Chip

iTrader: (3)

Greater lift and greater duration are two different things, so need answering seperately really.
Not something I profess to be an expert on, cams are a very black art, but I will try and answer from what little ive learnt over the years from my relatively limited meddling with different cams.


More duration in principle should lead to more power at the top end but gives you a problem of where you choose to time the cam and hence where you acutally have that extra duration in the cycle, too much overlap with the exhaust cam can result in blowing the mixture straight through the engine at low rpm, but time it out the other way so it starts later and you can end up with a situation where its not actually open at the right time in the cylce and potentially in an extreme case can result in the cam open too long when the piston starts to come back up, which means effectively a drop in VE due to the mixture coming back out again, and hence effectively a drop in dynamic CR (this will even show up on a compression test of the engine).



More lift without extra duration (although that will depend on where its measured of course!) will have a less negative effect at low RPM typically and can still yield good gains higher up but the problem then is massively increased strain on the valve train as it results in greater valve acceleration rates, so for really big lift short duration cams you need to be moving over to a proper set of lifters instead of the hydaulics.


To answer your next question that you havent asked yet but no doubt are about to, the standard valves are FAR more than big enough, and there is no point changing them until you are talking absolutely huge power levels IMHO.

Chip

iTrader: (3)

Ps

With regards to surge, you can DRASTICALLY alter your position on the map by playing with cam timing.
Dial it in for overlap and you lessen the chance of surge, dial it in too far after BDC for closing and you greatly increase the chance of surge.


Do you know about compressor maps JJ? as to understand cams on a turbo car (especially in terms of surge), I feel a fairly good grasp of them is an absolutely essential bit of background education.

JjCoDeX75

Chip is correct - I am trying to understand the subject as well as I can (albeit I accept that this may be a lengthy an tiresome process for others!!! )

Nonetheless, your input is appreciated, Phil.

Chip

Thanks for the description on the duration and the lift - this kind of makes sense now. This kind of also explains why often when people put different camshafts in their cars, they end up with a rather lumpy idle and low end performance!

So also, the idea of increasing the lift causing additional stress is usefully explained.

Lets come back to the Compressor Map. To be honest I dont know. If I had to hazard a guess, this would relate to the shape of the boost that is created by a turbo. Presumably you would need this data before you could map the car at all? I believe we did touch on this the other day when chatting about the cheaper turbos that do not come with that kind of data....

Is this correct? PErhaps someone has a sample sheet that I could see?

JJ

Chip

iTrader: (3)

JJ, as per Phils excellent suggestion of giving you URL's rather than me having to type things out again, here is a link to a brief introduction I wrote about compressor maps, read it, get a genuine understanding of the basics, and then come back and read again what I wrote about surge and you can tell me WHY inlet cam timing effects surge so much in terms of wether the mixture blows out the exhaust or reverses and comes back out the inlet on a big duration cam

http://www.vauxhallsportforum.com/ph...urbos-t92.html


Utterly no chance of you ever understanding surge properly without fully understanding where it sits on a compressor map.

Deleted by Request

...you can help eliminate surge by porting the shroud on the turbo....BUT this looses BHP..BUT its recommended on GT turbos.

Chip

iTrader: (3)

Another piece of information you have remembered, now if you could just explain why, then it might be useful to JJ

Chip

iTrader: (3)

JJ, a ported shroud, like Phil remembers someone mentioning to him as being useful against surge, is essentially a way of allowing boost to bleed away from the compressor wheel of the turbo, what this then results in is a movement of the surge (aka choke) line on those maps ive just linked to over towards the left of the graph, ie decreasing the amount of flow required before surge stops occuring.
The downside is that this will (especially at higher boost levels) reduce the amount of flow from the turbo for any given amount of work done by the wheel (ie the turbo speed).
This then results in a net loss in VE of the engine due to the increased pumping losses of the engine from the increased resistance on the hotside of the turbo required to increase the speed to a point where you get back to the same airflow from the turbo (doubled edge sword this as you also now need more boost, making the intake temps increase which will result in both another decrease in VE, and also a drop in efficiency), and it also will slightly reduce the peak flow potential of the turbo as the safe operating speed of the turbo remains the same, but the amount of flow at that speed is now reduced for the reasons Ive just given.

This all comes back to what I was explaining to you the other day of looking at the turbo (BOTH sides) and the engine as a package, rather than trying to look at items in isolation.

Deleted by Request

Ported shroud is done to the compressor housing...It allows some airflow to exit the wheel through the port to keep surge from occurring....This basically allows you to use a bigger compressor...but it does have a minor negative for the compressors effeciantsy.

Chip

iTrader: (3)

I dont think you have ruined it just yet, im sure it can still progress when JJ comes back from reading that article I pointed him at (if he does so)

I like your 16 year old ferrari, and although I prefer the house I own (well co-own with the bank till the mortage is finished) to the bungalow you do, I dont dislike your bungalow either, you misunderstood my point, it wasnt that there is anything wrong with those things, I dont think there is at all, its just that my own dreams go beyond a few basic material posessions thats all


Im sure you will stick to that as long as you stuck to not replying to anything by me that you said at the top of this page and how long you stuck to leaving the forum forver the last few times you announced you were doing it

Chip

iTrader: (3)

Cool, speak to you soon then mate when you have finished with all the pesky work nonsense.

Jay,

iTrader: (4)

right ive cleaned the shit out of this thread KEEP IT ON TOPIC

ECU Monitor Enthusiast

iTrader: (1)

I must admit, cam shafts and timing them have always been a mystery to me.

Still are even after all the good advice here

Chip

iTrader: (3)

Thanks

Jay,

iTrader: (4)

Same here simon i think thats where my cars lacking

Chip

iTrader: (3)

Go back to the standard exhaust cam mate

Jay,

iTrader: (4)

yes i think thatll be the way , got a road trip to wakes a week on sunday to finish that 21t off on the megasquirt so gareth was guna give me a practical lesson on it

JjCoDeX75

Right

I see that the thread has been face lifted - Thanks Jay . ?Just for the record - all relevant is appreciated irrespective from whom

I have now read the article on the Compressor Maps - Nice one Chip - top article


Soooo Lets see if I can answer the question posted - Basically, surge happens when the turbo is providing more air flow than the engine can manage - which leads to the excess air rushing back towards the compressor.

Therefore - lots of boost showing against minimal flow is effectively like a blockage in laymans terms,. Is that corect?

JJ

Jay,

iTrader: (4)

has a rs500 got a flowed head seeing as there s a t4 fitted ?

Chip

iTrader: (3)

Yes, thats correct, so to actually answer the question I asked then......


Which was IIRC more along the lines of:

If on a fairly hot cam you have the timing of the cam moved so its overlapping the exhaust cam, and the timing of the cam moved so that its got seperation from the exhaust cam (and hence the valve still open long after BDC) which of those two will help to prevent surge and WHY?

Deleted by Request

jaycos there isnt enuff boost as standard to cause surge so i would say not....you'd need to wack a T4 to around 25psi to start seeing it.

...the issue with surge is the amount of air the engine tries to consume at lower revs...hence roller bearing turbos are famous for it....if you getting surge on your engine the turbo is not suitable for your spec....so say you have a std YB engine your only turbo to have is a t34 .63 MAX.

Chip

iTrader: (3)

Not entirely true, perfectly possible to map your way around that and still see decent results, by artificially limiting the point at which boost is allowed to build to maximum.

As an example Doug Stirling had a GT35R on his YB, and it would *just* start to surge at about 4000rpm if it was allowed to go over 2 bar, Karl mapped it so it stayed just under 2 bar until it was at about 5000rpm IIRC, and that completely eliminated the surge and left him with what was a decent spec engine with effectively a well matched turbo IMHO, the fact it had to be limited by just a couple of psi for just a tiny section of the rev range was not something that would make you want to avoid that turbo on that engine.

Deleted by Request

...then dougs spec is not suitable...maybe different cam timing or camshaft to eliminate that 2bar surge or ported shroud would sort it...

your engine should be able to consume what the turbo can give at any rpm otherwise you are having to compromise by getting round it.

..i will say my MAD engine didnt have any surge at 2bar on the gt35 non ported shroud..but i cant say what cams or timing it had.


...be interesting to know wether Rod has had to map around or compromise on his new turbo?

JjCoDeX75

Basically, if I understand it correctly, where the cam separation is increased I guess the overall flow of the engine is reduced, which would lead to increased likleyhood of surge?

JJ

Chip

iTrader: (3)

Not suitable for what?

Ported shroud would compromise the top end potential, and altering the cam timing to avoid surge is likely to adversely effect the low down torque of the engine, how are those two options "right" and capping the boost slightly for a small portion of the rev range is "wrong", that makes no sense when ALL are compromises surely?

What are you having to compromise? and why for example is it better to make the engine waste boost by pumping it straight down the exhaust with lots of overlap?
Do you not understand that EVERYTHING about building an engine is a compromise?
Cam choice for example, if you want it up top you lose it down below, thats a compromise, CR, thats a compromise, exhaust size, thats a compromise, why is a slight boost cap ove a limited RPM range on an engine a worse compromise than any of those other factors?

Rods new engine is one enormous compromise, just like his last one was, his engines have never been anything but compromise, thats the whole point of his car, to be both a top speed car AND a road car, you cant get a bigger compromise than that!

Chip

iTrader: (3)

Thats my understanding of it too mate

Remove all the overlap, and increase the duration its open for after BDC and you effectively cap the flow through the engine, and hence move yourself to the left on the compressor map for that amount of boost, by flowing less, which in turn increases the chance of surge.

I suspect there is far more to it than us mere mortals realise though, but its the one subject no tuner on here ever seems prepared to go into depth on from what ive seen!

Deleted by Request

...you say every engine is a compromise...it doesnt have to be....yes i can see Rod's is with the top speed mind frameset.

The bit you missed out from your quotes is...an engine should be properly specced enuff to be able to consume all the air that turbo can give at any rpm of its range.

I concede,as i practice not preach,my 7.1-1 engine with a T4 that vcould only run 1.5bar before surge was utter shite ....but i learnt from there on...open the fooking ports and bin the bd10's for this application ...to me my 7.9-1 engine was a no compromise unit with the T4 and 4 x 1000cc injectors....could take 2.3bar at any rpm of the boost area 4k to 8k ...with nice off boost driving...as a road car it didnt get any better imo.

Chip

iTrader: (3)

Phil, if you could just answer the question as to why BD10s (which lose you bottom end torque) and bigger ports (which lose you bottom end torque) are NOT a compromise, but that a 2psi cap on boost for 1000rpm of the rev range IS a compromimse, that would be great, cause at the moment you are saying one is a compromise and one isnt, and I cant understand your thinking for that?

Why is getting rid of surge by putting in a bigger cam somehow more acceptable than getting rid of surge by altering the boost map slightly?



As for your comment that "an engine doesnt have to be a compromise" can you name one that isnt? cause I can think of NO engine in existance that isnt a compromise in multiple ways.

Deleted by Request

.......but thats silly as you could say having to use SUL fuel and not std cheaper unleaded is a compromise...lol.....its horses for courses you build the engine in a way you want it to perform.....the main thing is on a turbo charged engine the turbo has to be the right specification that your engine can take all its got....or go down a turbo...lol

Chip

iTrader: (3)

Indeed, its very silly to say you can build an engine without compromise, hence I dont know why you said that?

Not just how it performs either, many other factors:


Formula 1 engine:
Compomised between
Needing to make good power
Reliability of having to last a couple of hours
Needing to be fuel effecient to minimise the weight of the vehicle during the race and/or number of pitstops


Drag engine:
Needs to make good power
Has to last the full 6 seconds
Has to have a cam choice that allows good torque low down to hook up with, and also has good power at the top end to keep pulling


Toyota Aygo standard car engine:
Needs to make enough power for sprightly performance
Has to be reliable for 100,000 miles+
Needs to be economically viable for mass production


There is not one single engine on the whole planet I can think of that isnt built around multiple compromises, can you name one Phil?

If not why are you saying engines dont have to be compromised if every single one on the planet is?



And if you concede that every engine is compromised, then why is a 2 psi cap on boost for 1000rpm a bad compromise but all the other things mentioned are not bad compromises, you are making NO sense at all.

Deleted by Request

...no Chip you are not making any sense as you are taking it to seriously....the whole world and everything on it is compromised if we looked deep into it LOL.


..LETS LOOK MORE TO IT AS HORSES FOR COURSES


Good job i am replying to this or it would have died a very premature death

Chip

iTrader: (3)

Surely if that were the case, we would all run with no boost controllers of any type ever other than maybe one that exactly maxed out the turbo to its maximum possible sustainable RPM?

Going down a turbo like you suggest sounds like a big compromise to me, espeically in terms of reliability in many cases, as the last thing you want on a reliable engine is a turbo thats absolutely revving its tits off constantly.

Chip

iTrader: (3)

Im taking JJ's thread seriously, as he has REPEATEDLY expressed a wish that people do so, if you dont wish to do so then you shouldnt be in the thread, you should be in one about shagging PON and Rainbird like you normally start yourself.


Ok, so looking at it from that point of view, why is a boost compromise worse than a cam profile, lift or timing compromise between high and low rpm?

JjCoDeX75

Okay, so lets have a little recap....

What we know:

The more we can increased the volume of airflow through the engine, the more power that will be produced, subject to the following design constraints;

1. The turbo itself restricts air flow out of the engine. In order to maximise air flow, a larger exhaust housing assists. Notwithstanding this, the larger housing inevitably increases the boost threshold, which is bad.

2. Given that the above statement is true, and ceteris paribus, the flow the engine itself is generally dictated by the size of the openings (inlet and exhaust), the amount of lift and duration of cams, as well as cam timing.

3. The simple action of changing the duration of the exhaust cam will inevitably lead to the increase of boost threshold, as a greater volume is required to spin up the turbo (?)

4. If you overly increase the inlet size, and do not modify the exhaust cam timing, you are in danger of increasing cam separation, which may lead to turbo surge.

We touched briefly on the benefits of the action of simply porting the engine. Basically increasing the size of the inlets and exhaust ports. This allows a greater volume of air to travel through the engine for a given amount of boost.

However we also agreed that the increased port size will lead to potentially adversely affecting the performance of the car at low rpm levels.

Why is this? Can we discuss this a little further?

JJ

Deleted by Request

...to sum up

an F1 engine is built in such a way it will be good at what it does..that aint a compromise thats perfection as they know exactly what is needed..it doesnt need to have a smooth idle or run on unleaded fuel or do 40 mpg etc etc

..the dragsters again ,its built to do that job it doesnt need to do more

..the road car engine again is designed to do a job where is the compromise its economical fairly fruity and does what it says on the tin nothing more nothing less

puddy

If you are quoting pro mod 6 sec drag engines,surely they only have

to perform at wot.

They leave on the 2 step on wot and run clutchless gearboxes so they

never lift.

puddy

Deleted by Request

...building a compromised engine is like choosing a girlfriend with one breast

JjCoDeX75

Okay, so lets have a little recap....

What we know:

The more we can increased the volume of airflow through the engine, the more power that will be produced, subject to the following design constraints;

1. The turbo itself restricts air flow out of the engine. In order to maximise air flow, a larger exhaust housing assists. Notwithstanding this, the larger housing inevitably increases the boost threshold, which is bad.

2. Given that the above statement is true, and ceteris paribus, the flow the engine itself is generally dictated by the size of the openings (inlet and exhaust), the amount of lift and duration of cams, as well as cam timing.

3. The simple action of changing the duration of the exhaust cam will inevitably lead to the increase of boost threshold, as a greater volume is required to spin up the turbo (?)

4. If you overly increase the inlet size, and do not modify the exhaust cam timing, you are in danger of increasing cam separation, which may lead to turbo surge.

We touched briefly on the benefits of the action of simply porting the engine. Basically increasing the size of the inlets and exhaust ports. This allows a greater volume of air to travel through the engine for a given amount of boost.

However we also agreed that the increased port size will lead to potentially adversely affecting the performance of the car at low rpm levels.

Why is this? Can we discuss this a little further?

JJ

Deleted by Request

JJ simply put,more air down low is not good but it is good at higher revs.

The best YB built would be the Rouse engine for allround greatness

JjCoDeX75

But why is more air low down bad? what exactly makes it bad?

JJ

Chip

iTrader: (3)

Absolutely bollocks, MPG is one of the most important factors about formula one engines, getting the power is easily, getting it without drinking lots of heavy fuel you have to carry around with you is the ultimate compromise, and is the key to winning races NOT outright power.

Built to do what job?
Work well at low rpm for the all important holeshot, or built well for massive top end power to carry the acceleration on to the end of the track?
The answer of course is a compromise of the two.

Ah, the "road engine" as found in an FQ400 evo, a 1.1 fiesta popular plus, and a hummer, all doing nothing more and nothing less than each other.