AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
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- 25,866
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I wanted to put up a post for 318 newbe's to show what happens to low-rpm torque with various cam sizes and compression ratios, so that we all don't have to keep going over the same old questions. And since almost everybody has, or has had, a 318, usually an 8/1 variant, with a stock 240* cam, lets start there.
First of all,
forget about SCR (Static Compression Ratio). This number has little meaning other than being a tool to arrive at what we really want to know, which is the running cylinder pressure at WOT...... which we cannot actually ever know. What we are trying to do with our builds is to always keep the finished engine from self-destructing by way of detonation, when the running CYLINDER PRESSURE is too high for street gas. But, with every psi less than optimum, the engine loses power (EDIT:low-rpm torque). So most of us want to keep the pressure up as high as is safely possible. If you don't, then this post is not for you.
Ok
Since we cannot actually know the running cylinder pressure,
Smarter men than me, have by empirical data, determined safe "cranking pressure numbers", to get us into the ball-park, with various chamber designs. We'll get to that.
Most of us have or have had, a stock mid to late 70s, pos low-compression 318, and we know how those don't perform (EDIT; at low rpm)...... particularly when saddled with typical 2.76 gears and very low-stall convertors. So lets look at that first.
This smog-engine had a tiny cam IIRC, 240/248/112, with an ICA (Intake Closing Angle) of 48*(depending where you center it.) At sea level, this combo should crank about 140 psi, which reflects on the very low Dcr of just 7.1 to 1; and the Wallace Calculator predicts a V/P index of 116 remember this number. All compression pressures hereafter will be standardized at sealevel
For low-rpm
sub 3500rpm operation, this V/P number will be very important, as it helps us understand why some engines are so sucked out as compared to others that might even be smaller. The point is that we need to plump up this number, for street performance, which, a lot of it is below 3500 rpm.
Ok so lets organize that info to make it useful. What we need is the Scr/Dcr, the Ica, the predicted pressure, and the V/P, so for this combo;
Stk smog 318; 8:1Scr/7.1Dcr/48*Ica/140 psi/VP of 116 (this is lackluster and everyone knows it)
Now,
lets talk about the fuel. At a cranking cylinder pressure of 140psi, this 318 should run on the crappiest fuel imaginable. It is generally accepted that mid-grade 89 octane gas will support about 155psi CCP(Cranking Cylinder Pressure), in an open chamber design. So this 318 is already 15 psi down, and that is partly why the take-off is so lack-luster.
Since the upper limit is around 155, how do we get it there.
Well, there are three ways to change the pressure;
1) increase the SCR with a smaller total-chamber volume, or
2) Increase the Dcr with an earlier closing intake, or
3) by increasing the density of the air going into the engine, usually called, supercharging..
In the case of the factory engine, the cam is already very small, and supercharging is more money than most will want to spend, leaving us with increasing the Scr. So, we want to know how to get to 155 psi.
Going back to the Wallace Calculator, just keep upping the Scr until it spits out the desired 155psi, and I get, 8.7 as the number The New numbers are
Hi-comp 318; 8.7 Scr/7.7 Dcr/48* Ica/155psi/ VP of 130 (That's an increase of 12% !)
So your former smogger is now feeling 12% peppier, down low
But say, you had installed a 340 cam and made no compression changes. the New Ica is gonna be about 64*. and the new numbers are
Stk smog 318: 8.0 Scr/6.43 Dcr/64*Ica/122psi/ VP of 91 (thats a loss of pressure equal to 21.6%!)
So your former lackluster 318 is now feeling like a slanty, down low, unacceptable. Now you are forced to install a higher than stock stall, and hiway gears are also out.
So what do we have to do, to get the VP back up? Well 9.6 Scr will get you to 155psi, the numbers are;
hi-comp 318: 9.6 Scr/7.70 Dcr/ 64*Ica/155psi/ VP of 115 (notice that 115 and the original starting point of 116 are similar enough that at low-rpm your 340-cammed 318 is now feeling lackluster the same as it did when stock.
Now, my fellow 318 owners, I gotta tell ya that a VP of 116 is totally unacceptable with a 4-speed. And only works with an auto that has been stalled up, and for performance, forget the hiway gears. That 340 cam peaks around 5300 in a 340, with big-port heads. Put that on a factory open-chambered 318 with 2.76 gears and a 2000 stall, and you'll be timing your acceleration with a sundial; so don't do what I did.
So what's the answer?
Well firstly, we gotta figure out how to run more pressure.
So far, all my examples are with open chamber, no-squish heads, in which 155 psi is pushing the upper limit for 89gas. With closed-chambers and tight squish, and performance gears, plus 91gas, you might be able to push 165 psi. And lets get a different cam with a tighter LSA(Lobe Separation Angle), say 108, but still 340 specs. I get an Ica of 60*, the new numbers are;
very high comp 318; 9.8Scr/8.1Dcr/60*Ica/165 psi/ VP of 127 ( this is just barely on the lower edge of performance, but is as good as it gets with closed-chamber iron heads, and pump gas. If you need more bottom end than this, you are gonna have to sacrifice something.)
So what about alloy heads?
Well, with alloy heads, I myself have successfully run 195 psi without detonation and on 87E10. Others here on FABO say they have run a lil more pressure on better gas.
So let's pump up that 318 to the max with that 108*/340 cam. the new numbers are
max effort 318: 11.20 Scr/9.2Dcr/60*Ica/195psi/ VP of 150 ( now we're talking!) Lets go back to the 114* cam, with an Ica of 64*, and the number are;
max318/340 cam 11.55 Scr/9.1Dcr/64*Ica/195psi/VP of 145 ( notice the VP dropping)
So what should you do?
Answer; IDK, but I can tell you that I am addicted to a VP greater than 150, to the point of pulling out a nice cam that made 146 minimum. So, if I was ever stuck running a 318 again, I'm sorry but it will never have a 340 cam nor ever run at sub 165 psi pressures. Show me an iron head that can run 185 psi without detonation on 87E10, atta fair price, (EDIT: to compare with my alloy heads) and I would run it. Until then, I'm addicted to alloy heads.
For you, when on a budget, as most of you are, you do what you can afford, bearing in mind, as the VP goes down, the stall and gears have to go up, ........... at added cost. And since most of these old girls come with 2.76s and stalls between 1800 and 2200, those will have to go..... at added cost. By the time you add those up, it might have paid for new pistons and boring, and you just know a 50 yo engine needs a rebore anyway. For me, that's a given unless you just did it and like a DA stuck Low-C pistons in her....................................... lol. That's a joke, son, as Rooster Cogburn used to say. Don't do what I did
-----------------------------------------------------------------------------------------------------------
Other
I have run a VP of up to 170 with my 367 cuber, and I can tell you, it is very addictive. It runs any gear I want it to, and even with a clutch, it's a dump-it and go deal, and will set the tires on fire almost any time. The lowest I have run is about 146, and I pulled that cam before the summer was ended, it was just too soft.
But if you can afford to bore a 318, then maybe you can afford a core 360 too, which will solve just about all your low-rpm issues. Say you want to run iron open chamber heads at 165 psi with a 360, to burn 91 gas at WOT.
With q-pad pistons say down around .012, and an 028 gasket, and say open chamber iron heads around 70cc, this comes in around 10.0Scr and you just install any cam with an Ica of 62, and the new numbers are;
360 cubes; 10.0Scr/8.1Dcr/ 62*Ica/ 166psi/ VP of 147. (see how easy that was!) Let the fun begin! No stall, no gears, no fuss, no nuttin', just slam it together and let her buck.
Well a lil fuss;
with open-chamber heads, you gotta set the Q which may require milling the pads off the pistons just a lil.
Ya know, I have run this VP with a 2bbl and still had a chitload of fun.
If ever you can afford closed chamber heads, you just mill the pads off the pistons, as required, install a new cam with a later ICA, and yur ready to go.
Here is the Wallace calculator......................
To read about VP, go here; V/P Index Calculation
EDIT
As usual, some of you have comprehension issues.
Guys
This article;
1) is all about; How to save money by doing it right the First Time and not having to buy extras like hi-stalls and performance gears, and
2) is all about; How to get torque at low-rpm, from the commonly available low-compression 318 that just about everybody has at least one of, and almost nobody wants to bring into the modern era with pistons and/or compression.
3) is addressed to NEWBE's
who are usually on a budget, and usually on a bolt-on binge that will quickly slide into disappointment, when the wrong cam is installed..
4) is all about; Not listening to the freaks who just want to spend poor newbe's life-savings.
This article
has nothing to do with making absolute power.
I mean, I even put it in the title for you DAs. And then, I settled the issue in the opening paragraph.
When you see this line as below,
________________________________________________________________
that's a page break, indicating something else is to follow.
And so, right-after that page-break, I prefaced what was to come, with; "other";
And so then, I included a bit of my history
____________________________________________________________________
^^ page-break
I just don't get you guys.
I'm trynta help Newbe's with 318s to not get into a disappointing situation, that is just gonna cost more money, to get out of. Most of us have been down this road, so you all know what I'm talking about. But as so often happens, everybody shows up with guns ablazing.
First of all,
forget about SCR (Static Compression Ratio). This number has little meaning other than being a tool to arrive at what we really want to know, which is the running cylinder pressure at WOT...... which we cannot actually ever know. What we are trying to do with our builds is to always keep the finished engine from self-destructing by way of detonation, when the running CYLINDER PRESSURE is too high for street gas. But, with every psi less than optimum, the engine loses power (EDIT:low-rpm torque). So most of us want to keep the pressure up as high as is safely possible. If you don't, then this post is not for you.
Ok
Since we cannot actually know the running cylinder pressure,
Smarter men than me, have by empirical data, determined safe "cranking pressure numbers", to get us into the ball-park, with various chamber designs. We'll get to that.
Most of us have or have had, a stock mid to late 70s, pos low-compression 318, and we know how those don't perform (EDIT; at low rpm)...... particularly when saddled with typical 2.76 gears and very low-stall convertors. So lets look at that first.
This smog-engine had a tiny cam IIRC, 240/248/112, with an ICA (Intake Closing Angle) of 48*(depending where you center it.) At sea level, this combo should crank about 140 psi, which reflects on the very low Dcr of just 7.1 to 1; and the Wallace Calculator predicts a V/P index of 116 remember this number. All compression pressures hereafter will be standardized at sealevel
For low-rpm
sub 3500rpm operation, this V/P number will be very important, as it helps us understand why some engines are so sucked out as compared to others that might even be smaller. The point is that we need to plump up this number, for street performance, which, a lot of it is below 3500 rpm.
Ok so lets organize that info to make it useful. What we need is the Scr/Dcr, the Ica, the predicted pressure, and the V/P, so for this combo;
Stk smog 318; 8:1Scr/7.1Dcr/48*Ica/140 psi/VP of 116 (this is lackluster and everyone knows it)
Now,
lets talk about the fuel. At a cranking cylinder pressure of 140psi, this 318 should run on the crappiest fuel imaginable. It is generally accepted that mid-grade 89 octane gas will support about 155psi CCP(Cranking Cylinder Pressure), in an open chamber design. So this 318 is already 15 psi down, and that is partly why the take-off is so lack-luster.
Since the upper limit is around 155, how do we get it there.
Well, there are three ways to change the pressure;
1) increase the SCR with a smaller total-chamber volume, or
2) Increase the Dcr with an earlier closing intake, or
3) by increasing the density of the air going into the engine, usually called, supercharging..
In the case of the factory engine, the cam is already very small, and supercharging is more money than most will want to spend, leaving us with increasing the Scr. So, we want to know how to get to 155 psi.
Going back to the Wallace Calculator, just keep upping the Scr until it spits out the desired 155psi, and I get, 8.7 as the number The New numbers are
Hi-comp 318; 8.7 Scr/7.7 Dcr/48* Ica/155psi/ VP of 130 (That's an increase of 12% !)
So your former smogger is now feeling 12% peppier, down low
But say, you had installed a 340 cam and made no compression changes. the New Ica is gonna be about 64*. and the new numbers are
Stk smog 318: 8.0 Scr/6.43 Dcr/64*Ica/122psi/ VP of 91 (thats a loss of pressure equal to 21.6%!)
So your former lackluster 318 is now feeling like a slanty, down low, unacceptable. Now you are forced to install a higher than stock stall, and hiway gears are also out.
So what do we have to do, to get the VP back up? Well 9.6 Scr will get you to 155psi, the numbers are;
hi-comp 318: 9.6 Scr/7.70 Dcr/ 64*Ica/155psi/ VP of 115 (notice that 115 and the original starting point of 116 are similar enough that at low-rpm your 340-cammed 318 is now feeling lackluster the same as it did when stock.
Now, my fellow 318 owners, I gotta tell ya that a VP of 116 is totally unacceptable with a 4-speed. And only works with an auto that has been stalled up, and for performance, forget the hiway gears. That 340 cam peaks around 5300 in a 340, with big-port heads. Put that on a factory open-chambered 318 with 2.76 gears and a 2000 stall, and you'll be timing your acceleration with a sundial; so don't do what I did.
So what's the answer?
Well firstly, we gotta figure out how to run more pressure.
So far, all my examples are with open chamber, no-squish heads, in which 155 psi is pushing the upper limit for 89gas. With closed-chambers and tight squish, and performance gears, plus 91gas, you might be able to push 165 psi. And lets get a different cam with a tighter LSA(Lobe Separation Angle), say 108, but still 340 specs. I get an Ica of 60*, the new numbers are;
very high comp 318; 9.8Scr/8.1Dcr/60*Ica/165 psi/ VP of 127 ( this is just barely on the lower edge of performance, but is as good as it gets with closed-chamber iron heads, and pump gas. If you need more bottom end than this, you are gonna have to sacrifice something.)
So what about alloy heads?
Well, with alloy heads, I myself have successfully run 195 psi without detonation and on 87E10. Others here on FABO say they have run a lil more pressure on better gas.
So let's pump up that 318 to the max with that 108*/340 cam. the new numbers are
max effort 318: 11.20 Scr/9.2Dcr/60*Ica/195psi/ VP of 150 ( now we're talking!) Lets go back to the 114* cam, with an Ica of 64*, and the number are;
max318/340 cam 11.55 Scr/9.1Dcr/64*Ica/195psi/VP of 145 ( notice the VP dropping)
So what should you do?
Answer; IDK, but I can tell you that I am addicted to a VP greater than 150, to the point of pulling out a nice cam that made 146 minimum. So, if I was ever stuck running a 318 again, I'm sorry but it will never have a 340 cam nor ever run at sub 165 psi pressures. Show me an iron head that can run 185 psi without detonation on 87E10, atta fair price, (EDIT: to compare with my alloy heads) and I would run it. Until then, I'm addicted to alloy heads.
For you, when on a budget, as most of you are, you do what you can afford, bearing in mind, as the VP goes down, the stall and gears have to go up, ........... at added cost. And since most of these old girls come with 2.76s and stalls between 1800 and 2200, those will have to go..... at added cost. By the time you add those up, it might have paid for new pistons and boring, and you just know a 50 yo engine needs a rebore anyway. For me, that's a given unless you just did it and like a DA stuck Low-C pistons in her....................................... lol. That's a joke, son, as Rooster Cogburn used to say. Don't do what I did
-----------------------------------------------------------------------------------------------------------
Other
I have run a VP of up to 170 with my 367 cuber, and I can tell you, it is very addictive. It runs any gear I want it to, and even with a clutch, it's a dump-it and go deal, and will set the tires on fire almost any time. The lowest I have run is about 146, and I pulled that cam before the summer was ended, it was just too soft.
But if you can afford to bore a 318, then maybe you can afford a core 360 too, which will solve just about all your low-rpm issues. Say you want to run iron open chamber heads at 165 psi with a 360, to burn 91 gas at WOT.
With q-pad pistons say down around .012, and an 028 gasket, and say open chamber iron heads around 70cc, this comes in around 10.0Scr and you just install any cam with an Ica of 62, and the new numbers are;
360 cubes; 10.0Scr/8.1Dcr/ 62*Ica/ 166psi/ VP of 147. (see how easy that was!) Let the fun begin! No stall, no gears, no fuss, no nuttin', just slam it together and let her buck.
Well a lil fuss;
with open-chamber heads, you gotta set the Q which may require milling the pads off the pistons just a lil.
Ya know, I have run this VP with a 2bbl and still had a chitload of fun.
If ever you can afford closed chamber heads, you just mill the pads off the pistons, as required, install a new cam with a later ICA, and yur ready to go.
Here is the Wallace calculator......................
To read about VP, go here; V/P Index Calculation
EDIT
As usual, some of you have comprehension issues.
Guys
This article;
1) is all about; How to save money by doing it right the First Time and not having to buy extras like hi-stalls and performance gears, and
2) is all about; How to get torque at low-rpm, from the commonly available low-compression 318 that just about everybody has at least one of, and almost nobody wants to bring into the modern era with pistons and/or compression.
3) is addressed to NEWBE's
who are usually on a budget, and usually on a bolt-on binge that will quickly slide into disappointment, when the wrong cam is installed..
4) is all about; Not listening to the freaks who just want to spend poor newbe's life-savings.
This article
has nothing to do with making absolute power.
I mean, I even put it in the title for you DAs. And then, I settled the issue in the opening paragraph.
When you see this line as below,
________________________________________________________________
that's a page break, indicating something else is to follow.
And so, right-after that page-break, I prefaced what was to come, with; "other";
And so then, I included a bit of my history
____________________________________________________________________
^^ page-break
I just don't get you guys.
I'm trynta help Newbe's with 318s to not get into a disappointing situation, that is just gonna cost more money, to get out of. Most of us have been down this road, so you all know what I'm talking about. But as so often happens, everybody shows up with guns ablazing.
Last edited:
