Max Safe Inlet Air Temps?

So I'm now ready to go and set of the carb - on the road with a camera looking into the carb and a wideband lambda to see whats going on with fuelling. However I could do with know what is a safe max IAT... I have a thermocouple fitted in the outlet of teh supercharger and it would be useful to know what is considered to be a max safe operating air temperature. The pistons are 'standard' flat top oem spec. CR is 8.2:1. Distributor modified to limit max advance to 24deg (as per other thread in this forum). No intercooler fitted as the SC is bolted straight onto a manifold at the rear of the engine. 

Is there a rule of thumb that if you go over a certain OAT you should increase fuelling by a certiain amount? Or is there a point where I should think about doing something funky like water injection?

 

Thanks

 

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Not really, pressure & temperature being intrinsically linked all those factors that affect what the actual combustion chamber pressure is count too. The objective is to avoid detonation, watching exhaust gas temperature would be better for diagnosing that.

 

Somebody may well have some ballpark figures though, I think there was a bit in the Vizard bible relating to intercoolers.

Bear in mind that there will be a latency between the carb needle position and the AFR reading.Whether this is significant with a carb I would not know.I always think limiting the dissy is a band aid and may restrict performance over a large part of the rev range.MAP referenced ignition would be my only choice.

One of my injected cars had the almost universal Mesh filters in the engine bay.The engine was OK but misbehaved badly in traffic as the underbonnet temps rose.Refitting the original filter with grill mounted air intake saw great improvements.

Nice looking install.Steve..

Well the first drive out just now has shown that all my efforts so far have been worth it - the performance improvement from the 998 N/A installation is impressive to say the least, even with wild errors in the fuelling :)

 

I understand your point about fully mapped ignition using MAP as a reference, however I want to keep the car on a distributor as it'll be reverted back to N/A next winter for my newly 17y/o to have it as his road car (there's no way he'll be able to insure it for a few years with the SC installation), then when he's a bit older he can put the SC kit back on it.

 

Currently running pretty lean part throttle (~1.3lambda), but 0.95lambda at what over 4000rpm so gave it some beans and and no sign of detonation with IAT hovering around 85degC. Hope to get out in it with some richer needles later today to see how close we can get before getting the file out!

 

Very happy indeed so far!!

You say you have fitted a  thermocouple as opposed to an IAT sensor, would that not be more inclined to pick up inlet manifold heat and exhaust heat soak rather than air temperature . Have you wrapped the exhaust?I have no experience of supercharging but 85 I would think is high for the air but is correct temperature for the engine itself. If it was a turbo I would expect high air temp and preferably fit an intercooler although the Metro managed well without one.

85 doesn't sound unreasonable for forced induction. Though  an intercooler would likely give a noticeable benefit.

 

Programmable ignition can give just as much improvement on a regular engine, low speed tractability & economy, but it'd still be a mod to declare for insurance.

85 C is monumentally high for inlet air temps. Most turbos with an intercooler are circa 30 degrees (OAT dependant). If you're 'listening' for detonation, you won't hear it. It's something that needs to be detected at engine level - be it det cans or a electronic hardware incompassing a knock sensor on to the block.

 

As a guide, or idea, most EFI ecu's start to pull timing out of the MAP anything over 40 C, by 80, they're usually pulling around 8-10 degrees. Get it set up properly with knock detection equipment.

There will be a cooling effect by the fuel vapourising at the carb. One of the guys here was getting the intake pipe so cold it’d get ice on it

The intake temp pre and post supercharger would be interesting to see too

He isn't running an intercooler though. Increasing the pressure will increase the temperature, then there'll be friction on top. It's still a small difference compared to the effect of the engine's compression ratio. He's running 8.2, even Rover were running 9.4, or higher, with warranty claims to bother about.

 

If somebody has a yellow bible to hand I'm sure there are some actual figures in there.

As you’re running a suck through installation you have no option to intercool. Those roots type blowers are not particularly efficient & at 10psi for example your charge temp could be over 100deg.
These kind of applications is where water injection is highly effective. Increasing fuelling slightly will help but running excessively rich mixtures to compensate for charge temps isn’t the way to go. Any temperature sensor needs to be isolated to avoid heat soak distorting the figures.

Phil.

Thanks for feedback guys.

Fuelling is now set up. I’ve modified the needle to chiefs 0.82lambda at what over the operational engine speed range.
The thermocouple IS measuring IAT at the SC outlet and it’s reading is not affected directly by the ambient conditions around the exhaust manifold. With fuelling set up correctly as above I’m seeing max IAT of ~82degC under load at ~3000rpm. Interestingly this figure drops slightly to ~72degC as engine speed increases to 6500rpm. I’m not sure if fuel vaporisation is causing the drop in temp or if it’s some other effect. These figures are recorded in ~12defC ambient. Also now I’ve run the engine through its full operating conditions the max boost is 12psi at 6500rom - a little higher than I’d calculated it should be with the pulley ratio I have (std SC pulley and a crank pulley I made)
Hadn’t realised the subtleties of identifying detonation so will see if I can rig up something to help me listen for it.
I’ve not reinvented the wheel here by strapping an M45 to the back of the engine but it does seem particularly hard to find out the finer details of what conditions others who have done this are seeing.
Water injection is clearly a good thing to do so I will see what’s involved in applying it - I’m a fan of making, not buying..!

Ray

Edited by fatcatvera, 19 February 2022 - 10:10 AM.