IDK what your high-pressure goal is, but Suppose it's for fuel economy. If it's not then don't bother reading any further.
But if it is,
lemmee tell you a story:
>>My DD Orlando, has a Direct-injected, VVT Ecotec, 2.5 4-banger. It runs 220psi CCP. At 4500rpm, the cam-phasers do their thing. It is rated 167hp@6700 rpm.
I run it on 87E10
It makes 32mpg@55mph, pretty consistently.
It's really peppy even below 4500, and it very rarely needs more than 3500rpm for me. With me and the wife in it, and groceries in the back, I'm guessing she's over 4000 pounds. I'm always impressed with it's low-rpm power.
The point is this;
Pressure is heat is torque is power.
You really really want that pressure, both while cruising but most especially while accelerating normally at low to mid rpms. But the threat of detonation is ever-present. The Orlando, like most EFI cars solves that problem in several ways; Firstly, it has EFI, computerized timing control, and a knock sensor. And secondly, it has alloy heads.
Now, IDK the Scr, but to make 220psi, with a modest Ica, it's gotta be way up there.
If you screwed a 318 together to make that, I doubt it would make it very far before the pistons would give up.
>> so how can you achieve similar results with an SBM?
Well you can't. A 318 is 5.2liters, which is more than double of the Ecotec 2.5. And my Ecotec cruises at 65=about2000 rpm. And you'll never run a 318 at 220psi on iron heads, more than at idle, and besides that, it would need a lot of custom parts to get there.
But, getting to 200psi with a 360 is pretty easy.
If you're building for hiway fuel economy:
IMO
a high pressure 360 should be given consideration..... because, mine flew down the road, geared at 65=1600rpm and returned excellent fuel economy...... running at about 195psi CCP.
Will a 273 (or a 318) pull 65 =1600 @195psi?
IDK, probably; but it will, again, take custom parts to get to 195psi, and likely, the 273 would be pretty gutless, because to run there requires a 1.97 final-drive ratio.
If you are looking for fuel economy, you need:
>The smallest engine possible to keep internal friction down,
>To keep the rpm down,
>To keep the cylinder pressure up,
>To figure out how to cheat wind-resistance,
>To run the cheapest gas that will effectively suppress detonation, and
>To get the daymn ignition timing worked out; oh yeah and >To run the tiniest carb that will get you up to cruising speed.
But good luck driving this combo under any other conditions.
Sacrifices have to be made.
The smallest rear-end that Chrysler ever made was a 2.20 so, that makes 65=about 1800 Now, just try taking off with a standard A904 and 2.20s . So then, that's the First sacrifice. Oh and forget the automatic, it'll cost you roughly 2 mpgs which you'll never get back. So that's the second sacrifice.
And forget the A833od because the very wide ratios will force you, with a 2.20 rear, to carry each gear way to far up the rpm band, before shifting, if you don't, the engine will detonate itself to death. So that's the third sacrifice; you will need, at minimum a close-ratio 5-speed. OR a 4-gear auto with overdrive and a lock-up convertor. The auto will cost you a bit of Mpg, but the engine will live, AND, you no longer have to run the 2.20 rear.
Here's a study.
To get to 65=1600 in .69od will take 2.86 gears. Lets round down to the commonly available 2.76s for 65=1540. To get this moving, you will need the A500/2.74 low-gear, for a starter gear of 7.56, but the Convertor will make that bearable. With this combo, getting the cruise-timing right will take a computer.
Ok so the sacrifice here is cutting your tunnel up, to accept the overdrive transmission. And, you may need a higher stall for ease of drive-away and for a satisfactory rate of acceleration to the cruising speed, but you won't know this until you try the stocker.
Now, on to the engine.
To get the pressure up, to say 185psi, with a small cruiser cam, like the factory 240/248/112 (Ica of ~50*) the Scr is gonna need to be over 10.2. To get there with a 273, the total chamber volume needs to be less than 61cc.
To withstand 195psi you need something like the .039 FelPro, but with a more appropriately sized fire ring. So looks like a custom gasket. Say you get one at 6cc
Next, yur gonna need some really hi-compression pistons, installed high up in the bores.Let's say you achieve plus 6cc; for a net zero. . Now you need a head of 61 cc minus whatever eyebrows your pistons have. Lets say 4 eyebrows total 8cc, so the target chamber volume will be 53cc. But here's the catch; you can't run 195psi with iron heads, So you need alloy heads at 53cc (in this study). Next comes another catch, most of those come with 2.02 intakes, which; A) won't fit into the 3.63 bores, and B) are way too big for this application, and C) usually come with way too big chambers to achieve your target Scr. So ................ your looking at a lotta money for custom parts and/or custom machining, money that may take decades to break even for the economy savings ....... therefore .............
My advise is to not try to cruise at 65=1540, with a 273, in an attempt at getting fuel-economy.
Can a 273 cruise there?
I bet it can, but the cost for a fuel-miser is just not worth it, unless yur just trying to prove a point, and your pockets are deep.
what about with a 318?
Well sure, but the minimum Scr for 195psi is now 10.6, and the minimum TOTAL chamber size for this is 71cc. Now you can use the .039 Fell-pros, at 8.6cc, and you'll again need hi-compression pistons jacked up, which usually come with just two eyebrows totalling 5cc. Lets say you get them up high enough to net 0 cc. Thus your head chambers can be 62.4 which is doable in alloy.
So all of this is now both possible and affordable.
To make getting the timing, and drive-away a lil better, lets change the rear gear to 2.94s for 65=1640
This combo is now very doable; pistons, gaskets, and heads are all available off the shelf.
The only sacrifice here is the factory cam with it's ~50* Ica, which is a low-rpm cam, and to successfully run it, without detonation, at 195psi may take careful carb and ignition timing co-ordination, rendering the whole thing somewhat underpowered.
The solutions here are running better gas, octane booster, or computer-controlled, variable, water-injection, or some other tricks I have not thought about.
> or to get a lil more power, just run a small-cammed 340.
> or to get a lil more torque, a small-cammed 360
Both of these bigger SBMs easily accept 2.02 valves. and
Both of these can more easily achieve 195psi, with more conventional parts. The sacrifice here is a slight increase in point-to-point fuel consumption due to more internal friction.......... which can be offset by going back to the 2.76 rear gears.
And finally, you can install a similarly sized cam but ground for solid-lifters, to shrink the advertised duration and win more cylinder pressure, OR keep the pressure the same and win a lil more power/torque with about one additional cam size at .050.
But here's the thing, if this is an honest cruiser, limiting the rpm to say 4500, then you can get a custom cam ground strictly for that application. If you can get faster low-lift ramps, you can run hydros, for the same results. To get back some acceleration, you just use a bigger engine.
In the beginning of this post, I said something like, the 360 should be given some consideration. Maybe now you know why I said that.
If I told you mine, on a certain Day-Trip, made 32mpgs, would you believe me? and it fell together at 10.7Scr, with off-the-shelf parts. With just minor machining this can go to 11.3. With minor Ica adjustment, the pressure can easily go to 200psi. I ran it for a while at 195psi, still on 87E10, still on full-timing, without any tricks. other than the final-drive ratio. (3.55 x .71 x .78=1.97)
That's a stinking lot of flexibility.
Oh and when it was set up like that, it still went
106mph@12.9 seconds in the quarter, with it's street tune, and on 245/60-14 BFGs, at 3650 race-weight; and
she revved willingly to 7200.
Va-va-Voom!
I loved that combo........
The point of this story is,
that I didn't need any tricks to stay out of detonation. This engine has always run on 87E10 since I first bolted it together in 1999; and the pressures it has run have always been from 177 to 195psi, and typically at 185
Happy HotRodding
Oh BTW
the 360 is way overkill for this kind of application. Mine spins the 295s at-will below 45/50 mph, and in straight-line, they say, all the way to the 660ft traps with 325/50-15 BFG-DRs
In the case of max fuel-economy, a 3.91/4.04 short stroker should be fine. If you dress a 340 up with the 318 stuff, it's just a big-bore 318 ...... with benefits. I had one like that, for like 3 years, but it lived on in my memory as my favorite combo, for like 22 more years.Hmmmmmmmmmm
BTW-2
Years ago you could get a drop-in 3.58 crank for the 3.31 saddles. in a 318 that would make her a 344. I think I could suck some nice mpg out of that combo, with almost all stock bolt-ons...... and still have a good little streeter.
Sincerely,
AJ
Haa,
