Higher Compression vs. Timing Advance - Team Camaro Tech
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post #1 of 19 (permalink) Old Jan 20th, 09, 05:52 PM Thread Starter
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Steven
 
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Higher Compression vs. Timing Advance

I am trying to get some feedback on your theories of what makes more power on 91 octane: running a greater compression ratio with less timing advance, or vice versa.

I'm currently in talks with my engine builder who is building a 355 for me. The have told me they prefer not to run near the edge of compression, and often make more power with 0.5 to 1 point less compression, and make up for it with greater ignition advance. I understand dynamic compression well and would like my engine build to be in the 8 to 8.5 range with good quench, but I would like to hear some feedback on what you think.

Here is my potential combo, also seen here: https://www.camaros.net/forums/showthread.php?t=148886

They are talking about 9.5:1, when I was thinking 10.25:1 which would give me a 8.2:1 DCR

355ci
New rotating assembly, flat top forged pistons 4 valve reliefs
200cc 64cc aluminum Pro Topline Heads
Victor Jr. or RPM Air Gap
700 or 750 double pumper
XR280R solid roller: 242int, 248exh, 110 or 108 LCA

Tremec TKO 5speed 3.73 gears

Thank you,
Steven
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post #2 of 19 (permalink) Old Jan 20th, 09, 06:01 PM
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Re: Higher Compression vs. Timing Advance

i thought timing had more to do with chamber/piston design than compression ratio. then again i dont build motors for a living either.
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post #3 of 19 (permalink) Old Jan 20th, 09, 09:10 PM
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Re: Higher Compression vs. Timing Advance

I've never been a fan of high ignition timing to make up for low cylinder pressure caused by too much overlap. I'd widen up the lobe seperation to 114 or so to get the cylinder pressure back up. Your choice of cylinder head (aluminum) will like a little more timing, however. What I don't like are the large runners on a 350ci engine, your port velocity will leave a lot to be desired and the car will be a bit doggy down low.

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post #4 of 19 (permalink) Old Jan 20th, 09, 10:35 PM Thread Starter
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Re: Higher Compression vs. Timing Advance

Quote:
Originally Posted by SRGN View Post
I've never been a fan of high ignition timing to make up for low cylinder pressure caused by too much overlap. I'd widen up the lobe seperation to 114 or so to get the cylinder pressure back up. Your choice of cylinder head (aluminum) will like a little more timing, however. What I don't like are the large runners on a 350ci engine, your port velocity will leave a lot to be desired and the car will be a bit doggy down low.
They were talking about ordering the cam on a 112, which would help a bit, but I admit that I am a fan of a rougher idle, and top end power over off-idle torque. The manual transmission helps quite a bit. I just don't want bandaid fixes and it does sound like timing and lobe separation are bandaids.

I'm not sure that I would agree that 200cc runners are too big for this engine with this camshaft and my steep gearing. I envision this engine as really 2500rpm on up. The cam should make peak torque around 4800, with peak power around 6500. I think a set of 180cc heads would really stifle the engine towards the upper rpms.
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post #5 of 19 (permalink) Old Jan 21st, 09, 08:46 AM
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Re: Higher Compression vs. Timing Advance

Is this going to be a dedicated track engine or street/strip? If you are going street/strip or mostly street, I think the 200cc runners are a bit large. I would also run a dual plane intake. You say you have a new rotating assembly but you didn't say whether it was forged or cast. I think that would be one of the deciding factors on what speed you want to spin your engine too.
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post #6 of 19 (permalink) Old Jan 21st, 09, 10:18 AM
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Re: Higher Compression vs. Timing Advance

I think you will run into street performance problems trying to run 8.2 dynamic compression on 91 oct fuel and you will be pulling out advance to keep your mill alive. An 8.2:1 dcr is pushing 91 pump gas and everything will need to be perfect to get the most out of it. Hot weather, gearing, weight of the car etc will be a factor. On flat ground in 75 deg weather with 3:73 rear in 1:1 or lower at the trans the engine may run fine but crank up the heat to 95 or higher, and drive up and down hills and mountain hwys in overdrive and she'll be pinging for sure.

My 9.87:1 iron head 383 has a dcr of 7.82:1 with a .036 quench and 34 degs total timing seems to handle everything I throw at it fairly well. When I switched from 3.73:1 rear gears to 3.42:1's on really hot days I have to keep it out of OD until the car is going more than 50 mph or it will just start to ping if I step on it without downshifting. With the lower gears I could be in OD at 40 and step on it without a problem... Check this out for some good info.

http://members.uia.net/pkelley2/DynamicCR.html

...Dennis

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post #7 of 19 (permalink) Old Jan 21st, 09, 11:02 AM
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Re: Higher Compression vs. Timing Advance

Quote:
Originally Posted by SRGN View Post
I've never been a fan of high ignition timing to make up for low cylinder pressure caused by too much overlap. I'd widen up the lobe seperation to 114 or so to get the cylinder pressure back up. Your choice of cylinder head (aluminum) will like a little more timing, however. What I don't like are the large runners on a 350ci engine, your port velocity will leave a lot to be desired and the car will be a bit doggy down low.

I read that over three times, and have to step in,

"I'd widen up the lobe seperation to 114 or so to get the cylinder pressure back up. "

lets look at that a bit, its a very comon misconception

http://forum.grumpysperformance.com/...0&p=1697#p1697

btw generally the higher the compression the more torque youll produce provided you don,t get into detonation, and theres other factors besides static compression alone that effect that
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post #8 of 19 (permalink) Old Jan 21st, 09, 02:30 PM
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Re: Higher Compression vs. Timing Advance

I seriously think you would benefit from just running 11.5:1-12:1 with that cam instead of running more timing.
I really never have heard of an engine builder making a set up like that and planning on running higher ignition timing.
That cam would want 18*+ at idle, how much more do you want to run?
Why not do higher compression and run 91 octane, it won't detonate.
I have run 85 octane in a 11:1 333 with a cc286 just to put the "it'll rattle unless you have 7.8:1 compression!!!" talk in the grounds.

The only Mustang I'd ever own is a Fender.

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post #9 of 19 (permalink) Old Jan 21st, 09, 04:43 PM
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Re: Higher Compression vs. Timing Advance

Quote:
Originally Posted by grumpyvette View Post
I read that over three times, and have to step in,

"I'd widen up the lobe seperation to 114 or so to get the cylinder pressure back up. "

lets look at that a bit, its a very comon misconception

http://forum.grumpysperformance.com/...0&p=1697#p1697

btw generally the higher the compression the more torque youll produce provided you don,t get into detonation, and theres other factors besides static compression alone that effect that
Link doesn't work, says I need to register. By reducing the overlap with a wider LSA, you bleed off less cylinder pressure. If you bleed off less, you thereby increase it over a narrower LSA cam.

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post #10 of 19 (permalink) Old Jan 21st, 09, 06:49 PM
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Re: Higher Compression vs. Timing Advance

youve got it reversed

LOOK heres TWO cams IDENTICAL EXCEPT FOR THE LSA,the tighter LSA of the 110921 builds a bit more cylinder pressure and results in slightly more torque over a NARROWER rpm band so its better with a manual trasmission, the 114681 with its wider LSA tends to work better with an auto trans with its wider torque band but very slightly lower peak torque, the 110921 has more overlap and better scavaging in the mid rpm band, but it idles rougher at low rpms and that overlap doesn,t help if you use nitrous
COMPARE the TIMING
tight LSA

http://www.cranecams.com/index.php?s...21&lvl=2&prt=5

wide LSA

http://www.cranecams.com/index.php?s...81&lvl=2&prt=5


heres a chart (BELOW) showing degrees of rotation and piston possition, as you can see the valve on the wider LSA closes 5 degrees later on the wider LSA and the piston can,t compress ANYTHING untill both valves are closed, the tighter LSA alows the piston to compress almost a tenth of an inch more cylinder voluum, youll find the tighter lca usually makes more low and mid rpm power , and in many cases more peak power but has a slightly roughter idle and the power peaks faster in the rpm curvewere not thalking a huge change, maybe 7-8 hp/ft lbs but the tighter lca tends to lope more, idles roughter and be more responcive, keep in mind that if you had sellected a cam with 5-7 degrees more durration on the wider lca the overlap would be similar, but the effective compression would be even worse

http://www.iskycams.com/ART/techinfo/ncrank1.pdf
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post #11 of 19 (permalink) Old Jan 21st, 09, 07:04 PM
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Re: Higher Compression vs. Timing Advance

you pick LSA after you know what head flow curve is [email protected] good low lift wide LSA. not so good ports @ narrow LSA. change ISL. +/- 5* to adjust DCR.
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post #12 of 19 (permalink) Old Jan 21st, 09, 07:17 PM
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Re: Higher Compression vs. Timing Advance

you cannot make more power with more ignition advance .it is what it is. keep your quench under .050 with aluminum pro topline @ good chamber DCR.8.25 or less you will be fine IMO. good luck DOUG.
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post #13 of 19 (permalink) Old Jan 21st, 09, 08:23 PM
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Re: Higher Compression vs. Timing Advance

Maybe it should be phrased as "reasonable compression for pump fuel with optimum timing vs. higher compression with reduced timing to stop detonation". This is often the case when one goes too far with the compression & then tries to pull timing so it can be driven on pump fuel.

In this case the motor with the correct timing will do better.
Pulling timing will quickly take away the power that the compression adds.

Build the engine for a reasonable static & dynamic compression that allows optimum timing on the fuel you intend to use without detonation.
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post #14 of 19 (permalink) Old Jan 21st, 09, 10:11 PM
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Re: Higher Compression vs. Timing Advance

Quote:
Originally Posted by SRGN View Post
Link doesn't work, says I need to register. By reducing the overlap with a wider LSA, you bleed off less cylinder pressure. If you bleed off less, you thereby increase it over a narrower LSA cam.
Tighter LSA builds cylinder pressure by closing the intake valve early. The compression stroke begins when the intake valve closes. My motor used to have 11:1 SCR with a 264/274 @.050" .630" lift 106LSA cam. It would produce 215-230 PSI cold. Now it has a little over 13:1 SCR and the cam is 268/274 @ .050" .630" lift 112LSA. It now produces 195-205 PSI cold.

The SCR was increased by more than 2 full points but cylinder pressure went down because the wider LSA delays the beginning of the compression stroke.

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post #15 of 19 (permalink) Old Jan 21st, 09, 10:22 PM
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Re: Higher Compression vs. Timing Advance

ICL (intake centerline) determines the intake closing point, and therefore the dcr
LSA has nothing to do with it, unless the icl changes too.

There is an awful lot to this discussion.

Example: my previous combo, calc at 7.9 dcr, would ping under a high load, exactly as dennis described above, if I used 89 octane gas.

My current combo, with 8.5 dcr, does not ping ever, not with any gas, at any rpm, or any load. never. The diff: alluminum heads for one, but the biggie is the electronics, full computer control of fuel and spark advance. The ability to use a computer to truly optimize your timing is unreal.
And no, I'm not compromising. 26 degrees idling, 34 total at WOT, 52 at cruise.
It's just that I can control what's happening between those datapoints.





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