Recipe for rebuild for more mid range

[RDDave]

I have a couple of spare engines and would like to build or have one built for street duty with more mid range. Meaning keeping the top power range near the stock range. Of course more up there there would be perfectly acceptable, too.

Is there a recommended recipe for getting that? Like who to do the porting if it calls for that? Who to do the head mods if it comes to that? Recommended size carbs if bigger than stock would be able to help the mid range.

Again, this would be a street bike not a racer. It would just be nice to have even more mid range power and torque, if possible and practical. I already have a set of Bassani chambers to go with the combo.

I appreciate the consideration and information on this.  Thank you!

[racerclam]

Give me a call 702-858-9177 Check out my web site richstaylordporting.com  WE can have a counselng session

Rich

[RDDave]

I'll do just that. Is a particular time or day better?

[racerclam]

Prety much available most any time. Im in Utah MT time


Rich

[Jung]

RD350 engines? There's loads of power you can squeeze out without raising the RPM ceiling - that's what I wanted for mine. Porting, heads and custom pipes can do low / mid 50s HP, with power from 5.5 through to 8.5k. I think just the right pipes alone would be a big boost.

You could do a pipe to bring things even lower than that but there's already quite a lot of exhaust timing / blowdown on a stock 350 so that would limit things a bit.

[RDDave]

COOL! That's just what I am looking for.

Any tips you could pass along would be much appreciated.

[Brad-Man]

Don't forget that a programmable ignition can add to the low end AND the high end.

[Jung]

I'm just a hobbiest, but this is what I did, I'm sure others might do it similarly - you could probably go 1mm higher and not as wide on the exhaust and get similar and perhaps better ring life. I've not tried back to back.

I raised the exh port to 28mm and widened it to 43.5/44. Raised the transfers to a 42mm and milled out the roof of the little one as much as I dared to open it up and match the roof angle of the A transfer better. I also redirected them as best as I could to get the streams closer to Jan Thiels model. Cut back the base of the main transfer by about 8mm, and also got rid of the hump in the tunnel. Raised and opened up the boost port to 18mm and pointed it just in front of the spark plug. I skimmed the barrels for 400 gaskets and pinned the heads on centre. I recut 250 heads to give 45% squish by area and 0.85mm clearance - total head volume is about 24cc.

Running aftermarket chinese cylinders, prox banshee pistons, standard reed blocks with YZ85 reeds, banshee inlet rubbers with link pipe and stock 28mm carbs with the Dale F mod. Single large foam filter on the standard intake rubber. HPI ignition.

Made pipes to a design by Mick Abbey in the UK - It was quite close to what I came up with on the 2t exhaust calc sim, but with a few notable differences. The overall length was almost identical. You want pipes for an 8.5k peak or thereabouts. Speak to Mick or someone local here in the US and tell them what you want the pipes to do.

I've not dynoed mine but it's smooth all the way through to 9k and will pick up the front, smartly, in second from about 6k. It's a really nice engine.

[RDDave]

Is there a thread you can reference on the ignition and the curve that you recommend?

I am taking notes on the porting and head work. You are far more skilled than I in those areas, but they give me information.

Thank you.

[m in sc]

broad squish is a good practice for low to mid range, ive seen a few triple guys do it. most pipes quit making power at 9k anyway, unless oyu get real juicy wit them. with bassanis, you'll be fine. as far as a midrange ign curve, you can basically go 17 degree at idle, then right to 22 degrees at 2k, 24 at 5k, then 19 at 6.5k, 17 at 8k. 15 at 10k, and then 7 at 10.5k. (if you go programmable) this will sign if off nicely early. thats a rough idea, but you get the point. 

[Jung]

I got the bog stock HPI system which only has one curve - you can check it out here:

https://www.hpi.be/item.php?item=210K176

Seems to work alright but people do make decent gains dialing in a custom curve. I love the little flywheel.

I want to try making a 4mm stroker next time I build one. You pick up a load of time area from the longer stroke, which is tough to do with the limitations of the castings at 54mm. On a 65mm Bore, you'd have an RD385 and probably a genuine 60+hp somewhere between 8-9k, without crazy porting. Pretty sure it's possible with some fiddling around....

[RDDave]

What programmable ignition are you referring to?

I am very familiar with quench if that is the same as what you call squish. I have used it for many years on my big block Mopars.

Please keep the info coming, I am learning a lot.

Thank you.

[m in sc]

zeeltronic with a vape cdi is what i run. there are others but this is my preference.

its sort of related to quench, some people interchange the terms but the effects in a 2 stroke can be VERY different than in a 4 stoke.  (Particularly in an RB engine.)

squish will direct the mixture to the center and keep it cooler at the piston crown edges. the wider it is, typically the more midrange you can get. I know a guy with an H2 thar builds power, and a lot of it,  off idle to like 7k, but hes at like 50-60? percent squish area (might be more), its ridiculous. however, his head volume was almost stock after recutting. so, squish doesn't nec affect compression as long as the volume is addressed. but it can direct the mixture where you want it to land in the chamber, which can affect (greatly) how much gets out of the exhaust port on compression stroke, for one thing, and how the transfers can be shaped.   

[RDDave]

I get what you are saying. It would seem that it would take a good bit of trial and error to nail that down.

I also see 250 heads mentioned often. Is that only due to the smaller combustion chamber or is there more to it than that? And are they better to start with to modify or are the 350 heads about as good?

[Jung]

There's been a lot of research done on squish velocity, apparently the numbers to shoot for are something like 20-29m/s at peak power RPM. If peak power RPM is lower, you need a greater squish area to achieve it (squish angle also is a factor.)
Fortunately for us plebs, clever people have been experimenting with RD350s for decades, so anything 45-50% is going to work well. I'm pretty sure I set my compound angle to 14 degrees (will check notes later), which should be fine for RD/YPVS pistons.

250 heads are good because they're way cheaper and more plentiful than 350 heads and you have a little more metal to play with. The only prob with all US 250 and 350 heads is that they're all for the short plugs, so to get the sort of chamber volume you need for pump gas, you might have to put spacers under the plugs to pull them up a tiny bit once you're done.

This is how I do them. Pick out a pair of vaguely matching heads and lap the faces on a surface plate, just to clean them up. Then measure from the top of the plug. Put them in the mill and clean up the top of the plughole and head bolts until they are equal.

Make an arbor to hold the heads from the plug hole, which is just a piece of mild steel, machined down to 1" diameter with a 14 x 1.25mm thread on the end and a centre reference. You can use 5c collets on my lathe, which hold everything  accurately and are handy for this.

Once it is all mounted up, spin them up and clean up the gasket land, about 15 thou on each. After that, you can measure the height from the centre reference and work out the hyp, to scribe an accurate OD on the head for where to start cutting the squish.

After that, set the squish angle on the compound and cut until you hit the OD. It doesn't matter how wide it is at this point as you need to recut the bowl anyway.

For the next bit a radius turning tool is helpful. I worked out that a 28mm radius would be a good starting point for a bowl that would intersect the squish band about the right point (you can mark up the id of the squish on the part with a pen, same as before. This gives a single radius bowl of a little over 12cc total vol, which added to the vol under squish (about 5.5cc) and the tiny bit in the plug recess makes for a hair under 18cc. This might might work on 100LL, which could be fruit for a future experiment as it's a doddle to cut.

For pump gas you need more vol (how much exactly is a bit of an open question, depending on your gas / altitude / tuning etc) but I'd say min 22cc, which is about what the stock 350 heads have. I cut this a bit at a time with a large radius tool bit, using dyechem to mark up the bowl, a bit like wood turning. You need to stop and measure with a burette regularly, but you get a feel for the cuts and the vol and it doesn't take long.

Here's a couple of pics of the tools / process.