Mopar Muscle Association UK

Hi Dave,

Good to see you haven't lost your sense of humour!

Seen a few engines that have run 40thou squench over in the States and they have had piston damage!
50thou is definitely safer due to the fact of machining tolerances with the head chamber depth and block surface levels, crank-throw, rod length and the KB's should be perfect height if they are US made ones and not the new Mexican varieties? LOL.

Good luck with the rebuild Dave and see you at the HRD's

Bacca.

I was going to check clearance by fitting the heads with no gasket (39 thou compressed) and using plastigauge to check the raised quench area on each piston for one thou clearance or more. Adjusting the quench area in the chambers to suit if tighter.

But of course I can't do that at more than 7 thou and nor will it matter really so it is easier to aim for 50 thou for all sorts of reasons.

50 thou starts pushing the benefit of quench I believe. The brave go for 35 thou, most street cars are fine with 40 thou. Anyway, you will get some quench effect with 50 thou and at least you'll be safe. To be honest you should be able to pick a number by using a custom thickness head gasket (Cometic MLS for example), except of course that messes with your CR. Guess it depends how far you are already pushing it on CR?

I agree .040" quench is fine on a stock engine , although Ma Mopar put some pistons .100" down the hole , stroker motors are a different kettle of fish , some of these kits have oil rails intersecting gudgeon pin with short skirts to clear counter weights , some of these pistons can rock .025" @ TDC , in real world terms that .040" quench now becomes .015" , this is the reason some stroker assemblies have tight tuning windows & suffer detonation issues when running pump fuel (11.5 cr) , overheating is also an issue with some of these kits , guess you gotta pick the right parts & maybe sacrifice a few cubes in doing so.

40 thou should have given me about 10:1 compression with 85cc heads.

It's tricky to work out because by cutting the heads I am making the chambers smaller too. You loose about 1cc from the combustion chamber for every 10 thou you machine off a 906 head.

Brutus wrote:I agree .040" quench is fine on a stock engine

Yeah but take into account that I built it.

Dave wrote:40 thou should have given me about 10:1 compression with 85cc heads.

It's tricky to work out because by cutting the heads I am making the chambers smaller too. You loose about 1cc from the combustion chamber for every 10 thou you machine off a 906 head.

Yes , but it's a case of diminishing returns , the more material you take off the deck the lesser is removed from chamber.


Once you get the head back measure combustion chamber then compression ratio cam be calculated............. i would of measured combustion chamber volume & scribed a mark to where heads need to be cut , machinist will then cut to mark & jobs a good un , simples.

Yes I plan to measure the volume when I get them back so i can work out what the compression ratio is. But as long as it falls somewhere between 9.7:1 and 10.2:1 I will be happy. And I am pretty sure it will.

It was more important to get some quench going on in there.

Good luck Dave , hope it all works out ok..........PITA will be tuning those triple carbs again.

I have a pair of billet metering plates that take regular jets on order because I want to play with the jetting in the outer carbs to see if I can improve the mph any.

Dave wrote:I have a pair of billet metering plates that take regular jets on order because I want to play with the jetting in the outer carbs to see if I can improve the mph any.

Hi dave which billet plates did you go for???

Promax

http://promaxcarbs.bizland.com/id1.html

i went for those they are nicely made should be good fun tuning those in

Yeah but getting them sent out is turning into a pain. They promised today they are boxed ready to send out.

But they said that last week as well........

Ok so...

I have had a total of 29 thou cut from the head and I am using a 39 thou gasket instead of a 20 thou to leave me with a quench distance of 50 thou. Not sure if the heads have had any previous cutting. Quite often when re-faced they get 10 thou removed so that may have been done before I bought them.

My combustion chamber volume is now 80cc which I have just measured.

Now according to the tech sheets for this piston, on a stock rod and stroke I should be 25 thou down the hole on the deck height.

But measuring it (if I am measuring the correct bit) I get 16 thou.

So if I am measuring correctly (and doing my sums correctly) I now have a static compression ratio of 10.59:1.

If deck height was 25 thou i would have 10.3:1 compression ratio.

However "effective" piston volume at 1.8cc may depend on how well the dome fills the quench area? So with a generous quench distance I might have lower compression still?

You can maybe gather that I am a bit confused about this.