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Engine basics => Engine set up tips => Topic started by: Geoff Goodworth on December 07, 2007, 11:18:03 PM
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Hi folks
Does anybody know what the correct deck height is for a stunt-tuned FP40 and what the clearance between the head and the piston crown should be?
Regards, Geoff
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Hi Geoff - For a simple answer, I'd start with one gasket @.015. Real world, it depends on how the head was cut by the reworker. Stock, the combustion chamber is a squish band, a nearly flat ring about .25 inch wide around the outer chamber edge, with a deep bowl in the center. The piston comes close to the squish band at TDC, causing the mixture to rush to the bowl right at the instant of combustion. This causes high turbulence and speeds the flame travel. The combustion goes too quick, so it goes BAM instead of Whoosh. For stunt use, we want slower combustion, so the engine comes on and off the 2-4 break smoothly. So look and see if the head was cut to a true hemi, with almost no shoulder left at the edge, and one smooth radius all the way up to the plug, or if only the square shoulder at the edge of the squish band was cut to a radius. More material removed means you have to use less deck height, to keep the compression ratio the same. Typical numbers are .015 stock gasket, about .025 - .030 deck height for a 'shoulder raduis' hemi, and .005 - .010 gasket with .012 - .015 deck height for a real hemisphere. Tom H.
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Thanks Tom
I'm about to try a hemi head and an FP20/25 venturi on an FP 40 but I am going to modify Tower Heads. I will measure very carefully to see what we get. I also have a couple of GP 40s, a TT and a Magnum that have TT GP25 factory venturis and I will do the same head mod with those.
Thanks again, Geoff
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Hi Geoff - For a simple answer, I'd start with one gasket @.015. Real world, it depends on how the head was cut by the reworker. Stock, the combustion chamber is a squish band, a nearly flat ring about .25 inch wide around the outer chamber edge, with a deep bowl in the center. The piston comes close to the squish band at TDC, causing the mixture to rush to the bowl right at the instant of combustion. This causes high turbulence and speeds the flame travel. The combustion goes too quick, so it goes BAM instead of Whoosh. For stunt use, we want slower combustion, so the engine comes on and off the 2-4 break smoothly. So look and see if the head was cut to a true hemi, with almost no shoulder left at the edge, and one smooth radius all the way up to the plug, or if only the square shoulder at the edge of the squish band was cut to a radius. More material removed means you have to use less deck height, to keep the compression ratio the same. Typical numbers are .015 stock gasket, about .025 - .030 deck height for a 'shoulder raduis' hemi, and .005 - .010 gasket with .012 - .015 deck height for a real hemisphere. Tom H.
Hi Tom Geoff All
The above while is true for some engines with some setups is not true of all engines, there are many stunt engines with a very smooth 4-2 transistion that have a large squish band. The Aero Tiger for one has a very smooth 4-2 and has a huge squish band style head ,extreme to the point it may be the widest squish band head of any stunt engine out there.
I have tried almost every head shape you can imagine on this engine and none work better than the ultra large, wide squish band head. The deck height on the AERO Tiger is set partially by the top lip thickness of the sleeve, since we make these that is zero problems.
I have seen several of these engines ruined by cutting a hemi dome into the stock head, it just killed the performance, and did nothing to improve the 4-2 transistion.
There are also many other engines that have a really smooth and rapid 4-2. many of the older stunt engines such as the ST 46 and HP 40 come to mind. proper deck height seems to be much more important than the size of its band. Many of the PA engines have a wide squish band and can be setup to have a very smooth 4-2.
So as usual, there are many ways to get a great engine run, some involve cutting the compression ratio by cutting a very large hemi shape into the head, as Tom has described, others are a mixture of deck height / nitro percentage. not to mention the many other factors such as prop load and venturie size.
Ya pays your money and take your chances, even if you ruin one it pays sometimes in the ole learning experince, and there is nothing wrong that a new part won't cure y1
Regards
Randy
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Hi Rand - I almost agree. You can certainly ruin a head by going to a full hemi and not lowering the head/deck height to regain the compression ratio. I think the smoother running uncut heads probably have a steeper bottom angle at the base of the squish band. I have made button heads with the squish band level, parallel with the top of the piston, and gotten an overly harsh, undesirable run quality. Merry Xmas, Tom H.
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Thanks for the advice guys. Notice that I am machining replacement parts. If I don't like the outcome, I can revert to stock. Also, as I am the other side of the Pacific and a mechanical engineer, I am interested to have a bit of a play myself—without ruining a perfectly good engine.
Thanks again, Geoff
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Hi Rand - I almost agree. You can certainly ruin a head by going to a full hemi and not lowering the head/deck height to regain the compression ratio. I think the smoother running uncut heads probably have a steeper bottom angle at the base of the squish band. I have made button heads with the squish band level, parallel with the top of the piston, and gotten an overly harsh, undesirable run quality. Merry Xmas, Tom H.
HI Tom , Geoff
Well some may , some my not,the AERO Tigers have an almost flat large squish and work very well, I made about 150 OS 35S stunt engines and the wide squish (.150)I used on them was almost perfectly flat, They had as nice of a 4-2 transistion that you have ever seen. It was however ,not ultra high compression, When you compress those more the transistion got much harder. But they did work better with the wide Hemi squish than with the stock head.
Geoff since you machine your own parts you could make several differant types of head inserts with a button setup, maybe interesting to see what you find ;)
If you do that let us know what you find.
REgards
Randy