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Old 04-08-2010, 10:55 PM
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Dynamic CR

Hi,

From what I've read on pump gas you shoot for a 7.5:1 to 8.5:1 DCR. So I've got a 454 that if I build how I want it would have a 10:1 static CR and a 7.8:1 dynamic CR. Is the DCR a little low for a "performance" motor? Was originally thinking that I'd run aluminum heads but would 7.8:1 DCR be fine with iron heads or should I stick with aluminum?

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Old 04-08-2010, 11:33 PM
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Building a motor to a dynamic compression ratio number just means it will detonate at higher rpm where you won't hear it. 10:1 compression is too high for pump gas, I don't care where you live or how big a cam is in it.

At some point in the rpm range that big cam that bleeds off your low rpm compression is going to get efficient and...wham...you got 10:1 effective compression ratio at 5800 rpm.

Sound like a recipe for disaster?

If you want to build a big block with those huge chambers and have it live on pump gas on the street stick to 9:1 at the most no matter how big the cam is, you still want to be able to put some timing in the thing so it has some bottom end torque and going too high on the CR is going to limit what you can run for initial timing and how fast it comes in...something you don't want to do since it is counterproductive and slower.

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Old 04-08-2010, 11:50 PM
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makes sense. Does the DCR catch up due to at high RPM the exhaust port just cant keep up to bleed the same? I went to like a "wiki" site and an engine builder there (plus another site cant remember) said that DCR doesn't change with RPM..

I was following a build up in a mag that built a 468 and it ran 92 octane with iron heads and 10.2:1 CR and they said no problems. Why Do I see a ton of build ups with 9.5:1 to 10.2:1 on 91/92 octane with BBC?
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Old 04-08-2010, 11:55 PM
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Quote:
Originally Posted by ekacpuc
makes sense. Does the DCR catch up due to at high RPM the exhaust port just cant keep up to bleed the same? I went to like a "wiki" site and an engine builder there (plus another site cant remember) said that DCR doesn't change with RPM..

I was following a build up in a mag that built a 468 and it ran 92 octane with iron heads and 10.2:1 CR and they said no problems. Why Do I see a ton of build ups with 9.5:1 to 10.2:1 on 91/92 octane with BBC?

I'm wondering if they don't see anything wrong on dyno because there isn't a load. HOWEVER. I was reading on other forums that alot of guys are running 10:1 CR on pump gas... Why is DCR important if it doesn't mean anything?
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Old 04-09-2010, 06:15 AM
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Assuming you have a decent intake and exhaust system the most controlling factor for how much DCR you can run is piston and chamber design. You wouldn't believe the difference that the chamber design makes.
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Old 04-09-2010, 07:10 AM
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I also am a fan of low static compression ratios on a BBC. You just do not need 10.0:1 or higher to make tremendous power when it's so easy to make 600 hp at 8.9:1. Many of you fellows didn't live through the Arab Oil Embargo when all we had at the pumps was one grade of junk gas. That could happen again and you'd be left high and dry with your 10.0:1 motor.

The thing about DCR is using the same calculator all the time and comparing one build to another. There are several calcs out there and they are all different, so unless you use the same one all the time, you have no idea where you are.

Here, for instance is what I consider to be a real world build. It's a take-off on a Sim I ran for another fellow on here yesterday, but with a lower SCR. This one uses a 460 cubic inch block with flat-top pistons, Patriot 320 heads (with flow factored down 10% from published figures to hopefully get into the range of reality) with 119cc chambers, 850 carb, RPM or Stealth intake and 1 7/8" or 2" headers through the mufflers. The cam is a solid roller, Comp 11-700-9 installed straight up with stock ratio rockers. Engineer the block deck height with the piston deck height and gasket thickness to achieve a 0.050" squish.
RPM HP TQ
2000 163 428
2500 210 441
3000 268 469
3500 347 520
4000 428 562
4500 505 590
5000 566 594
5500 603 575
6000 622 544
6500 616 498
Peak volumetric efficiency 97.7% @5500
Peak BMEP 194.5 @5000

Need a little more power and run it on a lower grade of pump gas? OK, build this 505 with flat tops at 8.9:1, 4.350" bore, 4.250" stroke, Racing Head Service 320 heads, 119cc chambers, RPM or Stealth intake, 850 carb, 2" headers through the mufflers, CompCams solid roller 11-713-9, installed straight up with stock ratio rockers. I might be thinkin' a Scat cast steel crank and Scat Pro Stock rods with SRP forged pistons for this one.
RPM HP TQ
2000 179 469
2500 231 486
3000 294 514
3500 377 566
4000 462 607
4500 541 631
5000 607 638
5500 635 606
6000 651 570
6500 618 500
Peak volumetric efficiency 95.8% @5000
Peak BMEP 190.3 @5000
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Old 04-09-2010, 04:27 PM
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Okay so if I use the flat tops and a 100cc edelbrock performer head gives me 9.3:1 static CR any 2000-6000 hyd roller cam I've seen gives me a DCR of 7.5:1ish. This IS using the same calculator. That good to shoot for?

When I get bigger than 100cc like 110cc bumps my static CR to 8.7:1 and a DCR of about 7:1 even with 0" deck
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Old 04-09-2010, 06:52 PM
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Quote:
Originally Posted by ekacpuc
Hi,

From what I've read on pump gas you shoot for a 7.5:1 to 8.5:1 DCR. So I've got a 454 that if I build how I want it would have a 10:1 static CR and a 7.8:1 dynamic CR. Is the DCR a little low for a "performance" motor? Was originally thinking that I'd run aluminum heads but would 7.8:1 DCR be fine with iron heads or should I stick with aluminum?
You can push aluminum SCR and DCR higher by a full ratio for sure and a half more if you design carefully. The reason is that aluminum transfers heat to the cooling system faster than cast iron, so you need to run more compression to make as much power as a cast iron head. But in fact you can push that a little bit further than with iron and pick up a little extra.

Tight chamber heads with lots of squish/quench using a flat top or D dish piston work better than open chamber heads with domed pistons where more compression is desired. Regardless of head chamber volume pistons that have a dish under the valve pocket rather than a big circular dish under most of the combustion chamber. So where the ultimate compression ratio is determined by a dish in the piston a D dish is best and is optimized with a small chamber head. You, also, want the spark plug location to be as far toward the center of the cylinder as the the valve position will permit. Favoring the exhaust valve side is also an asset.

Taking all that apart:
Squish and quench are vital to producing power without detonation. So is spark plug location and the distance between the piston crown and the squish/quench step of the head (.040 to .060 inch max). The functions of squish and quench are performed by the closure distance of the piston crown and chamber step. The functions are separated by time.

Squish happens first, as the piston closes to TDC the intimate closing of the piston crown and chamber step ejects the far side mixture toward the spark plug with great force. This does two things:

1) The mixture is violently mixed, this breaks up globules of fuel and puts it into intimate contact with oxygen molecules. This results in a faster and more through burn. The faster burn allows a reduction in ignition advance so the pressure rise is not so premature that it becomes excessive before the piston is ready to start the down stroke. This helps prevent the mixture from exploding ahead of the flame front. This also extracts the maximum amount of heat energy from the fuel your paying for.

2) The mixture expelled to-gather in-front of the spark plug increases its density in that area making it easier for the plug to promptly ignite. This reduces miss and late fires, especailly under part throttle where this is a very big problem. This is also a case where centralized sparkplug is useful to reduce burn time. When the plug is off to one side of the chamber, the flame front must take the time to travel the entire bore diameter, this provides more opportunity for the unburnt mixture ahead of the flame front to suddenly explode.

The next function is quench:
Quench is simply a heat sink function offered by the high surface area of the piston and chamber against the small volume between them. The problem is that as the flame front progresses the temperature and pressure inside the cylinder become high enough that the unburnt mixture spontaneously ignites. The quench function pulls the excess heat out of the far side mixture below the point where it will self ignite. The result is a smooth yet fast burn across the chamber.

So large open chambers and circular dishes in pistons minimize these functions when burning gasoline as fuel,making the engine more prone to detonate. Alcohol and nitro-methane mixtures like open chambers because so much liquid is pouring in it just needs more space and in the case of alcohol the octane value is so high and burn temps so low that you can use a dome to squeeze the daylights out of it and way advanced timing to make up for the slow burn. Doing that with pump gasoline quickly puts you in the market for new pistons.

Details:

Obviously the time exposure has much to do with an engine's detonation resistance, this means that bore diameter is a function, large bore motors require more burn time and that increases the likelyhood of detonation setting in. This is why the Chrysler Hemi went to two spark plugs, it was an attempt the reduce the burn time. It is also one of the functions of dual plugs on aircraft engines, the other duplicate reliability. Even small airplane motors drop a couple hundred RPM when you go thru switching mags on and off during the run up check. Car engines could benefit from this technology, but space and cost limits rather over-run the usefulness of it except in rare and costly applications. But the thrust of this is bigger bores are bigger flame management problems and this is why the BBC has a reputation for being detonation prone. But you can minimize that with careful design and parts selection.

You, also, can control detonation with a richer than needed fuel mixture, the excess fuel slows the burn by simply obstructing the flame path on one hand and the extra fuel draws off heat by vaporization. Unfortunately it washes the upper end lube off the cylinder walls shortening engine life and that unburnt fuel going out the exhaust is lost power that you paid for at the pump.

Detonation onset can also be reduced with lower engine operating temps, colder intake air and retarded ignition. I suppose I could toss water injection in there as well. A car with stiff gears is less likely to detonate as the engine doesn't work as hard.

Frankly, I shoot for a DCR up in the 8s and work the hell out of the details.

Bogie
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Old 04-09-2010, 08:32 PM
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Thanks! That's what I was looking for.

So I was looking at a combo. edelbrock heads that are 300cc intake runner 106cc chambers with 0 deck and XR288HR gives me 9:1cr static with 8.6:1 DCR and they're aluminium.

The other one is some performer heads with 290cc intake runners (seems small) 100cc combustion 0" deck chambers gives me a 9.4:1 static CR and with same cam (I know its too big with those heads) be DCR of 9:1

Prob is the affordable (iron) heads are 119cc and gives me a static CR of 8.2:1 with the flat tops and the other common size (110cc) 8.7:1 (zero deck).

Depending on what I actually do I'm hoping for a 2400 or 2800 rpm stall with 3.73 or 4.10s.
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