Need some help.....

[RustyRatRod]

I need some pointers. I am in the process of acquiring parts for a stock bore and stroke 383 build. Well, ok, .030" over. I'm an old school mechanic/machinist/welder/fabricator. Built a LOT of engines through the years. Here's my delema. I WANT to build a higher compression engine. I want it to run on pump gas. Super would be good.

I've read a lot about quench engines and understand the basic principle, but I've never built a quench engine. Here's my basic question. I've already chosen my camshaft. I have also decided on pistons. KB400. I've done several calculations on the KB web site and with 80cc heads (what 516 closed chamber heads are usually around), I can get 10.5:1 real easy. Also, since the piston itself sits .024" in the hole, zero deck height is acheivable.....and I plan on doing that for additional quench. The camshaft I have is this one:

http://www.lunatipower.com/Product.aspx?id=1573&gid=283

With 261 and 271 @.050 on a 108 LSA, I'm thinkin that'll help my pump gas cause. Will it run on pump gas with good quench and that big *** old school cam with around 10.5:1? I can run a thicker head gasket to lower it, but keep in mind, with zero deck height, my head gasket WILL be the quench distance, so adding to the head gasket will decrease quench. Thanks for any advice.

 

[RustyRatRod]

I did some figuring off the cam spec card on the KB calculators. I came up with 10.421:1 static compression ratio and 7.779 dynamic compression. This doesn't take the quench into consideration. I think even without quench, that would work, much less factoring it in. This is figured using SUmmit copper head gaskets with 4.380 bore and .043" compressed thickness. .043" would also be the quench distance using the 516 heads. Opinions?

 

[rumblefish360]

What would the ratio be if it were at zero deck height and a .040 head gasket?
Currently, you can that cam easy.

 

[RustyRatRod]

What would the ratio be if it were at zero deck height and a .040 head gasket?
Currently, you can that cam easy.

Probably not much different. The figures above were calculated with a .043" head gasket. I think general rule of thumb for best quench distance is like .035"....but I am skiddish of going down that far with the head gasket. That might be too much compression. See....I have to keep in mind Georgia heat in the summer and possibly sittin and idlin in traffic. .040 might be a good compromise.

 

[RustyRatRod]

8.372 dynamic. That might be pushin the envelope....but see that's the grey area I am inexperienced with. Since the calculator doesn't take quench into consideration, I am betting I can get by with it.

 

[RustyRatRod]

Screw it. Nevermind.

 

[Meep-Meep]

Back in the 80's I built a 383 with 915 heads and zero deck pistons. Ran the MoPar .528 cam and it seemed to do very well. In a B body and a 4.88 Dana I went 12.65 @ 107. I have no idea what the compression is but since I still have the engine I can reverse engineer it now that I have the equipment. My quench distance was the composition gasket at ~.040". I added one valve relief for the intake otherwise the piston is flat. Back then I had 92 at the pump and no issues. I also don't recall my cranking compression.

Right now I have a 440 with -.015" deck KB's and 915 heads. With a shim gasket I CC'd at 10.1:1. I am running the .455 MoPar cam and the combo blows 190 PSI. With a factory cast iron intake and 750 AFB I can run pump 91 but it will ping on occasion or when you hammer it. I mixed some 110 and brought the octane to about 95 and that shut the pinging up. Recently I put a 6 BBL on and the pinging pretty much stopped probably because the fuel curve is richer. It does seem to make more power.

Hope this helps.

 

[rumblefish360]

The closest I have run is .039 in the 360's I build. This is good enuff for me. There has not been a need to get it closer. Part of that "Perfect distance" is the need for a good rod that is not going to stretch and allow the piston/head to connect. Info on rods can be looked up at the manufacturer of the rod.

If your going to stay with a OE rod, then .040 is fine.

Quench;

More the merry. Even if you can get a zero deck hieght and a .040 gasket, you can gain more compression and quench area with a milled down or closed chambered head. Of course if you went to high on the ratio, no matter how good the quench is, pump gas octane may be an issue.

IMO, with what you have now, your good to go no problem.

 

[oldkimmer]

.........If this is a street car i would tone down the cam...............kim......

 

[RustyRatRod]

.........If this is a street car i would tone down the cam...............kim......

Naw.....that would take all the fun out of it. LOL It's gettin a 8" converter and 4.56s. It'll be a good match. lol

 

[383man]

Did you figure the comp at zero deck or with the piston .024 down ? On my 493 I run a cam close to yours. I run 10.6 comp with aluminum EZ heads and my cam is a solid flat tappet with 264 & 270 at .050. Its a 110 LSA and I have it in at 106 centerline. The lift is right about .590 but I use 1.6 rockers for about .630 lift. With 37 total timing and on 92 pump it has never pinged and runs fine. You should get by as the large cam will help but its still a little close with 10.5 comp and iron heads. I will say my old 440 had 10.0 comp with 906 heads and the MP .557 cam and it had no pinging at all on 38 total. It had some quench as I used the KB quench pad pistons for the open chamber 906's. Ron

 

[RustyRatRod]

I am calculating zero deck height using the KB400 domed piston and 80cc closed chamber 516 heads with .043 compressed thickness gasket. So with zero deck height, the quench is the gasket thickness. According to the United engine calculator, that is 7.779:1 dynamic compression, which falls well below the 8.5 threshold for pump gas. I think it'll work. I know the domed pistons are not optimum for quench, but the calculator doesn't take quench into consideration, so any quench at all will help even further. I plan on a loose converter with 4.56 gears, so it'll have good leverage to move the car.

 

[AdamR]

How do you get zero deck with a domed piston ? Its going to have a positive deck height because of the dome. Is the dome going to clear the closed chamber of the heads ?

Piston choice for 383s are limited, You would be best off getting something custom made if your really worried about quench. The easiest way to get the quench right would be to have a flat top piston made that can be set a zero deckk, Run a .040" head gasket and mill the closed chambered heads to the desired CC to get your compression were you want it.

 

[IQ52]

How do you get zero deck with a domed piston ? Its going to have a positive deck height because of the dome. Is the dome going to clear the closed chamber of the heads ?

Piston choice for 383s are limited, You would be best off getting something custom made if your really worried about quench. The easiest way to get the quench right would be to have a flat top piston made that can be set a zero deckk, Run a .040" head gasket and mill the closed chambered heads to the desired CC to get your compression were you want it.

Argh! When did we start measuring the deck height at the dome?

 

[Garys1969RR]

I have 14 cc dome pistons in my 451, and they come to .017" below deck on the flat area of the piston opposite the spark plug. Thats where you determine your quench distance. So with closed chamber heads, and a .020" compressed gasket, I would have .037" quench distance. And I think thats about ideal. Right now running 906 heads, so I'm more like .137" quench distance. And thats close to what the factory motors were at in the 68 and 69 Magnum engines. Without the dome of course. Maybe a little more deck clearance, I dont know the blueprint spec. So with my C/R of 12.5 to 1, 100 octane Sunoco fuel, at 5400 feet it doesn't ping unless the motor gets too warm on a hot day. Alot of variables involved here. Over 2500 feet MSL you can use more compression compared to sea level.

 

[AdamR]

Argh! When did we start measuring the deck height at the dome?

Im a body guy not an engine builder. Its a legitimate question. Isnt the dome still to big for a closed chambered head ? Especially if the dome sits out of the bore ?

 

[IQ52]

The piston is designed so that the dome protrudes into the open area of the cylinder head, the area where the valves and the sparkplug are. The piston is flat and below the top of the bore where the cylinder head is flat. That flat area is where you measure the deck height.

And then someone is going to say that isn't so on a quench domed piston. Yer right but that piston isn't designed for the 516 head.

 

[AdamR]

But the KB400 isnt designed for the 516 head either is it ? And when figuring compression your not calculating at zero deck, your adding the positive displacement of the dome.

But yes I understand how the flat portion of the piston needs to be at zero deck to properly measure how many CCs the dome would take up.

 

[RustyRatRod]

Yes, the KB400 is designed to work with the factory closed chamber head all the way down to 72.5cc.

 

[IQ52]

It is designed for the 516.

For your compression you are caculating from whatever deck height you have AND you are considering the dome volume. The KB400 has a compression height of 1.908", the stock rod is 6.358" and 1/2 the stroke is 1.690". This all equals 9.956" and the O.E. block height is in theory 9.980" which would yield a deck height of -.024".

But it is always better to measure from the top of the block to the flat of the piston to know for sure the deck height. With the fly cut for the valves and the pop up on the piston, the KB400 will give you 6cc reduction in combustion chamber volume. The combustion chamber volume in this case would be the head volume + the deck height volume + the head gasket volume MINUS the piston head volume (6cc).

A word of caution, most online calculators require you to enter a negative number for a pop up (-6cc) and a positive number for a dished piston.