Crank timing advice, please? Still trying to wring more power out of a HPI .12R SS
 

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:

http://i.imgur.com/ElrdX2e.png

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.

That's not crank timing. That's sleeve timing. Roleof I think said there's a .1mm on closing crank timing available to grind as a first approach, I would start there.
http://www.rctech.net/forum/onroad-n...t-modding.html

I intend to modify the intake port on the crank, so by the definition in your own thread, it's crank timing. I provided pictures of the transfer and exhaust port timings as well, because the port timings have to all play nice with each other. I have no intention to modify the shapes of the transfer and exhaust ports unless someone says it would be a significantly better idea based on the information I provided.

You need to get things measured up the right way, at least degrees on everything.
Eyeballing things from a picture will not give you any answers that's useful. :)

I was talking about the pics, I could get a visual of durations because I'm accustomed to see them that way and I wouldn't touch sleeve timing, at least for now. Measuring with a rig should be easy for you.

I would make accurate measurements before actually cutting anything. The photos are just to get a rough idea from someone who's modded crankshafts before whether it's even worth putting in the effort to build a degree wheel to use on this engine. I'm happy enough with the performance of my other engines that I would only start modding their crankshafts if I ran out of absolutely anything else to occupy my time, so a degree wheel would almost certainly be used for this engine only. Therefore I'd rather not build one at all if it's unlikely I could actually use it to improve this engine.

For example, if someone were to say "yes, it looks like the intake timing is very conservative and you could keep the intake valve open for an extra 5 degrees and get a lot more top end", then that would make it worth my time to measure things carefully. Whereas if someone were to say "no, this engine is already optimized from the factory, mediocre though it is", then there would be no point in pursuing this project any further.

Then do the same for the crank, take off the carb and take a pic of where the crankpin is at crank closing and revealing the opening. I know most if not all RTR engines are strained in the crank, not so much the sleeve.

The problem with cranktimings with your crank size (12mm?) is that about 0.1mm shaving of the edge of the crankshaft is equal to 1 degree. Ofcourse you can remove something visable but there is a chance it will be too much.
That is why those pictures with positions does not tell a thing.... (athough I suspect the timing is very mild and has some room to enlarge)

The most simple thing to do is use a dremel and a caliper.
Take a caliper and measure the diameter of the crankshaft on the opening of the crankshaft. That is probably arround 9 to 10mm
Use a dremel to touch one edge at the opening so the diameter will be 0.05mm smaller. Do the same on the other edge so the total will be 0.1mm smaller.
In that way you have changed the opening and closing with both 1 degree, it should be noticeable.

Here a pic how to meassure
http://www.euronet.nl/users/tooms/ftp/meten_krukas.JPG

+1 Roelof

You could use something as simple as a CD, I even think there is a picture on the web (Google it) that you just glue on to a CD.
Or go by calculating from piston movement, that's how I usually do it.

30Tooth, do you still have that motor from your link? how did it do, perhaps your thread can be updated with your findings, as I am sure this thread too will be updated after the mods.

Yeah still have the engine, I ground a bit too much on the crank and am afraid to run it, it's the ring close to the outer bearing.

??

That's what I did. The picture labeled "intake port opens" was taken after aligning the crank so the leading edge of the intake port was just barely visible at the base of the carb socket, and the picture labeled "intake port closes" was taken the same way except based on the trailing edge of the intake port instead.

Thanks, I'll take the appropriate measurements. It sounds like the diamond-coated hand files I own will come in handy for this project.

Would I want to change the leading edge of the intake port, though? I'm looking for more top-end, not more bottom-end. I thought top-end was adjusted by changing the trailing edge of the intake port?

Also, how do I determine how much adjustment is actually useful vs. how much will cause damage?

More top end needs power, so the total opening needs to be larger. But with your enginge also the top-end is limited by a low exhaust port, just do not expect wonders with your engine, some small steps are noticeable.

Fair enough, thanks.