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Old 01-24-2011, 03:06 PM
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Engine build questions

Hey guys I am starting on my chevy 350 build for my 3/4 ton truck. Just got the block all tore down and have some questions about the parts I will be buying. The truck has turbo 400 4.10 gears and I am looking to build the motor for torque and decent mpg(for a sbc that is). This will be daily driven once and a while and used for camping pulling small trailers. What type of pistons should I be using. I want to keep the cr down so I can use 87 octane if possible. I see the cast pistions use a lower cr, and the hyper- use a little higher. Planning on the vortec heads so I know the 64cc chamber will bump it up some more. I don't care so much about the vortecs but they seem like the only option under $700. I would get the factory heads rebuilt but everyone says don't bother they are junk and not worth it. Can anyone reccomend some facotry heads that are worth rebuilding(carbed motor). Just a little lost when it comes to the cr in respect to the pistions and heads and what will increase it. Will not know for sure what I'm looking at for cr until the short block is built but just want a plan. Any help would be great thanks.

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Old 01-24-2011, 05:45 PM
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1st suggestion: Build a 383, if you are going to buy a new set of pistons then buy a new crank along with it. I don't think I would ever build another 350 after I built my 383.

2nd suggestion: Use the search feature and type in something like building 350 for torque or something like that, you will find so much info you won't know what to do with it.
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Old 01-24-2011, 06:08 PM
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Hey my old man has a 78 c20 with a turbo 400 and the corporate 14 bolt. Engine wise he just had his block bored 30 over and put the edelbrock performer intake, 600 carb, and performer intake. heads he got a set of 1.94's with z/28 springs because the factory ones were the small valved ones. To be honest he spent 2 grand on the rebuild and it runs great! He pulls the 20 foot bluewater and the bed camper with no problems! Keep the set up simple for daily driving and rather stock and it'll last you forever! good luck with the build!

Dylan
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Old 01-24-2011, 06:52 PM
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Couple things to consider...

Quote:
Originally Posted by 75chev4x4
What type of pistons should I be using. I want to keep the cr down so I can use 87 octane if possible. I see the cast pistions use a lower cr, and the hyper- use a little higher. Planning on the vortec heads so I know the 64cc chamber will bump it up some more.
Hypereutetic pistons ARE cast. You can use cast hypereutectic pistons w/o a problem.

The piston material (i.e. cast or forged) or the alloy silicone content has nothing to do w/how much compression they make, per se. That's purely a function of the piston volume (dome or dish) and compression height along w/the rest of the variables that go into calculation the CR.

Quote:
Just a little lost when it comes to the cr in respect to the pistions and heads and what will increase it. Will not know for sure what I'm looking at for cr until the short block is built but just want a plan. Any help would be great thanks.
Techinspector1 has posted the formula for figuring CR longhand several times, you can do a search here or google it for more.

You can also enter the figures into a CR calculator to give you your answer. This allows a quick way to recalculate changes in just seconds- like what will the CR be, if using a different chamber or piston volume, or a different gasket thickness, etc.

The quench measurement is the distance of the piston head to the cylinder head, including the compressed gasket thickness. The "ideal" quench figure is often cited as 0.040", and is often arrived at by "zero" decking the block and using a 0.041" head gasket (a common compressed thickness). 0.040" IS good, but it is by no means an 'absolute'- there's room to play with, within reason. Just don't get carried away and try to use 0.090"+ and expect the same detonation resistance as you'd see w/0.045".

The CR I calculated using a 0.030" over 350 SBC w/FT pistons (-6cc valve reliefs), a 0.041" head gasket, a 0.020" deck height (0.061" quench), a 4.090" gasket bore and 64cc chambers and got about 9.76:1 CR.

That's pushing it for ANY pump gas (unless you can get E85), and will detonate w/87 octane.

That leaves you w/the option of going wider yet on the quench, but that will, by itself, also increase the chance of detonation until you're so wide that the CR will be so low as to not have detonation. But this costs you economy, the engine runs dirtier, and won't make as much power.

The other option (talking hard parts here- not water/alky injection, at this point, anyway) is to use a dished piston to reduce the CR w/o compromising the quench.

Use a "D"-shaped dish as opposed to a round dish. The round dish does virtually nothing to promote the things that a tight quench can provide, given a surface to work with- a surface that's provided w/a D-shaped dished piston. Maybe not as good as a true FT piston, but far better than the round dish style piston.

One more thing to be aware of is the compression height of the piston. Rebuilder-style pistons are often 0.020" 'shorter' than other pistons. 1.540" as opposed to 1.560".
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Old 01-24-2011, 09:36 PM
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OK, Wow thats a lot of good info cobalt327. So how exactly do I need to build this to run correctly on pump gas and get some power out of it. I know I can just go buy all the parts and put it together, but I want to design it for my needs. So the pistons that I just pulled out of my old block are stock and they are flat top with 4 reliefs and it had the 624 casting heads on it which are 76 cc I believe. So how do I build this motor to run well on pump gas(lower compression ratio) with the flat top pistons. Or I guess how is everyone else running the vortec heads on their motors and still using pump gas or 87? I did search before I originally posted this but couldn't find my specific question. I will do some more searching though, thanks guys.
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Old 01-24-2011, 10:59 PM
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http://www.chevyhiperformance.com/ho...075/index.html

There's a good article that builds one like you want to do.

The cam they used has the intake valve closing a little later - when the piston is a little farther up on the compression stroke - which allows a little more of the compression pressure to build up which helps avoid detonation which allows lower octane gas.

Also they used a D cup piston with a large 21cc dish because they were using Vortec heads with the small 64cc chambers. They used a forged piston, I'm not sure if there are any hypereutectic pistons made with the large D shaped dish, but they'd be cheaper and they can be run with a tighter piston to wall clearance because the cast hypereutectic material does not expand as much when hot as a forged piston, so is actually a little better for a daily driver street vehicle.

As far as affordable heads goes, I found these heads.
http://www.rogersperformance.com/EQ.htm

And from that vendor, they're right about the same cost as an assembled oem Vortec head.

And you'll see they're available with larger combustion chambers which allows you a wider choice of pistons so you can probly find some less expensive ones, and maybe even use flat tops if it works out at or just slightly over 9-1 compression with one of the larger combustion chambers like the 76cc pnes, but I'd stay with the 1.94 intake version to avoid any piston to valve interference and maintain the best low rpm flow.

They're basically an aftermarket iron casting version of the oem Vortec heads, but with some improvements in exhaust port flow (the stock Vortec's are a little weak in that regard) and more material in the comb chamber around the valves to avoid exhaust valve seat cracking which can happen with stock oem Vortecs.

But remember you want to maintain that ~.040" quench clearance between the non-dish flat part of the piston and the flat area in the comb chamber. There are coated head gskets available in different thicknesses at Summit Racing at reasonable cost, such as .020", etc, so you can start by finding an inexpensive piston like a common flat top with 4 valve reliefs which has plenty of flat area for good quench, then plug that into a compression ratio calculator along with different comb chamber sizes and gasket thicknesses to see what you need to end up with for clearance between the top of the piston and the top of the block at top center ("deck height"). There are standard flat top pistons available with the 1.56 compression height (center of hte pin to top of piston) which would give around an .025 deck height, then use that with i.e. a .016 or .017 or .020 compressed thickness head gsket and a 67 or 76cc comb chamber and you probly would end up with a compr ratio around 9-1 or a dab higher which is what you want. That way you maybe wouldn't have to mill the deck on the block. Or if you do mill the deck to get the piston even with the deck at top center, then you could go with the common .039 compressed thickness head gaskets to get your .040 quench. Several ways you could play that, just have to run the numbers in
the calculator. http://www.projectpontiac.com/ppsite...tio-calculator or http://www.kb-silvolite.com/calc.php

Here's some good info on them in a magazine build, altho those were with more expensive stainless valves, etc, and on a fairly high hp engine, but it's good info on the heads.

http://www.popularhotrodding.com/fea...ine/index.html

These EQ heads flow a little bit more than stock Vortecs esp on the exhaust side and will work with higher lift cams. You can run either regular roller rocker and guide plates or the guided tip roller rockers if you want to run roller rockers. Or the roller tip Comp Magnum rockers are a good cost effective rocker.

I'd shoot for about a 9-1 compression ratio instead of the little bit higher ratio they used in the article and then a little bit shorter cam such as the Comp XE-262H for just a little better torque in the lower rpm ranges, altho what they built had a great torque curve over a wide rpm range including at lower rpms. But I've read articles on builds comparing the 268 & 262 cams that said the 262 was a little more responsive and had a little more torque
in the lower rpm ranges than the 268 and with the heavy vehicle that's important.

I'd go with a Performer style intake and a carb in the 600 or 650 range max. The Demons are great carbs but are more expensive. The Holleys are easy to tune but the Edelbrocks are a dab better in the low to mid rpm ranges and many folks will say also with mileage.

If you're thinking headers, by all means stay small diameter and go long tubes if you can, or mid length at the shortest. The closer to equal length the better, or tri-y's are great headers for torque, if you can find them and at a decent price. Long tube headers also increase lower and mid rpm range torque, shorties soften lower rpm torque.

Last edited by macx; 01-24-2011 at 11:05 PM.
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Old 01-25-2011, 08:32 AM
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thanks macx this is really good stuff. I am finding out that part of the problem I see anyway is there in no decent cast(affordable under $900) heads that are in the 72-67 cc range. At least that I can find anyway. This seems to be part ot the problem with getting the combustion ratio correct. I will read these now and get some ideas, thanks.
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Old 01-25-2011, 09:20 AM
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Can someone give me the values for the compression calculator, trying to use it, but not fully understanding all the inputs.

cylinder head volume- 64cc if I use the vortecs

Piston Head volume-?

Gasket Thickness- .020 can vary I get this part

Gasket bore-?

Cylinder bore Diameter- If Its bored .020 or .030 over it would be 4.020, 4.030? not sure?

Deck Clearance-?

Stroke-? Not sure off hand but I can find this one

If I can get some help with this I can mess around with some different combinations, thanks
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Old 01-25-2011, 09:50 AM
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Quote:
Originally Posted by 75chev4x4
Can someone give me the values for the compression calculator, trying to use it, but not fully understanding all the inputs.

cylinder head volume- 64cc if I use the vortecs
The early fulie heads were also 64cc, the later "smog" heads were 76cc (all figures nominal- they need to be "cc'ed" to measure the exact chamber volume- which can vary due to manufacturing tolerances and prior work done to them).

305 OEM heads are from the lower 50cc range to 58cc.

Quote:
Piston Head volume-?
The 4-valve relief FT pistons are somewhere around -6cc. Most manufacturers will cite the volumes of their pistons.

On the calculator I linked to, domed pistons are expressed using "+" before the size in cc's. Dished (or FT w/reliefs) are expressed w/a "-" before the size.

Quote:
Gasket Thickness- .020 can vary I get this part
Absolutely. There are gaskets that vary between 0.015" or so all the way to 0.055"-plus. Copper head gaskets can be had in virtually any thickness.

Quote:
Gasket bore-?
A 350 SBC will be between 4.09" to 4.100", generally speaking.

Quote:
Cylinder bore Diameter- If Its bored .020 or .030 over it would be 4.020, 4.030? not sure?
Correct.

Quote:
Deck Clearance-?
This is asking you how far down the bore (or above the deck in some cases) the piston top is located.

Quote:
Stroke-? Not sure off hand but I can find this one
The 350 SBC is 3.48", a 383 stroker uses a 3.75" stroke, like the SBC 400. The 400's bore is also bigger than the 350, at 4.125" nom.
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Old 01-25-2011, 10:54 AM
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cool cobalt thanks man. Hey I was looking in the summit catalog that I have and they have their own vortec heads that are 67 cc and they also have some 72 cc cast heads that they make that are better flowing than stock. Was thnking with the little larger like the 67 or 72 cc heads I could run the flat top pistions no prob. I will plug them into the calculator.
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Old 01-25-2011, 11:37 AM
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IMHO, use the Vortecs, they are very good heads for the money. They flow well, and build decent power throughout the rpm range you will be using. Also, they can support decent HP #'s, so are ready to run if you decide later for some more "oompf".

Unfortunately, nothing you build will get decent mileage, due to the fact it is a 5,000lb + truck, armed with 4.10 gears and a TH400. If you want better milage, an overdrive manual trans and some numerically lower gears (3.73, or so) would be next in line. Adapting in an NVG4500 trans from a newer chevy 4x4 truck (3/4 and 1 ton) might be worthwhile, later on. Shouldn't be too hard, IIRC they were availiable in a gas truck, as well as the diesels. Dodge and Ford also used them, so one from them would also work, just need a chevy bellhousing.
Good luck, and have fun!!
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Old 01-25-2011, 12:10 PM
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Mileage is fine its not daily driven much. Just want to get it to run well on 87 octane.
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Old 01-25-2011, 01:27 PM
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When I started looking for lower cost cast heads for my 81 Vette for a mild top end upgrade, I looked long and hard at those Summit heads you mention.

I asked for flow numbers and for exhaust flow to intake flow ratio or %.

The flow numbers are a little above oem Vortecs, but the exhaust to intake
ratio is under %60 which is not as good as it could be.

Those EQ heads I linked to flow about the same on the intake side but flow
more like 61 or 62% on the exhaust, a better ratio. Of course, a lower ratio
can be somewhat compensated for by a cam with more exhaust duration and lift than most dual pattern cams, but the EQ heads can do well with exhaust
duration that isn't abnormally more than the intake duration, besides being
resistant to the exhaust valve seat cracking. Also, depends on which Vortec
casting you use - if you use the common 906 (?) heads from trucks, they have hardened valve seats and the lip on those hurts exhaust flow even more than the car Vortecs. The EQ heads have some small differences in
the comb chamber around the exhaust valve and in the bowl behind the valve, 2 of the more important areas for good exhaust flow. That is borne out by the better flow numbers and exhaust - intake %.

Just FYI the deck height dimension you'll find on most SBC's is around .025",
if they have the normal 1.56" piston height (center of piston pin to top of piston) and you can use that for calculating purposes at least to start with, unless you plan on milling the deck anyway. Then you could vary that figure in
the calculator from .025" down to .00" to find a good combo with the gasket thickness & comb chamber volume. Piston listings usually have that figure so you can check what affect they'd have on the comp ratio.

I just ran some numbers on the KB calculator, using 76cc comb chamber, 4.1 gasket bore, 3.48 stroke, 4.03 bore (a larger bore such as an overbore will increase the ratio a little becuz the cylinder volume is a little larger), flat top pistons with 5 cc valve reliefs (in this calculator, a + figure adds volume and a - figure reduces volume, such as with domed pistons which reduce the comb chamber volume cuz they stick up into the chamber), and gasket thickness of .020. Using an un-milled deck height of .025 (nominal, just used for figuring as a likely figure) gives a ratio of 9.038 which should run OK with 87 octane as long as you don't use too much advance or too agressive of a curve with lots of advance at low rpm. Using a milled deck height of .00, and all the rest of the figures the same, gives a CR of 9.528 which I don't think would work with 87 octane. Besides varying the deck height, you can vary the gasket thickness. Using the same figures incl a .00 deck height but going to a .04 gasket drops the calculated CR from 9.528 down a little to
9.116, quite close to the 9.038. So you can see how varying the deck height, gasket thickness, and comb chamber size can change the comp ratio.
Of course remembering to try to maintain around that .040 quench distance.

Last edited by macx; 01-25-2011 at 01:32 PM.
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Old 01-25-2011, 02:16 PM
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wow thats cool, why is.040 the target for quench? What would it be with the 72 cc summit heads if you dont mind.
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Old 01-25-2011, 02:57 PM
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Quote:
Originally Posted by 75chev4x4
wow thats cool, why is.040 the target for quench? What would it be with the 72 cc summit heads if you dont mind.
DooD! You just got schooled on the why's and wherefores of using a CR calculator- why are you still asking what the CR would be when using "X"??

Anyway, should you decide to do your own calculations instead of depending on spoon feeding, pay attention to the fact that different calculators use different 'signs' for a positive or negative piston dome displacement.

You will also see this same thing among the different manufacturers and catalogs- some use "-" for a dish (like the calculator I linked above) and some are opposite that- like the KB calculator macx mentioned ("...in this calculator, a + figure adds volume and a - figure reduces volume..."). So be sure of what you're looking at when you enter the data.

Good luck.

Almost forgot- the 0.040" quench figure is so there's sufficient room for heat expansion and rod stretch so the piston cannot contact the head.

In a running engine at high RPM, this distance shrinks considerably.
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