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Old 11-13-2012, 07:09 PM
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Piston selection 377

Hello, I am new to the site, so.. Its nice to meet you guys and I look forward to hearing from you all.

I am in the process of building a 377 stroker motor for my 85 Monte SS. (350 block, stock bore, 3.75 stroke) Ive been looking at a few of the different rotating assemblies available, and now I have some questions about piston and ring selection.

My first concern has to do with compression ratio. (It will be a street/ occasional strip car) My block is decked to 9.010, and I will be using a set of Dart Iron Eagle heads with 64cc combustion chambers. With this combo, wouldn't a flat top piston put me out of pump gas range? Is there any reason why running a dished piston would be a bad idea? What is the max compression ratio I could run safely?

Second, I have heard good things about the reliability and temp resistance of moly faced rings, but I also just read that the cylinders need to have a special finish to seal properly with moly rings. I just got the block from the machine shop, and dont really plan on sending it back. Is it a bad idea to run iron rings in a mild performance engine? (I am hoping for 450+ hp and tq)

Thanks for the help.

PS I know dynamic compression ratio is what really determines pump gas driveability, but I don't have a specific cam picked out yet.

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Old 11-13-2012, 07:50 PM
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what end results are you looking for? how much power,or whats the performance you want for that car
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Old 11-13-2012, 10:49 PM
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Vinnie, I would really like to push 450+ horsepower and torque. Reasonable goal. But I want to be sure it is streetable. Am I basically forced to run dish pistons since I cant run a large enough cam to keep my dcr in pump gas range, BC I do plan to drive it on the street?
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Old 11-13-2012, 10:56 PM
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Hcompton,

would a cam large enough to keep my dcr in pump gas range w flat top pistons even be streetable? Im looking at somewhere around 11:1 with flat tops and a comfortable quench. If it were your build, would you choose flat tops and a large cam, or just go with the dish style piston?
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Old 11-13-2012, 10:57 PM
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Using a Keith Black #102 standard bore piston with 18cc dish, the static compression ratio would be 9.5:1. Using a cam with the intake closing point at ~35 degrees ABDC would yield a DCR of 8.4:1. Look, for instance, at the 119671 on this page.
http://www.cranecams.com/68-71.pdf
Second set of numbers from the right tell the cam timing @0.050". Notice that this cam opens the intake at 1 degree BTDC and closes the intake at 35 degrees ABDC. I'm not saying to use this cam. I'm just trying to show everyone on this board how to arrive at a cam that will work with a given static compression ratio. You can always PM me if you like the way I do things but don't want to ask on a forum.
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Old 11-14-2012, 07:00 AM
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Quote:
Originally Posted by 377MONTE View Post
Hcompton,

would a cam large enough to keep my dcr in pump gas range w flat top pistons even be streetable? Im looking at somewhere around 11:1 with flat tops and a comfortable quench. If it were your build, would you choose flat tops and a large cam, or just go with the dish style piston?
"Streetable" covers a lot of ground I'm sure you know. But to use that much compression (64cc heads and FT w/good quench) will mean a lot of cam and that means a lot of stall rpm and rear gear. It might be drivable on the street but I wouldn't call it streetable, at least not on a daily basis. Maybe when I was 18....

Much better to build it w/12-18 cc dished pistons, IMHO. I'd suggest a reverse dome or D-cup piston to maximize the quench effect.
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Old 11-14-2012, 07:32 AM
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These are I-Con's which are a pretty good deal given the offset pins to help cold piston slap and are .030 5.7 rod stroker pistons.Reverse dome 18cc's with a .010 down deck and a 67cc head ends up to be 9.3ish SCR.







I'm kind of confused why when your replacing the pistons anyway you didn't take the option of boring it .030??.I don't think you would find .030 pistons costing more.You do know that bore sizing should be fitted to the pistons you going to use.So the bore/hone should not be done until the machine shop has the pistons in hand.I would seriously question the machine shop that measured your bores on a block that old why they didn't find some of the bores oval.Not a bad thing in SBC's,just pretty common wear factor.How round on center the bores are with a very good hone is very important.
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Old 11-14-2012, 11:51 AM
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Gary,

I purchased the block directly from the machine shop. I didnt tell them how to prep it, it was already done. I kind of like the idea of doing something a little different, and everybody builds a 383, so i like the sound of a 377 stroker a little better LOL. They did give me a sheet w all the things that were done to the block. Clean, degrease, magnaflux, hone, and decked. I guess i may end up having to go with a 383 after all though, I didnt really know the bores needed to be matched to the piston in hand. Good thing I havent ordered the rotating assy yet.
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Old 11-14-2012, 12:01 PM
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there is also a 377 that has the larger bore and shorter stroke.
measure the block and look around to see if you can find pistons to fit,or better yet,get the machine shop to find the pistons to fit there machine work? If the cannot,then they can bore the block for free or take back their block. different pistons require different piston to block clearances
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Old 11-14-2012, 12:12 PM
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cobalt,

why am i under the impression that dish pistons are second best? maybe I am mistaken, but i figure lower compression by a whole ratio, maybe even more, would equal less power? Are my 450+ hp/tq goals still realistic with dish pistons?
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Old 11-14-2012, 12:15 PM
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ahh, thanks vinnie. makes sense now. bores need to be finished to the pistons clearance requirements.
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Old 11-14-2012, 12:39 PM
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Not sure I would want to deck a block either if I didn't have the pistons in hand.Do you know if it was decked to 0??.
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Old 11-14-2012, 01:01 PM
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gary,
it was decked to 9.010.
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Old 11-14-2012, 01:55 PM
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Quote:
Originally Posted by 377MONTE View Post
cobalt,

why am i under the impression that dish pistons are second best? maybe I am mistaken, but i figure lower compression by a whole ratio, maybe even more, would equal less power? Are my 450+ hp/tq goals still realistic with dish pistons?
The piston dish or dome volume is a means to an end. You need to lower the CR. Your choices are a larger combustion chamber, a larger dish volume, or both. There are a few heads available w/larger chambers than 64cc. But in the beginning you had specified cast iron heads having 64cc chambers, so the recommendations reflect that. That said, I hope you have the newer Platinum castings.

Using an inverted dome or D-cup piston places the dish over the deep part of the chamber, which maximizes the quench effect (there's an echo in here! lol). This is an important consideration because good quench action is the same thing as raising the octane of the fuel you burn in that the propensity for detonation is lowered in either case.

Good quench is sometimes referred to as "mechanical octane". This is why we do not recommend the factory-type round dish pistons- they do not offer as good a quench action. This is also why using thick head gaskets or lower compression height pistons is not such a good idea, unless they allow a quench in the area of 0.040" (distance from the block deck down to the toop of the piston at TDC, more on quench here).

Now you are correct in thinking that compression IS worth power. The generally accepted rule of thumb says there's about a 4% change in hp per number. So going from 8:1 to 9:1 on a 400 hp engine will add 16 hp. This is a relatively small gain to chance having problems w/detonation because the compression ratio is too high. If you have to retard the timing to prevent detonation, you will lose FAR more power and economy than you will by simply building the engine w/a lower CR to begin with. Since you have your choice of pistons, now is the time to build it using parts optimized for the application. You can thank us later.

Oh, and 450 HP is totally doable w/dished pistons and the right heads.

Last edited by cobalt327; 11-14-2012 at 02:02 PM.
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Old 11-15-2012, 09:52 PM
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You also have to remember that during the same era that CR was being lowered, the factories went to net power ratings.

As far as the validity of 4%, YMMV and that's fine w/me.
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