383 DCR and quench questions - Lateral-g Forums

**WSSix** · #1 07-12-2016, 06:12 AM

I've gotten my heads and cam back from Ai for my 383 and am working on selecting pistons. I've been reading about DCR and quench etc in an effort to know what I'm spending my money on and how it's going together or should go together. I've been using the calculator found here to calculate DCR for my engine https://www.uempistons.com/index.php...tors&type=comp. I've compared it to other calculators and the values are all close.

From what I've been reading I need to keep DCR to a max of 9:1 and quench between .050 and 0.035 inches for optimal results in a street engine running 93 octane fuel. I'm having a hard time achieving these numbers based on the parts Ai has sent me to use, and I'm not sure what's the best way to adjust in order to achieve these numbers.

I was wanting to use Mahle flat top pistons with four valve, 5cc valve reliefs as I've heard good things about these pistons. If I do this though, I end up with a DCR way over 10:1 using the gaskets Ai sent and zero decking the block. The only way I can get the DCR down is to use a 0.050 thick head gasket and leave the piston in the block 0.020 inches. That doesn't sound ideal.

I can use different pistons obviously, it just brings me back to not knowing what's the best way to adjust so that I can bring DCR down to an acceptable level yet some how maintain good quench. Is there such thing as going with too thick of a head gasket? Should I definitely zero deck the block and use dished pistons? Does it even matter so long at DCR is achieved? Any help is appreciated.

Relevant info is as follows:
Fully forged 383
6 inch rods
will receive a 4.03 bore
block is undecked so far
currently have 0.026 thick, 4.11 bore head gaskets
57 cc minimum combustion chamber heads according to Ai's info
Cam has a 110 LSA +5.9 adv, intake closes at 61.1* ABDC, and advertised duration is 269.8* Duration at 0.05 is 234.9/244.2 if it's needed.

I did a quick search on Summit just to see what else is out there. DSS makes a piston with a 13cc dish. Combined with a 0.04 gasket and zero decking, I can get a 8.9:1 DCR. Or, use the 0.026 gasket and leave the piston in the block 0.010 inches and have a 9.1:1 DCR. No idea on quality of the pistons. I'm just using them as an example.

So what would you do? I'm probably over thinking this as always.

Thank you!

Che70velle · #2 07-12-2016, 01:37 PM

Trey, quench is a great tuning tool to achieve desired results in an engine. For a sbc, .035 to .040 works well. I see nothing wrong with 10:1 compression in your engine, especially with aluminum heads. We have 93 octane available around here every 5 miles or so...

Tell me about the ignition system that your going to run in the new engine, from the time of first start up.

SSLance · #3 07-12-2016, 01:59 PM

From what I understand, the better you get your quench, the more compression ratio you can run safely...

Dave95z28 · #4 07-12-2016, 03:52 PM

Are you stuck with that camshaft? It seems like rather than playing with your static compression ratio, you should just build your motor with the static compression ratio you want and then get the camshaft with the proper timing events you need to get the desired DCR.

I had an LT1 that had over 11:1 that ran on the crappy 91 Octane where I live. Granted it had a relatively large solid roller, but no issues.

David

**WSSix** · #5 07-12-2016, 05:04 PM

Thanks guys.

The cam is one that's spec'd from Advanced Induction as part of their heads/cam package for LT engines. They recommended 11.25 to 12:1 SCR with this combo to produce the desired/claimed power. And, yes, it will work with 93 octane. I, wanting to understand what all is going into my engine and how it works, am digging deeper instead of just throwing it together. That's when I started looking into my DCR numbers and realized they would be well over 10:1 if I went with the Mahle pistons I was going to use. The SCR would be 12:1 with those pistons. Everything I'm reading says that can't work on 93 octane as the DCR is way too high. Yet, Ai has proven results and is one of the best in the country with LT1/4 motors. They charge a premium and get it for a reason. So do I just build it like they say and stop worrying about it or try to understand what's going on?

I haven't asked Ai about any of this. I guess I should. Part of me says they aren't responsible for educating me on engine theory. Another part of me says I spent a lot of money for their parts.

I'll be running the factory opti spark, Scott. I honestly didn't even think about upgrading it. Should I consider an MSD box?

Che70velle · #6 07-12-2016, 11:37 PM

Trey, your over analyzing the whole deal, just like an engineer...kidding bro.
Quench, when set up properly, gives you the ability to run as much compression as possible with the given fuel available. For one, it puts the incoming fuel air mixture into a turbulence, by forcing the mix over into the chamber. The turbulent air mixture will actually add a cooling effect in the chamber, allowing more compression, and burning more efficiently. Secondly, the closer the quench, the more the cooler piston cools the warmer head quench area. Both of these qualities aid in a more efficient combustion process, and a more efficient chamber will allow for a higher compression ratio.
Ignition timing, and the ability to control/adjust it plays into the role of combustion greatly, which is why I asked about what you are using. If the lt engines had a weak link, it was the factory opti. Personally, I'd set it up with an adjustable ignition box. Definitely not a have to have item, but useful in controlling a possible detonation problem, if you put too much DCR in the engine, which I don't think you are.