Heater Matrix bypass

[JustGav]

Okay,

 

This has been bugging me for a while, I've spotted a few cars which have had the heater matrix bypassed which have then gone on to have HG problems or overheating issues.

 

Is it really such a good idea to bypass it, in my mind no, you are shortening the coolant system, and while a lot might not pass through the matrix system it will certainly have some effect and may shed that little bit of heat that seems to be taking out the HG.

 

Gav

[Digsy]

If you bypass it with a restriction (mimicing the heater matrix) then its OK.

 

Why anyone would want to bypass it without a restriction is beyond me.

[JustGav]

If you bypass it with a restriction (mimicing the heater matrix) then its OK.

 

Why anyone would want to bypass it without a restriction is beyond me.

 

Exactly what I was thinking... and lines up with my thinking on the cars that I've seen with HG probs...

[JustGav]

Would I be correct in thinking that the matrix is just an alternative branch of the cooling system and as such will be slightly restricted to force the water through the alternative routes, when the matrix is bypassed the water will take the easiest route, ie thru the bypass and therefore not cooling the head/block properly.

[JohnA]

Would I be correct in thinking that the matrix is just an alternative branch of the cooling system and as such will be slightly restricted to force the water through the alternative routes, when the matrix is bypassed the water will take the easiest route, ie thru the bypass and therefore not cooling the head/block properly.

smart observation, I hadn't thought of that:cool:

Of course, getting rid of a restriction in the path would change the coolant flow balance on other paths, below or after.

Since the cyl head route is nearby, one has to be very careful. Even *more* flow can damage the head (too high coolant speed could mean less time spent around hotspots)

 

Gav, one brownie point from me:Pling:

[dangerous brain]

Would explain the BHG on that shed I had.

[JustGav]

Would explain the BHG on that shed I had.

 

It was yours that has made me think about it, and a certain car for sale which has had a similar issue...

[RobSheffield]

My car had a bypass of sorts, but not due to a dead matrix (i hope) that basically ran from the matrix pipes to the top rad pipe.

 

I have now removed it!

[Digsy]

Since the cyl head route is nearby, one has to be very careful. Even *more* flow can damage the head (too high coolant speed could mean less time spent around hotspots)

 

That's incorrect. The temperature rise of a flow of coolant is a function if the mass flow rate. A lot of coolant flowing fast over a "hotspot" will absorb the same amount of heat as a smaller amount flowing more slowly (up to a point). The difference being, of course, that the smaller amount flowing slowly will get hotter doing so.

 

Since the goal is to keep the head as cool as possible, faster flowing coolant is always better. Coolant flow should always be above 0.5m/s in all areas of the water jacket to avoid localised boiling.

[JustGav]

Does anybody have a coolant path diagram for the 2JZ..

 

It might worth looking it to see if the bypass is more destructive than perhaps blocking off the pipes and forcing the coolant down the alternative routes.

[Digsy]

Does anybody have a coolant path diagram for the 2JZ..

 

It might worth looking it to see if the bypass is more destructive than perhaps blocking off the pipes and forcing the coolant down the alternative routes.

 

 

There's one here. (GTE is the same through the engine, but the turbos just confuse things).

 

If you bypass the heater matrix then the heater velve (old skool or what? ) will be redundant. Coolant will circulate straight back to the water pump inlet causing:

 

1) Less flow through the radiator circuit.

2) Higher water inlet temps.

3) Less flow through the head. Note how the heater feed comes fromt he back of the head before coolant flows through it. This is interesting.

 

Because the block is seeing the same coolant flow rate, albeit at a hotter inlet temp, the temperature rise acorss the block will be the same. However, the head will see significantly less coolant flow, so the temprise across the head will be larger. Thus the temp rise across the whole engine will be larger.

 

In a U-flow cooling system (like the 2JZ), especially with engines with a cast iron block and an alloy head (like the 2JZ) the temp rise from coolant inlet to outlet is critical - especially on a long engine. The differential expansion between the head and block can go out of control and fcuk the head gasket if the rise gets too great.

[JustGav]

In a U-flow cooling system (like the 2JZ), especially with engines with a cast iron block and an alloy head (like the 2JZ) the temp rise from coolant inlet to outlet is critical - especially on a long engine. The differential expansion between the head and block can go out of control and fcuk the head gasket if the rise gets too great.

 

This is exactly what I suspected when I heard of the indestructible HG's going pop and it seemed all had a matrix bypass....

[VIL]

Strange and fortunate you should be discussing this issue now...

 

I have just done the heater matrix in my diesel estate and rather than change the matrix again (have already done it once) I was going to bypass it?

 

I cant see a huge amount of heat loss through a matix occurring as its pretty small so little radiating effect and equally it has no air flow?

 

I may be incorrect in assuming this so will watch this thread with interest....

[JustGav]

It might worth looking it to see if the bypass is more destructive than perhaps blocking off the pipes and forcing the coolant down the alternative routes.

 

This may be the way forward then, HOWEVER the matrix should be changed and not just blocked off...

[Digsy]

Blocking off the matrix would actually be better than bypassing it, as it would force all the coolant through the radiator.

 

On the 2JZ it looks like you can actually block the heater matrix off anyway by closing the coolant valve.

[JustGav]

I cant see a huge amount of heat loss through a matix occurring as its pretty small so little radiating effect and equally it has no air flow?

 

The heat loss due to the matrix isn't the issue as such, it is the water path changes that are the issue. In this case it appears that 'shorting' out the matrix will cause more problems.

 

I suppose another thought could be that when the heater matrix is open for long periods of time that the ambient temp outside the car will be cooler (Well, you don't have the aircon set to 30 when it is 30 outside), so the engine will benefit from that sufficiently to offset the cooling flow loss from having the matrix active in the equation, although being a radiator of sorts it will still provide a good level of restriction ensuring coolant flow is around the main routes.

[JustGav]

On the 2JZ it looks like you can actually block the heater matrix off anyway by closing the coolant valve.

 

My A-series engines on the morris minor's have had this... admitedly it was then cable driven to the dash *grin*

[Digsy]

Like I said - old skool, baby

 

Modern cooling systems have the heater matrix always in circuit and control the HVAC by altering the airflow thorugh it.

[carl0s]

On the 2JZ it looks like you can actually block the heater matrix off anyway by closing the coolant valve.

 

Does this mean that anybody spending the time to bypass the heater matrix with hoses etc, should really just turn a little tap in the engine bay - job done, properly?

[JustGav]

Does this mean that anybody spending the time to bypass the heater matrix with hoses etc, should really just turn a little tap in the engine bay - job done, properly?

 

Something along those lines, I was actually going to have a look on an engine quickly myself... BRB *grin*

[JustGav]

Just had look at Graham S's car with him, and the 'tap' is definitely not accesible, so the best plan would be to put bungs into the two pipes when 'bypassing' the matrix, however the best thing of course is to replace the matrix.

[Digsy]

I don't think there's a tap in the engine bay. The coolant valve is probably linked to the HVAC controls, and is what allows hot coolant to heat the cabin air.

[JohnA]

That's incorrect. The temperature rise of a flow of coolant is a function if the mass flow rate.

Theoretically.

In practice there is an acceptable range of speeds where this rule works. Too fast or too slow and the function is very different.

 

Since the goal is to keep the head as cool as possible, faster flowing coolant is always better.

not always, no. You are assuming that coolant/metal heat exchange is always constant. It isn't.

That's why sometimes the internal casting is intentionally rough and 'restrictive' so at that location the coolant will have to spend more quality time with the neighbours.

 

I remember on the Vaux V6 there is a thermal 'gotcha' when it is turbo/supercharged beyond a point, and speeding up the waterpump produced the opposite results to what we expected. It looked bizzare at the time, but there are all sorts of flow eddies in there. The heat exchange conditions are not always as smooth & simple as we would like them to be.

Same as aerodynamics.

 

Coolant flow should always be above 0.5m/s in all areas of the water jacket to avoid localised boiling.

yep, there is also a max speed as well.

[JustGav]

Tech sticky please on this?

[Digsy]

Cylinder head water jackets, especially those with longitudinal coolent flow, do commonly use what I would call deflectors above the combustion chambers to force the coolant down into a narrow passage above the hottest areas. These to all intents and purposes do look like restrictions. However, they are there to increase the flow over the hot surface, and hence reduce the temperature of the coolant as it leaves the area.

 

A well designed water jacket, however, will be of a very narrow cross section which hugs the flame face and exhaust ports, and hence has higher coolant velocities. This comes at the expense of system back pressure, so your pump has to work harder to deliver a given mass flow rate.

 

I don't pretent to understand the maths behind fluid dynamics, but I know that when I'm designing a water jacket, unless I can get rid of all the low flowing and recirculating areas, the people that do understand it don't stop beating me up. I've never had one come up to me saying "bit of a problem, mate. Far too much flow here. Can you slow it down a bit?" In practise its always too little flow at too low a speed that keeps you booking the overtime.

 

Incidentally, even though I don't understand it, I did have a quick Google. This Wiki page gives an equation for heat transfer coefficient which shows it is proportional to the Nusset number, which is in turn is proportional to the Reynolds number, which is itself in proportion to mass flow rate.

 

As for your Vauxhall V6, I can't comment on that objectively. Maybe your pump was cavitating or something. Maybe, as you say, there was a lot of recirculation going on which could have been caused by the increase in pump speed. That's not to say, however, that it was the increased cooling flow that was knackering the heat rejection to coolant - more like the increased flow was compromising the flow within the water jacket itself, leading to reduced cooling. Just a theory.

 

Maybe there are indeed upper limits on coolant flow, but they've never ever been a practical problem in any of the water jackets that I've designed.