Static vs Dynamic Compression Ratio Questions

[colnago]

Okay, I know that this topic might be tinged with angst (especially coming from me), but please bear with me.

I'm looking at swapping my current cam (generic RV cam, wide advertised angles) for the Crane Cam 343901 (much narrower angles, peakier lobes). From the numbers, my static compression ratio is 8.68:1. The Crane 901 is advertised to work with engines with a static CR of 8.0:1 to 9.5:1. However, the dynamic CR for this cam comes out to 7.11:1.

My basic question is, what does this mean? I thought the "optimal" dynamic CR was closer to 8:1. If I aim for an 8:1 dynamic CR, the static CR needs to be over 10:. What am I missing?

Inquiring minds want to know ...

Joseph

[Jacksdad]

Static CR is calculated with the crank at bottom dead center (BDC), while dynamic CR measures the swept volume based on the piston position when the intake valve closes. DCR is the real world number the engine sees as compression rarely starts as the piston leaves BDC, and is largely dictated by cam timing.
Are you still building your motor to run at a max of about 3000 rpm? If so, I would concentrate on your powerband more than things like DCR. That cam starts making power at 1500 rpm, which only gives you another 1500 rpm before you're at the limit you're setting yourself. Something that pulls hard from idle would be a better choice in my opinion. You seem to be concentrating more on when it runs out of steam as opposed to when it starts making usable power.

[Busboy]

Mechanical compression ratio is a mathematical equation and doesn't change regardless of camshaft profile. That said, the engine will literally run rough and inefficiently at low rpms with a "hot" cam. This is the lope sound we all enjoy. It's all about cylinder pressure. A supercharged engine will run good with a 7 to 1 mechanical compression ratio because the blower makes up for it with "artificial" cylinder pressure. Make your choice of a smooth idling engine with lots of vacuum or one that is cammed to provide more power higher up in the rpm range. Thus the beauty of variable valve timing, solenoid operated valves etc. They literally will give you the best of all worlds.

[DuckRyder]

Everyone read this:

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

I would probably not shoot for 8:1 DCR on an engine that I’d like to run on regular, go for the lower end of the scale.

What does your current cam calculate to?

Whats the ICA of the 343901 @ advertised? I can’t find the cam card at the moment.

[71cc]

Do you know your static compression ratio ? What size engine ? The higher compression and more cubes tend to tame a cam especially at lower rpm. The 112 lobe center would tend to be more forgiving than a 108 lobe center. Rule of thumb on a completed engine is do compression test. 180psi under will do fine on pump gas. Start approaching 200psi and you'll have problems. Whole lot of variables interacting. That can be influenced by installing cam straight up or advanced. Maybe state as much as you know about your combo to get specific recommendations

[colnago]

A lot of good info! Fantastic link! My engine is together, so my static CR is based on some assumptions. I am assuming that the compressed Fel-pro head gasket is 0.040". I am assuming that the deck clearance is 0.035" (I've heard anything from 0.032 to 0.038 for stock pistons). Incidentally, this brings up some questions on quench/squish, but my brain can only wrap around one thing at a time, so I'll start a different thread for that.

The card for the 901 doesn't give an advertised ICA, so I took an educated guess against my current cam (which is 73* ABDC), and came up with 62* ABDC. Also, I found a calculator that just used the advertised duration, and came up with near-identical results. My current cam gives me a dynamic CR of 6.7:1 (or thereabouts; my numbers are at work right now, and I don't remember exact numbers). I am assuming this is too low to provide for much efficiency (assumption on my part). The good thing is that I can't make the engine ping to save my soul!

The data sheet on the 901 says the static CR should be between 8.0 and 9.5. It also says the power range is 1200 - 4800. Finally, it gives a cruise RPM of 2200 - 2600. Is there a "more important" number to worry about? I figure that 90% of my driving will be at 45MPH (which gives an RPM of 2000 for my combo), 5% towing, and 5% freeway. Should I be looking at "lower" cams? Is this why someone recommended the Edelbrock 2106?

Oh, I thought I had C8AE heads, but they're C6AE, so my static/dynamic CRs will probably be lower. I haven't run numbers, though.

Joseph

[DuckRyder]

If you are running numbers without actually measuring volumes you could be off a bit, you’ll be ok in comparing between cams (ie: which is higher or lower,) but the absolute number may be iffy)

Since you don’t plan on much freeway, I’d give less weight to the cruise RPM and more to the power range.

Since we are now working with a relatively low compression motor it may be prudent to look at cams designed to increase DCR. Though I haven’t run the numbers, I think the performer cam might be the wrong direction here (and I know I said earlier that I wasn’t sure I didn’t like it better than the 901, but I didn’t realize we were now working with a low static compression).

So - once the static compression is set, DCR is controlled by the IC event. All other things being equal a cam with a shorter advertised duration should have a higher DCR. Also look at the ratio of advertised to .050 this should give a clue to the speed of the ramps.

There are variations in how advertised duration is given but they should not give very large differences. (some might be total, some at .006 and some at .004, and some at some other random value).

[DuckRyder]

I took the Liberty of having DynoSym calculate the ramp rates and advertised events for 3 cams that seem to fit your usage.

Edelbrock Performer:



Crane 343901



Lunati VooDoo 256 (Part Number: 10330701 Previous Part Number: 62001)



Edit, that doesn’t seem right for the 901 looking at it now let me check that because the centerlines don’t seem right based on the LSA....

[DuckRyder]

Came up with an old cam card for the 901.

Cam Timing: TAPPET @ .004
Lift: Opens Closes ADV Duration
Intake 19 BTDC 61 ABDC 260 °
Exhaust 69 BBDC 23 ATDC 272 °

Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake (5) ATDC 29 ABDC 107 204
Exhaust 45 BBDC (9) BTDC 117 216

[DuckRyder]

So based on that:

[colnago]

Robert,

Wow, you really went above and beyond with running all of the profiles! A big thank you!

One of the things that I have never looked at is ramp rates. None of the articles talk about it. It's all about durations, lobe separations, and overlap. Any insights or links on the ramp rate?

Joseph

[DuckRyder]

No worries, glad to help if I can and it helps me dig back into some of this.

On the ramp speed,

The upside is that of two cams with equal .050 durations and LSA the one with faster ramps will build higher DCR for a low compression motor, cam manufacturers use buzz words like “area under the curve”, “high efficiency” and “computer controlled” to describe this.

The down side is that they generally require stronger springs and put more stress on the valve train. The reality for cams as mild as we are discussing is that I doubt this is a factor in your situation.

Take the Lunati VooDoo 250 (Part Number: 10330700 Previous Part Number: 62000)
Advertised Duration (Int/Exh): 250/256
Duration @ .050 (Int/Exh): 207/213
Gross Valve Lift (Int/Exh): .503/.524
LSA/ICL: 112/108
Valve Lash (Int/Exh): Hyd/Hyd
RPM Range: 800-5000

This cam has basically the same .050 duration and very similar lift to the 343901 with 10 degrees less advertised duration and probably enough difference in ICA to make an appreciable difference in DCR.

Incidentally any Lunati with an old number like 305A1 is an old UltraDyne grind and probably has pretty fast ramps, though I don’t think any of them really fit your needs.

[colnago]

First, on the springs, I'll probably replace them. I'd be tempted to keep my current stock springs with the Edelbrock cam, but the valve peaks on the Crane and Lunati are probably a bit high for stock springs. I'm not sure what the max lift is before the stock springs bind (Google couldn't find it), but I think it's lower than either cam will lift.

Second, I got run over today by the reality bus. No, it wasn't the Magic Bus, and it didn't take me to Funky Town, but it crashed the gate doing 98, 'cuz it didn't have a @#$%&* dime.

Okay, back to reality, and real numbers. This is what I found, for a '66 352:

Deck height = 10.17"
Compression height = 1.816" (both for my piston and for Silv-o-lite 1129)
Connecting rod = 6.54" +/- 0.002"
Crank circle = 3.5" (divide by 2 to get max height)
Therefore, deck clearance = 0.064"

But, every link I find says that the SCR for a '66 352 is 9.1:1. Huh?!? It can't be, with such a large deck clearance. The closest I could get was to use an early head volume of 59.7cc, and a piston compression height of 1.839" (found on the white-and-green spec table that we've all seen). But that same table gives a SCR of 9.3:1 for the early 352.

What am I missing?

Joseph

[DuckRyder]

My reference show the compression height of a stock 1966 352 piston as 1.846 so that would cut your deck clearance almost in half - no?

Summit does show 1.816 for both Sealed Power 246AP and United Engine Machine/KB/Silv-O-Lite 1129 (not many 352 engine piston choices out there) any markings on your pistons? It had a Crane cam right so perhaps the pistons are already not OEM.

[colnago]

According to the Silv-o-lite catalog:

"The compression ratio is the ratio that will result when the standard (STD .) size Silv-O-Lite piston is substituted for the original equipment piston. These ratios have been calculated from the best published data available to us. While every effort has been made to use authoritative sources we cannot guarantee the accuracy of such data and, hence, cannot assume responsibility for the resulting compression ratios calculated therefrom."

So, regardless of what Ford used, Silv-o-lite said that their 1129 piston creates a 9.1:1 SCR for the '58 - '66 352FE. I guess when you're worth a bazillion bucks, you can say anything. You can even cherry-pick the data to make your case. Unfortunately, regardless of the hype, my SCR is closer to 8.1:1, and the only way of increasing it is to get really thin head gaskets (or shave the block/heads).

By the way, that Lunati cam is $$$.



Joseph