FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: FElony on April 06, 2020, 07:34:21 PM
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So we know the 360 has dismal quench and compression from the factory. We also know it takes the same pistons as the 390. So what happens when you load a 360 with L2292 domes instead of the flats?
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.110 in the hole, plus gasket at .040, 72cc chamber, .030 over = 8.91 compression with .150 quench, add .010 more with a standard blie Felpro.
At that point, it could be a mile of quench, no worse at .150 than .100, likely would be a healthy-ish 360, but man, a L2291 390 would run circles around it on cheaper fuel
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.110 in the hole, plus gasket at .040, 72cc chamber, .030 over = 8.91 compression with .150 quench, add .010 more with a standard blie Felpro.
At that point, it could be a mile of quench, no worse at .150 than .100, likely would be a healthy-ish 360, but man, a L2291 390 would run circles around it on cheaper fuel
What are you using for a dome value? I guess the question can be rephrased: Will the dome induce more swirl than the flat top and thus help offset the lack of quench?
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10.8 cc for the dome, and yes it could churn a little but nothing like flat quench pads tight against each other
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My thoughts about this have reverted to old ideas in light of Brent's failure in the junky junk build. Tossing around combos with junky I have AND the Futura.
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on the topic of quench, I know "everyone" says that ~.040 is a good number, but i also have heard that in certain racing circles, builders try to get as close to zero as they can. They look for ghosting or very light marks on the piston tops to indicate they are where they want to be; the tightest possible quench without causing any damage.
So, the question is, if .040 is a safe bet for any old build, how tight could you go ? Assuming good rods, pistons, crank and all clearances checked and trued up, deck true and level etc.
one thing ive always wanted to try, but no money or other means to make it happen, would be to build an engine with ~.040 CH, and the pistons at 0.000, set the quench with the head gasket. Then try progressively thinner gaskets and pull it apart and inspect after every swap to see where the limit is.
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.040 is a safe number, and I am not sure any tighter makes that much difference in terms of movement from the quench surfaces, if fact, .040 is probably more than you have to go. Regardless, I'd be interested in more or less too, but a lot of work to swap gaskets, set lash, potentially retune and make a pull.
I also think it is very much chamber and piston-dependent, a small chamber with a nice shallow dish will behave different than something with surfaces in the way.
Would be cool to play with if someone wanted to pay the labor :) I have heard guys going to .032 or so. I tend to shoot for .041-.045
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So when folks list the gap 0.040" is that the clearance with a cold engine? I guess I am more accustom to this type of study considering how the engine components change dimensions ever so slightly when at operating temperature. It's a stack up of several items. Block iron/Aluminum, Piston/Deck clearance at TDC, Head gasket type and crushed thickness etc.... So like a Harley street engine (air cooled) gets the clearance set at a minimum cold since the aluminum cylinders expand more than the iron/steel rotating assembly.
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I run .040" cometics on my stroker with the pistons @ .001"- .002" out of the hole. I turn it to 7000-7200 rpm and have never had an issue. Any time I've pulled the heads the quench pads are clean and there are no "kiss" marks of the piston to head. I'm not sure what is to be gained with going any tighter or looser. I'm going to be putting a 406 together for my summer daily driver in the following weeks with true flat tops and C4ae-g heads(75CC chambers). I'm not sure where the pistons will land in the hole but at 0 it puts me at 10.8:1 compression. I'm going to be trying to run 93 octane so I'm curious to see if running tighter quench will help with detonation or run a thicker gasket to knock the compression down but have a larger quench. This will be with an edelbrock pro flo EFI and a comp 270s cam.
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How tight you can go depends on bore size, and piston rock, as well as the other things.......steel crank and really good rods, as well. At 4.190, with about .008 piston to wall, and all good parts, the pistons will kiss the head with no damage or rod bearing issues at .035 quench. This is an 8400 rpm example. I have seen looser stuff touch harder. I am sure that a really tight clearance on the piston, and a 6000 rpm situation would probably make it closer to .030, but that engine will wear eventually, and might lead you to problems before it should be "time". I stop at .037-.038 on stuff that isn't coming back apart frequently, and clearance is generally tighter on streetable stuff to start with.
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Similar to Blair on data. I have had a couple engines at .036-.037 that put a visible witness mark from the surface of the head in the carbon on top of the piston, but seemed to run well. I could see signs of stress on rod bearings but cant really be sure that the contact was the cause or a contributor - booth were pretty far out on the fringe in terms of tuning. Both were +/-7000 RPM packages.
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I've ran .020" total quench without issue.
Just kidding.
Like the others, I will go about .038-.039, but usually about .040".
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So the Killer B's (Blair, Barry & Brent) have it.... If that isn't a consensus than I don't know what is.
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No kidding. Thanks guys.
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Blair's reply (#9) is as good as it gets. Minimum quench is affected by piston weight , stroke , rod length , rod material, RPM , compression ratio, wrist pin (wall thickness specifically), operating temperature, piston to wall clearance and piston rock. I recommend .035 absolute minimum with a steel rod. To generalize a number "standard" to which all engines should be built to avoid detonation is debatable. Certainly iron heads are more subject to detonation than aluminum as well as "open chamber" or large chamber volume head designs. .040 is the number "repeated" most often and is safe. That alone doesn't guarantee an engine to be detonation free. Tapping or "ghosting" the head should be reserved for drag race only engines where carbon build up is minimal and maximum compression is critical.
Randy
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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.
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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
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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.
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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.
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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
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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.
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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.
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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.
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10-4 that good buddy.
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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 :)
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Yep. Jay I promise last post from me on this thread hijack.