Contemplating top-end swap
10-31-2019, 01:23 AM
#12
Charter Member # 55
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Check out some of articfriend's posts on modifying your MPI intake to make more power.
11-01-2019, 10:00 AM
#13
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I've read that thread over and over... well before I even had MPI engines. His thread had an incredible amount of detail to it. Haxby had a thread that showed some promise especially with the aftermarket injection. 570 HP with a Erson .630/.630 230/236 cam and factory heads.
11-16-2019, 03:36 PM
#14
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NOSE WEIGHT is the huge difference-maker on a 311. Moving as much "movable weight" aft is important. Store the lightest items you need in the front compartments, and all the heavier stuff as far back as you can get it. Anchors, Batteries, wet weight (lubricants), tools, etc. If you don't plan on needing freshwater, keep the freshwater tank empty. The lighter the nose is, the less you'll have to run counterproductive trim angles to get it aired out. As far as "total weight", then NO, there is no "too light" when it comes to weight (for top speed purposes). You can't remove too much weight from the stern to adversely affect the way that hull runs. It's a deep (AND HEAVY) hull, so any weight reductions are a plus.
Roller lifters need inspected to make sure the needles are good. I would never reuse valvesprings unless KNOW they TRULY have less than 25 hours on them.
The MPI can be swapped easily with the engines in the boat. I would pull the motors and rework your 088 heads (some bowl blending and pocket porting will be worth the effort and the shards of metal you'll end up with in your hands and arms). Have them do a 3 angle valve job. Adding back-cut and clip angles to the stock valves is cheap and worth it. Be sure to inspect the seat width (with machinist blue) BEFORE they install the springs. Make sure the intake seat width is not crazy wide. The exhaust valve seat can be a little wider to help with heat transfer.
Gasket matching isn't crucial on this setup, but any time the long-side entrance to the intake port from the manifold has an internal step, remove that.
While the heads are off, check your piston-to-deck height. MOST of the time you'll see about .025" below deck deck. What you PREFER to see is .005" below deck, so that your assembled squish height with an .039-.046 gasket is under .050". That alone will allow you to shave .030" off your heads for a compression ratio that will be happier with your cam upgrade. If your pistons are deeper in the hole than that, you can't be as aggressive with decking the heads. There's a narrow range of squish clearance where the turbulence delivers the benefits of detonation resistance you want. Above .055" and it's gone. Tighter is better, but you also want to keep from slapping the head with the piston. A 572 Gen 2 Chrysler Hemi we set up with .070" clearance with BME aluminum rods showed signs of piston kisses on the heads after some aggressive street passes (with a 7500 rpm chip), so be aware that things stretch in real life..
A 502 MPI with the cam you mention and aftermarket exhausts will end up north of 500-525 hp. Probably not much more. But that will get you to 75 MPH (propped for 5500) with a moderately light load (less than half tank fuel, 2 people).
THEN you can decide if you want to ditch the MPI and run a big carb. Easy enough to swap that out in the boat.
It doesn't sound like you plan on doing any machining on the block itself. If you DO decide to do some block machining, I highly recommend zero decking it. Once you do that, you can run small combustion chambers to raise the compression ratio while still resisting detonation. All the old big block hotrods we play with, the ones running tight squish clearances are WORLDS happier, run harder, have incredible throttle response (cause you don't have to run lazy ignition timing). Same goes for boats, although, the throttle response thing isn't all that noticeable..
When I was a teenager, I helped a friend with a hotrod he bought. 350 smallblock with 12.5 popups. Solid cam. Ran great when he test drove it. Found out later the guy had straight race gas in it, and it wouldn't do anything but knock and ping on pump gas. We pulled all the timing out so he could get it back and forth to school. When he saved up some cash, we stripped the block and zero decked it. Set it up at .040" squish. Same pistons. Same heads. Running even MORE compression than before. We added 1.6 rockers, and installed the cam 4 degrees retarded to lower combustion pressure a touch. Can you say RAPED APE? On pump gas. 34 degrees timing. Never underestimate the benefits of running tight squish...
I'm thru rambling.
I'm at the shop intending to attack a huge mountain of paperwork, bills, invoicing, tax stuff, and I guess I'm procrastinating on here...
MC
Roller lifters need inspected to make sure the needles are good. I would never reuse valvesprings unless KNOW they TRULY have less than 25 hours on them.
The MPI can be swapped easily with the engines in the boat. I would pull the motors and rework your 088 heads (some bowl blending and pocket porting will be worth the effort and the shards of metal you'll end up with in your hands and arms). Have them do a 3 angle valve job. Adding back-cut and clip angles to the stock valves is cheap and worth it. Be sure to inspect the seat width (with machinist blue) BEFORE they install the springs. Make sure the intake seat width is not crazy wide. The exhaust valve seat can be a little wider to help with heat transfer.
Gasket matching isn't crucial on this setup, but any time the long-side entrance to the intake port from the manifold has an internal step, remove that.
While the heads are off, check your piston-to-deck height. MOST of the time you'll see about .025" below deck deck. What you PREFER to see is .005" below deck, so that your assembled squish height with an .039-.046 gasket is under .050". That alone will allow you to shave .030" off your heads for a compression ratio that will be happier with your cam upgrade. If your pistons are deeper in the hole than that, you can't be as aggressive with decking the heads. There's a narrow range of squish clearance where the turbulence delivers the benefits of detonation resistance you want. Above .055" and it's gone. Tighter is better, but you also want to keep from slapping the head with the piston. A 572 Gen 2 Chrysler Hemi we set up with .070" clearance with BME aluminum rods showed signs of piston kisses on the heads after some aggressive street passes (with a 7500 rpm chip), so be aware that things stretch in real life..
A 502 MPI with the cam you mention and aftermarket exhausts will end up north of 500-525 hp. Probably not much more. But that will get you to 75 MPH (propped for 5500) with a moderately light load (less than half tank fuel, 2 people).
THEN you can decide if you want to ditch the MPI and run a big carb. Easy enough to swap that out in the boat.
It doesn't sound like you plan on doing any machining on the block itself. If you DO decide to do some block machining, I highly recommend zero decking it. Once you do that, you can run small combustion chambers to raise the compression ratio while still resisting detonation. All the old big block hotrods we play with, the ones running tight squish clearances are WORLDS happier, run harder, have incredible throttle response (cause you don't have to run lazy ignition timing). Same goes for boats, although, the throttle response thing isn't all that noticeable..
When I was a teenager, I helped a friend with a hotrod he bought. 350 smallblock with 12.5 popups. Solid cam. Ran great when he test drove it. Found out later the guy had straight race gas in it, and it wouldn't do anything but knock and ping on pump gas. We pulled all the timing out so he could get it back and forth to school. When he saved up some cash, we stripped the block and zero decked it. Set it up at .040" squish. Same pistons. Same heads. Running even MORE compression than before. We added 1.6 rockers, and installed the cam 4 degrees retarded to lower combustion pressure a touch. Can you say RAPED APE? On pump gas. 34 degrees timing. Never underestimate the benefits of running tight squish...
I'm thru rambling.
I'm at the shop intending to attack a huge mountain of paperwork, bills, invoicing, tax stuff, and I guess I'm procrastinating on here...
MC
11-18-2019, 04:04 PM
#15
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Thanks MC! That was quite the write-up. I’ll need to go back through it again so that I can digest it all.
I have purchased a set of AFR 325’s from another member here. They are already well set up but require a little attention before moving forward. They’ll be going to my head guy after Thanksgiving to begin work. One chamber has an issue with then exhaust valve seat (sunken after 80 hours or so). These came off blown 540s running 741 cams, and I’m thinking the inconel valves on iron seats caused an issue. I’ll probably have powdered metal seats installed for the exhaust on all four heads to correct this. I also plan to mill them to get the compression up. My port engine has the pistons about .010 in the hole so I don’t think I’ll need to take much from the heads. I’m not sure of where the other is at until I get it out of the bot. The addition of these heads changes the game a good amount. A good single plane with a 1000 cfm carb should net some good gains.
I have purchased a set of AFR 325’s from another member here. They are already well set up but require a little attention before moving forward. They’ll be going to my head guy after Thanksgiving to begin work. One chamber has an issue with then exhaust valve seat (sunken after 80 hours or so). These came off blown 540s running 741 cams, and I’m thinking the inconel valves on iron seats caused an issue. I’ll probably have powdered metal seats installed for the exhaust on all four heads to correct this. I also plan to mill them to get the compression up. My port engine has the pistons about .010 in the hole so I don’t think I’ll need to take much from the heads. I’m not sure of where the other is at until I get it out of the bot. The addition of these heads changes the game a good amount. A good single plane with a 1000 cfm carb should net some good gains.
11-23-2019, 06:19 PM
#16
Platinum Member
Platinum Member
If your pistons are less than .015 below deck, run the .039/.041 head gaskets.
Do you know what the chambers are cc'ing right now? Yes, hardened seats are in your future and you have an opportunity to specify your seat angle treatment. 3 angle is adequate, dont let them try to talk you into 5 angles on an exhaust seat but you do want to grind the back angle on the valves too.
Do you know what the chambers are cc'ing right now? Yes, hardened seats are in your future and you have an opportunity to specify your seat angle treatment. 3 angle is adequate, dont let them try to talk you into 5 angles on an exhaust seat but you do want to grind the back angle on the valves too.
11-23-2019, 06:31 PM
#17
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Your AFR heads probably came with 119cc chambers. With an .040 gasket and .015 in the hole, your CR is 8.45:1. I would aim for 9.5-9.6:1 with your cam on pump gas. You'll need 100cc heads to get there.
Check w AFR to get an estimate on how much to mill them to get there. I'll see if I can find out.
Check w AFR to get an estimate on how much to mill them to get there. I'll see if I can find out.
11-23-2019, 06:37 PM
#18
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Hmm, looks like 110cc is about as thin as they recommend. If the chambers arent blended it ported, then maybe 108cc, but a ported chamber they say 114cc is the limit. Doesnt lookike you can get down close to 100cc with that head. At 109/110cc, you are just under 9.0:1 CR.
11-23-2019, 06:54 PM
#19
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Feel free to tell me to shut up, but I think you would be better served with a 99cc chamber and 2.2" intake valves. The 2.300 valves like additional material removed from the chambers, which makes them larger again. Not doing that creates a pinch point where the constricted side of the intake doesnt flow as well, especially at lower lifts. Your 325's will support 650hp, but need a lot more cam and more CR. The good news is that they are a huge improvement over the 088s. In every way, as long as you weren't planning on a lot of wakeboarding.
11-24-2019, 12:08 PM
#20
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Your advice is much appreciated so please feel free to keep suggestions coming!
No wake boarding with this boat that’s for sure.
I was going to aim for a little over 9.0. Based on the port engine’s deck height, I could get there without having to cut the heads down too much. 114cc would make about 9.2 based on the measured .010 deck height and Felpro 17048.
I’m not going to rush into this, and may look at doing the short block in order to fit it with the right pistons, etc. The lower ends on both appear to be in great shape so making changes is something I didn’t plan on doing, however, if this thing will be a turd at 9:1 then I’ll need to rethink the plan.
No wake boarding with this boat that’s for sure.
I was going to aim for a little over 9.0. Based on the port engine’s deck height, I could get there without having to cut the heads down too much. 114cc would make about 9.2 based on the measured .010 deck height and Felpro 17048.
I’m not going to rush into this, and may look at doing the short block in order to fit it with the right pistons, etc. The lower ends on both appear to be in great shape so making changes is something I didn’t plan on doing, however, if this thing will be a turd at 9:1 then I’ll need to rethink the plan.
