Big Bore Pinging

[sthomag]

I've got a big bore piston kit recently broken in on my '79. It runs great except for one thing: it pings on hard acceleration in top gear, running on 93-octane fuel.

I noted that the big bore pistons are based on the later versions, and have more of a dome than the standard '78-79 pistons, so that plus the bigger bore obviously means the engine is running more compression.

I've retarded the ignition timing a couple of degrees to little effect. Is anyone else running this combination? I'd like to know if there is a sweet spot for ignition timing.

[3Phase]

Is the mechanical advance in good condition, greased and not sticking?


REPOST: The Mechanical (centrifugal) Advance

The same thing goes for the vacuum advance timing plate and its bearing.
Remove the vacuum advance dashpot arm from the timing plate and turn the plate by hand.
The timing plate with the pick-up coils should rotate smoothly, like a Lazy Susan so if it's notchy or it sticks then it needs to be cleaned and greased.

[Bonz]

For timing advance, my FSM says 36 deg BTDC at 5200 rpm (which I assume is the rpm at which max advance would occur and stay there) with the vacuum adv disconnected and plugged. Correct me if I am wrong.

Have you run the bike like this, so the vacuum advance is out of the picture for the sake of trying it? That would either eliminate or bring into play an issue with the vacuum advance not releasing, IMO.

-Since the later pistons the big bore units are based on have a higher dome and the early heads have shallower combustion chambers, could very well have the bike pushing too much CR.


My opinion, worth what you paid for it... nothing... but could have some merit.

I would say you have gained at least .75 point of static compression ratio with the later style domes, and given they are big bore pistons you add even more CR on top of that. Stock CR is 9.0:1, and if you are up in the 10.0-10.5:1 range, on an old air cooled lump you could be above what the engine can do on even high octane pump gas.

With a big bore piston, at the bottom of the stroke the cylinder has more volume and it is filling that volume with A/F mixture. Then it compresses the higher volume into the same squish at the top, increasing compression ratio on top of the higher domes you said these pistons have in combo with the early head.

Below, I copied this from a thread Dan Hodges was part of. He is unquestionably the GURU when it comes to milking extra power from different combinations of XS1100 heads, cams and pistons. His thoughts seem to concur with what I know about big bores.

I have edited it down to what we are dealing with and the big bore, plus a little cam info in there too. He references Wiseco big bore pistons, and how they have slightly more volume in their domes for the reason I share above. I do not know if the big bore kit you used has this accommodation or not. But the fact you say they have the higher domes of the later pistons, makes me think they may not?



Dan Hodges:
Originally Posted by Dan Hodges
"The domes on the late pistons for the standard bore measure 6 cc's and the domes on the Wiseco pistons for the 74 mm bore measure 6.6 cc's. The domes on the Wiseco's are shorter because the bore is bigger and the dome volumes are essentially the same. A cheap upgrade for a 78-79 combo would be to use the late piston with the early head which would increase the compression ratio by about three quarters of a point or about 3% more power.
The late head in stock form will flow about 10% more than a 78-79 head in stock form which equates to about 5% more power.
The early cams will make about 3 HP & 2 ft. lbs. of torque more than the late cams from 4,000 rpm up, about the same at 3,000 rpm and less from idle to 3,000 rpm. I've checked this on the dyno and a fellow member named Gene from Florida found similiar results. I've posted the timing specs and event angles on these cams previously and so I won't list them again. Installing the cams on the stock marks will work/run but you should degree the cams no matter which you are using. I used a "Bridgeport" to slot my cam sprokets but you can buy them new from Mega-Cycle already slotted.
If you have a bagger with the 750-850 FD you don't want the early cams but should use the late ones with the intake advanced 5 degrees because it's low end torque that is most important to you because of the high gear ratio. You can fiddle with the intake valve closing and static compression but it's 150-160 pounds of pumping compression that you are after in a torquer motor. All things equal, the quicker the intake valve closes the more pumping compression you will have."

Hope that may help. Bonz.

[BA80]

The BB kits that are still available are for 80-82 engines. The dome on the piston is taller than the 79 pistons and the combustion chamber is smaller on the 79 heads. You have raised the compression ratio considerably.

Use higher octane fuel and it should clear up the pinging.

I have considered doing what you have done myself. I'll be interested in how it runs.

[Bonz]

I am always one to want to raise performance one way or the other on anything internal-combustion that I have ever owned in my adult life. . Thus the cams, higher redline ECU, double valve springs, higher compression pistons and base gasket delete along with larger carburetors on my ZRX 1200. However, because the ZZR 1200 and ZRX 1200 are the same engine from the head down, bolting on all of the ZZR parts together is a known system.

What he said initially has me thinking that high octane isn't going to fix it, because he is running high octane already. I concur his compression ratio is up considerably, and I had not included the fact the combustion chamber on the early heads is smaller like you just reminded.

[sthomag]

The vacuum and mechanical advances are in perfect condition, as are the rest of the ignition.

The traditional way to deal with this problem would be to install a hotter cam setup with more overlap. That's not really practical, so I think what I'll do is double up on the base gasket and play around with ignition advance. I'll let you know how it works out.

[3Phase]

There are other cams still available but you're right, the prices aren't really practical:-
Web Cams Racing Cams | Yamaha XS 1100 (78-81) DOHC 8v

[BA80]

The pinging at low end can be delt with by playing with the base timing with the higher octane fuel. Possibly playing with the timing curve also. A slower advance.

After my past experience with exhaust issues I would say that a more free flowing exhaust might help also. Stock exhaust is tuned to a stock engine.

[Bonz]

What I hear him saying is it only pings under hard acceleration in top gear. He could take away base timing to limit total advance, but I don't know that is needed. I think his idea of doubling up on a base gasket and starting with that is a cost-effective way to do it. My instinct says it's just a matter of higher compression than the current setup will allow.

[BA80]

What I hear him saying is it only pings under hard acceleration in top gear. He could take away base timing to limit total advance, but I don't know that is needed. I think his idea of doubling up on a base gasket and starting with that is a cost-effective way to do it. My instinct says it's just a matter of higher compression than the current setup will allow.

That's where the timing CURVE comes in.

[Bonz]

How do you effectively change the timing curve on and XS1100? I don't know, so I'm curious how that would work.

[3Phase]

For the early models you can you can change the springs and weights on the mechanical advance, maybe the slots too. Like any old distributor you can limit the max advance by filling in the slots or add advance by grinding them out.

You have to remember the advance runs at crank speed too, not half speed like the cams or it'll throw you off.

It won't work for the later TCIs with built-in advance and load curves.

[BA80]

81 & up has no centrifugal advance, it's built into the TCI.

[sthomag]

For the early models you can you can change the springs and weights on the mechanical advance, maybe the slots too. Like any old distributor you can limit the max advance by filling in the slots or add advance by grinding them out.

That's a good idea: limiting the final advance while leaving the base advance alone. I think I'll try that if lowering the compression a tad doesn't do the trick.

[Bonz]

Look forward to hearing how doubling up on the base gasket works. I don't know what a base gasket is worth on an XS however on my ZRX1200 removing it gained me a half point.