This winter, I am planning on building a 408 stroker using a 4.000" stroke, 6.125" rods, and Mahle 1.105" CH Pistons with a 12 cc dish. I will zero deck the block for a deck height of 9.230." I plan to use a .040" head gasket to optimize the squish/quench aspects. So far, so go. Here's my concern. With my 71/72 cc combustion chamber LQ4 heads, all this will yield about a 10.0:1 static CR and I want to continue running 87 Octane. As I see it, I have three choices:

1. Retard the timing a bit to avoid knock.
2. Enlarge the combustion chambers to about 76 cc's.
3. run a judiciously large and thick head gasket.

My thought is to do #1. Do you think that this is the best option?
Do you have other thoughts?

Thanks in advance,

Steve

Note: this is a daily driver weighing 6,900 pounds and I tow an 8,600 pound trailer so going with higher octane fuel is not a good option considering my gas mileage.

 

If you want to build a motor to use 87 octane, then build it for low octane.

You should start with a lower compression piston and leave the timing and
quench height to 0.040".

Pulling timing to satisfy knock is just counter productive for power and fuel
economy.

Adding quench height will reduce the efficiency of the motor.

That's my view anyway.

 

If I could find some shelf stock pistons that are about a 19 cc dish, that's what I would do. However, I've not found anything in a coated stock piston in that range. I could go to custom pistons, but that gets expensive.

Steve

 

Compression is your friend, and finding the fine line so you can tow with 87 octane is important.

Why not have a custom cam designed for your requirements? The designer will take into account the Dynamic Compression Ratio which 87 can live with, and perhaps use the Static Comp. Ratio (SCR) you are planning for.

My $.02.

 

I'm with OSS! I can't be bothered noodling out the trig formula, but my graphic solution says if you can run a cam with an actual (0.006" or 'advertised', not 0.050"), closing point around 73 degrees ABDC, your DCR will be around 7.5:1 and that's plenty low enough for 87 Octane. This might even be an off the shelf cam, possibly installed "straight up" or even a degree or two retarded, rather than the common 4 degrees advanced.

Other steps: Headers and a good exhaust, cold intake air, a lower temperature thermostat and a top notch cooling system. By the way, optimum power at a given octane is found with a CR high enough to require a few degrees retard from best power spark advance. (In other words, if say 30 degrees advance was best for power, but you had to either drop the CR to 9:1 to use 30 or back off the spark to 25 to use 10:1, you'd get better power (and significantly better economy) with the latter set up.

 

Mad Bill,
Where are you coming up with a dynamic compression ratio of 7.5:1 related to an IVC of 73 degrees ABDC? You have my cutiousity up.

Basically, this brings us back to my option number 1.

I believe that cam durations, intake and exhaust, as well as valve timing events (includeing lobe separation angle) should be chosen based on the RPM range where you want torque and HP to be optomized, whether you want to favor HP versus torque and whether you favor peaky power/torque versus a more broad band power range.

However, I'm interested in this business of dynamic CR. Tell me more.

Steve

 

Well, it's a little much to cover in full detail here, but basically, the concept is that compression cannot begin until the intake valve closes. The later that happens, the lower the DCR. (Two stroke compression ratio has been calculated this way for ever, based on the stroke remaining when the exhaust port is sealed off by the rising piston) David Vizard and others say that 7.5:1 DCR is a safe number for a regular street machine, although fast burn chambers, aluminium heads, etc. can raise it to 8:1 or so.

While there are formulas to predict it, in your case I just did a schematic of your engine, but first I determined the cylinder volume needed to give a 7.5:1 CR: I calculated your total Combustion Volume from the head, gasket and piston dish ccs (5.65 c.i.) and verified that it gave ~10:1 CR on a 408 c.i. engine: 408/8 = 51.0, (51.0 +5.65)/5.65 = 10.02. Then I calculated displacement to give 7.5:1 CR with your CV as 5.65 x 6.5 = 36.725 c.i. Just as a check: (36.725 x 5.65)/5.65 = 7.50.

Back to the schematic/graphical solution (which sounds a lot more complicated than it is): I drew vertical and horizontal lines on a sheet of paper, crossing at right angles, a 2" radius circle centered on the cross point (representing the stroke) and a 6.125" arc crossing the vertical and centered at the BDC point to represent the rod length and thus piston pin location at BDC. Now I calculated the stroke required to give 36.725 c.i. in your engine: (36.725/51.0) x 4.0 = 2.880" and subtracted it from 4.0", giving 1.12" and made a mark on the vertical line this distance above the BDC mark. Next I swung an arc of 6.125" radius from this point to intersect the 2" radius circle on the left side and drew a line through this point to the center of the circle. This represented the crank position at IVC, in order to achieve a 7.5:1 DRC. Now all I had to do was lay on a protractor and determine that this line was at an angle of 73 degrees relative to vertical. Nothing to it! (o:b

OK, if you're still with me, a reward for persistence: I just Googled "Dynamic Compression Ratio" and got a million and a half hits, the first of which is a jazzy little spreadsheet that does all the heavy lifting in an instant. Better yet, it agrees with my stone-age calculation! No doubt some of the remaining 1,4999,999 Google hits will provide more insight into this important concept. Happy reading!

http://www.rbracing-rsr.com/comprAdvHD.htm

 

MadBill,
Thanks for the information. I'll run down some of the Googled URL's. Basically, I think that I will proceed as planned with the .040 gasket/squish/quench and retard the timing as necessary. I looked at the rbracing calculator. I want to understand this better. I also own two or three of David Vizzard's books, I'll do some looking there too. I can set up any cam in DynoSim Pro and look directly at all the numbers. I plan to switch back to my 212/218 114 LSA cam with the 408 as a starting place.

Thanks so much,

Steve

 

MadBill, how much effect does the reverberation have from the intake air going back out the intake valves? I would think it would make some nasty harmonics in the intake.

 

Well, it does make fuel calibration a PITA, especially for carburetors, but that's just one of the challenges of building an 'outer limits' engine! BTW, when the OEMs do this, they call it a Miller Cycle engine, and extoll the fuel economy virtues of its high expansion ratio, as a half cylinder's-worth of combusted mixture undergoes a full stroke expansion. (Typically a supercharger is used to maintain power, because the engine is not designed to operate at a speed where the late IVC might be effective for optimum V.E.)

 

I cant agree more. Compression is awesome. i'll take ion over spark timing any day (within reason).

People seem to forget- its not just a high pressure a high CR will give you but also a great expansion ratio!

 

Mazda's Miller worked fairly well, and the one I drove ran well for it's displacement, but you didn't want to own one past warranty. It was rather complex, and didn't make many inroads into the basic Otto cycle mills.

Did any other OEM's do a production Miller Cycle?

 

Guys,

At what point do you draw the line between altering static compression and
customizing the camshaft to suit the octane needs?

In extreme cases, would the later closing intake valve bleed off too much
pressure on the lower side to make it feesible

or

will the higher SCR offset the late IVC and keep the torque curve fairly similar?

 

If you are building an engine, I don’t see why you compromise

 

If you haven't seen it yet I would check out the Cheap Date 408 article that was in the last Engine Masters: http://www.fuelairspark.com/Articles...ID=-1515255203

 

I'm not really sure if you guys are replying to my post, or just commenting in
general?

I did read the article, but unless I missed something, the SCR is 8.65:1 with
87 octane. That's much different than what the initial question is.

(P.S. Someone should shoot the proof-reader of that article!)

My question is directed to MadBill and OldSStroker specifically. I'd like to know
what the 'limit' would be between SCR and IVC. Which brings on another
question:

Is there a maximum ratio between SCR and DCR for a natural aspiration setup?

 

Good question. As the IVC and CR climb into the stratosphere, the builders are usually highly experienced/motivated and are likely to push the envelope to destruction, rather than relying on rules of thumb. (i.e., they will put in as much cam timing as the engine can use, then keep nudging the CR higher & higher until something breaks.) That said, very few cams have an IVC of more than say 95 degrees, which would give 9:1 DCR @ 15.8:1 mechanical CR in a typical 4" stroke LSx, so the DRC concept can't extend too far beyond the well-explored region of 11 -14:1 MCR.
This would be a good question to bounce off the Pro Stock guys, if they are willing to talk about it...

 

No doubt.

I can't see a street engine taking on 14:1 compression on pump octane
by hanging IVC later.

But then, in a race application, the padding of a high SCR can make up for the
pressure
loss at lower RPM.

For the Pro Stock setups, I don't think the engine will ever see 4000 RPM?

Would be nice if someone coughed up a secret or two.

 

Well, in the Engine Masters Challenge, CR's run close to 13:1 on 92 octane, using fairly conservative IVC cams for a 6,500 RPM max and gunning for best averages from 4,500...

 

EMC has become (unfortunately) a study in either minimizing detonation in the 2500-6500 test range, or living thru/with it. Kaase's engines seem to be designed to minimize it to the maximum extent possible.

The first year, 2002, the 91 octane Union 76 gas, formulated especially for the contest, was quite good. SG was .735, and when run against Sunoco 94, it fared well. I suggest that because rated octane is (R + M)/2, and M or Motor Octane number is more biased toward WOT running, the gas could be blended to be a little more detonation "friendly" (higher M and lower R) without exceeding the 91 average. From what I hear, the 2005 gas wasn't as friendly.

A 11.7 SCR / 9.25 DCR (65* IVC) on a 2002 EMC SBC on the test gas only had a slight rattle in the 3000 rpm area. It surprised me how good that gas was.