Redline and overheating (CG/CD/HT engine)

Kehkou

Active Member
Local time
6:32 AM
Joined
Jul 21, 2022
Messages
134
Location
The Duke City
Based on experience, what would you say is the max safe RPM and the max safe temperature for your typical Yuyuan Garden-variety China Girl Doll Happy Time engine? I know this is extremely subjective; the many posts about it here or on that...other site...seem to just wander off-topic and never produce a definitive or even approximate answer. My hope is that this thread will at least provide people with a numerical starting point for entering Multifunction Meter (tach+temp) redline values for our bikes. I'll get things started:

The fastest I have gotten my engine is around 7800 RPM. The stock CDI obviously limits the max RPM. I am also up at 5060' HAMSL with the stock 70 mm jet, so, very rich. I must have an unusually well-balanced crank, because if I modulate the throttle just right to eliminate four-stroking at these speeds, the undue vibrations vanish, and the engine runs quite smoothly. I can probably push 8000 RPM down a steep hill with my unmodded HT but, heh, I ain't about to do that.

The hottest I usually run is between 250°F and 370°F. I was told by another member here that this isn't close to the "danger zone", but never clarified what exactly that "danger zone" is (EDIT: His answer is below under "ANSWERS", and in the first reply):
350 is hot but it's not quite danger zone in terms of cylinder/head temps. My Phantom can get that hot pushing it on level ground lol.

ANSWERS:
The absolute hottest I ran (recorded) was 399°F, but that was during a runaway condition. I do not exceed 7800 RPM.
As a general rule with an aluminum air cooled 2 stroke engine (in continuous operation) you want to avoid exceeding 450 degrees Fahrenheit at the head. I try to keep things at 400 and below personally. The "Danger Zone" - aka beyond 400-450 degrees - is where the aluminum starts to lose strength and enters into a state where it can fail.
I believe I have 17-18 hours on it, and they have all been hard hours. A lot of wide open at 8000 rpm, and recently 9000. I don't recommend doing that of course, and I'm sure it has accelerated the wear on all the components. I'm not afraid to push it though, seeing how it's held up.
...which sounds a whole lot like redline of other non-interference engines; no valves to smash up, just the piston etching its name into the bore.
 

ImpulseRocket89

Well-Known Member
Local time
7:32 AM
Joined
Apr 16, 2014
Messages
840
Location
Omaha, Nebraska
As a general rule with an aluminum air cooled 2 stroke engine (in continuous operation) you want to avoid exceeding 450 degrees Fahrenheit at the head. I try to keep things at 400 and below personally. The "Danger Zone" - aka beyond 400-450 degrees - is where the aluminum starts to lose strength and enters into a state where it can fail.

There are of course other aspects to running temps as well. Unfortunately a cylinder head temp reading doesn't give the best clue as to the piston temps during operation which is affected by fueling, sqush gap, and ignition timing in relation to the operating temp of the cylinder and head itself. This means that you can have an engine running at say 430 degree head temps that is in no danger of soft seizing a piston while another engine could be running just 390 at the head and the piston is getting hot enough to cause said soft seizure.

The world of engine tuning dynamics can be an interesting but confusing world if you don't understand the relationship of everything involved. It can also be a difficult thing to really explain as well. Just as an example of this you can get lower operating piston temperatures with a functional squish band in the head and proper squish gap even though this generally means the engine itself has higher compression which generally means more heat. That heat is often found in the form of higher head temps but the piston itself is cooler than an engine that lacks that squish band despite it.
 

Sidewinder Jerry

Well-Known Member
Local time
8:32 AM
Joined
Mar 14, 2011
Messages
3,921
Location
Rockwood, TN
As a general rule with an aluminum air cooled 2 stroke engine (in continuous operation) you want to avoid exceeding 450 degrees Fahrenheit at the head. I try to keep things at 400 and below personally. The "Danger Zone" - aka beyond 400-450 degrees - is where the aluminum starts to lose strength and enters into a state where it can fail.

There are of course other aspects to running temps as well. Unfortunately a cylinder head temp reading doesn't give the best clue as to the piston temps during operation which is affected by fueling, sqush gap, and ignition timing in relation to the operating temp of the cylinder and head itself. This means that you can have an engine running at say 430 degree head temps that is in no danger of soft seizing a piston while another engine could be running just 390 at the head and the piston is getting hot enough to cause said soft seizure.

The world of engine tuning dynamics can be an interesting but confusing world if you don't understand the relationship of everything involved. It can also be a difficult thing to really explain as well. Just as an example of this you can get lower operating piston temperatures with a functional squish band in the head and proper squish gap even though this generally means the engine itself has higher compression which generally means more heat. That heat is often found in the form of higher head temps but the piston itself is cooler than an engine that lacks that squish band despite it.
What would be the recommended max rpm for both the standard engine and for performance mods?
 

DAMIEN1307

Moderator
Staff member
Local time
6:32 AM
Joined
Jul 23, 2020
Messages
6,371
Location
Alamogordo, NM, USA
what would you say is the max safe RPM and the max safe temperature for your typical Yuyuan Garden-variety China Girl Doll Happy Time engine?
What would be the recommended max rpm for the standard engine and for performance mods?
I would say to answer this question for Kehkou just the way he asked..."for your typical Yuyuan Garden-variety China Girl Doll Happy Time engine"...Not for anything else, not for any performance modifications etc, just straight out of the box, standard motors for MBs...It would be nice to have something of a base line to work with in the first place.
 

Kehkou

Active Member
Local time
6:32 AM
Joined
Jul 21, 2022
Messages
134
Location
The Duke City
As a general rule with an aluminum air cooled 2 stroke engine (in continuous operation) you want to avoid exceeding 450 degrees Fahrenheit at the head. I try to keep things at 400 and below personally. The "Danger Zone" - aka beyond 400-450 degrees - is where the aluminum starts to lose strength and enters into a state where it can fail.
Okay, I do try to keep under 400 also; it's not too hard even in the New Mexico summer. I have my alert set at 399.
What would be the recommended max rpm for both the standard engine and for performance mods?
I would say to answer this question for Kehkou just the way he asked..."for your typical Yuyuan Garden-variety China Girl Doll Happy Time engine"...Not for anything else, not for any performance modifications etc, just straight out of the box, standard motors for MBs...It would be nice to have something of a base line to work with in the first place.
For mine I'd say 7800 RPM stock, less so if you have bad primary balance (vibration), but my experience is...limited (just finished breaking in my first HT). It seems counterintuitive, but at high RPM range (and my richness), letting off the throttle a little when it starts to four-stroke at 'top-speed' actually gets me a boost of acceleration up to said RPM, especially if I then very slowly retwist the throttle. It's a balance of throttle, ear, and feel. I'd keep anything above that to very short spurts (or "oopses"). If you can balance the crank better and use a high-quality CDI, there's no physical limitation to mod up to, say, 12000 RPM, other than the limits of heat transfer and secondary and tertiary balance (it is still a single-cylinder, after all).
 

ImpulseRocket89

Well-Known Member
Local time
7:32 AM
Joined
Apr 16, 2014
Messages
840
Location
Omaha, Nebraska
I cannot answer the maximum RPM for these engines as I have no experience with pushing the envelope. I have read several posts from other builders who race these engines and many of them tend to give the same figures in regards to reliability vs. performance - 8000rpm or less for continuous running. For most box standatd CG engines they state keeping things below 7000 for long distance cruising rpms.

Beyond that there are a number of factors that will determine the type of RPM an engine can turn reliably.
- Crank balance factor and trueness. - less vibrations and runout means the bearings are under less strain.
- Crank bearings. - Proper tolerance spec for operating temps and rpm rating (My C3 spec 14k rpm rated bearings I used as an example)
- Rod and wrist pin bearings. - self explanatory. Your typical CG engine standard bearings are probably not the most high quality units
- Rod piston and wrist pin quality. - Casting, machining, and material quality being able to withstand the G forces before failure
- Piston acceleration. - See above quality of parts and other factors that this plays a big part in
- Ring quality and material. - Ring material and thickness at work. At a certain point ring flutter starts to happen and thicker iron rings are the most prone to failure at high RPM.
- Port timing, size, and shape. - all of the potential issues associated with ports can play a role
- Thermal capacity of the engine. - The more RPM an engine spins the more heat it is going to make. A given design can only handle so much heat

Part of the reason I put the parts and work I did into my Phantom 85 was to improve many of these factors. From my C3 spec 14k RPM rated FAG bearings and Stihl factory wrist pin bearing to the Meteor piston with the steel caber rings. I balanced my crank and trued it within 1 thousandths of total runout and took the time to clean up and chamfer my port edges. I wanted to build the engine so that it could spin higher RPM with greater levels of reliability and I do believe I could cruise along all day at 8000-9000rpm without too much issue. My only real limiting factor at this point is the stock rod bearing and the engines ability to shed heat. I setup my bike so that it achieves the top speeds I want to operate at around 6000 - 7000rpm and it runs along under load at those RPMs without blinking an eye.
 

Sidewinder Jerry

Well-Known Member
Local time
8:32 AM
Joined
Mar 14, 2011
Messages
3,921
Location
Rockwood, TN
Okay, I do try to keep under 400 also; it's not too hard even in the New Mexico summer. I have my alert set at 399.


For mine I'd say 7800 RPM stock, less so if you have bad primary balance (vibration), but my experience is...limited (just finished breaking in my first HT). It seems counterintuitive, but at high RPM range (and my richness), letting off the throttle a little when it starts to four-stroke at 'top-speed' actually gets me a boost of acceleration up to said RPM, especially if I then very slowly retwist the throttle. It's a balance of throttle, ear, and feel. I'd keep anything above that to very short spurts (or "oopses"). If you can balance the crank better and use a high-quality CDI, there's no physical limitation to mod up to, say, 12000 RPM, other than the limits of heat transfer and secondary and tertiary balance (it is still a single-cylinder, after all).
Ears and feel can deceive you, a tachometer and temp gauge will let you know exactly what your engine is doing.
 

Sidewinder Jerry

Well-Known Member
Local time
8:32 AM
Joined
Mar 14, 2011
Messages
3,921
Location
Rockwood, TN
Top