Retarding intake cam timing on twincam engines with supercharging?

Here is an idea that has crossed my mind that I thought I'd bounce around on here.

Has anyone experimented on the dyno with intake cam timing under boost?

I had been looking at ways of running a stock whiteblock motor in the short term under boost without having to reduce its compression ratio. If the intake cam timing is retarded (valve closes and opens later), there will be less valve overlap. The valve will also close later on the compression stroke and therefore reduce the effective compression ratio, but the turbo or supercharger will still be able to achieve the same pre-ignition cylinder pressures.

The expansion ratio will remain the same, so in effect the turbo or supercharger can be allowed to do more of the work in compressing the intake charge in the cylinder.

The reduced valve overlap that could use high exhaust gas velocity to help get the intake charge moving in its port initially on a naturally aspirated engine is not so necessary on a turbo/ supercharged engine anyway.

This of course is really only practical on a twin cam motor like the whiteblock or 16V redblock .... just curious if anyone has tried playing with their inlet timing.

Any thoughts?

Sounds similar in concept to an Atkinson/Miller cycle engine.

You're trading off the preferred cam timing (your proposed changes being 'not ideal') in order to run boost. Seems like robbing Peter to pay Paul.

Without extensive dyno time and different grinds of cam profile and timing to test you're pissing in the wind. As a purely experimental thing, cool. If you expect it to work out of the box, gtfo

munday said:

Sounds similar in concept to an Atkinson/Miller cycle engine.

You're trading off the preferred cam timing (your proposed changes being 'not ideal') in order to run boost. Seems like robbing Peter to pay Paul.

Without extensive dyno time and different grinds of cam profile and timing to test you're pissing in the wind. As a purely experimental thing, cool. If you expect it to work out of the box, gtfo

Click to expand...

That is pretty much exactly what I was thinking, but not taken to anything like the same extreme as on a Miller cycle engine.

I don't believe that the standard N/A cam timing will be 100% ideal when subjected to high levels of boost either though... adding boost (and with a turbo some extra exhaust restriction as well) changes the whole internal airflow dynamic during the valve overlap period quite dramatically.

I would never suggest an alternative cam grind to do this on a single cam motor, but on a twin cam it would be very simple to make small angular changes between dyno runs if you have vernier cam pulleys. Making slight camshaft timing changes like this is necessary and standard practice to establish the best timing on tuned N/A motors or with aftermarket cams, and there is no reason that this should be any different when running stock cams but with the influence of a turbocharger thrown into the mix.

Given the levels of work on some of the twincam engines on here, I'd have been more surprised if a little dyno time hadn't been dedicated to establishing the best cam timing characteristics by some people. No special camshaft grinds involved, just minor timing alteration testing. Yes there are likely to be trade offs, but it is also perfectly possible that the optimal turbo cam timing positions are a little different from the stock N/A settings.

Obviously a timing change like this will also have a small effect on the effective compression ratio as well.

Given that the majority of cars here are probably non-16V redblocks though, none of this will apply in most cases.

I just wondered if anyone running a +T twincam motor had any experience of trying this and were prepared to divulge their findings or results if they had.

I have no personal experience of trying this with a turbo (or supercharged) engine myself, which is why I asked the question, just out of curiosity.

It will do the same thing (to low speed compression and torque) as more duration, but only on the closing side, and without the benefit of more time open to fill the cylinder. Not as detrimental at low speeds as closing the exhaust too late with a blower.

I totally agree about using verniers to get the cams set for the performance you want, my skepticism is about trying to use intake cam adjustment in order to reduce the dynamic compression ratio for turbocharging. That part of the equation would require the real time and effort.

If anything you're going to want to advance both cams.
You would tighten them up by advancing the intake cam less, but the cams aren't exactly wild and woolly.

I'm thinking, most 16v guys not running a bone stock Penta with the $180 timing belt have degreed their cams, or at least checked them.

Edit: That was vague. To reduce overlap, you would retard intake in relation to exhaust or advance exhaust in relation to intake.

mikep said:

It will do the same thing (to low speed compression and torque) as more duration, but only on the closing side, and without the benefit of more time open to fill the cylinder. Not as detrimental at low speeds as closing the exhaust too late with a blower.

Click to expand...

When turbocharged in particular, the peak cylinder pressures and exhaust back pressure will be greatly increased and even the increased intake pressure will not offset this. Flow reversal is more likely at lower rpm and the turbine section itself will be a restriction on a true extractor effect at higher speeds where exhaust tuning might normally be expected to work in a N/A engine.

A little less valve overlap could be useful here, remember that only the intake cam would retard.

The intake valve time-area stays exactly the same, but the turbo or supercharger now provides a little more of the total compression pressure as the valve only closes a little further into the compression stroke. This would be detrimental to a N/A engine, but not necessarily so where the turbo/ supercharger is just making up for the later closing valve.

With a +t, you'd be more worried about blasting your intake charge into the exhaust manifold.

munday said:

I totally agree about using verniers to get the cams set for the performance you want, my skepticism is about trying to use intake cam adjustment in order to reduce the dynamic compression ratio for turbocharging. That part of the equation would require the real time and effort.

Click to expand...

Those are my thoughts exactly. One change that doing this would make would be an effective CR reduction, but whether the nett difference in the later closing inlet valve under boost but with lower compression in the cylinder itself but keeping the original expansion ratio would be of any benefit even after allowing increased levels of boost I can only guess.

It could reduce peak cylinder pressures to allow the use of stock parts where a CR reduction isn't practical, but the higher stock expansion ratio would not offer the same benefits as if the CR had been lowered. On the other hand, the balance of total compression (turbo/ blower AND cylinder compression) with expansion ratio wouyld be better?

It would be very interesting to compare the factory turbocharged and N/A cam timings for the same engine to see if this has had any influence on them or not?

Assuming that they are different (does anyone have these to hand... even for stock a turbo redblock cam?), a factory turbo cam might be a good starting point for experimentation with retimed stock atmo cams on a +T motor

.

poulson01 said:

With a +t, you'd be more worried about blasting your intake charge into the exhaust manifold.

Click to expand...

With more charge in the chamber, exhaust gas pressures are going to rise much faster than intake charge boost. The more boost you use, the more exhuast gas pressure is increased as a result, I believe for most street use turbo cars the ratio of exhaust to intake pressures will lie in the range of about 2-3:1. A turbine section (where turbocharged) will likely offer a fairly fixed restriction, so exhaust pressures should rise disproportionately.

In a typical N/A motor, the greatest single pressure differential responsible for pulling in the intake charge is caused by the extraction effect from the outgoing exhaust gas momentum.....far higher than any pressure difference caused by the descending piston on the intake stroke. This relationship is all changed when a turbocharger is added, and the boost pressure compared with the exhaust gas and back pressure along with the additional exhaust restriction of the turbocharger will have an iunfluence on the optimal amount of valve overlap.

Reducing the valve overlap period should be beneficial here.

I can see a point in opening an exhaust valve a little earlier (advancing the exhaust cam), but certainly not in advancing the inlet cam?

There is a lot of potential experimenting in this area, but has anyone actually tried any of it?

Strap it on a dyno with the cams straight up and run it, then drop the intake a tooth and do it again. Post results.

Daviticus said:

Strap it on a dyno with the cams straight up and run it, then drop the intake a tooth and do it again. Post results.

Click to expand...

Ha Ha... if my new engine was as close to running as that it would be in the car already!

A whole series of more minor adjustments would be needed to see what difference it made and in what direction things were going, but doing just that is more or less the basic idea.

Turbeam said:

When turbocharged in particular, the peak cylinder pressures and exhaust back pressure will be greatly increased and even the increased intake pressure will not offset this.

Click to expand...

For that comment, I was leaving boost out of the picture.

mikep said:

For that comment, I was leaving boost out of the picture.

Click to expand...

Ah OK, but there is no point in doing this at all if the engine isn't going to have forced induction. There probably isn't much worthwhile gain in doing it in a turbocharged engine either if it has a large turbo that has a high boost threshold...the off boost losses would hurt it badly.

I was actually thinking specifically of how this might work in combination with supercharging, which further narrows down the possibility of anyone on this site having tried anything along these lines.

NA cams are often added to the T5 motor, as they often have greater lift. You might get some reference info on Volvospeed/performance in the pinned threads. Capt. Bondo ran a nice T6 in a 240, so of anyone on here he would likely have played with cam timing. I know the early 960 cams are better.

lookforjoe said:

NA cams are often added to the T5 motor, as they often have greater lift. You might get some reference info on Volvospeed/performance in the pinned threads. Capt. Bondo ran a nice T6 in a 240, so of anyone on here he would likely have played with cam timing. I know the early 960 cams are better.

Click to expand...

That is a good point.

On a T5 engine fitted with a much larger turbo, the N/A cams might well work better for exactly the same reason as stated above.

In honesty I think this whole idea is probably much more relevant to use of a mechanically driven supercharger than to turbocharging.

I know somebody posted that an intake retard/ exhaust advance on whiteblock cam timing was worth 40 ft-lbs of torque. 2 degrees each if I remember. All the whiteblock cam gears are adjustable, and somebody used to make a little trick sheet metal degree wheel to use to do it.

IPD sells a tool like this:



Works on T5 gears, don't know if T6 gears are identical or no.

ARD has some suggestions for cam timing (turbo) changes on their website

I'll be watching this thread. I plan to experiment with this in my 242 when I finally get it running.

For my 245 I have a 2.5 16v that I will run NA. The static compression ratio is going to be around 12:1 but I will do late intake valve closing to really decrease the dynamic ratio. I won't really be going for power though, more for efficiency on that car, so not sure if I will end up making changes on the dyno or if the datalogging will be enough. We will see.

Poik said:

I'll be watching this thread. I plan to experiment with this in my 242 when I finally get it running.

For my 245 I have a 2.5 16v that I will run NA. The static compression ratio is going to be around 12:1 but I will do late intake valve closing to really decrease the dynamic ratio. I won't really be going for power though, more for efficiency on that car, so not sure if I will end up making changes on the dyno or if the datalogging will be enough. We will see.

Click to expand...

I'm not so sure that is a good idea Eric... not so much on the inlet valve closing point, but in reducing the amount of overlap. With a 12:1 arithmetic compression ratio on a N/A engine a cam change might more often be in order, depending on what these engines can normally stand on your usual fuel... but it might be a touch high for an efficiency build?

Reducing the overlap might actually improve VE at lower engine speeds, but is likely to hurt it overall at higher rpm and reduce peak torque rpm.

Nothing wrong with experimenting with it to confirm any of this though.... in fact I believe that different cam timings really out to be tested by experimentation like this more often than they are.

The whiteblock pulley wheels do have a small degree of adjustment as standard, but that is really just to allow an initial set up to be made on first building and to allow for retiming after a head skim or similar.

The stock pulley fixing holes could be slotted a little to allow some adjustment, but some means of indexing this to the cam position like the degree marked part shown above would be a better idea.

I've just had a look at some of Kent Cam's performance grinds, and other than their assymetric timings, even their 'mild turbo' KC P274 has ten degrees more exhaust duration than intake, listed as 34/60 75/29.

On a T5 or T6 motor used with a non-stock turbo then, a stock N/A '204' exhaust cam could probably be fitted straight up or advanced a couple of degrees and used along with either a factory turbo intake cam, a N/A '204' or the later N/A intake cam retarded possibly by 3 or 4 degrees to good effect.

SOMEBODY must have got some hard data from having tried this sort of thing in the past?