Cold intake gas = Good?, Hot exhaust gas = Good? 4

[Gary_321]

thread71-29226

I have a BMW M Roadster and I've been gently modifying using labour and minimal cost - my rules.

I have insulated the intake side of the engine with aluminium shielding and loft insulation and gold insulation tape...

I didn't have any testing software before the exhaust modification (see below), but the "before and after" for the intake modifications show a decrease in the intake temperature and an increase in the air mass flow rate.

Until now, I have been using automobile forums to influence my thoughts and ideas. Since I have found this site, I'd like to seek your thoughts about my future ideas...


I have been looking at ceramic spacers to thermally insulate the throttle body from the engine to further cool the intake gas

Is hot exhaust gas a good or a bad thing?
I have also been looking at spending big bucks on having my stainless steel headers ceramic-coated (Zircotec) to insulate the exhaust gas and further cool the engine bay. Automotive forums think this is a good idea, but engineering forums seem to think it is a bad idea. I'm trying to get a definitive view (with evidence) as to why I should, or shouldn't do it.

I welcome your views (both pros and cons) for my latest ideas.



As an aside. To increase flow (I know that you need a certain amount of back pressure) I have gutted the exhaust mufflers...

Original

Modified

 

[EdStainless]

An ICE is nothing more than an air pump. By supplying colder (denser) air you can move more pounds of air through it.
That is if you can provide enough fuel to keep the F:A where it needs to be.
Insulating/isolating the intake can help performance.
Insulating exhaust can have various unintended consequences, it will move more heat further down the exhaust system. This will change the normal running temp of various parts and also change the thermal expansions. It might cause issues, or it might not. Where is your O2 sensor located?
Have you done any work on the heads or valves? That is the next place to look related to improving the flow.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Tmoose]

"Needing back pressure" may prove a debateable conclusion.
Have any controlled acceleration or top speed tests confirmed the benefit of your modifications to date?

good luck,

Dan T

 

[Compositepro]

The only place that exhaust temperature is important is in the cylinders. If you have a turbo you want the inlet gasses as hot as possible, as long as the materials can tolerate such high temperature. Jet engines must use cooling to keep the turbines from melting. Ceramic liners in cylinders have been proven to increase efficiency of engines and gun barrels.

 

[GregLocock]

If you don't have a turbo and you don't care about emissions then a cold exhaust is good, except that the heat will go somewhere, and inside the engine bay is not the right place. Some OEMs use double skinned downpipes to keep the exhaust heat in the exhaust.

If you've got a cat then most of the back pressure is in that, I doubt the rest of the exhaust adds another 50%.

I don't think 4 strokes 'need' back pressure.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[EdStainless]

The only real issues that I have seen with backpressure are related to the shape of the power curve at WOT.
There is some interaction between the length of intake runners and the exhaust backpressure. This can result in shifting the peak torque either up or down in RPM.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[GregLocock]

Strictly speaking that's not back pressure, that's dynamic wave propagation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Gary_321]

Many thanks for the feedback

Most other fora that I use have a "Quote" facility. I can't find it here, so I've constructed this reply in Notepad and pasted it as a new post. I can only apologise until I find my way around .

EdStainless

An ICE is nothing more than an air pump. By supplying colder (denser) air you can move more pounds of air through it.
That is if you can provide enough fuel to keep the F:A where it needs to be.

- Exactly as I understood it. I'm glad that my advice on other forums agrees exactly with what you have written - thanks.

Insulating/isolating the intake can help performance.

- It is well insulated and the air intake is drawn from an air duct to the brakes (it is not scooped, so the effect of vehicle speed is negligible - I'm not sure if the engine would be able to handle scooped air. I know that carbureted engines ran overly lean if they were supplied with scooped air.) I've managed to reduce the intake temperature (measured inside the plenum) from 10°C above ambient to around 3°C above ambient. Although the best improvement is the ability to reduce the temperature after a stop. Previously, the temp would rise to around 40°C and not reduce when I started driving; now, the temperature will come down when I restart driving .

Insulating exhaust can have various unintended consequences, it will move more heat further down the exhaust system. This will change the normal running temp of various parts and also change the thermal expansions.

- This is my concern, and why I have come here for advice. My headers are stainless steel. I've read that ceramic-coating stainless steel can induce stress in the headers and cause them to crack. I've not seen much evidence for this, but the engineering logic / material science would indicate this to be the case. Mild steel is less brittle and might be able to handle ceramic-coating better - is there a grain of truth here?
- Here in the UK, moving the heat further down the pipe may be a good thing, as the catalytic convertor will get hotter. Is it likely to be over-heated?

It might cause issues, or it might not. Where is your O2 sensor located?

- I have two O2 sensors. One on each manifold...

(I'm obviously not working to McLaren's standards of hygiene!!)

Have you done any work on the heads or valves? That is the next place to look related to improving the flow.

- On this engine (S50B32), the head is already ported and polished. There are improvements that can be made, but I don't think that I will get much improvement without spending big bucks and having to do a full engine strip-down.

This winter, I plan to measure the actual cam curve and measure the effect of different tappet shims. They are available in 0.05mm stages (but not all 2.75mm shims are 2.75mm ). I will attempt to maximise the valve opening time, but still staying within tolerance. Ideally, I'll find somewhere that can make (or modify) shims to within 0.01mm.

I plan to do a full engine strip-down at some point, but not this winter. When I do, I will be aiming at lightness and balance as well as gas flow.


Tmoose

"Needing back pressure" may prove a debateable conclusion.
Have any controlled acceleration or top speed tests confirmed the benefit of your modifications to date?

- I would think that there is still sufficient back pressure in the system. I did rig-up a compressor to test the welds (using talcum powder), and there was a back pressure in the muffler, even post-modification.
- I haven't done any controlled acceleration or top speed tests, but I have done several tests using the same test route. Obviously, the driving conditions won't be the same and there are too many variables to eliminate, but I have got some graphs...

Temps before intake mod...

Temps after intake mod...

Something that I have noticed is the MPG. The best that I used to achieve was 26mpg and the worst was 17mpg - for enjoyment purposes, you can swap the words best and worst (if you know what I mean!). Since the intake mod, the best is 30mpg and the worst is 14mpg. To me this indicates that the engine is very efficient on slight throttle and is using a lot of fuel (and air) at WOT (wide open throttle). I can also accelerate faster than my friend in his M Roadster - again, this is subject to too many variables to be conclusive.


Compositepro

The only place that exhaust temperature is important is in the cylinders.

- Interesting. I thought that hot exhaust gas was less dense and could flow faster. This has always been one of the automotive forums' reasons for insulating the headers.
- I'll use this information to consider ceramic-coating the combustion chamber, valve faces and piston crowns. I don't think that I'll be able to do the bores as the piston rings will damage the coating.

If you have a turbo you want the inlet gasses as hot as possible, as long as the materials can tolerate such high temperature.

- My engine is normally-aspirated.

Ceramic liners in cylinders have been proven to increase efficiency of engines and gun barrels.

- When I go for the big strip, I think that I'll look at ceramic-coating the combustion chamber and the piston crowns. I've read that it reflects the heat "back into the fire".


GregLocock

If you don't have a turbo and you don't care about emissions then a cold exhaust is good, except that the heat will go somewhere, and inside the engine bay is not the right place. Some OEMs use double skinned downpipes to keep the exhaust heat in the exhaust.

If you've got a cat then most of the back pressure is in that, I doubt the rest of the exhaust adds another 50%.

I don't think 4 strokes 'need' back pressure.

- I don't have a turbo. Why would a cold exhaust be bad for emissions? If the exhaust pipe is cold, the gas inside the pipe will be hotter than usual. Is very hot exhaust gas bad for the emissions?

- The engine runs very well (feels more free). I agree about not needing back pressure. I have run my Ford diesel without a manifold - and it idled very well - except for the noise. I didn't have the courage to try and rev it, or the noise police would have paid me a visit

Strictly speaking that's not back pressure, that's dynamic wave propagation.

- Do I need to be concerned with this?




What I would really like is to have an engineer's perspecive on whether coating the exhausts is worth it. At the moment, I can see pros and cons...

Pros
- Reduced underbonnet temperature - Am I just trying to keep the intake air cold? If so, I could probably achieve this for a lot less than the cost of ceramic-coated headers.
- Less dense exhaust gas that can move faster - seems like a good reason to do it, but

Cons
- It costs a lot of money - around £1000
- Possible cracking of stainless steel headers - I've not heard of this on the many M3 forums that I have visited.
- Tuned exhaust header / collector lengths will be wrong as the exhaust density is different - what will the difference be?
- Hotter exhaust gas that may overheat other components downstream of the headers - if you mean the internals, e.g. the catalytic convertor, will it be be too hot for it? If you mean the O2 sensors, will it be too hot for them? Do you mean the externals, such as the gearbox, propshaft, plastic undertray? Most of the underside of the car are already protected with insulation...

w/o undertray
http://www.bimmerfest.com/forums/attachment.php?attachmentid=144166&d=1208582578

w/ undertray
http://www.bimmerfest.com/forums/attachment.php?attachmentid=146730&d=1210658988

At the moment, the cons outweigh the pros.

 

[GregLocock]

Yes, tuned length headers are a great way of getting more torque at one speed. Of course the more you optimise the tuned length the narrower your torque peak will be.

Yes you can overheat catalysts. It is common for full throttle calibration to include excess fuel just to cool the cats with.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[enginesrus]

It depends on what your looking for as far as cold induction and hot exhaust.
For economy I would say a bit of heat on intake is good and the colder the exhaust is really good, that is non turbocharged. If you can extract full designed HP and be able to put your hand on the exhaust manifold that is the best deal ever.

 

[SomptingGuy]

One comment in the previous thread was: "keeping exhaust system hot - air exits more quickly".

I fail to see any physical basis for this. The velocity will be higher through the whole system. I've no measurements or analysis to support, but I'd suggest that this increases back pressure (because of higher friction losses), which isn't what you want. An hour or two with a cycle simulation would give some insight.

Similarly, running a simulation with heat transfer coefficients zeroed in the other areas you are thinking of coating would give you an estimate of the maximum you could get in return (and the side effects). Doesn't need to be a model of your engine. Any NA PFI gasoline engine would do. And any cycle simulation would also do. Some are free.


Steve

 

[Gary_321]

It depends on what you're looking for as far as cold induction and hot exhaust.

- The car is a toy. I'm interested in optimising the car as much as possible to enable it to be a fun car to drive. This (to me) means that I don't want it to attract too much unwanted attention, yet I want it to be fast (acceleration). Top speed isn't an issue as it's already limited to 155mph (and that's enough for me!). It can be a beast to drive, especially when there is any moisture on the road. It has ABS, LSD, but no traction control.

I'm not particularly bothered about economy when I'm booting it (I'm happy to pay for my smiles!!). I have a Smart car for economy.

If you can extract full designed HP and be able to put your hand on the exhaust manifold that is the best deal ever.

- this is what I'm after. I hope to get even more BHP than it left the factory with.
I hope to do this by using tolerances that are tighter than BMW built the car with. For instance, the tappet shims clearance is 0.18mm - 0.23mm (Inlet) and 0.28mm - 0.33mm (Exhaust). If I can get all the gaps to be 0.18mm and 0.28mm respectively , the cam duration will be at its maximum.

I'm not just aiming at BHP, but also weight (both mass and mass-moment-of-inertia). I've already fitted a lightened flywheel (I measured it to be 13% less MMI than the original). Obviously, I don't know what the MMI of the whole transmission system (from crank to wheels), but every little helps!

In the future, I plan to balance the conrods and pistons. BMW's tolerance is 4g. I will be aiming for 1g. I could go for lightened pistons and con-rods, but that would be a major expense. I don't mind spending money on something that I can't do myself, but begrudge spending it on something that I can.

I've done some calculations and I estimate that the throttle body throats are tuned for around 4250rpm. I have still to do the measurements on the exhaust primaries, collector and tailpipe, but they don't look to be the same as the throttle. I'll start a new thread to discuss intake and exhaust tuning later (unless I can find an appropriate thread)

One comment in the previous thread was: "keeping exhaust system hot - air exits more quickly".

I fail to see any physical basis for this. The velocity will be higher through the whole system. I've no measurements or analysis to support, but I'd suggest that this increases back pressure (because of higher friction losses), which isn't what you want. An hour or two with a cycle simulation would give some insight.

Similarly, running a simulation with heat transfer coefficients zeroed in the other areas you are thinking of coating would give you an estimate of the maximum you could get in return (and the side effects). Doesn't need to be a model of your engine. Any NA PFI gasoline engine would do. And any cycle simulation would also do. Some are free.

- I'm going to do a google search and have a play on a simulator. I don't think that I will be playing for just one or two hours, though. More like one or two days (maybe weeks).



- I also think that the nail has been put in the ceramic-coating coffin for me. I suspect that its benefits are aesthetic and under-bonnet cooling. Engine aesthetics don't bother me (as long as it's clean) and I can find a much cheaper way to cool the intake. If I had a mild steel exhaust, I'd consider it for the added protection.

- I am looking at the possibility of 3D-printing some spacers for the throttle bodies in ceramic. I have the printer, but not the ceramic material or the nozzle for that material. I will also need to make a kiln, but I've been planning to do that for ages.

Just for interest, if you are at a loose end for a couple of hours, here is the build thread of my car so far Linky

Many thanks for your feedback.

 

[RodRico]

Everything you're doing to cool the intake should yield better power for the reasons given (more air mass = more fuel mass = more power). Ram air can result in over pressure, but I believe the computers limit the amount of fuel in order to prevent it, and this is why it often yields lean operation (either that or the air mass sensors saturate). Insulating the intake path and pulling intake air far from heat sources should be just as good and much simpler than cooling the exhaust system (which can have collateral effects as others have noted).

 

[Compositepro]

"the colder the exhaust is really good"
There is more to it that that. Extracting work from the hot gases in the engine does cool them, so one might say that therefore you want the exhaust gases as cool as possible. But that does not mean that cooling the exhaust gas after it passes the exhaust valve provides any more power. The only possible benefit would be that due to higher density the gas velocity is reduced, and thus the back pressure.

 

[Gary_321]

Everything you're doing to cool the intake should yield better power for the reasons given (more air mass = more fuel mass = more power). Ram air can result in over pressure, but I believe the computers limit the amount of fuel in order to prevent it, and this is why it often yields lean operation (either that or the air mass sensors saturate). Insulating the intake path and pulling intake air far from heat sources should be just as good and much simpler than cooling the exhaust system (which can have collateral effects as others have noted).

- I've decided that this is the way for me to go. I'll have a go at further insulation with ceramic spacers between the throttle bodies and the cylinder head (the throttle bodies are 75°C) and maybe I'll try to insulate the top of the plenum as well.

- I've done a lot of reading about ceramic coating in the past week, and I've concluded that it is not worth it for me. Some of the potential downsides can be very problematic. I've read about chipped piston coatings leading to hot spots. I've read about heat being transferred to places that the engine wasn't designed to handle. I've read about tolerances being altered to the point where different piston rings are required. It has the potential to be more trouble than it's worth.

"the colder the exhaust is really good"
There is more to it that that. Extracting work from the hot gases in the engine does cool them, so one might say that therefore you want the exhaust gases as cool as possible. But that does not mean that cooling the exhaust gas after it passes the exhaust valve provides any more power. The only possible benefit would be that due to higher density the gas velocity is reduced, and thus the back pressure.

- Thanks. I took on board your earlier point about the important place to keep hot is the combustion chamber. That is what lead me to look at ceramic-coating of piston crowns, combustion chambers and valve faces. From what I have read, it is a great thing to do if you are only running an engine for a few thousand miles (or as a race engine), but I plan to run mine for tens of thousands of miles.
- As I said above, I think moving the heat away from the places designed to take it is a concern. I don't want to melt the propshaft giubo joint, for instance.
- I've also read that there are many unintended consequences of having an exhaust temperature that is hotter than designed for. The gas is less dense, so it can move faster. This can lead to more friction, which leads to more back pressure. Much as you say

 

[RodRico]

Given constant fuel/air ratio, you'll have higher combustion temperature and pressure as well as more back pressure by increasing intake air mass. It's unavoidable. I suspect you're unlikely to be able to increase air mass so much doing what you're doing to make it a large problem, however. A common trick among street racers, by the way, is to pull intake air through a box of ice. Granted, it only works for a while, but it's cheap and easy. Another not so cheap or easy but intriguing approach is to put Peltier heaters/coolers on the exhaust and intake such that exhaust heat generates electricity used to cool the intake.

 

[LionelHutz]

The engine doesn't care if the heat stays in the pipes or radiates out. Just build the exhaust to achieve the minimum practical back pressure to match what you do.

 

[kjoiner]

Gary,

If you want to try some ceramic and have access to 3D modeling software you might take a look at Shapeways porcelain materials. You could experiment with different concepts without having to purchase a kiln.

https://www.shapeways.com/materials/porcelain-ceramic

Kyle

 

[Gary_321]

If you want to try some ceramic and have access to 3D modeling software you might take a look at Shapeways porcelain materials. You could experiment with different concepts without having to purchase a kiln.

https://www.shapeways.com/materials/porcelain-cera...

I think this would be my preferred solution. Thanks for the link.

 

[CWB1]

One major mistake when thinking of exhaust flow is assuming it to be laminar and/or constant pressure rather than a series of individual turbulent pulses. The former tends to be true downstream of catalysts and mufflers and at the end of longer open pipes, the later true with short open systems typically found on performance/race engines. Quite often yes, the upstream exhaust components are ceramic coated to hold heat and aid expansion/improve flow as its often worth a few horsepower. Needless to say, design at this end of the system is a bit more critical than elsewhere downstream. Whether or not the cost of coating is worth the money invested for a mostly on-road toy is another topic entirely.

As mentioned above, on the intake side colder is better within the engine's performance limits due to the increased density.