1.25 or so HP/C.I. from a 390

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rmchrgr

rmchrgr

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Looking at combos - I currently have a 318 in my Dart. I'm considering the 390 because I might like to use my existing block instead of searching for a 360.

Without getting into what parts I should use, here's my theoretical question: is 1.25 hp/c.i. an achievable figure from an N/A stroker 318/390? Looking for 475 gross HP, netting in the neighborhood of 390 to the wheels. I want to go 11's in a 3,200lb car. I'm figuring at least 10.25:1 cr.

For comparison, a 375 hp 440 is 1.17hp/c.i. and the 275hp 340 is 1.23hp/c.i. These are the generally accepted factory stock ratings. Based on these figures, I think 1.25hp/c.i. is attainable from the 390.

Am I being wildly optimitstic? Anyone care to enlighten me?

- Greg
 
I think you've got your math backward on the 440 and 340. 375hp/440ci=0.85hp/ci. 275hp/340ci=0.81. That aside, there are alot of 360s running 425-450hp. A 390 should be able to get there easy with the correct combo.
 
1967DartGT said:
I think you've got your math backward on the 440 and 340. 375hp/440ci=0.85hp/ci. 275hp/340ci=0.81. That aside, there are alot of 360s running 425-450hp. A 390 should be able to get there easy with the correct combo.
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How is the math incorrect? To the best of my knowledge, it's not a percentage measurement.

Or is it?
 
You said "a 375 hp 440 is 1.17hp/c.i. and the 275hp 340 is 1.23hp/c.i."

However, 375/440=0.85. 275/340=0.81.

You just got your numerator and denominator mixed up is all. Suppose each engine made 1 hp/ci. The 440 would make 440hp and the 340 would make 340hp. However both have advertized hp below these numbers so the hp/ci values have to be less than 1.
 
1967DartGT said:
You said "a 375 hp 440 is 1.17hp/c.i. and the 275hp 340 is 1.23hp/c.i."

However, 375/440=0.85. 275/340=0.81.

You just got your numerator and denominator mixed up is all. Suppose each engine made 1 hp/ci. The 440 would make 440hp and the 340 would make 340hp. However both have advertized hp below these numbers so the hp/ci values have to be less than 1.
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Doh! I'm a dumb ***. Thanks.
 
Well.... You give 1.23hp/c.i. for the 340 at 275hp

1.23 x 340 = 418hp, not 275hp.

You need to divide the HP by the C.I. not the other way around.

340c.i. divided by 275hp = .808 or as stated above, 81hp/c.i.

81 x 340 = 275.4hp

or something like that....


(Someone beat me to it....)
 
It's late!

Still your 390 should be able to get the numbers you are after. There are alot of 318 combos making 400hp and 360 combos making 450hp+. The limiting factor to make the power you want will be the heads.
 
I'm making 1.55 HP/CI out of my 322 CI engine NA on pump gas. So you don't need a stroker to achieve this.
 
It's fairly easy to do. As BJR says, he's doing it with smaller inches and rpm. IMO, the stroker will do it with less inches and rpm. In any event, it's not that hard. You will need good heads, and the cam will negate power brakes, but otherwise it's easy.
 
moper said:
It's fairly easy to do. As BJR says, he's doing it with smaller inches and rpm. IMO, the stroker will do it with less inches and rpm. In any event, it's not that hard. You will need good heads, and the cam will negate power brakes, but otherwise it's easy.
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I'm thinking the same thing, should be easily within reach. Just wanted to confirm that the architecture will support this type of output without a lot of speciall considerations.
 
Like any other stroker... sonic test the bores to make sure they are at least .150" thick on the major thrusts. Stud the mains. I would also look into a set of modified EQ 1.92 intake Magnums and port them because a 2.02 valve will have some shrouding issues. Or plan to notch the bore to reduce the shrouding.
 
moper said:
It's fairly easy to do. As BJR says, he's doing it with smaller inches and rpm. IMO, the stroker will do it with less inches and rpm. In any event, it's not that hard. You will need good heads, and the cam will negate power brakes, but otherwise it's easy.
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But I don't turn the engine over 6000 rpm's. peak Tq. is at 4500 rpm's and peak HP is at 6000 rpm's so theres no need to turn it any harder. Peak Tq is 484 ft. lbs. @ 4500 and 501 HP @ 6000 rpm's.
 
BJR Racing said:
But I don't turn the engine over 6000 rpm's. peak Tq. is at 4500 rpm's and peak HP is at 6000 rpm's so theres no need to turn it any harder. Peak Tq is 484 ft. lbs. @ 4500 and 501 HP @ 6000 rpm's.
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So what is the torque at say, 2500? Because my memory sucks... what does your car weigh, and what is the rest of the car's combo?
 
Don't mean to be incredulous here but how come there is so much concern over shrouding?

Let's say you have a thick block and you bore it .060" over - that's 3.970" Can't be much shrouding going on there, if any.

Dulcich has stated unequivocally that the 318 bore will 'swallow a 2.02" valve no problem' and even goes on to call the shrouding issue an urban legend.

Does anyone have any proof/images of shrouding?

I'm planning on a set of 63cc Edelbrocks, with 2.02 valves and possibly with a 'street' port job.
 
489 hp @5500 rpm. (I plugged some simple numbers into the desktop dyno)
toolman
 
moper said:
I would also look into a set of modified EQ 1.92 intake Magnums
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What about the MP Magnum heads # 4876624? Clearly not cheap like the EQs, but they're aluminum, 53cc chamber with 1.92 valves.
 
hi, I was running math on a stocker 340 motor. seems they get 1.25 hp per c i with stock heads ,manifold, carb, stock comp ratio, stock stroke, stock lift hyd cams.stock rocker arms. NO alum heads,after market int, no ported heads. no light weight rods.
 
moper said:
So what is the torque at say, 2500? Because my memory sucks... what does your car weigh, and what is the rest of the car's combo?
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329 ft,lbs. @ 2500 rpm's, the car is a 71 duster and weigh's 3330 lbs. with me, in pro street trim. 30" tires and a looser converter and 4.88 gear to help with the tire size.
 
Bobby, what is the compression ratio? Here's a couple difference-of-opinion details I see. 4.88s with 30 inch tires is equal to 4.10s with stock sized tires. That's not a highway car, and 330 pound feet is 30 more than a stock Magnum in a RAM in the rpm range the engine will be operating in. IMO, totally opposite of what was being asked about. The stroke increase will push that figure up in peak, and down in rpm, to avoid needing the convertor and gear, and allowing cruising. It's a win win in terms of street car.

On the shrouding, here's something to consider. On a typical flow bench, you can skew numbers by simply testing a head on a larger bore fixture. Like many mopars are flowed on a 4" fixture, regardless of engine bore size. Thisis because 4" is popular and easy to get, where trying to get a 4.10 or 3.91 bore isn't cheap. The size difference is minimal.. but it can mean as much as 10cfm higher because of that couple tenths of an inch. Chevy felt it was important enough to notch the bores on the 396/402s. I know I know my big block 452s that were tested flowed 272 at .600 on a 4.25 bore, but 288 at .600 on a 4.5" fixture. Same bench, same day, same operator. I open up my chambers to the gasket size on all porting i do. I know shrouding makes a difference, and the smaller the bore, the more it effects things. On a typical set of J heads, the chamber will be about .050 larger than the bore on each side. So as the intake valve opens, the 2.02 will clear the bore (on a 273 it does not) but as it opens, the intake charge has to get turned around the valve head to avoid the cylinder wall. This also is the case on 383 bores vs 440. I think one of the reasons the 302 heads do as well as they do on 318s is the less shrouding with the 1.88 intakes and a chamber that fits the bore properly.
 
Well I did a little more research and it turns out I didn't quite get what was going on. Just because the chamber will fit over the bore does not mean that shrouding won't occur. I can't find it now, but in one of Dulcich's articles there was a picture of the top of the cylinder where he notched it to 'reduce shrouding'. He kind of contradicts himself in the text by saying a 2.02 valve will fit no problem but I guess he actually believed otherwise. Gotta kind of read between the lines I guess. I stand corrected.

I think once you really get into the design aspects of the 318 block, it starts to become clear why they're difficult to work with and why they were never popular as a performance engine. Sure the possibility is there but you're limited by several inherent factors that are tough to work around, mainly the 3.910" bore which limits valve size. If you were lucky enough to have a super thick block without too much core shift, you could go .090" over but that might be asking a lot. Plus to find this out, I'd need to pull the engine, tear it down and take it to get sonic checked. Maybe I get lucky but if not, then I have a disassembled 318 and a non-running car which amounts to nothing. I guess if you REALLY just want to say you have a 390 then it's fine, but the 360 is already at the magic 4.00".

That's not to say I can't achieve the goal of 475hp with a 390 but it might be 'easier' to do with a 360 due to the greater availability of parts that fit. You can go around and around arguing the point.

But then there's also the guy in the Engine Masters challenge who made 477hp with a 323". He used Chevy 6.2" rods, Comp flat tappet cam, 1.92 Magnum R/T heads, Air Gap intake and a 950 carb. Tough to duplicate since i'm not a pro engine builder, but I guess the architecture can support over 1.2hp/c.i.

http://www.popularhotrodding.com/tech/0901phr_mopar_318_magnum_engine/index.html
 
Moper,
The compression ratio is 10.82:1. on race fuel it make 536 HP @ 6000 and 501 ft. lbs. Tq. @ 4500.
 
Of course you could always use a poly block. That WILL accept an 0.090 over bore...

But that brings up other issues like;how the hell do I make this fit my Dart and,What about headers etc etc..

Not to mention the extra weight.

Notching the bores of an LA block should alleviate most of the shrouding.
 
it's not a question of will it fit... but more important is will it work well. I ran a few 318s with 2.02 X and J heads. They ran well, but I'm sure i left some on the table. The most important thing to remember is to keep the cost per hp down if you can. You can build anything out of anything really. But doing it and getting a result that you are happy with in terms of expense and reward is the key. A 360 can be built easy, and they will make the same power at a slightly higher rpm than the 4" arm 390 will. The parts for the 360 will be slightly less expensive because you can safely run hypereutectic pistons and the factory heads with a little work can support them well.
 
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