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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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#32 ·
Thanks for all the good advice, I'm not concerned about having the compression ratio up around 10.5 anymore. I've already ordered some other parts which are due in soon including the Fel-Pro 1014 head gaskets. They are drilled for the 400's steam holes but oddly, the Edelbrock Performer Heads I've been looking at (with 70cc chambers and roller cam compatibility), their instructions will have me only drill '3' steam holes per head, not '6' to match the block. The three they say to drill are the outboard holes nearest the exhaust side. Not sure if any of the other cylinder head manufacturers suggest only '3' 1/8-inch holes to be drilled but figure Edelbrock has been doing this for a while so I should be good to go. I also bought one of those Cloyes two piece aluminum timing cover, the one with the adjustable cam roller-stop so now have to go out for some socket head 1/4-20's. On the subject of carb size for the engine and usage I've described, would any of you think that an Edelbrock Performer 1406 at 600cfm be a bit too small? A friend has an almost new Quickfuel 680 carb that he no longer needs due to going to a Holley Sniper EFI setup so that might be an option too.
Mike T - Prescott AZ
 
#33 ·
What is the part number for those heads? IMO, a 600 cfm carburetor is way too small for a 400 inch engine.
 
#34 ·
Pops - I've heard that comment a number of times, that a 600 cfm carb would be a bit on the small side for a 400 SBC. I'll be talking with my buddy about the 680 he had (briefly) on his Smeding 383.
I had thought that Edelbrock had a Performer head with straight spark plugs and 195cc intake runners but all I see are 185 intakes offered by them so maybe time to expand my search a bit. On the other brands, like AFR's, I see that most of their products have raised exhaust ports and don't know if that would be a problem or not but would rather not find out the hard way after I've bought the heads.
Mike T - Prescott AZ
 
#36 ·
the raised exhaust runner is not usually a problem. and the 680 is the smallest carb to run on a 400 sbc
 
#35 ·
#37 ·
Without steam holes in the heads on the exhaust side of the engine air will be trapped between the siamese cylinders & the head = hot spots & ------------ you probably guessed it -- STEAM.

Match the intake runner size to the head port size an intake that is to large for the heads will suffer from air reversion @ the intake to head joint & kill rpm.

If HP & rpm is the goal get big port heads & intake.
If torque is the goal stock size ports will work fine.
 
#45 ·
Without steam holes in the heads on the exhaust side of the engine air will be trapped between the siamese cylinders & the head = hot spots & ------------ you probably guessed it -- STEAM.

Match the intake runner size to the head port size an intake that is to large for the heads will suffer from air reversion @ the intake to head joint & kill rpm.

If HP & rpm is the goal get big port heads & intake.
If torque is the goal stock size ports will work fine.
Churchkey - Your comment caught my attention. Because of building smaller inch smallblocks like 327s and 350s in the past, I'd use a mid 60's Corvette Aluminum intake or a Performer 2101 Intake on cast iron stock heads or on aluminum performers and never gave a thought to a potential mismatch by using heads with larger ports with the intakes mentioned above. The AFR 195 SBC Eliminator Street Head is one I'm looking at now and it has intake ports that are pretty decent size, like 2.110 High by 1.265 Wide. Decided to check out the 2101 Edelbrock Performer intake I have waiting in the wings and couldn't find the port dimension specs on their website so measured them myself and noticed the ports are quite a bit smaller, along the lines of 1.950 H by 1.140 W. The difference shows the AFR port to be .160 taller and .125 wider but I suppose since the flow is going from smaller port to larger port with no 'wall' hindering flow, I should be okay?
Mike T - Prescott AZ
 
#38 · (Edited)
I used to live 40 minutes from Sears Point Raceway (now Sonoma Raceway) but I'm a long distance from the closest track so this engine will be built for torque. The intake I intended to use is the existing Performer intake that's on the 350 in the Nomad now.
Cylinder Head Choices - Pops, your right, the 70cc chamber 195 intake runner Edelbrocks are all for flat tappet cams. The 70cc chamber heads they sell for roller cams all seem to be smaller 185 runners. With 70cc chambers the compression will be in the mid 10 to 1 range. Since the raised exhaust ports on AFR's don't seem to be a big problem, time for me to check out their products since they do offer larger 75cc chamber heads. A friend offered a pair of new/old AFR 72cc chamber 190 intake runner heads. For torque purposes and intake charge velocity, understanding this engine is larger than the typical 350 I've built in the past, what would you say the intake runner 'range' is I should be looking at...low of 190, high of 210?
Carb - The friend with the Quickfuel carb that came on his Smeding crate motor, went to the Holley Sniper EFI so when I asked which particular Quickfuel carb his is, he said today he could find no stamped identifying numbers on it. He had thought it was a Quickfuel SS-680. I haven't seen the carb up close but anyone have an idea of where to look (i.e., airhorn, front of the baseplate)?
Mike T - Prescott AZ
 
#39 ·
While the Ebrok 1406 is a decent Carb., it is a compromise targeted to increase fuel economy. If that were my 400 I’d be inclined to top it off w/ a Quadrajet 750. Two different engines between those two carbs.
Jim
 
#40 ·
the 70cc chamber 195 intake runner Edelbrocks are all for flat tappet cams. The 70cc chamber heads they sell for roller cams all seem to be smaller 185 runners.
What does it matter what the camshaft lifter type it is when picking a head chamber size and runner size?

The only logical correlation is that low compression and small runners might be classified as lower peak power expectations and a smaller budget, maybe with the possibility of using less than premium gas, as would a flat tappet cam. But as long as we're talking about streetable applications, one can buy either type camshaft in just about any duration and lift, for just about any rpm range. So the link there is not hard and fast.

On the carb, measure the throttle bore size and venturi size and post that - it will tell what cfm rating the carb has.
 
#41 ·
Two things I've come away with from asking the original question. One, having compression in the low to mid 10's here at 5500 foot elevation won't be a problem and the corresponding combustion chamber sizes I'd need to achieve that would be either 70cc for a 10.6 compression ratio or 75cc which would still be good at 10.1 compression. The second thing I learned was that the intake runner size should be no less than 195 and some folks felt that it should run larger like 200-210. The largest Edelbrocks Performers are available in 70cc, tops but just about all of their 70cc heads (with roller cam compatible valve springs) have small intake runners, smaller than what's been recommended on this thread. So, I'm looking at who can offer a cylinder head with chambers anywhere from 70 on up to about 75 and have the larger intake runners that have been recommended, somewhere around 195 on up to 200/210.
Mike T - Prescott AZ
 
#42 ·
Heads like that won't be hard to find. Here are a few to check out.

AFR 195 or 210's. Should be available in 74cc (you can have them milled to what you want).

Dart SHP 200's. Same deal.

Profiler 195's or 210's. Same deal.

Brodix IK 200's. Again, same deal.

Edelbrock E-Tec 200's (these are Vortec intake pattern heads...a little different than a standard SBC intake, not a big deal, but they do take a Vortec specific intake manifold which costs a little more). Probably not too high on my list, but an option.

One more note. I'd consider having your heads custom built by Eric Weingartner or Chad Speier. They both sell a bunch of different heads and will hand assemble them for not much more, or sometimes less than what you'll pay for heads from Summit or Jegs. They can also give very solid, experienced advice.

http://www.wengines.com/SBCweingartner.html

http://speierracingheads.com/index.html
 
#44 ·
There's no free lunch. Less peak side load = longer time side load is applied.

Same with dwell time at TDC that you get with longer rods. You get longer dwell time @TDC but peak piston velocity increases.

The other half of all this is that the differences in any characteristic you want to measure are insignificant for a given bore and stroke. Increase the stroke the same amount and you get more side load, less dwell time, higher piston speed. So I guess there's an argument not to build a 383 instead of a 355? I do think there is an argument in favor of a 377 (big bore/smaller stroke) over a 383 (smaller bore/longer stroke). But everyone builds 383's because they're cheaper.
 
#48 ·
Personally, I would not choke the 400 sbc with an Edelbrock Performer...I'd go for the Performer RPM or Air Gap.....JMHO

Let it breathe!
 
#50 · (Edited)
First off I will definitely check out the intake manifold fit now that the block has been decked. That crossed my mind a couple weeks ago and I ran it past the machinist who said basically the same thing, put it together and check the fit. He felt it should be fine but we'll see how well everything lines up.
Partial Update - Still haven't pulled the plug on a set of aluminum cylinder heads but while checking both Edelbrock and AFR's, they seem to have different approaches to dealing with steam holes for a 400 small block.
Edelbrock includes instructions to drill three 1/8-inch holes using the head gasket as a template but oddly, they say to drill the three holes only on the exhaust side of the head.
AFR will drill the steam holes for you, for a cost obviously, but in their case, they drill all '6' per cylinder head.
Mike T - Prescott AZ
 
#51 ·
Edelbrock may have heavier castings in the area for top steam ports = cannot drill into the water jackets.

The Brodix heads I ran required the steam port holes be drilled on an angle I
machined them on a Bridgeport mill.

I have a pair of Warren Brownfield aluminum heads in the shop that swallowed an intake @ about 7500 = junk now I kept them for posterity. They were a darn good set of heads they made + 600 hp on the blocks they were on. For the straight line gang the small blocks were NA & one 750 carb.

Warren Brownfield is now AFR.
 
#52 · (Edited)
Got the Cloyes 2-piece timing cover installed and the Summit 8" balancer. Had an assortment of the stamped chrome plated timing markers but two are obviously for smaller diameter dampers, the one that 'almost fits' is not right either. Don't necessarily need one of the higher end adjustable timing pointers but might go there if needed. With the thicker cast aluminum Cloyes cover and 8" diameter balancer, anyone have recommendations for timing pointers that fit and clear well?
UPDATE - Couldn't find one of the higher end billet timing pointers that didn't have a number of negative reviews (typically required some slicing and dicing to fit right) and the one stamped steel 'chrome' timing tab I had was not quite right either so...a little tweaking here and there (as in hacksaw work) and it'll do fine. Same sorta issue as a friend with a Smeding 383, the generic stamped timing tabs project way too far out over the damper so I ran the white paint tab indicating the 'zero' line and 12-degree line down onto the face of the balancer.
Mike T - Prescott AZ
 

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#53 ·
Oil Pan Choices - Engine Will Use 3/4-Inch Forward Mts

I'm still putting together that 400 SBC and wanted to button is up as much as possible to store since the installation will have to wait till another project is wrapped up. I have two SB factory oil pans on the shelf but both have fairly long sumps. The pan on the 350 currently in the Nomad has the right/shorter dimension, about 8.250 inches from rear of sump to front where it radiuses upwards. I've got about a little more than 2" of clearance between the front of the sump and the steering linkage and know this will work even when I install the 3/4-inch forward motor mounts I got from Earl Williams.
I'd prefer to find a pan now that has the same basic dimensions and ran across one from Dorman (#264-104). Also saw Pops mention this fits Tri-Fives in an older 2015 thread. https://www.trifive.com/forums/showthread.php?t=148778&highlight=oil+pan. Post #7.
Anyone by chance have this pan and can tell me the dimension of the sump, front to rear?
Thanks, Mike T - Prescott AZ
 
#57 ·
The proper way to use the dial indicator is to measure and record the crank angles where the piston is down some arbitrary amount, for instance .015" or .020", then adjust the mark as needed. This makes it equivalent to the piston stop method in that regard. You can't just go to 0, as said there's too long a dwell.

If the heads are off that's the preferred way. The piston stop method is only for engines that have the heads already installed.
 
#58 ·
Disagree using an indicator tells you where the piston is at not where the crank is at.

When assembling an engine: Always use the positive stop method BEFORE the heads are installed, use the head bolt holes & bolt a strap across #1 cylinder, if dished slugs or if they are down in hole drill the strap & use a bolt & 2 nuts to adjust for piston contact. Your set up should contact the center of the piston. Install the degree wheel & pointer & find TDC. If not familiar with the how to process check Google. Use a degree wheel that bolts to the back of the crank not to the snout.
After finding TDC: Install the cam & check / adjust the cam timing then the cover & timing pointer & balancer.
Dial in the timing marks.
 
#60 ·
I don't have to think about jack.
FYI: When the piston stops so does the crank.
The piston can dwell @ TDC for 4 to 6 degrees of crank rotation the # of degrees is dependent on stroke, rod length & pin compression height.

One needs to know where the crank is at in order to check / set the cam timing & initial timing.

If assembling a pump gas street engine non of this is relevant it will still run.
 
#61 ·
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.

This is first paragraph from the original post asking about compression ratio and the effect of altitude or more correctly density altitude. I realize that the engine is an - in progress - build and many of the postings are about various components. I feel that the discussion about altitude and CR has been lost. So I would like to get back to CR and its effect on altitude.

With standard atmosphere you lose about 1" of mercury (HG) at WOT per every 1,000' of altitude up to about 10,000'. Starting with almost 30" HG at sea level, when at 5,500' you'll be down to less than 25" HG. Turbo or supercharging will compensate for this and many systems will allow sea level power to be available at close to 20,000'. At 5,500 feet your un-supercharged or un-turbocharged engine will suffer from reduced performance as compared to sea-level operations - with all things being equal. If your car is always going to be operated at this altitude you can run a higher CR than you could at sea level and leaner air/fuel mixture and improve its performance.

I'm not an aeronautical engineer but as an airplane driver for 50+ years this is well known... Al
 
#62 ·
Has anyone actually figured out approximately how much horsepower is lost due to altitude? I'd heard numbers kicked about in the range of say 10-15% when talking about sea level versus our altitude of 5500 feet. I know it 'feels' like that much is lost but wondered if anyone has more definitive data on it.
Mike T - Prescott AZ
 
#64 · (Edited)
The general rule of thumb is 3% loss per 1000 feet in altitude. So at 5500 feet you're looking at 15%-17% down (ish). There are probably too many variables (barometer, relative humidity, temperature, for example) to come to a definitive answer.

To complicate matters more barometric pressure doesn't necessarily increase or decrease just because of altitude alone (although it generally does, weather affects it). Right now, where I live at 7200ft, the barometer is at 29.76. That doesn't mean my engine has the same power as it would at sea level though. Go figure.
 
#63 ·
It's directly proportional to the pressure ratio. So if the barometer at 5500' elevation is 25" mercury instead of the sea level's approximate 30", then power at 5500' elevation is the power at sea level times 25/30, about 17% less power. A 400 hp engine would be 333 hp.
 
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