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Old 12-25-2002, 12:01 AM
The Vagabond The Vagabond is offline
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Compression Ratio

A tech article on this site suggested to me that 10.5:1 compression would run fine on pump gas. Does that seem realistic to everyone here?
I've read before that anything over 9.5:1 risks pinging if driven at length, especially in-city.
I'm about to up the compression on my straight six. Obviously I want to press it as high as reasonably possible, but my truck is my daily driver. I can't have it pinging or detonating when the engine gets warm.
On a semi-related note, I've thought about getting an aftermarket head instead, which would solve both the compression problem and the valve problem all at once.
Does anyone know where to get my hands on a cylinder head... I'm looking for bigger valves and smaller combustion chambers.
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Old 12-25-2002, 02:30 PM
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mantta mantta is offline
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Compression Ratio

If you are talking about a 4.9, the 240 ci head is a bolt on and the combustion chamber is smaller but I do not know how much.
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Old 12-25-2002, 02:35 PM
tetraruby tetraruby is offline
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Compression Ratio

>A tech article on this site suggested to me that 10.5:1
>compression would run fine on pump gas. Does that seem
>realistic to everyone here?
>I've read before that anything over 9.5:1 risks pinging if
>driven at length, especially in-city.
>I'm about to up the compression on my straight six.
>Obviously I want to press it as high as reasonably possible,
>but my truck is my daily driver. I can't have it pinging or
>detonating when the engine gets warm.

Anything over ten seems a bit on the high side. You'll be needing premium gas all the time. Last year that was over $2 a gallon here in CA. I'm sure your machine shop can give you an answer on the optimum compression to go with. Two advantages to staying a little conservative with lower comression is that 1) The decrease in explosiveness happening in the combustion chamber will be better for long engine life (less brute force on the pistons, rods, and bearings) a concern for a daily driver -The HD's had lower compression for a reason. 2) Lower combustion temps will also occur - this will definetly help with emissions realted issues if you must deal with them..

>On a semi-related note, I've thought about getting an
>aftermarket head instead, which would solve both the
>compression problem and the valve problem all at once.
>Does anyone know where to get my hands on a cylinder head...
>I'm looking for bigger valves and smaller combustion
>chambers.
There aren't any reasonable sources other than a junk jard. A 240 head will bump up the compression, the larger valve install is a custom job. I just bought a 300 head with the SBC valves and ported bowls (a stage II). That was $700 - and a deal at that. I'm planning on running dished 351W pistons to keep the compression low for future turbo install. But first things first, I need to save $$ for all the goodies. Good luck.
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Old 12-25-2002, 07:04 PM
Silver Streak Silver Streak is offline
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Compression Ratio

With EFI you can run a lot more compression than 90% of the old timers will tell you. It's pretty much been beaten to death for the last 25 years that you can't run over 9.5:1 on pump gas, but technology has come a long way. Some engines will tolerate nearly 12:1 on pump gas with the right cam and most performance engines are now coming from the factory with over 10:1. On a 300 9.5:1 is probably reasonable with most cams. The more cam you run, the more compression you can run without problems. A general rule of thumb I tell my customers is that they can run 9.5:1 with any cam up to about 270 degrees duration, then compression can be increased about 1/2 point for each additional 10 degrees. You can go slightly higher with EFI and roller cams. Fords typically have better chamber designs and rod ratios than other engines, so you could run slightly more because of that. 10.5:1 doesn't seem unreasonable, but I don't like running premium in my vehicles. I cringe when I take the Lightning to the pumps.
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Old 12-26-2002, 09:24 AM
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Compression Ratio

[updated:LAST EDITED ON 26-Dec-02 AT 10:25 AM (EST)]The EFI and 240 heads have a decent chamber design but the rod ratio is pretty bad on these 300's. Overall though, 9.5 is a good limit. If you do swap in a 240 or an EFI head, make sure to get your distributor recurved. Both of those heads are a 'fast burn' design which needs alot less advance than the slower bathtub design of the carb 300's.


-=Whittey=-

[edit] Don't forget that the new motors running high compression ratios have much better design CC's and are running aluminum heads, which handle higher CR's naturally.
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Old 12-26-2002, 10:17 AM
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Compression Ratio

A cast Iron head, regardless of the fueling system, is good for 9.5:1. Period. I know lots of you remember running 11:1 Mustangs or Camaros, but remember, leaded gas was good for 130 octane. I have a 70-72 240 head on my truck, which actually has slighlty larger valves from the factory, along with stock size pistons. With the overbore in a rebuilt engine, this comes out to 9.2:1. This is about the ideal for these engines, as I can run this thing on 89 octane day in and day out.

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Old 12-26-2002, 05:41 PM
Silver Streak Silver Streak is offline
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Compression Ratio

What is the rod ratio of the 300 anyway?
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1990 F-150XLT Lariat Stillon the original engine with no squeaks or rattles at over 300k miles
4.9/M5OD/3.08
16.89@77.09
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2001 Lightning #127
13.18@104, no chip, no pulley, no nitrous

Confuscious say "Man who have no use for jackstand have promising career as jackstand".
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Old 12-27-2002, 07:10 AM
whittey whittey is offline
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Compression Ratio

Ford 240 Six 3.18 6.79 2.13
Ford 300 Six 4 6.21 1.55

The 240 is good. The 300 is baaad..... There aren't that many common motors that have a worse ratio from the factory:
http://www.angelfire.com/ar/dw42/rodratio.htm


-=Whittey=-
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Old 12-27-2002, 05:03 PM
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Compression Ratio

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Old 12-27-2002, 05:58 PM
Silver Streak Silver Streak is offline
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Compression Ratio

That is pretty bad, but not as bad as I thought it might be. It's definitely low for a Ford.
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1990 F-150XLT Lariat Stillon the original engine with no squeaks or rattles at over 300k miles
4.9/M5OD/3.08
16.89@77.09
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2001 Lightning #127
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Confuscious say "Man who have no use for jackstand have promising career as jackstand".
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Old 12-27-2002, 05:58 PM
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