[Aaron Quinton]

Why?

Originally Posted by cfmcnc

WE have a soft chamber cnc program for 72 cc Eddy RPM heads It requires that the seats be sunk .030 first,After CNC and hand blending it measures 84 ccs.Bill C.

[BruceWilkie]

Originally Posted by ponjohn

The soft chamber is completely useless n/a correct?

It will slow combustion rate too much... pretty much why hemis get touchy in high rpm NA apps. The link I posted ^^^^ will explain that better.

Unfortunately that article doesnt mention that combustion rate increases and ignition lead requirements decrease when the intake charge is made denser via supercharging/turbocharging.
Boosted the hemi doesnt need as much ignition lead as it would NA so it can start the spark(begin combustion) closer to tdc than it can NA and still reach Peak Cylinder Pressure at 14 degrees atdc.(variables in engine design, fuel etc can alter that 14 degree point as you will see anywhere from 14 to as late as 20 degrees in some writings)

[BruceWilkie]

Originally Posted by wolf

Bruce, I agree that detonation can cause pre-ignition (from the resultant hot spots), but I should have specified "late" pre-ignition often results in detonation.

The link you posted is a good read, but there's some key information missing (in my opinion) and a couple of things I don't necessarily agree with, but for "basics" it's pretty damn good. I'm not so arrogant to think I know everything (I learn something new everyday), but I literally have my own library on internal combustion engines (engineering texts), in addition to thermodynamics, heat transfer, etc., and I've got many years of experience with engine calibration, so I do have a little knowledge on the subject. But given new information and data, I can change my thinking.

Thinking more about the "big boost" situation this morning, I can see how quench areas can become hot spots themselves, especially in billet heads (no coolant), and with aluminum rods you need to run much larger quench clearance which creates an opportunity for end gases to detonate in that space, but in "moderate boost" situations I've always found fast chambers (tight quench) to be more octane tolerant (this is how you can get away with 15 psi boost on pump gas with modern high compression ratio engines, for example).

Right now the only reason I can see for slowing down the burn is if it's so fast you're getting peak cylinder pressure that's too high (hard on the components), so you're trying to spread out the pressure peak over a greater (and typically later) crank angle, but I would think a short rod and less spark advance would accomplish the same thing.

Read Uratchkos comments and give quench more thought... Quench speeds the rate of combustion... n2o speeds the rate of combustion because of extra fuel and oxygen present than there would be NA, Boost speeds the rate of combustion because the denser charge has more molecules of air/fuel in same space than it would NA.
Peak cylinder pressure from the combustion event needs to occur 14-20 degrees ATDC for peak HP(engine design and fuel variables)... keep in mind combustion events are not identical and the rate of burn does vary.

A fast burn chamber combined with a mix that wants to burn fast narrows the tuning window.

[JD311]

Originally Posted by cfmcnc

WE have a soft chamber cnc program for 72 cc Eddy RPM heads It requires that the seats be sunk .030 first,After CNC and hand blending it measures 84 ccs.Bill C.

How much for this? I have a pair of 87cc E's and also a pair of 72cc. Building a turbocharged 59 block. I had planned on smoothing up the 87cc, but if this would be better....

[Stan Weiss]

Originally Posted by BruceWilkie

Read Uratchkos comments and give quench more thought... Quench speeds the rate of combustion... n2o speeds the rate of combustion because of extra fuel and oxygen present than there would be NA, Boost speeds the rate of combustion because the denser charge has more molecules of air/fuel in same space than it would NA.
Peak cylinder pressure from the combustion event needs to occur 14-20 degrees ATDC for peak HP(engine design and fuel variables)... keep in mind combustion events are not identical and the rate of burn does vary.

A fast burn chamber combined with a mix that wants to burn fast narrows the tuning window.

Bruce,
How does a long verses short rod (rod / stroke ratio) effect where in that window?

What is your definition of "speeds the rate of combustion"?

1) Burns more fuel and air because it is denser

2) Changes the rate (M/s) that the fire front move at.

3) Maybe some of 1 and 2

4) ???????

Stan

[BruceWilkie]

Stan dont have an exact answer to 1st question other than rod angularity could likely affect the pcp point(14-20 degree window) most favorable to power.
FWIW... over the years pcp from 15 to 18 atdc seemed more common answer to questions of what point pcp should occur at in discussions on the topic.

Your answer choice 2 for your second question... molecules closer together in a denser environment than in a less dense environment will lite off each other faster because they are closer. Analogy.............starting a fire with match flame far away from kindling would be slower than if flame was closer to the kindling.

[Stan Weiss]

Originally Posted by BruceWilkie

Stan dont have an exact answer to 1st question other than rod angularity could likely affect the pcp point(14-20 degree window) most favorable to power.
FWIW... over the years pcp from 15 to 18 atdc seemed more common answer to questions of what point pcp should occur at in discussions on the topic.

Your answer choice 2 for your second question... molecules closer together in a denser environment than in a less dense environment will lite off each other faster because they are closer. Analogy.............starting a fire with match flame far away from kindling would be slower than if flame was closer to the kindling.

Bruce,
The reason about the rod stroke ratio is you rarely see turbo builds on Pontiac's but plenty of Chevy's. If we have two engines both with 4.125" bore and 3.75" stroke (400 ci) but one (Chevy) has a 5.565" rod and the other (Pontiac) has a 6.625" rod. Now the question is how does some of what works in a Chevy translate to a Pontiac?

Stan

[Tom McQueen]

We went short rod (6.700") on my 4.580" stroke Pontiac twin turbo for a reason.

[wolf]

Originally Posted by Chris Uratchko

The more timing you can run in a power adder engine(in the normal window of timing), the more hp it will make and the faster it will be on the track.

Originally Posted by BruceWilkie

Peak cylinder pressure from the combustion event needs to occur 14-20 degrees ATDC for peak HP(engine design and fuel variables)... keep in mind combustion events are not identical and the rate of burn does vary.

A fast burn chamber combined with a mix that wants to burn fast narrows the tuning window.

Guys, let's see if I'm finally getting this. The problem is cyclic variation? Is that what you're referring to as the tuning "window"?

i.e. if we target say a PCP at 15 degrees ATDC, a fast chamber might get there with 10 degrees of spark advance, a slow chamber may take 20 at the same boost/etc. If everything isn't consistent from cycle to cycle (or cylinder to cylinder), then the fast chamber will push PCP around more than a slow chamber (and into more troublesome situations)?

Is that why the EFI guys can use modern fast chambers with big boost? For example, just off the top of my head, Larry Larson (Proline engine) and Andy Frost both run BIG boost turbos, methanol, and make 5 second power with heart shaped, fast burn, high quench chambers.

[wolf]

Originally Posted by Stan Weiss

How does a long verses short rod (rod / stroke ratio) effect where in that window?

A short rod results in faster piston motion at TDC, and slower motion at BDC.

Thus it pulls the piston away from TDC faster, reducing the tendency for detonation, and creating slightly less peak pressure, but a somewhat wider peak.

And slowing piston speed at BDC allows for better cylinder filling before IVC, especially with a super/turbocharged engine that can keep ramming charge into the cylinder after BDC, before the piston motion creates reversion. So better volumetric efficiency.

[v869tr6]

Originally Posted by wolf

Guys, let's see if I'm finally getting this. The problem is cyclic variation? Is that what you're referring to as the tuning "window"?

i.e. if we target say a PCP at 15 degrees ATDC, a fast chamber might get there with 10 degrees of spark advance, a slow chamber may take 20 at the same boost/etc. If everything isn't consistent from cycle to cycle (or cylinder to cylinder), then the fast chamber will push PCP around more than a slow chamber (and into more troublesome situations)?

Is that why the EFI guys can use modern fast chambers with big boost? For example, just off the top of my head, Larry Larson (Proline engine) and Andy Frost both run BIG boost turbos, methanol, and make 5 second power with heart shaped, fast burn, high quench chambers.

Do Larry and Andy have high quench chambers?
Does, The Chief, Rodney or John Welter or Curt G. have high quench chambers??

[BruceWilkie]

Originally Posted by wolf

Guys, let's see if I'm finally getting this. The problem is cyclic variation? Is that what you're referring to as the tuning "window"?

i.e. if we target say a PCP at 15 degrees ATDC, a fast chamber might get there with 10 degrees of spark advance, a slow chamber may take 20 at the same boost/etc. If everything isn't consistent from cycle to cycle (or cylinder to cylinder), then the fast chamber will push PCP around more than a slow chamber (and into more troublesome situations)?

Is that why the EFI guys can use modern fast chambers with big boost? For example, just off the top of my head, Larry Larson (Proline engine) and Andy Frost both run BIG boost turbos, methanol, and make 5 second power with heart shaped, fast burn, high quench chambers.

Boosted engines on methanol typically run much higher compression than gas or ethanol(E70-E99).
Charge temps are usually considerably lower with methanol. Cool enough they dont intercool the charge vs guys running gas or E85. Consider that most racers running methanol have difficulty keeping heat in their motors as well, and that would indicate the chamber surfaces would also be cooler. So it seems to indicate burn rate would also be slower than if gasoline or ethanol.
Nitro powered cars run lower compression than gas... and hemi chamber seems to be preffered choice especially top fuel.

Seeing you cited Larson as an example... the question becomes... can he run more compression or more boost or both without softening the chambers if he starts to seek further power gains???

Do we know his current chambers arent softened some? If it were NA could they be "tightened up" more?

[Tom McQueen]

Calvin set my CV-1's up. Flow work, turbo friendly convertible exhaust and a soft chamber.
Like this.

[Dragncar]

How in the world can a short rod have anything but the same effect on dwell time at TDC and BDC. A circle is a circle. What changes in the geometry ?

[wolf]

Originally Posted by Dragncar

How in the world can a short rod have anything but the same effect on dwell time at TDC and BDC. A circle is a circle. What changes in the geometry ?

Piston motion is NOT simple harmonic motion because of the rod angularity. It's the same reason why piston speed is not maximum at 1/2 the stroke length.

Think of it this way: Same stroke = same distance to cover from TDC to BDC for the piston, right? If you change only the rod length, and RPM also remains the same, then the time for the piston to move from TDC to BDC is still the same, and equal to the time for 1/2 the crankshaft revolution, right? But if the piston is moving faster at one end of the cylinder because of a rod length change, it has to move slower at the other end so the total time from TDC to BDC stays the same.

[Chris Uratchko]

Short rod vs long rod don't mean a thing unless we're measuring peters ..

[Dragncar]

I don't think so Wolf. This all was explained to me a long time ago by Dan Whittmore. Using real long rods in Pontiacs is old school thing guys did because for so long all we had were iron heads. You ported your basic RAIV head to 300cfm, maybe 310 and you made the most power you could with a Warrior intake because that was all that was available then too.
But if you stuffed a 7.125 rod in your 400 with a .927 pin and oil ring supports and a piston that looked like it belonged in a motorcycle you could get 200-300 more RPM of "pull" out of your power band. It was because the long rod dwelled more at BDC to fill the cylinders more and at TDC you got a little more cylinder pressure.
Well, if you want to believe a very well respected old school Pontiac engine builder.
And Cris is right, long rods do not mean much anymore. As soon as high flowing aluminum heads came out the long rod game went out the window.

[LPI]

This is the Edelbrock head that we developed with Dave at SD Performance for boost.

It runs around 93 to 96 cc's. It is based off the 72cc head.

[wolf]

Originally Posted by Dragncar

I don't think so Wolf. This all was explained to me a long time ago by Dan Whittmore. Using real long rods in Pontiacs is old school thing guys did because for so long all we had were iron heads. You ported your basic RAIV head to 300cfm, maybe 310 and you made the most power you could with a Warrior intake because that was all that was available then too.
But if you stuffed a 7.125 rod in your 400 with a .927 pin and oil ring supports and a piston that looked like it belonged in a motorcycle you could get 200-300 more RPM of "pull" out of your power band. It was because the long rod dwelled more at BDC to fill the cylinders more and at TDC you got a little more cylinder pressure.
Well, if you want to believe a very well respected old school Pontiac engine builder.
And Cris is right, long rods do not mean much anymore. As soon as high flowing aluminum heads came out the long rod game went out the window.

I choose to believe in the actual math. Let's try a completely extreme example: say you shortened your rod all the way down to exactly 1/2 the stroke length. If you can picture it, you'd get 180 degrees of crank rotation at BDC without the piston moving.

[taff2]

Long rods still have their place, a long rod puts less thrust load on the bore walls which means less bore wall distortion and less piston/ring friction. You can then go to a lower tension ring pack for even less friction. The rod journal has also moved around its arc further,putting the crank in a more favourable position for the combustion pressure on the piston to act on it.