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Old 12-20-2009, 11:09 PM
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L-31 compression ratio and piston type?

I bumped into some on-line debate about L-31 CR with 64cc heads and stock pistons and would appreciate some input from the forum. Someone was saying that Chevrolet used two different dish cc's depending on the application - the heavy duty trucks being set up for less CR for some reason. I'm asking because I have a stock L-31 short block salvaged from a 2001 CK2500 (yes they did put some L-31's into 2001 vehicles, although the computer at Auto Zone won't agree). I'm considering the Edlebrock top end kit (E-Tec heads, RPM "Gap" intake, and a cam). Edlebrock claims 400 plus HP on a 9.5:1 350ci. What HP can I expect out of the stock-piston , stock bore and stroke Vortec with this top end kit installed? What will my compression ratio be? Finally, if someone can settle this one - are the stock GM dished pistons used in these late model Vortec engins hypereutetic??
Thanks!!

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Old 12-21-2009, 12:10 AM
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Yes the pistons are hyper pistons. The factory compression ratio was 9.4:1. There should be no problem converting it over as you wish. I`d say you`ll land in the 385 and up horsepower range but this is just a guesstimate. At any rate your sure to like the results.
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Old 12-21-2009, 02:50 AM
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The pistons from a Suburban L31 I own have exactly 11cc dishes that are 0.080" deep.


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Old 12-21-2009, 08:37 AM
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F-BIRD '88, okay, I'll do that. My engine is on a stand and I'm sure it will be important to use a level, shim wheels, etc. to ensure all is level before I start, to avoid trapped air. I do have a dial indicator and stand, so no problem finding TDC. I'll do this over this weekend and report back. Not looking to have my hand held, so I'll try to research the calculations best I can without bothering you, but I hope you wouldn't mind giving a little more help if I get stuck. The reason this is important to me is the short-block has only 39K mi. on it (it's out of a wreck and has been stored oiled and covered for several years). Budget is tight, so I would like to reuse it as is but still make maximum HP, and I know that takes decent compression numbers. If, say, the CR was in the "8's", with the particular dish cc I have (as some have speculated per my first post), I would find the dollars to replace the pistons with a shallower dish type. But since it's confirmed that the stockers are hypereutetic - and if after the volume measurements you kindly described come out favorable (9 plus points), I'll be sitting pretty. Still need to inspect the rod and crank journals, and measure bearing clearances, but I expect all will be well. the bores measure at 4.00" + - .001". There is no ridge at all, no scuffs or glazing. The honing is still visible. Cobalt327, that dish appears slightly deeper than mine, but it could just be the angle of the photo. Wish I had a nice depth gauge like that. I do have a good Starrett dial caliper, and I suppose I could play around with it by placing a flat bar across the piston top, resting the bottom of the scale on it and dropping the depth measurng device down from that, then subtracting the thickness of the bar. We'll find out soon enough.
Thanks!!

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Old 12-21-2009, 08:48 AM
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sounds like you could hit it with a hone and put in some std. flat tops. I know budget may be tight, but $100 for pistons, and $50 for rings, with some work done to the heads and a cam swap would give you 400hp AND cost less than the edelbrock conversion.

Its a lot cheaper to swap pistons than heads and Vortecs with $100 of machine work and new springs can support 400hp with no problem.
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Old 12-21-2009, 09:46 AM
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ap72, I thought flat-tops with the 64cc Vortec chambers would run my octane requirement over pump-gas range? FYI- I don't have heads or intake for the motor at present. The aluminum Vortec heads are attractive because of the weight savings. The project car once had a much lighter engine (rotary) and I would like to avoid too much weight up front.

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Old 12-21-2009, 10:05 AM
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you can save some weight by going to aluminum heads, and the heads you have listed are decent heads, your route isn't the most cost effective way to 400hp though, I was just trying to help your bottom line.

As far as pump gas and Vortecs, flat tops and Vortecs are ran a LOT. anything larger than a 268XE cam seems to run just fine on premium. If you are going for high RPM power you could probably get by on 89 maybe even 87 depending on a lot of other factors.

It all depends on what route YOU want to take. I was just trying to show you a cheaper route to make the 400hp you are after.
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Old 12-21-2009, 01:39 PM
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Quote:
Originally Posted by Zeeb
I bumped into some on-line debate about L-31 CR with 64cc heads and stock pistons and would appreciate some input from the forum. Someone was saying that Chevrolet used two different dish cc's depending on the application - the heavy duty trucks being set up for less CR for some reason. I'm asking because I have a stock L-31 short block salvaged from a 2001 CK2500 (yes they did put some L-31's into 2001 vehicles, although the computer at Auto Zone won't agree). I'm considering the Edlebrock top end kit (E-Tec heads, RPM "Gap" intake, and a cam). Edlebrock claims 400 plus HP on a 9.5:1 350ci. What HP can I expect out of the stock-piston , stock bore and stroke Vortec with this top end kit installed? What will my compression ratio be? Finally, if someone can settle this one - are the stock GM dished pistons used in these late model Vortec engins hypereutetic??
Thanks!!
With aluminum heads you can up the compression by at least a full point many be a point and a half. But the factory L-31 piston follows GM keep it cheap as possible formate of a circular dish. These things are the bane of performance as they lead to a less tan ideal compression ratio for the fuel being used because of their lack of adequate squish and quench. As you can see from the picture that COBALT was so kind to include, the dish is .080 inch deep. Add to the dish's depth the typical production head gasket of about .020 inch crushed and the typical Chevy having another .025 inch between the top most part of the piston and the block's head deck. All told that adds up to .125 inch between the bottom of the piston cup and the head's squish, quench deck. The optimum squish/quench is achieved at .040 inch from the pistons head surface to the heads squish/quench deck. That's a long way from the .125 (at best) the factory lets you live with. You make up the difference in squish/quench function with the octane's you buy at the pump. Now some part of the OEM piston, that being a rim around the outer edge does get close to the head but it is too small to be of much value.

Now you're thinking of an aluminum head. An aluminum head gives off heat at a rate much higher than cast iron, so if you don't bump up the overall compression ratio to something like 10 or 10.5, you will end up with less power than a cast iron head at 9 or 9.5 to 1. And even if you stick with an iron head the squish/quench needs to be fixed. So I'd surly put the fancy Edlebrock fuel injection in the can and start worrying about basics first. Wiz bang toys can always be bolted on later, but fixing incorrect fundamentals is a lot more work later on.

So the pistons have to go. They need to be replaced with flat tops or with a D dish piston. The selection will be that which dials in the SCR to what the fuel octane of what you want to use is. Go to the Keith black site, they have a ton of pistons and some compression ratio calculators.

Squish and quench are functions that build what's called mechanical octane into the engine, they also improve off idle and high axle ratio cruise performance as well as optimizing performance against the available octane fuels. The same parts perform both functions, which are merely separated in cycle time. Squish happens first, as the piston closes to TDC the mixture on the far side of the chamber is ejected by the close closing of the piston and head decks toward the spark plug. This stirs the mixture and increases its density before the spark-plug making it easier to light off and faster to burn.

As the burn proceeds from the plug, the temperature and pressures go up very quickly. The so called end burn on the far side of the chamber wants to spontaneously ignite creating and explosion, colliding flame fronts, and high pressure waves. The mechanical way of reducing this is to have an area on the far side of the chamber that has a lot of surface area to its volume to quench the explosion by being a heat sink. These two functions work together to improve the burn giving more power and economy and to delay the onset of detonation, especially under high loads and part throttle operation. Certainly not as sexy as EFI, the basics seldom are sexy, but the foundation is what keeps it together.

Bogie
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Old 12-21-2009, 02:43 PM
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Understood. I don't mind buying 92 - 93 octane, nor do I mind ditching the stock pistons. I get it that new pistons are a minor expense relative to everything else, and fortunatly I'm not so budget constrained that I would have to pass on them, what with the engine out and mostly apart, this would be the time for them. I thought I was stuck with having to keep CR close to 9 points in order to use pump gas (that's what the mags seem to preach), so the factory dished units made sense once I confirmed they were hyper., but I see how the thermal advantages of aluminum heads will allow more CR. So if flat top pistons (or another design with less volume than stock) running 10:1 or so, with aluminum heads, will make for a stout, and reliable engine on 92 octane, we'll do it. It's all good. BTW, the Edlebrock top-end kit I was referring to is not fuel injected, it's simply a set of their 170cc alum. "Vortec" heads, an RPM "Gap" intake manifold, cam, timing set, and some gaskets. Edlebrock claims 435 hp on a 9.5 CR roller-cam 350 with that set of goodies installed. Plan is to have a bottom-end that will safly get the most out of my further investments (even if Edlebrock's claims are a bit idealized), and you guys are a big help. This is a wonderful forum, full of eager to help type people. I try to dig up facts before I post; if i don't do a great job of that, your patience is appreciated. Here's what me and my 15 year old son, Wesley, are up to (I'm 52, going on 18 I think). Check out the pic, you'll see in the foreground the hood of a supercharged Mazda Miata that we run on club lapping days at road courses, its looking out of my garage at an '88 RX-7 being preped for a SBC and TH2004R (yes I know the chain is not supposed to come out the front of the hoist, but it's heavily reinforced all around and I needed the extra couple of inches). Not a classic hot rod, I know, but Chevy-power none the less. This swap is popular. Check out "granny's speed shop" on line if interested. Thanks!
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Last edited by Zeeb; 12-21-2009 at 10:52 PM.
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Old 12-21-2009, 03:10 PM
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you can run 10:1 on stock iron vortecs too, especially in that light of a car.

Aluminum can tolerate more compression, but it also requires more compression to match the power of iron heads, so you really don't gain anything.

I'm not saying those heads are a bad choice, just that they won't get you the most power per dollar. Edelbrock cams aren't that great either. By getting a custom ground cam (for about the same price or maybe even less), you can pick up a lot of power.

Those parts you chose are not terrible, but if you're going to spend that much money I would do some more shopping and I would read up a lot more about waht you can do. Also, define your exact goals. At this point you're basically at ground zero and can build anything from 300-500hp at roughly the same price (give or take $500). 500hp isn't alwasy what you need though, some times off idle control is much more important, sometimes you want something that can run on 87 octane, sometimes you want something else.

Define your goals.
Educate yourself.
Shop around.
Ask Questions!

Good Luck on your build!
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Old 12-21-2009, 07:13 PM
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Well, the Car Craft guys are big on the Scoggins Dickey modified iron Vortec heads, and they seem to be a value at about half the cost of Edlebrock aluminum (comments?). What I need to do is talk with the folks at Granny's, the conversion parts supplier, about the weight issue. I'm lightening the car significantly otherwise, so the iron heads may be ok. So many choices make things difficult, in a good way, so thanks all for the input. It's easy to run in circles with all the options. The top-end "kit", I figured, would keep me out of trouble, keep me from picking the wrong cam for example. Yes the car is light, and the rear-end gearing is 4.1:1, and the wheels are 15". I'm getting the point, perhaps the Edlebrock cam included in the kit is a compromise to cover a middle weight car. Since I'll be very light and have an easy-turn rear, perhaps it's not the best cam for me. Perhaps I can trade more low-end torque for HP up top. We do plan to drag the car from time to time. No need to hurry the motor because we have a lot of work to do yet on the car, mainly the wiring harness and instruments. I'm not letting myself off the hook to cc the pistons and calculate CR either, because it will be a learning experience.
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Old 12-21-2009, 09:06 PM
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I'm really excited for your build. If you run vortecs you can just machine what you have to be better than the SD heads.
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Old 01-31-2017, 07:43 AM
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compression ratio

I see this is a pretty old thread but it does touch on the in formation I am looking for. I am putting a 2002 5.7L L31 in to a 2003 chevy asrto van AWD. I am having the motor done by a family that runs a race shop that I have known for a long time. A regular motor isn't their forte but they are doing it for me. They do great work on all the race stuff they do.


They are building and assembling the motor for me with increased HP from in the neighborhood of 325 to 350.
My goal is to have a good running motor that will run on 87 octane. I am not going to race it down the street, the 4.3L just cant handle the lift with the tires I have and the weight I carry. So good milage and going down the highway easily are my objectives (with maybe a little fun from the increased performance once in a while,lol).


Going to put a GM ramjet cam # 1409739/ LS6 springs/ and Comp 787 retainers (suggested by another member of this forum)
The builder asked me what compression ratio would I be looking for to run on 87 octane?


He is going to keep the quench the same or perhaps a little less. Most of these trems are new to be but I do have a very basic understanding of what they mean.


So what should the compression ratio be and what is thought about the cam set up? Thanks for the help.
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Old 01-31-2017, 08:02 AM
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Quote:
Originally Posted by Rinny View Post
I see this is a pretty old thread but it does touch on the in formation I am looking for. I am putting a 2002 5.7L L31 in to a 2003 chevy asrto van AWD. I am having the motor done by a family that runs a race shop that I have known for a long time. A regular motor isn't their forte but they are doing it for me. They do great work on all the race stuff they do.


They are building and assembling the motor for me with increased HP from in the neighborhood of 325 to 350.
My goal is to have a good running motor that will run on 87 octane. I am not going to race it down the street, the 4.3L just cant handle the lift with the tires I have and the weight I carry. So good milage and going down the highway easily are my objectives (with maybe a little fun from the increased performance once in a while,lol).


Going to put a GM ramjet cam # 1409739/ LS6 springs/ and Comp 787 retainers (suggested by another member of this forum)
The builder asked me what compression ratio would I be looking for to run on 87 octane?


He is going to keep the quench the same or perhaps a little less. Most of these trems are new to be but I do have a very basic understanding of what they mean.


So what should the compression ratio be and what is thought about the cam set up? Thanks for the help.


You wanna stay around 9.5:1 for that setup.The Ramjet cam has very early timing & can build excessive cylinder psi if you get past that range.Stock quench from the factory on an L31 is .053" using the .028" gaskets.While this works out ok for the factory setup & tuning,I would strongly urge you,if possible to get the quench down to .045",or,even less.Even if it slightly raises the compression to do so,it will still have better detonation resistance.You computer will pull timing if it senses knock,so,the more resistance you have to detonation,the less timing it will pull & less often.It will also allow you to tune more aggressively for better power & efficiency.Bogie & others often refer to the quench effect as "mechanical octane".A good way to look at it.
As has been mentioned,the best piston type is the flat top,or,D cup.Alot of circular dish pistons don't have a wide enough quench band to be of much help.Especially stay away from dish pistons that have a chamfer on around the outside piston top.
A lot of this will also depend on your tuning.The factory L31 tuning is geared more for milage,emmissions,& to run on 87 octane.There is a good 20 HP to be gained in tuning alone,if,you were OK with 91 octane.For 87,the tune would be milder,but,can still be improved a lot over the factory tuning.

Last edited by jokerZ71; 01-31-2017 at 08:21 AM.
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Old 01-31-2017, 08:26 AM
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Joker thanks for the info. This isn't easy for me to get a hold of but I am getting there.


9.5:1 suggested compression
.045 or less quench
is there a different piston you would recommend or you think the KB is the best piston for what I am trying to do? Do you know the different part numbers of the choices that you might suggest on the pistons and rings. (should I go low tension?)
I might sound a little all over the place but I am making progress.
Hope I am not a PITA
Thanks again
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