Tuning an n/a

[Pulley]

Who said I'd cast one?, I've sure done plenty of proto mould and flow tests on rigs, even various electro valve lifters and such, remember most production head and valve gear are built to a cost and for mass production, you'd be surprised how many manufacturers have made prototype vvtl on both cams, end of the day it has to be financially viable to go into production, even say on a mclaren supercar, there are always limits to budget ect

Scratch built head btw, I wouldn't bother with trying to modify an existing 2jz head

Casting moulds cost a fortune, I've only actually cast 3 heads in my time, and one turbo snail housing, prototyping and real life testing is far too costly in this country for a small company without the backing of a big cheque book

[Digsy]

Who said I'd cast one?, I've sure done plenty of proto mould and flow tests on rigs, even various electro valve lifters and such, remember most production head and valve gear are built to a cost and for mass production, you'd be surprised how many manufacturers have made prototype vvtl on both cams, end of the day it has to be financially viable to go into production, even say on a mclaren supercar, there are always limits to budget ect

Scratch built head btw, I wouldn't bother with trying to modify an existing 2jz head

Casting moulds cost a fortune, I've only actually cast 3 heads in my time, and one turbo snail housing, prototyping and real life testing is far too costly in this country for a small company without the backing of a big cheque book

 

Got your PM. Don't worry - I know what you are talking about.

[Noz]

Who said I'd cast one?, I've sure done plenty of proto mould and flow tests on rigs, even various electro valve lifters and such, remember most production head and valve gear are built to a cost and for mass production, you'd be surprised how many manufacturers have made prototype vvtl on both cams, end of the day it has to be financially viable to go into production, even say on a mclaren supercar, there are always limits to budget ect

Scratch built head btw, I wouldn't bother with trying to modify an existing 2jz head

Casting moulds cost a fortune, I've only actually cast 3 heads in my time, and one turbo snail housing, prototyping and real life testing is far too costly in this country for a small company without the backing of a big cheque book

 

I always understood the NA head to not be as free flowing at the TT head, therefore being different flow rates adding to the number of limitations for the NA when under boost?

[johnny g]

I've looked into it extensively as a "works" project here, even done a couple of proto-mould heads with vvt/lift on both cams , but decided it's not worth the machine time and money given that the first response if I pulled up at a dyno with just over 300 bhp would be "why didn't you charge it instead" for far less money and far bigger horsepower

 

If you could make that for a turbo application, you'd be on to a winner

[Pulley]

I think for the hassle I'd rather just lightly modify the breathing on the tt head and run a twin turbo, small into large sequential system with a high enough volume first waistgate to allow the engine to breath freely while still having early boost, then big turbo power from then on, with water cooled turbos and variable vein on both first and second turbos

Or replace the first smaller turbo with a centrifugal sc

[Scooter]

I'm not sure of the limitations going from a 4 to a 6 cylinder engine but a straight 6 with the BEAMS technology/design from the 4 cylinder engine in late MR2s and the Altezza would be nice (assuming a real 210bhp 4 cyl could result in a +280bhp 6 cyl).

[Scott]

I'm not sure of the limitations going from a 4 to a 6 cylinder engine but a straight 6 with the BEAMS technology/design from the 4 cylinder engine in late MR2s and the Altezza would be nice (assuming a real 210bhp 4 cyl could result in a +280bhp 6 cyl).

 

 

The revs alone would make a reasonable difference to the N/A. Does the beams not rev to 7500rpm+?

[Scooter]

The revs alone would make a reasonable difference to the N/A. Does the beams not rev to 7500rpm+?

 

not sure but I think it has variable intake and exhaust valve timing and trick bits like titanium valves? just shows that an NA with over 1bhp per 10cc was possible a little later on as supra production came to an end. A 6 cylinder version was probably too costly to look into especially as at that time they had nothing left worth putting it in!

[Tricky-Ricky]

VVT works well because it can be tailored to effect both low load and high load areas, and can on the same bottom end increase output across the

board, your average engines with fixed valve timing is limited, as the VE is calculated at the maximum safe RPM at WOT, but at lower throttle openings and RPMs it becomes very inefficient,

If you then add VVT you can increase airflow dynamics in both low and high loads, and gain better power and fuel efficiency as well,

 

A good example is the VQ35DE engine in my skyline, it has 276BHP with VVT on only the inlet cams, the rev up version added VVT to the exhaust cams and gaind another 23BHP, and the HR version with an increase of 500RPM and redesigned intake resulted in 330BHP and even better fuel efficiency.

[superdooper]

port it yourself , if u have a metal rasps.. u can port and flow your self , ive done many heads. Not hard to do, but take your time and look where you are grinding out ok, especially around the valve guides, exhaust ports people always match the port to the manifold!!!!!!!!!! wrong ...doh,, leave a 1.5 to 2 mm of the port smaller then the manifold ok, most important becose that step stops the back flow into the exhaust port ,becose you get a slight stopage of exhaust flow when the valves closes, not much , but enough, it can affect cylinder scavenging, but not to a great extent, just thought i would say. Best to lower the compression if u r going to up the boost to a high degree on a TT......

[Pulley]

Vvtl on inlet and exhaust will become manufacturer standard eventually in the same way that diesels (and originally petrols) went from mechanical injector pumps to "common rail" fed injectors an efi

[Digsy]

Yup reading that back I can see where I went wrong there.

Now correct me if I'm wrong here, lets say for example an engine with cylinders of a 100cc and a 14:1 CR is only filling 50cc of fuel and air into the cylinder itself that would make the VE of only 50%. Theoretically speaking that engine would only be running on a ratio of 7:1 am I right in saying? Obviously though for an engine to be that volumetrically inefficient at full throttle it would have to be seriously restricted but you understand what I'm saying I think.

Well, despite the term "volumetric" efficiency, what really matters is mass of air entering the cylinder, but assuming the air density entering your 14:1 cylinder is the same when operating at 100% VE and 50% VE then, yes, that's pretty much correct. Youve hit the nail on the head in your second sentence saying for an engine to be that inefficient it would have to be seiously restriced. That's precisely what a throttle does. Let's say if you half close the throttle, your cylinder still pulls a full cylinder's worth of air past the throttle plate. The air coming through the airbox cannot refill fill the plenum at the sam erate as the air is extracted and thus a partal vacuum is produced (this is why engine load is measured using a pressure sensor in the plenum). The effect of this is that the air in the cylinder is (let's say) 50% less dense, and thus has 50% less mass, or 50% less stuff to go bang when mixed with fuel - hence you get lessd power out which is of course what you want when the throttle is parially closed.

The problem with this is that engines are designed and optimised for those big peak power and torque figures, but in reality spend most of their time at relatively small throttle openings. Running at part load with a vacuum in the plenum not only increases the pumping work in drawing the air into the cylinder but also makes the compresson stroke and expansion strokes horribly inefficient because in the example above you are compressing air that is already 50% less dense to start with. Modern engines are starting to address this by running four-stroke engine cycles that are sublty different from the norm:

1) Load control by variable intake valves.

This is what BMW "started" through use of their valvetronic technology. By making the intake valve opening fully variable from zero to maximum lift you can do away with the load control throttle and run the plenum at atmospheric pressure. At WOT the engine behaves normally, but at small throttle openings the valves open for only a small fraction of the intake stroke, taking a small "sip" of air. While the valve is closed, the piston moves through the induction stroke creating a large vacuum inside the cylinder, but importantly when the compression stroke is started almost all of this pumping work is recovered because the vacuum acts like a gas spring which helps to pull the piston back up. You are still left with the tiny effective compression ratio at the end of the compression stroke, but the efficiency is increased because of the elimitation of a lot of the pumping work.

2) Downsizing.

Put simply, this is making a small engine which has to run at a relatively high load even for "normal" operation, and then supplimenting the top end power by adding a turbocharger. The current state of the art for downsizing is approx 25%, so in cars that used to have 2.0 litre NAs, we now have 1.5 litre turbos, and so on. The downside is the lack of bottom end response from the smaller engine when off boost. This can be addressed by advanced turbos or even hybrid electrics in some cases.

3) Miller / Atkinson cycles.

These are both engines which have unequal length comprsssion and expansion strokes. In both the intake valve is left open well into the compresion stroke. In an Atkinson engine this allows for part of the intake air to be expelled back out into the plenum. By controlling the intake valve closing point you can trap less or more air and hence control the load. There is no "gas spring" effect but the compression stroke is now geometrically longer than the expansion stroke, meaning that a lot more energy can be extracted, making the engine overall more effiecient than a normal (Otto cycle) four stroke operating at part load. The problem is that size for size you cannot make an Atkinson cycle engine more efficient as an Otto engine at full load (because there would be no difference between the two). To get around this, the addition of a supercharger means that you can still get large amounts of air into the cylinder even with a late intake valve closing (and hence different length compresison and expansion ratios). A supercharged Atkinson engine is called a Miller cycle engine.

4) Variable compression ratio.

This (IMHO) is the holy grail of engine design. So many have tried and none have yet made it a cost effective production solution. Virtually everything on an engine associated with controlling combustion is now variable apart from the compression ratio. If and when VCR is finally brought to market, you could have an engine that runs at a normal 10:1 at full load but could be as high as 40:1 at low load. It wouldn't knock because of the rarification of the air at the start of the compression stroke, but at the end of the stroke it would be ready to burn just as efficiently as if it was at full load. The efficiancy gains would be tremendous, but at the moment there are far easier ways of getting smaller, but appreciable, benefits, so the industry is following those.

 

What I meant by that statement is if the VE of the engine is poor to begin with then just upping the CR wouldn't net you much of a gain without first improving VE.

 

Upping the CR will always get you an efficiency gain, even when VE is very low, but, yes, using this to get high power at high load makes little sense.

 

I'm probably wrong but after all I only have my L plates in terms of engineering with much to learn at the minute

 

Nope, you were pretty much there. I probably did some "teahcing you to suck eggs" in the paragraphs above, but I thought you might find it interesting. Unfortunately, all of this technology is going into making engines smaller and greener while being just as owerful, rather than bigger and more powerful.

[bigbloodyturbo]

Variable compression ratio.

 

This (IMHO) is the holy grail of engine design. So many have tried and none have yet made it a cost effective production solution. Virtually everything on an engine associated with controlling combustion is now variable apart from the compression ratio. If and when VCR is finally brought to market, you could have an engine that runs at a normal 10:1 at full load but could be as high as 40:1 at low load. It wouldn't knock because of the rarification of the air at the start of the compression stroke, but at the end of the stroke it would be ready to burn just as efficiently as if it was at full load.

 

 

Has Saab not already done this? I thought they had a variable compression turbo engine back in the late 80's early 90's?

[Digsy]

Yes they did (late 90s). The whole cylinder head (complete with induction and exhaust systems) and cylinder bores was on a hinge with a rubber bellows sealing the head to the block. By tilting the head you could vary the compression ratio. The engine was a monster, size wise. They never put it into production.

 

The idea of variable compression ratio is as old as the hills. Loads of companies have tried it. Its just that no one has managed to do it elegantly and cheaply yet.

[bigbloodyturbo]

Yes they did (late 90s). The whole cylinder head (complete with induction and exhaust systems) and cylinder bores was on a hinge with a rubber bellows sealing the head to the block. By tilting the head you could vary the compression ratio. The engine was a monster, size wise. They never put it into production.

 

The idea of variable compression ratio is as old as the hills. Loads of companies have tried it. Its just that no one has managed to do it elegantly and cheaply yet.

 

Thats the one. I remeber reading it in a book I bought years ago. I was almost sure it was in a production car. Was written by A graham bell (im sure you know the book, its like the bible for petrol heads i believe). I remeber it hinged on bellows and was immense. I would imagine having the cylinders and whatnot titlting all the time would have its own issues though, maybe longevity? Strength?

[Digsy]

Well it had no cylinder head gasket as the cylinder barrels hung from under the head, so that was a plus. I believe its achilles heel was the longevity of the rubber bellows going still and cracking due to exposure to crankcase gases. There are a few images of the engine on the web, I think. A quick Google should bring them up.

 

I was at an IMechE conference last year and Graham Bell was a guest speaker. He comes across as a thoroughly nice chap in person. I really wanted to think of an intelligent question to ask him but in the end I decided to keep my gob shut

[Nic]

Interesting

 

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[Nic]

Digsy have you been involved in this?

 

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[Kirk]

Nope, you were pretty much there. I probably did some "teahcing you to suck eggs" in the paragraphs above, but I thought you might find it interesting. Unfortunately, all of this technology is going into making engines smaller and greener while being just as owerful, rather than bigger and more powerful.

 

Always a good read your replys. Very insightful for me. Thanks

[Digsy]

Digsy have you been involved in this?

 

Yeah, in part. Not proper hands on design (I don't get to do that much anymore) but I did a lot of the maths behind the variable compression ratio mechanism to make sure that it would always move against the cylinder pressure.

[Pulley]

Digsy I think I want to marry you,

 

I've worked on variable compression diesel designs, coupled with hybrid vv turbo systems,

 

You give me nerd horn lol

[Colin-mkiv]

*shields his eyes*........