[Fot] Piston to head clearance

Fri Jan 12 15:45:56 MST 2024

Interesting discussion. I think I read the optimal is to run as close to zero as possible, but to add a slight angle to the head surface, increasing gap in the direction of the chamber. I can see an issue with a very small flat squish, as the mass of air moving increases in that direction, so more area provided would be logical.
          I have normally run my gap at gasket thickness, ie zero deck, but I am just building a motor that will (maybe ?) run 8 thou proud.

Thank you,
      Andrew Devenish

Sent from my iPad

On Jan 12, 2024, at 1:33 PM, Chris Marx via Fot <fot at autox.team.net> wrote:

The measurements were made on a rolling road….BUT on the rolling road at the Nurburging.
This dyno was in charge to measure the balance of performance power of the race cars under controlled regulations.
This dyno was equipped with a custom made software to be most exact. You can say the reference dyno.

Why did we get a little gain?
My GUESS -  Two things come into play:

  1.  Turbulence – turbulence is good to the point when it gets so strong that it blows the flame out or it pushes against the flame front and hinder it
  2.  Mathematical….assuming that you can make the gap zero between piston and head. The TR’s do have a chamber half the size of the piston surface. If you get the gap down to zero the pressure of the combustion forces on half of the piston deck: Force = pressure x area. If the area is half the size, the force is half as strong. Certainly this is only true on TDC as soon the piston moves downwards the full area of the piston deck comes in to play again.

I guess, these little things might make this little gain happen.
But, I’m not an engine developer, I only make some thoughts about it…maybe not true and the result is just luck.
On the other hand…..my car has many wins with standard crank and standard valve sizes. Does perform quite nice and seem to hold together.

Cheers
Chris

From: fubog1 <fubog1 at aol.com>
Sent: Friday, January 12, 2024 5:30 PM
To: Lorne Fritz <lorne.fritz at gmail.com>; tr4racing at googlemail.com; Ken Knight <kknight at klaenv.com>
Cc: fot at autox.team.net
Subject: Re: [Fot] Piston to head clearance

I'm still burning up (precious) brain cells on this, yes interesting, but that's one of my questions also, exactly how was the reported gain established?
I've always gone with the theory that more is better when it comes to turbulence, which requires minimal squish clearance.
I would imagine that chamber & piston top configuration, ie flat-tops or a lump on top, may be part of the equation as well...
Yes compelling discussion!

Glen Efinger

On Friday, January 12, 2024 at 10:57:08 AM EST, Ken Knight <kknight at klaenv.com<mailto:kknight at klaenv.com>> wrote:

Interesting discussion and I would agree on point.  What would interesting to know is how the HP change was measured; chassis dyno or engine dyno?

Ken

Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows

From: Lorne Fritz via Fot<mailto:fot at autox.team.net>
Sent: Friday, January 12, 2024 7:13 AM
To: tr4racing at googlemail.com<mailto:tr4racing at googlemail.com>
Cc: fot at autox.team.net<mailto:fot at autox.team.net>; fubog1<mailto:fubog1 at aol.com>
Subject: Re: [Fot] Piston to head clearance

Good point.

On Thu, Jan 11, 2024 at 6:45 PM <tr4racing at googlemail.com<mailto:tr4racing at googlemail.com>> wrote:

That is one thing to consider.

When is turbulence supporting and when is turbulence overdone and disturbing.

I could imagine to force too much turbulence with a very small squish gap the flame front expansion is compromised, thus slows down, and lowers the hp output.

That’s why we had the best power with 0.047” gap.

Cheers

Chris

From: Fot <fot-bounces at autox.team.net<mailto:fot-bounces at autox.team.net>> On Behalf Of fubog1 via Fot
Sent: Thursday, January 11, 2024 8:01 PM
To: Lorne Fritz <lorne.fritz at gmail.com<mailto:lorne.fritz at gmail.com>>; Steven Belfer <steve at artwithcars.com<mailto:steve at artwithcars.com>>
Cc: fot at autox.team.net<mailto:fot at autox.team.net>
Subject: Re: [Fot] Piston to head clearance

Steve nailed it, both on the ##s, & the importance of it.

It depends on what rods & crank, piston to wall clearance & what RPM, but for most applications that's right in the ballpark.

I've run as little as oops not enough ie .025 or less, .028 minimum (you won't get any carbon deposit on the squish area), but to play it safe nowadays I use .030-.032

Also on these engines it's not only a CR related issue, the squish is very critical, you want to minimize the clearance between piston top & head to maximize the squish effect, which creates turbulence, which assists in good combustion...

Glen Efinger.

On Thursday, January 11, 2024 at 01:00:50 PM EST, Steven Belfer via Fot <fot at autox.team.net<mailto:fot at autox.team.net>> wrote:

You want as little as necessary because this is where your high CR comes from.  The thickness of the head gasket is the major part of P to H clearance.  I ran .026 P to H clearance at Cal Speedway and the piston kissed the head and I spun a bearing.  My machine shop guy likes .040 for safety but I’ve been running .032 and it’s working out great
~Steve

> On Jan 11, 2024, at 9:13 AM, Lorne Fritz via Fot <fot at autox.team.net<mailto:fot at autox.team.net>> wrote:
>
> 
> Hi.  In the past I have had pistons hit the head and leave a mark(TR3/4).  What piston to head clearance are most of you running? Thanks in advance,  Lorne
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