SBC Primer

[Dave70968]

I followed all of that, but I still don't see any way of compensating for air density other than changing jets or metering rods (is that the proper name for what I've always called the "needle?").

As to the larger carb running in the smaller circuit at cruise, that sounds very much what I'd like to do for my application. I didn't understand the idea of multiple circuits like that, but it certainly sounds slick.

If they weren't so damned expensive, it might be fun to try building an SBC with an aircraft carb. I kinda suspect it wouldn't work out well, though, because automotive carbs have to deal with frequent and (relatively) quick power changes (thus the multiple circuits) that aircraft just don't see.

 

[Dave70968]

Also--to show you how little I know--I need to ask you to clarify how you described that cam. "Stupid big cams like 260@ .050," I'm taking to mean a lobe 260° wide, and .050" taller than standard? If that's correct, how do you measure the width: from the point where it starts to diverge from the base circle (starts to open), or where it reaches full height/full open? Also, as I recall, the cam turns at half the speed of the crank, so a 260° cam lobe would cover 130° degrees in the physical steel, doubled due to the divided rotation rate, right?

 

[swampratt]

I would really like too see how those aviation carbs work.
Might be cool. I know of 1 350 olds that had a 2 stroke chain saw carb on it to go down the road.. now top speed was 24 MPH and took some time to get there but MPG was 30+ I think I recall 38 MPG.
No i did not cobble that together ..another old guy pulled that trick.

260@ .050 is a time in degrees of how long the valves are open.
In a 4 stroke there is 720 degrees of rotation to complete 1 cycle.
The 26 would be 260 degrees of valve being open.
For a ball park .
factory 180HP smogger 350 used around 196 to about 204 degrees @ .050.
325HP or 350HP about 224 @ .050.
The ZL1 427 from way back used 260 and then some more on the exhaust side. That bugger put out over 500 HP

 

[Dave70968]

So I correctly read the rotation, but I'm still unclear on the .050". That's only 1/20", and every cam I've ever seen has lobes bigger than that, so is that .050" taller than the standard profile? The link you provided also talked about 1.5:1 lifter arms, so that .050" looks like it only translates to .033" at the valve, which doesn't seem like much at all.

As to 720° of rotation in a cycle, that means the space between TDC and BDC is 180°. Is the "extra" open time on the intake before TDC (pushing exhaust back up the intake), after BDC (expelling good charge back up the intake), or both? On exhaust, does it open before BDC (which would seem to be wasting part of the power stroke), stay open after TDC (which would seem to suck exhaust back in instead of a fresh charge), or both? Or is there more at play that I don't understand that doesn't waste part of the charge one way or the other?

For the aircraft carbs, have a look at https://www.aopa.org/news-and-media/all-news/2016/june/flight-training/how-it-works and http://www.flight-mechanic.com/float-type-carburetors-mixture-control-system/ . Aircraft carbs are supremely simple devices. They're also smaller than most automotive carbs, no doubt because our engines don't produce nearly the power of auto engines. You mention a 350 cubic inch plant producing an easy 400HP, and some going to 700; a 360 cubic inch aircraft engine will typically be in the 180-200HP range, and a 540-inch 6-cylinder typically tops out around 300 HP. On the other hand, they cruise at 55%-75% power for hours, and are air-cooled, so there are some limitations there (also, weight is a serious limitation; an O-360 is typically going to come in at under 300lb; a quick check of Google puts the 350 Chevy at about double that).

Edit to add: here's a quick overview of some commonly-available aircraft engines. They're "Experimental" in that they're not certificated designs (everything in aviation has paperwork, and is built exactly the same way, or else is experimental and limited in what it can be used for), but are highly representative of the equivalent certificated design (in four-cylinder engines, Lycoming is the household name; in sixes, Lycoming and Continental are both common): http://www.superiorairparts.com/xp-series-engine/engine-models/ . Superior is a manufacturer down in Dallas that also makes some certificated parts, including cylinder assemblies (our cylinders are external and individually-removable).

 

[swampratt]

Oh yes .050"
A camshaft is rated in Lobe Lift. and Duration and overlap and lobe separation angle and many other things.
Lift is how far the peak of the lobe is away from the radius of the base circle of the camshaft. Or you can say Lift is how much it will lift the valve open.
Lobe lift of say .300" will have the peak of the cam lobe .300" away from the radius of the base circle of the cam.
Say you have .300" lobe lift you also have a 1.5 :1 rocker arm ratio. it will multiply .300 x 1.5 =.450" there you have .450" of valve lift. valve is opened .450"
Now the .050" and duration.
Duration advertised on a camshaft can be advertised as 290 duration or say 268 duration.. now that can be misleading .
Because it is advertised and many times that advertised duration number could be at .006" valve lift or .012" valve lift.

Manufactures do not always tell at what Valve lift "advertised"duration is calculated at.

So a basic industry standard was set so we could compare a camshaft more easily and choose what we want.
SO the .050" spec was set forth.
Almost all cam manufactures will have the .050" spec for you too see in their advertisement.

so 260 duration at .050".. means 260 degrees duration of that cam beginning the measuring process after the lifter was lifted .050" and end the measuring process when the lifter or cam follower is .050" from closing.
So basically sliding up the lobe to start your measurement. now you can see advertised at 292 at .006 will be different than advertised 292 at .012" .

It does not get any better ...050" is not the end all in comparing cams or trying to pick one and how they will act.
You have duration at .200" lift also. you can have the same say 260 duration at .050" lift on 2 cams and at 200" lift one cam may have 138 duration and another will have 168 duration.. that 168 duration will make more power as it is holding the valves open much longer and allowing the cylinder to fill up with more air.

This may explain some of it better.
http://www.lunatipower.com/Tech/Cams/CamSpecTerms.aspx

There are exceptions to the rule when reading what cam manufacturers post about overlap and Lobe separation angle.

Most will state LSA Lobe separation angle if you make it wider say from 106 to 115 LSA the 115 LSA will have a more broad power curve and carry power farther past the peak HP rating.
Say peak HP is at 5700rpm.. they are saying the 115 LSA will not drop power off a rapidly past 5700 rpm as a 106 LSA cam will.

I found in my testing that is NOT true all the time. It has a lot to do with lobe events.. when the exhaust lobe is opened and closed and when the intake is opened and closed.

I have had Odd cams speced for my little 350's and just recently I had an odd ball made in search for more MPG it had a wider LSA 112 but power past the peak fell off like a brick compared to the narrow 106 LSA.

Both cams i compared in the same engine back to back both cams were ground with the same master lobe I just juggled the LSA .. but this experimental cam I only changed where the ex lobe would be.. left the intake lobe opening and closing rates the same between the 2 cams.

My theory was I would not loose my Low speed power because I had 225 PSI cranking pressure and intake valve closing determines what that will be ..I did not want that to change as with high cranking pressures comes brute power out of the hole.

Get the heavy vehicle moving.
Big juggling act.
That cam is what i am running now the 112LSA experimental in my 350.
MPG went from 12-14 to 17-18 MPG.
Peak HP was made at the same RPM on both cams 5700rpm but by 6500rpm the 112LSA was down over 40 HP.

Normally you do not get to 112 LSA by moving the ex lobe only you move both lobes.. in doing that the intake valve would close later in the cycle and add a little more power on the top.

Confused yet??
My cam grinder said I change cams like he changes underwear.
I am always wanting to experiment.
I almost wanted to have another made but go to 117 LSA and see if i could pull 22+ MPG.. this is normal driving with some spirited tossed in. not hypermiling. NO OD no FI and not a MPG friendly gear

 

[swampratt]

there are auto carbs very similar to the economizer type in aviation seen at the top of this link.
http://www.flight-mechanic.com/float-type-carburetors-economizer-system/

Where mechanical linkage moves the economizer needle out of the jet or into it dependent on throttle angle.
They do not seem to be much different really from the auto carb.
But the auto carb needs the needle moved in and out of the jet or fuel passage many more times than the aviation carb due to stop and go driving and flat and hilly areas and passing and cruising.
In the air you are steady state cruise.

Could you imagine how much in an automobile you would have to pull the lever in and out just to make it to the store with the correct air fuel ratio.
I would think you have exhaust temp readings so you know if you are rich or lean in an airplane..But i do not know.

 

[Dave70968]

Many (not all) airplanes have an exhaust gas temp (EGT) indicator, though they're often not marked as to specific temperatures, so all you can see is relative temp (which is enough--lean to peak, or whatever amount of rich- or lean-of-peak you want). For simpler airplanes, we can also just lean to peak RPM, which would be best power. Operating lean-of-peak is the subject of much heated discussion in the aviation world, with some swearing by it and some swearing at it. If you think threads get contentious here....

Some airplanes have cylinder head temp gauges on one or more cylinders. Very modern stuff may have a CHT probe on each cylinder and an EGT probe on each cylinder, so you can assess performance with great specificity; very handy for assessing cylinder balance. Not much you can do to adjust cylinder balance in flight, but certainly good diagnostic info to give to the mechanic on the ground.

 

[Dave70968]

This may explain some of it better.
http://www.lunatipower.com/Tech/Cams/CamSpecTerms.aspx

There are exceptions to the rule when reading what cam manufacturers post about overlap and Lobe separation angle.

Most will state LSA Lobe separation angle if you make it wider say from 106 to 115 LSA the 115 LSA will have a more broad power curve and carry power farther past the peak HP rating.
Say peak HP is at 5700rpm.. they are saying the 115 LSA will not drop power off a rapidly past 5700 rpm as a 106 LSA cam will.

Good article; it explains a lot. In particular, "At high engine speeds, overlap allows the rush of exhaust gasses out the exhaust valve to help pull the fresh air/fuel mixture into the cylinder through the intake valve. Increased engine speed enhances the effect. Increasing overlap increases top-end power and reduces low-speed power and idle quality" sounds like I want reduced overlap for my application, as I'm more interested in a broad power band than in raw high-end power.

 

[Dave70968]

So I'm clueless about the meaning of a 268H cam, and not sure why you'd put a 302 or 305 head on a 350 block.

 

[swampratt]

I assumed again that most people or guys have dreams of going fast since they were tens and were in the loop as to what is what.
Reading all the car mags they could get their hands on. But it is not so.
https://www.summitracing.com/search?SortBy=BestKeywordMatch&SortOrder=Ascending&keyword=268H
That link is for many offshoots of the 268H.
The 268H is a part number or short description of a Competition Cams camshaft.
It came out in the late 70's to early 80's along with other cams in that high energy line and magnum line.
Harold Brookshire designed that 268H as his answer to the 350HP 350 camshaft.
His idea was it would make as much or more power than the GM cam and have better manners and MPG all around and he nailed it.

Harold worked with/at Ultradyne cams and made some really great grinds and he worked at Comp Cams and Lunati Cams.
His design was the VooDoo line of cams.
Just a lot of UD harold grinds out there.

GM made a 302 as did Ford.. when I speak of 302 I will usually talk of my Ford.
305" heads from a GM vehicle will have a smaller chamber than a 350 head the reason to run them is to get that smaller chamber to raise compression.
If you know how to match and massage parts you can run a lot of compression with pump gas. say 91 octane.

Say we were stuck with the 76 CC (Cubic centimeters) head from a 350 That means the chamber will hold 76cc of liquid.

The bore is 4.00 on a 350 and the stroke is 3.48 and the piston is .025" below the top of the block (Deck)
You run a head gasket that is .039" thick and the vlave releifs on the piston are 5.5cc.

Your compression ratio would be 8.5 :1 You can easily run 9.5 with 87 or 89 octane.
Mill the 76cc head .030 and get 72 cc chamber and be at 8.82 compression.
Run the GM vortec heads which came out in 1996 with their 64 cc chamber and have you get 9.57: 1 compression.

You can run a smallish cam with that and make great power.

I like more power more low end TQ more top end power so I go with an even smaller chamber and I then port the head to up the airflow of it.
I have taken the 64cc head and milled it to 54cc.
With the above that would put you at 10.73 :1 compression..You better have your ducks in a row to make that work.

I went even farther and decked the block to get the pistons closer to the top of the block .013" and ran an even thinner head gasket .015".. border line psycho for pump gas at an actual compression of 11.95:1

It worked very well.
But I started young and my first car could not muster 20 seconds in the 1/4 and tune tune tune I did and started with that kind of junk and eventually went mid 11's in the 1/4 on pump gas with no power adders.

Learning the hard way what does and does not work and learning how to tune every last ounce of power from what you have at hand..and I was on a tight budget with 3 kids and a wife.

Confused even more?