My potential compression ratio

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octanejunkie

octanejunkie

Mopar Padawan
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OK so after much-ado about quench and what not, here is the workup I did on kb-silvolite.com using a $180 worth of Cometic head gaskets, I guess felpro's .040 compressed thickness will work too and lower the CR a wee bit with it's larger bore diameter
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360CRCalc.jpg

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I'm obviously going for optimal quench so the SCR doesn't bother me too much, but the DCR seems high... commensurately so using the XE268H's IVC of 38 plus 15 degrees

I estimated my chamber volume for stock magnum 360 heads cause I haven't CC them yet, appreciate any input here
 
FYI, my magnum heads cc'd at 64. Also, the bore of the Felpro head gasket is 4.150".
 
Now that I have CC'd my heads and got 62.5cc for the average chamber volume, I plug that into the handy CR calc at kb-silvolite.com and I get
10.852 SCR and 9.355 DCR
This is using the 53 ICV (38 +15) of the XE268H cam, zero deck height and an .040 x 4.060 gasket in my .030 over 360 block with 5cc pistons and stock rods

Seems the only way to reign back my DCR would be to go to a cam that has the intake valve closing later at like 55 degrees... not an easy or practical find - but having a dynamic compression over 9.3 seems scary and somewhat detonation prone...

Dave, pls remind me the part number of the Fel-Pro gasket you used, I came up with this mopar performance gasket looking at summit
 
octane,
those numbers seem really high to me as well. go on line and find "pat kellys" DCR calculator. and confirm those numbers with that.
It requires more input but seems to be very reliable.
If I remember correctly, A DCR in the 9s puts you at or above 93 octane.
Andrew
 
blue missile said:
octane,
those numbers seem really high to me as well. go on line and find "pat kellys" DCR calculator. and confirm those numbers with that.
It requires more input but seems to be very reliable.
If I remember correctly, A DCR in the 9s puts you at or above 93 octane.
Andrew
Click to expand...

Following Pat Kelly's article and CR calc found here I get confirmation that my DCR will be up the 9s, according to Kelly's calc, 8.93479 to be exact

Best available fuel around here is 91 octane

I either have to scrap the idea of having optimal quench or go to a different piston or live with a detonation-prone engine - none of these options excite me greatly
 
Static ratio is fine. The trick is finding the cam with the closing event you need to get the dynamic down to no more than 8.5:1 and fit your Magnums' lift limitations.
That might be worth talking to Scott Brown or a custom grinder. (rather than talking to Comp's tech guys who can be ignorant of the details)
 
Octane,

According to the gasoline refiners association the numbers are as follows:
Based on real or “dynamic” compression ratios 7:1req 87 octane, 8:1 req 92 octane, 9:1 req 96 octane. These numbers are based on an engine with a carb and no engine management system.
Based on that, extrapolated numbers are as follows:
7.0:1=87 octane
7.1=87.5
7.2=88
7.3=88.5
7.4=89
7.5=89.5
7.6=90
7.7=90.5
7.8=91
7.9=91.5
8.0=92
8.1=92.4
8.2=92.8
8.3=93.2
We happen to have 93 octane here in Texas but to be safe 92 is offered nation wide.
I think your best bet is to revisit the cam, as others have said the timing events are the easiest way to manipulate the DCR. With 92 octane gas 8.0 will be fine. If you are close to the edge consider some thermal management with coatings, that can be self applied to the valve faces and the combustion chamber.
Andrew
 
moper said:
Static ratio is fine. The trick is finding the cam with the closing event you need to get the dynamic down to no more than 8.5:1 and fit your Magnums' lift limitations.
That might be worth talking to Scott Brown or a custom grinder. (rather than talking to Comp's tech guys who can be ignorant of the details)
Click to expand...

Agreed, planning on this...

blue missile said:
Octane,

According to the gasoline refiners association the numbers are as follows:
Based on real or “dynamic” compression ratios 7:1req 87 octane, 8:1 req 92 octane, 9:1 req 96 octane. These numbers are based on an engine with a carb and no engine management system.
Based on that, extrapolated numbers are as follows:
7.0:1=87 octane
7.1=87.5
7.2=88
7.3=88.5
7.4=89
7.5=89.5
7.6=90
7.7=90.5
7.8=91
7.9=91.5
8.0=92
8.1=92.4
8.2=92.8
8.3=93.2
We happen to have 93 octane here in Texas but to be safe 92 is offered nation wide.
I think your best bet is to revisit the cam, as others have said the timing events are the easiest way to manipulate the DCR. With 92 octane gas 8.0 will be fine. If you are close to the edge consider some thermal management with coatings, that can be self applied to the valve faces and the combustion chamber.
Andrew
Click to expand...

Awesome info Andrew, thanks. And it does look like there might be a custom cam in my future...
 
FWIW, I just noticed you are using the Comp XE268H cam. My 360 has KB107 pistons at zero deck, 64 cc magnum heads and the FelPro 1008 head gasket with the XE268H cam. I plugged in the numbers into the KB calculator and came up with a dynamic compression ratio of 9.16:1. The actual cranking pressure measured with a compression tester is 185-195 psi.

I run 33 degrees of total timing (magnum heads require less than open chamber LA heads) with it all in at 2500 rpm.

Anway, I run it on 89 Octane and have in a pinch run it on 87. I have to be careful putting a load on the engine below 3000 rpm or I can just detect a little detonation with 87.

The magnum heads with a proper quench and the high swirl ports makes for a very detonation resistant engine. I would think you will be fine with 91.
 
Octane,
do me a favor and plug these numbers into the calculator:
comp cams XE275HL-10
110*,
276-288Adv
232-238 @.050
.523"-.525" w/1.5:1 lifters

SS= IVO@32*, IVC@64*
EVO@78*, EVC@30*


Comp Cams XE285HL-10,
110*
285-297Adv
[email protected]
SS= IVO@37*IVC@69*
EVO@83*EVC@35*
.545"-.545" w/1.5:1 lifters

See if that doesnt help the situation, and give you some more power.
I myself will be using the 285 with 1.6:1 lifters for a final lift of .581"


Andrew
 
blue missile said:
Octane,
do me a favor and plug these numbers into the calculator:
comp cams XE275HL-10
110*,
276-288Adv
232-238 @.050
.523"-.525" w/1.5:1 lifters

SS= IVO@32*, IVC@64*
EVO@78*, EVC@30*


Comp Cams XE285HL-10,
110*
285-297Adv
[email protected]
SS= IVO@37*IVC@69*
EVO@83*EVC@35*
.545"-.545" w/1.5:1 lifters

See if that doesnt help the situation, and give you some more power.
I myself will be using the 285 with 1.6:1 lifters for a final lift of .581"


Andrew
Click to expand...

I'd be happy to but I need valve timing at .050 rather than S2S

Also, for me using the stock magnum heads, I am looking to keep my lift around .500

Finishing my calls to the cam companies and will post their recommendations
 
So I have been calling around to get custom grinds to lower my dynamic compression, looking for the magic intake valve closing point.

Here is what I got so far for my efforts

Isky Racing Cams
They recommended their 280-Mega cam, an off the shelf grind
280 adv dur
232 dur @ .050
.485 lift w 1.5s
.517 lift w 1.6s
108 Lobe center

Lunati
They also recommended an off the shelf cam, their 30402 grind
228/235 dur @ .050
.495 INT lift w 1.6
IVO @ 8, IVC @ 40 (.050)
EVO @ 51.5, EVC @ 3.5 (.050)
110 LC

Howards Cams
They recommended a hyd flat tappet cam, custom ground on any LC I want.
230 dur at .050
.496 INT lift w 1.6
.504 EXH lift w 1.6
110 LC would be my choice
IVO @ 9, IVC @ 41
EVO @ 51, EVC @ 3
12* overlap

Comp Cams
Spoke with technician at length, got 3 cams recommended, 1 Hyd Roller and 2 hyd flat tappets

Hyd Roller
230/236 dur @ .050
.480 INT lift w 1.6
.490 EXH lift w 1.6
IVO @ 5, IVC @ 45 (.050)
EVO @ 48, EVC @ 8 (.050)
On any LC I want from 106-112, I would choose 110
Overlap of 13*

Hyd Flat Tappet #1
-flat tappet version of the Hyd Roller Cam above

Hyd Flat Tappet #2
236/240 @ .050
.480/.480 lift w 1.6
IVO @ 8, IVC @ 48 (.050)
EVO @ 50, EVC @ 10 (.050)
On any LC I want from 106-112, I would choose 110
Overlap of 18*

Still waiting to get info from Elgin and am willing to get more grinds from other mfgrs if anyone can recommend a few

I think I am actually leaning towards the Comp HFT#2 at this point but I am not at the point of making any decision, but I will take any opinions offered
 
blue missile said:
Octane,
do me a favor and plug these numbers into the calculator
Click to expand...

Andrew - using Pat Kelly's calc, which requires SS timing, I come up with the following using my specs and the IVC values of the cams you listed:

Comp Cams XE275HL-10 = 8.687 DCR

Comp Cams XE285HL-10 = 8.340 DCR
 
Octane, Why are you so hung up on getting the DCR down into the mid 8's when myself and others that have 9+ are getting by on 89 octane?

Don't underestimate the benifits of a closed chamber with a tight quench. If you really feel the need to lower the compression ratio you can without losing the quench. KB has step dish pistons that allow the tight quench under the closed portion of the head and have a dish under the chamber. These will get you static compression down into the mid 9's
 
dgc333 said:
Octane, Why are you so hung up on getting the DCR down into the mid 8's when myself and others that have 9+ are getting by on 89 octane?

Don't underestimate the benifits of a closed chamber with a tight quench. If you really feel the need to lower the compression ratio you can without losing the quench. KB has step dish pistons that allow the tight quench under the closed portion of the head and have a dish under the chamber. These will get you static compression down into the mid 9's
Click to expand...

Truthfully Dave, I am not sure if I am making a bigger deal out of this than it should be... I am almost willing to buy the same cam as you are running and run it and see what happens, I can always pull it out if it doesn't behave. I already have the pistons mounted to the rods and swapping those would be more expensive. The only extra machine work I am planning on is squaring my decks to zero, and only if required, otherwise I am almost ready to start mocking up my build.

I take solace in hearing from guys like you attesting to their success using the same formula I am looking at, but I am getting hung up on the numbers game. Honestly I don't know first hand what quench alone will net me when other indicators are in the "hazardous" zone - it is my lack of experience and anal retentive, academic nature to over analyize before plunging into the unknown - but it's only a $200 sacrifice if it doesn't work out, I've wasted more than that on less

Perhaps I should just drink the koolaide and let this subject rest
 
Just one more wrench, maybe I missed it, what is the elevation you drive at? Make a diff too. Above 5000 you can most times get buy with 2 less points of octane rating per calcs figures.
 
I'vwe always found it safer to err on the side of lower than higher dynamic because I never know what altitude or the quality of fuel available where-ever the engine ends up. I do think many people overthink a lot of things...lol. But there's making a decision, and hemming and hawwing and never making a choice. Overthinking is only overthinking when you can't make a choice.
 
Moper,
I feel that in the long run I have an advantage in that I am building the power plant last.
I can not tell you how many cams, "new and improved" I have plugged into the engine dyno, including all sorts of roller cams, and nothing happens to fit my combination like the 285. Now the 295 gives me more power but the power band climbs up into territory im not interested in going to.
I also have to agree with DGC333, in that chart is pretty much a worst case scenario put together by people who are not taking into account such things as quench and coatings which we have access to today.Its just a baseline reference when talking about true or Dynamic Compression. It also takes no account of altitude adjustments.
Andrew
 
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