Don't confuse a race engine with a daily driver. An engine that is most efficient at 9,000 rpm won't be at 3,000 rpm. We've wildly strayed from the OP's original question.
Everything you said is true, I agree. Except the DCR will be the same at sea level or at 5000 feet. The amount of pressure that compression generates will be less at 5000 feet. But the DCR will remain the same as will the SCR. Just less pressure as measured by a compression gauge.
No, problem! Let the thread go where it goes. I enjoy theses discussions! Might even learn something! Lol Bones
Its all about the column of air and things that are in motion tend to stay in motion at high RPM the column is moving fast so if you leave open longer more air will get in because of the ram effect. But at slow RPM with less ram effect keeping the valve open will let some of the air back out reducing compression.
The rougher the idle the richer the idle mixture needs to be. With a carb the idle circuit is somewhat in play to about 2,000 rpm.
My future 327- 10.1 CR. with flat top HyPr pistns-(EDV~5) w/2V-Rlf wth- AlumHds TrickFlow 64cc, O-deck- .039-FP-HdGask, Cam-Howards~#CL112591-12 Lift 470/470 Dur @.050-231/241 LS-112 & IC-108 1800-6000 rpm (HydFlattap-cam) Holley 4150-3310 Carb Edy-C3BX Intak-Manfld TH350 Wth/2600 rpm stall convrtr. Rear gear ratio 4:11 LongTubeHeader-1 5/8" X 2 1/2" OL Rear tires 28" tall In my 63 C10-3100lbs Pickup Truck Will I be wasting my time with this cam for a nice street cruiser ? I don't want a stinky rich exhaust just a nice hump & bump runner... Let me know what y'all think of my parts combination. I also enjoy this type of reading & finding answers from everyone's knowledge and input... Thank You All. 1Nimrod
Yeah, higher compression is only good up to a point, as soon as you have to run it richer and/or back of ignition timing to keep it in one piece at full throttle you are pissing away power. You can still get improvements at part throttle such as better fuel economy (assuming the engine is properly tuned), but more isn't always better.
Sorry, it doesn't work like that. The octane rating is simply a rating of the resistance to detonation or pre-ignition. A higher or lower octane rating does not create more power than the other. There is nothing about a lower octane fuel that would allow you to tune an engine for increased power vs what you could with a higher octane rating.
I think using a turbo 350 is the wrong choice for a "nice street cruiser". Its the same old question about gearing. You build a performance engine. Then you don't drive it very much because it sucks gas and the rpms are annoying at highway speeds. So then you use a lower numerical rear gear and give up your performance. Gas mileage still sucks because you put a high stall converter in it. What you need to consider is that when Turbo 350s were new, highway speeds were 55 (or maybe 65). At that speed you kept up with traffic and your engine could run an acceptable rpm. Today, you have to be able to run at least 75 mph and will still have people on your bumper. Your engine has to run a higher rpm to maintain that 75 mph and its downright annoying.......especially on a long drive. If you truly want a "nice street cruiser", you need to get a transmission that has overdrive. Then you can keep a decent rear gear ratio and your performance as well.I'd suggest a 3.50/3.70 rear gear. Also, I'd stroke a 350 to 383 since you are scratch building. An additional 50 cu in will make it more fun and it won't really cost anymore than building a 327 except for buying a 4" crank. 50 more cubic inches for under $300 is a bargain.
Hmmmmm, I remember when Turbo 350's were new, and the speed limit was whatever was "safe reasonable". I remember making cross country trips at triple digit speeds for hours on end.
This is missing the point. Dynamic compression is not about overlap when both valves are open at end of exhaust beginning of intake. It's about the intake valve closing point on the compression stroke. Longer duration will close the valve later ABDC. At lower rpm the piston will push some mixture back into the intake manifold. Compression will start when the intake valve closes. At higher rpm the incoming mixture will have more momentum and continue to enter the cylinder even as the pressure increases with the rising piston. The later ABDC, the higher the rpm needed for the intake momentum to overcome the pumping loss. This is why intakes have runners, injectors have stacks. The mass of the moving column of air will hammer the mixture past the valve as the cylinder pressure rises (inertia tuning). As the valve closes the moving column will create a pressure rise behind the valve. The length of the runner will determine the timing of when that pressure peaks. Ideally it will peak as the intake valve opens (wave tuning).
Good Catch. I was thinking about the 4" bore of a 350 I guess. I don't really fool around with the original small block engines these days, and all the unmentionable aluminum descendants mostly use one crank stroke.......so a 4" is the common go to for making any larger displacement.......including a 383. Had a brain fart there. The 383 smallblock uses a common 4.030 BORE and a 3.750 Stroke vs the stock 3.48 Stroke. You can buy complete stroker kits with pistons for less than $800 ready to install. Thanks for pointing out my mis-statement. Always like for my posts to be factually correct......or corrected. Jimmy six a 327 can run @ 3000 for hours and many had 3.08 gears in countless midsixtys cars with 327’s getting over 20 mpg That goes back to my recommendation to use an overdrive. I don't see any reason to drive at 3,000 rpms or more, or to use a 3.08 gear and give up any semblence of performance. To me, I never see the value in giving up some performance when the additional performance costs essentially the same. Back when we were all scraping for every dollar, and there really weren't many fast cars, it wasn't so bad. If you had any kind of overhead V8 you were a king. Today a 3.08 geared 327/ 350 trans in a truck won't keep up with many soccer moms SUVs. I thought we were building "Hot Rods" here. You would be building a 16 second or slower truck. Wheres the fun in that?
Prior to the 55 mph speed limits most roads were not 4 and 6 lanes or more wide, bias ply tires and steering geometry weren't the best for high speeds and many cars were incapable of continuous high speeds. If you remember, the predecessor to the Turbo 350 was the 2 speed Powerglide. Not exactly neck snapping performance. While many of us did drive faster than 55, it was usually not for extended periods of time because roads were generally 2 lanes and there were other cars in our lane. Conditions "back in the day" usually dictated how fast we drove even when not regulated. Still, you cannot get away from the fact that "gearing" or the lack of it determines a lot about the usefulness of a vehicle. Many "hot rods" sit in the garage everyday because they aren't enjoyable to drive because of gearing choices. So why not address the problem before building instead of after.
Assuming a stock 4 inch bore and 5cc valve relief pistons your static compression ratio(SCR) should be 9.65:1. With stock 5.7 inch rods that would make a dynamic compression ratio(DCR) of 7.33:1 at sea level. IMO not a very good choice of a cam for a street cruiser with a 2600rpm converter,relatively low compression and a dual plane intake. That cam has LOTS of duration for a 327 and a pretty wide LSA, it will rev to the moon with good flowing heads. Peak power will probably be around 6400rpm and peak torque around 4700rpm. Not much going on at 2600rpm... That cam is an old school cam with lots of duration and not much lift , it is a "low energy" cam with pretty mild lobe acceleration. A better choice IMO would be to spend some money and upgrade to a modern "high energy" Roller cam with with more lift and less duration. Tightening up the LSA a bit would would also help low end torque and increase the DCR. I think this Howard 219/225 0.510/0.520 cam would be a good fit for your 327: https://www.summitracing.com/parts/hrs-111145-10/make/chevrolet The aggressive lobe acceleration and high lift would let you make almost the same peak power with a lot less duration. The 110 LSA would bump your DCR up to 7.84:1 and make lots of low end torque. Peak power with this cam will probably be around 5900rpm and peak torque around 4300rpm. I think this cam would be a huge improvement over your current one if you are planning to keep the 2600rpm converter. Upgrading your intake to a modern Performer RPM would also help improving your power curve. Just my two cents..
There should be no controversy as to what is meant by "static compression ratio": it is a simple geometric calculation, the ratio of the volume of the cylinder at BDC to that at TDC. I can think of three different things we might take into account to arrive at something called "dynamic compression ratio," or any combination of them: The point at which the intake valve closes during the compression stroke; The gas pressure of the incoming mixture; and The effect of gas inertia. Of these the first is not as simple a calculation as it seems, because the intake doesn't close instantaneously. The second is simple enough if you know what the pressure is at that time: a vacuum gauge should give a good clue. The third I'd submit is best measured empirically, or deduced after the fact from results. Together they define volumetric efficiency, as the ratio of whatever this metric turns out to be as a percentage of static CR. And from this it should be clear that VE varies according to conditions, notwithstanding that it is conventional to use a single estimated figure for calculating carb CFM etc., presumably that at WOT. The theory behind progressive setups is that the primary phase provides drivability and that secondary phase provides WOT performance. But how often is VE at part throttle at 1800rpm considered when designing a primary phase? (It just occurred to me yesterday that the primary component of a progressive setup really ought to comprise constant-vacuum carbs like SUs or Stromberg CDs, because that is where the ability to self-adjust to vacuum is most useful.)
How does overlap "bleed pressure from the cylinder"? Overlap begins as the piston approaches TDC on the exhaust stroke, exhaust valve is on it's was closing. The only pressure in the cylinder are remaining exhaust gasses. The intake valve begins to open, prior to TDC, exhaust valve still open, beginning the overlap period, both valves now open. The remaining exhaust gasses exist either the exhaust valve, helping to pull in the fresh mixture through the intake valve (good) or push some if it back the intake (not so good), depending on RPM, tuning etc.. The exhaust valve closes shortly after TDC, the intake valve stays open through the intake stroke, closing at some point past BDC on the compression stoke. Pressure now begins building in the cylinder until ignited near TDC on the compression stroke. After TDC, the resulting explosion pushes the piston down during the power stroke. At a point approaching BDC the exhaust valve opens and pushes the exhaust gasses out on it's way again to to TDC and the overlap period. The only compression pressure in the cylinder (not counting power stroke) is made or lost during the compression stroke, opposite the overlap period. At least that's how I understand it.
I think one thing not brought out here is that the exhaust gases are exiting under some pressure as the piston rises on the exhaust stroke. Thats why exhaust valves can be smaller than intake valves. Its a balancing act as to when the intake can be opened without exhaust gas pushing against the incoming air that only has atmospheric pressure pushing it. With flat tappet cams, you can increase the duration somewhat, but adding lots of lift is much more difficult because of the rubbing nature of the lifter against the lobe. Think about pushing something up a steeper ramp and how much harder it is. The friction at the point of contact increases and may ruin the cam/lifter. With a roller lifter you can also increase duration, BUT you can add lots more lift TOO. The nice thing about the roller is that you can JUST add LIFT and keep the same duration or only change it slightly. The idea of opening the valve longer (duration) is to provide more time for ingesting air. The valve is open longer. If you can open the valve more quickly and even more open, you accomplish the same thing. Combine the two things and you get the best of both worlds. Here is a general chart of what happens when you change things on a cam.
As others have noted, the intake valve closes (opposite of the overlap phase when it is opening) not at the bottom of the intake stroke but at some point afterward, when the piston is beginning to rise on the compression stroke. This is when some of the static compression ratio is 'bled off', in theory, that is unless the incoming mixture velocity is able to overcome the rising piston attempting to push it back out the still open intake valve. This is when volumetric efficacy can exceed 100%, if all the other intake and exhaust tuning variables are in order. Advancing the cam, thereby closing the intake valve sooner on the compression stroke, tends to eliminate some of this bleed off that occurs at low engine speeds, and helps low speed power, however at the cost of power lost higher up, caused by the valve closing while the intake charge is still capable of overcoming the rising piston. Retarding the cam has the opposite effect, allowing more time for the higher RPM intake charge to enter the cylinder. The intake valve closing is generally the most important of the valve events. I'm a bit rusty on all this, been 10-20 years since I played with valve timing in any seriousness. But that's how I remember it.
^^ This rightch 'ere! The automotive forums are chock full of threads with folks slapping on high dollar intakes, carbs, cams, and the rest of it and then bitching when they discover the futility trying to drive them on the street. What did they expect?
^^^ Same can be said about higher compression ratio's, 9.0:1 is plenty for any street driven car. If the cam needs more then that to run at its full potential, then maybe it's not the right application for the camshaft.
Back in 1982, when advancing or retarding cam timing , changing dynamic compression, was done using offset dowels (I understand they have hex adjusts and what not nowadays), I made this with my little Post Tool lathe. Bolts to the cam and gear, and allows adjustment through the top of the timing cover, even with the short water pump. It worked great, but at that time all my testing was done at the strip, and the usual variables in track temp, wind etc., and my own incompetency, made improvement testing hit and miss. The local machine shop/circle track engine builder was super excited about it at the time, but with no manufacturing experience and more interested in my own exploits, I never made any more than the one. Was looking for it some time ago and realized it's still on the car 30 years later! As I recall I left it fully advanced, seemed best for my combo. Bottom photo take today.
....Sitting in Chuck Finders shop years back..... looking at the previous discussion.... we were discussing same stuff.... he looks at me and says.... just run a blower, it fixes all those problems, you can run junk heads too.
Well I've done my best to explain how no compression is lost during overlap, maybe you'll explain how this bleeding occurs. I believe the opposite to be true, although I don't expect you to believe me. Here is what Comp Cams has to say. Smaller LSA increases compression and chance of knocking, while larger LSA doe the opposite. This would primarily be due to the smaller LSA closing the intake valve sooner on the compression stroke, increasing compression. https://www.compcams.com/cam-timing-lobe-separation-angle
After having to $@k around with it and then learning just how important, it's a little surprising valve timing or cam phasing was never externally adjustable from the factory. Would be really handy. It just isn't possible without a lot of contortions.
Here is what Summit Racing says that may help............or add fuel to the fire so to speak. Exhaust Reversion Definition & Description Reversion happens when exhaust gases flow backwards and contaminate the intake charge. (It is the opposite of The Scavenging Effect.) What causes it? Backpressure in the exhaust system causes reversion. It gets worse with excessive Valve Overlap and improper Valve Timing. Performance camshafts take advantage of scavenging at high rpm. If the exhaust system is too restrictive for the cam, reversion can occur. Reversion usually happens at low and mid rpm. Remember, air will always try to move from high pressure to low pressure. So, at low rpm, during valve overlap, exhaust gases can flow backwards due to: Low pressure in the intake manifold (vacuum) High pressure in the exhaust system (backpressure) The upward motion of the piston How does it affect performance? Leftover exhaust in the cylinder means less room for fresh air and fuel. Less clean air and fuel equals less power. At idle and low rpm, reversion will also cause: Lower intake vacuum Poor idle quality How is it identified? In carbureted engines, fuel may be seen “standing” above the carb. The fuel can also get pushed up into the air cleaner. If your air cleaner is wet and smells like fuel, reversion is likely to blame. Another indicator is exhaust soot in the intake runner. Look for discoloration in the cylinder heads and intake manifold. How do you prevent it? Some valve overlap is needed to maximize the scavenging effect. So, all high-performance engines will have some reversion, especially at low rpm. However, it can be minimized. Improve Your Exhaust Flow: A stock exhaust can have 8 psi (or more) of backpressure at 6,000 rpm. Log-style manifolds can cause exhaust from one cylinder to be sucked into another. Upgrade your exhaust system with a set of Headers and a free-flowing Exhaust Kit. Pick the Right Parts: Long Duration cams with lots of Overlap make more power at high rpm. However, the intake, heads, and exhaust must be upgraded to allow for proper airflow. Mismatched components will make the engine run worse, not better. Warning: DO NOT port match the cylinder head to the headers. Many headers have ports that are larger than the ports on the head. This design helps resist the reverse flow of exhaust. 2 Camshaft Duration Definition & Description Duration is how long the valve is off its seat. It is listed in degrees of crankshaft rotation. When someone refers to a "big cam," they mean it has longer duration, not higher Lift. Manufacturers often list 2 different duration values: Advertised Duration is the degrees of crankshaft rotation that the lifter is raised more than a predetermined amount. This predetermined amount varies between manufacturers. Duration at 0.050" is the degrees of crankshaft rotation between when the lifter is raised 0.050" and when it is 0.050" from its resting position. This is standard among all manufacturers. You should use this value to compare camshafts. How is it measured? Most Cam Cards will list duration. However, if you want to find the duration for the camshaft in your engine, you can calculate it. Using a Dial Indicator and Degree Wheel, find the opening and closing points of the valves at 0.050 in. of lift. If the intake valve opens AFTER TDC, use a negative value. If the exhaust valve closes BEFORE TDC, use a negative value. Add the numbers together. Add 180° to find duration. For example: Intake: Opens at 7° BTDC, closes at 39° ABDC. Exhaust: Opens at 51° BBDC, closes at 3° ATDC Intake Duration = 7° + 39° + 180° = 226° Exhaust Duration = 51° + 3° + 180° = 234° How does it affect performance? At high rpm, longer intake duration fills the cylinder. It also allows more exhaust to escape. This creates more power. (It is why a camshaft's RPM Range is based on its duration.) However, at low rpm, the open valves reduce the pumping pressure of the piston. This results in lower cylinder pressure and less low-end power. Long duration camshafts also create more Valve Overlap. At high rpm, this helps promote the Scavenging Effect. But at low rpm, it also contributes to Exhaust Reversion.
Probly didn't want anyone messing with their R&D on their stock engines. I've seen ads for newer cars with variable valve timing, on the fly. Changing mine with offset buttons several times, and only the strip the following week to test, I made my little gizmo where I could change it right in the staging lanes. On an engine dyno I suppose you can change it with buttons quickly enough to easily retest. Or use the gizmos Cloyes and whomever sell. I haven't raced other than bracket in many years, life's priorities change.. If anyone wants to talk about Cadillacs 4-6-8 valve system, I adapted that to a 327 years ago too. Figured if it was good enough for Caadillac..
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