OK. So the 390 in the truck is pretty worn and I'm starting to casually consider a rebuild. So. I've been lookin aroung for pistons. The stock compression ratio is about 8.6/1 iirc. I'd like something more like 9-9.5/1.

All the pistons I can find are just stock, even the flat top ones are the same cr/compression height. What do I need to do and where can I find this stuff?


thanks alot guys...

SuperCab said:

OK. So the 390 in the truck is pretty worn and I'm starting to casually consider a rebuild. So. I've been lookin aroung for pistons. The stock compression ratio is about 8.6/1 iirc. I'd like something more like 9-9.5/1.

All the pistons I can find are just stock, even the flat top ones are the same cr/compression height. What do I need to do and where can I find this stuff?


thanks alot guys...

Click to expand...

Summit sells a bunch of Pistons for the 390

http://store.summitracing.com/egnse...94840105+4294889097+4294925232+115+4294891096

I found 13:1 pistons at a swapmeet, so there available.

Could you get fuel high enough octane for 13:1??? It'd ping something terrible unlessyou had like 120 octane fuel...

Also, how do you tell what c/r it will give you?

Did you check Summit?

OK. How about for a 360? I have an old one in the garage, bought it from an old guy that thought it was a 390, but never got around to doin' whatever he was gonna do with it. paid 160$$ for it. anyway, it'd be easyer to rebuild it and than swap them than to take my 390 out and redo it. what parts can you get for a 360? are they the same as the 390 ones???

Ya, parts should be the same.

so i can just get a set of hi comp 390 pistons and stick em in the 360?

SuperCab said:

so i can just get a set of hi comp 390 pistons and stick em in the 360?

Click to expand...

You will need to check the bore size to make sure there the same, most FE parts interchange they change the bore and stroke to get the various sizes.

SuperCab said:

Also, how do you tell what c/r it will give you?

Click to expand...

Static Compression Ratio is a moving target and depends on several things. You need to know the following information to determine the Static Compression Ratio:
Bore diameter
Stroke length
Rod length
Deck height
Combustion chamber volume
Cylinder head gasket compressed thickness
Cylinder head gasket bore diameter

Here are a couple of links to Static CR calculators
www.rbracing-rsr.com/compstaticcalc.html
www.csgnetwork.com/compcalc.html

Rough rule of thumb, 87 octane fuel is good to about 8-8.5:1 CR; 91 ocatane is good to about 9.5:1. Aluminum heads are good for one full point of compression more since they reject heat better than cast iron.
Also some engines respond better to high comprerssion ratios than others.

However the "Static Compression Ratio" is not the actual compression ratio the engine sees. The "Dynamic Compression Ratio" is what the engine truly sees.

Dynamic Compression Ratio is less than static compression ratio because the intake valves are open when the piston is at BDC. The longer the intake valves are open after BDC the lower the Dynamic Compression Ratio. So Cam timing has a serious effect on Dynamic Compression Ratio and overall engine performance.

Other things that effect how high of a compression ratio can be run before detonation occurs, computer controlled engines, knock sensors etc allow for higher compression ratios.

In addition, compression ratio is only one component of engine power, you also need to properly select the cam & cam timing along with the ratio of the rod length to stroke length. The internal geometry of the engine has a large effect on the power that the engine produces. For most applications you want an internal geometry ratio of 1.75:1
http://victorylibrary.com/mopar/cam-tech-c.htm

Here is a link to properly measure Combustion Chamber volume and Above Piston volume.
www.automedia.com/ViewPrintArticle.aspx?AutomediaId=pht20020701cr

Advertised piston compression ratios are an approximate guess by the piston manufacture, you will not know the actual static compression ratio until you select and install all the engine parts then measure everything to properly calculate the CR.

I am runnin about 10.5:1 and I can run it on 87 with lead addative and it only pings a little but when I put 91 in it it dont need the lead.

I thought putting a engine together was easy but now its looking like i'm going to need to hire Steven Hawkins to build a good preforming engine.

1985 Ford F-150 said:

I am runnin about 10.5:1 and I can run it on 87 with lead addative and it only pings a little but when I put 91 in it it dont need the lead.

Click to expand...

Tetra-ethyl lead was added to gasoline starting in the early 1920's specifically to reduce knocking/pinging since led is a cheap octane booster.
www.uwsp.edu/geo/courses/geog100/Lead-Science.htm
www.runet.edu/~wkovarik/ethylwar/
http://en.wikipedia.org/wiki/Tetra-ethyl_lead


Did you cc your cylinders to determine your compression ratio or just go off the manufactures published numbers?

Also cam grind and timing effect compression ratio.

TexasNomad said:

I thought putting a engine together was easy but now its looking like i'm going to need to hire Steven Hawkins to build a good preforming engine.

Click to expand...

Tex,

It really depends on what you are trying to accomplish. If you are just doing a stock rebuild because the engine is tired and you want to refresh it back to stock numbers, the rebuild is fairly straight forward.

However, if you are trying to build a performance engine then proper pre-planning and parts specifications are neccessary, followed by diligent machine work, and meticulous build procedures & habits. When building a performance engine research all the information you can find on the specific engine you want to build, read all the information you can find & talk to everyone you can find who has built the type of engine you want to build. Generally someone has already built the specific engine you want to build and so they have worked out many of the bugs & problems. Also take the information supplied by the performance parts manufactures with a grain of salt until you can locate people who are successfully running the same setup.

Well the pistons I have were 10:1 and with the stock 390 heads I could run 85 without lead and it wouldnt ping. But we put some non cj 428 heads on it with the large intake ports and it started to ping with 87 so the way I figure it is that it bumped the compression rario up at least half a point.

what did you do with the old heads??? Put them back on, it won't ping and I'll take the 428cj off your hands.

Also, Does anyone know where I could find the crank rods and pistons for a 390? If I put them in the 360 block I'd have another 390.

Cheap, too. Someone must know of some setting in a garage somewhere being tripped over and I could get 'em for $50 or something???...

Just a crank would work too...

It wasnt a cj just a reglar 428 and the block is cracked in two cylinders and two rods are gone ill go take a couple pics of em theyll be in my gallery. The crank is good but im hangin on to that for a 410 in the future. I also like the power that the 428 heads give me instead of the 390 heads. Them heads are just sittin in the garage for somethin else.

are they good or do the valves need ground???

I got em all checked and had em do what ever needed to be done to the heads so they put new springs seats and they shimmed the valves and put brand new ones in.

SuperCab said:

OK. How about for a 360? I have an old one in the garage, bought it from an old guy that thought it was a 390, but never got around to doin' whatever he was gonna do with it. paid 160$$ for it. anyway, it'd be easyer to rebuild it and than swap them than to take my 390 out and redo it. what parts can you get for a 360? are they the same as the 390 ones???

Click to expand...

360, same bore as a 390 (4.05"), the 360's stroke is 3.50" / 390 = 3.78"

Difference between the 360 vs the 390: Pistons, connecting rods and crankshaft.

Everything else is the same.