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420A Engine compression / turbo?

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Culon1997GS

Proven Member
84
8
Sep 22, 2023
Laredo, Texas
Hello, yesterday I ran a compression test, and this are the results,

4 152
3 160
2 160
1 155

From left to right,

Is that good to run turbo? (8psi) I want to build the internals as I go, but I wanna know if my compression is okay, any tips or what should I buy first as an internal mod will be greatly appreciated, thank you
 
That compression is low for a 420a.

You need to worry about how to tune the car more than anything. The stock internals can live around 6-8 psi for a while if you have good tuning software and a good tuner. Any more PSI than that and they won't live long. The block is actually quite strong on the 420a but the ring lands on the pistons are very thin and the rods are small so they break or bend under even moderate power.
 
For reference, my Neon (equivalent engine) had 215 - 200 - 125 - 200 on a compression test. The 125 is from a head warp from overheating, and that cylinder isn’t running right.

The 200 is what a healthy cylinder should look like for you, and 150 is the lower-end service limit according to the manufacturer. Your engine therefore has a lot of blow-by from wear. I would much rather have a healthy engine with high compression, than an unhealthy engine with low compression - that doesn’t really benefit turbo setups.
 
Further to the point, if you have a lot of cylinder or ring wear, as indicated by poor compression, I'd be highly suspect of the crank & main journals, since these engines don't just wear out one component. Most used Neon/420A cranks need at least a 0.010" grind to even them out, and the bearings usually look pretty bad. Not really something you want to be doubling the stress of.

For further reference, this is what the Neon version of the 420A with 161k miles looks like inside, even with great compression:

The walls have very little wear but the pistons are battered.
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All crank and rod journals have tapers
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The crank bearings were not happy.
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In all likelihood, you're about to take an engine that looks worse than mine does, and strap the ultimate stressor on it. If you want to go faster for cheap for a little while, sure, but it's already on crutches and you should expect to be causing significant damage. I'd be more concerned about refreshing this engine before turbocharging it, but that's just me.
 
Should I be worried about that compression? I know that turbos like lower compression.. :hmm:
You are confusing a compression test with compression ratio. Common mistake. Compression ratio is a calculation of the volume at TDC vs volume at BDC. A low compression ratio engine will generally have lower compression test results but in your case you are loosing compression past the valves or piston rings which isn't something you want.

I would agree with @pauleyman to definitely do a leak down test and find out where you are losing compression and repair it from there. You don't want to mod a car that is already hurt.
 
You need to do a leak down test and then tear the engine down and figure out if you need to have machine work done or if it just needs new rings, bearings, gaskets, ect. If the cylinder bores are tapered or crank journals are out of round you need machine work done and need undersize bearing and/or oversize pistons. So you can't buy parts until you know what size you need.

I would buy individual parts myself and get quality name brand parts. And if you plan to turbo it you may as well get forged pistons and rods. A "rebuild kit" for engines generally is just a kit of the absolute cheapest garbage someone could import from china and put in a kit on ebay, amazon, ect.
 
Further to the point, if you have a lot of cylinder or ring wear, as indicated by poor compression, I'd be highly suspect of the crank & main journals, since these engines don't just wear out one component. Most used Neon/420A cranks need at least a 0.010" grind to even them out, and the bearings usually look pretty bad. Not really something you want to be doubling the stress of.

For further reference, this is what the Neon version of the 420A with 161k miles looks like inside, even with great compression:

The walls have very little wear but the pistons are battered.
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All crank and rod journals have tapers
You must be logged in to view this image or video.

You must be logged in to view this image or video.


The crank bearings were not happy.
You must be logged in to view this image or video.

You must be logged in to view this image or video.



In all likelihood, you're about to take an engine that looks worse than mine does, and strap the ultimate stressor on it. If you want to go faster for cheap for a little while, sure, but it's already on crutches and you should expect to be causing significant damage. I'd be more concerned about refreshing this engine before turbocharging it, but that's just me.
Those pictures raise some questions about this engine you've torn down. Some being, had the oil pump gone out? Oil changes every 30k miles? Water in the crankcase? That is not "usual" wear for a 161k mile engine. Maybe for an old air-cooled trap.

Not too long ago, I opened up a 230k mile 420a, completely stock and had nowhere near as much wear as what you're showing. The ring gaps were large, but the piston skirts had probably 25% as much wear as you've shown, the rod bearings were obviously worn but no scoring on the crankshaft journals, which I did not measure but nothing to catch the eye. Just a couple of years ago I sold a running 420a, it had 205k miles on it but ran great after I freshened up some leaky gaskets. It had consistent compression reading across, at almost 170psi. No knocking or smoking, and there was no reason to even investigate piston skirts.

The point of not recommending to turbocharge an unhealthy engine is valid; the point of recommending forged internals for a turbocharged engine is valid. But there's no valid point in saying that the OP's engine is probably worn worse than yours just from a compression reading, or that heavily worn internals are normal. The wear level of OP's engine is to be determined by his own investigation, but to what matter?

The crankshaft I use in my 2g as of right now was removed from a 98 Neon R/T block. The car had 132k showing on the odometer, but who knows if it were still counting. The bearings looked good enough, and the crankshaft measured within specs with zero scoring. I'm still using that crankshaft with STD size bearings 3 years and 10k HARD miles later.

You're biggest indicators of blowby with a 420a will be a dipstick getting pushed out or excessive oil in the intake manifold. Keep in mind the 2 other common causes of 420a low compression: headgasket sharing gasses between cylinders, and worn valves/head. If the valves have float/shaken, their ability to seal will be compromised very rapidly, and surprisingly even. That's also a common diagnosis for the random cylinder misfire codes.

There's a million factors that play into the wear rate of the engine, so every case will be different. But you can't just assume the engine is completely and utterly worn because it has 161k miles, or because it has low cold compression readings. If one were strictly and properly maintained having a "heathy-ish" running engine at 250k miles is not unreasonable. But on the contrary, if it were never maintained and raced by some Fast and Furious kid (like many of them), I could see internal wear and failure at just 75k miles.

My rambling aside. OP, you should go for forged pistons/rods, headstuds & MLS gasket at a minimum. Not only will this eliminate any potential existing issues/wear, but I have zero faith in the ringlands of stock pistons. I've seen broken ringlands in some non turbo builds. So you're already running that risk without the turbo, add the stress of it and you're seriously gambling. Also the factory head bolts and their original torque specs are not to be trusted. Often I see leaky 420a headgaskets, all of them are factory equipped. MLS + studs is my trusted fix, but even aftermarket head bolts solve the issue since thay have corrected length and torque specs.

But just for reference, here's a compression reading from a freshly broken in 420a with small ring gaps, no planned boost or nitrous.

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Last edited:
damn that’s a lot of information, I appreciate you taking the time to write all of that, by any chance do you have links to those parts that you recommended me? Considering that I’m going turbo, I had in mind a Crower stage 2 cams, but I’d like to hear from you what I should get
 
The options for pistons are JE or Wiseco. I've preferred JE but the Wiseco are more affordable. Make sure you order the correct bore size. 87.5mm being stock bore, every .020" over is .5mm.



Eagle rods are what you want. Make sure you order correct length. A standard rod is P/N5608 (5.608").


At this time, Crower cams are special order only for the 420a. The demand is not high enough to keep shelves stocked. Expect higher than normal wait time. Stage 2 turbo grind are P/N 64462-2. Submit that cam card as requested specifications.
Crower part numbers are as follows:
Stage 1 64461
Stage 1 turbo 64461-2
Stage 2 64462
Stage 2 turbo 64462-2
And forth, changing the 5th digit for stage and adding "-2" for turbo grind.


With a stage 2 camshaft, you will need a spring upgrade. The Brian Crower spring & retainer kit is preferred.


The head studs are fairly simple. There aren't many options.


The FelPro PermaTorque MLS 9922PT headgasket is an excellent choice, and reasonably priced. You want the head and block to be flat and clean though (resurface both). You can order these across the web, but I get from RockAuto. They also sell Clevite rod & main bearings, which are my brand of choice. The gasket I linked is standard thickness, you may consider a .010c oversized if the head has already been resurfaced before, to avoid a compression bump.


These are just a few pieces of the puzzle. You'll need much more, but these ensure integrity of the engine. Also prepare for fuel system upgrades, engine control upgrades, custom exhaust & intake setup. And maybe get yourself some gauges.
 
If you haven't set a budget yet, start there. What you are comfortable spending on this project will steer your decisions. Does no good to budget $5000 only to spend $4500 on a beastly engine and not have enough $ left over to get the peripheral upgrades you need to use it.
 
The options for pistons are JE or Wiseco.
@Culon1997GS You have two more options for pistons. In case if you want the highest compression forged off the shelf pistons, Wossner pistons have 11.5~11.7 compression ratio piston kit (Fits better in a non turbo build). Or you can customize pistons as you desire by any piston manufacturer. It's not expensive. Usually pricing would be a bit higher than standard off the shelf kits if it's a basic custom piston kit.
 
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