360 compression ratio, Am I wasting my time?

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SGBARRACUDA

SGBARRACUDA

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This is going to take some speculation on your part but let me give you the facts that i have. I have a 360 that has been rebuilt by the previous owner who has since passed away, so I have little knowledge of the facts. I took it out of my 69 Dart to do alot of detailing under the hood, interior etc. The car was somewhat gutless when it was together so I'm planing on doing some engine mods while I have it out. It did run great, sounded great, ( has a nice sounding bumpstick in it) Though I don't know what it is. I plan on taking it apart after hunting season. I figured it was a low compression engine and this is what I know about it. It has 915 J heads which are open chambered, looking thru a plug hole I can see it has good size dished pistons. I'll cc the chambers, piston dish, and how far down in the hole the pistons are once I take it apart. My Question, Is it possible to get the compession up in the 9.0-9.5 range by cutting the heads? And or using a closed chambered head? Or am I wasting my time and should start over with a new build? Thanks upfront for your imput or suggestions.
 
Milling the J heads to get 9.0 to 1 compression with the factory dished pistons is nearly impossible. Those factory pistons were about .080 in the hole and with the dish are only about 7.5-8.0 compression assuming the heads are stock. If the bottom end is rebuilt like you say a swap to the newer magnum heads would be the easy way to go. They have much smaller chambers. They spec at 59 cc but are usually around 61-62 cc. Mill them .020 and install them and that will bump the compression up to approx. 9.0-1 compression. They also have better low lift flow numbers than the old J-heads an a better exhaust port design. So not only will you benifit from more compression but you'll benefit from better flow. You will have to buy an intake made for the magnum heads and conversion pushrods but everything else will bolt up.

The only other route to take is like Prostreet duster recommended and go with a new set of pistons which will mean a total rebuild of the bottom end and you will have to have it balanced because the KB pistons are allmost 100 grams lighter than the stock pistons. If you don't balance it it will shake itself to death.

You should also try to determine what cam it has in it. When I bought my Cuda it had an awesome sounding cam in it but it was a major slug. It was an old grind with alot of duration and only .440 lift. I switched it for a comp xe274 and the power increase was dramatic.
 
Yea, I'd agree with Fishy. First find out what you got for a cam and compression ratio, everything up to that point is just speculation. Once you've done that sit down and decide what ratio you're looking for and spec out a new cam and don't forget to look at your converter stall and rear end gearing in the overall formula. Engines are like a big blender, put in the wrong ingredient and it'll taste like crap.

Terry
 
Well the major concern with going with the magnum heads is I can't use the Ld 340 intake that I want to use and the stock 340 valve covers. This was a factory 273 Auto, P/S,A/C car. Witch I'm in the process of putting the A/C and P/S back on. Not looking to make a race car just a nice street car with 68-70 340 type performance. The car has a 727 with the 10 3/4 factory high stall convertor and a 8 1/4 3.21 SG. I will ID the cam when I take it apart and I'll post my findings along with the calculated Comp. ratio. Thanks for everyones feed back. What about using the 273 #920 Head?
 
SGBARRACUDA said:
Well the major concern with going with the magnum heads is I can't use the Ld 340 intake that I want to use and the stock 340 valve covers. This was a factory 273 Auto, P/S,A/C car. Witch I'm in the process of putting the A/C and P/S back on. Not looking to make a race car just a nice street car with 68-70 340 type performance. The car has a 727 with the 10 3/4 factory high stall convertor and a 8 1/4 3.21 SG. I will ID the cam when I take it apart and I'll post my findings along with the calculated Comp. ratio. Thanks for everyones feed back. What about using the 273 #920 Head?
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It's quite easy to redrill magnum heads to accept the LA intake. There was even someone that had a fixture they loaned out to do it but a machine shop should be able to handle it. My magnum heads cc'd a 64 and that is what the vast majority of folks that have cc'd theirs get.

Stock valve covers work just fine on magnum heads. The 5 bolt holes of the LA valve cover line up with 5 of the 10 on the magnum head. I have been running LA covers on my magnum heads for 20k miles now and not a drop has leaked out.
 
Seriously, I think the best way to go about is to do a set of pistons. The KB 107 units are an excellent choice and you'll just need to add rings and a balance job.

OK, it' expensive by compare to a milling, but that also avoids mis-aligned bolt holes and intake sealing/bolt hole alignment. I hate going down that road of milling to fit. It is a pain in the donkey butt!!!!!

You'll be ahead of the game very well with new pistons. A .039 gasket will hand out a 9.8-1 ratio. A .050 cometic can drop it about another 1/4 point. to near 9.5-1 pump gas super freindly. The limit of a good quench.
 
rumblefish360 said:
Seriously, I think the best way to go about is to do a set of pistons. The KB 107 units are an excellent choice and you'll just need to add rings and a balance job.

OK, it' expensive by compare to a milling, but that also avoids mis-aligned bolt holes and intake sealing/bolt hole alignment. I hate going down that road of milling to fit. It is a pain in the donkey butt!!!!!

You'll be ahead of the game very well with new pistons. A .039 gasket will hand out a 9.8-1 ratio. A .050 cometic can drop it about another 1/4 point. to near 9.5-1 pump gas super freindly. The limit of a good quench.
Click to expand...

And this can be done with the J heads I have now? I mean the 9.5 compression ratio?
 
Stock valve covers work just fine on magnum heads. The 5 bolt holes of the LA valve cover line up with 5 of the 10 on the magnum head. I have been running LA covers on my magnum heads for 20k miles now and not a drop has leaked out.

I didn't know that the stock valve covews would work! See 52 yrs old and still learning. That is good to know.
 
SGBARRACUDA said:
And this can be done with the J heads I have now? I mean the 9.5 compression ratio?
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Yes it works out good. I have a 360 with the KB107 pistons and X heads (same chambers as your J heads) that have been shaved .020 and they cc'd out to 67 cc. The pistons are .020 in the hole and the compression figures out to be 9.8 to 1

I do have to run premium in it but it's fine on pump premium. If your heads aren't milled any the comp would be real close to 9.5 to 1
 
SGBARRACUDA said:
And this can be done with the J heads I have now? I mean the 9.5 compression ratio?
Click to expand...


Just like fishy, I did this, what I wrote, with J heads cc'd to 72cc's. The pistons were KB107+.030 that sat @ zero deck and a Fel-Pro gasket was used. I do not remember the part number to it, but it was .039 thick.

If you have a desk top dyno, or other ratio calculater, know that there is a 5cc valve relief in the piston. If you can not make this adjustment on the program for piston relief, you'll just need to add 5cc's to the chamber (Taken from the valve relief area) and gasket diameter.

I just opened up the Desk Top Dyno for the ratio calc.

The Fel-Pro @ .039 thick will yeild 9.63-1 and gasket @ .050 will yeild 9.39-1 .
Since I do not remember the diameter, I used 4.1. Typical Fel-Pro spec or close. But the change is minimal.
With my Edel. heads, 65cc chamber, the ratio went to 10.43 or right close to it. (4.04 gasket @ .039 thick)
Very good for pump gas and a sizeable cam.

Of course type of valve face can change this along with a head mill to clean up and true the surface.
 
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