Re-Profiled Cams

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Hawaiis0
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Re-Profiled Cams

Post by Hawaiis0 »

Who can explain how cam profiles control an engines performance and what different profiles offer?

What is the QED Q420 etc offering?
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Re: Re-Profiled Cams

Post by richardw »

Hi Stu,

From what I can see, the QED Q420 cam is for the Lotus Ford twin cam, not the 9xx engine.

There are lots of sources that will explain the theory and practice of cam profile design, but my simple understanding is that they determine two things - the duration and the degree of lift of valve openings.

The greater the lift, very roughly the freer an engine will breathe. The length and timing of the valve openings has to be taken into account alongside this of course. For efficient engine performance, a degree of valve overlap is needed - that is, the time during which the inlet and exhaust valves are open at the same time. Again very roughly larger overlaps (up to a point) offer greater performance but will increase emissions. Camshaft design and timing will also affect where torque and hence power is developed in the rev range - big overlaps are used on higher revving engines for power further up the rev range.

I hope others more knowledgeable than I will chip in and correct my amateur speculations!

Cheers, Richard
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Re: Re-Profiled Cams

Post by rbgosling »

What Richard says is broadly accurate, but I will append....

Cam profile is largely a compromise between outright power at high revs, and driveablility at low revs.

Higher lift will allow the air/fuel charge to enter the cylinder more easily, and the exhaust to leave more easily, which is particularly useful at high revs as the gases don't have much time to flow.

More lift requires a longer duration (unless you reduce the rev limit), since there is a maximum acceleration of the valve that can reasonably be achieved, at least without some other mitigating measures (like the pneumatic valve springs on the F1 engines that are my day job! :D ). Longer duration also gives more time for the gases to flow.

More duration means a longer overlap time - the period when the inlet valve is starting to open, while the exhaust valve is still not yet fully closed. This is the downside - overlap is bad for smooth engine running, low-speed drivability, emissions and consumption.

So, if you are using your car mostly for racing, or track day work, feel free to go for a super-aggressive cam profile. There are some reasonable compromises out there that should improve power a bit while keeping the car reasonably drivable. If you want the car easy to drive around town and car parks (and to be fair, the Excel doesn't really excel in those areas anyway), then you probably don't want anything more aggressive than what Lotus designed in the first place. Those guys generally knew what they were doing, and they wouldn't turn down an opportunity to give the car more power if there wasn't a downside.

A final note - if the car has done many miles (and particularly if any previous owner has used less-than-the-best oil) then wear on your cams may mean the profile now is not what it was 30-40 years ago. It could be worth getting it measured, and if necessary re-ground (to the original or some not-too-different profile). I didn't do this when I rebuilt my engine, and in retrospect I probably should have.
"Farmer" Richard

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Re: Re-Profiled Cams

Post by rbgosling »

Hopefully this graph will be informative about the terms I'm using. The grey area is the overlap. In this case the overlap includes the first 0.050mm of valve lift, which is the take-up of the hydraulic tappets, not relevant to our engines since we don't have those.

You can see that, if you increase the lift, either the slope at the side of the graph will get steeper (faster acceleration of the valve, which causes issues), or the graph will have to get fatter, i.e. more duration, and more overlap.

You can also see that the exhaust valve starts to open long before the piston reaches BDC, and similarly the inlet valve remains partially open well beyond BDC - again this is not ideal, but a compromise to allow a larger duration and thus lift.

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"Farmer" Richard

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2011 Nissan Leaf (Ragly - EV pioneer, must be due to be a classic one day)

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Re: Re-Profiled Cams

Post by Lotus-e-Clan »

..and there is more... :D

An area that is neglected when amatuers tune engines is selecting an optimum static compression ratio in order to maintain a good DYNAMIC compression ratio for the cam. My knowledge on this is gained from experience building hot Imp engines over the years, so the examples below refer to Imp engine cams (ie Climax road and race).

The table below shows the standard Imp engine cam at the top with ever-increasingly-hot cams (ending in full race) going down the list.

the key parameters are:

Inlet closiing After Bottom Dead Centre ABDC - the LATER the inlet closing ABDC (when the piston is on the compression stroke) the greater the loss of gas volume/pressure with less cylinger filling at LOW RPM ..and therfore the greater the loss of DYNAMIC compression ratio DCR at low rpm when the engine is 'off-cam'.

So you MUST increase the static compression ratio for the hotter the cam in order optimise driveability @ low rpm (i.e. maintain DCR @low rpm for efficient cylinder filling) when the engine is off-cam. Obviously at HIGH rpm for hot cams the gasses don't have time to escape so the cylinders fill more efficiently.

Image

The end colum labelled 187/9.08 contains the SCR values needed to get close to the drivebility of the std cam at low rpm and maintain a DCR of 9.08:1 The DCR column shows the loss of DCR (for a std SCR 10:1) as the inlets are closing later and later with the hotter cams going down the list. The CP psi column shows you the calculated compression test figure if you just maintain the SCR of the std engine (10:1)... i.e. you are losing compression at low rpm (cranking speeds). All figures are mine from about 15 years ago using DCR compression calculator.

I have a Frazer B type cam in my 1040 so I tweeked the combustion chamber volume, piston valve pockets, piston squish, and head gasket thickness in order to get an 11.8:1 SCR. For the same reason Lotus will have increased the SE engine's (104 inlet cam) SCR to 11:1 in order to maintain a good DCR at low rpm.

Conversely, if you simply increased the SCR but used a std cam, the DCR would go extemely high and you'd run into detonation isssues.
Peter K

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Re: Re-Profiled Cams

Post by Esprit2 »

Here are a few examples of some cams for the Lotus 9XX engines. Note that many of the profiles are available for more than one engine type, and some of these are offered for both Lotus 9XX and Twin Cam engines.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dave
Manufacturer . . . . Kent. . . . Lotus. . . Lotus. . . Smith . . . QED
Cam ID . . . . . . . . L14 . . . . 104 . . . . 107 . . . . DS2 . . . . Q420
In. Opens, BTDC . . 35° . . . . 32° . . . . 22° . . . . 36° . . . . 37°
In. Closes, ABDC . . 67° . . . . 60° . . . . 50° . . . . 64° . . . . 68°
Ex Opens, BBDC. . . 67° . . . . 60° . . . . 50° . . . . 64° . . . . 68°
Ex.Closes, ATDC. . . 35° . . . . 32° . . . . 22° . . . . 36° . . . . 37°
Duration. . . . . . . . 282° . . . 272° . . . 252° . . . 280°. . . . 285° <<<<<<<<<<
Overlap . . . . . . . . . 70° . . . . 64° . . . . 44° . . . . 72° . . . . 74° <<<<<<<<<<
MOP. . . . . . . . . . . 106° . . . 104° . . . 104 . . . 104° . . . 105.5°
Total Lift . . . . . . 0.415" . . 0.410" . . 0.378" . . 0.415" . . 0.418"

People usually look at a cam's duration first as a prime indicator of how mild or hot the cam is. Note that the 107-cam, with 252° duration is Lotus' most mild cam for the 9XX, while the 104-cam is Lotus' hottest stock cam at 272° duration. And both are mild compared to the other aftermarket cams mentioned above. The QED Q420 in question is the hottest of the lot, but it's still considered 'Street Hot'. "Race" cams are going to be up around 300°-plus duration.

Overlap has a lot to do with how 'streetable' a cam is. Less overlap will produce a mild mannered engine with a smooth idle. More overlap results in a more tempermental idle with a roughness... a 'lope' at idle. But overlap isn't something you normally choose first. It's a function of more or less duration, and how the cams are timed (a tuner's option to play with the MOP). Make your primary choices for duration and MOP, and overlap is the result that goes along for the ride. "OR"... you might lust after the top end power that comes with more Duration, but not be willing to live with the lumpy idle that comes with the greater overlap that results. Choices... you can't have 'everything' you want.

Overlap and the Opening & Closing ramps do both occur at the low end of the "Lift" bell-shaped curve... but they're not directly related/ or simply different names for the same thing.

Everything has mass and momentum, and needs time to accelerate to where you want them to be... velocity-wise.

The tappets & valves move at a very high peak velocity, but they can't jump to that instantly... they'd shatter. They need time to accelerate... and again to decelerate. The opening and closing ramps do that, and result in the curved ends, or 'feet', at the bottoms of the bell-shaped curves. The length of the ramps/ curves are a matter of design opinion, and vary from manufacturer to manufacturer.

Some manufacturers use ramps that are 0.050" tall, some go as far as 0.100" tall. But the slower, more relaxed the ramps are, the less "full open time" the valves will have for a given duration, and the less airflow that will get into the cylinder. In othr words, two cams of the same duration and lift could flow very different amounts of air-fuel mixture, depending upon how much of the lobe was "wasted" on opening & closing ramps.

The Lotus 104 & 107 cams have very short, very aggressive opening & closing ramps, and really 'bang' the valves open and closed very aggressively. That results in the Lotus valves having more "full open" time than they would with other cams with more duration, but which open & close the valves more gently. Lotus later ran into some warranty expense issues related to broken tappets, switched from chilled cast iron tappets to steel tappets, and issued a Tech Service Bulletin stating that the iron tappets should not be used with cams that have very aggressive opening & closing ramps, like the 104 & 107 cams.

rbgosling wrote:
You can also see that the exhaust valve starts to open long before the piston reaches BDC, and similarly the inlet valve remains partially open well beyond BDC - again this is not ideal, but a compromise to allow a larger duration and thus lift.

Even air needs time to accelerate, especially on the intake side where there's only 14.7 psi of atmospheric pressure motivating it. That, plus the fact that the first few degrees of cam rotation barely crack open a tiny gap between the valve and seat. If you want to have a meaningful flow at 'X' degrees of cam timing, then you have to start the intake process before you get to that point... or continue the exhaust process beyond that point. Nothing is 'instant'. So the intake valve starts to open before it gets to TDC and the start of the intake stroke, and the exhaust stays open until well after TDC and the end of the exhaust stroke. And that results in overlap.

Okay, maybe you don't like the lumpy idle that comes with more overlap, but if you installed a cam with no overlap, you'd be complaining loudly about the gross lack of performance. Overlap is not what you want, but it comes with the package when you want more duration, more aggressive timing (MOP), and greateer lift. It's the price you pay for what you want, and you take the bad with the good. So, how much bad can you put up with?

Regards,
Tim Engel
Last edited by Esprit2 on Tue Sep 14, 2021 19:23, edited 1 time in total.

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Re: Re-Profiled Cams

Post by Pete Boole »

My main reason for supercharging is so that I can stay with "mild-mannered" cams but still get the performance I want. Since "hotter" cams also tend to like revving hard as well that would be no good for me with the ZF auto - peak rpm ~ 6k. I'll be using 107s or 104s probably. Still need to do more research on this as well. From what I understand less overlap tends to be better for blowers.

Good thread!

Pete

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Re: Re-Profiled Cams

Post by Esprit2 »

Pete Boole wrote:
Mon May 24, 2021 21:15
(Snip)... I'll be using 107s or 104s probably. Still need to do more research on this as well. From what I understand less overlap tends to be better for blowers.
Pete,
Whenever you use boost (blowers or turbos), less overlap is better. Both intake and exhaust valves are open during the overlap period, and it's hard to build boost while the back door is still open. The 910 Turbo engine uses two 107 cams for that reason... mild cams with less overlap. The 107 cams won't deliver big horsepower, but the boost will more than make up for that.

A pair of 104 cams run at considerably more overlap, making them less than optimal for use with boost.

107 cam = 44° overlap = 912LC & 910 Turbo on both intake & exhaust.
104 cam = 64° overlap = 912HC, intake only.

I have installed a 104 cam on the intake side of a couple of 910 Turbo engines (stayed with a 107 on the exhaust), and they run great. But the performance benefit over the 107 cams was at the upper end of the rev range. If you want to avoid that with the auto transmission, then forget the 104 cam... it likes to rev.

Regards,
Tim Engel

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Re: Re-Profiled Cams

Post by Pete Boole »

Thanks Tim. Good advice. I believe Gary Kemp also offers a blower-specific cam but I haven't talked to him yet - I don't know how it differs from the 107.

Pete

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Re: Re-Profiled Cams

Post by DavidOliver »

Stu, for a good understanding of Cam profiles, valve train dynamics and a degree of gas flow criteria I recomend you download the book Valve Timing for Maximum Output, available if you Google it. Written many years ago by Ed Iskendrian, who developed Cams and Valve Trains for racing applications in the States this publication describes Cam modifications explaining the very complex inlet, compression, ignition and exhaust sequence of the Internal combustion engine. While dealing principally with in-block cams for V8 long stroke American engines you can extend the principles to our 4-cylinder square and overhead cam configuration.
Current Injection and Engine Management systems still adhere to gas flow dynamics principles.

Some concepts to bear in mind when considering Cam choice:
1. Gas flow through an engine is not Stop-Go, more a gas flow of pulsating pressures. Gas is compressible.
2. Gas pressure immediately after ignition cannot push the piston down until the crank is off TDC, maximum leverage is half way down, and almost zero effective at BDC.
3. The cam profile has to be considered with valve train weight and rigidity, particularly spring choice for the Lotus 9XX engine, where spring weight is around 30% of valve train weight.
4. Valve overlap is a feature for scavenging, inducing fresh air/fuel inlet using the kinetic energy of exhausting gases.

If you want to delve deeper have a look at Scientific Design of Exhaust and Inlet System by Smith and Morisson available at Amazon.
Not worth buying but will show how complicated Gas-flow in engines is.

Most practical discussions on Cam profiles consider that the only way to get a good result is following advice from individuals with wide practical experience of trial and error. A wrong choice can provide a disappointing result, or even engine breakeage.

Dave the cog.

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Re: Re-Profiled Cams

Post by Pete Boole »

Thanks for those recommendations Dave :D. I'll take a look at them.

Pete

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Re: Re-Profiled Cams

Post by DavidOliver »

Pete, seeing that you are using a blower and an Automatic gearbox, you may find the following article interesting, through Google.
Blower Cam Profiles VS Nitrous and Unblown by Alkydigger.

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Re: Re-Profiled Cams

Post by Pete Boole »

Thanks very much Dave - good read.

I've also got a set of adjustable cam pulleys that will allow me to alter the MOP. Lots of research yet to do!

Pete

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Re: Re-Profiled Cams

Post by shaunw »

From a practical point of view:

my car has L14 profile cams. I didn't fit them but I understand that they are designed to deliver more top end power and having driven other Excels with healthy standard engines, the differences, good and bad are very noticable.

On the plus side my engine has extra zip from around 4000rpm and revs more freely to the red line, there also appears to be a more aggressive intake growl. I love it because it's so much fun revving it out.

On the negative side, low rev torque is compromised a bit so it's harder work in traffic, standard cars definitely pick up better up to about 3000 rpm, great when you just want to cruise around

Personally I really like the extra power and free revving nature of the cams but do sometimes long for a bit more torque below 3000rpm. It's a compromise. Adding a lightened flywheel which I did when I replaced the clutch slightly enhanced low and hi rev performance. She really flies beyond 4000 rpm.

I've been wondering recently if mapped ignition would restore some lower rev torque by allowing a bit more ignition advance and make it a more relaxed cruiser when I'm not in the mood to cane it.

Shaun

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Re: Re-Profiled Cams

Post by Lotus-e-Clan »

Shawn,

Definitely mapped ignition will help ...

...but .... for the L14 cam look at Esprit2's chart figure for inlet closing after bottom dead centre (ABDC) ...it's 67 degs rather than 60 degs ABDC for the 104 cam on the SE inlets.

Lotus matched the static compression ratio (SCR) for the 104 cam with 60 degs closing ABDC to 10.9:1 SCR.

Look at the chart I calculated a few posts up and you can see a 59 degs ABDC cam (it doesn't matter that it's not the same cam - the closing figure is universally useful for tuning) matched to 10.7:1 SCR (the end column is the SCR! :oops: badly labelled). This is consistent with Lotus' SCR spec for the for the 104 inlet cam.

For the L14 cam on an inlet with 67 degs closing ABDC you need a SCR of about 11.4:1 rather than the std SE 10.9:1. This to a large extent will restore tractability at lower revs and along with mapped igniton will make the car much better in traffic. I know 0.5 doesn't sound like much of an increase but it will make all the difference.

I have practical experience of this with Imp engine tuning. For years me and my mates (we where just 17/18 yearolds) fitted hot cams without thinking about it and wondered why they should be so shite when off-cam. Then a pro tuner told me I needed to increase the SCR to match the cam ... I hadn't the foggiest idea what he meant, plus having no money to mess about with I gnored the advice ... but now I know better.

I now run a cam with 72 deg closing ABDC with an 11.8:1 SCR on a 1040 cc Imp engine and it will pull in top from 25 mph ( ok top gear is only 12.5 mph/1000 rpm and it's a light car) whereas I'd be otherwise reaching for second gear when I knew nothing about proper tuning and would run a 'tuned' Imp on its std 10:1 SCR with a hot cam.
Peter K

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