Crank timing advice, please? Still trying to wring more power out of a HPI .12R SS
12-13-2016 | 05:14 AM
#16
Tech Master
Joined: Apr 2010
Posts: 1,098
One of my cars runs a HPI .12R SS engine. Using the awesome power of elementary-school math, I calculated based on the top speed, tire size, and gear ratios that the engine is peaking at about 30,000rpm. I did a little grinding and polishing on the sleeve (and the hole in the side of the piston) to improve breathing through the transfer ports, and testing showed the car gained ~1mph, i.e. ~600rpm. Not a huge improvement.
I've been doing more reading and it looks like the next thing to try is modifying the intake port timing on the crankshaft. The general rule seems to be "opening earlier = more low-end power" and "closing later = higher max RPM". I wouldn't complain about more low-end power, but it's really the max RPM I want to raise. This engine just doesn't rev as high as I think it should, and it has the same conrod as HPI's .18 engine from the same time period, so there should be no concern about the conrod breaking from increased stress.
Having said that, I'd like someone with more experience to look at the pictures below and tell me if they think there's actually room to improve the intake port timing. If air had no inertia, then the answer would obviously be yes, because the intake port closes well before before the exhaust port and transfer ports open -- but of course air *does* have inertia, *and* it's elastic too, and I don't have enough experience to account for those effects at 30,000rpm.
Here are the crank positions when the various ports open and close:
What do you think? Is there enough room to have the intake port close later, to get more max RPM out of this engine? Or do the port timings suggest a different approach that would work better? Or am I S.O.L. and I should accept the engine as-is?
Also, I should mention that I recently got a Novarossi .12-.15 pipe tuned for higher max RPMs, but I haven't had a chance to test it compared to the stock HPI pipe, so take that into account if you think it will matter at all.
I've been doing more reading and it looks like the next thing to try is modifying the intake port timing on the crankshaft. The general rule seems to be "opening earlier = more low-end power" and "closing later = higher max RPM". I wouldn't complain about more low-end power, but it's really the max RPM I want to raise. This engine just doesn't rev as high as I think it should, and it has the same conrod as HPI's .18 engine from the same time period, so there should be no concern about the conrod breaking from increased stress.
Having said that, I'd like someone with more experience to look at the pictures below and tell me if they think there's actually room to improve the intake port timing. If air had no inertia, then the answer would obviously be yes, because the intake port closes well before before the exhaust port and transfer ports open -- but of course air *does* have inertia, *and* it's elastic too, and I don't have enough experience to account for those effects at 30,000rpm.
Here are the crank positions when the various ports open and close:
What do you think? Is there enough room to have the intake port close later, to get more max RPM out of this engine? Or do the port timings suggest a different approach that would work better? Or am I S.O.L. and I should accept the engine as-is?
Also, I should mention that I recently got a Novarossi .12-.15 pipe tuned for higher max RPMs, but I haven't had a chance to test it compared to the stock HPI pipe, so take that into account if you think it will matter at all.
12-14-2016 | 02:56 AM
#18
Tech Lord
Joined: Aug 2007
Posts: 14,044
From: Holland
Raise the ports and so the port timing.
Raising the exhaust will give more RPM and raising the intake will give more power but creates also also more consumption.
Raising the sleeve by adding shims is an easy trick which can also be undone. But it is depending the amount of headshims you have, if currently there is only one 0.1mm headshim there is no room to add shims under the sleeve because what you add under the sleeve you must take away from above the sleeve to keep the combustion chamber on the same level. It is wise to buy a shim set because you have to modify one to fit it arround the sleeve.
Raising the exhaust will give more RPM and raising the intake will give more power but creates also also more consumption.
Raising the sleeve by adding shims is an easy trick which can also be undone. But it is depending the amount of headshims you have, if currently there is only one 0.1mm headshim there is no room to add shims under the sleeve because what you add under the sleeve you must take away from above the sleeve to keep the combustion chamber on the same level. It is wise to buy a shim set because you have to modify one to fit it arround the sleeve.
12-14-2016 | 10:55 PM
#19
Thread Starter
Joined: Mar 2014
Posts: 8,054
From: Virginia, Near DC, USA
Ah, I see, raising the sleeve effectively increases the duration that the transfer ports and exhaust port are open, all at once, without having to cut anything. That's a very neat trick, I'll definitely have to try that. Is it okay to raise the sleeve more than 0.1mm, if the shim stack allows for it? Will it reduce the tightness of the seal between the piston and sleeve, since the piston won't travel as far up into the "pinch zone"?
I would have to enlarge the inner diameter of a head shim in order to fit it under the lip of the sleeve. Or maybe I can find some Kyosho ball-bearing shims with a suitable diameter.
I would have to enlarge the inner diameter of a head shim in order to fit it under the lip of the sleeve. Or maybe I can find some Kyosho ball-bearing shims with a suitable diameter.
12-15-2016 | 01:58 AM
#20
Thread Starter
Joined: Mar 2014
Posts: 8,054
From: Virginia, Near DC, USA
Well, I decided to go for it:
I reduced the thickness of the crankshaft at the intake port from 7.69mm to 7.44mm, meaning the intake port will be open ~ 2.9° longer. (the crankshaft diameter is 9.98mm.) Most of the grinding was done on the trailing edge, but per Roelof's recommendation some of it was done on the leading edge as well. I started the job with a Dremel diamond cutting wheel and finished the job with a diamond-coated jeweler's file. I also did a small amount of polishing inside the crankshaft, because I had to go in there anyway to make sure the edges I rounded-off didn't have any burrs still attached.
I haven't found any shims I can use to raise the sleeve, but there is room, so that's something I can try if I don't see any noticeable improvement from the crankshaft porting job.
As I was putting the engine back together, I suddenly remembered there was something I wanted to do the next time I had the engine apart -- open the lowermost area of the swirl-port channels. They were almost completely blocked off at the bottom originally:
The channels are nice and open now. (yes, I made sure to cover the bearings as best I could and then flush them thoroughly afterwards.) When I shine a light down through the sleeve, you can see plenty of light making its way down through the channels, and in through the side of the piston. There shouldn't be any intake-related breathing difficulties with this engine anymore.
I'll try to run it briefly tomorrow if the weather isn't too terrible, just to make sure it still works, but it's way too cold to run it hard so a speed test will have to wait until spring, or at least an unusually warm day this winter.
I reduced the thickness of the crankshaft at the intake port from 7.69mm to 7.44mm, meaning the intake port will be open ~ 2.9° longer. (the crankshaft diameter is 9.98mm.) Most of the grinding was done on the trailing edge, but per Roelof's recommendation some of it was done on the leading edge as well. I started the job with a Dremel diamond cutting wheel and finished the job with a diamond-coated jeweler's file. I also did a small amount of polishing inside the crankshaft, because I had to go in there anyway to make sure the edges I rounded-off didn't have any burrs still attached.
I haven't found any shims I can use to raise the sleeve, but there is room, so that's something I can try if I don't see any noticeable improvement from the crankshaft porting job.
As I was putting the engine back together, I suddenly remembered there was something I wanted to do the next time I had the engine apart -- open the lowermost area of the swirl-port channels. They were almost completely blocked off at the bottom originally:
The channels are nice and open now. (yes, I made sure to cover the bearings as best I could and then flush them thoroughly afterwards.) When I shine a light down through the sleeve, you can see plenty of light making its way down through the channels, and in through the side of the piston. There shouldn't be any intake-related breathing difficulties with this engine anymore.
I'll try to run it briefly tomorrow if the weather isn't too terrible, just to make sure it still works, but it's way too cold to run it hard so a speed test will have to wait until spring, or at least an unusually warm day this winter.
Last edited by fyrstormer; 03-29-2018 at 04:26 PM.
12-16-2016 | 03:00 PM
#21
Thread Starter
Joined: Mar 2014
Posts: 8,054
From: Virginia, Near DC, USA
I HATE TRYING TO START A FRESHLY-REBUILT ENGINE. That being said, it did eventually start, and I got it to hold an idle with the glowplug heater removed after about 30 seconds of fiddling with the carb. It's nighttime now, so I can't do a speed run, and the weather is supposed to be sloppy through the weekend, so I won't be able to do a speed test for quite a while, but at least the damn thing still runs.
Also, I need to recharge the receiver pack. It's so low the servos won't even go into failsafe positions.
Also, I need to recharge the receiver pack. It's so low the servos won't even go into failsafe positions.
12-17-2016 | 11:27 AM
#22
Thread Starter
Joined: Mar 2014
Posts: 8,054
From: Virginia, Near DC, USA
Raise the ports and so the port timing.
Raising the exhaust will give more RPM and raising the intake will give more power but creates also also more consumption.
Raising the sleeve by adding shims is an easy trick which can also be undone. But it is depending the amount of headshims you have, if currently there is only one 0.1mm headshim there is no room to add shims under the sleeve because what you add under the sleeve you must take away from above the sleeve to keep the combustion chamber on the same level. It is wise to buy a shim set because you have to modify one to fit it arround the sleeve.
Raising the exhaust will give more RPM and raising the intake will give more power but creates also also more consumption.
Raising the sleeve by adding shims is an easy trick which can also be undone. But it is depending the amount of headshims you have, if currently there is only one 0.1mm headshim there is no room to add shims under the sleeve because what you add under the sleeve you must take away from above the sleeve to keep the combustion chamber on the same level. It is wise to buy a shim set because you have to modify one to fit it arround the sleeve.
12-22-2016 | 01:37 PM
#23
Thread Starter
Joined: Mar 2014
Posts: 8,054
From: Virginia, Near DC, USA
I bought an extra shim and trimmed the inner diameter so I could fit it under the lip of the cylinder sleeve, and I installed it and removed a shim from on top of the sleeve to compensate, and I ran the engine today because the weather was relatively nice.
The top speed is exactly the same as it was originally. The engine is louder, but still not powerful enough to move the car faster. Considering that all these modifications accomplished nothing measurable, and considering that the engine revs much higher when the wheels are in the air, I suspect the engine just doesn't have enough power to push the car any faster, and changing the gearing would result in the car reaching the same top speed and taking longer to get there. The only other things I can think of at this point are to try increasing the compression and/or using a hotter plug to advance the ignition timing. I suppose trying a hotter plug would make sense, since it was about 50F outside today, whereas I initially tuned the engine in ~80F ambient temperatures. Or I could just wait until the weather gets hot again, and the ignition timing will advance on its own. I suppose it's noteworthy that the car achieved the same top speed in 30-degree cooler weather with a used glowplug and no significant re-tuning.
In my mind I know the engine must be better for all the work I've done, but maybe the car is just too heavy or un-aerodynamic for it.
The top speed is exactly the same as it was originally. The engine is louder, but still not powerful enough to move the car faster. Considering that all these modifications accomplished nothing measurable, and considering that the engine revs much higher when the wheels are in the air, I suspect the engine just doesn't have enough power to push the car any faster, and changing the gearing would result in the car reaching the same top speed and taking longer to get there. The only other things I can think of at this point are to try increasing the compression and/or using a hotter plug to advance the ignition timing. I suppose trying a hotter plug would make sense, since it was about 50F outside today, whereas I initially tuned the engine in ~80F ambient temperatures. Or I could just wait until the weather gets hot again, and the ignition timing will advance on its own. I suppose it's noteworthy that the car achieved the same top speed in 30-degree cooler weather with a used glowplug and no significant re-tuning.
In my mind I know the engine must be better for all the work I've done, but maybe the car is just too heavy or un-aerodynamic for it.
Last edited by fyrstormer; 12-22-2016 at 01:52 PM.
12-22-2016 | 02:16 PM
#24
I bought an extra shim and trimmed the inner diameter so I could fit it under the lip of the cylinder sleeve, and I installed it and removed a shim from on top of the sleeve to compensate, and I ran the engine today because the weather was relatively nice.
The top speed is exactly the same as it was originally. The engine is louder, but still not powerful enough to move the car faster. Considering that all these modifications accomplished nothing measurable, and considering that the engine revs much higher when the wheels are in the air, I suspect the engine just doesn't have enough power to push the car any faster, and changing the gearing would result in the car reaching the same top speed and taking longer to get there. The only other things I can think of at this point are to try increasing the compression and/or using a hotter plug to advance the ignition timing. I suppose trying a hotter plug would make sense, since it was about 50F outside today, whereas I initially tuned the engine in ~80F ambient temperatures. Or I could just wait until the weather gets hot again, and the ignition timing will advance on its own. I suppose it's noteworthy that the car achieved the same top speed in 30-degree cooler weather with a used glowplug and no significant re-tuning.
In my mind I know the engine must be better for all the work I've done, but maybe the car is just too heavy or un-aerodynamic for it.
The top speed is exactly the same as it was originally. The engine is louder, but still not powerful enough to move the car faster. Considering that all these modifications accomplished nothing measurable, and considering that the engine revs much higher when the wheels are in the air, I suspect the engine just doesn't have enough power to push the car any faster, and changing the gearing would result in the car reaching the same top speed and taking longer to get there. The only other things I can think of at this point are to try increasing the compression and/or using a hotter plug to advance the ignition timing. I suppose trying a hotter plug would make sense, since it was about 50F outside today, whereas I initially tuned the engine in ~80F ambient temperatures. Or I could just wait until the weather gets hot again, and the ignition timing will advance on its own. I suppose it's noteworthy that the car achieved the same top speed in 30-degree cooler weather with a used glowplug and no significant re-tuning.
In my mind I know the engine must be better for all the work I've done, but maybe the car is just too heavy or un-aerodynamic for it.
12-22-2016 | 04:14 PM
#25
Tech Apprentice
Joined: May 2010
Posts: 84
I bought an extra shim and trimmed the inner diameter so I could fit it under the lip of the cylinder sleeve, and I installed it and removed a shim from on top of the sleeve to compensate, and I ran the engine today because the weather was relatively nice.
The top speed is exactly the same as it was originally. The engine is louder, but still not powerful enough to move the car faster. Considering that all these modifications accomplished nothing measurable, and considering that the engine revs much higher when the wheels are in the air, I suspect the engine just doesn't have enough power to push the car any faster, and changing the gearing would result in the car reaching the same top speed and taking longer to get there. The only other things I can think of at this point are to try increasing the compression and/or using a hotter plug to advance the ignition timing. I suppose trying a hotter plug would make sense, since it was about 50F outside today, whereas I initially tuned the engine in ~80F ambient temperatures. Or I could just wait until the weather gets hot again, and the ignition timing will advance on its own. I suppose it's noteworthy that the car achieved the same top speed in 30-degree cooler weather with a used glowplug and no significant re-tuning.
In my mind I know the engine must be better for all the work I've done, but maybe the car is just too heavy or un-aerodynamic for it.
The top speed is exactly the same as it was originally. The engine is louder, but still not powerful enough to move the car faster. Considering that all these modifications accomplished nothing measurable, and considering that the engine revs much higher when the wheels are in the air, I suspect the engine just doesn't have enough power to push the car any faster, and changing the gearing would result in the car reaching the same top speed and taking longer to get there. The only other things I can think of at this point are to try increasing the compression and/or using a hotter plug to advance the ignition timing. I suppose trying a hotter plug would make sense, since it was about 50F outside today, whereas I initially tuned the engine in ~80F ambient temperatures. Or I could just wait until the weather gets hot again, and the ignition timing will advance on its own. I suppose it's noteworthy that the car achieved the same top speed in 30-degree cooler weather with a used glowplug and no significant re-tuning.
In my mind I know the engine must be better for all the work I've done, but maybe the car is just too heavy or un-aerodynamic for it.
12-23-2016 | 01:56 AM
#26
Thread Starter
Joined: Mar 2014
Posts: 8,054
From: Virginia, Near DC, USA
Link to this "rexon" engine you're talking about? I can't seem to find it.
Yeah, increasing compressing is the next thing I'm going to try. I tend to err on the side of not enough compression, because it reduces the risk of rapid overheating if I forget to check the engine temperature every single minute during initial tuning. When I first started with nitro I started with off-road trucks, which put much more continuous load on their engines as they drag themselves through the grass, and I still haven't successfully adjusted my thinking for on-road engines to account for the lower continuous load and much higher airflow. I already increased the compression of this engine once, but I have a little more room to increase it some more; it seems to be running unusually cool right now, and takes several full-throttle passes in a row before it gets up to what I would consider a high temperature.
When the weather warms up again and the engine starts running hotter because of it, will installing a colder plug be an acceptable adjustment, or will I have to reduce the compression and/or tune it richer again, thus giving up whatever power increase I might get from the increased compression?
Yeah, increasing compressing is the next thing I'm going to try. I tend to err on the side of not enough compression, because it reduces the risk of rapid overheating if I forget to check the engine temperature every single minute during initial tuning. When I first started with nitro I started with off-road trucks, which put much more continuous load on their engines as they drag themselves through the grass, and I still haven't successfully adjusted my thinking for on-road engines to account for the lower continuous load and much higher airflow. I already increased the compression of this engine once, but I have a little more room to increase it some more; it seems to be running unusually cool right now, and takes several full-throttle passes in a row before it gets up to what I would consider a high temperature.
When the weather warms up again and the engine starts running hotter because of it, will installing a colder plug be an acceptable adjustment, or will I have to reduce the compression and/or tune it richer again, thus giving up whatever power increase I might get from the increased compression?
12-23-2016 | 11:20 AM
#28
Tech Apprentice
Joined: May 2010
Posts: 84
Link to this "rexon" engine you're talking about? I can't seem to find it.
Yeah, increasing compressing is the next thing I'm going to try. I tend to err on the side of not enough compression, because it reduces the risk of rapid overheating if I forget to check the engine temperature every single minute during initial tuning. When I first started with nitro I started with off-road trucks, which put much more continuous load on their engines as they drag themselves through the grass, and I still haven't successfully adjusted my thinking for on-road engines to account for the lower continuous load and much higher airflow. I already increased the compression of this engine once, but I have a little more room to increase it some more; it seems to be running unusually cool right now, and takes several full-throttle passes in a row before it gets up to what I would consider a high temperature.
When the weather warms up again and the engine starts running hotter because of it, will installing a colder plug be an acceptable adjustment, or will I have to reduce the compression and/or tune it richer again, thus giving up whatever power increase I might get from the increased compression?
Yeah, increasing compressing is the next thing I'm going to try. I tend to err on the side of not enough compression, because it reduces the risk of rapid overheating if I forget to check the engine temperature every single minute during initial tuning. When I first started with nitro I started with off-road trucks, which put much more continuous load on their engines as they drag themselves through the grass, and I still haven't successfully adjusted my thinking for on-road engines to account for the lower continuous load and much higher airflow. I already increased the compression of this engine once, but I have a little more room to increase it some more; it seems to be running unusually cool right now, and takes several full-throttle passes in a row before it gets up to what I would consider a high temperature.
When the weather warms up again and the engine starts running hotter because of it, will installing a colder plug be an acceptable adjustment, or will I have to reduce the compression and/or tune it richer again, thus giving up whatever power increase I might get from the increased compression?
and fast for sure. This would be much faster and would respond to mods well.
12-23-2016 | 12:41 PM
#29
Thread Starter
Joined: Mar 2014
Posts: 8,054
From: Virginia, Near DC, USA
Increasing the compression gained 1mph. 
I know the engine can rev much higher than this, because it revs much higher when the car is sitting on a work stand. Of course there's almost no load on the engine then. At this point I'll have to wait for warm weather and an opportunity to drive it for an extended period of time, so I can really tune it properly. I leaned-out the HSN by almost a quarter of a turn today and it was still blowing smoke, so it's nowhere near lean. Maybe all the extra power is waiting for a proper tune before it will show itself. I *have* made a bunch of changes that ought to require re-tuning, after all.
I know the engine can rev much higher than this, because it revs much higher when the car is sitting on a work stand. Of course there's almost no load on the engine then. At this point I'll have to wait for warm weather and an opportunity to drive it for an extended period of time, so I can really tune it properly. I leaned-out the HSN by almost a quarter of a turn today and it was still blowing smoke, so it's nowhere near lean. Maybe all the extra power is waiting for a proper tune before it will show itself. I *have* made a bunch of changes that ought to require re-tuning, after all.
Last edited by fyrstormer; 12-23-2016 at 12:55 PM.
12-28-2016 | 09:27 PM
#30
Thread Starter
Joined: Mar 2014
Posts: 8,054
From: Virginia, Near DC, USA
I ran the car with the body off today to see how much speed was being sacrificed to generate downforce. The car went 2mph faster, and was significantly harder to control. Obviously that energy is being put to good use keeping the tires stuck to the ground instead of making the car go faster, but damn I wish I knew where the extra power is hiding. I leaned-out the fuel mixture a bit and it *still* goes 45mph with the body on. It just seems implausible that all these changes I'm making could have no measurable effect whatsoever. Even the exhaust note is the same pitch at top speed, so the GPS speed readings are definitely accurate.
The only things I haven't tried at this point are:
- Hotter/colder glowplug (maybe advancing the ignition would help? I dunno, but I don't want to waste a new glowplug to test it; maybe I can get away with borrowing a hot plug from a different engine)
- Higher gearing (unlikely to do anything at all unless the engine is operating significantly above its power peak at its current top speed)
- Degrease all bearings and re-lubricate with oil (impractical for a basher vehicle)
- Replace the T3.0 carburetor with the stock carburetor (this would decrease the venturi size, which I'm pretty sure would *reduce* its top speed, unless there's something I'm misunderstanding)
The only things I haven't tried at this point are:
- Hotter/colder glowplug (maybe advancing the ignition would help? I dunno, but I don't want to waste a new glowplug to test it; maybe I can get away with borrowing a hot plug from a different engine)
- Higher gearing (unlikely to do anything at all unless the engine is operating significantly above its power peak at its current top speed)
- Degrease all bearings and re-lubricate with oil (impractical for a basher vehicle)
- Replace the T3.0 carburetor with the stock carburetor (this would decrease the venturi size, which I'm pretty sure would *reduce* its top speed, unless there's something I'm misunderstanding)