23 replies to this topic

[jakejakejake1]

I have recently changed my head from a standard 998 head to a skimmed 295. This has upped the CR of the engine and using the same advance causes pinking (no pinking whatsoever before).
I tried using 97 octane fuel rather than the standard 95 but this doesn't seem to have helped too much. So the question is this, what is the best way to help reduce this problem?

I could use a lead additive as these claim to increase the octane rating of the fuel, or I could retard the ignition. Or a combination of the two. Or any other ideas?

Also I think the mixture may be slightly on the lean side as I am currently using a HS4 with AAU needle with freeflow manifold and rc40 exhaust, would I be better off with something like an AAA as I have heard that the richer the mixture the more it resists pinking?

And with regards to altering the timing, am I right in thinking rotating the dizzy anticlockwise will retard the ignition?

Any help or advice is welcomed.

Cheers, Jake

[bmcecosse]

Yes - retard the Ignition - you MUST NOT let it pink! Don't waste money on silly additives.........

[Kerrin]

Basically all of what you have said is possible as a solution. But I think you need to go back to basics, what is the timing set to? And how high is the compression now? Have you any idea what the CC of the head was before you fitted it? If it is way too high there is probably no easy fix. Don't run it too long as it is, or you will damage something. It could be something as simple as mixture, but the change in head makes me think you have a compression issue. Retard the ignition first too see if it will stop the pinking.

Edited by Kerrin, 21 February 2012 - 12:05 AM.

[Cooperman]

Before you run the engine again, you MUST retard the ignition to prevent that pinking. If it is allowed to continue you can expect broken piston rings and scored bores.
You cannot set the engine up until you know what the compression ratio is. So take the head off and measure everything, then calculate the C.R. It should be no higher than about 10.5:1 for a road car and at that you will probably need 97 RON petrol.
Lead additive will not increase the octane rating, for that you need an Octane Booster. Sometimes this comes mixed with a lead additive (Castrol Valvemaster Plus).
Running too rich will not correct too much ignition advance, all it will do is cause 'bore wash' which in turn causes high rate of bore wear.
Once again this is proof that you can't build an engine by guessing.

[racingbob]

shell optimax is 99 i use this when passing a shell garage
I always use use 97 as no shell near me
my compression is 200 on each pot

[Cooperman]

shell optimax is 99 i use this when passing a shell garage
I always use use 97 as no shell near me
my compression is 200 on each pot

What is the measured and calculated compression ratio?

[racingbob]

shell optimax is 99 i use this when passing a shell garage
I always use use 97 as no shell near me
my compression is 200 on each pot

What is the measured and calculated compression ratio?

mines 10.1 on my mg metro

[ACDodd]

At 200psi you are always goiung to have to be careful. Engines run better when they are not over compressed. With modern fuels in the UK 180 to 190 psi is about the limit before you run into problems. If using a stock A+ 998 cam consider 9.5:1 the sensible limit on modern fuels.
Secondly you need to see what advance curve you have actually got in the dizzy, this needs to be matched to the cam and compression ratio for best results.

AC

Edited by ACDodd, 22 February 2012 - 12:09 AM.

[jakejakejake1]

Apologies for being an idiot, but...
I have tried retarding the ignition, although I'm not entirely sure by how much. I may have retarded it too much, what would the symptoms be of this? Because now I'm getting some odd noises but don't know what they are, they don't sound as horrible as the pinking from before, but aint much better. Happen under much the same conditions and are similar, but less metallic.
I retarded the ignition a bit, went out for a drive, then this new noise happened once when accelerating hard, I assumed it was pinking so went home to retard the ignition a tad more just to be on the safe side, but now its even worse. Any thoughts on what this could be?
Also the car is down on power with these new timing settings.

And for a better solution to this whole problem, does anyone have any idea to how much it would cost/ how hard it would be to have some chamber enlargement work done? I'm not sure on exact figures, but would guess it cant be that much that would need removing.

Thanks again everybody

[Cooperman]

It sounds as though the C.R. Is quite high and that your distributor is not matched to the engine.
Unless you know what the C.R. actually is you could cause yourself some expensive problems.
Take the head off and measure what you have. Then do a C.R. calculation. From that you can get a re-curved distributor, fit it at a nominal static setting then take it to a rolling road for proper setting-up. If the C.R. is high, say over 10:1, you should use 97 or 99 RON petrol and if your 295 has not been modified with hardened seats you should use a lead replacement additive. Castrol Valvemaster Plus has both an octane booster and a lead additive.
It must be said, again, that you cannot just put an engine together without measuring and setting everyting correctly and expect it to run properly.

[jakejakejake1]

I know I shouldn't have just fitted it without checking, but I just wanted the car back on the road as its my only car and the head was all I had.
I believe the CR is over 10:1 as it was a normal high compression 998 before and now it clearly has a higher CR.
I am currently using RON 97 as its the best my local station sells.
The car it came of was running a non-standard cam, would this have much of an effect due to the lower dynamic compression ratio? As I have heard that at high engine speed the effect is negligible.

[Cooperman]

I doubt it's a cam issue. I built a 998, well 1061 actually, with a gas-flowed 295 with larger inlet valves, 10.3:1 CR, a Cooper 'S' 510 cam (quite mild) and an Aldon Yellow dizzy. It ran very well and was easy to set up (66 bhp at flywheel at 5900 rpm).
You really do need to know what the C.R. is. It's no more than a 3-hour job to remove the hes, measure the combustion chamber, the piston dish if they are dished and the distance from block deck level to piston crown. That's all you need to measure, plus check to see if it's been re-bored, then do the calcs and see what you have.

[liirge]

Do you know what spark plugs you are using?

[jakejakejake1]

What would the next steps be based upon the CR found, say if it were too high what options would I have?
And what CR would be deemed 'too-high'?

[jakejakejake1]

Im using NGK BPR6ES plugs, although I think they might be the wrong ones, I was assured these were correct for my car (no electronic ignition).