Power losses from reducing compression ratio?
 

On another forum I came across this and responded to it using the Wallace racing calculator. The OP came back with his response saying the Wallace calculator was wrong. I'm curious as to what you all think. Thanks!

Here are the exchanges, names omitted.

OP: That's what I thought, it's no rap on anyone who replied, but I still have seen no one who has actually run E 85 through a '66 tri power. I've seen comparisons between the racing gas and E 85, but I'm not buying 55 gallon drums (at what, $400 or more?) of racing gas to store in my garage! And I do want to drive the car at least 5K miles per year. Changing to open chamber heads, and re-rebuilding the motor with dished pistons to drop down to a 9:1 compression ratio would lead to a huge loss in torque and h.p., right? Even though everyone says watch out for the winter E 70 versus summer E 85, why is that so bad, and how do you fix it? Add gallons of pure ethanol to the tank right away if you're not at 85% at the pump? What happens if you temporarily go from E 85 at 105 octane, to E 70 at about 100 octane in a 10.75 to 1 motor? Retard the timing a couple degrees til you get the ethanol back up? I live in the corn belt so E85 is cheap and readily available at all the local Kwik Trips. bigD, thanks for the links and the advice, but we know that a tri power is more than a two barrel carb in that when the end carbs dump open the mixture is probably not the same as a Holley 4 bbl or a Q jet and I'm wondering if you can get all six barrels to work right with E 85, and I've never heard that tale. If anyone has, please let me know.


Me: Actually it is not as "huge" a loss as many think. Check out the Wallace Racing Calculator:

Wallace Racing - Calculate New HP From Change In Compression Ratios Calculator

Hope this helps.

("Your old Compression Ratio of 10.5:1 and HP of 350 is now calculated
as a Compression Ratio of 9.5:1 and 346.29 Horsepower.")


OP: Hmmm- I think that Calculator might need some new batteries in it. You can find online an article I read from Hemmings magazine, Aug 2005, called "Tin Indians." They interviewed "Jim Taylor, the legendary Pontiac engine builder from Phillipsburg, NJ." He said; "One point lost in compression equals 50 lbs ft of torque and 50 hp." If I started at 390 hp, that means I'd lose about 80 going from a 10.75 to 9.00 CR. My experience makes me believe Mr. Taylor is correct, all else being equal. That's why I'm still interested in E85, and interested if anyone has tried it in a tri power motor. (The calculator also said I'd lose about 3 hp, which is fantasy IMO.) I think Jim Taylor Engine Service is still in business; I'm going to contact him directly and see if he's changed his belief since he contributed to that story.

How much you lose highly depends on the cam being used. 50 ft. lbs., and 50 hp per 1 point reduction? I don't believe that for a minute. Maybe 15 HP, and Ft. lbs., but not 50. I wonder if Taylor was mis-quoted, as someone misunderstood him.

Thanks, Paul.

I would lay odds that you'd lose less power dropping compression to stay pump fuel compliant then having to pull out 5+ degrees of timing to get it to run without pinging.

Related to what Paul has eluded to....

"If the engine has a HUGE camshaft, where 11.2 just doesn't provide 'ample' cylinder pressure, raising it to 13 will provide big dividends. If the cam isn't that huge, and it has decent cylinder pressure to start with, then you are into the point of diminishing returns, and the gains won't be as significant."

Source: Compression vs HP

http://www.speedtalk.com/forum/viewt...14016&start=15


Maximizing the compression ratio delivers more bang for the buck than you may imagine.
http://www.hotrod.com/articles/0311em-power-squeeze/



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Just a thought when discussing iron heads and compression ratios...Was there ever a stock Pontiac engine that has what we consider an ideal compression ratio? I don't think so? The early engines up to 1970 have too much compression for todays gas and after 1970 have too little compression to be ideal.

1970....Sunoco's dial went to 102 or better back then. 100 for premium was common most places.


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My understanding is that all other 'things' being equal, dropping SCR is a loss by a percentage, and you really can't say by X hp.

I know that for some builders, SCR is more an afterthought, like, if you're targeting a particular usage & power level, for example, drag racing, you know you want/need to run cam with X duration/lift, then it will require X SCR.

If it's a car that will be driven on the street using pump gas, then you already have some limitations set forth. That will help set some hard limits on engine parameters, then you work with the rest, and tailor it to preferences.

So with a street/strip car, with iron heads, a 1 point SCR change, difference between say 10.5 and 9.5, the level of power loss is nominal.

If it's more than 1 point, then the 'factor' will be greater. Like if you're going from 13.5 to 8.0, yeah, the loss will be greater.

That's why you can have a generic X hp lose per point. The further you go from the 13.5, the rate of loss increases more rapidly.

Not sure that makes sense to some, just having troubles putting my thoughts into words.

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Exactly. It's really camshaft dependent.

I knocked the compression down in this to be pump gas friendly, but the 70 RAIII isn't a large camshaft anyway. My thought is that 10.75:1 factory is too much for this little cam anyway. It didn't hurt the performance of the engine at all as far as I can tell compared to other stock RAIII's.

https://youtu.be/er1z7PpqsnY

Now if it were a RAIV it would be a different story, that camshaft needs compression to work properly.

That chart in that Hot Rod article about theoretical change of HP, in percentage, does a good job of saying what I was trying to explain:


So dropping from 10.1 to 9.1 is roughly a 2.9% HP difference. Nominal. An example would be for a 500hp engine, you would lose less than 15hp.

Yeah, so one drawback to running a low SCR, like 9:1 or less, it limits your cam selection, which in turn limits the amount of power you can make.

Lots of people already know what cam they want to run before they even start a build, so targeting an SCR is already done for them. There's no reason to go beyond X SCR with a particular cam profile.

Curious what the actual SCR is for some of the factory engines, I know the SCR numbers from the factory are inflated, by at least a half a point, if not more.

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In the article posted....

"Bumping up the CR one point from a low ratio has a greater effect then bumping it up from an already high ratio. This means the bigger the cam the more responsive it is to a raise in CR, especially in the lower rpm range."


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Yes yes, and that chart shows that. Also, if you're already below the recommended min SCR for a particular cam profile, and you bump it up, bringing it into the min or above range, the increases will be more. (percentage)

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Yes they are inflated. Every factory engine I've torn down has never had it's advertised ratio. Usually pistons are down in the hole .025" or more, and some heads don't CC as tight as advertised either. Then throw in the fact that most decks are uneven which changes compression ratio from cylinder to cylinder. No wonder these engines run better when they are rebuilt properly ;)

Getting back to your original thoughts, in PS they allow a 1.5 compression bump over stock advertised numbers. For my RAIII engine that's 12.25:1. Like Steve mentioned above, I see very little benefit doing so on this particular engine because the RAIII cam you have to run is so small anyway. Matter of fact the cylinder pressure would be extreme. This little cam just doesn't need that much compression to run well.
The RAIV's however benefit greatly from the compression bump.

Another example on the other end of the spectrum is my 69 Z. Advertised at 11:1. It actually had closer to 10.5:1. I built the engine with a true 11:1 ratio though. Because the cam in these things is pretty radical for a little 302. 254 degrees @ .050. It needs compression to run properly. Dropping it to what some consider pump gas friendly would quite frankly make this engine a dog. I'm running the factory iron heads and 91 pump gas with 36 degrees of timing, runs beautifully, thanks to that camshaft bleeding off cylinder pressure.

Didn't the factory use thin gaskets....compared to aftermarket replacements?

Related, the article goes into combustion chamber dynamics. Something to consider.

I've mentioned this before. Years ago I was in the process of beginning on a pump gas build using a set of older style 87cc chamber Edelbrock cylinder heads. In conversation with Dave Bisschop he stated 10.25 max for Edelbrock 87cc castings and 10.5 max for their 72cc castings, this based on his dyno testing at the time. Difference being the chamber design. He felt you can't protect all of these engines once they leave your shop so he purposely kept it conservative for most that don't want (or shouldn't) push the envelope and stay on top of the tune, bad gas, weather, etc. This was his opinion with the older chamber design Edelbrock heads. The newer "heart shaped" design is a different story and a higher compression can be run. As is the chamber design of the KRE d-port head.

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I don't necessarily put too much into that.

When dad built his engine, and used the old Edelbrock 87cc bathtub chamber heads, Tony Bischoff built that engine with 10.84:1 compression and it's running fine on 91 octane pump. When dyno'd it made best power at 34 degrees timing as well. Didn't need a bunch of timing in it like others preach for those heads. With timing dropped or raised the engine lost power.
Usually the more modern heart shaped chambers I've been involved with do in fact like less timing. Usually a range of 26 to 32 degrees, using a dyno to find the sweet spot.

We found our best with 37 degrees timing on that combo using the 87cc chamber Edelbrock head mentioned. So it will vary.

Yes we know you can run more compression than Dave stated, again opinion because he had no control for engines once they leave his shop.

Stated during a similar discussion with the Edelbrock head:

"Butler has customers run 9.9 CR so they don't have problems should they make a mistake and run 87 octane. What the man told me."

Edit: On a Ken Crocie build using the same Edelbrock head he found ignition timing was varied from 37 degrees to 46 degrees, with 42 degrees being the sweet spot for the 455 based combo, 10.2 compression ratio was designed to run on 92-octain pump gas.


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Man you'd have to be a numbskull to damage an aluminum headed engine with only 9.9:1 compression LOL With the right camshaft that would run on cat pee :D

That's where I'm at with the example I posted above, and a stock "iron" head and RAIII cam and running pump gas at 34 degrees timing.