Selecting a compression ratio

[Wesman07]

Hi,

I’m looking for some experienced advice on selecting a compression ratio and ignition theory. The motor I’m building is a Ford 300 six. Im familiar with what the other inline guys are building and come here for some variety of experiences.

Most of these guys are using a DCR of 7.0:1 for 87 octane builds and typically don’t adjust timing much. That puts me at about 8.5:1 static for the cam I will be using. My machinist thinks that would be a mistake and I should be in the 9’s. Using EA pro, the only way I can use a compression in the 9’s would be to retard the spark quite a bit. So, I guess my real question would that be is that a good move?

Bore 4.050”
Stroke 3.98
Cam 260/204 268/ 210 on a 112LSA, 109ILC
Good flowing iron heads, intake and headers
Peak hp 4,500
9.5:1 static?
Estimated 9 degrees base, 21 total??

Stock cam is 268/192 268/192 on a 110LSA, 114ILC
8.8:1 static
10 degrees base, 29 degrees total
Will get some detonation under load if the cam is set to 110ICL

The only thing I forgot to mention is the head was a heavily shrouded fast burn chamber and after porting will likely require 36 degrees of timing or so with all else staying the same.

[BillK]

Street driven or strictly track ?

Vacuum advance distributor ?

[Wesman07]

Street/ off-road truck. Fuel injected, TFI distributor, stinger pimxs (ms3) ecu

[Dave B]

Is dcr even a thing now days?There is allot of variables that make those numbers seem worthless to me.Piston design,chamber design etc etc.One dcr number can work in one layout and not another.I gave up on dcr having a meaning at least 30 yrs ago.

[Dave B]

In 2023 it should be "DC or R"
Meaning doubt credible or repeatable.
If I was designing a performance engine off of dcr formulas ,my dcr would be higher than allot of guys static number they think to be safe.Spark plug location ,heat ,quench ,plug heat range.Change anyone of these and dcr formula is pretty useless imo.Work on fundamental engine design as far as getting all sharp edges out of combustion area ,get your piston and valves sealed up etc.I wish guys were as interested in learning how to hone a block properly and correct valve jobs and chamber layout.But everyone seems to waste time on useless online formulas that mean less than squat .What happened to r&d and testing what you have and learn how it works?Dcr is a number and at the end of day has no given meaning .

[Wesman07]

Sounds like you’re still pretty hung up on DCR.

Soo do you have a system? Trust a computer program? eeny meeny miny moe?

[rfoll]

The cylinder head and it's chamber design can dictate compression ratio. I have a 9.6:1 350 with Vortec heads that almost never pings on regular fuel. The predecessor with different heads pinged terrible at 9.3:1 and required retarded timing. I have no idea how the 300 chamber works. It might be useful to task to people who know these engines well. An extra point of compression can matter.

[PackardV8]

Most of these guys are using a DCR of 7.0:1 for 87 octane builds and typically don’t adjust timing much. That puts me at about 8.5:1 static for the cam I will be using. My machinist thinks that would be a mistake and I should be in the 9’s.

With all due respect to Wesman07, he has his own reasons and we should give the requested advice and not second-guess.

If one has EFI, with all of today's available sensors active, 87-octane can be accomodated; OEMs do it with 11:1 and a warranty. But with carb and distributor, not so much.

Having said that, it boggles the mind when a customer comes in with a build request which will be between $5,000 and $10,000 long block, says he's using a carb and distributor and then closes with "and it's gotta run on 87-octane."

The most recent rationale we got was, "Well, I might take it to Mexico someday." We're 1600 miles from Mexico and no one he's ever known has driven an off-road truck that far and back, but the internet told him to plan for that contingency.

[David Redszus]

If I was designing a performance engine off of dcr formulas ,my dcr would be higher than allot of guys static number they think to be safe.

Perhaps the problem is a lack of understanding of DCR and its function and usage. DCR can never be higher than SCR; but it is still just a ratio of volumes. DCR is much more useful than SCR which has limited value.

Spark plug location ,heat ,quench ,plug heat range.Change anyone of these and dcr formula is pretty useless imo.

While each mentioned parameter does contribute to the combustion process, they have nothing to do with DCR.

Work on fundamental engine design as far as getting all sharp edges out of combustion area ,get your piston and valves sealed up etc.I wish guys were as interested in learning how to hone a block properly and correct valve jobs and chamber layout.

Yes, I agree completely. Chamber design trumps sharp edges and incorrect honing and block preparation is often poorly performed. Seems like a body shop that does no surface preparation before squirting paint.

But everyone seems to waste time on useless online formulas that mean less than squat .

Everyone does not use formulas of anykind, much less understand them. But OEM engineers and professional racers all use advnced formulas and calculations. No factory would function without them. Specifically, which formulas do you think are useless and a waste of time?

What happened to r&d and testing what you have and learn how it works?

Theoretical work must be performed before prototyping can proceed. Proper R & D testing is much too expensive and very complicated to perform correctly. But proof of concept and component validation must/should be done.
The alternative is trial and error, and error, and error, and error, etc.

Dcr is a number and at the end of day has no given meaning .

Not true at all. DCR is a useful measure which allows the calculation of in-cylinder compression temperature and pressure.

SCR is a number that at the end of the day (or morning) has no given meaning.

[David Redszus]

DCR is not as valuable as commonly believed.

Actually, DCR may be even more valuable than commonly believed.

SBC 350 V8, 3.48" stroke, 5.7" rod, same chamber, same spark (using as 1.25 ratio of variable heats)

In order to produce the SCR and DCR values listed below, the two engines cannot have the same chamber volumes. The ratio of specific heats is incorrect; it should be 1.33 which makes a difference.

Tune 1: static CR 9:1, IVC 33° ABDC, DCR 8.54:1, CCP 200 psi,

The compression pressure at TDC will be 254.9 psia, chamber volume 89.5cc, the compression temperature will be 795F at TDC, and 541F at 33deg BTC (ignition).

Tune 2: static CR 12:1, IVC 77° ABDC, DCR 8.55:1, CCP 200 psi,

The compression pressure at TDC will be 255.3 psia, chamber volume 65.15cc, the compression temperature will be 796F at TDC, and 484F at 33deg BTC (ignition).

Tune 1 will have good power immediately above idle, and knock badly. The nominal CR dominates very quickly.

Note that the peak compression pressure and temperature are the same for both configurations.

Tune 2 will have no power until much higher RPM, and may not knock with a high stall converter.

In spite of identical temperatures and pressures at TDC, there is a substantial difference in
chamber temperature at 33deg BTC when the spark plug fires. (541F vs 484F). Due to the huge difference in chamber volumes (89.5cc vs 65.15cc) there will be a difference in ignition delay, chamber turbulence, and combustion flame speed and burn angle. Tune 2 will need ignition advance to reposition the combustion pressure curve.

Compression ratios, either SCR or DCR, have little direct value. What has value is the information regarding an engine's compression temperature and pressure which can be obtained from DCR, but not SCR.

[Tom68]

Is dcr even a thing now days?There is allot of variables that make those numbers seem worthless to me.Piston design,chamber design etc etc.One dcr number can work in one layout and not another.I gave up on dcr having a meaning at least 30 yrs ago.

Yer it's one of the things, but when you're familiar with combinations it's a thing you don't need to look at as a calculated number, you inherently know the combos that don't work.

[panic]

Even with all my errors, which of you already posted that DCR results may vary widely?
[crickets]

[David Redszus]

Even with all my errors, which of you already posted that DCR results may vary widely?
[crickets]

Good point. As you said; crickets. But it was not the DCR that varied widely. It was the fact
that the engines were not the same.

But who knew that DCR, by itself has limited value; its only a ratio and engines don't do math.

But who posted that DCR is only a tool to be used for a bigger purpose?
Who understood the use of DCR to obtain compression pressures and temperatures at any
crank angle?

And who, besides yourself, even bothered to attempt the mathematics of understanding
the effects of compression? Kudos to you.

[Wesman07]

I’m following you David on how DCR can be useful. I guess I'm questioning if all the other guys are set up correctly?

Using this program, I should be able to run as high as 10:1 static which gives an 8.5:1 DCR IF I tailor the spark curve back. Power picks up everywhere.

[skinny z]

Yer it's one of the things, but when you're familiar with combinations it's a thing you don't need to look at as a calculated number, you inherently know the combos that don't work.

Agreed. Or conversely, what combinations do work. Very much application specific.
Case in point: I know from several builds that a 350 SBC with iron heads will be OK with unleaded premium pump gas if the DCR remains below 8:1 or so. And this is for the given set of parameters I place that engine within.
I've also learned that this won't necessarily cross over if I venture outside of those parameters or to any other iteration of an SBC let alone another engine platform.