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Folks,

Keen to here the thoughts and opinions of the simple adjustment made to the exhaust cam by advancing by 1 tooth which should reduce overlap (a common improvement to a FI engine to ensure too much air isn't being lost out the exhaust valves).

Check this post out ... http://www.mx5nutz.com/forum/index.php? ... try1194567

G

gslender wrote:Folks,

Keen to here the thoughts and opinions of the simple adjustment made to the exhaust cam by advancing by 1 tooth which should reduce overlap (a common improvement to a FI engine to ensure too much air isn't being lost out the exhaust valves).

Check this post out ... http://www.mx5nutz.com/forum/index.php? ... try1194567

G

No 2 engines are exactly the same, there will be slight variations in cam timing due to production tolerances, and other factors effect where it needs to be set (inlet runner and exhaust pipe length, port shape etc etc).

In theory, the idea is sound. Generally the lobe separation inlet/exhaust should be wider on a turbo application to give less overlap, BUT i would suggest that just grabbing the exhaust cam and advancing it a tooth is A: way too much adjustment and B: not true for every engine.

Cam timing specs vary on every engine, and the only place to dail the correctly is on an accurate dyno.

Cheers
Woz

yep,what he said,1 tooth is probably far too much.but a n/a cam used in f/i motor should ideally be separated a little,but best done by adjustable cam gears/pulleys.
i'm just fitting some myself this weekend along with new timing belt etc,so I can do this when(if) I fit turbo later.
my intention was to just move each cam 3-4 degrees and call it done.(lazy,I know).

what I should do is fit dial guage and degree wheel and plot lift/timing with stock pulleys first so I know exactly what I've got.
(I just fitted a couple of weeks ago some aftermarket el cheapo OBX 256/9mm cams with no real information of actual profile,but they are running ok with a noticeable widening of my smile)
then fit new belt,pulleys etc,plot again just to make sure they are where they should be.
then gather information on other peoples setups,ask people with more knowledge about these engines than me,(wozzah for example).
then probably end up moving each cam 3-4 degrees,just as a starting point.
it can be complicated(it is,a little)or it can be simple.the less dyno time you can afford the simpler it gets,lol.you just make the best decision you can and live with it.
Mick

Not sure if the link I posted work - http://www.mx5nutz.com/forum/index.php?showtopic=100096

I'll repost the relevant parts...

1989-93 man Intake 5 BTDC 51 ABDC Exhaust 53 BBDC 15 ATDC Valve Timing 20 deg overlap
89-93 Auto -- Intake 5 BTDC 40 ABDC Exhaust 55 BBDC 5 ATDC Valve Timing 10 deg overlap
1988-89 GTX Intake 5 BTDC 51 ABDC Exhaust 69 BBDC 1 ATDC Valve Timing 6 deg overlap

So what happens when the manual exhaust cam is advanced 1 tooth(12deg)? We then have:-

Inlet is the same of course, but notice the exhaust timing.
1989-93 man Intake 5 BTDC 51 ABDC Exhaust 65 BBDC 3 ATDC Valve Timing 8 deg o/lap

The poster from MX5Nutz in the UK measured the impact of the single tooth change to exhaust and it is within the tollerances that you would have recommended I change anyway?

Ok - so is the general consensus that you should adjust the cam lobe separation to favour FI conditions (away from what you would do for a NA engine), which is interesting as I see heaps of folks going turbo/super charger and throw in some cams in an attempt to further improve "breathing" but they might actually reducing the benefits of any lift/duration by the lobe separation being unfavourable to boosted applications.

Thanks
G

Not so sure its as simple as that.

Sure for a factory car you would lower overlap, so drivability is better.

All this means is that if you were after 290* cams with 40* overlap when you boost it you go for some 280* ones with less overlap.

Bigger cams still equal more power.

But you need a dyno to prove it actually helps power to reduce overlap.

Dann

NitroDann wrote:Bigger cams still equal more power.

Once again I'm not sure I get where you are coming from. Why does a cam profile whereby intake incoming gas can rush right past into the exhaust gas = more power? vs a cam profile whereby it closes the exhaust valve earlier to ensure all the incoming air is forced by maximum pressure of the supercharger into the expanding cylinder ??

Wouldn't the stock profile with much more overlap = some intake air escaping (under pressure) out the exhaust side? Why is that better and why would that even help at all? I totally understand from a NA point of view (that scavenging effect helps draw air into the engine), but think that it isn't needed (or even becomes less beneficial) when used in FI. Turbo might be different (as it needs exhaust gas to turn) but not so for SC.

Why not?

...actually the more I think about it... it would make sense to retard the intake cam, and leave the exhuast cam as is. The intake is under positive pressure so you can afford to retard the intake opening (when it would begin to be less effective in an NA engine), gain some less overlap (so the positive pressure is maximised) and still get the correct timing and maximum effective opening for the exhaust side.

gslender wrote:

NitroDann wrote:Bigger cams still equal more power.

Once again I'm not sure I get where you are coming from. Why does a cam profile whereby intake incoming gas can rush right past into the exhaust gas = more power? vs a cam profile whereby it closes the exhaust valve earlier to ensure all the incoming air is forced by maximum pressure of the supercharger into the expanding cylinder ??

Wouldn't the stock profile with much more overlap = some intake air escaping (under pressure) out the exhaust side? Why is that better and why would that even help at all? I totally understand from a NA point of view (that scavenging effect helps draw air into the engine), but think that it isn't needed (or even becomes less beneficial) when used in FI. Turbo might be different (as it needs exhaust gas to turn) but not so for SC.

Why not?

What you need to understand is that EXACTLY the same theories apply for making more power on a forced induction (turbo or s/charged, no difference) application as they to on an n/a application. You still need to fill the cylinder as efficiently as possible for a giving amount of power. This point is missed by alot of people. The situation you desribe about overlap still applies to an n/a engine if it is working correctly, as scavenging will pull the intake charge through just the same as a forced induction set up pushes it through. In fact, on an N/a application the intake charge is still travelling in even when the piston is moving up, explain that to me! The key is making everything work together, matching cylinder head flow figures at given amounts of lift, matching cam profiles to suit, and piston acceleration rates to suit also. It is a highly technical process.

If you want to learn about what goes on, I highly recommend the new David Vizard book on measuring and testing cylinder head flow.

You're right in theory about the REDUCED overlap for a given application, BUT Dann as 100% correct in the way you choose boosted cam profiles. If you had an engine of "X" specification, and you wanted to add boost and keep a similar rev limit, you would usully reduce the duration, and/or move the lobe centres apart, but not to a point where it is detrimental to air flow/cylinder filling. Usually by retarding the inlet cam, and advancing the exhaust cam,thus reducing overlap for "X' engine.

Stuffing up cam timing and cam profiles on a boosted application is the same as stuffing them up on an N/A application, either will be bad for the power curve. If you think that the std cams, cylinder head, exhaust etc etc simply can't be improved on in a boosted application you're completely wrong.

Cheers
Woz

Hi Wozzah - I must admit you've only confused me here as I'm unsure if you are saying "yes, reduce the overlap", or no, "you shouldn't"...

wozzah1975 wrote:What you need to understand is that EXACTLY the same theories apply for making more power on a forced induction (turbo or s/charged, no difference) application as they to on an n/a application. You still need to fill the cylinder as efficiently as possible for a giving amount of power. This point is missed by alot of people.

I guess I appreciate the theory is the same, but the forcing of air into the engine by either Turbo or SC is totally different to the vacuum created and air being sucked into the cylinder. Clearly there would be characteristics of the CAM that are suited more for one than the other.

In general, I always understood that forced induction engines like camshafts that have as little overlap as possible. Overlap is the time that the intake valve is open and the exhaust valve is open. With forced induction setup, overlap will cause the forced air/fuel mixture to go past the intake valve, through the combustion chamber and past the exhaust valve. You've just wasted all of your work in getting that mixture in there. Forced induction camshafts also don't require as much duration or lift to make power as a normally aspirated engine.

wozzah1975 wrote:The situation you desribe about overlap still applies to an n/a engine if it is working correctly, as scavenging will pull the intake charge through just the same as a forced induction set up pushes it through. In fact, on an N/a application the intake charge is still travelling in even when the piston is moving up, explain that to me! The key is making everything work together, matching cylinder head flow figures at given amounts of lift, matching cam profiles to suit, and piston acceleration rates to suit also. It is a highly technical process.

Sure. But I'm only talking about making a change to the overlap. I'm only pointing out that adding forced induction to an engine where the CAM overlap has been designed to benefit a naturally aspirated engine where scavenging is used to ensure the intake air is helped into the engine - this CAM wouldn't be the right solution when air is being fed into the intake & cylinders under positive pressure. With FI and std CAM with large amounts of overlap, there is a real chance the intake charge is also being forced into the exhaust side and fuel/air is being pushed out of the engine. As such, reducing overlap would be a simple and beneficial improvement (without having to replace CAMS).

wozzah1975 wrote:You're right in theory about the REDUCED overlap for a given application, BUT Dann as 100% correct in the way you choose boosted cam profiles.

Ok. I'm not sure what you're saying here - is it that the "right thing to do" would be to get an engine engineer to make me a custom CAM? If so, then that is obvious - but isn't simply reducing the existing overlap a step in the right direction too?

wozzah1975 wrote:If you had an engine of "X" specification, and you wanted to add boost and keep a similar rev limit, you would usully reduce the duration, and/or move the lobe centres apart, but not to a point where it is detrimental to air flow/cylinder filling. Usually by retarding the inlet cam, and advancing the exhaust cam,thus reducing overlap for "X' engine.

Excellent. I fully agree with this. My simple point is that making a minor change to the intake or exhaust CAM timing (I think retarding intake by 4-6 deg and only slightly advancing exhaust by about 1-2 deg) should be (in theory) beneficial. I cannot see why it wouldn't, as the reduced overlap would minimize boost leakage, and the longer duration allow a slightly more advanced spark.

wozzah1975 wrote:Stuffing up cam timing and cam profiles on a boosted application is the same as stuffing them up on an N/A application, either will be bad for the power curve. If you think that the std cams, cylinder head, exhaust etc etc simply can't be improved on in a boosted application you're completely wrong.

Don't disagree - and don't think anything I said did, but I guess you are warning against making bold changes like adjusting by a tooth etc. In which case I agree.

Way better to use vernier pulleys and dial in the needed overlap timing changes.

G

Building exhaust so I dont have time for a full explanation.

Basically the only difference between boost and no boost from the cams perspective is that the pressure differential on each side of the intake valve is greater, this quickens cylinder filling, in turn this affects the duration and overlap needed to give the same shaped torque curve.

Overlap simply narrows and concentrates the peak torque band.

So does a turbo, so in order to get THE SAME PEAK TORQUE BAND, you need less overlap.

Thats not to say that less overlap is beneficial to peak power or area under the curve, its to say that to get the same shaped torque curve you need less overlap.

Dann

Got it!

gslender wrote:Hi Wozzah - I must admit you've only confused me here as I'm unsure if you are saying "yes, reduce the overlap", or no, "you shouldn't"...

wozzah1975 wrote:What you need to understand is that EXACTLY the same theories apply for making more power on a forced induction (turbo or s/charged, no difference) application as they to on an n/a application. You still need to fill the cylinder as efficiently as possible for a giving amount of power. This point is missed by alot of people.

I guess I appreciate the theory is the same, but the forcing of air into the engine by either Turbo or SC is totally different to the vacuum created and air being sucked into the cylinder. Clearly there would be characteristics of the CAM that are suited more for one than the other.

In general, I always understood that forced induction engines like camshafts that have as little overlap as possible. Overlap is the time that the intake valve is open and the exhaust valve is open. With forced induction setup, overlap will cause the forced air/fuel mixture to go past the intake valve, through the combustion chamber and past the exhaust valve. You've just wasted all of your work in getting that mixture in there. Forced induction camshafts also don't require as much duration or lift to make power as a normally aspirated engine.

wozzah1975 wrote:The situation you desribe about overlap still applies to an n/a engine if it is working correctly, as scavenging will pull the intake charge through just the same as a forced induction set up pushes it through. In fact, on an N/a application the intake charge is still travelling in even when the piston is moving up, explain that to me! The key is making everything work together, matching cylinder head flow figures at given amounts of lift, matching cam profiles to suit, and piston acceleration rates to suit also. It is a highly technical process.

Sure. But I'm only talking about making a change to the overlap. I'm only pointing out that adding forced induction to an engine where the CAM overlap has been designed to benefit a naturally aspirated engine where scavenging is used to ensure the intake air is helped into the engine - this CAM wouldn't be the right solution when air is being fed into the intake & cylinders under positive pressure. With FI and std CAM with large amounts of overlap, there is a real chance the intake charge is also being forced into the exhaust side and fuel/air is being pushed out of the engine. As such, reducing overlap would be a simple and beneficial improvement (without having to replace CAMS).

wozzah1975 wrote:You're right in theory about the REDUCED overlap for a given application, BUT Dann as 100% correct in the way you choose boosted cam profiles.

Ok. I'm not sure what you're saying here - is it that the "right thing to do" would be to get an engine engineer to make me a custom CAM? If so, then that is obvious - but isn't simply reducing the existing overlap a step in the right direction too?

wozzah1975 wrote:If you had an engine of "X" specification, and you wanted to add boost and keep a similar rev limit, you would usully reduce the duration, and/or move the lobe centres apart, but not to a point where it is detrimental to air flow/cylinder filling. Usually by retarding the inlet cam, and advancing the exhaust cam,thus reducing overlap for "X' engine.

Excellent. I fully agree with this. My simple point is that making a minor change to the intake or exhaust CAM timing (I think retarding intake by 4-6 deg and only slightly advancing exhaust by about 1-2 deg) should be (in theory) beneficial. I cannot see why it wouldn't, as the reduced overlap would minimize boost leakage, and the longer duration allow a slightly more advanced spark.

wozzah1975 wrote:Stuffing up cam timing and cam profiles on a boosted application is the same as stuffing them up on an N/A application, either will be bad for the power curve. If you think that the std cams, cylinder head, exhaust etc etc simply can't be improved on in a boosted application you're completely wrong.

Don't disagree - and don't think anything I said did, but I guess you are warning against making bold changes like adjusting by a tooth etc. In which case I agree.

Way better to use vernier pulleys and dial in the needed overlap timing changes.

G

The point that I was try to make was that in your previous post you made particular reference to cam changes and more overlap not equalling more power, and other references to these issues only being specific for forced induction set ups. It's not that simple.

quote:
"Once again I'm not sure I get where you are coming from. Why does a cam profile whereby intake incoming gas can rush right past into the exhaust gas = more power? vs a cam profile whereby it closes the exhaust valve earlier to ensure all the incoming air is forced by maximum pressure of the supercharger into the expanding cylinder ??

Wouldn't the stock profile with much more overlap = some intake air escaping (under pressure) out the exhaust side? Why is that better and why would that even help at all? I totally understand from a NA point of view (that scavenging effect helps draw air into the engine), but think that it isn't needed (or even becomes less beneficial) when used in FI. Turbo might be different (as it needs exhaust gas to turn) but not so for SC."

The fact is, that all the issues you raised are present on any engine, they are not FI specific, and whilst scavenging changes, it is still very important on any application. Cam changes still react in the same way, and you still find a "sweet spot" when dialling them on any engine.

Bottom line, don't just adjust your cams to what you think suit a FI setup. Do it on a dyno and KNOW your making the right change, and you may find that they don't go the way you think they need to.

Cheers
Woz