Ford Mustang Forum banner

11:1 ford 302 compression

23372 Views 13 Replies 5 Participants Last post by  1968poney
i have a tread about what octane i should use but heres a kicker the whole engine is a stock 1980 block with 1966 289 heads will the compression affect the internals of the engine like the rods crank bearings or pistons heres a list of exact whats in the 302

Stock cam
Stock flat toped pistons with valve reliefs
Stock rods
Stock crank

Engine bore is .030 over
Mains are .010 over
Cam is .010 over

The heads have a chamber of 54.5cc

a bonus the stock 250 289 302 starter doesnt have enough power to get it turning to start but i got it too sounds great but only had it running for a couple of secounds and turned it off. is there any strenght concerns regarding the fact every thing is a stock 1980 302 i believe i dont know if for sure it is fully stock but would it be fine with 11:1 if it is all stock?

Thanks Tom
1 - 14 of 14 Posts
11:1 sounds a little high on the compression. However, there's people running blowers, turbos and ALOT of nitrous to some stock bottom ends. Unless you're running the crap out of it, I believe you'll be ok.
thanks only one person has advice what am i already hatted already but i should be fine with the internals right i dont know the stress its excerting and im not doing test to find maximum weight forces but as long as its fine i shouldnt worry i hope
Hey with you combo your not running 11 to 1 compression more like 10 to 1.
Here is your average flat top piston and these compression ratios are with .030 over bore yours would be a little less. Sealed Power L2488F30 - Speed-Pro Powerforged Ford Pistons - JEGS
ahhhh thanks sooo much by the way the engine is .030 over bore and those are the pistons in my engine so with 54.5cc heads which i have will produse 10.15:1 i feel sooo much better lol now i could use 93 gas instead of 104 wow that helped but i did the math and came up with 11:1 is the cam in the 1980 302 difrent i have stock was it changed to lower compression i know the heads were larger to lose power and compression but was the cam changed from the 1968 cam ? ?

Thanks Tom
the cam does not change compression. the cams are about the same. like 72 said you will be fine on premium.
sweet that makes me feel good but 10:1 i calc 11:1 on a calc i hope your right i had to get a stronger starter which doesnt make sence if it should be fine the starter was built to handle 10:1 so im thinking its over that but the only way to know for sure is to replace the starter and put a guage on it.
Check all of your wiring my 302 would always give me trouble. I just completed a 351w windsor conversion with a little more compression than yours and replaced all of the cables checked all the grounds and it turns over with no problems .
In your calculation, did you make an allowance for the valve reliefs. If you didn't, the calculated compression ratio will be higher.

The fact that the starter has a problem turning the engine over makes me wonder if the starter's performance is being affected by a low amperage output from the battery or the wiring isn't heavy enough to deliver the amperage to the starter. The ground wire to the battery's negative post needs to be at least the same size as the positive wire. Is there a good ground connection between the body and the engine? If everything is in order with the starter, it may be that the compression is high or the timing is too far advanced.

You at first mentioned that you had stock pistons then you say that you have the forged pistons. I would be hesitant in using stock cast pistons with a high compression ratio. I hope that you have the forged ones as the cast pistons won't hold up in my opinion.
i have cast but they look like the ones in that pic but the skirts were not colored then again i have no idea if there stock or not the starter is copletley grounded i have the engine on a test stand and its hard wired so every conection is perfect i tried 3 factory starters and just gets it to turn but not the speed it should turn at thought it was the batery so jumped it with a car still didnt work i put 3 batterys on it still had the same speed so came to the conclussion that the starter isnt strong enough and to acount for the 4 reliefs i add 2.5cc to the over all chamber i did the clay thing and messured then converted to cc. it would be a tottal of 57cc so with the valve reliefs the compresion is .31 lower so its around 10.6:1 so still not 10:1 which would explain why the starter isnt turning fast enough. now that the compression is 10.6:1 could i run it off 93 or is it still to high i know i can but i have to change the igntion timing but i lose power but it gets rid of the ticking

Thanks Tom
See less See more
Another question, did you account for the thickness of the head gasket? Most head gaskets are between .039" and .041" thickness.

I think that I would pull all of the spark plugs to see how the engine turns over. There might be a binding in the rotating assembly.

The old stock starters used an electro magnet system for the field magnets where the newer factory starters and aftermarket performance starters use permenent magnets and reduction gears. If the engine rotates OK without the plugs, try one of the newer small starters. They don't require as much amperage because of the fact that there isn't those electro magnets drawing a load on the battery. It should turn over an engine with high compression.

Watch out for too much timing advance with 93 and high compression and cast pistons. It won't take much to blow a hole through the top of the piston. Are you sure that you want to use your present heads?
good idea i will try that and would it start with binding or no what could be binding i can turn the engine with a wrench so define how much binding the engine has all new bearings and everything is prelubed the only thing i could think of is because of the new rings they might be really tight it was hard slipping them in they were a nice fit. the only reason why im going with the 289s is because ford lowered the hp on the 302s during 76 something like that and a big part was the 1980 heads they have a 69cc chamber with the valve reliefs thats about 71.5cc compared to 57cc so thats a whole 14.5cc difrence should give me at least stock horsepower to 1968 compared to 1980 i hope your right about the starter i bought a mini high torque for about 14:1 compression i hope it and junk but i will def try the spark plug thing man the simplest things slip your mind thanks
I took the plugs out and the engine turned freely so its the starter
1 - 14 of 14 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.