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Old 04-26-2010, 05:34 PM
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Opinions needed, Dished or flat top pistons

Well since the machine shop borked up my block its out getting bored now to .040 over. Now I have to exchange the pistons. Problem is all of the dished pistons aren't available to ship until mid may. I can get some summit flat top pistons shipped out today. Now here is whats melting my brain down. The motor has dished pistons in it now, thats fine and dandy, well i got to thinking about it, and was wondering if i really want to run the flat tops and bump the compression up. I'm deathly afraid of detonation and melting down some pistons again. The issue is, its a marine engine (makes no difference, i know) and the boat lives at the marina, now the only fuel available is 89 non ethanol fuel. I am just worried about possible bumping the compression ratio up too far into the range of needing to run 93 octane which wouldn't be available for the boat. Sure i could run octane booster, but when you are trying to add some sort of additive to a 55gallon tank, it takes a lot. Specially when the boat only sees action every week to two weeks sometimes three depending on the weather, and gas like to go stale after a little while. I tried to calculate out the compression ration but I don't know the deck height, or the thickness of the gasket that I ordered. So whats the experts opinions, run the flat tops? or play the safe card and run the dished pistons? I'm sure I'm probably over thinking the hell out of this whole engine build, I just really want to do everything right the first time and get some good longevity out of this motor, specially since its a lot more difficult to pull out then it is a car. Sorry for more noob questions, and sorry for the novel. Let me know what you guys think.

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Old 04-26-2010, 05:57 PM
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Without knowing the cc(volume) of the combustion chamber in the head you are using(or the casting # so we can look up the cc) it is impossible for us to even guess at the compression ratio you might have with flat top pistons. You will have to give us a little more info.
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Old 04-26-2010, 06:29 PM
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Ifthe engine was a stock motor it likely had 8.5 or something around there to start with unless the boat is a performance boat by design. If he does nothing else other than increase the boer by .040 he will have effectlvley raised the cr just by that act. The flat tops might not be a good idea is a marine environment. I had a twin engine for a number of years and it took 100 gallons to top it off, at 3.50 that could be a task. The machine shop should be capable of telling you what you have and what you will have with the new set up.
Bill
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Old 04-26-2010, 06:59 PM
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They are the double hump or fuelies or what ever they hell they are called, according to the casting number i looked up they are 64cc heads
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Old 04-26-2010, 07:13 PM
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3782461 and 3917291 are the casting numbers
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Old 04-26-2010, 07:14 PM
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Quote:
Originally Posted by titan uranus
They are the double hump or fuelies or what ever they hell they are called, according to the casting number i looked up they are 64cc heads
Knowing this and the fact that it is a marine engine, which is a high load enviroment, I would not reccomend flat top pistons, the compression will be too high to be reliable.

I can't believe you can't get a dished piston until May unless you are unwilling to look for another source.

I would reccomend a reverse dome piston(also known as a D-dish piston) and not a full round cup shaped dish piston so that you retain a quench pad on the piston to decrease the engines octane sensitivity. Kieth Black has several that meet this criteria at www.kb-silvolite.com
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Old 04-26-2010, 07:16 PM
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Well since i originally ordered the pistons through summit and then the whole machine shop mishap happened, summit already has my money so i was just going to exchange the pistons with them for an oversize set... I didn't look into any of the D cup pistons yet, and i was looking at some of the keith black pistons.. I will go look at those now and see what is available.
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Old 04-26-2010, 07:18 PM
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2nd that on the kb's good pistons at a good price . keep the compression down .
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Old 04-26-2010, 07:22 PM
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availability would probably be a little better then me if i wasn't being so picky too, I'd like to stay with hypers over cast and would like a coated side skirt, unless i'm just being sold by fancy words
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Old 04-26-2010, 07:26 PM
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Hyper is fine for what you are doing, I wouldn't worry about the fancy skirt coating, engines have been built for years before the bells and whistles coating showed up on the scene.
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Old 04-26-2010, 07:34 PM
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Now is there a major difference between 4 valve reliefs and 2? which should i stick with? Man.. building jap engines was soo much easier, way less choices lol
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Old 04-26-2010, 07:38 PM
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2 or 4 valve relief doesn't really matter in your case. The 2 valve relief will give a slightly higher compression than the 4 valve relief due to the lesser volume cut out of the piston(the 2 extra reliefs). Amounts to about .15-.2 of a point of compression.
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Old 04-26-2010, 09:14 PM
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What would I do if I had to run on 89 octane fuel, knowing that every once in a while the marina would get a bad load of gas?

As I have stated on this board, I'll never build another flat tappet motor, so here we go with a marine build that I would do if it were my boat.

We know the stroke radius is 1.74", the rods are 5.7" and that we are going to use a piston with a 1.561" compression height. That would be a Keith Black #KB142-040, 18cc D-cup using 5/64" rings. The stack would be 9.001", so we will cut the block decks to 9.015" block deck height. We'll use a Victor Reinz composition head gasket, #5746, 4.100" bore, 0.026" compressed thickness, 5.4cc's. That will finalize the squish at 0.040" and the static compression ratio at 9.08:1.
SCR was calculated with 731 in the cylinder, 64 in the heads, 5.4 in the gasket, 18 in the piston and 3 in the piston deck.

650 carb mounted on RPM or Stealth intake, marine exhaust manifolds.

Cam is a CompCams 12-408-8, XR258HR retrofit hydraulic roller installed straight up with 1.6 rockers.

RPM HP TQ
2500 174 336
3000 228 400
3500 275 413
4000 314 412
4500 344 401
5000 362 380
5500 354 338
Motor makes 400+ ft/lbs of torque from 3000 to 4600 rpm's. Dynamic compression ratio is 8.247:1 on KB's calculator.

And by the way, I used flow figures from Brzezinski's bench testing average double hump figures.

Last edited by techinspector1; 04-26-2010 at 09:37 PM.
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Old 04-26-2010, 09:31 PM
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Not real familiar with the described cam and or grind. I actually lied on my own dive boat for several years and depending on the boat in question idle quality and dependability are real important as they don't have brakes. Reversing the prop rotation at low throttle position frequently while docking or moving in close quarters is done frequently. If it's a speed boat then this post isn't applicable.
Bill
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Old 04-26-2010, 09:37 PM
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Quote:
Originally Posted by Bill Adkins
Not real familiar with the described cam and or grind. I actually lied on my own dive boat for several years and depending on the boat in question idle quality and dependability are real important as they don't have brakes. Reversing the prop rotation at low throttle position frequently while docking or moving in close quarters is done frequently. If it's a speed boat then this post isn't applicable.
Bill
Hi Bill, here's the cam card...
http://www.compcams.com/Cam_Specs/Ca...?csid=156&sb=0
I kept the scr down to run junk gas and consequently used a relatively short cam in order to make power down low for docking, but still able to make reasonable torque for planing the boat. Everything is a trade-off.
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