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Location:
Brisbane
Registered:
November 2002
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An actual technical discussion!
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Wed, 03 March 2004 03:33
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Unlike some people, I am limited to a budget 
Down the track I was planning on getting a set of cams for a turbo engine running a large amount of boost, but $2000ish in one hit (including adjustable cam gears) is a bit rude if you ask me.
So, I was wondering what kind of benefit the replacement of one cam would be, and which one would offer the greatest improvement? Obviously the second cam would come in time, but I would be interested to know people's thoughts.
The way I'm thinking is that greater lift and duration on the exhaust cam would be of greater inital benefit than it would be on the inlet side. Why?
As the inlet side is seeing air forced in at a particularly high pressure, I would guess that it is less likely to be causing a flow problem. The exhaust side already has a turbocharger in the way so any extra flow capability before then should be taken advantage of!
On the other hand, it's entirely possible that the combustion process is already forcing the air out of the exhaust side rather efficiently, and perhaps the fact that the turbocharger is providing a greater restriction means that greater benefit may be received on the inlet side.
Interested to hear people's feedback as this is prettymuch uncharted territory for me. Just got the old cogs turning a little, that's all!
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Location:
NSW
Registered:
September 2003
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Re: An actual technical discussion!
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Wed, 03 March 2004 03:48
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Well traditionally guys run a longer duration cam on their inlet side. So i would say go inlet side 1st.
Trouble is that you might want to go up 2 sizes on your inlet side and 1 size on your outlet side... That means if you only do the inlet side 1st then it will be too greatly mismatched to the OEM outlet (approx 2 sizes smaller). IE sizes = 256, 264, 272, 282, 304. Hope you get what i mean by "sizes" and this makes some kind of sense...
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I supported Toymods
Location:
I renounced punctuation
Registered:
May 2002
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Location:
Perth
Registered:
September 2003
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Re: An actual technical discussion!
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Wed, 03 March 2004 07:01
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i was under the impression that on 4-5-6-7MG engines, the most gain is to be had on the exhaust side...?
i'm not sure if you're running a turbo or not though. if you are, exhaust side. if not, then as gianttomato said, inlet is the way for more power. 
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Location:
Tasmania
Registered:
May 2002
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Re: An actual technical discussion!
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Wed, 03 March 2004 08:38
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From what I have heard...
With non-turbo engines you get the most gain buy increasing the duration of the inlet cam. This raises the rpm range of usable torque, and so increases power as the torque is made at higher rpm. (Power = torque x rpm).
With a turbo engine, it is very different. Mostly the best cams to use on a turbo engine are the stock ones. Note how most turbo upgrade kits on new cars do not include cams, just computer and injectors etc. But to modify a turbo engine, you get the most gains increaasing the duration of the exhaust cam. This is to get exhaust flowing better to the turbo, which inturn increases boost. But this also would raise the rpm range and down low may feel worse, or feel like there is more lag.
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Location:
Brisbane
Registered:
November 2002
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Re: An actual technical discussion!
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Wed, 03 March 2004 12:00
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i'm not sure if you're running a turbo or not though
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What, people actually modify non-turbo engines? 
Yes, turbo!
And it's actually for a 1JZ
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Location:
Tasmania
Registered:
May 2002
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Re: An actual technical discussion!
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Wed, 03 March 2004 12:17
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Some people do, but most don't modify the actual engine. Changing the turbo, injectors, exhaust, computer etc are all external. Engine is often left alone.
And if you are on a budget, maybe you should leave the camshafts alone and just play with boost control, computer stuff, exhaust etc. Getting a new camshaft would mean sorting out all the shims again, getting a adjustable cam wheel, timing the cam in etc. All takes alot of time, and probably won't give much more power with the standard turbo and computer, if at all.
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Location:
Brisbane
Registered:
February 2003
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Re: An actual technical discussion!
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Wed, 03 March 2004 13:46
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my (limited) understanding of turbo cams is that you:
-want to improve intake breathing - either thru duration or lift
-want to minimise exh/int overlap so you don't waste unburnt, compressed gases out the exhaust (no point having the turbo do all that work for nothing)
so i would guess that my cam has:
-slightly higher lift - exh and int
-longer duration for int
-faster exh closing
the longer intake duration makes for the sluggish motor (sub-optimal manifold vacuum when not on boost) at low revs
the crux is finding the best balance between better breathing and low-end performance - i think that once on boost, a whole lot of other factors (int and exh manifold designs, turbo trim, wastegate, dump pipe) have a greater influence on engine behaviour.
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Location:
South Australia
Registered:
July 2002
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Re: An actual technical discussion!
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Wed, 03 March 2004 13:53
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I would go with johnny 2tg on this one. Internals are expensive and not the most economical way of modifying an engine - especially if you already have a turbo.
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I supported Toymods
Location:
I renounced punctuation
Registered:
May 2002
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Re: An actual technical discussion!
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Wed, 03 March 2004 21:47
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http://tru-442.tripod.com/camselect.htm Mostly pertains to NA motors - ones that are actually modifed.
http://www.grapeaperacing.com/GrapeApeRacing/tech/ turbocams.cfm Specific to turbo engines for the handful of people that decide that there is more to "modifying" an engine than a bleed valve and a FMIC. It's aimed at Yank bent 8s, but the principles are valid.
Also this:
Cam: On naturally aspirated engines that run high RPMs, usually there is a lot of overlap because the air doesn't start moving instantly. By opening both valves at the same time, it gives the intake a head start and helps to flush out the cylinder of the exhaust. The exhaust by now is a column of air that is already headed out the pipe and helps pull in the intake charge. On turbo cars, this valve overlap will allow the increased cylinder and exhaust pressure to flow backward into the intake. (I'm led to believe that this is worsened if the turbo is too small and results in significant back pressure. A well matched [read non restrictive turbo] will have reduced back pressure)
Some turbo cam grinds also wait with opening the intake valve until the piston is already headed down. This is because on a boosted engine the exhaust back pressure can be as high as 30 PSI. The intake pressure under boost is only 15 PSI and will be blown backward. By waiting until the piston has started downward, the cylinder pressure drops and the trick is to open the intake valve just as the cylinder pressure is crossing 15 PSI (in this example).
Good high performance cams for naturally aspirated engines will have a intake and exhaust duration of say 270, 280, or 288, 298 etc. Notice the exhaust event is longer than the intake. On turbo grinds, it is always shorter, say 270, 260 (intake, exhaust), or 275, 255.
So Leigh, get a 272 intake and 264 exhaust. Oh, and that big TA45.
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Location:
Madrid - Spain
Registered:
August 2002
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Re: An actual technical discussion!
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Thu, 04 March 2004 05:28
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Just to throw in another variable. Exhaust gas velocity is important for turbo spin up. Could this then be a driving factor in exhaust cam selection??? I guess it would need to be matched to the turbo specs for an optimal setup.
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