DOHC cam timing changes and effect
Discussion
So .. not being an engine builder .. what are the effects of cam timing changes?
On the interweb you can read all manner of tech articles written by people of varying degrees of competence, but the ones you tend to see are limited to the effects of changing single cam engine timing .. advance for more low down, retard for more top end blah blah.
The problem with this is that the effect is from changing both the inlet and exhaust cam timings together, so ..
which has most effect from a pure full throttle power point of view.
For instance .. If chasing top end power is it better to retard the inlet, keeping the exhaust at manufacturers figures ..? retard both cams? retard the exhaust only? retard one and advance the other ? Are there even any rules that generally occur or is it more a case of "stop being a stupid idiot, this is basic stuff why do you not know this stuff already? "
Does anyone have first hand DOHC knowledge of the kinds of trends they've experienced (if there are any).
Is this topic even too wide to cover generally bearing in mind the numerous combinations of engine specs out there?
On the interweb you can read all manner of tech articles written by people of varying degrees of competence, but the ones you tend to see are limited to the effects of changing single cam engine timing .. advance for more low down, retard for more top end blah blah.
The problem with this is that the effect is from changing both the inlet and exhaust cam timings together, so ..
which has most effect from a pure full throttle power point of view.
For instance .. If chasing top end power is it better to retard the inlet, keeping the exhaust at manufacturers figures ..? retard both cams? retard the exhaust only? retard one and advance the other ? Are there even any rules that generally occur or is it more a case of "stop being a stupid idiot, this is basic stuff why do you not know this stuff already? "
Does anyone have first hand DOHC knowledge of the kinds of trends they've experienced (if there are any).
Is this topic even too wide to cover generally bearing in mind the numerous combinations of engine specs out there?
Edited by spitfire4v8 on Monday 10th December 12:07
We have only set DOHC cams by going through all the combinations of advance, standard and retard on each/ both cams until we get the desired result. Last time we did it on a VX red top we bug*ered about for about 3 hours only to find the hairy cams fitted were best set to the grinders recommendations! Very time consuming and labourious which shows how beneficial it was in terms of time spent to do it like Karl did on the ST170.
Every engine is obviously different as to what degree it is applied, but generally speaking compared to manufacturers figures you want more overlap at TDC for maximum high rpm power. So intake closes later, exhaust opens earlier. Some high output motors can end up with both the in and ex being 3mm open at TDC. You are basically using the exiting gasses of the exhaust to start to pull the inlet charge in.
Downsides; poor tickover, emissions, loss of low down torque.
That's why VVT is king - especially double VVT as closing the inlet is only part way there to making an improvement.
Downsides; poor tickover, emissions, loss of low down torque.
That's why VVT is king - especially double VVT as closing the inlet is only part way there to making an improvement.
Edited by 227bhp on Monday 10th December 17:22
How can there ever be an easy answer when there are so many variations in cam profile, heads and head flow, intake, exhaust, application etc.
And often OEM tend to use it for power/emissions, to allow milder cams to offer better power all round.
I know on Subarus, they tend to advance the intake cam at low loads, ramping it back to 0 at higher.
The best thing about VVT, is you can easily try different things on the dyno to see what works best for each application.
And often OEM tend to use it for power/emissions, to allow milder cams to offer better power all round.
I know on Subarus, they tend to advance the intake cam at low loads, ramping it back to 0 at higher.
The best thing about VVT, is you can easily try different things on the dyno to see what works best for each application.
The simple answer is that, assuming you all you want is to make the most power possible, you need to set the cams in a position that maximises the engines volumetric efficiency.
See, easy!
(BTW, VVT on OE engines is not about being able to make peak power, or about maximising torque, or about minimising fuel consumption or engine out emissions. No, it's about ALL those things, and hence is a compromise!)
More usefully, ime, the only real way to know what to do, assuming you haven't got a handy room full of very smart and very experienced analysis engineers at your beck and call, is to try it! Ie, get your engine on a dyno (engine dyno preferably, but bulk changes are detectable on a chassis rolls if you take care to control boundary conditions for each run), swing the cams around and measure the result....
See, easy!
(BTW, VVT on OE engines is not about being able to make peak power, or about maximising torque, or about minimising fuel consumption or engine out emissions. No, it's about ALL those things, and hence is a compromise!)
More usefully, ime, the only real way to know what to do, assuming you haven't got a handy room full of very smart and very experienced analysis engineers at your beck and call, is to try it! Ie, get your engine on a dyno (engine dyno preferably, but bulk changes are detectable on a chassis rolls if you take care to control boundary conditions for each run), swing the cams around and measure the result....
stevieturbo said:
How can there ever be an easy answer when there are so many variations in cam profile, heads and head flow, intake, exhaust, application etc.
Well the SingleCam mantra is retard for top end power, advance for low end power. I was wondering if there was a general rule that applied to DOHC engines too (other than retard them both / advance them both of course)I have done some testing myself on this in the past for the TVR SP6 engine but I'm going to revisit this again at some point and do the tests again. The graph results were on my old dyno and I need to do the cam timing swings and get the graphs again. Sadly that engine is chain drive / cam sprockets under the cam cover, so not terribly quick to do the changes and re-test, but I will certainly do these tests again and report back, but will be into the new year now.
If anyone happens to do any tests on a belt drive / vernier sprocket engine (which will significantly reduce the test time) that would be great!
I think the problem is extrapolating test results from one engine species to another can be full of pitfalls. For example the cam timing on the TVR V8 engine that has cropped up recently seems most odd and folk seem to alter the timing a lot!
Once one has some cam figures for a specific engine and set up it can be used for others of the same ilk but I cannot see how one can draw specific conclusions from one engine type to another. We have mainly played with A and B series over the years and settled on cams we were happy with. Piper sent us an A series cam to run in a 1380 with high lift rockers, it absolutely blew the other cams we had used into the weeds at all rpms especially mid range. One would have thought the venerable A series would have been worked to death but seemingly not so! With the B series race stuff we fell short when the FIA 1840 engine size took the place of the old tried and trusted 1950 engines. The cam that seemed 'just so' with the big engine was cr*p in the smaller engine, top end above 5000 rpm was good but bottom end very poor especially out of bends. We were hoping for a silver bullet cam to come along like the Piper A series cam but we are still playing with the Crane profile which is known as a Kent 719 in the UK. We have been experimenting with phasing of the 719 to get the best we can as well as scatter pattern. We see a pattern of overlap to engine size but nothing to predict results!
Once one has some cam figures for a specific engine and set up it can be used for others of the same ilk but I cannot see how one can draw specific conclusions from one engine type to another. We have mainly played with A and B series over the years and settled on cams we were happy with. Piper sent us an A series cam to run in a 1380 with high lift rockers, it absolutely blew the other cams we had used into the weeds at all rpms especially mid range. One would have thought the venerable A series would have been worked to death but seemingly not so! With the B series race stuff we fell short when the FIA 1840 engine size took the place of the old tried and trusted 1950 engines. The cam that seemed 'just so' with the big engine was cr*p in the smaller engine, top end above 5000 rpm was good but bottom end very poor especially out of bends. We were hoping for a silver bullet cam to come along like the Piper A series cam but we are still playing with the Crane profile which is known as a Kent 719 in the UK. We have been experimenting with phasing of the 719 to get the best we can as well as scatter pattern. We see a pattern of overlap to engine size but nothing to predict results!
Well mentioned Dave. The hairier the cam the worse the effect of retarding over grinders recommendations we find. Mild cams are much more tolerant of advance or retard. Many years ago while still at the Stonebroom unit an engine builder running on our heads with a 1500 Midget had to run a standar cam, he ran this at some 9 degrees retarded to force it to rev out. Thinking this was 'cool' we tried 5 degrees retard on a race Mini on a Kent 310 and it was as flat as a f*rt, with up to 3 degrees advance it worked very well. Like you are saying, the swing effect on the Manta will depend on the cam spec and vary greatly.
Remember that a river meanders and errodes the longside of the bend until it forms an oxbow lake of stagnant water, until then it is producing a new direction, after the oxbow lake of useless(to the river) water has been shed the river has formed a new straighter path until the next oxbow lake is formed, no one knows where the flow will lead.
Lol, they are though, there aren't many people left tuning or remotely interested in 1950s boat anchors 
It must be terrible to have to buy a cam, then find it doesn't work because of the LCA and not be able to do anything about it very easily, or at all. Or if it's got rockers would you alter the ratio to change the LCA? Sorry, I was brought up on a diet of modern DOHC....
It must be terrible to have to buy a cam, then find it doesn't work because of the LCA and not be able to do anything about it very easily, or at all. Or if it's got rockers would you alter the ratio to change the LCA? Sorry, I was brought up on a diet of modern DOHC....
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