Thanks Jmac

Just called the cam grinder, he didnt know what seat pressure was needed... these are guys that do cams for most of our racing industry he also told me now that it needs .4mm clearance not .3/.35 like I was told when they cut it...

Will check the seat pressure tonight, do any of you know if the shim is a fairly common size id/od? going to change the shim with the head on, got a tomei head gasket in there would rather just use the slight pressure method to hold valves in place.

Does .4mm rocker clearance sound right? sounds a big much?

Wish i had the money for a nice Tomei 304deg

Quote:

Assuming the 15.1 referred to is 1/4 Mile time that sounds pretty unlikely if revving to only 4500rpm. Are you sure your rev counter is correct/accurate? or Maybe its set for 6 cyls and 4500rpm is actually 6750rpm... but then 6200rpm would therefore be ... something horrendus!

yeah it is right, sits at just under 4000rpm at 120 in 5th gear

Got video of the 1/4mile run somewhere, motor only had 300km's on it, did a few 15.1's and a 15.2, mus still take it to the track reving it to 6000rpm

The car is very quick, just needs some extra RPM now

You say you have tried timing changes, have you removed vacuum can from distributor and regraphed it to suit the motor? Without the correct advance curve you could be doing serious damage to your motor at those rpm.

What about fuel flow and pressure, do you have a pressure gauge hooked up at top revs? You want about 2-3psi at idle never dropping below 1.5psi at high revs. If it does drop you need more flow ie. larger pump or 2 inline.

Hope this helps.

good point, also is it vibrating if you rev it out of gear? Put it in neutral and try to get the desired revs. If it vibrates then at least you can eliminate a couple of things

Is that MSD coil ok to use with the Pertronix?
If I'm not mistaken, you can only run certain coils with it.

Ive never actualy tried to rev it that high out of gear, will try this afternoon.

Fuel system is sorted, runs a holley red pump and holly regulator at 3-4psi at all times.

Thing about this problem is that it is continualy repeatable, it will do it in 1st after idleing for 20 mins, will do it in 5th down the main straight of the track after being held at high RPM's for ages, nothign seems to make it better or worse which is why spring pressure could be a very real cause.

As far as the distributer goes,,, heh, we have no one here that can recurve it, I run my A12 dissy (adapted to fit the A15 bracket) with and without vacuum (makes no difference really in power) I usualy run it at base timing (ie stock timing) I have run it -5 and -10 under base and obviously besides lack of power it still does the same thing at 6300-6400, with +5deg over base besides loads more power still the same miss at same RPM and with +10deg over base with race fuel it pulls even better but agian same thing at 6300-6400

Just for interest sakes, what type of timing do you guys run in your built higher comp A-series& how does it compare to the stock curve? I find the base timing works well on 97octane even with the medium to high compression... I presume you remove the vac advance to stop det at lower rpm with the higher comp?

try a few other cam grinders, possibly just contact them on the internet, and ask about cams with similar lift/duration and their recommended valve spring spec/settings. You might have trouble checking the tension, and absolutely don't 'wing it' - as you might get to the point of the coils hitting one another/binding, and that'll be potentially catastrophic (I've seen an engine push the valve stem so hard against them it ripped the groove/lip out of the collets and the valve dropped into the cylinder, ended up jamming, smashing through the bore wall, twisting a rod, you name it, all at semi moderate rpms too!)

If you are stuck for a tool to measure the valve springs, if you have someone with a drill press, you can mount everyday bathroom scales under them, and use a lump of steel in the jaws of the drill that helps keep the spring centred (not too long or it will hit the scales of course!) and then you twist the lever handle on the drill press down until the spring is XXmm high (whatever the installed height on the head is) and then look at the poundage/kg on the scale. It might not be the most dead on accurate thing in the world, but it _should_ get you close enough to see whether it's out big time, or at least in the ball park.

I haven't played with a lot of datsuns but I have done a fair few high revving minis, I mention this because they are similar sized valves. I can maybe look at what I used to run (I'd have to dig out some old paperwork, or worse case, dig the heads/springs out of the shed, as I held onto the 'good' stuff, it might offer some sort of a ballpark as to the seat and nose loads??

.4mm sounds right as far as valve clearances go. Apart from preventing thermal expansion from lifting the valve off the seat, it allows a harder/longer contact and helps the exhaust in particular transfer heat from the valve to the seat and off and away, which is good to prevent detonation (to say nothing of ex valve lifespan) - and it'll improve it above and beyond whatever tiny amt of lift you lose.. From memory the rockers are a 1.5:1 ratio, so 0.4 (15-16 thou in the old money) is pretty right - used to use around 13-14 on the minis (which were 1.2-1.3:1 rocker ratio std, but sooner or later went up to 1.5:1 when budget allowed for something better). If you allow for the rocker ratio difference, conceptually it's actually the same 'actual' clearance in the system.

If you pick up a couple of hundred rpm (not a thousand, just a couple of hundred) from moving the gaps from .3 to .4mm it's a _very_ strong sign that the valve springs are too soft or not preloaded/shimmed enough.

If you wanted to check if it's running out of fuel, don't do it for a long stretch, because it might be on the threshold of detonation as it leans out, but it's easier to 'find' it in top gear or the highest gear you can safely/practically get up to highish rpms in. In first and second, the motor just doesn't spend enough time revving before the upshift, whcih gives the fuel pump time to top up the bowls. A longer drawn out run in a higher gear will have the fuel starvation show up a lower and lower rpm, and the dropoff will or should likely be more pronounced.

But based on the footage, and the difficulties of a 'round the world' diagnosis, I don't think it's fuel.

sorry for a double post - but I just noticed your reply - imho keep the vac advance - it only comes in at part throttle, so you never get it at full throttle, it shouldn't hurt detonation.

Very very generally speaking the 'bigger' the cam, the more intial advance you want/need as the motor is less efficient down low, but conversely,. the less total advance you want as it's better up higher. So you need to alter the mech advance mechanism to reduce the total amount of travel. I can't speak for overseas fuel, but generally speaking, for hotter cams, having the mech advance all in by around 2800-3000rpm used to be 'the go' but with more modern fuels (even of similar octane) they seem to be happier with slightly stronger springs to slow the mech advance so it's all in by around 3200-3500rpm.

As a very very very rough guide, I'd suggest you'd want something like 12-14 initial timing, and 32 degrees total (and that's a guess, in all honesty) timing all in by 3500rpm. To find out what total timing to run there is a 'trick' of sorts - and it is this - at the dragstrip, start with 28 degrees, then do a few runs, then 30, then 32, then (if required) 34 and beyond. Basically what you are doing is completely ignoring the ET, (though you still drive it to run as good as possible) and you look purely at the mph. The mph is a good indication of the power (whereas et is indicative of traction, power to weight, gear ratios, powerband, you name it) the motor is producing on average from a couple of thousand rpm shy of and up to and including redline. So basically keep advancing the timing whilst the mph improves, and if it pegs/levels off at say (for arguments sake) 34 degrees - i.e. it is no quicker than at 32, then go back to 32 degrees and you have the 'magic' figure. Then of course you go back and alter the advance mechanism so that you can dial in the right amont of initial advance (i.e. the 12-14 degrees or so) and still hit that 32 total timing at higher rpm.

The other reason I suggest sticking to vac advance, as it'll clean it up more at part throttle, and the bigger the cam, the more it'll 'appreciate' that at part throttle. It can also help slightly crispen up the throttle response, as it takes a split second for the vac advance to spring back to 0 when you slam the throttle, and so it gets a fraction of residual advance just as you do that (and let's not forget that's the one time when fuel metering is taking a split second to respond to increased airflow - of course there is the acc pump to help, but this is a little bit extra help on top, and for free no less.

Thanks for all the info! ( one of the reasons i love my AEM's and Apexi managment systems on the turbo cars I build )

I just phoned most of the guys in this town, non of them know of anyone with the equipment to properly curve a dissy so I will order in the MSD spring kit and fit some slightly softer springs to get it all in a touch earlier than the 4300-4500 stock, though honestly im not worried about the power under 5000rpm, the midrange is fantastic already, but will check tonight where my total advance is (neibours will love me making all that noise)

I am leaning towards a spring pressure / valve clearance as the route cause of drama by how repeatable it is. going to try it at .4mm clearance and from there look at seat pressure if it clears up a few houndred RPM. One thing I did pick up is that Tomei say you should use the Nismo retainer for anything over 7mm lift @ cam, I wonder if their double valve springs were not design for the nismo retainer only which I would imagine drop the outer edge of the retainer down to give it rocker clearance, and in dropping it also give it more seat pressure? just a thought,,,, kicking my own ass for not checking like I do on the turbo cars

p.s. to the guy who said MSD coil & pertronix, it did the identical thing on the stock coil b4 I upgraded.