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502 MPI Gen V freshen w/ possible cam upgrade questions for the pros

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Old 02-05-2016, 12:37 AM
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Originally Posted by articfriends View Post
If you go to the main technical forum you will see my dyno test of various modified 502 parts and throttle bodys recently, testings not done yet, will be posting more info next week after testing a stock throttle body and k/n flame arrestor
Essentially I made 541 hp at 5700/5600 and 568 ft lbs tq from a 502 with 4.470 je flat tops, iron rectangle ports that I spent 4-6 hours porting the casting flash, ridges, bumps and port matching, a 500 efi mercruiser roller cam, fully ported mpi intake with cut down runners, 58 mm TPIS throttle body. Motor made at least 540 ftlbs tq from 3300 to 5300. Bolting a flat plate gen 6 flame arrestor on killed almost 30 hp, comparing the 58 mm TB to a smaller bored out 56 mm throttle body lost 13.5 hp at 5600, I expect a similar drop again going to a stock throttle body. Would still nd up being a fairly expensive upgrade to copy as the hp 500 efi cam wont work with cast iron stock manifolds but some of the info should give you a idea of what will do what, Smitty
I have been following that post thank you! Some great info!
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Old 02-05-2016, 05:07 PM
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cylinder pressure and the dynamic compression ratio are the governing factors of cam duration and lift. Not the headers. Valve lift is governed by the coil bind of the valve spring and valve seal to the valve keeper. This all has to be taken into consideration for cam selection. Stock heights on valve train components can be very restrictive. The good news is it can be corrected easily with longer valves, springs, and pushrods. The bad news is If you have low static compression you are extremely limited on cam duration and lift. to fix this you need to cut back on the piston dish or use a smaller combustion chamber on the heads. Definitely a little more pricey to correct that aspect. Number one cause of reversion is low cylinder pressure which is generally caused be over camming the dynamic compression ratio. A roller cams geometry allows the valve to open and close more abruptly which creates more HP then a slow transitioning flat tappet.. I recommend you go forth with the roller cam upgrade. I'll bet your water intrusion was from a leaky riser gasket. It happens more than not and can be undetectable, Especially with through the prop exhaust. You need to know what your static compression is, What size combustion chambers you have. Then you run those numbers through a dynamic compression calculator with the intake closing angle of your cam. Dynamic compression should fall between 7.0 to 9.1 for pump gas. Aluminum heads give you a bonus of .5 Hope this helps you with your endeavor.

Last edited by minx163; 02-05-2016 at 05:19 PM.
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Old 02-06-2016, 02:35 AM
  #53
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Originally Posted by minx163 View Post
cylinder pressure and the dynamic compression ratio are the governing factors of cam duration and lift. Not the headers. Valve lift is governed by the coil bind of the valve spring and valve seal to the valve keeper. This all has to be taken into consideration for cam selection. Stock heights on valve train components can be very restrictive. The good news is it can be corrected easily with longer valves, springs, and pushrods. The bad news is If you have low static compression you are extremely limited on cam duration and lift. to fix this you need to cut back on the piston dish or use a smaller combustion chamber on the heads. Definitely a little more pricey to correct that aspect. Number one cause of reversion is low cylinder pressure which is generally caused be over camming the dynamic compression ratio. A roller cams geometry allows the valve to open and close more abruptly which creates more HP then a slow transitioning flat tappet.. I recommend you go forth with the roller cam upgrade. I'll bet your water intrusion was from a leaky riser gasket. It happens more than not and can be undetectable, Especially with through the prop exhaust. You need to know what your static compression is, What size combustion chambers you have. Then you run those numbers through a dynamic compression calculator with the intake closing angle of your cam. Dynamic compression should fall between 7.0 to 9.1 for pump gas. Aluminum heads give you a bonus of .5 Hope this helps you with your endeavor.
My water intrusion was from and awesome leaking head gasket.

I have read about 525 and 575sci engines with silent choice on boats I run across in the classifieds. Is what you are explaining making it possible for these engines to run the cams that they do and not have the reversion issues? OR is it that they have CMI header type exhaust and I have Stainless Marines?
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Old 02-06-2016, 09:27 AM
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A misconception is that reversion only happens in a water cooled exhaust . In fact any engine can get reversion. Engines have actually spit fuel out the venturi in extreme circumstances. I'll bet there are tons of people running Stainless Marine exhaust with out issue. There was a study done when Merc's were having trouble with there motors hydro locking from reversion. The study found that the old time engineers had put a vacuum break in the riser, to prevent the water from trailing back. It was omitted on modern risers and boom a Monte Vesuvius of problems arose. A longer LSA or less overlap is usually the proclaimed cure. LSA is also the means to control the torque curve. It has a direct relation ship to the amount of lift and duration so it doesn't necessarily mean you need a huge LSA. to prevent reversion. Cylinder pressure is key to keeping it at bay. If you took a SBC 400 with 5.7 rods and a 9.4 Static compression ratio (STR) and threw in a HR270. Your dynamic compression ratio (DCR) would be 7.5 :1 and dynamic cylinder pressure would be 146psi , if you used a HR264 with all the same variables your DCR would be 7.7 and DCP would be 151psi. You can see how duration and lift effects the cylinder pressure. V/P (Volume to Pressure Index) is another way to target you cylinder pressure for what works in other applications. So if you know a guy that's running a certain cam with a specific SCR. You can calculate what worked for him, will work in your application or meet the same VP with a completely different setup. Here's a link to some automotive calculators. http://www.wallaceracing.com/Calculators.htm

Last edited by minx163; 02-07-2016 at 09:49 AM.
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