390 vs 408 - the real difference?

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Rat Patrol

Rat Patrol

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Yes, there's ~ 18cui difference , but I'm interested in hearing learned opinion on how much the slightly smaller bore on a stroker 318 would have on performance compared to a similar build 408.

Lets assume same heads and valve size (2.02) ..., same combo, same pipes, etc....

Would the narrower bore have significant effect on potential HP?
 
In my opinion the smaller bore would cost a little bit of intake port flow due to slight valve shrouding. I don't believe it to be any significant amount though and what ever potential intake flow the shrouding creates is probably offset by the reduced bearing surface of the smaller mains.

Now the 18 cubes, well at 1.5 HP per cubic inch that's 27 horsepower.
 
Now the 18 cubes, well at 1.5 HP per cubic inch that's 27 horsepower.
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Good point.

I guess if you had to buy a 360 block to build the 408, it might not be considered "cheap HP" though...... Just a thought.
 
If you look around long enough and talk to enough people you can probably get one for nothing. I got my 74 360 block from another racer that was moving and just wanted to get rid of it, free in other words.
 
Lets say the engines was done the same.The only differance is 18cubic inchs.quote [Now the 18 cubes, well at 1.5 HP per cubic inch that's 27 horsepower}you can,t say that but you can in a dyno stand point but real world rpm in the 390 can make up the differance of the 27hp.
 
I've been wondering if the smaller piston would make the 390 wind up higher? I wonder what the max rpm comparison would be.
 
If I'm not mistaken, RPM capability is partly due to piston speed and since both engines utilize a 4" stroke the piston speed would be the same so the RPM range would be basically the same. The only difference would be the slightly less mechanical drag due to the smaller pistons and smaller mains so the smaller 390 might make usable power at an extra 100 rpm. JMHO
 
When trying to tie this to a real-world build there are too many variables to give a definitive answer, but my feelings are as follows.....

IF you have a cylinder head that actually flows well, the larger bore could give a horsepower advantage over and above the presumed 1.5 per CID discussed above, especially when we're talking an engine that could use bigger than a 2.02 valve.

At the likely RPM usage for the average users stroker engine, my thought is that the added girth of the 360 main is going to lose exactly zilch.

If the advantage were only 1.5*18 hp for the 408, that would make the cost of a 360 block pretty cheap horsepower/$$
 
I only used the 1.5 hp/ci figure as an example because the application wasn't specified. It takes a pretty stout package to make that kind of steam. So if you are talking a hot street/strip engine you probably aren't going to get that, maybe more like 1.2-1.3. But the question was what effect the smaller bore has on the potential power it could produce. In my opinion in order to see the potential limited by the smaller bore you'd have to be building something capable of producing at least 1.5 hp/ci and probably really more. Without quantifying the application there isn't really anyway to determine.
 
The 390 is an undersquare design (stroke measurement is larger than the bore measurement 4.0" x 3.910") These designs typically develop/favor peak torque at relatively low rpm. The Slant 6 is a famous undersquare design. Think about what a Slant drives like - idles low, has a teeny bit of grunt off the line and the pedal is on the floor when you're trying to get on it. Unless you start modifying it, you can't rev it past 4,500 or so. The 390 is the same principal. I guess you can infer the rest. Not too many 390s around I'm guessing for those reasons. Plus the smaller bore does not utilize the valve area as well as the 4" bore of the 360 so it can't ingest as much air. Pretty simple. 390 would likely be an OK truck engine or maybe good for rock crawling.

The 408 is square (4" x 4") which is a little better than undersquare.

Best architecture for hot rodding is oversquare - 340, most SBCs, 5.0. (3.xx" stroke x 4" bore) Basically most performance engines that you can rev the piss out of.
 
This is a bit apples to oranges but the famous honda 50cc engine has a 39.0mm bore and a 41.4mm stroke, peak hp is at 7k rpm but the engine can twist up to 9k rpm. smaller piston than stroke.

Also according to wiki "The Honda Integra Type R's B18C5 engine has one of the highest redlines (8,400 rpm) of any production engine, yet features an undersquare design."

Not saying that all undersquare engines are like this.

I suppose the best answer would be to find out what 390 and 408 owners spin their engines up to, specs on the engine certainty would also help.
 
Personall I'd take the motor that makes usable h.p and torque WITHOUT having to rev to 8000 rpm. Hi rpm = lots of wear and possibly breakage,plus lots of money to mod it for those rpm's
 
SirDan said:
This is a bit apples to oranges but the famous honda 50cc engine has a 39.0mm bore and a 41.4mm stroke, peak hp is at 7k rpm but the engine can twist up to 9k rpm. smaller piston than stroke.

Also according to wiki "The Honda Integra Type R's B18C5 engine has one of the highest redlines (8,400 rpm) of any production engine, yet features an undersquare design."

Not saying that all undersquare engines are like this.

I suppose the best answer would be to find out what 390 and 408 owners spin their engines up to, specs on the engine certainty would also help.
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LOL... That's more like comparing apples to watermelons.
 
I appreciate the views - I'm not after a quantatative result, which is why I delberately avoided suggesting a "combo".

My real interest was in the different configurations - especially bore size - and how much effect it might have.

One poster said this:

Lets say the engines was done the same.The only differance is 18 cubic inchs. [Now the 18 cubes, well at 1.5 HP per cubic inch that's 27 horsepower
You can't say that but you can in a dyno stand point but real world rpm in the 390 can make up the differance of the 27hp.
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Why can't we say if both engines make 1.5 Hp per cube, the final result will be 18 x 1.5more in favour of the 408?

Is there something wrong with this assumption - all other things being equal?
 
Rat Patrol said:
I appreciate the views - I'm not after a quantatative result, which is why I delberately avoided suggesting a "combo".

My real interest was in the different configurations - especially bore size - and how much effect it might have.

One poster said this:



Why can't we say if both engines make 1.5 Hp per cube, the final result will be 18 x 1.5more in favour of the 408?

Is there something wrong with this assumption - all other things being equal?
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All other things being equal? That depends.
I may be wrong but if using the same heads and cam specs I'd expect the 390 to be a little peakier and lose LESS than 1.5hp per cube at peak. However, if you cam the 408 a little bigger and go with a slightly bigger, higher-flowing port to retain the 390's 'characteristics' i.e. power and torque rpm peaks then I think 1.5hp per ci difference is a reasonable, or maybe conservative estimate. JMO.

Once again, there's a fair amount more to it than this. I'd say that in terms of ultimate potential on a SBM the sub 4-inch bore becomes a limiting factor before anything else. 1.5hp per cube would make for one hell of a stout 390!
 
i have a 408 stroker in my dart,

all machine work, deck, align bore, .040, hot tank, etc
scat forged rotating assembly
forged pistons
complete built shortblock with all ARP hardware ran me $4000

now heads, $3000
intake, cam, roller rockers, and head bolts $2000
headers starter, waterpump, etc $2500

so for around the cost of a 408 475hp crate motor, i got my 408 built the way i wanted it with 550is hp
 
Piston speed, as noted above, is also limited by weight. The 390 slug is smaller in diameter, larger in length Then there's the underside which we can not see.

G Jones idea and 1.5 is a good illustrative number.

OH, also, the smaller bore .... While you could point out a few issues or draw backs to it, (And I'm not saying this company is great, but...) Chevy doesn't seem to mind so much. The engines still make power.

The valve shrouding is the only issue (And it isn't for me) to even concern yourself with. Other real issues that come with building a stroker should be addressed.
 
I spent 8000 for my 410 stroker , that included port work on the heads , a roller cam , air gap , holly 950 hp , msd billet dist , scat alloy crank , etc... It makes 519 hp thats 1.26 hp per cubic inch so I don't see making 1.5 that would be 630 hp in a 410 which is a bit much.
I know guys have made that and more but how long did that sb hold together ? I think not long especially the way I pound on em , As for the origional question I'll let you know next summer after I get my 390 built by the same guy who built my 410 and we can compare dyno sheets
 
cannucky said:
I spent 8000 for my 410 stroker , that included port work on the heads , a roller cam , air gap , holly 950 hp , msd billet dist , scat alloy crank , etc... It makes 519 hp thats 1.26 hp per cubic inch so I don't see making 1.5 that would be 630 hp in a 410 which is a bit much.
I know guys have made that and more but how long did that sb hold together ? I think not long especially the way I pound on em , As for the origional question I'll let you know next summer after I get my 390 built by the same guy who built my 410 and we can compare dyno sheets
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And what will this "390" be going in Chris?8-)
 
pettybludart said:
And what will this "390" be going in Chris?8-)
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It might go back where the block came from so the 410 can go somewhere ? got my eyes open for the right notch/ragtop roller ,I should have a complete spare drivetrain by then . I was going to do a stock rebuild but you know what they say "Power corrupts and absolute power corrupts absolutely " and I sir am absolutely corrupt
 
I think, the real difference would be less. You have offsetting details. The smaller bore is more friction due to smaller area of skirt to distribute, but less ring friction because there is less ring (shorter overall length). The hieghts and overall lengths are identical. So it's not a "taller/shorter" thing. Quench could be tighter as the smalelr bore will have less rock at TDC. The 318 piston is slightly lighter. The 408 has the larger bearing main surfaces. The 309 bore will shorud the valve. No denying that and that is huge in terms of a well thought out head/cam package. To me, the offsets when applied, should make the most "affecting" differences the lower friction of the 390 ring stack vs the more flow potential of the 408 bore. the bearing and cylinder wall friction bits offset each other. When it's all said and done, the PRICE will be the greatest difference. The engineering differences are so minor. If the debate was 390 vs 416 I think it would be much seperation between the two. 390/408 are virtually the same engine.
 
If it just a street car and you wanna burn some rubber every once in a while a 390 will get it done.
 
Rat Patrol said:
Thanks and lets hope that saves some "boat anchors" from an early grave :D
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I've retained my boat anchor and gone 390. Sure, I'd have taken the extra bore if I was tripping over 360s and 340s, but they're not exactly common round these parts. And now I'm more excited to see just how far I can push this thing with iron heads and pump gas.
 
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