Add Fuel On Decel To Cool Pistons?

[No Crust Racing]

Just had a random thought, willing to get laughed at if I'm way off the mark, but if I can add fuel on throttle close events, under a given throttle percentage, is that going to help in piston cooling or is it going to cause more backfire and higher EGTs?

The SAFC II has a Decel Air setting which is used to add some fuel in those situations, for avoiding stall when using an Atmo BOV, my BOV is plumbed back in, so I'm thinking I can use those two throttle points to add some fuel in when the throttle is closed and get some extra piston cooling as the car is a track car (CA18 with a GT2560 on 16-17psi). As for WOT mixtures, it doesn't go above 11.5 for safety so there's plenty of fuel in the map as is.

Forgot to add, I already have an oil cooler, oil temps generally don't exceed 85-90 on a real hard session. Was just thinking that if I have the functionality and it will help then why not.

Edited July 11, 2012 by ActionDan

[Lithium]

I like the way you are thinking outside the square, but in this case I think you are overlooking or missing a lot which might make you think otherwise.

Here's one of the easier things to consider though - I assume you've heard of overrun injector cut-off (/ various other names meaning the same thing)? Imho that would have more of an overall cooling effect than anything you're going to be able to do by adding fuel. Any time you have fuel and spark you are going to be generating much more heat than if you have no fuel at all.

[No Crust Racing]

I'm aware they can be cut off but I do not have that level of control with the management I have.

I could test this poperly but I'm not sure if the dyno I use has an EGT probe.

I'm in two minds about it, My dads mate who builds the mufflers for the V8 supercars said the over run absolutely cooks the mufflers but I'm guessing there's some benefit.

[No Crust Racing]

Seems the consensus on NS is not to do it based on EGTs and the amount of fuel needed to have any notable effect.

[JonnoHR31]

Assuming you have a standard ecu with the safc? If so the stocker ecu should cut fuel on decel until lowish rpm's (~2000rpm) anyway which will cool them down more than any combustion even, no matter how rich.

[HYPED6]

I dont think you are grasping the EGT thing.........

The extra fuel will cool the piston. End of story. That is one of its jobs.

Now when you have no bang.... you have no heat..... so your temps drop.

Heat only becomes a problem when there is too much. A piston will last a limited time at 1000+ temps, very long time with EGT's of 500deg and even longer with EGT's of room tempreture.

So if your EGT's of 750 at full throttle and you cool them another 70deg under decel your not going to gain anything in longevity just use more fuel.

[Dose Pipe Sutututu]

Good way of shooting massive flames out of the exhaust..

You should be adding fuel on firing to cool the pistons, on decel you're just spitting it out of the exhaust.

[Rolls]

I remember hearing from karters that ran carby powered motors who would put their hand over the air filter to make the motor run extra rich to cool it down, to the point where they could even see the coolant temp drop in the time they had their hand over it.

So if your EGT's of 750 at full throttle and you cool them another 70deg under decel your not going to gain anything in longevity just use more fuel.

This is where we need some hard science, if you are to run standard size injectors at 20% duty cycle for 5 seconds on the overrun how many degrees are we likely to see drop?

Saying 70c drop is just a purely guess, we really need some hard datalogging to make any real conclusions. Has anyone actually done this in a race car or know of people doing it? Would be interesting to see some real world results, otherwise all we are doing is guessing.

[R31Nismoid]

So if your EGT's of 750 at full throttle and you cool them another 70deg under decel your not going to gain anything in longevity just use more fuel.

Would it perhaps aid in keeping water & oil temps down? In a circuit application, say 5+ laps.

[89CAL]

I remember hearing from karters that ran carby powered motors who would put their hand over the air filter to make the motor run extra rich to cool it down, to the point where they could even see the coolant temp drop in the time they had their hand over it.

I used to do karting in this was mainly done at first startup to richen the mixture to aid the engine on the first run of the day. Holding your hand over one of the two inlet holes on the filter housing chokes the engine to much.

I've never heard of people doing this in competitive karting to cool the engine done. There are two mixture adjustments on the carby and once I set these I never worried about it again. I had EGT on my steering wheel and you can hear when your running to lean anyway. You can drastically drop EGT by slowing down to say half throttle around 2 corners.

Its much easier to see for yourself

I don't see the point of injecting fuel on decel, with the possible of hardcore race cars doing long sessions

[HYPED6]

I remember hearing from karters that ran carby powered motors who would put their hand over the air filter to make the motor run extra rich to cool it down, to the point where they could even see the coolant temp drop in the time they had their hand over it.

This is where we need some hard science, if you are to run standard size injectors at 20% duty cycle for 5 seconds on the overrun how many degrees are we likely to see drop?

Saying 70c drop is just a purely guess, we really need some hard datalogging to make any real conclusions. Has anyone actually done this in a race car or know of people doing it? Would be interesting to see some real world results, otherwise all we are doing is guessing.

I have done it on the dyno. The EGT's are not fast enough to get an accurate result. It cooled if there was fuel added under decel for sure, it also cooled if no power/heat was generated in the combustion chamber. If the engine is tuned correctly you should never need to add extra fuel on decel. If your running a carby and you engine is over heating from excessive lean mixtures then yes...... running more fuel will help.... when its too lean

[discopotato03]

Feeding unburnt fuel through an engine achives nothing . The only way fuel could ever cool anything is when it evaporates and you get a charge cooling effect . When you burn fuel you generate heat - thats its purpose . Heat = thermal expansion = pressure rise to push the pistons down on their power strokes . In theory a rich mixture burns cooler than a lean one but thats a simple rule of thumb because too many other factors ie ignition timing play a part in combustion temps .

If you were paranoid about piston temps you could ceramic coat the crowns and use oil squirters to transfer some of the heat to the oil .

I'm guessing that you have read about running slightly richer mixtures under load to reduce combustion temps .

I reckon the point is an engine with a closed throttle (over run) has a very low dynamic compression ratio because the throttle is literally strangling it and the cylinder filling efficiency is obviously very low . Because its so low the mass of air going through the engine will be also and the AFR to get even a reasonably rich ratio ie 11 to 1 is going to take stuff all fuel . The piddling amount won't have the capacity to absorb much heat so I think it would achieve SFA .

I think the best over run cooling idea is to have no fuel at all because reasonably cool air goes through the engine unburnt meaning no combustion heat what so ever and the engines cooling systems are still operating so for any significant amount of time the engines internals will cool further .

Most will tell you that the highest temps occur in modern engines at part throttle cruise because of the lean running conditions . Turbo engines manufacturers style typically run reasonably rich under load beacause they are not intended to run at full load for any length of time and the extra cooling adds reliability .

The question I ask is how often and how long do you expect to have your engine in over run mode ? If you can tune your computer by all means richen it up on over run , I think all it will do is cough and fart and foul you plugs , and make throttle response a bit wooly until it cleans itself out once on throttle .

You may also risk slight bore washing and some extra cylinder/ring wear plus a bit more fuel contamination in your oil .

Your call , A .

[No Crust Racing]

I wont be going this route based on everything I've read and heard, the mix is plenty rich in my opinion anyway at 11.5 at its leanest as I wanted some headroom.

We may also end up with some more overlap when I put the DET EXhaust Cam on the intake side and dial in the gears which would mean more fresh air coming in sooner on the exhaust stroke which would also aid cooling.

[discopotato03]

If you can move your cams around with adjustable pulleys you can change your ovelap timing . Uh , not really sure what you expect to achieve with more overlap because thats more a scavanging thing than a cooling thing .

Generally turbo engines don't like a real lot of overlap timing because the static compression ratio is often lower than NA versions and you have an exhaust manifold and turbine housing designed to get a turbine up to speed quickly . Until a turbo actually does something it acts like a small system restriction on the exhaust side and a low engine speeds and loads (low exhaust gas velocity and pressure) mostly all wider valve overlap timing does is increase exhaust gas reversion back into your cylinders . Hot gasses going backwards before the exhaust valves close and shut it off .

The other thing wider overlap timing does is decrease you cylinders "trapping efficiency" at low revs which reduces your dynamic or effective compression ratio .

Any engine and moreso a turbocharged one is a "system" meaning a certain combination of components achieve a good overall result .

Things like static CR cam timing ITBs exhaust manifold and turbos turbine sides all have direct affects on how the system works . Changing one thing can mean the system is now incomplete and does not work as well as it should . Manufacturers change things and alter others to compensate to get the good overall outcome .

All this aside you obviously have concerns over piston temperatures and it would be good to know whats raised these concerns you have .

If its to do with running std or even std NA pistons there are ways to make them better but they have to be removed and coated and it could be easier and cheaper just to use new OE turbo pistons . Again generalising but OE Nissan turbo pistons are pretty good but hardly F1 standard and mostly don't need to be .

The thing with turbocharging NA engines is doing everything you can to reject more heat because the NA system was not designed to do this . Better radiators and particularly oil coolers are a must and do anything you can to eliminate detonation .

Warmer cams are about making an engine breathe better and make power at higher revs than standard . OE turbo cams are about making a lower CR engine have acceptable power (and economy/emissions) off boost and good mid range torque with boost .

Using OE turbo cams with NA pistons isn't always the answer because again they were not designed to work together as a system . I would think that slightly warmer cams would be Ok on a NA+T engine though a little less hotside restriction would help too .

My old Subaru RX Turbos EA82T has NA cams in it but its running a tad higher CR and a much higher flow turbos hot side and manifold than it originally did . Needed all the breathig help it could get and NA 256 cams were better than the std I think 248s , I don't think the few extra degrees of valve overlap did anything significant for it .

A .

[No Crust Racing]

Cheers for the considered and lengthy response.

Car is factory turbo so not an NA, I'm not overly concerned about piston temp, but I know the SAFC has the Decel air function and the thought occurred to me that I might be able to put it to good use, though it doesn't seem to be the case. That's really all the thread was about, can I use that setting for some extra cooling and it looks like the answer is know.

If I get concerned I'll just swap out my 13 row cooler for the 19 row cooler I've got here and I'd say that move would do a lot more, but it requires work, so my thought was if the SAFC has the function and it's literally a 5 min job to set it then why not, but it doesn't seem to be the case so I'll leave it as is.

As for the overlap, simply something I'd read and I'm no expert, you get a small bonus as there's more time available for fresh air to flow in and out. When I get it on the dyno, my aim will be mid range more so than top end but that's a completely new topic for another thread

Cheers

[Dose Pipe Sutututu]

here's an example of too much fuel on decel on my old R33... I am not going to mention names on who tuned it (when I had no idea on how to tune cars)

[No Crust Racing]

While I realise that's not a good thing, it looks cool as hell - no pun intended.

[Dose Pipe Sutututu]

While I realise that's not a good thing, it looks cool as hell - no pun intended.

I had a donkey tune my car, actually made less power afterwards with more knock not to mention on full song A/F was at 12.3 with 1.3bar of boost, surprised my motor didn't die, he belongs on this forum and is praised by many.

I now trust no one and do it myself.

[No Crust Racing]

I've got a donkey tuning mine too, me

That's why I bought a CA, cheap and tough for me to butcher.

Edited July 12, 2012 by ActionDan