408 Build help

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360D@rt

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Hi guys,
I've searched the theads and found lots of information on how to build a monster 408 but I'm looking to build a solid, reliable 408 in the high 300 low 400 hp range. I wouldn't be stroking it except the deal I'm getting on the stroker kit is way too good to pass up. The car is a 72 dodge dart swinger with a 76 360 with in it. I don't remember the castings for the heads but I looked them up and they are nothing special. I currently have dougs headers, a standard performer intake, a holly street dominator 670 carb. and a full msd ignition. It's mated to a 904 built to handle 400 hp with a 2400 stall converter it's got a factory 8 3/4 posi in the back but I'm not sure what the gears are. I'm getting scat forged 4" crank, I beam rods, .30 over pistons (I'm told they'll give me 11.1 ratio on stock heads), bearings, hardware and gaskets. This is my first build ever but I am mechanical inclined and have tools at my disposal. I'm trying to do this project as inexpensively as possible but I also want to build it once and build it right. I am hoping to be under 3000 not including the stroker kit and components listed above.

My questions are
1) heads. Can I get away with freshening up the heads I have or do I need new ones? If so iron or aluminum and where would be my best bet on getting them?
2 ) cam. This is a street car, it's driven regularly and often by my girlfriend so I can't go to wild. I have the 2400 stall so I would like to take advantage of that.
3) carb. Will my 670 work if I adjust it or do I need to upsize? If so, what would be best? Again, it's a street car. I don't expect 20 mph but I don't want to burn my 12 gallon tank to go around the corner either.
4) like I said, this is my first ever build so I know I'm missing stuff, any advise at all is greatly appreciated.
Thanks in advance for all your help!
very respectfully,
JE
 
360D@rt said:
Hi guys,
I've searched the theads and found lots of information on how to build a monster 408 but I'm looking to build a solid, reliable 408 in the high 300 low 400 hp range. I wouldn't be stroking it except the deal I'm getting on the stroker kit is way too good to pass up. The car is a 72 dodge dart swinger with a 76 360 with in it. I don't remember the castings for the heads but I looked them up and they are nothing special. I currently have dougs headers, a standard performer intake, a holly street dominator 670 carb. and a full msd ignition. It's mated to a 904 built to handle 400 hp with a 2400 stall converter it's got a factory 8 3/4 posi in the back but I'm not sure what the gears are. I'm getting scat forged 4" crank, I beam rods, .30 over pistons (I'm told they'll give me 11.1 ratio on stock heads), bearings, hardware and gaskets. This is my first build ever but I am mechanical inclined and have tools at my disposal. I'm trying to do this project as inexpensively as possible but I also want to build it once and build it right. I am hoping to be under 3000 not including the stroker kit and components listed above.

My questions are
1) heads. Can I get away with freshening up the heads I have or do I need new ones? If so iron or aluminum and where would be my best bet on getting them?
2 ) cam. This is a street car, it's driven regularly and often by my girlfriend so I can't go to wild. I have the 2400 stall so I would like to take advantage of that.
3) carb. Will my 670 work if I adjust it or do I need to upsize? If so, what would be best? Again, it's a street car. I don't expect 20 mph but I don't want to burn my 12 gallon tank to go around the corner either.
4) like I said, this is my first ever build so I know I'm missing stuff, any advise at all is greatly appreciated.
Thanks in advance for all your help!
very respectfully,
JE
Click to expand...

1: for your power goals freshening the stock heads would be the better choice, no need to buy aluminium heads. It depends on the price for the valve guides, seats, surfacing etc.....and i´d lower the CR if using iron heads to around 10:1 (risk of detonation).

2: I´d choose a cam around 235°@.050", should idle nice at 750-800 rpm. Mine is 242°@.050", i don´t think that´s what you want - rough idle. The mentioned cam should work fine with your converter.

3: a 670cfm carb will work, but will be limited on power and rpm band.

4: the forged crank is not necessary for you, maybe you can save some dollars getting a cast one (scat´s cast crank is fine). I suggest to get a internal balanced kit, no trouble if ever changing converters etc. Be prepared to modify the distributor to get less centrifugal advance, i´m running 24° in idle, 34° fully advanced.

Have fun!

Michael
 
Thank you Michael for your rapid response. I already have rotating assembly, it's forged, internally balanced and knife edged (I don't even know what knife edged means). It's a lot more kit than I need and I wouldn't have bought it if it wasn't for the fact that I paid 750 for everything.

Being that the pistons I have are flat tops for 11.1, am I right in assuming that the only way to lower my compression is to mill out the chambers in the heads? With the machining costs would aluminum heads at around 1500 for the pair be better? Plus then I could keep compression up and still run pump gas 89 or 91 octane right?

As far as the advance goes, how would I modify the distributor? It is an msd electronic system with the 6A box.

thanks again for your time and insight.
JE
 
360D@rt said:
Thank you Michael for your rapid response. I already have rotating assembly, it's forged, internally balanced and knife edged (I don't even know what knife edged means). It's a lot more kit than I need and I wouldn't have bought it if it wasn't for the fact that I paid 750 for everything.

Being that the pistons I have are flat tops for 11.1, am I right in assuming that the only way to lower my compression is to mill out the chambers in the heads? With the machining costs would aluminum heads at around 1500 for the pair be better? Plus then I could keep compression up and still run pump gas 89 or 91 octane right?

As far as the advance goes, how would I modify the distributor? It is an msd electronic system with the 6A box.

thanks again for your time and insight.
JE
Click to expand...

If I recall correctly, flat top pistons in a stroker application will get you 11.5:1 compression or more depending on the heads...which on an iron head is too much compression for pump gas. You will probably want a dished piston to get the compression down. A large cam would bleed off some cylinder pressure, but if you want "low power" for a daily driver you will want a small cam...so scratch that idea. You really need to bring the compression down to meet your goals.

The rotating assembly you have is race kit. No need for knife edge crank in a street car...well, unless it is like my definition of a "street" car, lol.
 
Just my opinion:
Since 11:1 compression is to high for pump gas I think aluminum heads might be a bettor choice. However, it still may be a little too high. If you could get dished pistons say a 16 cc dish that might solve the compression problem for the iron heads. You would have to go through all your components to determine that.Bore, stroke, chamber size, gasket compressed thickness, deck height. I have never heard of milling out the chambers but I suppose it could be done. Polishing the combustion chamber may reduce the likelihood of detonation but that wouldn't be enough by itself for your application with the iron heads.
 
As far as heads it's up to you. I like running aluminum heads and you can run a little more compression, I think iron is 10:1 and aluminum is 11:1. But i've seen some cool stuff done with iron heads too. I run a 408 in my truck and I have a 750cfm carb. For a cam you can always call which ever manufacturer you like and ask them. They'll take all your info like car weight, rear gear ratio, ect. combine that with how you want it to drive and recomend something for you.

Knife edge is when you grind down the cranks counter weights to a "knife edge" to make them more aerodynamic. I've never looked into it but it sounds cool.

Good luck!
 
StrokerMan said:
As far as heads it's up to you. I like running aluminum heads and you can run a little more compression, I think iron is 10:1 and aluminum is 11:1. But i've seen some cool stuff done with iron heads too. I run a 408 in my truck and I have a 750cfm carb. For a cam you can always call which ever manufacturer you like and ask them. They'll take all your info like car weight, rear gear ratio, ect. combine that with how you want it to drive and recomend something for you.

Knife edge is when you grind down the cranks counter weights to a "knife edge" to make them more aerodynamic. I've never looked into it but it sounds cool.

Good luck!
Click to expand...

With a 400 hp goal from a 408 stroker, I think 10:1 will even be high with cast iron heads and 91 octane. The cam would be around a Comp XE268. And I think that won't take a lot of compression.
 
autoxcuda said:
With a 400 hp goal from a 408 stroker, I think 10:1 will even be high with cast iron heads and 91 octane. The cam would be around a Comp XE268. And I think that won't take a lot of compression.
Click to expand...


True. Those pistons are making it harder than it has to be.
 
So the guy I'm buying my rotating assembly from just sent me part numbers and the stuff isn't nearly as good as he'd said it was. It is a cast crank P5007258 with mopar performance rods P5249746 and dished pistons. He's also including bearings, gaskets, hardware and a transmission overhaul kit P5007569 all for $750. I think this is still a really awesome deal.
My questions now are
1) will the cast crank hold up ok? It's a street car, it might make 5 track passes in its life time. It has tacky tires and at some point I might mini tub it to put a 10" tire but it will never have slicks.
2) with dished pistons will I still run into compression ratio issues with my iron heads?
3) what would I be looking at for a cam with the dished pistons and lower compression ratio?

Thank a lot guys, I appreciate all your I'm put and apologize for the earlier misinformation
JE
 
Hey guys,
So the guy I'm buying my rotating assembly from texted me the part numbers today. They aren't nearly as good as he'd said they were. It's a cast crank P5007258, mopar performance rods P5249746 and dished pistons that I don't have the number for. He's also including bearings, hardware, gaskets and a tranny kit P5007569 all for $750 which I think is still an awesome deal.

My questions now are
1) will the cast crank hold up? It's a street car with street tires. At some point it may get mini tubs in the back for a 10" tire but never a slick. It might see 5 passes on the track in its life.
2) with the dished pistons, will I still run into compression ratio issues?
3) with the new, lower compression ratio, what will I be looking at for a cam/carb combo and timing?
4) am I still going to be able to make my goal of low 400hp?

Thanks again for all your input, it's greatly appreciated. And I apologize for the earlier misinformation.
JE
 
cast crank is good for 400hp no problem. probably at around 500hp is when you should be forged.

i'd get what you got to a machine shop. they can actually measure the comp ratio and help you get it to where you need. alum heads would be great for $1500 but you could probably get away with not using them. especially since the pistons have a dish to them.

that is the best advice i could give without measuring stuff. there are calculators on the silvolite pistons website, but again you need a lot of measurements.

measure twice cut once. carpenters do it, machinists do it. they have all the stuff to measure anything and they know what and how to measure. they will teach you any of them i've ever worked with are great people
 
That Mopar crank is a Scat product - good for 600hp+. The rods I'm not familiar with but any factory type rod using a rod bolt and nut will need the bores notched to clear it. Not a big deal but figure it into the work because you'll have to mock it up, grind it away, then take it apart and clean everything prior to fianl assembly.
Dished pistons are what you want. Who made them and what are the part number?
The iron heads are fine, and using the dished pistons will mean great on any pump fuel. Have them redone by a real performance shop and upgrade the valves to 2.02/1.65s along with a 5 angle valve job and gasket matching. Have the guides cut down and the spring seats cut while they're at it.
A tried-and-true low maintenance cam is the Crane H-302-2. It runs very well in that combo and will clear 425hp with ease if the heads are any good. Run a set of adjustable 273 rockers (or any iron adjustables) and you're good.
 
those rods are a MP 4340 rod that weights the same as a stock rod...Had 3 sets of them ...went mopar dumped them years ago...

the rod was supposedly made by Manley for MP.....
 
So are those rods any good or do I need to look into buying new ones?

Thanks again,
JE
 
I would go with Scat I-beams. They are stronger (good to 650hp), lighter, and you don't have to clearance your block to use them....and they are relatively cheap....and you can sell the crap rods to help make up the difference in price.

I run Scat I-beams in my Barracuda, very nice rods for the money.
 
you could just sell that forged race kit and build a 400hp 360 for near nothing which would be easy just a suggestion .
 
What would it take to get a streetable 400-450 horse 360? The engine is a 76 so I don't think it's a steel crank, right? Also, how much power is a factory sure grip rear end good for?

Thanks
JE
 
360D@rt said:
What would it take to get a streetable 400-450 horse 360? The engine is a 76 so I don't think it's a steel crank, right? Also, how much power is a factory sure grip rear end good for?

Thanks
JE
Click to expand...

All 360 cranks from the factory are cast iron....

the rods you have are a good rods...just heavy....at 725 grams vs 590 grams for scat I Beams...
 
You could put KB flat top pistons in that 360, go with scat I-beams, some head work, nice cam, and that would make 400 hp and a lot of torque for cheap. Stock 360 cast crank is plenty strong.

If you get the 4" kit get some Scat I-beams like suggested by others. But for your goals a 360 with flat top pistons to bring the compression up will be plenty stout for what you are trying to do.
 
360D@rt said:
What would it take to get a streetable 400-450 horse 360? The engine is a 76 so I don't think it's a steel crank, right? Also, how much power is a factory sure grip rear end good for?

Thanks
JE
Click to expand...

Streetable is something that is only really defined by what you can put up with. If you don't mind steeper rear end gearing, and slippery stall convertors then it's really easy. If you need power brakes, high rear end gears, and low cruise rpm then 400hp gets a little harder and 450hp gets a lot more difficult.
You still need the basics - good heads, good cam, and solid lower end. Then you need the rest of the driveline to match and deliver the goal. So - what's the goal? What is streetable for you? What are you doing with the car?
 
The car is a street machine, I drive it to work frequently and we take weekend road trips in it from time to time. As far as road manners go, I'd like to maintain a reasonably smooth idle, keep the rpms in the 3000 range on the freeway (between 60-70), sit in the 10-15 mpg range and not break down every other week. The car does not have power breaks now but I would like the option to add them in the future. If I'm lucky, the car might see 5 passes at the track in its life. I may, at some point give autocross a shot but that's down the road. I just want a reliable, fun street car that can light the tires up every now and then. From what I'm hearing I can likely stick to the stock stroke for that, which would be nice.

Thank you again everyone for all of your much needed advice.
Very respectfully,
JE
 
For what you want the 400hp level 360 is fine. To get it to the 450hp level you won't be able to use power brakes without a vacuum can or some other help. If it was me I'd go with the stroker route. It will give you the power you want in the range you need and with a cmashaft that will pull enough vacuum for power brakes.
 
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