Discuss........

I thought I would start a few series of discussions on some hotly discussed topics that seem to extract supreme conjecture and/or regurgitate BS from someone else. Hopefully we can help understanding of what works and why.

Please add your thoughts and reasons why you do/do not believe the motion:-

"Turbo cars do not like camshaft overlap"

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

seen cosworths power pumping out 780 hp running overlap

seen other engines pumping power that don't .

seems the more stock it is the less requirements for overlap.

according to an engine builder that own's his on business building for major player told me its all down to design and the specs used.

ranging from camshaft design to crank ,stroke, bore , rpm etc.

don't see many custom parts for 3sgte apart from run of the mill stuff thats not exactly trick which probably limits R & D .

Even the stock setup produces good torque increases from retarding the exhaust cam

I presume that this is due to creating a little overlap promoting better chamber filling at low rpm

Without exhaust manifold pressure measurements it's impossible to say for the general stock case, never mind custom turbo setups

It will vary enormously from turbo to turbo but given the restrictive stock CTxx hotside I'm not suprised it creates some benefit to have the turbo activly pressurizing the cylinder to try and eek a little more burn stuff out and a little more fresh stuff in
The eye (not always a bad tool) tells me that ex manifold pressure will be higher than inlet most of the time. Hence the propensity for boost creep with stock setups and large bore exhaust

I think that there is a common miss-conception that with overlap and when both inlet & exhaust valves are open at the same time that the inlet charge can just flow in to the cylinder and out of the exhaust port hence not providing any real gains (and only negatives)

My thoughts are that the exhaust manifold including CTXX hotside (as steve has pointed out) is pretty restrictive. Typically turbo cars have considerably higher exhaust side (pre turbo) port pressure than inlet pressure (or else the turbo would not actually have enough energy to drive the compressor wheel) and as such the inlet charge is very unlikely to just flow in and straight out of the combustion chamber/cylinder.

However surely the exact same exhaust pressure is likely to cause flow reversion if overlap is too great? Hence you will end up with hot, spent gases back in the cylinder.

I would also guess that rod ratio will have an effect on how an engine responds with overlap as the rod ratio alters the speed with which the piston transfers from going up to going down again.

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

Some definitive guidance below from the 'bible':

With turbo engines more cam duration and higher lift always takes away low speed torque.

For a turbo road car it is recommended that you use 10 degrees less duration and 5% less lift than for an equivalent normally aspirated engine e.g 3S-GTE vs 3S-GE

For a turbo competition car it is recommended you use 20 degrees less duration and 15% less lift than for an equivalent normally aspirated engine e.g 3S-GTE vs 3S-GE

The cam lobe displacement angle in a turbo engine is very important because of the relative pressure differences between the inlet and exhaust ports

In an N/A engine there is only minimal gas pressure in the exhaust port at the time the inlet valve opens during the valve overlap period and at a certain engine rpm there will actually be a vacuum in the exhaust port if the exhaust headers are the correct length.

With a turbo, the exhaust back pressure will often be higher than the intake boost pressure due to the flow resistance caused by the turbine wheel and housing

It follows that during the valve overlap period exhaust could flow back into the inlet tract. This gas then dilutes the inlet charge, taking up volume which should have ben occupied by fresh air/fuel mixture. Also, the hot exhaust gas reduces charge density by adding heat to it, then in severe cases this extra charge heat can set off a chain of events leading to detonation. This problem is much greater in engines where a small turbine wheel and housing is fitted to improve bottom-end power and reduce turbo lag. This effect is more evident in road cars than competition.

The most effective way of reducing this effect is to move the inlet and exhaust lobes further apart. Fortunately the twin cam design of the 3S engine makes this a practical option. Also, if you fit a big free-flow exhaust system the corresponding reduction in back pressure means that the lobe separation can be reduced by between 4-8 degrees

You can only successfully run narrower lobe displacement angles (more overlap) when you free up the exhaust and using very mild cams

Source of information: Forced Induction Performance Tuning by A. Graham Bell

_________________
Don
GT4DC Chairman
1994 Toyota Celica GT-Four ST205WRC JDM 269bhp @ 0.9bar
1994 Toyota Celica GT-Four Special GT 590bhp @ 1.8bar
1989 Van Diemen RF88/89 Formula Ford 1600
2008 Nissan Patrol GU 3.0L ZD30DDTi 154bhp

Hmm,
Didn't realise "Mr Telephone" also referenced the 3S engine in his text.

This is pretty much what I expected, some of the reasoning being banded about did not however make any sense.

I think "Mr Telephone" makes generalisations whereas DJ is right to point out that it will be influenced by target power, max rpm, (what I assume is rod ratio), bore/stroke ratio etc.

Maybe it wasn't a myth as such but of a miss-understood process.

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

LOL!...I've paraphrased to add clarity to our specific discussion.


Quite the contrary, he is quite specific about the differences between a road based setup and a track/race engine. Target power and max rpm will be determined by the sort of use you will put the engine to. i.e road vs track. Aspects like bore/stroke are fixed for any given engine and the basic principles of valve overlap still apply. He has even included a table of the various options as a guide to the sort of changes you'd expect to have to make to your engine. I omitted this in my posts in the interests of brevity.


I think that's fair comment. We all know that until Adrian got the cams set up on his 3S-GTE he wasn't really seeing the horsepower that you'd expect from his spec. Originally I was going to set up my 3S-GTE on an engine dyno but my man seems confident that he has got the cam timing sussed so we can do it all in-car. Sometimes there's a benefit in following rather than leading

_________________
Don
GT4DC Chairman
1994 Toyota Celica GT-Four ST205WRC JDM 269bhp @ 0.9bar
1994 Toyota Celica GT-Four Special GT 590bhp @ 1.8bar
1989 Van Diemen RF88/89 Formula Ford 1600
2008 Nissan Patrol GU 3.0L ZD30DDTi 154bhp

Indeed, but always best to have an understanding to ensure that you are not following someone up the garden path

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

I was just about to quote the same passage from the same book. Good job I obthered to read the thread.

It really is an excellent book which i would recommend to anybody with a passing to extensive interest in the subject.

_________________
JDM ST205 (286bhp and 281lb/ft)
JDM ST202 (Finally sold )
1982 Pontiac Trans Am 5.7L V8 (See my website)

Maybe I should re-read this section

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

we had a cosworth engine 4 potter than ran over 11.60 mm lift with valves making the tidily 3sgte ones look like toys.t when it came to the crunch produce over 750 hp..and revved safely to 9k all day long.

da guys who built these engines came from cosworth racing ,mountune, all carry very extensive knowledge producing many types of race engine from v12's v10's to little 4 potters pumping over 1000 hp, dutch guy in fast ford 2006 had 900 using almost an identical engine we ran.

on the subject of engine design apart from the block casing , cylinderhead casing every other part was bespoke. crank, rods, pistons, cams, valves, shimless buckets, manifold etc completely design for the job.

can remember the engine builder explained that many engines had been built and dyno'd to destruction before the final recipe was set, cost was massive.

not saying that bell is wrong, however the proof is these engine been running for many yrs producing big power without letting go every other event.

these engines started life on an engine dyno as its the only way to do it.

rouse sport their engines used high lift cams over size valve running over lap which was clear when servicing cambelts which was preformed on a regular basis back in the late 90's running 550 - 640 hp

don't get me wrong i know jack *bleep* compared to corky bells books, but going by hands on experience to many ran this configuration for it to be none productive.

one thing i will say again ,its down to engine design the sum of parts used for the job that determines the formula used.

the 3s engine is more like a motorbike engine

hartmans over a bell, could list a whole load of guru's each to there own.

many ways of skinning a cat , following is something we all do to different levels, just make sure to follow someone thats not already a follower.

hanging around with peaple who are a million time brighter pays off than just reading books...

have found an article thats been in the back of my mind bit more upto date on turbo engine, when i get a chance will scan it in . must worn you its heavy and conflicts with the bible yet used today on some of the most powerful turbo charge engines in the world.

This one?
http://www.amazon.co.uk/Turbocharging-P ... 338&sr=1-3

I already have this one...
http://www.amazon.co.uk/Modify-Manageme ... 427&sr=1-1

_________________
JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

well for now the scanner is saying no.

anyway will dig out a few links from my moon long bookmarks that need organizing..

anyway something that might be of intrest

A certain amount of valve overlap is beneficial, as without it the volumetric efficiency of the engine would be compromised. Too much valve overlap can have a negative effect on clean running engine of the engine. Valve overlaps one of the areas where road and race engines differ massively. On the road engines, overlap is typically limited to less than 20° at the crank, while on the race engines; this figure is more like 100°.


The reason for this is that the outgoing exhaust gas helps the incoming air move before the piston begins its travel down the bore, leading to more air entering the cylinder.

Inevitably some of the incoming air/fuel can flow past the exhaust valve and out of the engine, but that’s a small price to pay for the additional efficiency that the added air provides. The scavenging effect provided by valve overlap is very useful tool but only tends to work at a limited range of engine speed, giving benefits at that speed but at the expense of loses at the other speeds. You can end up in a situation where exhaust gases actually flows back into the intake system.

Its due to this reason that race engines normally use barrel or port throttles. Having a throttle near the cylinder prevents large amounts of backflow when it’s closed (during cornering, braking or gear changes). When a conventional intake manifold is used with high overlap, closing the throttle means the whole manifold gets filled with exhaust, causing poor combustion until it gets clean. This is why some oval racers use inlet manifolds, while for the street circuits, port throttles are more common.

probably just some school boy's project..

Great feedback DJ ...... most engine builders keep so much up their sleeve that its difficult for mere mortals like us to find out the hard facts. I think the general principles are well understood but its in the detail execution that the engine builds vary. Unfortunately that's when it all goes secret squirrel

_________________
Don
GT4DC Chairman
1994 Toyota Celica GT-Four ST205WRC JDM 269bhp @ 0.9bar
1994 Toyota Celica GT-Four Special GT 590bhp @ 1.8bar
1989 Van Diemen RF88/89 Formula Ford 1600
2008 Nissan Patrol GU 3.0L ZD30DDTi 154bhp