I will chime in with my experience on Spitfires. Bill is right in regards to
the importance of flame front and complete combustion. When I remove a head the
first thing I look at is the top of the pistons. If there is black carbon build
up or shiny areas around the edges you are leaving Hp on the table.
Flame travel is not only dependent on piston/head clearance but also combustion
chamber contours. Just like in the intake runners, air/fuel does not like sharp
corners. After a head is decked I go back in and put a lead in radius on the
squish area and a matching radius on the piston dome/popup to get 0.030"
clearance. Piston design is another hole topic. I will just say that I leave an
area for the flame front to travel to the piston OD.
Complete combustion is a much more complex subject. Cam timing, fuel mixture,
ignition timing, combustion chamber design, even exhaust header design can
influence how complete the combustion is. An engine is an air pump, Air/fuel in
and exhaust gases out. The more "fresh" air/fuel in, the more potential power
produced. We are talking about 4 cycle engines so flame travel and combustion
is only 25% of the complete process. Once you maximize complete combustion on
your current combination of components, you need to look at where the choke is
in the system. The choke is whatever is limiting flow through the engine.
Typically on Spitfires it is valve size in SCCA. Thus you need to push more air
in and push/suck more air out (intake /exhaust cycles). Then with all that
extra air you need to add more fuel and you are back to where this started.
Ron Krantz
ReK Racing
> On August 24, 2017 at 12:19 AM Bill via Fot <fot at autox.team.net> wrote:
>
> The theory about squish area in two strokes is that besides generating
> turbulence (thus mixing) and moving the majority of the mix to the middle of
> the head, that the relatively cool surfaces in close proximity to the mix
> slow the flame front headed for the cylinder wall and prevent detonation. If
> the squish band is too tight the gasses cool too much and don?t burn to the
> edges. The piston is still moving upwards at that point so partially burned
> gases push back into the non-squish area as well and screw up complete
> burning. One of the reasons that head and piston tops needed to be
> decarbonized was closing up the squish gap and losing horsepower.
>
> At least that?s what Gordon Jennings said. I always believed him.
>
>
> > > On Aug 23, 2017, at 6:46 PM, Kas Kastner via Fot <fot at
> autox.team.net mailto:fot at autox.team.net > wrote:
> >
> > This is a very interesting subject. In the thousands of hours of
> > dyno testing I did in the 60's, the squish area clearance was never
> > addressed with exception of making certain the piston did not hit the head.
> > Off hand, when the clearance is too close it rather sounds like there is
> > insufficient volume in the squish at TDC to do its job of properly exciting
> > the balance of fuel mixture when it squirts back into the combustion
> > chamber thus making a better mixed and thus more powerful combustion
> > moment. Or maybe something else :-)
> >
> >
> > Never be beaten by equipment.
> >
> >
> > On Wed, Aug 23, 2017 at 3:47 PM, John Hasty via Fot <fot at
> > autox.team.net mailto:fot at autox.team.net > wrote:
> >
> > > > > Interesting... We found 1.5 mm in the 250 two
> > strokes was the best....
> > >
> > > Sent from my iPhone
> > >
> > > On Aug 23, 2017, at 6:31 PM, "Chris Marx via Fot" <fot at
> > > autox.team.net mailto:fot at autox.team.net > wrote:
> > >
> > >
> > > > > > >
> > > > On dyno test we found a disturbing issue that lowers
> > > > the power if the piston to head clearance is less than 0.039?.
> > > >
> > > > We didn?t find a cause for this but tested thoroughly
> > > > clearances from 0.0236? upwards.
> > > >
> > > > Best power reading was with 0.0472?.
> > > >
> > > >
> > > >
> > > > Cheers
> > > >
> > > > Chris
> > > >
> > > >
> > > >
> > > > Von: Fot [mailto:fot-bounces at autox.team.net
> > > > mailto:fot-bounces at autox.team.net ] Im Auftrag von fubog1 via Fot
> > > > Gesendet: Mittwoch, 23. August 2017 23:52
> > > > An: timmmurphh at gmail.com mailto:timmmurphh at
> > > > gmail.com ; fot at autox.team.net mailto:fot at autox.team.net
> > > > Betreff: Re: [Fot] FW: Grattan VSCDA next weekend?
> > > > piston/head clearance
> > > >
> > > >
> > > >
> > > > With the good rods and reasonable rpm you have plenty
> > > > of clearance.
> > > > You don't want too much clearance either cuz then it
> > > > lessens the "squish" effect.
> > > > The idea is to have enough clearance for all running
> > > > conditions, plus a few thou safety margin, and no more.
> > > > Glen
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: timmmurphh <timmmurphh at gmail.com
> > > > mailto:timmmurphh at gmail.com >
> > > > To: 'fubog1' <fubog1 at aol.com mailto:fubog1 at
> > > > aol.com >; fot <fot at autox.team.net mailto:fot at autox.team.net >
> > > > Sent: Wed, Aug 23, 2017 3:38 pm
> > > > Subject: RE: [Fot] FW: Grattan VSCDA next weekend?
> > > > piston/head clearance
> > > >
> > > > The Payen head gasket last used was compressed to 0.055
> > > > at the fire ring. So, with the 0.010 pop-up that I have from the all
> > > > efforts to deck the block, I will have 0.045 clearance. I have
> > > > Carrillo rods and we try to stay to under 6000 RPM for engine
> > > > ?longevity?. (Not sure ?longevity? is a term that can be used with a
> > > > race engine!) At the most we?ll hit 6200 or 6300 RPM briefly before
> > > > upshifting. Therefore, I should be within the 0.030 clearance that you
> > > > recommended. It will be interesting to see what the top of the pistons
> > > > looks like on the next rebuild with regard to carbon build up in the
> > > > squish area.
> > > >
> > > >
> > > >
> > > > Thanks for the advice and all the other comments on
> > > > this subject. All much appreciated. Still a lot to learn?
> > > >
> > > >
> > > >
> > > > Tim
> > > >
> > > >
> > > >
> > > > P.S. Finally got everything together on the engine and
> > > > about ready to mate up to the transmission and put back in the car.
> > > > Looking forward to ELVF next month. That will be the test.
> > > >
> > > >
> > > >
> > > > From: fubog1 [mailto:fubog1 at aol.com]
> > > > Sent: Wednesday, August 16, 2017 5:27 PM
> > > > To: timmmurphh at gmail.com mailto:timmmurphh at
> > > > gmail.com ; fot at autox.team.net mailto:fot at autox.team.net
> > > > Subject: Re: [Fot] FW: Grattan VSCDA next weekend?
> > > >
> > > >
> > > >
> > > > Check the gasket thickness, it depends on what
> > > > piston/wall clearance, rods, and RPM you use.
> > > > You generally need around .030 piston/head with stock
> > > > rods, so add that to your pop-up of .010 for minimum gasket thickness.
> > > > If you run good rods you can run a little less before
> > > > the pistons smack the head.
> > > > It's perfect when there isn't any carbon build up on
> > > > the piston tops under the squish area.
> > > > When it's not right, it goes BRRRRRRRRRRRRR...
> > > > HTH
> > > > Glen
> > > >
> > > >
> > > >
> > > > > > > > >
> > > > > After all the machining to get the stick out
> > > > > right I now have the pistons sticking up about
> > > > > 0.010 above the tops of the
> > > > > liners. No problem with the composite Payen head
> > > > > gasket.
> > > > >
> > > > > > > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: timmmurphh--- via Fot <fot at autox.team.net
> > > > mailto:fot at autox.team.net >
> > > > To: fot <fot at autox.team.net mailto:fot at
> > > > autox.team.net >
> > > > Sent: Wed, Aug 16, 2017 5:48 pm
> > > > Subject: [Fot] FW: Grattan VSCDA next weekend?
> > > >
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: timmmurphh at gmail.com mailto:timmmurphh at
> > > > gmail.com [mailto:timmmurphh at gmail.com]
> > > > Sent: Monday, August 14, 2017 12:20 PM
> > > > To: 'Tony Drews' <tony at tonydrews.com mailto:tony at
> > > > tonydrews.com >
> > > > Cc: ryan.murphy at fdlco.wi.gov mailto:ryan.murphy at
> > > > fdlco.wi.gov ; Steve Yott <tr4 at wi.rr.com mailto:tr4 at wi.rr.com >
> > > > Subject: RE: [Fot] Grattan VSCDA next weekend?
> > > >
> > > > Sadly, we won't be able to make it.
> > > >
> > > > I just got the block back from the machine shop last
> > > > Tuesday. Not enough
> > > > time to get everything together and in the car and
> > > > ready to go by Thursday.
> > > > The good news is the liner stick out is the 0.004 the
> > > > machine shop said they
> > > > machined the liner seats and deck to. It's still a
> > > > bitch to measure, as the
> > > > liners "wobble" on the seats and when you're looking
> > > > for 0.004 it's a
> > > > problem. I finally have a decent dial indicator gage
> > > > setup to at least give
> > > > repeatable readings. The figure 8's used was a set of
> > > > copper ones I had
> > > > that measured 0.016. The machining was done on a single
> > > > setup on a Rottler
> > > > floor type vertical 4-inch spindle milling/boring
> > > > machine. These are
> > > > special purpose machine tools built specifically to
> > > > machine engine parts,
> > > > such as blocks and heads. After all the machining to
> > > > get the stick out
> > > > right I now have the pistons sticking up about 0.010
> > > > above the tops of the
> > > > liners. No problem with the composite Payen head gasket.
> > > >
> > > > Thanks for all the responses to the Figure 8 gasket
> > > > question. I did
> > > > eventually hear back from Moss UK. They said their
> > > > steel fig. 8's are 0.016
> > > > and their copper fig. 8's were 0.018. No explanation
> > > > regarding the
> > > > different thickness other that they have not had
> > > > reported problems with the
> > > > gaskets. I'm not sure where the 0.020 thick copper ones
> > > > that I measured
> > > > came from or for that matter, the 0.016 thick copper
> > > > ones I'm using.
> > > >
> > > > Tim & Ryan Murphy
> > > > 1961 ('61, it is Comm. No. 511 and was built in 1961
> > > > per British Heritage)
> > > > TR4 #317 BRG
> > > >
> > > > -----Original Message-----
> > > > From: Fot [mailto:fot-bounces at autox.team.net
> > > > mailto:fot-bounces at autox.team.net ] On Behalf Of Tony Drews via
> > > > Fot
> > > > Sent: Saturday, August 12, 2017 6:21 PM
> > > > To: fot at autox.team.net mailto:fot at autox.team.net
> > > > Subject: [Fot] Grattan VSCDA next weekend?
> > > >
> > > > Anyone else going to Grattan next weekend?
> > > >
> > > > Vroom, vroom, Tony Drews
> > > >
> > > > _______________________________________________
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> > > >
> > > >
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