Static CR too high?

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MidWestCoast442
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Static CR too high?

Post by MidWestCoast442 »

Hi, everyone. Longtime lurker. Firsttime post.

Changing fuel types and cylinder heads on my 461cid Olds. The car had iron heads and E85. It will soon have aluminum Edelbrock heads and 92 pump gas with throttle body EFI.

On the advice of an Olds engine builder, I had the new heads milled to 68cc for 11.05:1 static CR. Then I got PipeMax and talked a bit with Larry Meaux. Now I’m a little uncertain about the relative safety of my soon-to-be static CR. It seems that my previous understanding of dynamic CR was lacking.

I’ll post some pics relating to static CR, dynamic CR, and cranking pressure. I’m without access to my PipeMax program right now.

Cam is a hydraulic roller. Seat-to-seat IVC (@0.006 is 79*). Will post the cam card, as well. I’m at sea level.

My current plan is to get everything reassembled, start-up the engine, get it to operating temp (180*), and do a compression test to see where it’s at on the gauge. Will probably set the target A/F ratios a little on the rich side and keep an eye on the plugs to look for signs of predetonation. The car will be going to a chassis dyno for final tuning of the fuel map & timing curves (electronic timing control).

Let me know what you think and if you have any suggestions. Thank you!
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cjperformance
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Re: Static CR too high?

Post by cjperformance »

I am not 101% up on the exact comparison specs between our Australian 98 RON fuel and US 92 AKI but if they do represent a comparable fuel and your quench height is tight , ignition and fuel curves set up well then I dont see a problem. It may be a little testy if loaded up at lower rpms but correct gearing, convertor/manual gear use will see it fine.
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Re: Static CR too high?

Post by MidWestCoast442 »

From what I can find, 92 AKI is equivalent to 96.4 RON

Another concern that I have is that I don’t have much quench to speak of. The pistons have large/wide 16cc dishes. Dish depth is 0.120” if memory serves me correctly.

Rear gear is 3.90

Not sure where the converter will stall with the new top end package. Probably in the 2800-3000 range. We’ll see on the chassis dyno.
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Re: Static CR too high?

Post by cjperformance »

Even though you have a large diameter dish, how much piston/head clearance will you have on the perimeter of the crown? This could be your possible make or break area. I like full round dish pistons but do set the quench tight on them.
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Re: Static CR too high?

Post by MidWestCoast442 »

The distance between the perimeter of the crown and the head will be 0.043.

Head gasket = 0.039
Deck height = 0.004

FWIW, the chamber design is a more modern vortec style design. Will attach a picture.
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Re: Static CR too high?

Post by plovett »

I'm no expert, but I think you will be okay. I'm assuming this is a 3800-4000 lb. A-Body?

With the long advertised duration and decent octane, I think it will be fine. A good timing curve is a must as you know. And a smart right foot. Don't load it unnecessarily. Don't floor it in high gear going uphill in 100F heat. Especially if you have no kickdown.

JMO,

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MidWestCoast442
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Re: Static CR too high?

Post by MidWestCoast442 »

It’s an A-Body. 1965 442. Probably around 3600lbs. It has gone on a bit of a diet. Need to weigh it after the aluminum heads are on.

The timing will be controlled by way of the EFI. We can make the curve whatever we want it to be when it’s on the chassis dyno.

It doesn’t have a kick-down. The 2004R has a reverse manual valvebody.

Also FWIW, the air cleaner will be sealed to the cowl induction scoop and the intake manifold is an Edelbrock RPM Air-Gap.
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Re: Static CR too high?

Post by Newold1 »

With aluminum heads and those DCR numbers you will be fine.

Another area that you can probably use some possible protections that most EFI control systems provide is "knock control" .As we all know knock is generally detonation and that's the 800 lb. Gorilla! If your system will support knock sensors and you can mount one or two on the block itself it will afford more protection especially with summer temperatures and inconsistent street fuels.

One other area to think about and take some care is your engine cooling system and how you can try to insure that your engine operating temperatures stay in that 175 - 190 operating range where most know` and feel street driven engines should stay when operating. When performance engines get in temperature ranges where they start spiking above these ranges they can sometimes get into detonation that is possible from higher combustion chamber temperatures with street fuels. JMHO
Last edited by Newold1 on Mon Jun 18, 2018 11:05 am, edited 1 time in total.
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MidWestCoast442
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Re: Static CR too high?

Post by MidWestCoast442 »

Thanks for the responses, so far. I appreciate the input.

Not sure if my EFI system supports knock sensors & related timing retard. But I do know of a stand-alone knock sensor & timing retard system. I have thought about purchasing such a system for piece of mind. The SafeGuard from J&S is the one that I was looking at. Need to do some more research into what Holley EFI supports.

The cooling system has been upgraded. Saldana aluminum radiator. Derale electric fan. The EFI has a temp sensor and will be controlling the electric fan relay. My goal is to keep it in the 175-185* range.
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Re: Static CR too high?

Post by MidWestCoast442 »

Anyone else care to chime-in?
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Re: Static CR too high?

Post by cjperformance »

MidWestCoast442 wrote: Mon Jun 18, 2018 9:46 am The distance between the perimeter of the crown and the head will be 0.043.

Head gasket = 0.039
Deck height = 0.004

FWIW, the chamber design is a more modern vortec style design. Will attach a picture.
On our 98 fuel that would be fine. Id be pretty sure it will be ok on your 92 also BUT seeing i have never used it thats about all i can say.
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Re: Static CR too high?

Post by David Redszus »

MidWestCoast442 wrote: Mon Jun 18, 2018 10:43 pm Anyone else care to chime-in?
Using the engine specs posted, my TCR calcs come up quite a bit different.

With a SCR of 11.05, and a TCR of 7.72, I would expect to find a compression temperature
and pressure at TDC of 223psi and 748F.

At an ignition point of 33deg BTC, we would expect a compression temperature of 445F,
which should be low enough for most gasolines.
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Re: Static CR too high?

Post by MidWestCoast442 »

Thanks, Craig.

Thanks, David. I do have some questions for you, if you don’t mind. I am interested in learning more about these aspects of engine dynamics.

What do you mean by TCR? What software/formula did you use to arrive at the compression pressure and temp at TDC? How about the compression temp when accounting for timing spark BTDC? How does that compression temp change if we change the spark to 34* BTDC? 35?

Also, is there a resource that I can use to determine what my temp/pressure limits are with 92 AKI pump gas?

When I was talking to Larry, he made the point that dynamic CR can be significantly influenced by volumetric efficiency, especially after bolting-on a set of good-flowing heads (which I happen to be in the process of doing). Is there any way to account for how VE could potentially influence the pressures & temps that you mentioned?

Thank you
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Re: Static CR too high?

Post by David Redszus »

What do you mean by TCR?
Trapped compression ratio is the term used by Blair and others. Same as dynamic CR.
What software/formula did you use to arrive at the compression pressure and temp at TDC?
i write my own software using Excel and formulas extracted from various engineering textbooks.
How about the compression temp when accounting for timing spark BTDC?
The compression temperature (and pressure) will change with each change in piston position. So, at any ignition angle we know the piston position and therefore we know the temp and pressure.
How does that compression temp change if we change the spark to 34* BTDC? 35?
The later the timing, the higher the chamber temp and pressure.
Also, is there a resource that I can use to determine what my temp/pressure limits are with 92 AKI pump gas?
I find the best indicator is the autoignition temperature of the fuel compared to chamber temperature. The exact relationship will depend on rpm and chamber/piston geometry.
When I was talking to Larry, he made the point that dynamic CR can be significantly influenced by volumetric efficiency,

As much as I respect Larry's work, I disagree with him on this specific topic. Volumetric efficiency concerns air mass principally driven by density. We are not considering air density.
Is there any way to account for how VE could potentially influence the pressures & temps that you mentioned?
No, not that I am aware.

Other factors to consider are:
Fuel evaporative cooling, which will change with type of fuel, mixture ratio and evaporated fraction.
Changes in polytropic constant, which result from changes in composition of chamber gases such as fuel vapor and EGR.

Compression temperature, unlike pressure, is very difficult to obtain directly and must be calculated from other measured parameters.
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Re: Static CR too high?

Post by MidWestCoast442 »

Thanks, David. That’s really helpful.

Is there a reference that you know of to find the autoignition temp of 92 AKI gasoline? I have scoured the web and found nada. From your initial post, it sounds like I should be safe in that department. Yes?

I may have misquoted Larry and his remarks regarding the use of DCR to determine the relative safety of using pump gas. I think he was referring to cylinder pressure as it relates to VE: better VE = more air/fuel trapped into the cylinder = higher cylinder pressure & possible preignition even though the DCR/TCR remains constant. Does that sound more accurate?

Were your calculations (based on my stated engine metrics) made with an assumption that the engine is operating at 100% VE and air density at sea level? How would bolting-on a set of heads that are capable of flowing more air/fuel change the results of the calculations? Or would they?

Overall, it sounds like you believe that my combination should be safe with 92 pump gas. Yes? Do you think that a knock sensor & timing retard may be necessary or helpful in this application?

Thank you for answering my questions. Please forgive my ignorance.
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