I searched and searched so here I am. I am doing a Magnum swap and I am rebuilding the motor before I drop her in. I talked to Ryan from Shady about 6 mos ago, and he reccomended some pistons to raise CR, and they actually would keep the motor close to factory balance because of their weight. Any ideas which pistons those might have been? Better yet, which ones did you use? like to get the compression around 10:1 ish
Pistons to raise Compression Ratio
- Thread starter chaditotx
- Start date
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These pistons are designed for the magnum and should put your compression arond 9.3-1. There are several variables that will effect that number. I am rebuilding a 5.9 now with these pistons. When I get evrything back from the machine shop and can measure it, I will report back. I do not know if they will come out to factory balance specs or not, mine is getting balanced.
Keith Black Pistons - Pistons - Engine Components | CNC Motorsports | Car Parts | Truck Parts | Competition Engines
CNC Motorsports sells these cheaper than anyone else. A little less than Hughes and a lot less than Summit or jeg's.
Keith Black Pistons - Pistons - Engine Components | CNC Motorsports | Car Parts | Truck Parts | Competition Engines
CNC Motorsports sells these cheaper than anyone else. A little less than Hughes and a lot less than Summit or jeg's.
Thanks so much man.... that's a good start. I'm really hesitant to start rebuilding this motor, I think a divorce is on the horizon for me. Sucks really. BUT I plan on keeping the ole car. Maybe one day my boys will be able to drive it.
magnummopar
Well-Known Member
How many miles were on the engine? Most magnums dont need pistons unless they had tons of miles on them. Whats the build that you are planning and why do you need more comp than mid 9's? What size cam do you plan on using?
Shainesboostin
Well-Known Member
In for answers. Would be cool to build a Mag with some compression. Actually make some good power NA.
magnummopar
Well-Known Member
Factory short block at 9-9.5 will make 450. Just what do you consider good power?
Shainesboostin
Well-Known Member
Thats not bad at all. Just in general, most SBM stuff is lower compression. Compression always helps. Im a newb with Magnum motors. Thought they were pretty low compression, hence alot of people run the .028 headgaskets.
magnummopar
Well-Known Member
Magnums have closed chamber heads with small 62 cc chambers so unlike LA engines, they are between 9-9.4 to one from the factory. The variation depends on jjust how far down in the bore the pistons are. If you have a .070 engine you will have right at 9-1 and if you have a .040 you will have closer to 9.5-1. Machine tolerances account for the difference. People typically run .028 head gaskets to get to 9.5-1 from an .080 in the hole engine. Half a point of comp is not going to make more than 10 HP on a 400 HP engine. As IQ52 posted in another magnum thread, a stock 5.9 with 216 @.050 .505 lift 114 LSA cam made 400 HP with not much more than an Edelbrock AG EFI intake, that cam and a pcm flash. If you put a good cam in your engine, you will have smae power if properly tuned.
Exactly. Unless your cam is of a generous duration......
I plan on running a pretty heavy cam. I will be running ported Eddy heads, and would like to keep the compression up around 10:1. I am shooting for around 500hp, I'd like the car to be dangerous to drive, and fast. Modern fast. The 340 ran great, but it's been together since the 80's and popped a head gasket. Not an original 340 car (318) so I don't feel bad going Magnum and roller... and serpentine. I really need to get the block to the machine shop, and let them measure how far down in the hole I am right now. The block may need cleaned up on the deck anyway. I hope it doesn't for labors sake and getting the intake milled costs money id rather spend on the roller rockers anyway. I need to let my machine shop take a look at her. The bores look great imo, and hopefully I can get away with a hone and run standard. Should have block completely torn down tonight, and at the machine shop tomorrow. I've noticed most of the pistons are around $350 for hypereutectics so that works for me. Cam selection will depend on vacuum. I picked up a second hand MSD Atomic for $900. I love the thing, but it is vacuum limited, and they only recommend cams up to 250 duration. Realistically i'd keep it around 234 int 244 ex. Not sure if it being a roller will impact that number much, but I have a lot of figuring to do.
Well I think that is a good bit of duration to run a street car on and capable of making very good power. While it is a limitation, it's not really to bad of a bad one.
I have KB107's with Edelbrock mag heads 58cc with a .048 head gasket. Ended up with 10.81 compression.
The KB107's are heavier than the stock mag pistons so balancing was required.
The KB107's are heavier than the stock mag pistons so balancing was required.
Shainesboostin
Well-Known Member
Do you still have to run the external balance stuff? Ie: Weighted flexplate and harmonic
Yes, I used a professional products SFI balancer with bolt on weights, the truck is a 4-speed so I used a McLoed flywheel with the bolt on mag specific weight.
Used standard LA stuff for oil pan timing chain cover, pulleys etc.
Used standard LA stuff for oil pan timing chain cover, pulleys etc.
I finally have some numbers using the Keith Black pistons. First off, with out decking the block, my pistons were .009 above deck. The block checked out square so decking was not needed. My EQ heads checked out at 60 cc's, and I am using a Fel-Pro head gasket that is supposed to be .054 thick installed. After running the numbers through a calculator, I have come up with 9.4:1 static CR and with my cam, 8.4:1 dynamic CR.
Shainesboostin
Well-Known Member
What is the stock magnum gasket thickness? About to pull my heads and wanna go ahead and do the .028 if the stocker is infact more like .048.
Shainesboostin
Well-Known Member
Found the Mr. Gaskets (Mr Gasket 1121) have heard mixed options on them. But, cant seem to get the MP thin gaskets anymore. May give these a shot.
I would leave it alone as far as compression in a street application. The Magnum has a shorter deck height, which adds compression compared to the LA engines, plus the smaller chambers. As already said, the compression is around 9.3, which is perfect for a street engine.
That is good to know. I am having the deck shaved a bit, and I noticed quite a few choices in gasket thickness available from most parts places. I think I'll keep the stock dish pistons and go from there.
Probably the best idea, you'll get close to 9.5:1 with a little shaving and a thinner head gasket, but it's just not worth investing a bunch of money for a little compression bump
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