So, I've required this bike that I'm told needs 110 octane fuel and I'd like to establish whether it actually does or not. If I were to measure cylinder pressure with a compression tester would that be enough information for an educated guess?
Reading about the subject, pressure is not the only factor in octane requirement and I imagine experience and careful jetting are key?
I haven't even found a table that shows compression versus octane.
Would a compression test be done with a cold engine or a warm engine?
Should a compression test be done before a rebuild, would assembly lube affect the compression test, I imagine it would.
Thanks
Compression is an indicator of potential octane required, that's it.
How well that octane combusts is they key.
Squish/ign timing/pipes/transfers all play a big or moderate role adding up to more that just a compression test.
You can get a basic idea of your compression ratio, if you have compression psi numbers, with this calculator -
https://best-calculators.com/everyday-life/compression-ratio-to-psi-calculator/
squish needs to be measured as does head volume. as stated above, its an indicator. imho, anything over 135-140 psi on an aircooled rd350 is pretty high and would def need different considerations. I have seen where porting was aggressive and cold cranking showed 110-120 but as the rpm rose the cylinder pressure got out of control. so to automans point, compression is just an part of it.
Thanks for the replies. At some point in the near future I'm going to disassemble the engine to change the main seals, have a look at the clutch etc. I'll measure the height and width of the ports, measure the amount of squish etc. If I could post the information here then perhaps you guys could take a look at it?
I would, love to here, many others also.
I'm ready to measure things to figure out what state of tune this bike is in and what octane fuel I need to run it on.
Measure in millimeters?
Top and bottom edge of ports in relation to top of cylinder?
Width of ports? Can I measure the width in a straight line or should it be expressed in an angle?
Squish band clearance and squish bandwidth?
Cylinder head volume?
How about the ports in the piston skirt?
Carburetor diameter and jets?
Expansion chambers?
Ignition timing before top center?
Any other information that I could add that those who know far more about it than I do could use?
compression?
Yes, compression. I have measured cylinder pressure.
I've collected some measurements of this motor of mine for your consideration. I drew the exhaust ports out the same way as vintagewannabe's clymer modification post and included them as a photo, which I hope is legible.
I've also included a picture of the carbon deposits on the piston crowns, which I thought looked interesting, something to do with the way the fuel burns and the squish band?
There's also pictures of the cylinder heads showing tiny pits which are perhaps something to do with preignition? They are on the forward half of the head on the centerline closer to the exhaust port on one side and closer to the plug on the other. These pictures also show what might be someone's initials? And some information about volume?
When I cleaned the pistons, there was only one pit on one and about five on the other again closer to the exhaust board.
There are oversize read cages with a spacer to fit them and greenish FRP read valves, Boyson's?
32mm Mikuni carburetors with a 300 main jet.
Moto Carrera millennial expansion chambers.
Ignition timing set at 1.8mm btdc.
The Squish band clearance measures 1.47mm at the outer edge and 1.74mm at the inner (measured with lead solder over the gudgeon pin)
I measured cylinder pressure with two gauges and a cold engine,
161lbs left and 155lbs right. The other gauge read 10lbs less. Both gauges were older, Taiwanese, and had schrader valves in the plug adapter.
This all started with me wondering whether I needed to run this on 110 octane and I think I know the answer, but I would be interested in what you have to say.
id say no. thats a pretty big squish clearance to be honest though, and that itself can cause detonation. set timing to 1.8 and run 93 and should be fine. i run 155+ on a motor i have with a tighter squish w zero issues.
A couple of observations. The ports are close to TZ350 or TZ750 dimensions, and at 17.5cc the heads have been machines to raise compression. Looks to me like someone wanted a full race tune. Squish is much too large. The inner squish dimension is supposed to be larger than the outer measurement. It may have been designed to run without a head gasket, but inserting a gasket would increase squish even more.
Looks like the head was machined to be dowelled to the cylinders. The squish band looks to be very wide which is usually for lower RPM motors and the transition from squish to the combustion chamber should not be as radiused. It's as if the tuner wanted to use that head to stock the motor from revving too high even though the ports would support very high revs.
If you are racing it, use 100 octane race gas, but for the street, I would be looking for a pair of stock heads or get that pair machined to take O rings and reduce the compression and make the squish band less wide.
all depended what the were going for. you can crank the squish up to 60% and reduce rpm but come out of the hole like a rocket, theres an H2 im familiar with that doesn't rev over 7k thats set up this way. its weird, but who knows what the intent on this motor was.
i agree though, id get another set of heads or get these recut.
17.5 cc heads?? Most RD350/400 are around 24cc.
Quote from: 1976RD400C on May 25, 2026, 12:22:36 PM17.5 cc heads?? Most RD350/400 are around 24cc.
yeah but when you raise the exhaust port.... you need to reduce it. that's the problem with a vintage race port job on a street bike w no power valves. also why for the street, wider is better vs taller. (on the exhaust port). JUST went thru this wth an rz that had the exhaust port cut SO high the head had to be cut an insane amount to get any bottom end, but it ran like absolute ass on the street... until 7k, and this was with fully functioning power valves. was about 19cc and just cracked 135 on cold cranking compression. the cyls were ditched and it was converted to a 421. oh happy day. :metal:
So to recap, this Engine has an extreme porting job and a rather unusual squish band? It probably won't make any power until 6000 or so and I will quickly tire of it on the street and I'll be looking for another top end suitable for the street? I can lower the cylinder pressure with new heads, but that still doesn't deal with the aggressive porting.
But I do have the bike with the engine that it has for now. I didn't build it as it is, it just came to me and I couldn't say no, always regretted having sold my RD 400 a few years ago. I've been working hard, cleaning it up and changing a few details, it had been sitting for about 10 years collecting dust and light oxidation and I am going to ride it as it is even if it's just once or twice. I'd rather not buy a 5 gallon pail of race gas, but I will if I have to. I guess I feel a little nervous putting 93 octane in it, if I were to then would the only way to tell if it was OK was the audible sound of pre ignition, right around without my helmet on with my good ear turned towards the bike, or is there a better way?
How about some 100 octane low lead Av gas. I can get it locally and it wouldn't be a permanent solution to the aggressive tune of the bike, that would be a new top end which would be a project for the coming winter.
Thanks for your replies, I appreciate your help and admire your accumulated knowledge
if it were me, I'd drop some pump 93 in it and try it. ive heard people claim high octane requirement a lot, where its absolutely not needed.
listen for detonation. it'll probably be fine. if it pings back the timing off.
Check timing, B9 spark plugs, AV gas if you are worried, ride it, come back and say "holy crap!"
Quote from: m in sc on May 26, 2026, 07:13:09 AMif it were me, I'd drop some pump 93 in it and try it. ive heard people claim high octane requirement a lot, where its absolutely not needed.
listen for detonation. it'll probably be fine. if it pings back the timing off.
Many engines will run better and make more power with
just enough octane to prevent detonation - and no more.
exactly.
Ok, that's what I'll do, 93 and keep an ear out,
Thanks