Stavros

I didnt know wether to add it to this post--

http://www.passionford.com/forum/vie...er=asc&start=0

But as nobody really posts on it now, i decided not to.

Just wondered, one of the ways of decreasing/eliminating compressor surge is making the head flow more air per rev, by cams, headwork, etc, but never seen any specifics on this and also wondered if that was all there was on the engine internals side of things.

Does compression ratio and engine capacity have an effect on compressor surge? (if head spec was identical) And if so, in what way and to how much effect.

Is either the inlet or exhaust side more important to combatting surge (providing both sides were equally good/bad in the first place)?

I presume as well as removing any obvious resrictions, enlarging the ports will help, or am I getting the wrong end of the stick here?

Presume less exhaust/manifold/turbo back pressure will help too.

Cheers (tho obv no help this time of night, hopefuly be a good post tomorrow)

bud-weis

i think compression ratio will have an effect, if you lower it then presumably you can increase the size of the turbo, but obviously you can't just chuck on a massive turbo that's produces more air than the engine can swallow, so then by porting and increasing how efficient the engine is, it is then able to swallow more of that air and make more power.

but i guess you would need to have similar sized turbine/compressor wheels?? a tiny exhaust wheel will spin very fast,in turn making the large compressor wheel turn very fast and then produce more air than it can consume ( would this make the boost gauge rise ?? i guess it would as all that air is backed up and not being consumed)

i don't reckon engine capacity as such has an effect on surge i.e the principal is the same no matter what the capacity, you still need to make that engine more efficient with headwork etc so it can consume as much as is needed. old F1 turbo engines were only 1.5 ltrs from memory with massive turbo's, so i guess all they did was get the head spot on and run big boost through it.

in answer to your question though, cam choice and porting,along with a correctly specced turbo would be a good way of combatting surge.

kinda thinking as i'm writing so probably sounds like bollox and nothing you already didn't know,but at the very least i have bumped it up for you

foreigneRS

i disagree with bud-weis.

the reason a turbo surges is that the compressor is supplying more air than the engine can consume.

increasing the capacity or improving the volumetric efficiency will allow it to consume more of that air that the compressor is trying to supply, so the surge will be reduced.

i don't see how the compression ratio would really change anything.

Anonymous

Lowering the CR is more likely to make it worse as it lowers the efficiency of the engine at emptying its cylinder at the end of the exhaust stroke as the cylinder pressure for any given volume of air will be lower, if the engine doesnt empty its cylinders as well then it cant refill them as well on he next stroke.

Thats only my guestimate though, ive never had the opportunity to test it.

With regards to large ports etc, depends on the size to begin with, bear in mind that compressor surge happens at lower-mid RPM and for best VE at lower rpm you dont actually want huge ports as the gas speeds drop, same goes for excessive amounts of cam duration.

One thing i personally am going to test at some point is the effect of cam overlap, as ive got a theory that overlap will reduce surge as it effectively allows the turbo to flow more air by blowing some straight through the engine during the overlap period.


Im guessing that the people who know the answers to all of the above are going to be reluctant to share their findings sadly, i cant imagine any tuner other than possibly Stu posting on this thread as most of them guard this sort of knowledge very closely, and TBH in this case, i really can see why!

Mike Rainbird

iTrader: (3)

It might be cheaper to just make the compressors smaller .

Anonymous

Thats at the expense of peak flow potential then though

he could also make the turbine housing larger, but thats going to increase lag


I suspect SteveN knows both of those things already though, so if you could stick to asking Harvey only for comments about the actual subject matter, ie the engine internals, that would be ace

dumped

WTF - as mike says lol

Mike Rainbird

iTrader: (3)

I disagree. If it surges, then the turbos are mismatched for the engine. It would just be so much easier to change the compressors than change the engine.

You can reduce the effects of surge by increasing the valve size / porting and changing the cam profile, but unless you know the engine inside out, you would struggle to get it exactly right first time.

sunny

iTrader: (1)

Might be even easier to use a correctly spec'ed turbo and engine.

Anonymous

So if someone had built an engine that is spot on at high rpm, perfectly matched compressor at those RPM and makes great power etc, you would advise that rather than try altering the cam timing slightly (for example) to try and eliminate surge they instead fit a smaller compressor and lose their top end?

Interesting approach


As you arae hinting, like always the answer is to do what you would do and just go to a tuner who has played with all these parameters and pay them to build you something that works, but some people (like itsmeagain) enjoy doing things for themselves and want to learn along the way.

Anonymous

Is that not what he is trying to do?

Understand the interaction of the engine spec and the turbo spec?

Every question on here could be answered with "do it correctly" or "get someone who knows how to do it correctly to do it for you"

But then it would be a fucking crap site TBH!

sunny

iTrader: (1)

You should be able to look at a compressor map, look at the engine spec (air flow etc) and make a good decision as to what turbo to use instead of guesses and seeing if something will work without surging and flow the air to make the horsepower. Too many people take a chance and HOPE the turbo will not surge on their engine without looking at the technicalities in the first place.

I would say the surge issues are more top half related...ie cams and head not the c/r but if your tuner or you understand the maths at this point having the ability to spec the correct turbo shouldn't be too much of a problem.

Mike Rainbird

iTrader: (3)

Chip,
As ever, you're over-simplifying things to suit your own response . If it was only surging slightly, then yes adjusting the cams may help. But if it was surging its tits off all the way until 6k, then the easiest solution would be to go down a size on the compressor (just as Doug has done on his car ).

Anonymous

Mike as ever i was giving an extra example to get a point across clearly.

itsmeagain isnt stupid, and he wont be talking about massive amounts of surge for a totally mismatched compressor housing, by only considering that scenario in your response it was YOU that was over simplifying things by bascially replying saying "if its wrong fit one thats right, if you dont know whats right then pay harvey to decide"

Why not try and actually answer the question you KNOW he is really asking (if you have alreay had chance to ask Harvey that one), or just STFU?

Mike Rainbird

iTrader: (3)

Just wondered, one of the ways of decreasing/eliminating compressor surge is making the head flow more air per rev, by cams, headwork, etc, but never seen any specifics on this and also wondered if that was all there was on the engine internals side of things.
Yes, but you have to get it just right, or you will remove the surge, but drastically increase the lag. It is a fine art matching the VE of the engine to the turbo.

Does compression ratio and engine capacity have an effect on compressor surge? (if head spec was identical) And if so, in what way and to how much effect.
As Chip has said, anything that improves the VE of the engine will help remove surge. Anything that worsens the VE will heighten the surge. Obviously raising the compression to improve the VE will give an increase in internal pressures, which may require the boost to be knocked back (which will help remove the surge as well). Obviously you can only go so far with this, and it is difficult to do on an engine already built (unless the gasket is currently of suitable thickness that you could increase the c/r by putting a thinner one in. Otherwise it is a case of skimming the head, which may not be possible depending on how much material has already been removed.

Is either the inlet or exhaust side more important to combatting surge (providing both sides were equally good/bad in the first place)?
The inlet is the most important, but if they are completely mismatched, you won't be able to get rid of the gasses as quickly as you require and internal temps / pressures will build up.

I presume as well as removing any obvious resrictions, enlarging the ports will help, or am I getting the wrong end of the stick here?
Yep and valves if necessary.

Presume less exhaust/manifold/turbo back pressure will help too.
Yep, same effect of reducing the compressor size would be to increase the turbine size, but will result in noticibly more lag, but possibly more power at top end, if the housing was restrictive.

Anonymous

Nice useful post there Mike, although still only skimming the surface of course, hopefully Stu or someone will come with some more details

One thing to point out though, is that where you say "anything that increases the VE of the engine"
Dont forget that some things that increase the VE at certain points will actually decrease it at other points, its not just a case of "bigger = better VE" irrelevant of rpm etc

Philip

Practically speaking, you need to get the engine to take more air in, so you would advance the inlet cam and retard the exhaust cam.

I think a BOV would help, to stop the air backing up along the inlet tract. I'm going to put mine back on to see if it makes any difference.

How would adjusting the duty cycle on the boost controller help?

Phil

Anonymous

Its the effects of cam timing that i am most interested to see peoples experience of, and its also most useful to people in terms of being so easy to adjust

I dont see how a BOV would help as it wouldnt be operating during surge anyway as there is no pressure differential across the throttle butterfly

By preventing the turbo reaching boost levels above its surge limit at given points in the range you can "map out" surge completely even on a very badly specced engine, but thats a different thing to what steve is asking about.

Philip

Yes, not thinking straight.

Phil

Anonymous

Potentially some form of managed boost leak would actually solve the problem, by allowing the turbo to flow more air for the same boost level, but im not aware of anyone who has ever done that, effectively you would need a wastegate on the inlet that was electronically mapped to the surge line of the turbo, but i think thats one of those ideas thats best left as just an idea, lol

Mike Rainbird

iTrader: (3)

Chip,
There is already something like that, it's also called a BOV (not to be confused with a "dump" valve) .

However, I don't know of a mappable one, just the fixed variety. It does EXACTLY what you say, and bleeds off any air above a set FIXED pressure. It is there to stop engines from grenading themselves from over-boost and also there to stop any cheating in fixed pressure race car series with sealed ECUs (just in case someone figures out how to hack into them). Eg, the Indy cars use BOVs in the above-mentioned way in an attempt to keep the power levels equal. I "think" they are fixed at 2 bar, so anything above that, the spring allows the valve to lift and bleed the excess air off, so the engine only ever sees a max of 2 bar, no matter what the turbos try and supply.

Obviously there is no need for a mapped one, as this is what the wastegate is "supposed" to do if the boost curve is mapped properly.

Philip

As seen here, if you look closely:



Phil

Anonymous

Mike, thats no good though cause you cant set it to only vent boost when lower in the rev range.
Usefulfor stopping boost spikes when the wastegate cant act quickly enough though

When i said about "no one has ever done that" i meant in the context i was referring to it, in order to allow a turbo not to surge.

What you are on about is purely overboost protection, they get used a lot on superchargers like that where there isnt a wastegate to control boost.

bud-weis

up for more discussion, i'd like to learn more

Stavros

hmm, good so far, but died quickly too.

TBH ive been very careful on spec and dont think i will have any issues, but from the findings that came from a recent skyline engine dynod got me thinking.

It came on boost far sooner than expected, 2bar boost was easily do-able by 3k, but was surging a bit so will have to be adjusted for the road.
I didnt expect my engine to produce full boost till about 4-4.5, but going by this one, it probably will do it way before, and id rather not have it spoiled by surge.

Also the head isnt great, far worse than is usually made out, so headwork will be killing two birds with one stone hopefully if done right.

Bearing in mind i got nearly 500cc more in my engine that should help with regard to surge too, obviously...

I should be fine, i dont think it will surge from what I worked out, but it got me thinking and hence this post.

Its good to learn, and not just the usual "take it to your tuner" bollocks...

EscortWRC

Just a random comment from the peanut gallery here, but I can't see how the CR would affect anything because by the time its relevant, the intake valves are closed, and you're at dead-head pressure in that intake runner.

In my experience, surge is typically caused by a mismatch of turbo and engine.