Is 9.7:1 compression on iron heads too much for 91 octane

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cawcislo said:
Looking to boost my compression and did some calculations with kb399 pistons. These domed pistons would put me at 9.75:1 static compression. Would this cause me detonation problems on 91 octane with open chamber iron heads? Current cam has intake closing @0.050 38 ABDC and at 400 feet above sea level which according to the Wallace calc gives me 9.04:1 dynamic compression. Let me know what you think.
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You can look at it two ways. You either have too much compression or you have input the wrong info.

Best of luck!
 
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Without quench domes or a wedge head and a tight quench he is fighting a loosing battle
if he is stuck with the parts he has put on a really thick gasket but hard to keep the Intake side of the chamber from lighting off
or hammering hard burn which requires retard
try e-85 or Water Injection till you get past the torque/ BMEP peak
 
I honestly don't see any issue with this, even without the quench/squish. Folks get all hung up on getting the ignition timing to 34* or 36* total, but just back off the total timing a bit. So you lose .1 sec in the 1/4 mile... no big deal if you are not on the strip running absolute times. The higher compression at low RPM's makes the engine a pleasure to drive around.

And, of course, the OP can put in the cam at 110 ICL and use the Felpro stock kit gaskets (8553PT) and his DCR will now be at 7.7, assuming his cam is something like a VooDoo grind; we don't know that exactly as he has never replied with the cam PN.
 
nm9stheham said:
I honestly don't see any issue with this, even without the quench/squish. Folks get all hung up on getting the ignition timing to 34* or 36* total, but just back off the total timing a bit. So you lose .1 sec in the 1/4 mile... no big deal if you are not on the strip running absolute times. The higher compression at low RPM's makes the engine a pleasure to drive around.

And, of course, the OP can put in the cam at 110 ICL and use the Felpro stock kit gaskets (8553PT) and his DCR will now be at 7.7, assuming his cam is something like a VooDoo grind; we don't know that exactly as he has never replied with the cam PN.
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He gave a closing of like 38 degrees.
He has a higher dynamic than I ran in my 410 and it pumped 184 psi cold.
I was 9.98.1 with a 8.8 dynamic.
 
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Yeah I know... but that 38* ICA is totally mistaken. That intake closing angle is taken from the .050" duration numbers.... which is not at all what you do to run a DCR or cranking compression numbers.

For something like a Voodoo grind, with a pretty quick closing and opening between .006" and .050" lifts, the .006" advertised duration works pretty well for DCR and crank compressions.... and those cams are typically around something 44 degrees longer duration than the .050" duration number for that class of cam grind. So that intake closing angle changes is not 38* but actually 60* degrees... which gives the real DCR numbers, which are down in the 8.1 to 7.7 number range that I listed in the posts above. (I ran all the OP's dimensions, volumes, to get to the SCR and DCR numbers, etc. So honestly not just guessin'......)

For most of our street-ish builds in this type of engine size and the typical cams that go with them, the DCR runs around 2.0 + or - below the SCR. If it is way off of that number, then either the cam is odd, or the stroke is reeeealy weird or something is a mistake, like in this original post where the .050" duration was incorrectly used for DCR.
 
I just went by what he said. If people give fkd up information they'll get a fkd up answer.
I ran it based on his numbers and 68cc 675 heads.
9.4 static
8.7 180psi
If 60 close, it would be fine.
Funny how approach is everything.
 
he might not have problems but he would be leavng torque on the table
let's start over with good data
 
I dunno if this is the correct way of lowering your CR by using thick head gasket .54 felpro head gasket? or install a more aggressive cam?
 
Slantsix64 said:
I dunno if this is the correct way of lowering your CR by using thick head gasket .54 felpro head gasket? or install a more aggressive cam?
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Sure nuff. Other than changing heads or pistons this is the simplest way. Many use thin gaskets to raise compression. Thicker ones can lower it.
 
toolmanmike said:
Sure nuff. Other than changing heads or pistons this is the simplest way. Many use thin gaskets to raise compression. Thicker ones can lower it.
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I always have problems raising compression lol never lowering. what about retarding the camshaft couple degrees.
 
I think cawcislo will be fine. Note - post #3 - pistons are up .018", enter your data - deck = 0
 
cawcislo said:
Looking to boost my compression and did some calculations with kb399 pistons. These domed pistons would put me at 9.75:1 static compression. Would this cause me detonation problems on 91 octane with open chamber iron heads? Current cam has intake closing @0.050 38 ABDC and at 400 feet above sea level which according to the Wallace calc gives me 9.04:1 dynamic compression. Let me know what you think.
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This is a test right;
this is what I get.
from your 38*@.050, I generate a compression duration of 142*
From that, I back calculate the intake duration at 218*. I assume an LSA of 110* And I assume a split of 6* so the exhaust comes to 224*.
This brings us to a common cam size of 218/224/110.
Adding the typical acceleration ramps of 46*, this gives us advertised specs of 264/270/110
installing that at 109* gives us an Ica of 61*. And so
Mr. Wallace says;
Static compression ratio of 9.75:1.
Ica of 61*/400ft elevation
Effective stroke is 2.64 inches.
Your dynamic compression ratio is 7.97:1 .
Your dynamic cranking pressure is 160.61 PSI.
V/P (Volume to Pressure Index) is 123

That will make a heckuva 318 running 91 under power, and 87 while cruising.

With a 904 automatic, I see a 2400TC with 3.73s down to 3.23s with a 2800.
With 47* of overlap, I see headers and hi-flo duals.
I see a dual plane and personally, I'd like a 650DP, but anything over 450 is actually adequate.

I installed it at 109 because with a VP of 123, and a 2800TC, she will have plenty of low-rpm grunt, and the 3.23s will make awesome fuel mileage at 65=2600@ zero-slip. Therefore, 109 versus 106 is an extra 100 rpm on the top. If yur lucky, the power will peak at 5000. With just a lil work on the heads, you may be able to pull a 5600rpm shift, which is what the 904 wants.

As already said; there is no good reason to try to run max timing at idle; that is just nuts. Even if you are 4* short, the power loss does not come until after ~3500, and it is minimal. IDK maybe 7hp at 5000.
But there is nothing stopping you from engineering a a two-stage timing curve, being conservative at idle (cuz of the 2800TC, building fast to something like 28*@2800, and then slowing to 34* at 3600. That is easily doable with the 2800TC.

I would build this combo in a heartbeat, because;
The Wallace doesn't know that at the advertised Ica of 61* , the intake is still open at rocker ratio times advertized spec; say 1.5x.008=.012! So compression doesn't actually start at 61*ABDC, but might not start until as much as 15 degrees later, on a particularly slow ramped cam !

Of course; garbage in/garbage out, and so if I made wrong assumptions then you get wrong answers,lol.
 
AJ, you're running the numbers in the same way as I am, and getting the same range of results. I was figuring about a 270 advertised duration and a similar ICA. And putting in the cam at 109-110 ICL and controlling the ignition timing as suggested will make this 318 a heckuva lot better all around than anything that can be done with the run-of-the-mill lower compression stock replacement 318 pistons.

OP is MIA, so this all may be moot.... and he still ought to balance the thing as these pistons are lighter than stock, by enough to put the bobweight well outside even the broad factory tolerance.
 
cawcislo said:
Looking to boost my compression and did some calculations with kb399 pistons. These domed pistons would put me at 9.75:1 static compression. Would this cause me detonation problems on 91 octane with open chamber iron heads? Current cam has intake closing @0.050 38 ABDC and at 400 feet above sea level which according to the Wallace calc gives me 9.04:1 dynamic compression. Let me know what you think.
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You have quite a few responses from very knowledgeable people here, many well versed in engine building and that actually measure things. If you haven’t done as they do to determine actual static you are left to doing math based on specs given for all the components you are incorporating. It can be done, IMO you need to play it safe with cam timing and the dynamic numbers they yield. The cam you list looks to be very small, not sure what size engine we’re talking here? While many may think every engine build must be a maximized and precisely blueprinted deal or else forget it....if you do your homework on everything, and error on the conservative side vs pushing things to “living on the edge“ you’ll be alright. Run premium and dial in the carb and ignition, obsess on it, and you’ll be good. That’s my take.
 
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Just for some perspective, first full engine I built from the block up was a 360 with Magnum heads and KB flat tops. Static CR came out to 10.42:1. It should have ran fine but it pinged like crazy in warm weather on 91-octane; problem was I was stupid and reused the small Voodoo cam I had in a previous 318. The 256/262 adv. duration version that's intended to boost cylinder pressure in smaller low-compression engines, the OPPOSITE of what I needed. However even with all the pinging it still lasted about 25,000 miles and I beat the hell out of it, made maybe 9 passes down the drag strip and lots of screwing around in "Mexico".

If I had simply changed the camshaft for one that was designed for my engine combo I'm confident I could have stopped the pinging and that engine would still be running today. Live and learn...

Now I'm running a stock junkyard 5.9L Magnum short block (stock pistons etc.) with open-chamber Edelbrocks so my static compression is just about 9.0:1. It has noticeably less bottom-end grunt than my 10.5:1 engine but still a lot more than I expected (it has no problem pushing my Duster from a dead stop with 2.94 gears and a cheapo 2600-rpm stall converter). This time I went all the way and called Jim at Racer Brown for a custom-ground hydraulic roller cam for my exact setup. It runs like a champ, screams to 6000+ RPM and has great street manners, also runs fine on regular pump gas (85-octane here at high altitude) which is a nice bonus. The best part is the heads came with ARP head studs which I installed with Cometic MLS head gaskets so whenever I feel like it it's ready to take some boost.
 
Marcohotrod said:
I think cawcislo will be fine. Note - post #3 - pistons are up .018", enter your data - deck = 0
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Right, I have KB 243’s which have deep valve reliefs. The pistons ended up .018” over the deck, and the published data is just the reliefs. The calculator doesn’t deal with the over the deck piston and the valve relief volume correctly. So the 7.3 is the displacement of the piston that’s above the deck including the valve reliefs. So the deck height of 0 and the 7.3cc displacement tells the calculator what it needs to run the math right.
 
AJ/FormS said:
Hyup I think a h-compression 318 with a 218 cam would be a real sweet deal. I would build it just for the timing challenge!,lol
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Spoke to a guy at our local track with a beautifully restored valiant sedan 318 with 11-1 comp a comp cam with 216 duration @ 0.050 Airgap 650 DP and headers otherwise looked factory stock. Ran 12.7's @ 105 from memory on really small tyres too. He also said he had to tune around the 11-1 comp.
 
Thanks for all the replies. I think the domed pistons may cause me too much grief and limit my options so I’m going to stick with the kB199 flat tops for my 318. The flat tops will also allow me to run a closed chamber head should I so desire.
 
Hi Slant
he has no quench so he could go to thicker gaskets without changing his rev band
or wider LCA
or
 
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