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  #16 (permalink)  
Old 05-07-2009, 01:35 PM
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if you have flat tops and don't plan to change them then use the 290 head (101cc).

call the shop and tell them to stop working on the 781 and to work on the 290.

You can get the compression ratio up to 9.1:1 with the 290 heads and a thin head gasket.
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Old 05-07-2009, 03:23 PM
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In the past I've used modeling stiff modeling clay to get an impression of an irregular shaped dome. Coat the top of the piston with something like crisco, that way it doesn't stick and you can get a pretty good negative of the dome. Then measure the amount of liquid it takes to fill the dome. A CD with the silver scratched off makes a pretty good seal and has a hole in it to pour the liquid in to the cavity.
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Old 05-07-2009, 06:07 PM
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BBC heads

If it's an all out race car, go with the 781's. If it's for the street, go with the small heads. You will loose alot of low end torque with the 781's on the street unless you want to run $7.50 a gallon race fuel to help it out some and a 3500 or more stall converter.
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Old 05-07-2009, 07:35 PM
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I wish i can stop the machince shop from working on the 781 heads he already has them apart already tanked these heads are going in jet boat not street car
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Old 05-07-2009, 07:42 PM
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I will take picture tommorow of top of piston
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Old 05-07-2009, 08:23 PM
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I dont know if this makes any diffrence my stock heads that were on my boat were also oval port casting number 14092360 . 2.06/1.72 118. These 781 are 2.19/1.88 119 cc .Im just hoping I didnt make a big mistake with these heads the 781"s .Thanks for all your imputs greatly appreciated
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Old 05-08-2009, 06:45 AM
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The 781's are great heads. They flow well and work with mild cams to wild and low compression to high compression. The only problem is these heads have big chambers which require a domed piston to make compression. And you need compression to run a bigger cam.

The 781's were used mostly on low compression 454's with a very small cam. On torque engines even through the they are considered big ovals.

So, the compression will barely change going from 118cc to 119cc. That isn't a concern, but you need to have enough compression to support the cam. that is the problem.

The 781's will run better than those peanuts, no doubt about that. and you will be able to run 87 octane if those pistons are flat tops or even slightly domed.
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Old 05-09-2009, 01:14 PM
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Maybe the easiest way out of this situation is to have the heads milled as much as is safe, buy steel shim head gaskets, and give a cam grinder all the specs so he can work with you on a grind that will match the application. There is probably too much cam, but that is a guess since I did not see cam specs in this post.
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Old 05-09-2009, 04:40 PM
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here are the cam specs I did go with all the recommened parts as well,double roller timing set, springs,roller rockers push rods ,lifterscam specs

rpm range 1600 to 5800 valve lash: intake exhaust
valve timing 0.006 HYD HYD
lobe -center angle 112 duration 270 286
Intake centerline 110

duration @.50" lift 226 236

valve lift 0.544 0.547
lobe lift 0.32 0.322

valve timing @ 0.006" lift

my machine shop is installing recommened springs for cam ,hardened seats valves etc.
Thanks for you help everyone
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Old 05-10-2009, 12:01 AM
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Sounds to me like he's doing the work your paying for on a set of heads that aren't going to work well in your set-up. If you have told him what your bottom end consists of he should have let you know before he even laid hands on the heads. He may be a fantastic machinist, and you may end up with a great set of heads. But my personal opinion is that he should have informed you that somethings gonna have to give to get things where you want them performance wise, If you are looking for decent power your gonna have to get new pistons, if you wanna have a smooth running engine that cruises around for a sunday drive , you need to pick a smaller cam.

If all else fails, let him finish the 781's and then sell them on eBay or Craigslist. The 781's are one of the most desirable oem heads out there so I doubt you'll have much problem getting them sold. Once you have that cash in hand start looking for a set of closed chamber heads that flow well.
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Old 05-10-2009, 10:47 AM
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A smaller cam should probably be in the works at this point, whether or not more compression can be obtained. Correct me if I am wrong, but I do not think the A impeller will see much over 5000 RPM with this setup...?
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Old 05-14-2009, 10:58 AM
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the cam in the description is already installed to late for a smaller cam im hoping this set-up will work the heads are at the machine shop getting longer valves installed,my jet with the previous stock engine set up ran about 5100 Rpm .Its to late to return the comp cam etc .I will keep my fingers crossed,Oh to your question regarding A impellar on jet ,the A impellar is the biggest Berkley makes ,was told from jetboat shop should turn 5300-5400 rpm.Thank you
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