Help me understand cam timing, advancing or retarding a SOHC for mpg

[cRiPpLe_rOoStEr]

Originally Posted by Madmanmax

they seemed like a good option because they are super easy to re-jet and tune.

Besides that, nowadays an easier availability for motorcycle carburettors has been a reason why they're so used for adaptations. Odd enough, I have even seen vehicles originally fitted with EFI adapted with a motorcycle carburettor.

[Madmanmax]

Originally Posted by cRiPpLe_rOoStEr

Besides that, nowadays an easier availability for motorcycle carburettors has been a reason why they're so used for adaptations. Odd enough, I have even seen vehicles originally fitted with EFI adapted with a motorcycle carburettor.

and when Chinese knockoffs are a dime a dozen it makes for a cheap upgrade.

[cRiPpLe_rOoStEr]

Originally Posted by Madmanmax

and when Chinese knockoffs are a dime a dozen it makes for a cheap upgrade.

Not sure if there is still any carburettor-fed street-legal motorcycle above 50cc available new in my country. Not even one imported from India, and fitted only with a kick-starter.
https://ecomodder.com/forum/showthre...nly-40728.html

[Ecky]

3 degrees likely won't amount to much, and whether advancing or retarding will benefit power or fuel economy will depend on where it started.

~

I only have personal tuning experience with intake cam VVT. Higher in the rev range, retarding tends to move peak torque upward in RPM, and advancing moves it down, but things get muddy below... say, 3500rpm. There, "it depends".

Most newer engines run a higher static compression ratio than can be readily (or efficiently) controlled by ignition timing alone. Sufficiently advancing or retarding cam timing effectively limits how much air gets into the cylinder, reducing the absolute peak cylinder pressure to a manageable number, while also allowing for very high expansion ratios. (as an aside, most companies choose to control this via retard, likely because it more reliably meters cylinder fill).

Tuning on Hondas is typically done in 10 degree increments, with ~50° total variation of the intake valve - and the full range is typically used under various driving conditions. Mazda's high compression engines have 74° of intake adjustment and 56° of exhaust adjustment (and a maximum of 92° overlap). 3° likely disappears into background noise. That said, I'd probably favor the more advanced position, without knowing anything else about the engine.

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

Most newer engines run a higher static compression ratio than can be readily (or efficiently) controlled by ignition timing alone.

Even though some flexfuel engines didn't resort to such an "extremely" high compression a few years ago, and Fiat introduced intake and exhaust VVT to a SOHC engine in the "people's car" segment when it released the GSE/Firely 3-cyl 1.0L and 4-cyl 1.3L in Brazil. Some folks often say the VVT was meant to provide a lower dynamic compression, yet it doesn't seem to be really the case, otherwise the power and torque difference would be much greater on gasoline compared to ethanol.

Sufficiently advancing or retarding ignition timing effectively limits how much air gets into the cylinder, reducing the absolute peak cylinder pressure to a manageable number, while also allowing for very high expansion ratios.

AFAIK adjusting the ignition timing works to overcome turbo-lag too, but I'm not sure why it would limit how much air gets into the cylinder.

[Ecky]

Originally Posted by cRiPpLe_rOoStEr

AFAIK adjusting the ignition timing works to overcome turbo-lag too, but I'm not sure why it would limit how much air gets into the cylinder.

Whoops. Corrected my post. I meant to write "cam timing".

[Madmanmax]

In theory if you had direct injection you could keep the intake valve closed to increase vacuum in the cylinder so that when the fuel was injected it would vaporize and then you could open the valve to mix in the air. this could drastically increase burn efficiency.

[Isaac Zachary]

Originally Posted by Madmanmax

In theory if you had direct injection you could keep the intake valve closed to increase vacuum in the cylinder so that when the fuel was injected it would vaporize and then you could open the valve to mix in the air. this could drastically increase burn efficiency.

There are some engines that open the intake valve twice at low loads.

[Madmanmax]

Originally Posted by Isaac Zachary

There are some engines that open the intake valve twice at low loads.

interesting, I've not heard of that one before.

[Isaac Zachary]

Originally Posted by Madmanmax

interesting, I've not heard of that one before.

Here's an example from 11 years ago:

https://arstechnica.com/features/201...rther-on-less/

For stop/start traffic—the natural home of both the Fiat 500 and the average American commuter—the intake valves can be opened and closed twice during a single intake stroke.