Cams choice, Low CR vs VE%

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273

273

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Read a few Dozens threads over the years and a couple recently about Cam choice with low CR engines.

Not saying CR ain't important but most are stuck with what they got for one reason or another.
There's always a few who get super concerned about adding any cam not to lose any more dynamic CR and scare the OP's from doing anything. My feeling is that adding duration to low CR ain't ideal but it's better than stock low powered engine. eg adding cam and even 360 heads to a stock 318 short block with little concern for it's final CR or dynamic CR, worked for me a few times and others I know and people on here. Not recommending that especially now that we have magnum and other high flowing closed chambered heads.

What I am wondering is adding cam and even head flow while sacrificing compression, does it work sometimes, cause even though there's less cr but probably have more VE%? so there's more air to compress, people leaving out VE% when factoring dynamic CR?
 
Well, for conversation's sake, the 400 I am building will have under 8:1 with a camshaft up in the 240s @.050" LOL Dynamic will be under 7:1. And you know what? I couldn't care less. It's going to run on any available octane, run cool, run reliably and sound nasty. In other words, it's going to be FUN.
 
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It is all about the exhaust scavenging effect that starts at mid RPM. If you are always operating up in the mid to high RPM's, then the DCR becomes less important. The exhaust scavenging makes the VE's approach 1 regardless. Drag racing with a higher stall TC is a prefect example; you don't want to spend any time at low RPM's; that won't get you anywhere! And some folks want high RPM hooliganism on the streets LOL

If you want the torque band to start in low RPM's where the exhaust scavenging effect has died off, then the engine is just a simple low RPM air pump and evaluation of DCR becomes important if you are going to maintain good torque down in the low RPM's. Street cruising and some forms of racing emphasize a wider torque range engine design.

We all know that too low a static CR and too much cam ends up with too low a DCR for a car/engine to get out of it's own way at low RPM's. There have been plenty of threads here of someone heading down that road and not realizing that will be the case. I would hope that telling folks the why's and offering an explanation and help to see the numbers would not be a problem; if the builder decides that budget dictates the compromise of CR and they can live with it, that is fine. At least they are going into the situation with their eyes open. And they may decide to make a cam or other change to better mitigate the low rpm torque loss, if that is important to them.

Any customer-oriented engine builder will first explore what the customer wants to use the car/engine for (figure out the application), and then help them make the best decisions to get there.
 
If you can't build combustion efficiency in the lower rpm's move the rpm higher to where the engine does more work due to more combustion events for the time allotted and burn more fuel but inefficiently.
 
I'm not talking about putting a like an 285 in a low cr 318 or something like that or scavenging per say. But adding mild to slightly over cammed. Idle to 4500 - 5000.

What I'm trying to figure out, does added air flow "VE%" from bigger cam offset the lower dynamic cr.

If 318 stock say is 80-85% VE so would act more like 250-290 cid if cam adds 5-10% VE thats like 15-30 cid of air.
So if dynamic CR drops 5-10% wouldn't that possibly just even out or ?
The intake charge is being compressed less but there more air to compress.
 
273 said:
I'm not talking about putting a like an 285 in a low cr 318 or something like that or scavenging per say. But adding mild to slightly over cammed. Idle to 4500 - 5000.

What I'm trying to figure out, does added air flow "VE%" from bigger cam offset the lower dynamic cr.

If 318 stock say is 80-85% VE so would act more like 250-290 cid if cam adds 5-10% VE thats like 15-30 cid of air.
So if dynamic CR drops 5-10% wouldn't that possibly just even out or ?
The intake charge is being compressed less but there more air to compress.
Click to expand...
Only if the intake closing event happens later. Some cams with more duration can Infact have an earlier closing than say a smaller one. Which would bring up dcr.. interesting thread. I wish I had an engine dyno to do unlimited tests in.
 
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All 3 have the same short block, The Base HP/TQ on 1st one is just a 4bbl dual plane and headers on a stock 5.9l magnum there another dyno out there that have manifolds instead of headers and is like 10 hp less. The Mod is air gap eddy heads and comp 275 roller cam, and last one is 380hp crate engines 288 cam with single plane and headers.

None of these engines have ton of cr, the main one I'm comparing here is the 300 hp and 380 hp crate engines from 2500-4000 rpm there both have about the same power. For one we're comparing dual plane vs single plane and about 7 steps up in cam size, the 380 hp I know from have it, it drop off under 2000 rpm. Which is exaggerated by 2.96 gears I have. , and I have a stock 5.9l in my jeep and pulls strong under 2000 rpm. Can't see why a mild cam in the 300 hp crate wouldn't be all gain with maybe with slight loss just off idle depending on cam choice.

To me it seem like the added air flow allow by the cam can offset any or some of the loss in dynamic cr or even gain depend on cam etc..,
 
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mini said:
Only if the intake closing event happens later. Some cams with more duration can Infact have an earlier closing than say a smaller one. Which would bring up dcr.. interesting thread. I wish I had an engine dyno to do unlimited tests in.
Click to expand...

True but get the felling a lot of the cams people run don't, but if that the case could pick cams with near stock intake closing event.
 
All dyno curves are at WOT.
Streeters are almost never at wot, unless the cam is just way too big; then they use up all their available throttle just to struggle off the line.
So you hear about the guy who bolts on a 340 top end and cam onto his 318 engine and he says it ain't so bad. What he doesn't tell you about, is the 3000 stall and 3.91s he's running, so he's never in that gutless sub-3000 rpm zone, to start with, and if his engine is lazy, the 3.91s don't squawk about it.

But put that same engine in front of a regular A833 with 3.55s, and suddenly she's a turd below 25 mph and it lays over on every "normal shift" on the primaries, just to to keep accelerating, at a reasonable rate.
This engine will respond completely differently with a matching to the 340 cam, boost in cylinder pressure.

Another thing you need to always keep in mind is that lo-compression engines cannot draw the cylinder pressure down sufficiently low enough,on the intake stroke, for atmospheric pressure to want to be eager to dive in there. And then with a long period cam, at low-rpm, the piston starts pushing some of the meager amount of air it got, back up into the intake. and the bottom-end torque takes a dive. Adding compression ratio at this point is a real game-changer. The air now being eager to get in there builds a good head of steam, and only the tail end gets pinched off as the piston charges up the cylinder.
To be sure, for a streeter, it's a somewhat delicate balancing act. But if your combo includes a manual trans and hiway gears, and lots of city mileage, it is well-worth it to crank the pressure as hi as you dare, with a small engine. The 360 will make more low-rpm torque inherently, and is less sensitive to the pressure loss at low rpm. And it usually makes more than the chassis can handle anyway. You hardly ever hear a guy come on FABO complaining about how gutless his 360 is off the line. But every 318 owner out there, has this as their number one frustration.And they all want to cam it bigger. and they all want to not put pistons in it. And they all don't understand that you can't make a silk-purse out of a sow's ear.
But I digress.
Max VE usually occurs with peak torque, which usually occurs close to 1500 rpm under the power peak. So if your cam peaks at 5000, then your peak torque might come in at 3500. And that means from 3500 back to the stall-rpm, VE is dropping off. And without changing the cam, just about the only way to get the torque up down there, is with more cylinder pressure.
So hang on where is 3500?
In first gear with 3.55s and 27s, 3500@zero-slip =32 mph.
Is this gonna be a problem? Well maybe yes and maybe no. If you have a 360 it will have plenty of torque to not require WOT, to accelerate briskly. But if you have a smaller engine, it might require WOT to keep up to that 360.
What about second gear?
32 mph is 2050 at light throttle/zero-slip. So with a 3000 stall TC no problem. But with a 2000, you are going no where fast with a lo-compression, small displacement engine. So it will have to back-shift to get moving.
So where is 3500 in second gear? That would be 55 mph.
If you take first gear to 5000, then the Rs will drop to 2950 in second. And there is your engine struggling again, from 51mph at WOT all the way to 55mph before it starts marching up the horsepower curve. So,if you don't want your engine to be struggling from 51/2950 to 55/3500, you are gonna need more cylinder pressure, or a high-stall TC.
OK so lets look at it another way; when is the lo-compression engine actually alive? Answer; from 32 to 38 mph in first gear, and from 55 to 60 and beyond to 71 mph. Then comes the 2-3 shift.
So there you go; the low compression engine from zero to 60mph,by the numbers is doing ok for 11 mph, the rest of the time it varies from lazy-dog to lazy pony....... unless you put a hi-stall on it. Which doesn't actually take the laziness away, it just lets the engine not be in the lazy-zone, at take-off, ...... but doesn't solve the problem in second gear, until the stall is something like 3500.

But the thing I want to stress is all the above is happening at WOT. In the real-world I'm gonna guess that 95% or more of the time, my engine is at sub 3000 (2800 even). And the reason is because with 180psi,and being a 360 it makes mountains of low rpm, Part-Throttle torque, and with 355s she doesn't need any more rpm to accelerate briskly.In fact, the 230* cam in it today is a compromise to reduce the cylinder pressure, simply because I did not know that I could run higher than 180psi. I thought I was already pushing it. then I come to FABO and hear about some of you pushing 200 and more, still on pumpgas.So if this 230* cam ever dies,then I will likely back up a size or maybe two even, and let the pressure climb close to or into the 190s.

It's like Rusty said; his low-compression 400 is gonna be fun strictly because it has the cubes. It doesn't need the pressure nor even want it; because the surplus will just go to heat, mechanical trouble, or tirespin.

Really, IMO, its the 318s and smaller that need to run on the edge. That might include the lo-C 340s
 
AJ/FormS said:
But the thing I want to stress is all the above is happening at WOT. In the real-world I'm gonna guess that 95% or more of the time, my engine is at sub 3000 (2800 even). And the reason is because with 180psi,and being a 360 it makes mountains of low rpm, Part-Throttle torque, and with 355s she doesn't need any more rpm to accelerate briskly.In fact, the 230* cam in it today is a compromise to reduce the cylinder pressure, simply because I did not know that I could run higher than 180psi. I thought I was already pushing it. then I come to FABO and hear about some of you pushing 200 and more, still on pumpgas.So if this 230* cam ever dies,then I will likely back up a size or maybe two even, and let the pressure climb close to or into the 190s.
Click to expand...

At that point the carb and how you tune it becomes critical.
 
All of this is just thinking too much. It's a hobby. Build whatever you want and HAVE FUN!
 
AJ/FormS said:
All dyno curves are at WOT.
Streeters are almost never at wot, unless the cam is just way too big; then they use up all their available throttle just to struggle off the line.
So you hear about the guy who bolts on a 340 top end and cam onto his 318 engine and he says it ain't so bad. What he doesn't tell you about, is the 3000 stall and 3.91s he's running, so he's never in that gutless sub-3000 rpm zone, to start with, and if his engine is lazy, the 3.91s don't squawk about it.

But put that same engine in front of a regular A833 with 3.55s, and suddenly she's a turd below 25 mph and it lays over on every "normal shift" on the primaries, just to to keep accelerating, at a reasonable rate.
This engine will respond completely differently with a matching to the 340 cam, boost in cylinder pressure.

Another thing you need to always keep in mind is that lo-compression engines cannot draw the cylinder pressure down sufficiently low enough,on the intake stroke, for atmospheric pressure to want to be eager to dive in there. And then with a long period cam, at low-rpm, the piston starts pushing some of the meager amount of air it got, back up into the intake. and the bottom-end torque takes a dive. Adding compression ratio at this point is a real game-changer. The air now being eager to get in there builds a good head of steam, and only the tail end gets pinched off as the piston charges up the cylinder.
To be sure, for a streeter, it's a somewhat delicate balancing act. But if your combo includes a manual trans and hiway gears, and lots of city mileage, it is well-worth it to crank the pressure as hi as you dare, with a small engine. The 360 will make more low-rpm torque inherently, and is less sensitive to the pressure loss at low rpm. And it usually makes more than the chassis can handle anyway. You hardly ever hear a guy come on FABO complaining about how gutless his 360 is off the line. But every 318 owner out there, has this as their number one frustration.And they all want to cam it bigger. and they all want to not put pistons in it. And they all don't understand that you can't make a silk-purse out of a sow's ear.
But I digress.
Max VE usually occurs with peak torque, which usually occurs close to 1500 rpm under the power peak. So if your cam peaks at 5000, then your peak torque might come in at 3500. And that means from 3500 back to the stall-rpm, VE is dropping off. And without changing the cam, just about the only way to get the torque up down there, is with more cylinder pressure.
So hang on where is 3500?
In first gear with 3.55s and 27s, 3500@zero-slip =32 mph.
Is this gonna be a problem? Well maybe yes and maybe no. If you have a 360 it will have plenty of torque to not require WOT, to accelerate briskly. But if you have a smaller engine, it might require WOT to keep up to that 360.
What about second gear?
32 mph is 2050 at light throttle/zero-slip. So with a 3000 stall TC no problem. But with a 2000, you are going no where fast with a lo-compression, small displacement engine. So it will have to back-shift to get moving.
So where is 3500 in second gear? That would be 55 mph.
If you take first gear to 5000, then the Rs will drop to 2950 in second. And there is your engine struggling again, from 51mph at WOT all the way to 55mph before it starts marching up the horsepower curve. So,if you don't want your engine to be struggling from 51/2950 to 55/3500, you are gonna need more cylinder pressure, or a high-stall TC.
OK so lets look at it another way; when is the lo-compression engine actually alive? Answer; from 32 to 38 mph in first gear, and from 55 to 60 and beyond to 71 mph. Then comes the 2-3 shift.
So there you go; the low compression engine from zero to 60mph,by the numbers is doing ok for 11 mph, the rest of the time it varies from lazy-dog to lazy pony....... unless you put a hi-stall on it. Which doesn't actually take the laziness away, it just lets the engine not be in the lazy-zone, at take-off, ...... but doesn't solve the problem in second gear, until the stall is something like 3500.

But the thing I want to stress is all the above is happening at WOT. In the real-world I'm gonna guess that 95% or more of the time, my engine is at sub 3000 (2800 even). And the reason is because with 180psi,and being a 360 it makes mountains of low rpm, Part-Throttle torque, and with 355s she doesn't need any more rpm to accelerate briskly.In fact, the 230* cam in it today is a compromise to reduce the cylinder pressure, simply because I did not know that I could run higher than 180psi. I thought I was already pushing it. then I come to FABO and hear about some of you pushing 200 and more, still on pumpgas.So if this 230* cam ever dies,then I will likely back up a size or maybe two even, and let the pressure climb close to or into the 190s.

It's like Rusty said; his low-compression 400 is gonna be fun strictly because it has the cubes. It doesn't need the pressure nor even want it; because the surplus will just go to heat, mechanical trouble, or tirespin.

Really, IMO, its the 318s and smaller that need to run on the edge. That might include the lo-C 340s
Click to expand...

I understand what your saying but I'm looking more to figure out, say you got two engines of same size one 8.5:1 dynamic cr but 80% VE and the second 8:1 cr but 90% VE, wouldn't you think that would have to be a factor, the lower cr compressing more air would act similar to the other?
 
273 said:
I understand what your saying but I'm looking more to figure out, say you got two engines of same size one 8.5:1 dynamic cr but 80% VE and the second 8:1 cr but 90% VE, wouldn't you think that would have to be a factor, the lower cr compressing more air would act similar to the other?
Click to expand...


It’s trapped volume that matters, and how you convert that to heat.
 
If I have to make the choice between the two, in a race I'll take flow over compression any day of the week and twice on Sunday.
 
318willrun said:
If I have to make the choice between the two, in a race I'll take flow over compression any day of the week and twice on Sunday.
Click to expand...

I'm on the same page.
I'm just try to figure out is increased VE% is why that's so.
 
318willrun said:
If I have to make the choice between the two, in a race I'll take flow over compression any day of the week and twice on Sunday.
Click to expand...


It needs to be good flow over bad compression, but I agree. I’ve seen it go both ways, but 99% of the time if you have to lose some compression for flow, it’s a win.

Now, if you do something to that gives up compression and the reverse flow gets worse (better as it may be said) then you can lose a ton of power at and around peak torque. Which should be right in the gear change. Which is an ET killer.
 
318willrun said:
If I have to make the choice between the two, in a race I'll take flow over compression any day of the week and twice on Sunday.
Click to expand...

The problem begins when people say an engine is just an air pump. Its not, they totally ignore the fueling component.
 
Hysteric said:
The problem begins when people say an engine is just an air pump. Its not, they totally ignore the fueling component.
Click to expand...
I don't think it's ignoring it, I think it's assumed that the air/fuel ratio needs to be correct. But CFM's it a measurement of airflow, and airflow creates hp in combustion chambered engines.
 
318willrun said:
I don't think it's ignoring it, I think it's assumed that the air/fuel ratio needs to be correct. But CFM's it a measurement of airflow, and airflow creates hp in combustion chambered engines.
Click to expand...

Read the thread I just posted. There's a reason you have to run more comp with a bigger cam. Think about what you trap in the cylinder before commencing the burn. AFR has very little to do with it.
 
@273
I think I get what you are saying; Not saying I understand your thinking completely, but at one time, I had those thought as well. But me, I wasn't smart enough at thre time, to pursue it; probably still not,lol. But here's my thinking;

Let's talk a streeter at WOT;
Peak VE, and peak torque walk hand in hand.
In an LA318, with the stock cam, peak torque is around 2400rpm.
If you want more power at 2400, you have to trap more air.
Compression ratio won't "pull" more air in.
You can compress that ingested volume to kingdom come; it will never increase your VE. Well actually, depending on how you compute VE, yes it is possible; but that is a numeric increase not a real-world increase. That is how science and math can be used to manipulate the world around us; but I digress...
All you can do is make it easier for the atmosphere to soldier it's way into that cylinder before the intake closes. With the Stock cam, what are your options for increased induction at 2400rpm? I mean besides supercharging, and besides a CID change.Well;
#1 is a free-flowing exhaust,to let out the spent gasses, so the falling piston on the Intake stroke immediately following, doesn't have to deal with them anymore. Forget backpressure; that is your engines worst nightmare. and
#2 is a free-breathing induction system,obviously. IMO the bigger the carb the better until you hit low-rpm response driveability issues. and
#3 is a bigger intake valve,but not so big as to induce shrouding, and
#4 is better ports, and
#5 which should be #1, is the often overlooked, increased air density. Sucking hot underhood air is a sure recipe for strangling an engine's power.and
#6 which should be #2, I reasoned, was the ring seal.
If your rings are leaking, the piston cannot properly evacuate the chamber, making it less inviting for the atmosphere to dive in there. And IMO this is the part about high-compression that nobody talks about. This is why I chose a very small chamber volume and Plasma-Moly file-fit rings. I was searching for zero% Leakdown, in an effort to "pull in" as much cold-air as possible at peak VE and below. I already knew my 367 had plenty of high-rpm power, now I wanted more bottom-end with the same cam.
Every time I hear a 318 fan say; " I'm not changing the pistons", I cringe; knowing that 40 year old bores are tapered for sure, and probably oval, and sloppy loose on the pistons and she has ring gaps you could drive a bus thru.. But the first thing they want to know is what cam to put in it.
Even if you put brand new rings in deglazed holes, you will still not get a decent cylinder evacuation. On the induction stroke, everything just slowed right down, and on the power-stroke,all your imagined power is going straight past the rings.
#7 is crankcase pressure; If the pistons and rings have to work against 4psi underneath of them, that is lost power to the crankshaft.

But lets talk about a bigger cam;
so with no other changes, that means more rpm, and that means a "bigger" cam has to move the operating window up higher, sliding the VE window to a higher rpm as well. So the VE slides to a new higher rpm, and swells to a greater number. If you put a 340 cam in there, the VE might slide to 3200rpm. And might increase the VE by 5%. I'm just throwing numbers out there. But to get that extra 5% at 3200, the engine might sacrifice that same 5% at 2400.
To get the 5% back at 2400, your only option is to figure out a way to trap the original volume of air back in there. How are you gonna do that?
IDK either.
IMO, this starts with ring-seal and fresh-cold air.
Now; you can compensate for the lost VE, with say; stall,Scr,and rear gear; but you can't get the 5% back, at least I don't think you can, at least not with the same type of cam.

Now lets talk cruising;
If you are cruising at 65mph,and say that requires 300cc of air in an 800cc chamber; just saying, that is 37.5% VE.
If your engine is a low-compression model, it might take 40% primary throttle to get that 300CC.
If your engine is a hi-compression model, it might take 35% to get that.
If a very hi-compression model, maybe 30%.
This is where Hi-compression pays huge dividends. You are making the same power on less throttle opening. This allows you to install a smaller rear gear, and that is where your better fuel economy comes in. As long as you stay on the primaries, lol.
Of course this filters down at all Part-Throttle rpms as well.

Now lets talk WOT power only, at like racing or something;
If you are targeting a certain amount of power,it is easy to compute your fueling requirement. which will dictate your air requirement, which will dictate your rpm requirement, which will dictate your camshaft requirement. If you can't get the air requirement out of the chosen cubic-inch displacement, at a reasonable rpm, then you back up the bus and get a bigger engine.
Was VE mentioned? Was Scr mentioned? Did I forget,lol?

So the point I guess I'm trying to make is this;
What VE are you talking about? If at WOT, as is usually the case, you are blasting thru the peak just once on your way to where the power is. So the absolute peak value is rather un-important. Higher VE through-out the upper rpm is hepfull but Higher compression may make more power by virtue of the higher expansion ratio. But VE will be little affected by the higher Scr.
But if your peak VE is at 2400rpm, then you might want to put your cruise rpm close to that. If it's at 3200, well you might find that uncomfortable on long trips.
So choosing a perfect blend of cruise-rpm and camshaft, with a small engine, in a streeter, when you are limited to non-overdrive applications, gets tough,

My solution was a small cam, tight ring gaps, plenty of pressure, cold air, full-length 3" duals, and lots and lots of gears, seven of them at one time,lol..

I think I understand your thinking, but for a streeter, I chose pressure over absolute VE.
Besides, you can fudge the absolute VE, by changing carb sizes, and messing with the exhaust. And you can move the VE with intake-sizing and cam-timing.
Or you can just start with a bigger engine .......... like Rusty mentioned. Then who cares,lol.
 
Hysteric said:
Read the thread I just posted. There's a reason you have to run more comp with a bigger cam. Think about what you trap in the cylinder before commencing the burn. AFR has very little to do with it.
Click to expand...
In all respect, I don't need to read it to understand. Bigger cams want more compression, I've known that for many years.

What do you think would be the winner in a 1/4 mile race if both were in a Duster with 3.23's and a stock 904 (just to favor low end torque).
  • A 360 with a 318 2bbl on top of a 318 2bbl intake, a set of stock 318 "302" heads, 318 2bbl cam, flat top standard bore pistons, 318 exhaust manifolds with single exhaust.
  • A 360 with a 750 holley, Eddy RPM intake, Lunati 703 cam, 360 heads that are fully ported, headers and dual exhaust BUT keeping the stock dished pistons and using a .050 thick head gasket.
One has a lot of compression but no flow, one has a lot of flow but no compression. So which one crosses the line first?
 
318willrun said:
In all respect, I don't need to read it to understand. Bigger cams want more compression, I've known that for many years.
Click to expand...

Then keep building a bigger pump. But for those who want to extract more HP/TQ per cubic inch read it and think.
 
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