5.9 magnum compression ratio

[Camtron]

Finally looking into a cam to finish up my 98’ 5.9 magnum I’ve been putting together.
Moved away from doing the full swap with injectors and 45re to just swapping the engine; found out I have a baby on the way. Just want the engine off my stand at this point and as much money left in my wallet as possible.
Was emailing with Dave over at, Hughes Engines and he informed me, Magnum blocks only have about 8.2:1 compression...is that really the case? I was expecting a little closer to 9.1:1, is the compression really that low on Magnum blocks?

 

[Aspen500]

I've seen numbers anywhere from 8.4:1 to 9.1:1 for the magnum engines. What the true spec is, no idea. Don't know if it's different depending on year or not either. Problem with advertised numbers is they aren't very accurate. Production tolerances might have an advertised 9:1 really be only 8.3:1 or something. I'd guess Hughes has measured the actual ratio(?)

 

[Duke5A]

Everything I've read puts it at 9.1:1. Seeing as the Magnum 5.9 was good for almost 70 horses more than the 360LA I would be inclined to believe it.

What cam are you looking at? I would keep it roller.

 

[Camtron]

Right, I was a little perplexed when He responded with that. I thought I was getting rid of a 8.2:1 compression ratio by ditching the LA engine.
this is what I’m leaning towards for a roller cam.
I like the guys over at Hughes Engines. They’re local to my state, ship quickly and always entertain my questions.

 

[BudW]

8.2 sounds more like a '80's LA compression ratio (if not lower).

 

[9secRR]

Hughes Engines is mopar only and been in business forever. i'm sure they know 5.9's very well.

 

[Duke5A]

That is a modest cam that will make for a motor with excellent street manors. I distinctly remember though that Magnum heads need machine work to run a cam with more lift that .520" or somewhere around there. I'm sure Hugh's knows all this.

 

[Aspen500]

I think the guides need machined down some so the retainers don't hit above a certain lift, is what I've been told.

 

[Camtron]

From the man himself
“ I would recommend cutting the guides and installing bronze guide liners“

- Dave Hughes

 

[Hayzoos]

My '87 FA should be 9.1 also. I thought '85 and later LA went roller and increased compression, but not necessarily any more power, maybe so in certain cases.

 

[AJ/FormS]

The 5.2 Magnum cam actually has less exhaust duration than the 318LA.
You can't get to this powerlevel with just EFI
Nor with just a 4bbl.
It takes a combination of things and compression is one of them. The thing about Compression is it multiplies all other changes so you always apply it last.
For instance, if you had a 5.2 engine that made 200 dyno hp and then upped the compression from say 7.8 to 8.8 , and say it made 8hp more.
Ok, but; say same 5.2 was making 225 hp; the same change in Scr might now get you 9 or 10 hp.
What compression ratio does NOT tell you, is what is going on at stall rpm or cruise rpm. More compression is always better at low rpm because you do not have to drive as deep into the carburator, increasing response and fuel-economy, as well as increasing Torque at all Part Throttle settings..
Here are two examples of a 360s with the only change being the Scr

Static compression ratio of ...................................7.8:1.
Effective stroke is 3.06 inches.
Your dynamic compression ratio is 6.81:1 .
Your dynamic cranking pressure is 132.22 PSI.
V/P (Volume to Pressure Index) is ..............................122


Static compression ratio of ...................................9.2:1.
Effective stroke is 3.06 inches.
Your dynamic compression ratio is 8.01:1 .
Your dynamic cranking pressure is 163.82 PSI.
V/P (Volume to Pressure Index) is ..............................151

Notice the huge increase in VP; Plus 23.8% .. 122 is the stocker at sealevel, you may know what that feels like. In comparison 151 is like as if you just changed the rear gear of the stocker from 2.76s to ~3.42s. You may know what that feels like. Nothing else was changed!


read about VP here
V/P Index Calculation

What disappoints me is that people cry about the cost of installing higher compression pistons, which totally magnifies the bottom end, right where a streeter spends maybe 90/95% of it's time;
yet are willing to drop money on everything else that mostly magnifies the top-end where you hardly ever go and when you do, yur only there for a few seconds atta time.
The only time you will ever be sorry you increased your Scr is if it leads to incurable detonation.
Whereas too many people show up on FABO saying how sucked out the bottom end of their 318 became after they bolted on big-port heads and installed the 340 cam. Well duh.................... you just dropped your Scr nearly 2 points!
When you go that route, your 318 will need BOTH a higher stall TC and a bigger-number rear gear.
For a Streeter, more Scr is always a win-win until the pressure produces more heat than the octane of the fuel can deal with.

Following is a low compression 318s; the first is factory stock, and the second is with a 340 top end and cam

Static compression ratio of ..........................7.8:1.
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 6.93:1 .
Your dynamic cranking pressure is .....................135.34 PSI.
V/P (Volume to Pressure Index) is....................... 114

Static compression ratio of ........................... 7.5:1.
Effective stroke is 2.57 inches.
Your dynamic compression ratio is 6.04:1 .
Your dynamic cranking pressure is ...................... 112.52 PSI.
V/P (Volume to Pressure Index) is ........................ 84

Notice the huge losses in both pressure and VP. At 84/114 this will feel like the gearing has been changed from 3.73s to 2.76s

Ok then, the 318 will make more power at 5000rpm with the 340 stuff, but where you actually drive, it will be a total dog until you re-stall and up-gear it

The point I'm trying to make is this;
for a street Engine the single most important thing IMO is to have decent working Dcr and VP numbers, and for the Combo, it is stall and gears.
Everything else is secondary.
For instance; I would rather have my Dcr adjusted to the max for pumpgas, than any other bolt-ons; AND would rather have a high stall with common gears than race gears with a stock stall.
Together, in a streeter, the high pressure and higher stall will produce the most fun.

 

[AJ/FormS]

Magnum blocks only have about 8.2:1 compression

I don't think that's right; the pistons are only down a short ways, and the heads are closed-chambers. to be down at 8.2 the total chamber volume would have to be; about;
(360/8 x 16.387)/(8.2 less1)=102.42cc, lol. Naw that'll never fly.
To be at 9.1, the total chamber volume maths out to (360/8 x 16.387)/(9.1 less1)=91.04, still pretty high.

IIRC the closed chamber Magnum heads are about 60cc; so with an 8.8cc gasket, the pistons would have to be ;
91.04 less (60 +8.8)=22.24; sounds close for a dish. If the piston was down .012( 2.5cc), that leaves 19.74cc for the dish.
So by the specs, 9.1 seems more plausible. I have never taken one apart, tho.....

But no matter, you can/ may have already corrected all that at the rebuild. Just keep in mind that the street engine only cares about Dcr, which includes the cams Ica or Intake closing point. Scr is just the tool we use to get to the required Dcr.
There are a few ways you can calculate your Scr:
If the heads are not yet on, you just measure all the pertinent parts and a lil math spits it out.
If the heads are on and the engine is not yet installed, here is what I have done;
I roll the engine over on the stand and put either #1 or #6, up as the high point, then adjusted the sparkplug hole to be the highest point. But you can use whatever plug is conveniently the highest.
Then crank the piston for that chosen cylinder to TDC (TDC-compression if the cam is in); try to be very accurate. Next
I get my trusty 100cc pig-sticker medical syringe, and fill it full of oil, to exactly 100cc. Next
I inject as much of that oil into the chosen cylinder as will fit, to the bottom of the plug hole. Be careful, as it approaches full, the level will rise very rapidly. Finally
I measure how much oil is left in the syringe and do the math, according to; 100 less what is still in the syringe. then
the formula is
Scr= (CV+cv)/cv, where
CV is the swept volume and cv is the amount you injected. The swept volume is that part of the cylinder that the piston sweeps thru as it travels up and down. Which is
(Bore/2)squared x pi x stroke x 16.387( the conversion factor to get cc)
pi is 3.1416
example at stock bore and stroke;
(4.00/2) squared x 3.1416 x 3.58 x 16.387 =737.214cc

If you measured 91cc, then
Scr = (737.2 + 91)/ 91 = 9.10

Here are some examples of various cams installed in a 9.1 Scr engine.
If the cam you install has an Ica of 60* then your Dcr will be
Static compression ratio of 9.1:1.
sealevel/ Ica of 60*
Effective stroke is 2.88 inches.
Your dynamic compression ratio is ...................7.52:1
Your dynamic cranking pressure is 150.79 PSI.
V/P (Volume to Pressure Index) is ................... 131

if the Ica is 56* then
Static compression ratio of 9.1:1.
sealevel/ Ica of 56*
Effective stroke is 2.97 inches.
Your dynamic compression ratio is .................... 7.72:1 .
Your dynamic cranking pressure is 156.09 PSI.
V/P (Volume to Pressure Index) is ..................... 140

If your Ica is 52* then
Static compression ratio of 9.1:1.
sealevel/ Ica of 52*
Effective stroke is 3.06 inches.
Your dynamic compression ratio is .................. 7.92:1 .
Your dynamic cranking pressure is 161.41 PSI.
V/P (Volume to Pressure Index) is..................... 149

If you can get a tight Q down to .040 or less,
AND your elevation is 800ft, then
Static compression ratio of 9.1:1.
800ft/ Ica of 48*
Effective stroke is 3.13 inches.
Your dynamic compression ratio is .................... 8.08:1 .
Your dynamic cranking pressure is ....................161.41
Your effective boost compression ratio, reflecting static c.r., cam timing,
altitude, and boost of PSI is ...............................7.92 :1.
V/P (Volume to Pressure Index) is ....................... 152

VP of 152 makes a firecracker street engine.
Pressure of 161 is pushing the limit for pumpgas, but with a tight-Q should not be a problem
Ica of 48* is pretty small, ending up at something like a 200@.050 or smaller.
Speaking of VP:
Anything over 140 with 3.55s and a manual trans is already fun. Same for an auto with 3.23s and a 2800TC.
By 152 you have a serious tire-fryer.
131 is ho-hum, could be fun with the right TC/Gear combo. It will likely cost you more for the TC and gears than hi-compression pistons installed. And the Hi-C pistons keep giving back forever.
Read about VP here
V/P Index Calculation

But if the engine is already installed you are kindof screwed.
You can get a rough idea by back-calculating from the Cranking cylinder pressure BUT
you will need to also know your Ica pretty accurately and your elevation. However, if your pressure is over 150/155 you're in pretty good shape

 

[Camtron]

AJ, you’ve posted quite a bit here. I’ll take the time to read through it when I’m off work; and then spend a day and a half cross referencing and trying to understand things better.
But, heads will still be stock aside from guides, springs and retainers, TC has a 2,400rpm stall and rear end has 3.23 gear set.

thanks for taking the time to write that all out, I do appreciate it.

 

[AJ/FormS]

In a street machine;The fixed stall and gears point to the engine needing to have it's Scr and Ica well matched. Else the lower rpms from stall to in the zone of 3000 to3500 will be somewhere between "somewhat disappointing" to very disappointing".
Within limits, the Scr needs to be "high", and the Scr needs to be "low". This is easiest to accomplish by targeting a very narrow Dcr window, respecting the final pressure, in accordance of the octane you are willing to pay for.
Setting up for 91gas is quite a bit different than for 87.
So, if the hard parts are already chosen and fixed, then the Scr window,will already be very small, whether it's 8/1 or 9 or 10, the window will be small, and for best results will point to a very specific Ica range of perhaps 3 degrees. If you miss it one way you may get into incurable detonation. If you miss it the other way, you get into a soft bottom end. Both are a disappointment, but detonation has to be eliminated at all costs.
The soft bottom end is only really soft at zero MPH. Getting less soft with rpm and in the window of 3000 to 3500 in a streeter, is no longer a big deal. Except that 3000/3500 "catch-up" window, with 3.23s, is from 26 to 30 mph, in 2.45 first gear. So if it's soft there, you can see how disappointing that is gonna be

 

[jasperjacko]

Hey AJ, Do you think I can run 11-1 compression with aluminum heads? on 91-93 octane?

 

[Duke5A]

Hey AJ, Do you think I can run 11-1 compression with aluminum heads? on 91-93 octane?

That is really a function of the cam. It's not the static compression ratio you have to worry about it - it's the dynamic ratio. SCR (static compression ratio) is simply a measurement of the total volume of the cylinder gets squeezed down to. DCR (dynamic compression ratio) actually measures your true compression as it factors in how much pressure is bleed off. Camshaft duration effects this. The more duration the greater the overlap, or a time when both the intake and exhaust valves are open. This bleeds off a bit of compression. It's DCR that determines how susceptible your are to detonation.

Alloy heads do allow you to run a bit more compression due to their ability to transfer heat. I've read they're good for a half a point of compression.

There are other factors in this like how aggressive your timing curve is and whatnot, but yeah, long winded answer. Need to know what cam you're running.

FWIW my motor is 10.5:1 with alloy heads and a 230@.050" cam. Timing is 20 degrees initial with 34 total all in at (I think) 2500 RPM. More than that and it starts to rattle.

 

[jasperjacko]

That is really a function of the cam. It's not the static compression ratio you have to worry about it - it's the dynamic ratio. SCR (static compression ratio) is simply a measurement of the total volume of the cylinder gets squeezed down to. DCR (dynamic compression ratio) actually measures your true compression as it factors in how much pressure is bleed off. Camshaft duration effects this. The more duration the greater the overlap, or a time when both the intake and exhaust valves are open. This bleeds off a bit of compression. It's DCR that determines how susceptible your are to detonation.

Alloy heads do allow you to run a bit more compression due to their ability to transfer heat. I've read they're good for a half a point of compression.

There are other factors in this like how aggressive your timing curve is and whatnot, but yeah, long winded answer. Need to know what cam you're running.

FWIW my motor is 10.5:1 with alloy heads and a 230@.050" cam. Timing is 20 degrees initial with 34 total all in at (I think) 2500 RPM. More than that and it starts to rattle.

Yeah I should have posted cam specs. I'll try to get them.

 

[jasperjacko]

cam specs duration a .050 260 int, 266 exh
lift .597 int, .582 exh. 108 lobe separation, 108 intake centerline.

 

[AJ/FormS]

What @Duke5A said,

I have run three cams in my OOTB Eddie-headed 367. With each of them I readjusted the Scr to achieve a pressure of 177 to 185.
The Q has varied between .028/.032/.040
My initial has always been 14 and the Power timing has been 32/34, all in at around 3200/3400
All three cams were fine with 87E10
The Scrs to get that pressure were;11.3/10.7/10.95

My experience is mostly limited to small-block streeters. So if you have more than a 367 or are NOT a streeter, than I cannot comment. Otherwise;

IMO, with .050specs of 260/268/110,in an alloy-headed beast,you are gonna need more than 11/1 to get the bottom-end up, and it will still burn 87E10 at 180 or less psi,lol. With allow heads, guys over on FABO are running 200 cranking-psi with alloys, they say, on pumpgas.
If you are running a hi-stall and race gears, in a streeter, then your bottom end is pretty much not important.
But I don't know and cannot imagine why, anyone would ever run a 260@.050 in an SBM on the street,lol....

 

[M_Body_Coupe]

cam specs duration a .050 260 int, 266 exh
lift .597 int, .582 exh. 108 lobe separation, 108 intake centerline.

Wow...that's a boat-load of duration @ 0.050"...11 static and 91-93 is nothing to worry about...I would say 11 is in fact about 1 to maybe 1.5 points too low?

I'm not sure what combo you're putting together, but do you have the matched components for a cam that big in a street motor?

Stroker? High stall converter? Matching gearing?, etc...