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-   -   Compression and 2.0S MFI (http://forums.pelicanparts.com/911-engine-rebuilding-forum/236704-compression-2-0s-mfi.html)

kenikh 08-18-2005 01:55 PM

Compression and 2.0S MFI
 
I wonder what the realistic maximum CR is for a 2.0S w/ MFI on pump gas. When reading Wayne's book, I noticed that (on p. 112) that a 2.4L w/ 2.2 pistons makes compression of about 10:1. It also says this motor will run on pump gas with single plug. From that, the question has arisen: WHY?

I realize that the 2.4 heads don't have piston dome interference problems like the 2.0, thus aiding flame propagation and increasing knock resistance, but they are larger, increasing flame travel distance. It is thus reasonable to assume you could run a 2 liter engine with even higher comrpession in single plug form and even HIGHER compression in twin plug.

The problem with the stock 2 liter is the heads. Can a 2.2 or 2.4 head be run on a 2 liter to increase flame travel efficiency and compression when coupled with custom pistons? For comparison, the 2.0L HSR motor runs 11:1 compression and needs race gas (again from Wayne's book), but does not mention plug configuration. I will assume single plugs.

In a nutshell, here are my big questions:
What CR could you get on pump gas with single plugs in a stock 2.0? I assume the stock 9.8:1 is near the limit.
What CR could you get on pump gas with dual plugs in a stock 2.0?
What CR could you get on pump gas with single plugs in a stock 2.0 w/ 2.2 or 2.4 heads?
What CR could you get on pump gas with dual plugs in a stock 2.0 w/ 2.2 or 2.4 heads?


Any expert insights will be greatly appreciated.

(edit) What are the squish band repercussions of using larger diameter heads on smaller pistons and cylinders?

kenikh 08-19-2005 12:04 PM

Anyhoo...no one knows or no one cares?

Steve@Rennsport 08-19-2005 04:23 PM

Kenikh:

You have asked some VERY complex questions that go far far far beyond the ability to answer/discuss on a BBS or by e-mail.

If you want to call me sometime when I'm not swamped, I'll help as much as I can,.....:)

kenikh 08-19-2005 05:59 PM

Yikes, I must be reading too many books. Steve, thanks for your offer. Let me know when you are not swamped. :D

Steve@Rennsport 08-19-2005 07:44 PM

Call me near or at the end of a business day,....:)

kenikh 08-22-2005 08:23 AM

Thanks again to Steve. We haven't spoken yet, but I have thought more about my motivations for asking the previous questions and perhaps they are too complex for the actual answers I am seeking. I am getting ready to install a long absent, correct MFI for my 2.0S. My basic questions were:

1. Is it worth twin plugging my stock 2.0S during this install?
2. Would it only be worthwhile if I go with custom pistons to raise compression?
3. If so, how high is reasonable for a twin plug 2.0S?
4. How would these mods affect the character and longevity of the otherwise stock 2.0S?
5. Should I just be happy with the single plug MFI and spend my money elsewhere?


My other questions were only tring to get to the heart of this basic issue. Sometimes I just geek out a little too hard and forget my basic motivations.

(edit) I just did some perusing of the Rennsport site and it looks like the 2.2S heads are a bolt on upgrade for the 2.0S and help immensely with detonation. Very interesting. I promise I won't ask any squish band questions here...I promise...must resist. :o

ttweed 08-22-2005 09:12 AM

Quote:

Originally posted by kenikh
1. Is it worth twin plugging my stock 2.0S during this install?
That is still a "value-laden question" which depends on many personal aspects of the situation, such as "how fat is your wallet" and "how much do you want the cool-factor of twin plugs?" The performance aspect of twin-plugging the stock motor will not be huge, compared to the expense.

Quote:

2. Would it only be worthwhile if I go with custom pistons to raise compression?
IMHO, yes, unless the afore-mentioned "cool-factor" means a lot to you.

Quote:

3. If so, how high is reasonable for a twin plug 2.0S?
Depends on what fuel you use. If you are going to run the cat-piss we get for gas in CA, I wouldn't go any higher than 10.5 or 11:1 on current 91 octane fuel, depending on how far you push ignition advance. If you are willing to run race gas, 13:1 or greater would get you a lot more power.

Quote:

4. How would these mods affect the character and longevity of the otherwise stock 2.0S?
Raising compression will generally be detrimental to longevity, as it will increase stress on the pistons, rods, crank and case. The higher you go, the greater the stress. There is a reason that race motors are "freshened" every 40 hours. The "character" will be changed by giving the motor more grunt due to the higher compression, and it will probably run crisper and cleaner, with fewer misfires, due to the twin plugging.

Quote:

5. Should I just be happy with the single plug MFI and spend my money elsewhere?
IMHO, yes. So far, I have had no problems with my '67 2.0S carb'ed motor with 9.8:1 compression running our CA 91 octane, except for having to increase idle jet size a little bit and installing a CD ignition, to make up for the increased volatility (RVP) of the newer fuels. I think a mixture adjustment with your MFI will be all you need to get satisfactory performance from premium pump gas with your motor, and you already have a CD ignition, so there is little to be gained without spending cubic dollars on it. Just drive it and be happy. If you want more power, swap in a bigger engine. "There is no replacement for displacement."

TT

kenikh 08-22-2005 09:25 AM

Thanks, Tom. You know how slippery the slope goes: MFI turns into twin plugs, turns into high comp pistons, turns into 906 cams, turns into... In the end you could have just built another motor for less money.

To answer the question: I am not into cool factor for cool factor's sake. All I was looking into was making my 2.0 the best it can be without going into stupid money like Pauter rods and a titanium valve train. My thoughts were that if I could do smart upgrades to my 2.0 in the $1500 range in addition to adding the MFI, I'd go that route vs. possibly building another motor. Plus, I like the looks from people when I tell them I only have a 2.0. :D

P.S. I know that some of you will be skeptical of adding twin plugs as cheaply as stated above. It can be done utilizing a Ford crankfire setup, using a MegaSquirt II brain and twin EDIS coil packs or MSDs. Hardware total is less than $700. Some would say that using MegaSquirt without its fueling capabilities is stupid, but you can't argue with the economics of it.


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