dished or flat top pistons?
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Topic: dished or flat top pistons?
Posted By: mufflerboltz
Subject: dished or flat top pistons?
Date Posted: 21 Jan 2014 at 12:39pm
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I tried to do a search and find if anyone prefers one over the other but all I found was a certain "someone" trying to talk smart then they actually were. So any advice on this subject would be great considering I have a set of both and which ones would work best for my build. My flat tops have a 2 3/8 compression height and my new dished have a 2 5/8 but the center of the dish is 1/8 lower then the tops of the flats. Will the dished create a better swirl and still bump the car ratio up?
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Replies:
Posted By: mattb
Date Posted: 21 Jan 2014 at 1:13pm
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flat tops should be higher comp but the only way is to figure cc of piston bowl and subtract it when u figure comp ratio
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Posted By: Glockhead SWMI
Date Posted: 21 Jan 2014 at 3:53pm
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Most people do believe the dished piston makes more power as long as the compression ratio is the same it seems. I am curious to see what the experts have to say about this.
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Posted By: patrickmull
Date Posted: 21 Jan 2014 at 6:20pm
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I like the 4.125 M&W my old engine was 5" stroke i used sp46 pistons and trimmed the tops so they were at the top of the block she was 13:1 comp ratio she ran real good on 112 octane VP fuel
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Posted By: wi50
Date Posted: 21 Jan 2014 at 9:59pm
You have a flat head, a flat piston makes for a bad chamber. A dished piston maybe is better, but you can't quench it, the piston isn't tall enough and the dish isn't big enough..... it's still a bad chamber, it's still to far away from the head to do any good. It's a simple question of which one is worse and I bet you would never tell a difference.
I built a D-17 engine with M&W pistons from a WD, with those I was able to machine the bowl large enough to build a quench engine. Weather it's better or not I don't know but it should work better in theory. It pulls well for what it is.
pank tried to argue with me on YTmag that it was impossiable, the math doesn't work out, but then again he didn't have parts in front of him to measure and is pretty proud of his tape measure and likely not bright enough to read a caliper. When he got some measurements then he changed his story to some other line....then changed it again and probably wound up off on some tangent about a crankcase evacuater system. He spent more time trying to argue than I did to build the engine. That's probably why he isn't having any fun pulling.
Bottom line is if you have one set that is significantly better condition than the other set, use them. If one set has a higher ring, use them. If you still can't tell use the dished ones unless the compression ratio is way off from where you want it.
Lastly for what it's worth I was ordering a set of custom made pistons from Diamond and the engineer strongly suggested a dish because of the flat head. That wide squish band was going to be better off with a dish break it up than a flat top even though I'm no where near being able to quench it.
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"see what happens when you have no practical experience doing something...... you end up playing with calculators and looking stupid on the internet"
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Posted By: mufflerboltz
Date Posted: 22 Jan 2014 at 6:12pm
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Thanks again wi, I think if i did my measuring right ( and not with a tape measure) the dished might be a little higher on the comp ratio.
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Posted By: Kip-Utah
Date Posted: 22 Jan 2014 at 11:54pm
I'm probably in the minority here but I still prefer the dished "power crater" type pistons for these kind of builds. In my last WC pulling engine I used the 4.128 Power Seal 170/175 pistons with the heart shaped dish like the older M&W designs. I used a 4.5" stroke WD45 crank. I'm really pleased with this one and have collected a bunch of trophies with it. Kip
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HANSEN'S OLD ORANGE IRON. Showing, Pulling, & Going!!
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Posted By: mufflerboltz
Date Posted: 23 Jan 2014 at 6:15am
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Well the pistons I bought are WC/WD 4 1/8 dished pistons and running them in a 4.5 crankshaft.
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Posted By: DrAllis
Date Posted: 23 Jan 2014 at 6:26am
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So, when you get it assembled, please run a compression test on it to see how high compression it actually is. I'd like to see 240 psi if it was mine. That takes 103 Octane fuel also.
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Posted By: DrAllis
Date Posted: 23 Jan 2014 at 6:37am
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Kip, if your pistons are the real M&W replacement for WC/WD, you had to machine about .150 to .170" off the top of each one to keep them from hitting the cylinder head, right ?? The material removed is the outer rim which makes the bowl a little shallower rasiing the compression even more. I had one like that 30 yrs ago and it ran well and was very reliable.
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Posted By: mufflerboltz
Date Posted: 23 Jan 2014 at 6:39am
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I'll do that and that's where I'd like to be. Going to try and my this tractor run in the 4000-4500 4mph modified antique class.
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Posted By: Kip-Utah
Date Posted: 23 Jan 2014 at 7:33pm
Doc, my pistons were 170/175 M&W type knock offs so they worked right out of the box with 4.5" crank. Kip
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HANSEN'S OLD ORANGE IRON. Showing, Pulling, & Going!!
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Posted By: mattb
Date Posted: 27 Jan 2014 at 11:19am
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I have a couple sets of the m and w pistons for a 226 if anybody needs them
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Posted By: bmartin55
Date Posted: 27 Jan 2014 at 2:58pm
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will they fit the gleaner 17 if so what $ thanks e mail open bob
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