Royal Enfield tuning 4 - possible reasons why retarding inlet cam timing often improves performance

Videos discussing 'how to' retard the inlet cam timing have already been made and can be found in my recently started playlist titled 'Royal Enfield tuning', to which I will be gradually adding older videos already made, as well as any new material which may be relevant as it is produced.
Here, I ponder and discuss some of the potentially beneficial effects of retarding the inlet timing, which certainly seems to work for some reason, or reasons, whether my theories as to why are correct or not!

[@notwocdivad]

Thanks for that clear and concise explanation Paul, Even my thick head took it in, I'm sorely tempted to try it on my Bullet Sixty 5 500! Wish I could afford one of your ASBO engines but my pension won't run to the expense!!!

[@noeldoran8884]

That info only comes with experience that's what you do Paul this type of knowledge will be lost without this effort!!!

[@johnmahoney4523]

Fantastic video, well done Paul, you explain things brilliantly. Keep up the good work 👏 👍😎

[@keithchessun5085]

Thanks for taking the time to give that excellent explanation. Surprised at the difference between piston position at opening and closing times

[@thomasrose7713]

I love the Royal Enfield focus of your work Paul thank you for the time and effort you put in to helping all of those of us with old royal Enfield and triumphs. Hope to see more on the infield big twins thank you for sharing your extensive knowledge and the great views from the bench and the road all the best from the US👏🏻

[@paulhenshaw4514]

Many thanks!

[@alexanderpittman3849]

I did this modification a couple of years ago after watching your original video on the subject and I can certainly say it has improved the performance of my 500, it starts very well and is excellent under racing conditions.

[@norsehall309]

G'day Paul thanks for that timely lesson on tuning the engine l have been reading everything at the moment on that subject as my 500 bullet needs some work, cheers mate, Neil 🤠.

[@roythearcher]

One of the benefits of having the inlet valve close silghtly later is that the inertia of the incoming charge is still pushing the fuel/air mixture into the engine. Now, this wont be very strong at low engine revs but at higher revs it will make a considerable difference to the new charge coming into the cylinder as the speeds and thus, the inertia of that new charge will be higher so that the new mixture is still entering the cylinder, even when the piston is already on its upward path and could be part of the reason why you get more power out of the engine with the timing retarded by one tooth.

[@wheater5]

I’ve often wondered about the effect of changing the overall length of the inlet tract when combined with this modification. I fitted a 26mm concentric type carb with short bell mouth to my 350 and removed the air box and original filter. This was one of other mods, such as basic gas flow improvements to the ports, a Meteor Minor piston (raised CR) and a free flowing exhaust. I also discarded the aluminium spacer block that housed the redundant auxiliary air valve. So, the inlet length is reduced. The bike now seems quite quick compared to many 500s and will easily rev to the point where the valves bounce. I visualise engine gas flow/volumetric efficiency as a similar thing to tuning a trombone. Change the overall length of the tube and change the note.

[@56Model30]

I would have thought that the inlet valve timing impacts the cylinder filling. Retarding the inlet could improve things because the exhaust effect on the inlet charge is not so great when the inlet valve opens while closing the valve later allows the inertia in the inlet charge longer to fill the chamber….

[@bananabrooks3836]

So if he went even further with the retarding, at some point the valve would allow the charge to blow back into the inlet tract and carb?

[@markalton2809]

may be worth experimenting with longer lift duration on the inlet cam?

[@paulhenshaw4514]

Yes, go too retarded and a lot of fuel / air mixture will get blown back.

[@ferrumignis]

Retarding cam timing tends to tilt the torque curve , a little less at low RPM traded for a little more at high RPM. Having the valve close later allows better cylinder filling when the air in the inlet ports and throttle body/carburettor has enough kinetic energy that it continues moving into the cylinder even as the piston starts to reverse direction.

[@williamharris4426]

My BSA B31 goes well with 1 tooth retarded on inlet cam, glad I watch your channel

[@441rider]

Well explained I had the 441 years ago off one tooth and it would not start. Took me a week or two to narrow it down as the bike felt like it wanted to go but of course points were not set to compensate.

[@shug831]

That all makes good sense to me. 😊

[@markalton2809]

I think a Vernier-adjustable inlet cam wold be an interesting tool to find if there's any more optimal setting for the valve timing. I was also thinking that at higher RPM, perhaps the inertia of the incoming inlet charge would help reduce the effect of lost pressure from the later inlet valve closing.

[@hermangibbs8163]

Great science. Thanks !

[@neillawson4252]

Really enjoyable ,just which I understood better lol 👍

[@theforester3426]

Had a Royal Enfield 500 silver bullet back in the day found it to be near impossible to get the timing spot on many tried and many failed, it had a lethal kick back when trying to start it never got to the root of the problem.?

[@nigelpearson6664]

Maybe do a compression test.140 psi perhaps and 120 standard.

[@paulhenshaw4514]

I have honestly never done a compression test on any engine I have ever built.

[@441rider]

@@paulhenshaw4514 LOL! me once or twice but 165psi skids rear wheel on a hill trying to bump start. I have only used a strobe 2 times all my timing is by sound, dyno hills and intuition and scribe marks. Roadside mechanic,

[@thomasobrien5976]

Love your video's Paul you really know your stuff for sure...which is why Iwould like to ask a question! Could I fit one of those high compression pistons in my 52 350? Do you need to change valve spring? Timing? And lighten the crank Or is it ok just to do the piston? I think the std bhp is around 18 what would this increase it to? I don't want a 100mph bike but I would like a little more confidence when riding on bigger A roads at the moment it is comfortable at 55-60

[@paulhenshaw4514]

Thanks, Thomas. Yes, but you would need to increase the fueling at the very least. Depends on how far you want to go with it really.

[@mkdave1955]

Hello, when you talk, about 32 to 38° advance is the full advance or static? Thank you, David.

[@paulhenshaw4514]

Hi David, those figures are for full advance.

[@jeremyatkinson4976]

My asbo 535 had the cam retarded. I don't think it suits the unit 500 bike as one loses the punch at 3000. It certainly wouldn't suit a scrambles engine. The mod allows me to overtake without changing gear during the maneuver. It widens the rev range. I'm no sure if it adds much power at peak power but the power hangs on for another 700 revs or so. Basically it's moved the power band up by that much. One would probably get more power from this if the restrictive airbox was modded. I recently saw another 535 that had just had the airbox extensively altered, nothing else, and the rider said it now happily reached 85, whereas before it struggled to get much past 75. But as is obvious all this is just the 535, and yes the engine is smoother.

[@thomasobrien5976]

So would you reccomend leaving it where is is on a standard 350

[@paulhenshaw4514]

Yes, definitely.

[@nigelpearson6664]

The testers are very cheap. I have a hunch Asbo will be higher. Halfords did stock them. I had a Mini Metro that didn't like Shell petrol. The tester found it.60 psi on one cylinder and 75 psi on the others. It was early unleaded.

[@shyamdewan3160]

The head looks exactly like a 350cc bullet

[@paulhenshaw4514]

That is what it is.