What cam timing do I want?

[rlearp]

I've got my 907 engine apart AGAIN. I convinced myself at one time that I needed 115 EX/115 IN cam timing for top end power on the little motor. Then, at one time I convinced myself that I needed 100 EX/100IN for top end power. Now it has been 4 weeks and I've forgotten everything again. Someone convince me of what is right!

Remember, the motor is for a race car so if we drive it right we'll never see below 5k and we have to use the stock cam - so don't even think about suggesting different cams to go with the cam timing ;-). Thanks,

Ron

[Esprit2]

Ron,

What you "want" is up to you, and it may change from track to track. For a wide open track with long straights, you may wish to optimize peak, high-rpm horsepower above all else. For a shorter, twisty track where you spend more time accelerating out of slow corners, you will want more torque with a fatter, fuller power curve. Different horses for different courses.

Changing the MOP affects the two cams differently
Decreasing the MOP Advances the intake / Retards the exhaust
Increasing the MOP Retards the intake / Advances the exhaust

Advancing the intake and/or retarding the exhaust results in closer/ tighter lobe centers (ie, increases overlap).


Assuming constant duration cams (no hi-tech variable systems)…
Moving the lobe centers closer together (MOP1 + MOP2 = less, exhaust retarded / intake advanced)
… increased overlap
… more peak power,
… useable power RPM band gets narrower
… higher fuel consumption
… rougher idle
… higher emissions,
Wider lobe centers (MOP1 + MOP2 = more, exhaust advanced / intake retarded)
… opposite of above


With constant lobe centers & cam duration (ie, intake and exhaust on a single, rigid cam / or moving two cams together)…
Advancing the cam timing slightly (ie, advancing both cams together)
… improved low rpm torque,
… slightly reduced peak power
… imperceptible change in emissions, idle quality and fuel consumption
Retarding the cam timing…
… opposite of the above


Assuming a constant lobe center angle (MOP1 + MOP2 = constant, ie, intake and exhaust on a single, rigid cam)
Then Longer cam duration gives (I know, you're stuck with stock cams):
… more peak power,
… power peak occurs at higher RPM,
… less torque at low RPMs,
… higher fuel consumption,
… rougher idle,
… higher emissions.
And Shorter duration
… the opposite of the above results

Sorry, no direct, easy answers. But that's how it works. Now the fun decisions are yours.

Regards,
Tim Engel
Lotus Owners Oftha North

[benfell100]

Tim, I understand you had some input into the cam setup on Dermots car. I was wondering about the 'replace the red with the green pulley' thing he did. Did Lotus do this to meet emissions legislation?
If so how easy would it be to undo this and would it make a big difference to the engines character.
Dom

[Esprit2]

Ron,

The Jensen Healey used the Lotus C-cams... same as the ones used in Lotus' own Federal version of the 907. The C-cam's nominal design MOP is 110°. All the different cam timing data that appears in my copy of the JH manual is just pulley variations on the same C-cams. And the JH is the only application where I've ever seen the 115° MOP pulleys used.

JH engines prior to #4030 used a 115 IN / 115 EX set up. That will advance the exhaust cam 5°, retard the intake cam 5° and reduce the overlap by 10°. Overall, a pretty milquetoast set-up and not very race-worthy.

After engine #4030, JH used the nominal 110°IN / 110°EX set-up, except in the Federal emissions engines.

For the Federal engines, JH used 100° IN / 115° EX. That advances the intake 10°, advances the exhaust 5°, and increases overlap 5°. Normally, more overlap isn't good for emissions. But Lotus found that more overlap resulted in more exhaust residue mixing with the intake charge and that had much the same effect as exhaust gas recirculation upon oxides of nitrogen.

If you're using the trapezoidal timing belt, then the stock pulleys available are:
100°: ... Advance IN 10° / Retard EX 10°
102.5°:. Advance IN 7.5° / Retard EX 7.5°
104°: ... Advance IN 6.0° / Retard EX 6.0°
110°: ... Nominal
115°: ... Retard. IN 5.0° / Advance EX 5.0°

If you are trying to optimize peak horsepower at full scream, then some combination that increases overlap would probably be most appropriate... Advance IN / Retard EX.

Are you also stuck with stock carbs? Hopefully you are allowed to dump the Zenith-Strombergs and upgrade to Dellortos or Webers. If you are serious about full-scream horsepower, then you should be running two Dellorto DHLA 45's with 38mm (or larger) chokes. That's a lot of carb for a basically stock 2.0 907 and the low end will suffer. But it will really come on above 5000 rpm. Big ports will help too.

The stock cast iron tappets are only safe for around 7500 rpm. Install the later forged steel tappets (Lotus standard service replacements... the only new ones available now... that's your excuse). With them you can extend the rev range to around 9500 rpm. I'm not sure the C-cam will pull 9500 rpm, but at least you won't be rpm limited in your choices.

Not that I would ever encourage not following the rules to the letter, but Colin Chapman was a master of creative interpretation. And yes, I heard you say you had to use the stock cams. But the C-cam's duration is 272° and the 104 cam's duration is 272°. The difference is the ramp shape and a significant increase in lift (.410" verses .340"). A pair of 104 cams running 100° MOP pulleys... just thought I'd mention it.
;-)

Regards,
Tim Engel
Lotus Owners Oftha North

>> Edited by Esprit2 on Tuesday 29th March 19:48

[Esprit2]

benfell100 said:

(Sni)... I was wondering about the 'replace the red with the green pulley' thing... (Snip)...

Dom,
The 107 cams in the 910 turbo engine are designed to run with a 104° MOP. As Federal emissions standards tightened, Lotus first retarded the intake cam, and then both the intake and exhaust cams. Finally all engines regardless of world market came with both cams retarded. What cam timing a 910 has will depend upon the year and market for which it was made.

Putting it back to "right" is as simple as installing the nominally correct 104° MOP pulleys (green dots) on the cams. It's pretty easy, especially if you do it during a normal timing belt replacement since you'll be doing 99% of the work anyway.

On a "bang for your buck" basis, it's one of the best returns on your tuning investment available. It won't single handedly turn a stock 910 into a hot rod motor. But the improvement in low end torque will be noticeable and the engine will feel more responsive and "alive" overall.

New pulleys are running about US$165 (+/-) now. The stock auxiliary pulley is a 104/green dot, so if you don't mind a little extra work to move it to one of the cams. Then you can do the conversion for the price of one new green-dot pulley plus some labor.

Later,
Tim Engel
Lotus Owners Oftha North

>> Edited by Esprit2 on Tuesday 29th March 19:52

>> Edited by Esprit2 on Tuesday 29th March 19:54

[rlearp]

Great info and I appreciate it.

One comment though - the Strombergs I think are under appreciated and can work well. We've got a set on Jeff's TR8, exactly the same carb and size, and the 3.5L motor put down 160 rear wheel hp with stock carbs and no changes to them. That is about 180 crank hp.

We've now had the carbs reflowed and stuff removed that was not needed - it runs much stronger now and we'll dyno it in a couple of weeks.

My JH has the exact same carb setup as far as changes to the carb go and I'm sure it'll be enough for a 2L motor. We run the TR8 to 6500RPM too, not far from what the JH motor will run.

I'll get some dyno results in about 4 weeks or so.

Now to ponder those cam choices and setup.........

Thanks again,
Ron

[Esprit2]

rlearp said:

(Snip)... the Strombergs I think are under appreciated and can work well. (Snip)...

Ron,
Strombergs can work very well in terms of power output. I think the area where they suffer... where any constant depression carb suffers... is in throttle response. It's not as quick and lively feeling, even if the ultimate reading on the dyno dial is the same.

Also, I think Strombergs don't deserve the reputation they have for being a pain to work on. The only thing wrong with them is that there are so many SU carb's (and SU clones) out there. "Everybody" knows how to work on an SU. But when they use that knowledge on the ZS, it doesn't work. "Of course" that's the Stromberg's fault.

Despite being constant depression carbs, and looking loosely like an SU, they work differently. And you have to approach them differently. If the world had gotten used to Strombergs first, then everybody would be bad-mouthing SU's. In many ways the Strombergs are better than SU's, they just aren't as universally understood.

Regards,
Tim Engel
Lotus Owners Oftha North

>> Edited by Esprit2 on Tuesday 29th March 22:04

[JeffYoung]

I agree with that - the TR8 always feels like it is a bit "behind" where I am with the throttle. Tim, is that due to piston reaction? Also, what is the difference between teh SU and the ZS? I've only worked on ZS carbs, so I'm the opposite of those who think they are the same but have only worked on SUs. I thought both were CD carbs that operated off of a needle rising up and down to release fuel from the float into the horizontal air stream, which had been sped up by a venturi.

Back to Ron's dilemma -- given that the Stroms have slower throttle response than desired, and given that the JH/907 motor really needs to be stay on top of things to stay in its power range, what timeing would you recommend for better throttle response and high (5k plus) rpm performance?

[Esprit2]

JeffYoung said:

(Snip)... is that due to piston reaction?

Jeff,
Primarily piston reaction. When you open the butterflies on a Dellorto or Weber, you're in business. When you open the butterfly on a Stromberg, you have just started the chain reaction that finally results in opening the carb. Strombergs are great on hills or when the engine is lugged down and pulling. But for quick response to rapid throttle inputs, they are always behind the game. Even if horsepower numbers are about the same, a Dellorto/Weber equipped engine will feel more alive than a SU/Stromberg equipped engine.

JeffYoung said:

(Snip)... Also, what is the difference between the SU and the ZS? I've only worked on ZS carbs, so I'm the opposite of those who think they are the same but have only worked on SUs. I thought both were CD carbs that operated off of a needle rising up and down to release fuel from the float into the horizontal air stream, which had been sped up by a venturi.

In principle they are basically the same, as you noted. The devil is in the details. How you make adjustments, which way you turn screws, air bleed verses fuel bleed. Little things that make a difference.

JeffYoung said:

(Snip)... Back to Ron's dilemma -- given that the Stroms have slower throttle response than desired, and given that the JH/907 motor really needs to be stay on top of things to stay in its power range, what timeing would you recommend for better throttle response and high (5k plus) rpm performance?

I'm not trying to dodge the issue. There really isn't one correct answer. It will depend upon the track-of-the-day and what you want from the engine. Some things you just have to work out. That's why the crew chief gets the big bucks.

C-cams and 104 cams are both 272° duration, but the C-cam gets a 110° MOP and the 104-cam gets a 104° MOP (more overlap). I think the MOP difference is primarily due to the big difference in lift... 0.340" Vs 0.410". The 104 cam's high lift can flow enough air to feed the engine at high rpm and justify the greater overlap. The C-cam's low lift can't, so give in to a more mild timing geared toward mid-range and low end.

110° MOP is nominal for the C-cams. I'd probably start by going with 104 MOP pulleys on both intake and exhaust just to see what it can pull. That will increase the overlap and move the power curve up the rev range a bit (advances the intake 6° and retards the exhaust 6°... 12° more overlap).

If a general track day or Test-n-Tune day is available to burn (or dyno time), I'd try that for the first session. Then put a numerically "lower" MOP pulley on the intake before the next session. That will further increase overlap and advance the intake. Depending upon our budget for pulleys, you could make an intermediate stop at 102.5°, or just jump to 100°.

Increasing overlap trades low end grunt for high end horsepower. If an incremental change produces a noticeable horsepower gain, keep it. However, if there is no noticeable gain, go back to the previous last step that did show a gain. Each step toward more overlap costs on the low end and mid-range. Don't give up anything down low end that doesn't produce a gain on the top.

Where to go next depends. With a measly 0.340" valve lift, I'm not expecting the C-cam to pull lots of overlap and big rpm. If the engine doesn't seem to be running out of breath by 7400 (remember the tappets), then it might be worth trying a "lower" MOP pulley on the exhaust as well (retards the exhaust a bit more, increasing overlap).

If that shows more peak power gain, great. But if it doesn't, or shows a loss, then it would be worth exploring the other direction. Increasing the overlap will generally increase peak horsepower. But if the low lift cam just can't support that, then advance the exhaust cam a bit. In other words, if the C-cam isn't going to produce the full-scream power you want, then explore what you can have… more low end and a fatter power curve. Go back to the nominal 110° MOP on the Exhaust, but leave the intake at the last advanced MOP that showed an increase.

Ron's stated goal is high rpm power. That's great on a wide open course with lots of high speed sections. But if it's a tighter, twisty course, then there's a lot to be said for low end torque and a fat power curve, even if the peak is a little lower.

In drag racing, the highest trap speed doesn't always win the race. It's the shortest elapsed time that wins. Low end grunt and a quick start off the line will get you to the finish line quicker than a more leisurely start that just builds and builds to a high top speed. In many respects, road racing is just a series of drag races between corners. Torque is acceleration, and horsepower is top speed. Don't thumb your nose at acceleration.

If you change pulleys on a hot engine, then set the timing belt tension higher than spec. About 125 Burroughs (69 Krikit) on a hot engine. If you work slow and the engine's temp cools down to somewhere between cold and hot, then do a preliminary tension, start the engine to warm it back up, and finally do a hot tension. As the engine heats up it expands, stretching the timing belt a bit more. A belt that's properly tensioned cold will run about 125 Burroughs at normal operating temp. Set the belt on a hot engine to the book spec and it will be too loose.

Regards,
Tim Engel
Lotus Owners Oftha North


>> Edited by Esprit2 on Thursday 31st March 06:12