I have two questions for all experienced engine mechanics out there.

 

The more power question

What is the easiest way of squeezing 10 kW more from a Favorit/Felicia 135 type engine on carburettor? For now I am only thinking at raising the compression ratio from 8.8 to 9.5. Should I think also at a new camshaft? The idea is to make minimal changes, while using the same type of petrol.

 

The less power question

We all know any car has poor economy in town driving. Is there any solution to 'switch' somehow the engine to a low power / economic mode when driving in town? The question is addressing the 135 type engine on carburettor too.

Edited 3 January, 201412 yr by adurer

what saab did many moons ago was to have the entire cylinder head on a kind of hinge so that the compression ration could be dynamically adjusted to suit driving conditions, this is of no use to you because it would be impossible to engineer this, but it does at least seed a concept..

it all boils down to volumetric efficiency, and the grade of the available fuels in your locality. increasing the static compression ratio of the engine from 8.8 to 9.5 wont have a huge effect on power, in fact the only cheap way of doing this is to machine the cylinder head down and this will lead to gasket reliability issues.

to increase the volumetric efficiency you need to increase the cylinder filling ability at the points in the rev range where you need the best fuel economy, to do this you could decrease the size of the ports in the intake tract to up the speed of the gas flow into the cylinders, but obviously at peak output this would harm high rpm output from the engine.

so in essence what i'm suggesting is a variable length/bore intake tract size which is fine tuned to get the best volumetric efficiency..

unfortunately the combustion chamber design and porting design on these engines is not the best, ideally you need a pent-roof type with inclined multi-valves per cylinder with the spark plug bang in the centre for best results, these engines have a bathtub type chamber with an offset spark plug and minimal squish zone which is inherently flawed for efficient combustion especially in regards to making a homogeneous fuel/air mixture

of course the other thing to consider is turbocharging, with a small capacity turbocharger unit from a small diesel engine but modified to have a much lower wastegate opening pressure would give you the best of both worlds. this is the most efficient method becuase you are recovering spent energy from the exhaust gasket and you are increasing to volumetric efficiency of the engine to over 100%...

Very interesting information, Tom. Best of both worlds for me would be a lower fuel consumption in the low rpm range (< 2500 rpm) because we're snail driving at low rpm in town and 10 kW more at higher rpm (>2500 rpm). See illustration below showing original vs desired powertrain performance. Do you think this is achievable by a more clever design of the air intake only?

 

I think my money would go on low pressure turbocharging, it would be cheaper and easier to engineer, with a relatively low static compression ratio of 8.8:1 you could probably do that without any internal modifications, you could even use the original carb, set up the turbo to blow straight into the intake without an intercooler, and have the carb at the turbo intake to suck through a wet mixture through the turbocharger.

Now we're getting somewhere That looks like a match made in heaven. But before digging into some details, one last question about intakes. Browsing through my photo vault I came across the photo below. In terms of REAL efficiency, do you think that polish job is any good or is it just a waste of time?

 

 

Now getting back to LPT (Low Pressure Turbo) TurboCharger.

 

It's obvious I have to read a while about it, so from all possible books / articles out there, do you have one in particular to recommend?

 

Any brand / size of LPT suitable for Felicia?

Edited 4 January, 201412 yr by adurer

Polishing is a waste of time, when any kind of fluid or gas flows through a chamber the surface finish makes very little difference to it flow ability, a pressure front forms around the surface which acts a a barrier, the remaining fluid through through the centre or voids. There are lots of videos on YouTube illustrating fluid mechanics, but more specifically in relation to an engine, a polished surface allows fuel condensation to form which restricts efficient mixture presentation..

turbochargers... I would go for a small one, like the one fitted to the gm/Isuzu Diesel engines.

A very good book, well worth a read, all in English though http://www.amazon.co.uk/Forced-Induction-Performance-Tuning-H691/dp/1859606911/ref=pd_sim_b_1

A very good book, well worth a read, all in English though http://www.amazon.co.uk/Forced-Induction-Performance-Tuning-H691/dp/1859606911/ref=pd_sim_b_1

Yes, great book indeed. Very much praised all over the net. I'll try to get my hands on a used one. English is best for me, I'm much better at reading than writing where my spelling, grammar and style sometimes suck.

 

Overall I am 90% convinced to try a LPT. Bigger torque at low rpm is something I always envied at Diesel cars. Although rather modest to others, some 10 HP more seems achievable too, you never know when you need them.

 

Now let's talk about fuel efficiency after fitting a LPT. I get the worst mileage in town during winter, 32 MPG, on highway I get 48 MPG on average. My question is: without abusing the car, is tnere any benefit from LPT in terms of higher mileage in town?

 

I looked in here: http://ecomodder.com/forum/showthread.php/saab-light-pressure-turbo-technology-23061.html

Edited 4 January, 201412 yr by adurer

it all comes down to how the extra power is used, on a naturally aspirated engine it's pulling a heavy vacuum at cruise, with the turbo added the depression in the manifold is reduced to nearly 0 (higher volumeteic efficiency or in other words the cylinders are fuller) depending on how much thottle is applied, so in theory you need less throttle opening to achieve the same amount of prime power, plus the cylinder filling is more linear throughout the rev range of the engine rather than just having one sweet spot like it would on a naturally aspirated engine.. if however you burn it round at full throttle the boost will go positive and an increased amount of fuel will be burnt which will make more power.. the massive advantage of turbochargin over any other kind of pressure charging is that the energy is recovered from the waste heat and pressure in the exhaust system..

 

things to consider is the extra heat load on the cooling system, and the ignition system will need a few tweaks to cope with the extra temparature in the combustion chambers, possibly swtich to a slightly colder spark plug (more gap, shorter electrode nose) and also retard the ignition timing too.

 

a few thoughts on mounting the turbocharger.. it's quite a heavy assembly, i would suggest using your existing exhuast manifold and make an adaptor to fit it near to where the normal exhaust downpipe goes if possible, if you can also add some sort of bracket going to the cylinder block to support the weight of the turbocharger.. it needs to be fitted the right way up too so that the oil inlet union is uppermost so that gravity does the work and allows oil to naturally flow down to the outlet, in most cases the exhaust turbine housing can be rotated independently of the compresser turbine by undoing a few bolts to get the exducer facing the right direction if necessary, the turbo needs to be mounted above the level of the oil sump too, to feed oil to the turbo you could but a T peice in the oil pressure swtich hole on the engine and run a pipe to the turbo from there but you might need a restricter to decrease in-line to decrease the oil pressure to something that suits the turbo, also you need a free flowing retun back to the sump, possibly some engines have a bolt that can be unscrewed where you can put a return in, or you could drill a hole in the sump and weld a pipe in there..

 

the carbueretter needs to be mounted so that it's in the same attitude as it was before (same way up, same angle etc) also it would need to be on a rubber shock proof mount or similar otherwise the extra vibration may cause float chamber overflows. most carbs cannot be pressure charged unless they were designed for it hence why i suggest fitting the carb at the turbo inlet and drawing a wet mixture through the turbo rather than the turbo blowing through the carb.. this set up is a bit unorthordox on modern fuel injected vehicles because it means you cannot safely use an intercooler becasue of fuel vapour consensing in the intercooler core but the suck-through method was fairly common in the early turbo/carbs years, renault used it quite extensively on the gordini turbocharged cars of the 1980's, one advantage of a wet mixture in the turbo is that it helps keep it cooler and also helps mix up the air and fuel mixture more evenly.

 

hope this helps, i could probably write a book about turbocharging and supercharging myself to be honest