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High(er) Compression Because of Altitude

8K views 73 replies 13 participants last post by  finn210 
#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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#2 ·
Mike....I'm trying to plug all this into a CR calculator.

How much has your block been bored out?
 
#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
 
#4 ·
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)

 
#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
 
#6 ·
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.
 
#7 ·
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.
 
#8 ·
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.
 
#9 ·
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.
 
#10 ·
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.
 
#11 ·
Excellent observation John....Those measurements looked kind of suspect to me as well....But, didn't know for sure.
 
#12 ·
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.
 
#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/ccr...ci-small-block-chevy-torque-monster-for-2500/
Mike T - Prescott AZ
 
#14 ·
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
 
#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
 
#16 · (Edited)
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.
 
#21 ·
Ok, I'm not grasping what you guys are talking about. If the stroke is still 3.75, then the rod length shouldn't matter that much...right? Backing up a bit, for my combination of 5.7 rods and expected rpm range of mid teens on up to mid 5K range, even with the much larger amount of cubic inches (than what I'm used to), wouldn't I still want to err on the side of smaller intake ports versus larger? I'm all ears, so fill me in on what I'm not seeing here.
Mike T - Prescott AZ
 
#22 ·
I don't think I'd worry too much about the rod length. Would 6" rods be better...probably. Would 5.7" rods be ok...probably. It's not like you're spinning it that high anyhow.

Let's put it this way on heads. A 350ci truck motor with Vortec heads (1996-2000 vintage) had ~170cc runners. What are truck motors good at? Low and mid-range power. A 400ci motor is ~15% bigger (in displacement) than a 350. If you add 15% head runner volume to make up for the increase in engine displacement, you're at 195cc. Also, the cam timing on those truck engines was/is 191 at .050 (roller cams like yours). Add 15% to that 191 and you get 219 degrees (which is what your cam has!). That's pretty apples to apples...so now you have a 400ci motor good at low and mid-range power (which might be just fine with you). A 350 with 170cc intake runners will behave like a 400 with 195cc intake runners (generally speaking). Since you're not spinning your motor very high, that's probably fine, but hopefully you catch my drift here. The motor you're building will probably be good to about 5000 RPM give or take, but it would likely fall off in power pretty quickly after that (which again, may be just fine with you).

This is why a few of us say to run a 195 to 200cc head at a minimum. As it were, on your motor, I'd probably run a cam about 10 degrees longer in duration than what you selected for some extra RPM and power up top, but that's personal preference (and I like high winding motors!). FWIW, my 355 has a 225/234 at .050 roller cam and power brakes...it's fine and pulls around 12" vacuum at idle. My 421 runs 240 degrees at .050 (with AFR 220 heads) and peaks around 6300 RPM. I could've run bigger heads and cam on my 421, but I wasn't looking to spin it higher than about 6500 RPM anyway. You could easily run 230 degrees at .050 and still have good vacuum.

Good luck :)

If you wanted to spin it higher and/or make more power, you could *very* easily grow into a 210 or 220cc intake runner head without losing much, if any low end torque and it will really shine above 4000 RPM due to it's increase breathing capacity. Now, I'm sure a ~200cc head will be enough for a mild 400 SBC like you're building, but I wouldn't choke it with a small intake runner head.

At the end of the day it's a bigger motor so it's needs a bigger volume in the intake runner to fill the cylinders and it can take more cam duration as well without sacrificing street manners. Remember, you're not building a 350.

Personally, I think the smaller runner/intake velocity argument is WAY overstated. If you have good compression, a good cam (especially a roller), and a good intake...you don't need a small runner to keep good torque down low. Case in point. I used to have a 383 with Dart S/R Torquer heads (I think ~170cc intake runners) with a mild flat tappet cam. I swapped those heads for a set of Profiler 195 heads (and picked up about a point of compression in the process) and a similarly timed roller cam (albeit with more lift) and it REALLY woke that motor up. It had more low end grunt and way more mid range and higher RPM power. Best of both worlds. I also run a 355 with 195 heads on the street and it's a great little street motor! It keeps up with LS2 and LS3 motors until about the 1/8 mile where the top end of the LS motors pulls away a bit.
 
#23 · (Edited)
CR should be ok I run 11-1 on pump fuel.

Get you cam card & figure your DCR (dynamic (effective) compression ratio) to get operational CR to determine if pump gas will be ok. Its all about when the intake valve closes after BDC not the static CR..

Ignition timing is important suggest 10* in the dist (20 crank) with 14 - 16 initial & heavy springs for the dist weights. Put a timing tape on the balancer & use an adjustable vacuum advance & ported carb vacuum. Try 42-44* vacuum advance if light throttle ping occurs during cruise kill some vacuum advance. Your loose converter is a plus it will help with this issue. If FI with a knock sensor total timing is not that critical run whatever lead you like the ECU will think for you.

Replacement heads drilled for steam holes?

CR is more important than quench you can use thicker head gaskets.

An observation: GM does not give a rats butt about quench my stock 90 Vette pistons were .030 in the hole & the stock head gaskets were .050.

I ran +.030 400 blocks a number of times in the track cars with a 350 arm & 6" rods = 377 inches they would rpm past 7500. Returned to 350's when the tracks adopted an 8 lb per cu in rule & 2800 lb minimum weight.

Link to CR & DCR calculations

https://uempistons.com/rt-4-calculators.html

Luck with your build
 
#24 ·
The rod length discussion is one of those never ending deals. I personally don't think it matters a hoot.

I agree with hutchenc's comments on runner volume. The runners recommended aren't going to kill any low end power.

However, you may want to reconsider the cam timing. Don't go bigger, the lack of low end power will be noticeable at elevation. You might even want to go smaller.
 
#43 ·
The rod length discussion is one of those never ending deals. I personally don't think it matters a hoot.
I understand what you are saying (thinking), but I really like the idea of less side load with longer rods on both the pistons as well as the cylinder walls.

To me, all the complicated talk of stock length vs long rods concerning engine manners is pretty much bovine scatology. Who in the world would notice any seat of the pants difference anyway? It's not like a guy would change engines several times a day just to compare...

I still like the less side load deal though. As long as the pin hole isn't way up on the piston a long rod might be good for engine smoothness and longevity.

 
#25 · (Edited)
There was no 'cam card' in the box with the new Howards Hydraulic Roller but I contacted them and they sent the specs. I've got a link to Wallace Racing's Dynamic Compression Formula but not sure which particular spec on the sheet is the one to use for: Inlet Valve Closes ABDC. Is it 'Int Close 62'?
Here's a copy of the spec sheet.
P/N 111145-10 VALVE LIFT INT. .525
GRIND retro hyd roller VALVE LIFT EXH. .525
ENG sbc INT OPEN 30.0
ROCKER RATIO INT. 1.50 INT CLOSE 62
ROCKER RATIO EXH. 1.50 EXH OPEN 73
CAM LIFT INT. 0.350 EXH CLOSE 25.0
CAM LIFT EXH. 0.350 OVERLAP 55.0
L/C 110.0 INT PHASE 106.0
ADV OR RETARD 4.0 ANGLE or LC
INT. DUR 272
EXH. DUR 278 EXH PHASE 114.0
INT. DUR @ .050 219 ANGLE or LC
EXH. DUR @ .050 225
INT. DUR @ .200 x INT O @ .050 3.5
EXH. DUR @ .200 x INT C @ .050 36
CUSTOMER EXH O @ .050 47
EXH C @ .050 -1.5
LASH INT .000" OVERLAP @ .050 2.0
LASH EXH .000"

I just plugged that 62 into their formula and this was the result, based on 5500 foot elevation.

Static compression ratio of 10.6:1.
Effective stroke is 3.00 inches.
Your dynamic compression ratio is 8.68:1 .
Your dynamic cranking pressure is 152.38 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 7.58 :1.
V/P (Volume to Pressure Index) is 149
 
#26 ·
I don't understand why the effective stroke is 3.00 a 400 sbc has a stroke of 3.75"
 
#27 ·
Not sure about that one myself. This is what I filled in on their site.
Number of Cylinders: 8
Bore: 4.155
Stroke: 3.75
Rod Length: 5.7
Static Compression Ratio: 10.6
Inlet Valve Closes ABDC: 62
Target Altitude: 5500 Feet.

Mike T - Prescott AZ
 
#28 ·
Several terms there that are not familiar. To appreciate them I guess you have to be a dynamic compression aficionado. Which I'm not and never will be.

One thing I do know is that some of the Wallace Racing horsepower formulas don't work so good. So why would a DCR calculation full of terminology nobody heard of be any better?
 
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