# 2 Stroke motorcycle engines-why dont they use a spark advance curve like 4 strokes? **URL:** \ **Category:** Factual Questions\ **Created:** [August 14, 2013, 3:08am UTC](https://boards.straightdope.com/t/2-stroke-motorcycle-engines-why-dont-they-use-a-spark-advance-curve-like-4-strokes/666184 "2013-08-14T03:08:00Z")\ **Posts on this page:** 3\ **Page:** 2
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [May 19, 2016, 12:20am UTC](https://boards.straightdope.com/t/2-stroke-motorcycle-engines-why-dont-they-use-a-spark-advance-curve-like-4-strokes/666184/21 "2016-05-19T00:20:47Z")
> [@breakneck](#): > > yes i understand each stroke is a power stroke. As someone added before their rz350 having an advance curve. so indeed 2 strokes are using advance curves . I guess what i was getting at is how come the four strokes that i mentioned had advance units but not the 2 strokes of the same era. The concern of over whelming costs is hard to believe just a some weights and springs. so they were on the four strokes how come not the strokes untill cdi untis became main stay? my only guess is because two-strokes with expansion pipes have such a narrow powerband that it wasn’t worth the effort and cost to put in the complexity of spark advance. Just set the spark timing for the RPM range where the engine is “on pipe” (actually making power) and leave it at that. > [@breakneck](#): > > cost comlexity and weight do not seem to be issues involving a simple weighted advance unit . on a weed eater maybe but on a full size motorcycle i beg to differ . so once again is spark advance needed on a 2 stroke and if not why not all spark ignition engines _can_ benefit from ignition advance. but as has been said, a crankcase-scavenged 2-stroke has a fairly narrow powerband and since it isn’t expected to make its power over a broad RPM range like a 4-stroke, there’s no pressing need for a spark advance curve. but in any case, since two-strokes are _de facto_ banned from on-road vehicles, it doesn’t really matter in the slightest. ---
**Author:** ![Common\_Tater](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/common_tater/32/13846_2.png) [@Common\_Tater](https://boards.straightdope.com/u/Common_Tater)\ **Post date:** [May 19, 2016, 8:10am UTC](https://boards.straightdope.com/t/2-stroke-motorcycle-engines-why-dont-they-use-a-spark-advance-curve-like-4-strokes/666184/22 "2016-05-19T08:10:24Z")
> [@breakneck](#): > > Why do 2 stroke motorcycle engines not have a spark advance like a 4 Stroke? the advance is to compensate for saturation of the coils at higher rpms right? so why is this not needed on a 2 stroke? They do. While not a motorcycle engine, 2-stroke solid state LawnBoy mowers for example, use 5 degrees after top dead center for easier starts, above 800 RPM they switch over to 25 degrees before top dead center. They normally run at 3200 RPM. Not sure how this was accomplished from the illustration, some kind of centrifugal gee-gaws I guess. > **[lball25.pdf](https://lookup3.toro.com/ttcGateway/acrobat/manuals/lball25.pdf)** > > 1113.36 KB Advanced ignition timing actually reduces temperatures significantly, at least in 4 stroke engines. Anyone who has inadvertantly started an engine with severely retarded timing has discovered this, the exhaust manifolds will turn cherry red in an astonishingly short period of time. This is due to the unburned hydrocarbons passing through the combustion chamber. Any engine ideally runs with as much ignition timing as possible at all times under all conditions, short of pre-detonation or engine knock. I had wondered this myself, as a modern automobile can run 50 degrees or more BTDC when lightly loaded cruising down the highway. Maybe since a 2-stroke is half the engine cycles, 25 degrees BTDC for our representative lawn mower is an equivalent figure. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [May 19, 2016, 11:52am UTC](https://boards.straightdope.com/t/2-stroke-motorcycle-engines-why-dont-they-use-a-spark-advance-curve-like-4-strokes/666184/23 "2016-05-19T11:52:30Z")
> [@Common\_Tater](#): > > Advanced ignition timing actually reduces temperatures significantly, at least in 4 stroke engines. Anyone who has inadvertantly started an engine with severely retarded timing has discovered this, the exhaust manifolds will turn cherry red in an astonishingly short period of time. This is due to the unburned hydrocarbons passing through the combustion chamber. Unburned hydrocarbons in the exhaust will cook the catalytic converter (by releasing their energy there), but late timing causes hot exhaust manifolds because not enough energy is being extracted from the combustion products during the expansion stroke; instead of sending energy down the crankshaft as mechanical work, you’re sending it out through the exhaust port as (extra) heat. Same thing happens when an engine runs lean: people say that lean mixtures burn hot, but what’s actually happening is they’re just burning late (due to slow flame speed). > [@Common\_Tater](#): > > I had wondered this myself, as a modern automobile can run 50 degrees or more BTDC when lightly loaded cruising down the highway. Maybe since a 2-stroke is half the engine cycles, 25 degrees BTDC for our representative lawn mower is an equivalent figure. 2-strokes generally aren’t that big (lawn mower will be one cylinder with about 120cc, compared to maybe 600cc per cylinder on a car). The smaller the bore, the less spark advance you’re going to need in order for the flame to make it all the way across in an acceptable amount of time. [Previous page](https://boards.straightdope.com/t/2-stroke-motorcycle-engines-why-dont-they-use-a-spark-advance-curve-like-4-strokes/666184.md?page=1)