil starvation caused a spun bearing and scored crankshaft the Accusump has saved my engine numerous times. When I first fitted it I forgot to enable it on a wet Bedford trackday. Luckily I wasn’t putting the engine under huge load during oil starvation and it’s the only data I have of a session without the Accusump and then turning it on halfway through when I realised it was turned off !
The difference is obvious. Without the Accusump the oil pressure drops to just above zero around a dozen times before I realise and open the valve. For the rest of the session the pressure starts dropping but never goes below 1.8Bar as the Accusump discharges when needed.
This is the clearest example I can possibly give that shows how the Accusump maintains minimum oil pressure.
This is a close up of a couple of pressure drops. The first drops to 2.5Bar (1) and thesolenoid is requested to open by the ECU (2). The pressure immediately rises and after the corner the oil pressure starts to rise with the Accusump be refilled shortly after (3)
The second pressure drop (4) does NOT require the Accusump. It’s above the 2.5minimum I set in the ECU so the valve stays closed and the pressure recovers by itself after the corner as normal.
After 12 months of running with the Accusump and using it for pre-oiling during engine starts, the big end bearings were almost like new. That’s after a couple of thousand miles of track driving.
What trigger pressure should I use ?
The pressure switch trigger point can be tweaked slightly, so if you buy a 1.5Bar switch it can be adjusted to operate between approx 1Bar and 2.5Bar. This is done by adjusting a 4mm hex screw which changes the switching point of the internal spring. I’d highly recommend leaving it alone and going with the setting Canton recommend for your setup, but I’ve included this information for completeness.
You may think to yourself that simply setting the trigger to be just under your operating pressure would be obvious choice but the problem is the Accusump has a limited capacity. If you are running at 4.5Bar and trigger it at 4.0Bar, that will require almost all the contents of the Accusump just to maintain that pressure.
Just imagine a large glass of water, you can either tip it all out by turning the glass upside down in one go or tip it at a shallow angle and let it run out slowly. Once the glass is empty you have tipped out the same amount of water but over differing periods of time. You can have a large flow but it empties quickly or a slow trickle that lasts longer. It’s the same with the Accusump. The lower the pressure you try to maintain the longer it will last.
To show the difference between various trigger points I stopped the engine and discharged the Accusump several times using several pressure-switch settings.
These are purely to show the effect varying trigger points have on how long that can be maintained. The oil system is completely empty before switching on the Accusump so the time it lasts is not what you see with hot oil and a running engine that already has fully oilways and much thinner oil than was the case during these tests.
This sped up recording of the digital dash shows the Accusump emptying with a 1.6Bar target and the low warning alarm showing when it is 25% empty.
With a target pressure of 2.6Bar, the oil pressure jumps to 2.6Bar and is controlled for 5 seconds. After that, the contents bleed away over the next minute and a half until the Accusump is empty.
By lowering the target by 0.5Bar to 2.1Bar, the volume lasts twice as long before falling away.
Lowering the target to 1.8Bar sees the Accusump able to maintain that for 17 seconds
A reduction of only 0.2Bar to 1.6Bar sees a huge increase in the time the Accusump can maintain that pressure before depletion.
Finally with a very low target of 1.2Bar it takes almost a minute to empty.
Hopefully that has given you a better understanding of not only what an Accusump is but how it works. The differences between the manual and electric solenoid valve and the effect of changing the trigger pressure on electronic valve setups.
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