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Discussion Starter #1
I've read quite a few older threads where someone describes their engine build and asks advice towards picking the right head gasket and while I do grasp the idea of keeping the quench to a relatively low number, in the case of the 400 SBC I'm building, it appears that the compression is going to be a bit higher than I expected. I believe that I 'should' be alright due to living at the 5500 foot elevation I should be okay but I'd like to hear what you guys have to say about this juggling act.
Building a new engine for the 56 Nomad. Vehicle weighs about 3500 pounds, the trans will be a 700R4 with a 2200-2500 stall converter and out back there will be 3:55 gears. Expected rpm range to be off idle to about 5 to 5500 rpm tops but honestly doubt it'll see the high side of 5K too often. The engine is a 400 SBC, 1974 vintage and I've just gotten it back from the machine shop and have the shortblock mostly assembled. The rotating kit came from Eagle and included 5.7 rods, flattop hypereutectic pistons (with 4 valve reliefs that amount to +6cc when figuring compression ratio). Cam I bought is a Howards Hydraulic Roller with 219/225 duration at .050 and .515/525 lift. LSA is 110. Had the block decked and due to variances of rod length/piston height, etc, the deck height varies from .006 to .020. For this street engine, I didn't worry about having them all perfectly uniform.
Haven't bought the aluminum cylinder heads yet but was expecting to buy a set of Edelbrock Performers since they worked out well in the 327 in my '61 Vette. I waited till I had the engine somewhat put together before deciding on what combustion chamber size I'd need. With the info I've listed, and using Edelbrock Performers with 70cc chambers and a Fel-Pro 1014 head gasket (4.200 bore size, .039 compressed thickness), the static compression comes out to nearly 10.7. A tad high and no doubt going to need the best gas around here which unfortunately is only 91 octane but at this altitude of 5500 feet, I might not be too far off. If I've left out any details that are needed, let me know.
Looking forward to hearing your comments.
Thanks,
Mike T - Prescott AZ
 

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Mike....I'm trying to plug all this into a CR calculator.

How much has your block been bored out?
 

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Discussion Starter #3
Hi Pops, sorry about that...I knew I'd forget something :-(. The block was bored out .030. I did run across a thread you were involved with that listed that Wallace Racing 'Dynamic Compression Ratio' formula but haven't been able to figure out a few things such as the Inlet Valve Close ABDC.
Mike T - Prescott AZ
 

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Mike....Using this calculator:https://uempistons.com/p-27-compression-ratio-calculator.html I get 10.5....Which is very close to what you have....I agree that something a little lower would be more comfortable...I believe the only way you will be able to get it down a bit is with a thicker head gasket or larger head chamber...(or both)

 

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Discussion Starter #5
Thanks for running the numbers Pops. I was thinking I'd be a bit more comfortable with it having a compression ratio closer to 10 to 1 even than up around the mid 10's but was hoping that with aluminum heads and that cam with 219/225 duration I could get away with it at this high elevation. Granted, with nearly 407 cubic inches, that cam is considered to be mild but my other concern was with having enough vacuum to have decent power brakes. I have a buddy with a 383 from Smeding that has something like about 9.6 compression and a cam with duration around 220-222 on the intake at .050 and he's also here in Prescott at the 5500 foot elevation and his combination yields around 11 inches of steady vacuum at say 750/800 rpm idle speed. That works for him and I was trying to keep the cam around the same specs in order to have at least double digit vacuum readings.
Mike T - Prescott AZ
 

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Mike....Hopefully, one of our more experienced engine experts will chime in....Hopefully, someone that even has some big sbc experience.

Sounds like its time for me to PM a friend

Several heads are better in this instance.
 

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10.5:1 static compression ratio with aluminum heads will be fine. You'll have to run the 91 octane for sure though. The higher compression ratio will be a bonus at 5500' elevation.

I don't think you'll have a problem even if you take a trip to lower elevations with what you've described.
 

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On a 400+ SBC I'd run a minimum of a 200cc intake runner. The Dart SHP 200's are decent budget heads and are about on par with the Eddy's in terms of power potential and cost (but bigger runners which won't hurt you at all on a big inch SBC). Since you're running a roller cam, make sure you order heads with valvetrain that can handle a roller cam and NOT just a flat tappet (which is what the lowest priced Eddy's and Dart will be).

Personally I'd run a 210ish runner on that motor...maybe something like the GM Fastburns or if you have the budget, a set of AFR 210's. I run 195's on a 355 I have and it rips. 200-220cc on a 400+ SBC is no big deal. I run AFR 220's on my 421, and trust me, it has plenty of grunt down low.

On the compression...I wouldn't fret even running over 11:1 at your altitude even on 91. I'm running 11.23:1 on my 421 SBC with 91 octane at about 7000ft...I wouldn't hesitate to run it that way at 5000 either. Even at 5000 feet my effective compression is about a full point lower than at sea level.

You'll be fine in the high 10's or low 11's on compression provided that you run premium.
 

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personally I see no problem with the compression at 10.7. aluminum heads help there. a soft timing curve will help too. also I would avoid the fast burn heads . the smaller chamber will not help the afr or dart 210 head would be a good choice too. I would not use the edelbrock here the afr or dart would be a better choice.
 

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Had the block decked and due to variances of rod length/piston height, etc, the deck height varies from .006 to .020. For this street engine, I didn't worry about having them all perfectly uniform.
Mike T - Prescott AZ
this is not due to variations in pistons and rods. it is due to a poor deck job. Pistons and rods are usually made to .0005" tolerance in length and pin height. that no where near adds up to the .014" difference you have. with that much variation in deck height I would be checking the rest of the machine work real close.
 

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this is not due to variations in pistons and rods. it is due to a poor deck job. Pistons and rods are usually made to .0005" tolerance in length and pin height. that no where near adds up to the .014" difference you have. with that much variation in deck height I would be checking the rest of the machine work real close.
Excellent observation John....Those measurements looked kind of suspect to me as well....But, didn't know for sure.
 

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Pistons and rods are usually made to .0005" tolerance in length and pin height.
Maybe on the rods if they are aftermarket.

But I disagree on the pistons, they can vary far more than this. I had a set of Manley forged pistons once that looked like the pin holes must have been drilled with a pistol drill to locate them. That was extreme. A cast piston with an unmachined deck surface isn't even flat to anywhere near .0005".

The crankshaft comes into play here too. Stock cranks are usually not good to .001" of stroke which is .0005" of deck height.

The best way for the customer to see if the deck is square is to put the same rod/piston/bearing assembly in cylinders 1,2,7, and 8, checking the deck clearance in each location. That way you can see taper from front to back, and whether the two decks are equal height.

I have had to machine piston decks to get them equal right many times.
 

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Discussion Starter #13
Yeah, I wasn't totally believing the story of piston and rod size variations either. When I first took the block to him to see if it was in good enough shape to consider, he checked it out and mentioned both decks being off a bit, dropping towards the front of the block. In the end, my deck height measurements at the 4 corners came out as follows:
#1 - .020 #2 - .012
#7 - .013 #8 - .006
Since I had a '68 327 sitting on another stand that I had also partly assembled but not measured, I checked it too. That 327 had machine work done at least a dozen years ago when I was still in Northern California and the deck height measurements came out as:
#1 - .023 #2 - .015
#7 - .022 #8 - .017
In the end, on the 400, at least the quench will be better than if we hadn't touched it at all. I ran across a Hot Rod Network article where they gave a 400 SBC a shave and a haircut and were expecting to find deck heights in the .025+ range but were very surprised to find the best at .062 and the worst at
.074 down in the hole.
Here's that article. https://www.hotrod.com/articles/ccrp-1105-how-to-build-a-400ci-small-block-chevy-torque-monster-for-2500/
Mike T - Prescott AZ
 

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Yeah, I wasn't totally believing the story of piston and rod size variations either. When I first took the block to him to see if it was in good enough shape to consider, he checked it out and mentioned both decks being off a bit, dropping towards the front of the block. In the end, my deck height measurements at the 4 corners came out as follows:
#1 - .020 #2 - .012
#7 - .013 #8 - .006
Since I had a '68 327 sitting on another stand that I had also partly assembled but not measured, I checked it too. That 327 had machine work done at least a dozen years ago when I was still in Northern California and the deck height measurements came out as:
#1 - .023 #2 - .015
#7 - .022 #8 - .017
In the end, on the 400, at least the quench will be better than if we hadn't touched it at all. I ran across a Hot Rod Network article where they gave a 400 SBC a shave and a haircut and were expecting to find deck heights in the .025+ range but were very surprised to find the best at .062 and the worst at
.074 down in the hole.
Here's that article. https://www.hotrod.com/articles/ccrp-1105-how-to-build-a-400ci-small-block-chevy-torque-monster-for-2500/
Mike T - Prescott AZ
You have plenty of meat left to have it decked square to the mains.keep the Pistons down .003 in the whole and call it a day. Hopefully they torque ate bored and honend it
 

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Discussion Starter #15
While a bit disappointing it didn't turn out more square, I did ask if he used a block plate when boring/honing and he said yes. I will no doubt leave it as is and continue on. That brings me to another aspect of this build which is cylinder head selection which already came up with a suggestion from hutchenc that I find some heads with larger intake runners, like in the 200/210 size. For my intended usage, idle on up to around 5K rpm, I was thinking about nothing bigger than say 195 intakes in an attempt to maximize intake velocity. Sort of along the lines of the small 170 intake runners used in Vortec heads. Is there a tradeoff by going bigger than 195s for the rpm I'm expecting?
Thanks,
Mike T - Prescott AZ
 

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The 400 will like bigger runner. I wouldn't put any smaller than the 195. I wouldn't use a 170 on anything bigger than a 327.
 
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