Ignition Timing for Hemispherical Combustion Chambers

[malcolm]

My Renault 4 has a Renault 5 Alpine engine. It has a crossflow hemispherical head with a 10:1 compression ratio. See http://www.r5gordini.co.uk/mediawiki/Renault_5_Gordini_Atmo - Output is 3 times the power of the standard engine which is nice.

I've changed the distributor for a 123-tune distributor (programmable using a laptop) and have it running way better than the standard distributor at low rpm. My worry is to get the thing running as well as the original distributor at high rpm I've had to set the maximum advance to 45 degrees overall.

I'm terrified by 45 degrees advance. Although the engine just dies on full throttle at under 40 degrees so it does need more. Internet suggests hemispherical combustion chambers have a slow burn and need more advance than your average 35 degrees.

@langy - you might should have experience of this sort of thing

I've attached a picture of a spark plug which looks reasonable after 10k miles under much the same conditions.

 

[anjum]

What's the original advance curve for the Renault 5 engine. I wouldn't think that it would go much beyond that.

 

[malcolm]

I've got no data for the original distributor. In hindsight I ought to have measured what it was doing. No book data either. It's not in the haynes manual and the engines are rare so not much from other owners.

 

[anjum]

Have you tried contacting 123-Ignition themselves as they seem to have a curve for the Renault 5 Alpine. If nothing else it will tell you if you're in the right ballpark.
http://www.123ignition.nl/id/44.html

Quite fancy one of their units for the XJC but that's a long way off.

 

[dannyp]

an enigne needs what it needs, a hemi spherical chambers need silly amounts of advance. a mate who used to run a hemi V8 said if it's not knocking wind the timeing on untill it dose, then dope the fuel and add some more timeing.

hemispherical chambers need loads of advance and if they are high compression it's even worse as the piston ends up with a hell of a dome on it meaning the actual combustion chamber is a long cresent shaped thing, the shorter the stroke and bigger the bore the worse it gets.

prbaly goes best just where it's not quite knocking

 

[The Devils Reject]

As I get all curious about stuff like this I have been internetting about all things Alpine and come across a few bits and pieces.

These guys have done a few comp spec Alpine engines so might be worth getting in touch with? I don't know the company, it's just what I've found

 

[piman]

Hello Malcolm,

the only other hemispherical head engine I'm familiar with is the Jaguar XK and the absolute maximum for competition versions is 44 degrees total advance, i.e including static. If you figure does include static then not much difference. You probably know but it's important to keep away from lean mixture to avoid detonation. I mention that as there seems some erosion on the earth electrode of your plug?

Alec

 

[Shox Dr]

Excuse my ignorance.

Is that why Hemi's are called such?

 

[zx9]

Excuse my ignorance.

Is that why Hemi's are called such?

Hemi (spherical) combustion chambers. (Like half a sphere, ignoring the shape of the top of the piston)

 

[octo0072000]

What you using for fuel Malcolm that needs old fashoned 5star no longer available
are you using a octane booster

 

[malcolm]

It probably makes sense to book the car into a rolling road and check the mixture to be on the safe side. I don't want to be running too lean, and probably am a bit.

The fuel in there right now is 95 octane. That went in to help work out a tune. I was running 98 octane on the old distrubutor as it would pink at 2000rpm on 95.

 

[Spark plug]

Old 5 star was 100 octane, I don't think the closest you can get now is shell v power.

 

[Pete.]

Or look out for something like this Malcolm then you can use it over and again.


Or this http://r.ebay.com/1ch9YN

 

[Stueeee]

an enigne needs what it needs, a hemi spherical chambers need silly amounts of advance.

Absolutely. Hemispherical combustion chambers came about because early engine designers were obsessed with straightening out the ports (and therefore producing a wide incline between the valves) to get the maximum amount of mixture in and out. Once engine designers started to realise that this led to poor combustion (hence the huge ignition advance) valve angles became much reduced in race engines. Cosworth used a narrow valve angle in the FVA Formula 2 motor, but narrowed it some more in the later DFV F1 engine. None of this stuff has dented the "Hemi" mythology in most of the car enthusiast world though

 

[dannyp]

all you need to do is look at the specific power and torque of a hemi v8, then compare it to pritty much any european 4 cyl.

trouble is with all engine turning if you make one thing to be in theory amazing, then the horrible stark realisation come that it wasent that holding it back and you have infact in doing so completly buggerd up something you hadn't considerd that actualy has a far grater effect.

i chould see hemi chambers working in theory on a very long stroke narrow bore enigne as thats the only way you'd get good compression with relativly good chambers. but then you cant rev it because the stroke is so long and altho it may have really really nicely flowing ports the chamber is so small for the given displacement that there isn't enough room for valves to service the capacity of the cylinder at the higher RPM anyway.

why a lot of motors have gone with the pent roof chamber, you can nicely stuff 4 valves in it, nice spot for a spark plug in the middel and it dose most things allright. suboptimal head desing is a bitch i like my mk2 golfs and the 16v head regualy kicks me in the teeth and is the main reason for me wating a time machene

 

[AndyStobbs]

Has anyone ever dabbled with multiple spark plugs in a hemi?

 

[dannyp]

was going to say someone will have thought of and tryed it if there is any advantage to be had, the answer is defantly yes found a piccy of it as well ( nicked from wikipedia )

 

[graffian]

was going to say someone will have thought of and tryed it if there is any advantage to be had, the answer is defantly yes found a piccy of it as well ( nicked from wikipedia )

This is not actually a Hemi, it's a Hemi head made by Weslake and has flattened chambers. They are
double plugged, the original idea was symmetry, but there were some cool dual magnetos and
distributors made for them.

 

[normspanners]

just get a knock sensor and a set of cans, then you can drive it listen for the det and adjust to suit, though 45 degs seems a massive amount of advance on pump fuel,
just done a vaux red top on the rollers running on vpower and 28 was giving most power, 30 dropped it and that was with a decent set of cams with fair overlap. didnt hear any det at 30 but the figures dont lie.
ps, that plug looks to me like theres a bit of det going on.

 

[dannyp]

modern engines seem to do that. they need much less spark to make the power the going to make but wil often take a fair bit more spark without any detonation. i'm running 28 degrees on the golf under full throttel it winds out to to 40 odd on overun and mid 30's for cruseing. flat out is tuned for power everything else is tuned for feel dosent do enough miles to worry about economy