Quench Query

[Falcon67]

On my 351Cs I typically shoot for -.005 to zero deck, flat tops and a FelPro Blue 1013 which compressed comes in around .040~.041.  This seems to work well over all.  Pump gas is so junk now I don't run the door car on pure pump anymore, so detonation isn't an issue at any CR.  With iron 4V heads and nominal 74cc chambers, those "11:1 CJ motors" work out more like 10.5:1 after decking.  Ford was selling something IMHO.  I'd try something like a .035 Cometic but Cometic gaskets and me haven't found a way to get along.  When people usually ask about quench/is it worth messing with, I talk about the little 302 I use as a backup.  For years it was running .012 down in the hole with a Felpro Blue gasket, so around .053ish.  Several years ago I took it down for inspection and just decided to have the decks trimmed, took that .012 off.  Everything landed at .000 with maybe one at +.001.  Not bad for stock re-sized rods and old TRW forged parts, huh.  Engine picked up - gut feel - about 30 HP.  A solid 3 MPH in the 1/8 mile.  No other changes, nothing worn enough to replace. 

[gt350hr]

  Chris ,
      You picked up about a half a point in compression so yes TQ and HP went up for sure. As the compression ( in my experience anyway) goes above 13 the improvement is less than when the CR is around 10 or less. Others may have different experience.
   Randy

[Falcon67]

Yes, there's a chart somewhere that shows diminishing returns.   Haven't seen it in a while.  I've had other say same even with methanol.  Methanol seems to like a little less CR that a bunch more.  But for sure - bumps in CR from 8.5 to 9.5 would show a big gain while 12.5 to 13.5 not a whole lot. 

[mbrunson427]

Kind of off-topic, yet still on topic....

I have an engine builder friend that tells a story about dropping compression ratio down and actually gaining horsepower. It was an engine that was either 16:1 or thereabouts. The high compression was actually creating a pumping loss and was working against itself. Seems that adding compression produces diminishing returns as it gets higher and higher and can actually get to the point where it works backwards.

[gt350hr]

   Absolutely true! I was working with a famous 410 Sprint car engine in Ohio and he wanted to do 16.5 instead of 15"ish". We LOST  EIGHTY hp. He took it back apart and shortened the dome to drop back to 15.3 , didn't rebalance  , DID reuse the rings ( 6 dyno pulls only) and picked up all 80 he lost 13* All Pro headed 410 (or so). I felt it was hydraulic lock from so little space and so much fuel.
   On the FE side we used to use that super tall TRW 427 piston with the big "knot" in front of the plug and make decent power. One guy decided to mill the head for more compression and it was closing plug gaps but  made 10 or so more hp. I convinced him to pull the heads and relieve the knot with a "flame slot". Didn't re dyno but picked up a solid tenth on a 10.30 Mustang in 1975.
    289 Fords had a similar situation where you flattened the exhaust side of the dome , lost compression but made more power. All of this was OEM iron heads back then.
  Randy

[machoneman]

Kind of off-topic, yet still on topic....

I have an engine builder friend that tells a story about dropping compression ratio down and actually gaining horsepower. It was an engine that was either 16:1 or thereabouts. The high compression was actually creating a pumping loss and was working against itself. Seems that adding compression produces diminishing returns as it gets higher and higher and can actually get to the point where it works backwards.

I heard the same long ago about BBC's from my pro engine shop (Jerry Baker, Pro Motor Engineering here in Illinois back then).

However, they claimed that the pretty huge piston top 'bumps' needed to build high compression killed some flame travel (as noted by the piston top's burn) and hp. This with OEM chambers on open chambered iron and aluminum BBC heads. Today, aftermarket heads on many engine types have tiny, filled-in chambers with quite small cc volumes. These new design chambers no longer require tall piston 'bumps' to build quite high compression ratios w/o killing good flame travel.

[gt350hr]

  Exactly Bob. that is why you see the trend toward "kidney" shaped , small cc chambers on current "race heads". Many times the piston has a dish and still has a ton of compression. Nascar 358's are a perfect example.

[wowens]

Very interesting read about quench ! It's 5 o'clock here so I think I will have a adult beveridge and "quench" my thirst while I mull this over.

[allrightmike]

10-4 that good buddy.

[Dan859]

It's 5 o'clock here so I think I will have a adult beveridge and "quench" my thirst while I mull this over.

It's always 5 o'clock somewhere

[wowens]

Yep. Jay I promise last post from me on this thread hijack.