302 Engine mods

[Rsmach1 71]

According to the Edelbrock cam card, the intake valve closes 29 degrees (vs 8 degrees for stock 69 302) after the piston starts to move up. If the desired dynamic compression ratio is 8:1 then the static compression ratio needs to be 9.8:1 with this cam. Please let me know if your calculations show otherwise because I'm open to learning from all of this.

 

david

From the specs I have the stock 302 had an Intake closing of 26 degrees, you sure you didn't look at the exhaust closing by accident?

[moodster 55]

I attached the cam specifications I have for stock Ford camshafts.  It shows the 68-72 Ford 302 to have an Intake close event at 8 degrees ABC.

 

 

[moodster 55]

I think I see your mistake, the 289HP had an intake closing of 26 degrees.  However the compression ratio on that engine was 10.5:1.  The overlap on the 289HP was 82 degrees versus the Edelbrock's 51 degrees so this Ford cam really liked the upper rpm range.

 

david

[danno 128]

fyi, I am following this, interesting to read.  I am wondering about my own poor little 302...  maybe some day it will see improvements, but I will gain the knowledge from chats like this first!

Thanks, great suggestions....

[Rsmach1 71]

Actually we are both incorrect. If we figure the DCR of a 1970 302 with 9:1 compression based on the 9° figure you come up with an 8.97:1 DCR. This would hardly run on premium pump gas, not to mention the regular low octane gas it was designed to run on. After looking at the chart you posted I went and dug out my old Ford service manual and found that the 70° number (also on your chart) is actually correct, @.006 valve lift. Which is also the number you would want to use to figure our DCR (not timing figures @.050, I can elaborate later if your interested). If I remember correctly Ford also ground there cams with a small amount of retard, I don't recall how much but that amount has to be subtracted from the 70° when doing DCR calculations. As for recommended DRC's 8:1 is about the max limit for old style IRON heads, with aluminum heads it's around 8.5:1

 

Edit when I say max limit I'm referring to the ability to use premium pump gas without having to limit timing.

[Rsmach1 71]

Oops lol actually you would add the amount of retard ground into the cam to the 70°, sorry I'm tired lol.

[Rsmach1 71]

 

Here are the numbers from the Ford service manual.

 

[danno 128]

Novice here, but I was examining engine details and the difference between a 351 2v and high compression 351 4v.  I was looking, and the bore, stroke, and combustion chamber are the same in both engines.  So the only way one can have a much higher compression ratio is with the cam. Is this correct?   The higher compression in the 351 4v is obtained by the intake valve opening earlier and maybe closing later? Maybe also lifting the valve higher?  That gets more air into the cylinder on the intake stroke, and more air to compress.   It is making sense.

 

I have always wondered why my mileage is so bad in my 302, and maybe it is the cam?  The motor was rebuilt a long time ago, and I know before that the car got a lot better mileage. Who knows what cam was put in the car?  But I can make some measurements, based on the chart above, and find out. Those measurements will not be easy, as the precision needed will be difficult. But a worthwhile use of my time.

[moodster 55]

We should probably start a new thread however I'll try to succinctly answer your question.  Compression ratios listed for production engines are always static and not dynamic.  So the intake valve opening has no effect on the static compression.  I would have to guess that the difference in the compression listed is due to the pistons.  Ford probably used a flat top piston in the 4V versus a dished in the 2v engine.

 

For your engine, I would recommend just simply taking some cranking compression tests to determine what your dynamic compression ratio is.  Using this and measuring the valve lift/duration at the rocker you can get a pretty good picture of the cam and compression you're dealing with.

 

david

[Rsmach1 71]

Moodster you're guess is correct, 351w 2v had dish pistons and the 4v Flat tops.

[barnett468 418]

in 68 the 302 2v heads were 63 cc's and the 4v heads were 54 cc . . in 70 the heads were 58 cc's

 

ford used flat top pistons in the 289 with 54 cc heads in the 4 barrel engine and used dished pistons with the same heads for the 2 barrel.

 

the dynamic compression calculators are just a rough estimate and you can have the exact same cam but different dynamic compression numbrrs if one exhaust is restrictive and another is not . . this is due to the different scavenging abilities of each exhaust system . . in general, the better an exhaust system scavenges [sucks the exhaust out], the more intake charge the engne will pull in.

 

the dynamic numbers are more accurately reflected thru a professsional computer duny sim like dyno 2000 with the upgrades.

 

also, dynamic comp numbers change with rpm..

[barnett468 418]

I have always wondered why my mileage is so bad in my 302, and maybe it is the cam?  The motor was rebuilt a long time ago, and I know before that the car got a lot better mileage. Who knows what cam was put in the car?  But I can make some measurements, based on the chart above, and find out. Those measurements will not be easy, as the precision needed will be difficult. But a worthwhile use of my time.

 

 

if you set your timing curve so it is optimum for your particuar engine, it will increase power everywhere and wil also increase milege.

 

the hotter you run yur engine, the less it will wear if you use good oil and the leaner you can jet it which will increase mileage.

 

using oil thicker than you need will reduce mileage.

 

if your carb is jetted rich you will get less mileage.

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