How far in the hole 318 pistons

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James Clews

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Does anyone know how far down a 1972 318 pistons are? In a book I have it states in 1972-1981 they are .056 down from the top of the bore but with those calculations in the Diamond racing compression calculator it gives about 8.5 to one compression but our factory manuals state 8.2 compression ! I tend to think that the pistons are further down in the bore. I would like to find out the real comp before I pull the heads off as I would like to custom order a camshaft but need a more accurate reading on my compression ratio as it may take a few months to get a custom ordered roller cam and I would not like my engine sitting unassembled for many months.
 
Does anyone know how far down a 1972 318 pistons are? In a book I have it states in 1972-1981 they are .056 down from the top of the bore but with those calculations in the Diamond racing compression calculator it gives about 8.5 to one compression but our factory manuals state 8.2 compression ! I tend to think that the pistons are further down in the bore. I would like to find out the real comp before I pull the heads off as I would like to custom order a camshaft but need a more accurate reading on my compression ratio as it may take a few months to get a custom ordered roller cam and I would not like my engine sitting unassembled for many months.
Not sure it helps,, but I measured a 1986 318 block yesterday and the piston were .058 down in the hole.
I know different years had different piston heights. I had thought these roller block era had higher compression heights, apparently not.
 
If you really want to find true compression ratio, you need to cc the combustion chambers and find the downfill volume of the cylinders. Production machining tolerances were sometimes very........ "tolerant", lol. Do a Google search on "half inch downfill volume of a cylinder" if you are unfamiliar. As for cc'ing the combustion chambers, you can easily fabricate a buerette from a piece of thick clear plexiglass and a graduated syringe. Windshield washer solvent works well because the blue color is easily seen in the syringe and under the plexi
 
Furthermore
The factory put these together with 020 steel shim gaskets, with a bore hole suitable for a 318. I can't say exactly how many ccs those were, but I would think that ~4ccs would not be far off.
The thinnest I find now are 028s and based of a 340 hole, making them about 6.8cc.
The factory cam, installed straight up, has an Ica of, IIRC, of 48* and gives a CCR of about 140 psi at sealevel.
If you are at a higher elevation you WILL lose pressure.
If you install a cam with a later intake closing event, you WILL lose pressure.
If you give your heads a valve job, you will probably lose pressure from stock.
Pressure is heat is power.
Losing pressure will make the low-speed operation, and throttle response softer. At that point you will need a higher stall convertor to get off the line, with the same authority as what the stock cam gave you, and even more stall to make it more fun. and
forget any factory 318 gearing, Most cammed-up low-compression 318s are gonna need 3.91s . and
don't forget the headers. a bigger cam needs headers to shine. They work thru the overlap cycle, helping to get the A/F charge in the plenum moving. With the pistons down in the hole at TDC and dwelling there, this slows down and weakens the draw on the plenum, and the timing of the event takes a hit, nearly negating the whole event. Throw in the fact that most guys can't get the ignition events on time, and the whole idea of making power with an 8/1 318 just falls apart. and besides all that,
the factory cam already has timing events are nearly ideally suited for the low-stall TC, the hiway gears, and the log manifolds.
all that engine really needs is a higher stall, a 4bbl, better gears, and a free-flowing dual-exhaust system.

Now I heard you say "custom cam", but again; at your pressure level, every 4* later ICA is worth about 3>4psi, and the factory 140 psi is already super low.
for example;
the 340 cam has an Ica of 64*, compared to the 318s 48*, and with that cam, the pressure is likely to be down to 122psi versus the stock 140, both at sealevel. Thats an 18 psi loss for 16 degrees, equals 3.55 psi per 4 degrees. You think 140 psi is sucked out, wait til you drive 122psi!
That 340 cam wants a true 10/1 at sealevel to get the pressure up to 163psi, close to the limit for best gas in Canada, 91, (R+M)/2, and that would be best case.
Therefore
I highly recommend to get your pressure up to match your cam, or
just install a 2800 convertor and performance 3series gears tomorrow, and forget the cam. and, with a 4bbl you'll want a free-flowing exhaust, and I would slap on some cheap headers for good measure.
 
The pistons in my 1974 318 were .100” deep. Seriously. Measured with a dial indicator and deck bridge.
 
Now I heard you say "custom cam", but again; at your pressure level, every 4* later ICA is worth about 3>4psi, and the factory 140 psi is already super low.
for example;
the 340 cam has an Ica of 64*, compared to the 318s 48*, and with that cam, the pressure is likely to be down to 122psi versus the stock 140, both at sealevel. Thats an 18 psi loss for 16 degrees, equals 3.55 psi per 4 degrees. You think 140 psi is sucked out, wait til you drive 122psi!
That 340 cam wants a true 10/1 at sealevel to get the pressure up to 163psi, close to the limit for best gas in Canada, 91, (R+M)/2, and that would be best case.
Therefore
I highly recommend to get your pressure up to match your cam, or
just install a 2800 convertor and performance 3series gears tomorrow, and forget the cam. and, with a 4bbl you'll want a free-flowing exhaust, and I would slap on some cheap headers for good measure.
Instead of a 340 cam 64* how about something like a Comp xe250h it's got something like a 51* even better get a similar cam cut on a 107 lsa for a stock 48* ? But still be able to gain a good 30-50 hp over stock cam. Probably starting at least 2500 rpms and up.
 
So many variables here. You just need to measure.
 
They would have to be either down, dished, or some combination, and IIRC, they were at least dished.
and
none of the five early ones that I have were much out of the holes, at least not nearly .045..... In other words, they may not have been 10.5 engines. More like 10.0 with the thin steel gaskets, and as low as 9.5 with the FelPro .039s
 
Instead of a 340 cam 64* how about something like a Comp xe250h it's got something like a 51* even better get a similar cam cut on a 107 lsa for a stock 48* ? But still be able to gain a good 30-50 hp over stock cam. Probably starting at least 2500 rpms and up.
Wait; 30>50hp from nothing but a cam-change of ; 10* extra intake duration, and 19* additional overlap, that is only meaningful with headers and a free-flowing dual exhaust? I'm just not seeing it unless the rate of lift goes off the chart, or
unless
you are including the 4bbl, the headers, and the dual exhaust.
And besides;
friend;
yur still dealing with a measly 140psi ccp. Put a 4bbl on that and watch it get soggy. Sure you could do it but why?
Just to brag on your 1000rpm powerband 318?
That combo is gonna need about a 3500 stall and 3.91s for sure.
at Extra cost.

My guess is that Over 95% of the miles a 318 ever turns are gonna be under 3500 rpm, and what you're thinking is gonna give it all away, I mean, there is already nothing there at 140 psi. and now with a 3500 TC, it's all but disappeared.

You know the 1-2 split on the TF is .59 right, meaning that the revs drop to 59% on the shift. The 250* cam is gonna power-peak around 4500, so reving to 5000 is already going down the back of the power curve. Shifting at 5000, the rev's will drop to ~2950, with the stock 2000 or less, stall. With the stock rear gears, that are likely to be 2.76s, can you say sucked out? That's a hole yur 140psi cammed-up 318 is NOT gonna like. Put her in Drive and go home, cuz yur goose is cooked.
That combo is gonna need about a 3500 stall and 3.91s for sure.
at Extra cost.
For about the same money, you could bore it out and install some decent pistons, to get the pressure up to ~160, get a valve job, and reinstall the stock cam with better springs, and top it off with a small 4bbl, no headers, and finally, hang a 2800 on the back, and leave the rear-gears alone; then go have a blast.
And while yur at it, install a fresh air kit, cuz she's gonna need all the help she can get.
Yes you can make modest power with the short pistons; just not with 318 heads, a 250* cam, log manifolds, etcetera, and NOT at 140psi, and for sure not with a modicum of bottom end., not to forget the wet-noodle throttle response.

I'll say it again; IMO; the stock pistons have got to go! Notta chance would I leave them in there for any performance 318 in my stable.
I'll tell ya another thing, for a low-compression 318, here's a game changer;
4.10s with a valve spring swap, leave the 2bbl and factory convertor; or
a small 4bbl, a 2800 convertor, and 3.55s, is a reasonably good alternative. If you put a 4bbl on it, it needs more than the teeny weeny, 3-pass muffler, single-exhaust.
These are my opinions, based on my experiences.
Don't get me started on big-valve heads, and a 340 cam, on a low compression 318. I'll never do that again!
 
Wait; 30>50hp from nothing but a cam-change of ; 10* extra intake duration, and 19* additional overlap, that is only meaningful with headers and a free-flowing dual exhaust? I'm just not seeing it unless the rate of lift goes off the chart, or
unless
you are including the 4bbl, the headers, and the dual exhaust.
And besides;
friend;
yur still dealing with a measly 140psi ccp. Put a 4bbl on that and watch it get soggy. Sure you could do it but why?
Just to brag on your 1000rpm powerband 318?
Why is it only got a 1000 rpm powerband ?

Any time I've seen a mild cam swap over low performance stock cam the gains have basically been everywhere they measure 2000/3000 to 5000+ rpms, not just the top 1000 rpm.
That combo is gonna need about a 3500 stall and 3.91s for sure.
at Extra cost.
Why?

Your saying a 48* vs 51* is gonna murder the bottom end ?
What about if you carve the xe250h cam on a 107 lsa for the same 48*, shouldn't be all gain since it will have the same V/P ?





//// Explain the engines below why were not really seeing this torque loss and why we are seeing good midrange gains not just on top ? How does this square with what you are saying ?

I guess you could argue that the torque takes a huge nose dive starting at 2999 rpms and below.
I would like to see some dyno proof of that relatively mild cams killing 1500-2500 rpms.

//// Here's a BASE stock low cr 2bbl 318 with headers, MOD1 4bbl and xe262h cam, MOD2 same as 1 with spacer.

https://www.motortrend.com/articles/mopp-0312-318-long-block-bolt-ons/

Obviously it don't show Modded 2000-2999 rpm so it's a guesstimation but this cam is two sizes bigger than the xe250h were talking about and I don't see any signs this engine has loss any bottom end. From 3000 -5500+ rpms this engine obviously dominates a stock 318.

TORQUE SUPERFLOW 901 DYNO-TESTED AT WESTECH
RPMBASEMOD 1MOD 2
2,000292
2,500287
3,000287338336
3,500272335340
4,000248326330
4500218309319
5,000188285296
5,500250258

HORSEPOWER SUPERFLOW 901 DYNO-TESTED AT WESTECH
RPMBASEMOD 1MOD 2
2,000112
2,500137
3,000164193192
3,500182224227
4,000189248252
4,500186265274
5,000179271282
5,500262270

Here's a low cr 360 from 2bbl to 4bbl and stock cam to xe250h and xe268h and don't see any signs of this low end loss.




Here's a 380hp crate with a decent size cam and 9:1 cr probably poor V/P, making fairly large 2500 rpm torque numbers 381 tq @ 2500 rpm with single plane and a 288/292 cam on a 108 lsa.

https://www.motortrend.com/articles/mopp-0303-360-crate-engine-buildup/

opar Performance 360 cid 380 hp rated

RPMTQHP
2500380.6181
3000402.9230
3500415.1277
4000437.8333
4100439.5343
4500437.1375
5000416.3396
5400398.6409
5500388.6407
6000343.9395
 
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Your saying a 48* vs 51* is gonna murder the bottom end ?
What about if you carve the xe250h cam on a 107 lsa for the same 48*, shouldn't be all gain since it will have the same V/P ?
No, My comparison was to the 48* stocker.
Well that's the thing, the V/P at 116 is already at lawnmower status.
51* "only" drops it to 112.

Now pump the 8/1 engine up to 10/1 and run the stock cam at 48*Ica, and the pressure goes to 186psi and V/P to 155, now you got a chitload of bottom end with nothing but a change in compression.
Of course you can't run that much pressure with iron heads, without detonation.
But, you can run 9/1 @ 163 psi, 136V/P, about as good as it gets with iron heads. Personally, I wouldn't even run that low with my 367.

Now, I have run 197psi @ VP of 173 in my alloy headed 367 and can, without reservation say, that 173 is ludicrous on the street. So I took the motor out and apart and reduced it to 184psi, V/P of 158, and what do you know, I really missed that 173V/P. I tried all summer to get at least some of the missing bottom end back. In the End I installed a Commando transmission with 16% more First-Gear multiplication, and by the time I hit shift rpm in First, the rpm drops to just 4000, so V/P is no longer a thing, instead, now I'm riding on the 184psi. 11/1 Scr.
The point is this, a V/P of 112 or even 116 is so darn weak, you gotta get past it ASAP, and the 3500 will do that, But now the engine is trying to go fast on 140 psi@8/1 Scr, and it's all done by 4500 with that 250* cam, so the power band is from 3500 to 4500, and with 318 heads, and log manifolds, going down the backside of the curve, yur only gonna get a couple of hundred more rpm out of her. If you shift at 4800, the Rs will drop in at 2800, back in the crappy V/P zone, unless you get a higher stall convertor. Then, if the car is still running 2.76s, now yur in Second gear at 2800rpm, and pushing about 50 mph. Can you say sluggish?
Been there done that, it was no fun, I'll never do that again

The 2800TC works reasonably well with 3.55s , and the totally stock, but 4bbl engine, pumping thru a Free-flowing exhaust. Mine shocked a lotta riders.
Worked better with a 4-speed.
Even better with the A833od, 4.30s, and GVod as a splitter. This combo actually pleased me, but running a manual trans with snow on the ground, even with 4 studded tires, was not my cup of tea.
Eventually, I ran the A998 low gears, a 2800, with 2.76s, and a big Thermoquad. That combo turned out to be my favorite winter combo. I could hit 50 at the top of first gear, lol. Course at the shift, boom, she dropped to a tic under 3000, and with 2.76s, the rush was over.
But I learned to love that TQ carb. I installed a manual-shift on the trans, then I used to roll the barracuda out, and get her into "drive" as fast as she'd take full throttle, which with a 2800 is like right away. So then, the rpm goes to about 2800, and more or less stays there 'til I lift or like 80 mph, with the TQ howling all the way, what a hoot. Not fast but sure was fun. And the dual 3" cannons sung a sweet song the whole way.
But
I digress,
My 318 is a 1973 model, Still has the 73 cam in it. I gave it new rings in about year 2000. Mine cranks 140CCP at 900 ft, cuz I advanced the cam a lil. and I found some springs that will rev to just around 5200 IIRC; but otherwise, the long-block is 1973. The V/P at a calculated 116, is ridiculously low. I only used this engine for the winter, cuz it's what I had, and I doctored it with the 2800, and the gears.
Before the snow came, I sometimes still had the "meats" on the back, and notta chance could she break them loose, even with the 2800 TC. So, even tho it was, a ton of fun; I comforted myself by knowing that, come May long weekend, the HotRod367 was going back in.
 
No, My comparison was to the 48* stocker.
Well that's the thing, the V/P at 116 is already at lawnmower status.
51* "only" drops it to 112.

Now pump the 8/1 engine up to 10/1 and run the stock cam at 48*Ica, and the pressure goes to 186psi and V/P to 155, now you got a chitload of bottom end with nothing but a change in compression.
Of course you can't run that much pressure with iron heads, without detonation.
But, you can run 9/1 @ 163 psi, 136V/P, about as good as it gets with iron heads. Personally, I wouldn't even run that low with my 367.

Now, I have run 197psi @ VP of 173 in my alloy headed 367 and can, without reservation say, that 173 is ludicrous on the street. So I took the motor out and apart and reduced it to 184psi, V/P of 158, and what do you know, I really missed that 173V/P. I tried all summer to get at least some of the missing bottom end back. In the End I installed a Commando transmission with 16% more First-Gear multiplication, and by the time I hit shift rpm in First, the rpm drops to just 4000, so V/P is no longer a thing, instead, now I'm riding on the 184psi. 11/1 Scr.
The point is this, a V/P of 112 or even 116 is so darn weak, you gotta get past it ASAP, and the 3500 will do that, But now the engine is trying to go fast on 140 psi@8/1 Scr, and it's all done by 4500 with that 250* cam, so the power band is from 3500 to 4500, and with 318 heads, and log manifolds, going down the backside of the curve, yur only gonna get a couple of hundred more rpm out of her. If you shift at 4800, the Rs will drop in at 2800, back in the crappy V/P zone, unless you get a higher stall convertor. Then, if the car is still running 2.76s, now yur in Second gear at 2800rpm, and pushing about 50 mph. Can you say sluggish?
Been there done that, it was no fun, I'll never do that again

The 2800TC works reasonably well with 3.55s , and the totally stock, but 4bbl engine, pumping thru a Free-flowing exhaust. Mine shocked a lotta riders.
Worked better with a 4-speed.
Even better with the A833od, 4.30s, and GVod as a splitter. This combo actually pleased me, but running a manual trans with snow on the ground, even with 4 studded tires, was not my cup of tea.
Eventually, I ran the A998 low gears, a 2800, with 2.76s, and a big Thermoquad. That combo turned out to be my favorite winter combo. I could hit 50 at the top of first gear, lol. Course at the shift, boom, she dropped to a tic under 3000, and with 2.76s, the rush was over.
But I learned to love that TQ carb. I installed a manual-shift on the trans, then I used to roll the barracuda out, and get her into "drive" as fast as she'd take full throttle, which with a 2800 is like right away. So then, the rpm goes to about 2800, and more or less stays there 'til I lift or like 80 mph, with the TQ howling all the way, what a hoot. Not fast but sure was fun. And the dual 3" cannons sung a sweet song the whole way.
But
I digress,
My 318 is a 1973 model, Still has the 73 cam in it. I gave it new rings in about year 2000. Mine cranks 140CCP at 900 ft, cuz I advanced the cam a lil. and I found some springs that will rev to just around 5200 IIRC; but otherwise, the long-block is 1973. The V/P at a calculated 116, is ridiculously low. I only used this engine for the winter, cuz it's what I had, and I doctored it with the 2800, and the gears.
Before the snow came, I sometimes still had the "meats" on the back, and notta chance could she break them loose, even with the 2800 TC. So, even tho it was, a ton of fun; I comforted myself by knowing that, come May long weekend, the HotRod367 was going back in.
So your saying going from 116 to 112 V/P is gonna be so dramatic of a change that 3.91 & 3500 stall is a must it will a huge noticeable loss from stock if not ? Sorry I just don't buy it.

You seem to be very fussy, uncompromising on how the power curve works with your car and gearing, which is kool, it's what I like about you but doesn't mean everyone one else is looking for the exact same, maybe their willing to trade some V/P for more overall hp, everybody's level of compromise is different. A lot of people just want to add a 4bbl and mild cam, a lot of them on stock gears stall and cr is it the best way maybe not will they be happy a lot seem to be some not so much, you said before you mainly like to shift by 3000 rpm, a lot gonna hold on for another 2000-3000 rpms before shifting.

My car has the 380hp crate with highway gears and a 2800 stall is it ideal no but it's still fun enough I haven't bothered to change it yet, hoping to put some gears in it this year, but if not still works fine but bet with a 288 cam and 9:1 and no gears on paper should be a turd but it isn't, and yes stall gears and cr would probably make a night and day difference, but it what it is for now and I'm fine with it would others I don't know, some probably.
 
Was trying to be diplomatic :)
Well, I used to be. But I'm ready to rip him. What about all these newbies who think he's right about it all when he's not? What are they gonna do when they take his advice and they build a dead dog? Or worse? Something that can't run on anything but race gas?
 
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Well, I used to be. But I'm ready to rip him. What about all these newbies who think he's right about it all when he's not? What are they gonna do when they take his advice and they build a dead dog? Or worse? Something that can't run a like on anything but race gas?
318willrun's pretty much goes against everything he says and is a proven performer, And bet a good chunk of us has done similar with similar results, is it ideal to build a low cr engine no but it's what a good chunk of people have along with high gears and stock stalls again not ideal but a decent performers can be made. And a lot of people aren't asking for much, bet most be happy to have acceptable driveability and go pretty strong when they drop a gear and crush it for a gear or so, that's basically what my car does.
 
318willrun's pretty much goes against everything he says and is a proven performer, And bet a good chunk of us has done similar with similar results, is it ideal to build a low cr engine no but it's what a good chunk of people have along with high gears and stock stalls again not ideal but a decent performers can be made. And a lot of people aren't asking for much, bet most be happy to have acceptable driveability and go pretty strong when they drop a gear and crush it for a gear or so, that's basically what my car does.
I disagree about a low compression engine not being ideal. It can be. In the end, it all depends on WHAT you are doing and WHAT you want. Look what Jim Laroy has proven right on this site. Sure, higher compression will always make more power with parts matched for the application. I wouldn't go out of my way buying high compression slugs or different heads if I had something already in the 8-1 to 9-1 range. I'd build it. Like my Ford 400 for example. Rated at 8.5, but we all know how these factory engines blueprinted. Low. When I had the heads done, I had them knock .060 off the heads and mill the intake to match. I didn't even measure compression when I built it, because I didn't care. With the lumpy old school Crane Fireball cam, I knew it would run on 87 swill, which is what I wanted. .....and I promise you, this truck will drag dude's precious 367 Barracuda down the road sideways. I had him on ignore for the longest, but I took him off for a while. Now he's goin back on, because I know he'll respond and I don't even wanna see it. When these guys figure out how full of CRAP he is, it'll be too late.
 
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