Compression Ratio and Boost Query

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thrustbucket

thrustbucket

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I am in the process of a 318 LA budget turbocharged build, and there seems to be a lot of common info suggesting limits of around 6psi boost for an 8.5:1 static compression engine.

I am aware of how timing events affect dynamic compression, and how all things being equal, these principals typically apply to forced induction as well. Anyhow.. I have tried most of the formulas and seen most of the common (and uncommon) charts/graphs.

My question (and what they don’t seems to explain) is this:

I had a 1191 4G63T DSM (Talon/Laser/Eclipse) that ran 15psi on 91 octane with zero signs of detonation. It had no tune, and was bone stock other than a 16G ported and clipped turbo upgrade, along with the cylinder head being decked .0015”. My guess is the decking pushed the factory 7.8:1 compression up to around 8:1. I understand the ECM was likely pulling timing and the heads were fairly efficient aluminum castings. However, that alone doesn’t explain why most ‘boost compression’ charts say that would be a ‘race gas’ engine.

Why can’t I run 15 psi (max) on a properly prepped/tuned 318 with 8.5:1 compression?

Furthermore: If a 6psi limit is true, try finding a turbocharger compressor that maps efficient at such low boost pressures and engine RPMS that meets the potential max CFM demands of a small V8.

Anyone have real world experience boosting a 318 that can weigh in?

Thanks,
 
IDK anything about turbos, and IDK your answer, except that 25 years ago, I was on the same road that you are now on.

I remember back in the day, a 9/1 318 was advertised at 230 hp/crank, which translates to around 195 rear wheel.
And I seem to remember that 8 psi is likely to double your horsepower.
So at 460 hp/crank how close is a 318 to catastrophic failure? never mind 15psi.?
Whereas by 1973, the Net hp was down to 150hp(maybe 175crank), in the 8/1 engines with crappy timing controls and EGR. 350 hp would be doable.
and don't forget that these small-block Mopars only have 4 bolts per cylinder, and tiny cams, and AFAIK, you can't find a headgasket for a 318-sized bore. Which means that the sharp edge at the top of the cylinder bore, is now a part of the combustion chamber, which can't be good.
And then there's attrition. First the small rear end explodes, then the trans explodes, then the U-joints start shattering. and so on. IMO, there is no budget in this 15psi equation, as one thing just leads to another.

And then, there's the problem of finding traction. Lets not forget, that a 350hp 360 can break traction at 50 mph with just a foot-stomp in Second gear, and if you let that fly and the car gets off-axis more than ~7 degrees, depending on Driver experience and road conditions, that car is/or could already be in trouble.
With the more power, the car become a rocket, and the drum brakes can't slow you down anymore and the stupid thing keeps spinning around in a circle on nearly every corner. How much the more with a turbocharged 15psi 318?

Now, I get that if you re-engineer your 318 for a turbo, as opposed to just bolting one on then, ok, guys have made amazing power numbers; But again, forget the budget.
and like I said
IDK anything about turbos, and IDK your answer.

If it's bottom end you want, just bolt on a set of alloy heads onto a 360, and crank her up close to 200psi, and you'll already have more torque than the chassis can handle. I've never been sorry for the alternative to a turbo, that I chose.
Can you do it with a 318?
Sure, but Not on a budget. It's really hard to get to 11/1 Scr with those tiny cylinders.
It may require Specialty pistons and/or rods , and/or a lot of machining.

Now, if you just use the turbo to pump up the 318 bottom end, that is doable. But you can get torque with a higher stall, much cheaper, and way easier. But now you may need another gear, lol; criminy there goes the budget again.
 
I would read thru the builds on the forced induction threads, nd see what stays together, and what comes apart.
This will give you a good guide to building your car.
 
I have no real world experience with turbocharging a 318 but I have to think that engines that are engineered to run turbos from the "get-go" have a significant advantage utilizing the boost. Like you said, engine management via computers to keep them out of detonation territory.
 
I agree with you in regards to the budget factor. As a matter of fact: That is what has lead me to this build.
Backstory: I was building a parts grabber/daily Dakota pickup and I was gonna do an n/a LA build. My base ended up having a cracked magnum block at the freeze plug. Where I am at a junkyard V8 costs $375 you pull-it. Late model or not. There was no point in pulling/building a magnum 360 when I could pull a Gen III hemi with WAY better heads/architecture for the same price. Total build was 7K. For essentially a new truck. Rebuilt engine/trans/rear/suspension.

So.. my daily is pushing over 400hp at the crank. Its got 4.56 gears, a shift-kitted 42RE (yes.. its fine @ 400hp) and is easily a mid 13 second truck even with traction limitations. I don’t really need a ‘dependable’ hotrod at this point. As a result of the u-turn on the Dakota, I am left with a pile of LA parts, and a suitable chassis that was on the back burner.

So… the stuff below is essentially $0 at this point.

What I have:

1966 Plymouth Valiant Signet 2 Door sport top, mini-tubbed, springs relocated under frame, firewalled, all undercoat removed

Narrowed 8 3/4 (489) with 3.91 limited slip.

3000 lb super stock springs

318 Block .040 over, casting flash removed, bulkheads saddle cut and chamfered

Bushed heavy duty 360 rods with pin oil holes added. Casting ribs removed and balanced. HD rod bolts

Cast 318 crank. Really straight. (.000750 @#3 .002 @ 1+5)

Magnum 318 truck heads - stock. 58cc. Currently at the machine shop. Might be cracked… they usually are.

302 heads. 62cc chambers, some porting, 2.02/1.60 valves, bronze guides. 340 (red) mopar springs.

273 shaft rockers with banana groove shafts. (Clearanced a bit for bigger springs)

KB 167 Pistons .012” (below deck)

Mighty Demon 650 mechanical with annular boosters (probably too small for this..)

Edelbrock performer RPM LA intake

1973 A904. I will rebuild this. Probably go bolt in sprag and full manual.

I have way more LA crap lying around. Thats just what I plan on potentially using.

Like I said: I really don’t feel like building another 400hp n/a motor, nor do I feel like building another GEN 3 Hemi to boost.

Yes. The cast crank and A904 are very questionable @500hp+. However, people said a 42re can’t live over 350hp and I can tell you thats not true. I agree It would likely be different/blown if it were a track build.

The bore edges are a great point. I had not considered that.

My point is: I don’t care so much if it blows up. I want to see how much I can make with a single turbo blow-through setup.

I also realize that 14psi on a 2.0 liter engine is around 200hp, and 14psi on a 5.3 liter engine is like 600hp. Thats a big difference as far as stress on internals. Although.. the 2.0l 4G63T is all forged and can live (mostly) dependably @ 500hp with stock internals.
 
I have been thinking about your original post and there are a few things you are reading that I think are hogwash. I don't know much about turbos but I do run a 6-71 blower. The 6 lbs of boost limit is a joke. I have a static compression ratio of 8.1 to 1 and I run 9 lbs of boost with my timing locked at 31 degrees. The engine loves it and so do I. With a turbo it all comes down to timing management. My solution would be to start with timing based on the amount of boost you are making at any particular rpm. You may have to pull timing out a fair amount at high boost and then sneak up on it slowly until you find what your engine likes. The other problem you may have is with 15 lbs of boost you may get into a situation where you have to O ring the block. I don't know much about that either but with 9 lbs of boost I pushed out a head gasket last spring. Just a thought.

Jack
 
sireland67 said:
I would read thru the builds on the forced induction threads, nd see what stays together, and what comes apart.
This will give you a good guide to building your car.
Click to expand...
The "real world" reason you can do that with the Eclipse is called electronic fuel injection. Were that a carbureted build, it would act completely different. Also as you pointed out yourself, alloy heads are much better at dissipating heat, so there's another score for less detonation. I'm in the preliminary stages of a slant 6 turbo build, myself. I plan to try some unorthodox (read dirt cheap) timing control and I plan to make it work, even though some say I cannot. That just makes me want to do it even more and I will.
 
Coyote Jack said:
I have been thinking about your original post and there are a few things you are reading that I think are hogwash. I don't know much about turbos but I do run a 6-71 blower. The 6 lbs of boost limit is a joke. I have a static compression ratio of 8.1 to 1 and I run 9 lbs of boost with my timing locked at 31 degrees. The engine loves it and so do I. With a turbo it all comes down to timing management. My solution would be to start with timing based on the amount of boost you are making at any particular rpm. You may have to pull timing out a fair amount at high boost and then sneak up on it slowly until you find what your engine likes. The other problem you may have is with 15 lbs of boost you may get into a situation where you have to O ring the block. I don't know much about that either but with 9 lbs of boost I pushed out a head gasket last spring. Just a thought.

Jack
Click to expand...
So you don't pull any timing out whatsoever. That's an interesting approach.
 
Forget about all the stupid “rules” you’ve read. Just get out and put something together.
I have lots of experience turbocharging lots of different engines and the best way to learn is to do. Don’t let people tell you you’re doing it wrong or it won’t work or can “only” do this or that. Just put it together and go have fun. Run more octane than you think you need, and tune it conservatively at first then work your way up.
 
Coyote Jack said:
I have been thinking about your original post and there are a few things you are reading that I think are hogwash. I don't know much about turbos but I do run a 6-71 blower. The 6 lbs of boost limit is a joke. I have a static compression ratio of 8.1 to 1 and I run 9 lbs of boost with my timing locked at 31 degrees. The engine loves it and so do I. With a turbo it all comes down to timing management. My solution would be to start with timing based on the amount of boost you are making at any particular rpm. You may have to pull timing out a fair amount at high boost and then sneak up on it slowly until you find what your engine likes. The other problem you may have is with 15 lbs of boost you may get into a situation where you have to O ring the block. I don't know much about that either but with 9 lbs of boost I pushed out a head gasket last spring. Just a thought.

Jack
Click to expand...
Interesting. What you are saying is what I was thinking may be the case from my minimal turbo experience.

Just to be clear: My actual expectaions are 8-10 psi max @ 5800-6200 rpm with 8.5:1 compression.

I would like to avoid the O-ring approach.. but.. It is certainly a consideration, and likely necessary as you said at 14psi with that setup.
 
RustyRatRod said:
The "real world" reason you can do that with the Eclipse is called electronic fuel injection. Were that a carbureted build, it would act completely different. Also as you pointed out yourself, alloy heads are much better at dissipating heat, so there's another score for less detonation. I'm in the preliminary stages of a slant 6 turbo build, myself. I plan to try some unorthodox (read dirt cheap) timing control and I plan to make it work, even though some say I cannot. That just makes me want to do it even more and I will.
Click to expand...
I don’t wholly disagree. But, I do think a properly setup blow-through carburetor can deliver fuel just as efficiently/safely in a drag/wot situation. I do agree that the injection, in cooperation with the ECM and the sensor compliment working in unison gives a huge safety margin. Just not FI alone.

I got to thinking the main reason may be low speed cylinder pressure.
RustyRatRod said:
The "real world" reason you can do that with the Eclipse is called electronic fuel injection. Were that a carbureted build, it would act completely different. Also as you pointed out yourself, alloy heads are much better at dissipating heat, so there's another score for less detonation. I'm in the preliminary stages of a slant 6 turbo build, myself. I plan to try some unorthodox (read dirt cheap) timing control and I plan to make it work, even though some say I cannot. That just makes me want to do it even more and I will.
Click to expand...
I agree in part. I don’t think its the injection alone though. Its the injection and timing being controlled by the compliment of sensors via the ECM.

I got to thinking about it last night, and I think the primary difference between a 4G63T and an LA handling boost is low-speed cylinder pressure.

The eclipse made 14psi from about 6000-7200 rpm. I don’t think it made boost at all until at least 3800 from a cruising WOT pull.

My point is. That 318 will be lucky to reach 6500 N/A with those heads and springs. To have useable boost, it will need to come on lower in the rpm range. That means more pressure during less rpm = more dwell time = more heat soak. I suspect that is the primary limitation/advantage here. There isn’t much dwell @7200rpm compared to 5800rpm.

If you are building a 225, this is going to be something to consider. I don’t think the one I had would exceed 4200 rpm. (But it was a little sick)
A cam with a very late intake closing might help. But.. it might cause other issues with boost?
 
thrustbucket said:
Interesting. What you are saying is what I was thinking may be the case from my minimal turbo experience.

Just to be clear: My actual expectaions are 8-10 psi max @ 5800-6200 rpm with 8.5:1 compression.

I would like to avoid the O-ring approach.. but.. It is certainly a consideration, and likely necessary as you said at 14psi with that setup.
Click to expand...
O ringing the decks is not necessary until you get really serious with cylinder pressure. It shouldn’t even be part of the conversation. An 8.5:1 318 on 10 psi will be fine with regular old felpro head gaskets if you can keep it out of detonation. If you want some insurance stud the decks and use MLS gaskets from cometic.
 
Combustion chamber design and other constraints limit how much compression you can run.

One of my engine builders typically builds his boosted, LS based motors with 11 to 1. Another builder just built my Magnum based 408 with 10 to 1. These are California, pump gas builds, although spraying methanol can allow even higher ratios to be run.
 
jbc426 said:
Combustion chamber design and other constraints limit how much compression you can run.

One of my engine builders typically builds his boosted, LS based motors with 11 to 1. Another builder just built my Magnum based 408 with 10 to 1. These are California, pump gas builds, although spraying methanol can allow even higher ratios to be run.
Click to expand...
Inlet air temp and intercooling are big factors as well.
 
TT5.9mag said:
Inlet air temp and intercooling are big factors as well.
Click to expand...
Great point. I don't have enough room for an air-to-air or air-to-water intercooler, so I am using a "Chiller"(similar to the Hellcat system), which uses the A/C Freon to cool the water circuit and spraying straight methanol to drop the air intake charges.

The straight methanol can drop the air intake charge 70+ degrees nearly instantaneously. When combined with a chiller system, the air intake temperatures can be reduced even more.

68 408 Paxton.jpg


Dart Pro 427 LS Whipple.jpg
 
I'm planning 9.0:1 and 15PSI on an LA, but that's with computer-controlled distributorless ignition and MPFI. Timing and charge cooling are imperative, especially on gasoline. You can do this on a properly set-up blow-through, but you'll need computer control over the timing to really maximize power. I highly recommend picking up Street Turbocharging by Mark Warner. Excellent book on the subject. I've read it a few times to drill the contents into my smooth brain.

Another excellent read is Designing and Tuning High Performance Fuel Injection Systems by Greg Banish. Much of the EFI stuff doesn't apply (some does), but he gets pretty deep into ignition mapping, which is critical to how much boost you can push through an engine. I've read through this one multiple times as well. It's a lot to digest in one pass.

Both are great books that I highly recommend. Unlike the interwebs, there isn't a bunch of incorrect, misleading, or conflicting information. I think it was Banish that said something about armchair experts having plenty of time to post because they're not out there actually doing it. Banish has done extensive OE fuel and spark mapping at both GM and Ford, so he's probably pretty well qualified.
 
pretty sure the Chevy Cobalt runs 10 to 1 and way more than 8psi boost. the higher the compression you start with or the higher the boost you run the better your tune needs to be.
 
thrustbucket said:
I don’t wholly disagree. But, I do think a properly setup blow-through carburetor can deliver fuel just as efficiently/safely in a drag/wot situation. I do agree that the injection, in cooperation with the ECM and the sensor compliment working in unison gives a huge safety margin. Just not FI alone.

I got to thinking the main reason may be low speed cylinder pressure.

I agree in part. I don’t think its the injection alone though. Its the injection and timing being controlled by the compliment of sensors via the ECM.

I got to thinking about it last night, and I think the primary difference between a 4G63T and an LA handling boost is low-speed cylinder pressure.

The eclipse made 14psi from about 6000-7200 rpm. I don’t think it made boost at all until at least 3800 from a cruising WOT pull.

My point is. That 318 will be lucky to reach 6500 N/A with those heads and springs. To have useable boost, it will need to come on lower in the rpm range. That means more pressure during less rpm = more dwell time = more heat soak. I suspect that is the primary limitation/advantage here. There isn’t much dwell @7200rpm compared to 5800rpm.

If you are building a 225, this is going to be something to consider. I don’t think the one I had would exceed 4200 rpm. (But it was a little sick)
A cam with a very late intake closing might help. But.. it might cause other issues with boost?
Click to expand...
While all that may be true, a carburetor can never manage fuel and ignition like a computer.
 
Never heard of a 6psi limit but even that's good for a boosted hp of about 1.4 x NA hp.

That's probably consider an rule of thumb boost level on pump gas without an intercooler.

There's things you can do to raise that, but at least you can turn boost up or down as required then find ways to keep turning it up, water/meth, intercooler etc..
 
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jbc426 said:
Great point. I don't have enough room for an air-to-air or air-to-water intercooler, so I am using a "Chiller"(similar to the Hellcat system), which uses the A/C Freon to cool the water circuit and spraying straight methanol to drop the air intake charges.

The straight methanol can drop the air intake charge 70+ degrees nearly instantaneously. When combined with a chiller system, the air intake temperatures can be reduced even more.

View attachment 1716323408

View attachment 1716323409
Click to expand...
I wholeheartedly agree with everything you said. Except that you don’t have room for a big air to air or water intercooler. There’s always room if you’re not opposed to hacking sheetmetal.
:lol:
 
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