Cams choice, Low CR vs VE%

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Don't tell me what I ignore, because you don't know. An engine IS indeed an air pump, but a self driven one. Every air pump has to have a power source. An internal combustion engine just happens to create its own power.

But directed at @273, your question "as presented" is unanswerable. Though the forum gurus make think they can, they effectively cannot and neither can anyone else in the question's present form. For an answer, you "must" provide a basis of comparison. In other words "Does engine A have more VE% than engine B?" "If so why?" This is a comparison question and as such needs a basis for a comparison.

It's like all these stupid commercials on TV saying "You'll lose 40% more weight". Oh yeah? 40% more weight "THAN WHAT", yet people flock to them never realizing they just got took by the oldest trick in the book.

And the “self propelled” function is what we are considering.

The efficient use of fuel is way too often overlooked.

That’s how u can have two 500 HP engines (identical as they can be) off the same dyno and one is quicker than the other.

Every single time, the more efficient engine is quicker.

One of the many things the stroker engine has done is dumb down the public as to what’s important when putting something together. No consideration of the induction system and the exhaust is by far the most common. That means the cam is usually bigger than it needs to be. And bottom end suffers.

The HP and torque curves are backward because the exhaust limits RPM power production.

There is much more to the air pump than just making it bigger.
 
And the “self propelled” function is what we are considering.

The efficient use of fuel is way too often overlooked.

That’s how u can have two 500 HP engines (identical as they can be) off the same dyno and one is quicker than the other.

Every single time, the more efficient engine is quicker.

One of the many things the stroker engine has done is dumb down the public as to what’s important when putting something together. No consideration of the induction system and the exhaust is by far the most common. That means the cam is usually bigger than it needs to be. And bottom end suffers.

The HP and torque curves are backward because the exhaust limits RPM power production.

There is much more to the air pump than just making it bigger.

I think technology in general has dumbed people down. Made them feel like they don't have to learn things. Like EFI for example. If you know carburetor systems, you can come close to approaching EFI drivability with a carburetor. I've done it many times in the past. Haters will always be haters. That's why old farts who insist on carburetors and who know the ins and outs of point type ignitions will usually always know more about how an engine works. Purtnear every time.
 
Here's and old thread from I think the Old Innovate Forums that explains a lot this stuff:

shirnker - Fuel Vaporization


Incredible stuff that shrinker produced. Guy like shrinker, tuner and others whose names I have forgotten are a treasure of knowledge.

The above article is very good at explaining why vaporization is so important. And how atomization plays into vaporization.

Great stuff.
 
Incredible stuff that shrinker produced. Guy like shrinker, tuner and others whose names I have forgotten are a treasure of knowledge.

The above article is very good at explaining why vaporization is so important. And how atomization plays into vaporization.

Great stuff.
Just curious, how did you specifically implement this process in your car?
 
Not true. Hysteric just gave you an example of same air flow/more power.

Some guys never look at HP/CID and that’s much more important than what the HP is.

I’ll never understand why anyone they can’t have flow AND compression. And no. You don’t need a cam with 280 duration to do it.

Why I started this thread is about adding cams to stock low power cr engines, 273/318/340/360/383/400/440, and almost anytime the subject comes up DCR always enters the conversation, not saying CR and DCR isn't important obviously it is but people dealing with stock long blocks really are stuck with what they have.
So my question is adding a cam even if it sacrifices some DCR for added air flow, guess which means more VE%, not talking big cam high rpm but in the street rpm idle - 5000.
 
Why I started this thread is about adding cams to stock low power cr engines, 273/318/340/360/383/400/440, and almost anytime the subject comes up DCR always enters the conversation, not saying CR and DCR isn't important obviously it is but people dealing with stock long blocks really are stuck with what they have.
So my question is adding a cam even if it sacrifices some DCR for added air flow, guess which means more VE%, not talking big cam high rpm but in the street rpm idle - 5000.
because flow trumps compression. Ideally, flow with compression is optimal. But when bumping compression isn't in the budget, they chose flow over compression. Yes, they will bleed off a little compression to gain flow. And guess what? It works just about every time it's applied.
 
Incredible stuff that shrinker produced. Guy like shrinker, tuner and others whose names I have forgotten are a treasure of knowledge.

The above article is very good at explaining why vaporization is so important. And how atomization plays into vaporization.

Great stuff.

Yeah Shrinker passed too soon. When I first got into carbs and tuning I asked him about improving the Holley design and what to do and his response was "Why? By the time you fix the Holley you'll have a Carter". I didn't understand what he was talking about at the time but after a lot of questions and reading i caught onto what he was on about.

Have you ever looked into his Laminar Flow Design carb?
 
because flow trumps compression. Ideally, flow with compression is optimal. But when bumping compression isn't in the budget, they chose flow over compression. Yes, they will bleed off a little compression to gain flow. And guess what? It works just about every time it's applied.

I agree with you that's my feeling on the subject but there a camp of guys that always argue the opposite and are knowledgeable guys, so trying to see if there is a theoretical reason to why this is the case, We known DCR is important and it's easy to calculate, increase VE% is speculative, hard to measure without a dyno so harder argue it's merits plus don't how a cam is gonna work with a certain combo bit of a guesstimate.
 
Why I started this thread is about adding cams to stock low power cr engines, 273/318/340/360/383/400/440, and almost anytime the subject comes up DCR always enters the conversation, not saying CR and DCR isn't important obviously it is but people dealing with stock long blocks really are stuck with what they have.
So my question is adding a cam even if it sacrifices some DCR for added air flow, guess which means more VE%, not talking big cam high rpm but in the street rpm idle - 5000.

Adding a large cam to an engine with low compression will have the same effect as adding one to any engine. It will increase the peak power and torque, but move it higher in the RPM band. Just like any other engine. Probably "not as much" as one with an optimal compression ratio for the given camshaft, but it will increase nonetheless.
 
Why I started this thread is about adding cams to stock low power cr engines, 273/318/340/360/383/400/440, and almost anytime the subject comes up DCR always enters the conversation, not saying CR and DCR isn't important obviously it is but people dealing with stock long blocks really are stuck with what they have.
So my question is adding a cam even if it sacrifices some DCR for added air flow, guess which means more VE%, not talking big cam high rpm but in the street rpm idle - 5000.

I went a little faster with less head less intake less carb and less cam but with more cylinder pressure on the same bottom end.

Solid 243/245 vs 222/234 both on 112 lsa's.
 
I agree with you that's my feeling on the subject but there a camp of guys that always argue the opposite and are knowledgeable guys, so trying to see if there is a theoretical reason to why this is the case, We known DCR is important and it's easy to calculate, increase VE% is speculative, hard to measure without a dyno so harder argue it's merits plus don't how a cam is gonna work with a certain combo bit of a guesstimate.


Keep this in mind, too. When you measure static compression pressure with a compression tester, that's only by the starter's speed turning the engine. When the engine runs, the cylinder pressure is much more, because of combustion.....I am sure you know that. But what most people don't think about is, as the engine's RPM increases, so does the cylinder pressure. As the ignition timing advances, the cylinder pressure rises and the engine makes more power. It's not one dimensional It's a linear curve dictated by several inputs. Timing, camshaft size, induction, exhaust......everything has an effect.
 
Just curious, how did you specifically implement this process in your car?

You start at the carb. That’s where you get the process going.

The next big thing is the tune. Most guys run way too much fuel through the engine. More fuel equals less vaporization.

Booster gain is a big deal.

Then if you think you have fuel issues you have to deal with the intake manifold, reducing corners, getting the correct taper for the runner (if you can...you do what you can) eliminating section changes (most of these are reasons NOT to use any dual plane intake manifold) and maybe even surface finish on the runners.

As shrinker so aptly pointed out...pump gas is near impossible to get complete vaporization. It is what it is. That’s why when reading a plug you need to know what the fuel is and how to read it.

I’m not a fan of buying race fuel on sale and changing brands. You can have 5 different brands of race fuel, all the same octane and they all may read differently.

In fact, what shrinker wrote on plug reading should be a sticky in the race forum.
Yeah Shrinker passed too soon. When I first got into carbs and tuning I asked him about improving the Holley design and what to do and his response was "Why? By the time you fix the Holley you'll have a Carter". I didn't understand what he was talking about at the time but after a lot of questions and reading i caught onto what he was on about.

Have you ever looked into his Laminar Flow Design carb?


I have not. Do you have a link? I know when the other forum went down it was a big loss.
 
because flow trumps compression. Ideally, flow with compression is optimal. But when bumping compression isn't in the budget, they chose flow over compression. Yes, they will bleed off a little compression to gain flow. And guess what? It works just about every time it's applied.

Let me see if I’m following here.

If I’m building a 318 and I need heads do I chose a 318 head and keep the CR up or a 360 casting and get the flow?

Every single time I pick the 360 head. All engines are a series of compromises. The engine with the least amount of compromises is always better.

That’s why I pull my hair out when I see guys building 400 plus inches and use a stock casting or even an Edelbrock head. That is so induction limited that to get any reasonable RPM out of it you have a cam that is the dread bottom end killer.

But that torque number is up there.

That’s if I’m following your thinking.
 
Let me see if I’m following here.

If I’m building a 318 and I need heads do I chose a 318 head and keep the CR up or a 360 casting and get the flow?

Every single time I pick the 360 head. All engines are a series of compromises. The engine with the least amount of compromises is always better.

That’s why I pull my hair out when I see guys building 400 plus inches and use a stock casting or even an Edelbrock head. That is so induction limited that to get any reasonable RPM out of it you have a cam that is the dread bottom end killer.

But that torque number is up there.

That’s if I’m following your thinking.
Always a dilemma and a topic of discussion. A early 273/318 head ported would be the closest you could come to the best of both worlds. Edelbrock aluminum's or the like would be a good choice as well. Small efficient chamber, compression, and port flow. But you know that already.
 
Let me see if I’m following here.

If I’m building a 318 and I need heads do I chose a 318 head and keep the CR up or a 360 casting and get the flow?

Every single time I pick the 360 head. All engines are a series of compromises. The engine with the least amount of compromises is always better.
That’s if I’m following your thinking.
Correct.
 
Why I started this thread is about adding cams to stock low power cr engines, 273/318/340/360/383/400/440, and almost anytime the subject comes up DCR always enters the conversation, not saying CR and DCR isn't important obviously it is but people dealing with stock long blocks really are stuck with what they have.
So my question is adding a cam even if it sacrifices some DCR for added air flow, guess which means more VE%, not talking big cam high rpm but in the street rpm idle - 5000.
Yes. Match the intake, converter, gear, tire size etc to maximize that rpm range.
 
All i know which ain't much is i had ede open chamber heads , i cut them 60thou which increased compression and increased gas mileage and drive-ability as well . it did not have a big cam 513 lift with 1.6 rockers
 
360 heads with kb quenchdome pistons for a 360
or aftermarket heads
or ported 302 or 318 heads for daily driver
360 heads with pistons down the hole sucks
318's are tough to do right
 
I have not. Do you have a link? I know when the other forum went down it was a big loss.

I cant find it anymore so most probably its no longer up.

Motorsports village forum?

Motorsports Village • Index page

Shrinker on pump gas and cylinder pressure:

Compression is a relative question isnt it. How full is the cylinder, thats what you compress. How much exhaust residual is there supplying heat that you cant control or dont know about? How much wrecking of the atomization has been done to the fuel by the time it gets there? The thing that determines compression maximum is the amount of energy that has already been transferred to the fuel before its compressed and then also the droplet size that your compressing and how fast its gassing. vaporizing fuel removes energy from the atmosphere being compressed thus enabling more compression. I've seen street fuel lowest octane available engines with 240psi cranking pressure have no detonation issues and Ive seen engines with 165psi detonate on the highest octane available and be unfixable without a rebuild of the configuration.

240 psi on the lowest octane fuel.
 
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360 heads with kb quenchdome pistons for a 360
or aftermarket heads
or ported 302 or 318 heads for daily driver
360 heads with pistons down the hole sucks
318's are tough to do right

273/318 are tough that why there threads have the most views usually and most debate, and usually have most limitations put on them, stock gear, stall, cr etc.. usually gonna be tune duals 4bbl and a cam. and heads if their looking for over 300hp, an most aren't gonna port their heads that's an option for only a few.

So the debate usually becomes DCR vs Air flow, that's why I started this thread, I side with Air flow even though I have no direct theoretical evidence to support it.
I've done two 4bbl 360 heads and 340 cam on a stock 318 and drive train both times worked great my uncle and cousin done a few with bigger cams they all ran strong, obviously cr would of made them all better.
 
@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.


When talking say 318 could be any low cr engine but this discussion usually happens over 273/318 for big block guys imagine it's 383/400.

The VE I'm talking about is the air flow gain your gonna get from the cam from 3500-5000 rpm.

Yes bigger cams move the powerband up but I do believe you can generally at least add up to 10 degrees over stock cam before affecting anything in the 1500-4500 and some times even add power through this part of the powerband, usually your gonna move peak from 4500 rpm to around 5000 rpm with gain all above 3500/4000, 1500-3500/4000 is gonna be about the same, idle to 1500 might be slight effected but not enough to give up the 80-100 hp gain in power.
 
If you have two cams that open and close at the same exact spots, and have the same max lift, the one with more area may not make any more power down low,
but will make more top-end power, and hold it longer.
The one with less area, will normally be down on power everywhere.
There are some cases where more area doesn't help, but that's not the norm.

Adding area, without adding duration changes the RPM band very little in most cases,
and just increases the power in that power band.

Mike Jones 2011

use .200 as a proxy for area
and to apprpximate Mikes suggestion for seat duration
easy to overcam a low compression motor
so start the way aJ says and then figure out what happens when you increase duration
which BTW not only increases horsepower but narrows the HP range
 
If you have two cams that open and close at the same exact spots, and have the same max lift, the one with more area may not make any more power down low,
but will make more top-end power, and hold it longer.
The one with less area, will normally be down on power everywhere.
There are some cases where more area doesn't help, but that's not the norm.

Adding area, without adding duration changes the RPM band very little in most cases,
and just increases the power in that power band.

Mike Jones 2011

use .200 as a proxy for area
and to apprpximate Mikes suggestion for seat duration
easy to overcam a low compression motor
so start the way aJ says and then figure out what happens when you increase duration
which BTW not only increases horsepower but narrows the HP range


Since .200 # ain't a common measurement say you have two single pattern cams both have .500" lift and say 220 @ .050", and both measure advertise duration @ .006" but one was 265 and one was 275 wouldn't the 265 probably have the bigger .200" number ?
 
Draw that out on paper.
What you are proposing, is basically a difference in clearance ramps.
The lift after .050 can be manipulated by the camgrinder who knows his equipment, to fill a marketing need, or a perceived need for the combo.
If you gotta blow off some pressure, you might choose the long ramps,
if your pressure is already low, you might choose the shorter ramps.
If you have lightweight solid lifters, you could put some decent area under the curve.
If you have heavy-weight,retro-fit, Hydraulic rollers,you might get into trouble over the nose, so you might have to sacrifice rate of lift.
If you are drag-racing, you might go crazy on the rate of lift, knowing that the cam might not last very long.
As a streeter wanting long life, you might choose a slower,more durable rate of lift.
If you have a small engine, and want to prove a point, you might go both-feet-in, and choose the most radical rate of lift possible.
If you have a stroker on the street, you might choose the laziest rate of lift.
So there are as many ways to grind a cam as the grinders have lobes in their catalogs.
 
273
I'll give you an example from Jones cam which I did run through the camdoc
256 @.006 or 264 @.004
202 @.050
.305 lift
some others 256 @.006 and larger 204 @.050
but near as fat at .200
why?
Jones is real short at actual seat to seat - say .002
shorter than Howard or Lunati or comp 256 cams
so at .050 he has not caught up
but by .100 and .200 he has more area under the curve
go to .275 (no use using .300 on a .305 cam) he is 50% bigger than the MP 260 which is about 268 in .006 degrees
Jones really short actual seat to seat builds the most dynamic compression of any of these "256" or "204" class cams and has more area to boot
great lobe
he does not have any other Mopar HFT lobes
I think he advises to go solid if going bigger- can't argue with that
 
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