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Old 03-20-2009, 08:07 AM
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Street/Strip 408 SBC...Critique It?

Well, now that I've got my new garage straightened out and my hot rod parts out of storage, it's time to re-evaluate where I stand, and take a look at the combination I selected 10 years ago to see if it's any good.

I plan on (hope to?) buying another 1952 Chevy Pickup to put this motor in, for a pretty radical street/mild drag ride. How I get the power to the ground shall be a thread for another day.

My combination consists of:

400 block, 4-bolt, 511 casting, bored .040" over. w/ main studs
Scat cast steel crank, new, stock journal sizes
5.7" reconditioned (shot-peened, polished, etc.) rods
Hypereutectic dome (0.100") pistons (SCR should be about 11.1:1)
Edelbrock Performer RPM heads (#6073) 70cc chambers, 2.02/1.60 valves
Edelbrock Performer RPM intake (#7101), dual plane
Edelbrock Performer Carburetor (#1411), 750 cfm
Comp Cams 18-702-8 Solid Roller Cam 280/280 adv., 236/236@0.050", 0.550/0.550", LCA 110, overlap 60
Summit Extruded Aluminum Full Roller Rockers 1.6 Ratio

I also have a TH400 lying around too to pair it up with.

I used Dyno2003 to simulate these (I know the software is old, but hey...like the parts, I use what I've got on hand), and basically got a motor that has peak hp of 485@5500 rpm and 514 ft-lbs of torque @4500 rpm, with more than 450 ft-lbs from 1500-5500 rpm and more than 500 from 3500-5000. I figure this would make for a fun occasional strip run, and a beast on the street. I really had no guide in selecting these parts ten years ago, and I hope I got lucky and picked the right stuff (I might have picked it anyway, as I got really good deals on most of it).

I am somewhat concerned about the high compression ratio, but have heard that the somewhat large overlap of 60 degrees should help "bleed off" excess cylinder pressure at low rpms, helping prevent pinging. Thoughts?

I am also curious about your thoughts on intake port to cylinder volume ratios. My IP/CV ratio is about 0.20, which puts it at kind of the low end for pure street cars. However, my BMEP is 192@4500 rpm, which is more for a dedicated strip car.

Is there anything in my setup that I definitely need to scrap? Keep in mind that this engine has already been fully machined and balanced, so any changes might have a domino effect as far as costs and changing other components.

Thanks!

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Old 03-20-2009, 08:24 AM
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I think with a good, tailored ignition curve you might be able to get away with that CR, but you will be at the limit, IMHO.

The carb is what I might want to change. A 4779 would be a better choice, I think.

Might want to consider 1-3/4 primaries.

It'll be a beast on the street!
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Old 03-20-2009, 02:28 PM
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If the block is o decked the cr is going to be 11.62:1

Cheaper to do the correction now, than pay twice to fix it later.
Some one will buy those pistons from you.
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Old 03-20-2009, 03:24 PM
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Can you re-do that calculation? According to the paperwork that came with the pistons, while they are domed, the valve reliefs give them a net "dome volume" of -3.5 cc. That's how I came up with 11.14:1. Even that number has me concerned though. So even if I am right, I might want to make a change.
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Old 03-20-2009, 05:02 PM
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The dash (-) before the 3.5 indicates a positive volume, not a negative one. It's 180 degrees out of phase thinking unless you really pay attention to the way the piston manufacturers list their products. If this is the piston you're using with 70cc chambers, the SCR is 12.02:1.
http://kb-silvolite.com/performance....tails&P_id=208

Use these pistons for a SCR of 9.6:1. That'll work great with your 236/236 cam.
http://kb-silvolite.com/performance....tails&P_id=234
Plus, the wider rings will seal better in a street motor.
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Old 03-20-2009, 06:14 PM
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Tech,

I had considered the dome number might work that way, so I compared it with the piston literature, which stated that it should yield a 10.54:1 CR with 76cc chambers. If you do the math backwards from that, the piston has a net positive effect on combustion volume.

***EDIT***

Here is the piston I am using.

http://store.summitracing.com/partde...&view=1&N=700+

***EDIT***

Just so we're square on the exact numbers I am using:

70cc chamber
9.08cc head gasket (4.20" diam, 0.039 thickness)
3.50cc piston relief (-3.50cc "piston dome volume")

82.58cc Combustion Volume
837.2cc Swept Volume

Like I said though...even a CR of 11.1 is somewhat intimidating to me. I'm sure I would be always hearing phantom pings even if there weren't any.

Last edited by nofearengineer; 03-20-2009 at 06:18 PM. Reason: Adding Information
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Old 03-20-2009, 07:21 PM
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Have you given any thought to machining some of the dome height away?

I have done this many times to solid dome pistons in order to get a compression ratio to the specs I need when there is no exact off-the-shelf piston available. As far as the balance goes, you will probably be removing less than 10 grams (a sugar cube size piece of alum is 2 grams IIRC), so you really wouldn't be impacting the balance enough to worry about it at the low RPM you intend to run.

EDIT: I re-read your last post, and you are figuring dome volume into your calculation incorrectly. A negative listed volume is material above the flat of the piston's deck(i.e. domed). A positive listed volume would be a dish.
Because of their large cylinder volume relative to the common chamber sizes in SBC heads, 400's are easy to get too much compression in. Your combo with a Flat top piston that had valve reliefs of 3.5cc volume would be very close to the 11-1 you are figuring.

Last edited by ericnova72; 03-20-2009 at 07:32 PM.
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Old 03-20-2009, 07:35 PM
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I was using the KB .100" dome piston before.

The piston you are using has a 3.5cc net dome volume.
Here are the numbers based on a O decked block and a stock block with .025" deck clearance.

if you look around thru the different piston sepcs you'll see that summit often gets the +/- bass acwards.

EG a flat top piston with valve reliefs has a typical -4 to -7cc net volume.
But you'll often see it expressed as a + volume.

Typical .100cc domed SBC pistons range from .5cc to 3.5cc.
Again you'll often see this expressed as a - spec which add a lot of confusion.
This online cr calc makes things a lot easier than a hand calc
http://www.wheelspin.net/calc/calc2.html
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Old 03-20-2009, 08:29 PM
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Summit has some 'splainin' to do then, because I see absolutely no way that piston can make 10.54:1 with 76cc chambers...what do you think? They've been advertising that same kit for years with that verbage, and you'd think someone would have corrected them by now.
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Old 03-20-2009, 11:10 PM
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Using the calculator that F-Bird linked to, I came up with 10.58-1 with a 76cc head AND the piston down the bore .025" at TDC, close to what Speed-Pro lists. 11.3-1 w/70cc head and piston .025" down bore, and 12.06-1 w/70cc and piston zero decked.

Cutting these pistons down to flat tops will leave about 3.5cc of valve relief and gives 11.11-1 w/70cc and zero deck, or 10.48-1 w/70cc and piston .025" down bore at TDC. Just examine the piston to make sure it isn't a hollow dome, I've done this to the nearly same forged Speed-Pro 400 .145" dome LW2477 piston with zero problems three times

For what it is worth, I would not run a hypereutectic near the compression limit for pump gas without zero decking the block in order to get the minimum quench clearance down to .040". Been there, done that, if it rattles just a few times you will have junk.
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Old 03-21-2009, 01:06 AM
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Quote:
Originally Posted by nofearengineer
Summit has some 'splainin' to do then, because I see absolutely no way that piston can make 10.54:1 with 76cc chambers...what do you think? They've been advertising that same kit for years with that verbage, and you'd think someone would have corrected them by now.
That is a cop out
You are the engine builder. It is up to you to know how to calculate compression ratio and decide wether a dome is a + or a-. Relative to the math formula used or computer calc program used.
It's up to you to measure and verify all the specs of the actual pistons in your hands.

You bought the wrong pistons without checking the resulting actual compression ratio they would produce.

Do it right and get a -7 flat top or a -18 dish for this motor.

Nobody builds street pump gas 400 small blocks with a dome piston.

Do not get carried away with compression ratio.

This motor will make plenty of real power with a 10:1 cr.
Excessive cr will only bite you in the ***.

Take a look at the dome shape on the piston. It is designed for the old GM OEM heads. It may not clear the combustion chamber edge of a modern aftermarket head. I believe the RPM heads have a new revised chamber shape. it is up to you to check this.

Last edited by F-BIRD'88; 03-21-2009 at 01:24 AM.
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Old 03-21-2009, 01:42 AM
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This is how this motor should be built with a .040" net quench and a
true 9.64:1 . Follow the KB instructions for the KB hyper ring end gap required.
if you just got to have a little more cr, mill the 70cc edelbrock heads to 67-68cc
If the motor is built correctly you will never hurt a Hyper cast piston.
If the motor has excessive compression you will (eventually) bust or distort the most expensive, strongest super duper forged piston reguardless of brand or design.

Valve to piston clearance must be checked, also.
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Old 03-21-2009, 02:13 AM
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For those of you who are thoroughly confused by this whole mess, I'll try to explain.

If a piston is listed as 3.5 cc's, without the minus sign in front of the number, then there is a cut in the piston crown, either valve relief(s) or a shallow dish or both. No material sticks up above the flat part of the piston crown.

If a piston is listed as -3.5 cc's, then there is a dome sticking up past the flat crown of the piston. The valve reliefs which are present might be 7 cc's and the dome might be 10.5 cc's. Deducting the valve reliefs from the dome will leave you with a net -3.5 cc's protruding above the flat crown of the piston and this is how the piston manufacturer will advertise the piston, -3.5 cc's.

In order to figure the static compression ratio of a motor with domed pistons, visualize cutting the 3.5 cc's of dome off the piston crown with a sharp knife and glueing it into the combustion chamber. Now, for your calculations, the piston will have a value of 0.0 cc's and the combustion chamber will have a value of 3.5 cc's less than it was produced with.

Let's take this motor for instance. We will need 5 values to figure the SCR.

The cylinder on this 0.040"-overbored 400 has 837.24 cc's.

We sliced off the dome and glued it into the combustion chamber, so the 70 cc heads are now 66.5 cc's.

The piston is now the same as a flat-top with no valve reliefs, so its value will be 0.0

The head gasket, at 4.200" X 0.039" is 8.85 cc's.

The piston deck height is 0.025", judging by the stack of parts (9.000") and figuring a 9.025" block deck height. The OP has never said what it is, but computing at 0.025", the piston deck height has 5.58 cc's of volume.

837.24 + 66.5 + 0.0 + 8.85 + 5.58 = 918.17
66.5 + 0.0 + 8.85 + 5.58 = 80.93
918.17 divided by 80.93 = 11.34:1 static compression ratio.
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Old 03-21-2009, 02:43 AM
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To the original poster:
We've thrown a lot of technical info at you, but to simplify it, if you are uncomfortable modifying the parts you have follow what F-Bird is saying. Buy a different piston that gets you the compression that you could live with, weight match them to your current pistons and then sell off the domed set(EBAY?) to recover some of the cost. If you try to use what you've got as it is now without race gas it will go boom sooner rather than later. Hypereutectic pistons have zero forgiveness when put into the wrong operating environment.
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Old 03-21-2009, 04:18 PM
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I found the source of my error. It turns out their stated CR of 10.54:1 was indeed based on a stock block deck height of 9.025". The mistake of a novice: I bought a 5.7 rod/piston combo that would be zeroed at 9.000". Heck, even leaving the piston 0.025" in the hole like that would likely still have resulted in detonation, since with the 0.039 thickness gasket I got would have resulted in a quench of 0.064". Try to remember I bought this all as a kit, so I assumed it would work together. Somewhat expensive lesson learned though. I know the salesman can't be bothered to hold every newbie's hand when trying their first major project. I know more now and will be more careful.

This does bring me to a related question though.

In general, what are your thoughts on zero-decking a block vs. just using a thinner head gasket (assuming the deck is otherwise square) to end up with a quench clearance of 0.040"? It would seem to me that once material is removed, your future choices are limited, but maybe I'm overthinking it. (Note that I'm not thinking this is the solution to my current problem...just being curious).

I'm definitely going to look into buying some dished pistons to replace my domes, but am I correct in thinking I have no way of knowing if my cylinder wall clearance is exactly right? The last time, my machinist insisted I give him the pistons to get the clearance perfect.
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