RD350 High Compression with Points Ignition

[Pravin]

AM curious to know the experience here ,

Recently I has rebuilt my top end//

AM running on 150 PSI Engine with skimmed Heads I guess (as I borrowed them and don't have mechanism to confirm the squish immediately ) which is more than stock 125 PSI ?
My Points set up was not able to run properly and the bike use to do knockings on closed throttle..

What is the experience here guys , anybody running high compression engine on points ?

Cheers,
Pravin

[1976RD400C]

I don't think points should cause a problem if the coils and charging system is ok. You should use high octane fuel with 150psi compression.

[bitzz]

Calculate your theoretical compression, to see if you're in hi-test gas territory... AND compression adds to timing, adding more compression acts like adding timing... and modern gas is much better than 1975 gas, and needs/wants less timing
Whacked squish can make it ping... ping at idle is a PROBLEM

It's shouldn't be a "points" issue... timing: maybe. I'd try setting the timing at 1.6-1.7mm BTDC and see what that does

[m in sc]

check the squish with solder down the plug hole. anything under 1mm on your motor will probably be an issue. I agree set timing back a bit, 1.6-1.7. heads are usually cut to match porting

[Jung]

As much as I understand it, you want the squish to be as tight as you can get away with and then adjust the overall head vol to be at a safe combustion ratio. Actual dynamic compression will be different depending on the porting and pipes.

For mine, I had detonation at 22cc and 1mm squish, still a bit at 23cc and 1mm then totally clean so far at 23.5cc and 0.9mm.

Stock 350 heads have a big radius between the squish band and bowl, so even if you skim them, they're not a very efficient shape.

[m in sc]

that bears with what I have seen/experienced. 24cc typically, .8-1.0 mm as a rough guide for a hot street motor and yes, radiused but a lot less than stock is

[Jung]

Yeah, I think stock 350 heads measure out at 22cc, so there's a limited amount you can skim them before you're into detonation territory. I machined out some old 250 heads, which I think are a better starting point if you're going to the trouble of tuning....they're so cheap to buy too.

[Jung]

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[m in sc]

im not sure what stock cc is, but also w the thick ass stock gasket the effective volume goes up some i assume you are either o-ringing of using a 400 gasket?

[Jung]

I fly cut the top of the barrels flat and use a stock 400 gasket. Under 1mm squish, you really want to dowel the head to the cylinder. I did this with a little tool turned up to the piston diameter that screws into the underside of the plug hole and sits the head on square. With it clamped up, I drilled a couple of 1/8" holes through the head and fins. Seems to work well.

I've not tried O-ringing but one of the issues with the US 250 heads is a lack of material. Once you machine out the bowls, you need to run little spacers to get the plugs to sit right in the heads and avoid hot spots. I'd really like to get hold of some of the later euro heads which are thicker and have an extra fin - they seem even better as candidates.

[1976RD400C]

My stock 400 has 125 psi compression and my modified one has 140 with about 24 cc heads and .8mm squish. I use 105 octane fuel in the modified one as per the machinist that did the head and port work.

[teazer]

What is the octane rating of fuel there?  I know this is comparing apples to oranges, but I ran a GT750 at 165psi on 97 octane without any problems.  Pinging is a factor of temperature and pressure and ignition timing has a huge effect on combustion temperature and pressure and octane rating is what limits detonation.  So high compression, lean mixture with stock advance with low octane fuel will be sub-optimal.

Tight squish increases combustion turbulence which leads to faster combustion (higher flame propagation rate) and can lead to detonation.

Pinging at low revs is unusual except after a hard run where the motor is heat soaked and if ambient air temperature is high, that can exacerbate the situation.

Ignition source - points or electronic - should not impact that, but timing can.
   

[m in sc]

^this