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Boom.
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Discussion Starter #1
Ok...after all this confusion about CR on many different engines, let's see if we can collectively get at the truth.

Here's where we are right now:

-Pre-'96 4.6L SOHC engines are 9.0:1 CR
-Post '96 NPI 4.6L SOHC engines are either 9.0:1 or 9.3:1
-PI 4.6L SOHC engines are either 9.3:1 or 9.74:1.

Here are my thoughts...

In the MN12, '94-97 V8 engines are all rated at 205 horsepower. There is a difference of 15 ft-lbs. of torque between 94/5 (265) and 96/7 (280). The torque difference has been attributed to the new better head casting and the new intake manifold. Assuming that Ford's rating system is consistent from 94-97 (which might be a big leap of faith), I don't see how the compression could be different from pre 96 to post 96 NPI engines. The variations in power from '94-98 in MN12 and SN95 cars has always been attributed to other variables such as computer tuning in the case of the Mustang or heads/intake in the case of the MN12, not from a difference in CR.

Now onto PI engines. According to Ford, for the 2003 MY, the 4.6L SOHC PI engine in Expeditions, Mustangs, F-150s, and Explorers is 9.3/9.4:1. I was unable to find a spec on the Vic, but as the CV Sport is rated at 239 HP I assume that it is equivalent. As we all well know, PI engines have offered a great increase in power over NPI engines in stock form, to the tune of around 30-55 crank horsepower.

I would propose that this increase in power is largely attributable to three factors, higher lift cams, better flowing stock heads, and an increase in CR. As 2003 MY vehicles are according to Ford 9.3/9.4:1, I would have to assume that all NPI 4.6L SOHC V8s have a CR of 9.0:1.

There is obviously a lot of conflicting information floating around on this topic. I am not saying that I am necessarily right, I am only trying to get to the bottom of this so that we know for sure. Please feel free to comment on this subject.
 

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Here's my .02 (not trying to start a flame or anything, but this ranks up there with "NOS" with me):

I don't trust ANY written information about Compression Ratio. Even the OEM sources will say "nominal" combustion chamber sizes, etc. I feel that everything that is written is a “ballpark” number.

If I were going to "rely" on a documented CR, I'd still give it a + or - of 0.3. There are just too many things that can change it. Even from the factory, the pistons are sized based on the bore of the cylinder (there are usually three color coded sizes to be matched with the bore), which can change the CR. The cc’s listed for the heads usually have a range for them. Again that changes the CR.

The only way to get the true CR of an engine is to calculate it based on actual measurements of each component.

I’ve had friends quote a CR in an engine and not even know the cc of the heads or the compressed thickness of the head gasket. So unless it’s measured, it’s just a guess.

So all this discussion about 9.0, 9.3, 9.7, 10.0, etc….. just isn’t helping with passing along good information.

Just my .02 and I’ll get off my soapbox now. :thumbsup:
 

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Discussion Starter #3
I agree with the spirit of what you are saying. I guess what I think would be helpful for everyone would be if we could compile the chamber volumes and dish sizes on NPI and PI motors. That way people could use that as a reference and determine easily safe combinations of components. Obviously hardcore serious people are going to find things out on their own, which is fine, just thought it might be helpful to have it all in one place.

Meanwhile I have been trying to track down the actual numbers (CC Volume, piston dish, deck height etc) to varying degrees of luck. Obviously it is impossible to determine the CR of any given engine to the accuracy of Greenbird's significant figures, but you can get within a relatively small ballpark. Obviously I don't know the thickness of my 140k-old head gaskets, the exact volume of my combustion chambers taking into account carbon buildup and such. But this information could prove to be useful and has been necessary even in a generalized sense for people with headswaps who want blowers and such.

It's easy for someone to ask "will a blower work with a headswap" and have one of the higher ups say "no, your compression will be well over 10:1 and that won't work." If we can have a place where people can see, ok, I am going to put PI heads on my NPI shortblock, the dish and CC volumes are these, I can see my C/R will be too high to handle boost. Maybe that's not a good idea. I don't know. I thought it might add to the overall technical saavy of the readers here, including myself.
 

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Yes, I think we should find out what compression ratio the various 4.6's run. That's a good idea. It will also expose those that have incorrect info and hopefully they will correct it on their Web sites and/or books.

If manufacturers can put out horsepower and torque numbers, then they can also put out compression ratios for us.
 

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Palmguy said:
In the MN12, '94-97 V8 engines are all rated at 205 horsepower.
While the factory HP rating of the '94-'95 is the same 205 as the '96-'97's, my understanding is that the '94-'95's were over rated. Their actual HP was lower and the '96-'97s' actual HP come closer to the 205 rating.

I dyno-d my stock '95 back awhile for the heck of it and I'm thinkin the rwhp numbers were like 25-30 lower than my '96. I think, at the time, the '96 only had pullies, CAI and true duals. Good for a few HP gain, but certainly not 25-30.

I'll dig around and find the dyno graphs for the '95 and the '96 with the minimum mods and post the numbers. Maybe someone out there has some dyno numbers from an all stock '96-'97.
 

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Stock-156hp and 212tq with dynomax catback and K&N panel filter. Thats what I dyno'd before all other mods.
Road and Track Oct. 94' Compression Ratio for 4.6L bird-9.0:1
 

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What's the drive train loss on our cars... ??? if you add that in you're a little closer, cause the factory HP ratings are at the flywheel...

of course once again, we've wandered a bit far afield of Compression Ratios, the original topic..

Now maybe we need to start a "HP" thread....:D :D
 

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Palmguy said:
…Meanwhile I have been trying to track down the actual numbers (CC Volume, piston dish, deck height etc) to varying degrees of luck. .
I checked my OEM tech manuals and in the engine section, under specifications, it lists a lot of things.

Here are the specs that from the manuals that I have:
94 TBird: Head cc 50 +/- 1 (I checked the manual this morning)
96 Mark VIII: Head cc 52 +/- 1 (I didn't check the manual today so that number is from memory, but I’ll double check tonight and post if there’s something different.)

Unfortunately it doesn’t list the dish size, gasket thickness, or clearance height (which are the main items in figuring CR). I forgot to look for the deck height, but with the clearance height, it doesn’t really matter

It did list the different bore sizes and piston diameters to use, but I didn’t write them down. And the differences are very small 0.0001, (or something like that) so that measurement probably doesn’t matter.

If everyone would check his or her manuals we could probably have a rather complete list. Granted the manuals can be wrong, but at least it is a starting point. :D

What would be really nice would be a program in the tech section where you can put all the measurements and it calculates the CR. There are lots of web sites with that, but to have it here would be nice.

Just my thoughts on CR. :thumbsup:
 
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