my LeMons car

[JKav]

We've run three races at the same power/boost level and haven't blown up yet. In fact, once the rod bearing thing was sorted, the engine hasn't shown any signs of distress whatsoever. Compression is good, it never overheats (in fact the cooling system works staggeringly well).

Next weekend, we race at Thunderhill. So last week we decided it was time to turn up the boost. That sound you hear is caution being thrown headlong into the wind.

Carburetor jets come in nice incremental sizes and are clearly marked, making them ideal pee-holes for bleeding the wastegate signal. There was also a dust-covered box of them upstairs, making them even more ideal.

We started with a "200" (whatever that means) jet teed into the wg signal hose. On the dyno today, it raised our boost from its previous peak of 3.5- to 4-ish psi up to 6-ish. Fuel pressure hits a peak of 95 psi--fine. A/F is happy. EGT is happy. Best of all, it picked up about ~15-20 hp everywhere. Awesome! But there are a bunch of rattles. Crap!

We flip switches on the Bipes until all settings are at their most conservative. 5 LEDs light up in the midrange, meaning its pulling 10 deg. No change in power. Still rattles.

Base timing is 6-8 deg (it jumps around a bit at idle (?)), so we don't want to dial that back even further and end up killing our fuel economy (and raising EGTs). This is an endurance race after all. Fuel economy is important. So is keeping exhaust valves in the thing.

Stymied, we concede that we need to dial back the ambition. We try a "160" jet. 5-ish psi boost. +5 hp over our original state of tune. Slightly fewer rattles, but still rattles.

WTF? 5 psi is too much. Seriously. So we gave up and just put it back to the original state of tune.

Why is our low-compression (pistons from an automatic 1.6), low-boost, low-backpressure (no cat and 2.5-->3" exhaust with straight-thru perf-core 3" muffler), intercooled, rich a/f-havin' 1.6 engine so detonation-prone?

That's a serious question, not a rhetorical one. Why the f*ck is 5 psi too much? 5. Five.

[JKav]

Thinking about our knocky motor more... two things come to mind

First, the engine uses oil. Uses; not leaks. It's being burned. That's cuz the cylinder bores are rather, um, large. Valve stem seals/guides are crummy too. And oil in the combustion chamber is like antimatter for octane--it annihilates it.

Second, the timing jitter. At a fast idle (1500 rpm), the jitter gets worse. I suspect this is the bigger problem causing det.

Do 1.6 CASs cause timing jitter when they get old and crusty? Or what causes it?

[Trent]

could be mechanical. how is the timing belt tension?

[eyesoreracing]

That's a good idea to check, but I think the jitter Jkav was seeing was too low-frequency to be a loose belt. It would jump up for a few seconds, then jump back down for a few seconds. That right jkav?

I would expect something more random on each spark event. Still, we should check it.

Any chance the Bipes isn't actually working? It lights up LEDs, so the input side is working, but the fact that we didn't see any change in power is suspicious. Maybe we're not actually getting any retard?

-Dave

[curly]

It's in the middle of the CAS signal, if it wasn't working, you wouldn't have a CAS signal at all, and I'm not sure the engine would run.

[boileralum]

Originally Posted by eyesoreracing

That's a good idea to check, but I think the jitter Jkav was seeing was too low-frequency to be a loose belt. It would jump up for a few seconds, then jump back down for a few seconds. That right jkav?

I would expect something more random on each spark event. Still, we should check it.

Any chance the Bipes isn't actually working? It lights up LEDs, so the input side is working, but the fact that we didn't see any change in power is suspicious. Maybe we're not actually getting any retard?

-Dave

Are you using an early 1.6 crank? Is the keyway "wallowed out", allowing the crank pulley/timing gear to move around?

[eyesoreracing]

Nah, the small-nose crank is in perfect shape:


(FrankenMiata: Part I - Freebuilding the Engine > MotoIQ > Magazine Articles)

-Dave

[JKav]

Originally Posted by eyesoreracing

That's a good idea to check, but I think the jitter Jkav was seeing was too low-frequency to be a loose belt. It would jump up for a few seconds, then jump back down for a few seconds. That right jkav?

I would expect something more random on each spark event. Still, we should check it.

Any chance the Bipes isn't actually working? It lights up LEDs, so the input side is working, but the fact that we didn't see any change in power is suspicious. Maybe we're not actually getting any retard?

-Dave

The timing would jump from, roughly, 6 deg up to 14 deg and then back to 6, but it wasn't only those values. More often those numbers than others, though. Like everything on our car, it was irregularly regular.

I'll preface this next part with: I don't know how electrons work. Right, so with that said, is it possible that an improperly-wired Bipes could not only not be working, but could be the cause of the timing jitter too?

[curly]

Supposedly. I don't know the internal circuitry at all, but it controls timing, so if it's messed up it could be flickering between two settings. More likely a sensor feeding info to the bipes is bad, like your CAS. Try messing with the "degrees per rpm" setting on the bipes, see if it changes the timing jump.