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Old 08-05-2022, 12:47 PM   #1
Andy Somogyi
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Default B20 bottom end strength with turbo

Hey, thinking about a turbo B20 build, and wondering about bottom end strength of the stock crank / rods / pistons.

I'm really only thinking about a very conservative build, no more than 200 hp, using a 740 TD04 turbo.

Any idea what's the safe and reliable hp limit for a daily driven B20 turbo?





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Old 08-05-2022, 01:02 PM   #2
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I don't know a hp number to throw at you. But certainly a well built B20 isn't going to be overstressed at 200hp.
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Old 08-05-2022, 03:35 PM   #3
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Weak point would be the rods.
You could throw a set of MaxSpeed rods at it and not have a worry.
A surfaced deck and surfaced head and a good gasket.
I don't know about 200hp but you can probably push 10psi into that without breaking stuff.
Back to the rods.....
I'd guess 10# with and intercooler and a proper tuneup would run decent
Whats the NA hp? 110? 110hp+10psi=185hp
Going to need a fuel system too, not sure you can push that many ponies through the stock line.
It'll work for a return maybe.
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Old 08-05-2022, 03:43 PM   #4
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200 hp with good tuning, not an issue. The issue would be any slight tuning mistakes you make along the way. Stock cast pistons are not very forgiving to any detonation, same with rods. A little ping ping boom and you've got bent rods that will break soon, or cracked pistons, possibly a happy meal of piston mcnuggets.

Using an F head for lower compression? Avoid setting it up with a thick HG to lower the CR even more, that antique combustion chamber design really needs tight squish (clearance between piston crown and head at TDC) to avoid detonation.
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Old 08-05-2022, 04:18 PM   #5
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200 hp with good tuning, not an issue. The issue would be any slight tuning mistakes you make along the way. Stock cast pistons are not very forgiving to any detonation, same with rods. A little ping ping boom and you've got bent rods that will break soon, or cracked pistons, possibly a happy meal of piston mcnuggets.

Using an F head for lower compression? Avoid setting it up with a thick HG to lower the CR even more, that antique combustion chamber design really needs tight squish (clearance between piston crown and head at TDC) to avoid detonation.
Yes, will use a standard '72 F head, and want to leave compression at the factory 8.7:1. I do plan on some combustion chamber and port cleanup, but nothing crazy. I'll be running a Speeduino based ECU with sequential injection and coil pack ignition. I'm currently analyzing the output of a GM knock sensor controller, and writing some knock detection code for the ECU, but the knock detection is very experimental.

For the head, I'll unshroud the valves some, and do some smoothing of the flats to combustion chamber bowl, that'll drop the compression a touch, so I can run the thinner head gasket. Will have to measure compression ration when I'm done.
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Old 08-05-2022, 04:40 PM   #6
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Originally Posted by JohnMc View Post
200 hp with good tuning, not an issue. The issue would be any slight tuning mistakes you make along the way. Stock cast pistons are not very forgiving to any detonation, same with rods. A little ping ping boom and you've got bent rods that will break soon, or cracked pistons, possibly a happy meal of piston mcnuggets.

Using an F head for lower compression? Avoid setting it up with a thick HG to lower the CR even more, that antique combustion chamber design really needs tight squish (clearance between piston crown and head at TDC) to avoid detonation.
Are the rods in B20 a weak point? I would have thought it was the pistons as the rods are forged.
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Old 08-05-2022, 05:03 PM   #7
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It depends on which crank he has. 6 bolt cranks use lighter rods while 8 bolt cranks use the same rods as B21/B23. That means the stronger M rods can be used.

The arrow points to the weak spot in the B18 rods. You can see the 6 bolt B20 rod next to it has more material in this area. The 8 bolt rod on the far right is even beefier.The rod on the left is a Carillo for 6 bolt B18/B20.

http://hiperformanceautoservice.com/...//B20Rods2.jpg
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Old 08-05-2022, 05:19 PM   #8
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It depends on which crank he has. 6 bolt cranks use lighter rods while 8 bolt cranks use the same rods as B21/B23. That means the stronger M rods can be used.

The arrow points to the weak spot in the B18 rods. You can see the 6 bolt B20 rod next to it has more material in this area. The 8 bolt rod on the far right is even beefier.The rod on the left is a Carillo for 6 bolt B18/B20.
I see, thanks for the info!

Do you suppose it's worthwhile fitting a B21/23/230 crank + rods? If the nose needs to be machined, I've got a lathe, so no problem.

I'm assuming the 8-bolt and B21 use a larger rod journal diameter right?
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Old 08-05-2022, 05:33 PM   #9
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An 8 bolt B20 crank will work without machining and early B21 cranks are identical. They have a removable seal sleeve which was later incorporated into the crank. Not all B21 engines got M rods. I know turbos had them but I'm not sure which other engines came with them. B230 cranks and rods are completely different.

The 8 bolt rod is slightly smaller on the big end than the 6 bolt rods. I think you can machine the 6 bolt crank to fit 8 bolt rods. Also 6 bolt rods use a 22mm wrist pin while 8 bolt rods are 24mm.
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Old 08-05-2022, 08:07 PM   #10
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An 8 bolt B20 crank will work without machining and early B21 cranks are identical. They have a removable seal sleeve which was later incorporated into the crank. Not all B21 engines got M rods. I know turbos had them but I'm not sure which other engines came with them. B230 cranks and rods are completely different.

The 8 bolt rod is slightly smaller on the big end than the 6 bolt rods. I think you can machine the 6 bolt crank to fit 8 bolt rods. Also 6 bolt rods use a 22mm wrist pin while 8 bolt rods are 24mm.
I read the 6-bolt rod journals are wider than the 8-bolt. Wouldn't this cause an issue with the narrower rods floating?
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Old 08-05-2022, 08:48 PM   #11
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The greenbook shows the width of the bearing recess for 6 bolt rods is 32mm and for 8 bolt rods it's 30mm so yeah there would be too much clearance unless you had custom pistons made that steered the rods at the small end.
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Old 08-05-2022, 09:34 PM   #12
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The greenbook shows the width of the bearing recess for 6 bolt rods is 32mm and for 8 bolt rods it's 30mm so yeah there would be too much clearance unless you had custom pistons made that steered the rods at the small end.
Makes sense.

So, basically choices are, considering I'm only shooting for 200 hp / 10 lbs at most:

(1) use existing 6-bolt crank / rods, and hope for the best.
(2) find a B19/21/23 crank / rods and machine the snout for the B20 style block.
(3) not even going to try to look for 8-bolt B20, as these are likely unobtainium.
(4) forget the whole turbo idea and drive the B20 as-is until I swap in the B230ft I have.
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Old 08-06-2022, 10:09 AM   #13
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#4 is my choice
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Old 08-07-2022, 10:42 AM   #14
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#4 is my choice
Yes, a B18 or B20, even in stock form, is a pretty fun little motor for back roads and driving around town. Enjoy it while sorting out and collecting parts for a turbo swap.

They are also rather fuel efficient. Which is nice for the current high gas prices. I got 30mpg out of mine when driving the long-ish highway journey to my new place.

For a turbo swap with 200hp you will also have to sort out a transmission. I wouldn't trust an M40 or M41 behind a turbo B230FT with probably a bit north of 200 lb-ft of torque. You'll most likely be looking at an M46 or a T-5 swap. For which you will have to extensively modify the transmission tunnel to get the wider gearbox and the new shifter location to fit.

All this stuff will take time to plan and source parts before you even start turning a wrench. Slap a carburetor on it and drive it for now and sort out brakes and suspension if need be. I've found that I have more motivation to fix stuff and pursue my project cars when I can drive them more often.

Last edited by spock345; 08-07-2022 at 10:53 AM..
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Old 08-07-2022, 01:46 PM   #15
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It depends on which crank he has. 6 bolt cranks use lighter rods while 8 bolt cranks use the same rods as B21/B23. That means the stronger M rods can be used.

The arrow points to the weak spot in the B18 rods. You can see the 6 bolt B20 rod next to it has more material in this area. The 8 bolt rod on the far right is even beefier.The rod on the left is a Carillo for 6 bolt B18/B20.

http://hiperformanceautoservice.com/...//B20Rods2.jpg
On my first 2.1L B20 build in the PV I used some B18 rods because they're lighter. No turbo, but making around 170 hp. One of the pistons cracked and I took it apart and all the bearings were still in great shape, but there were little 'shadow' wear marks right where that red arrow is pointing, on all 4 top rod bearings. It's obviously thin enough that it somehow moves or flexes during some situations.

I replaced the rod bearings and used some later production B20 6-bolt rods on reassembly, they're heavier but noticeably beefier.
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Old 08-07-2022, 11:57 PM   #16
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Yes, a B18 or B20, even in stock form, is a pretty fun little motor for back roads and driving around town. Enjoy it while sorting out and collecting parts for a turbo swap.

They are also rather fuel efficient. Which is nice for the current high gas prices. I got 30mpg out of mine when driving the long-ish highway journey to my new place.

For a turbo swap with 200hp you will also have to sort out a transmission. I wouldn't trust an M40 or M41 behind a turbo B230FT with probably a bit north of 200 lb-ft of torque. You'll most likely be looking at an M46 or a T-5 swap. For which you will have to extensively modify the transmission tunnel to get the wider gearbox and the new shifter location to fit.

All this stuff will take time to plan and source parts before you even start turning a wrench. Slap a carburetor on it and drive it for now and sort out brakes and suspension if need be. I've found that I have more motivation to fix stuff and pursue my project cars when I can drive them more often.
I don’t do carbs, it’s a B20E. What I am doing as phase 1 is getting it running with modern injection. This is a Speeduino based board I designed, full sequential with crank trigger and knock detection. I do have a T5 on my shelf that I’ll be machining an adaptor plate for. Just got the B20 pulled out so now I’ll 3D scan the engine bay to see how much clearance I’ve got for a turbo.

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Old 08-08-2022, 12:53 PM   #17
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My setup was pretty much a stock bottom end 6 bolt. I think I mentioned this on instagram, but I can add some more color here. I have run 3 turbo setups on b18-b20s in my wagon.

First was a B18 D, cast +.040" pistons, on a worn out block, stock rods and a C cam running an HL-15g. Had about 5k of hard miles on the setup at 16psi with very minimal porting. Broke the ringlands after upping to 18psi, and polished the cylinders to a mirror shine. No other failures, rods were still straight, crank was fine, bearings were all perfect.

Second was a 73 B20 F, stock bottom end, used (probably 5k miles on pistons and block) +.020" pistons, stock ring gaps, D cam, 350cc/min injectors and megasquirt MSnS single coil, batch injection, and an HL-16t. That setup saw about 8k of hard miles at 18psi. Had a fueling issue at the track, and ran lean under WOT at 6k for a few too many hits, rings butted, polished the cylinders, and cracked a couple ringlands, and broke the m41.

3rd was a 72 block with 73 head, +.030" new pistons, ring gap opened another .005" from stock, K cam, 900cc/min injectors megasquirt 2 with wasted COP ignition, heavily ported head, 16t, shorter header design, better cooling, and huge intercooler, 3" exhaust. That setup saw about 10 miles mostly at 18psi, with a bunch of track/autox/drag days at 22psi. Did well until I ran one really short autox where I literally could not get out of first gear for the entire course, and was on the 7k limiter for way too long. That one died of... wait for it.... you guessed it. Butted rings, broken ringlands, polished cylinders, and a dead valve guide.

New setup in the works, going to run a bit looser on the rings. Forged pistons, Forged B230 rods resized big ends for my 6 bolt crank, and a new turbo/ic/fueling setup. This time I'm planning on 24psi on pump gas, 30 ish on e85.
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Old 08-09-2022, 08:36 AM   #18
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An 8 bolt B20 crank will work without machining and early B21 cranks are identical. They have a removable seal sleeve which was later incorporated into the crank. Not all B21 engines got M rods. I know turbos had them but I'm not sure which other engines came with them. B230 cranks and rods are completely different.

The 8 bolt rod is slightly smaller on the big end than the 6 bolt rods. I think you can machine the 6 bolt crank to fit 8 bolt rods. Also 6 bolt rods use a 22mm wrist pin while 8 bolt rods are 24mm.
M-rods came 79-80
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Old 08-09-2022, 10:12 AM   #19
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I don’t do carbs, it’s a B20E. What I am doing as phase 1 is getting it running with modern injection. This is a Speeduino based board I designed, full sequential with crank trigger and knock detection. I do have a T5 on my shelf that I’ll be machining an adaptor plate for. Just got the B20 pulled out so now I’ll 3D scan the engine bay to see how much clearance I’ve got for a turbo.

Nice, yeah if you are going with a DIY EFI solution I'd get that running first and then add a turbo and sort out the gearbox. Having a P1800 also makes it easier to run a T5 because of the shifter location.

What are your intercooler plans?
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Old 08-10-2022, 01:12 PM   #20
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Nice, yeah if you are going with a DIY EFI solution I'd get that running first and then add a turbo and sort out the gearbox. Having a P1800 also makes it easier to run a T5 because of the shifter location.

What are your intercooler plans?
I just found out my engine has an E head, and not the F head that it was supposed to come with, so that means no B20 turbo, compression too high.

My plans basically are:

(1): 3D scan engine bay, engine, and make CAD models of all the bits.

(2): Pull B20, inspect, address any issues, and get B20 running, normally aspirated, modern injection, with a few tweaks like head port cleanup. Fix gas tank issue, and drive car for a bit.

(3): Fit T5 trans to B20, fabricate adaptor plate and trans mount.

(4): Design fitment of B230FT, will probably make at least exhaust manifold, possibly new intake. Will likely have to tilt at custom angle to clear both intake an exhaust. That's what the CAD models are for.

Intercooler wise, my #1 goal is no cutting, and absolutely positively must look period correct, preferably look factory plausible. As any front mounted intercooler would require cutting holes in the radiator support, this is out, so I'll be fitting a air-water charge cooler in engine bay. Looks like the BMW 7-series charge coolers will probably fit. This keeps it all in the engine bay, and only have to run water lines in front of rad.
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Old 08-10-2022, 01:21 PM   #21
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If you want an F head, I've got one sitting... somewhere (probably in the basement). I took it off my PV's motor a long time ago when I found my #RARE R-Sport Stage 3(?) head. Nothing fancy, it's an unshaved (AFAIK) F head, threaded and plugged injector ports, some mild recession going on with the exhaust valves.

But really, if you're going to be putting a B230FT in there anyhow, save the money and time and effort on trying to turbo the B20 and put all that into the B230FT install.
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Old 08-10-2022, 01:34 PM   #22
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If you want an F head, I've got one sitting... somewhere (probably in the basement). I took it off my PV's motor a long time ago when I found my #RARE R-Sport Stage 3(?) head. Nothing fancy, it's an unshaved (AFAIK) F head, threaded and plugged injector ports, some mild recession going on with the exhaust valves.

But really, if you're going to be putting a B230FT in there anyhow, save the money and time and effort on trying to turbo the B20 and put all that into the B230FT install.
Originally I thought my 1800ES had the F head because I read that's what was supposed to have been on the car. So, I was thinking, 'hey, I've got a low-compression head, why not try a turbo???'. After I pulled it, someone pointed out that's an E, so inspected it, and yup, sure enough, it's an E. But yeah, I agree, I'll just get it running essentially factory, sort out the rest of the issues with the car, do some driving whilst I sort out the details with the B230FT.
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Old 08-10-2022, 09:36 PM   #23
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I just found out my engine has an E head, and not the F head that it was supposed to come with, so that means no B20 turbo, compression too high.

My plans basically are:

(1): 3D scan engine bay, engine, and make CAD models of all the bits.

(2): Pull B20, inspect, address any issues, and get B20 running, normally aspirated, modern injection, with a few tweaks like head port cleanup. Fix gas tank issue, and drive car for a bit.

(3): Fit T5 trans to B20, fabricate adaptor plate and trans mount.

(4): Design fitment of B230FT, will probably make at least exhaust manifold, possibly new intake. Will likely have to tilt at custom angle to clear both intake an exhaust. That's what the CAD models are for.

Intercooler wise, my #1 goal is no cutting, and absolutely positively must look period correct, preferably look factory plausible. As any front mounted intercooler would require cutting holes in the radiator support, this is out, so I'll be fitting a air-water charge cooler in engine bay. Looks like the BMW 7-series charge coolers will probably fit. This keeps it all in the engine bay, and only have to run water lines in front of rad.
Let me know when you are going to try doing a T5 to B20 adapter plate if it uses the B20 bell housing. I have some plans to get a T5 into my Amazon.
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Old 08-11-2022, 01:42 AM   #24
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Let me know when you are going to try doing a T5 to B20 adapter plate if it uses the B20 bell housing. I have some plans to get a T5 into my Amazon.
I’ve got both here next to each other, the M41 and T5, I’ll 3D scan them next day or so, and measure exact dimensions to get the thickness right.

Are you using an FOX, or SN95 trans? The SN95 has a longer input shaft (that’s easily swapped to early style). I measured and the T5 shifter is in the exact same spot as M41, but the tail of the T5 is about 1.75” longer.

And for driveshaft, I think the 2-piece is best. A 2-piece with a rear slip yoke AND trans slip yoke is what Chevy used on millions of cars, so can’t be anything wrong with that design. I do think the stiffer center bearings from a ford ranger would be better than the sloppy Volvo one.
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Old 08-12-2022, 10:39 AM   #25
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I’ve got both here next to each other, the M41 and T5, I’ll 3D scan them next day or so, and measure exact dimensions to get the thickness right.

Are you using an FOX, or SN95 trans? The SN95 has a longer input shaft (that’s easily swapped to early style). I measured and the T5 shifter is in the exact same spot as M41, but the tail of the T5 is about 1.75” longer.

And for driveshaft, I think the 2-piece is best. A 2-piece with a rear slip yoke AND trans slip yoke is what Chevy used on millions of cars, so can’t be anything wrong with that design. I do think the stiffer center bearings from a ford ranger would be better than the sloppy Volvo one.
Since I have an Amazon with the longer shifter I'm probably going to get a Chevy S10 or S197 (2005-2010 V6 mustang) T5 and just swap in the right gears and input shaft. That way I can have a longer shifter and keep somewhat of a stock look. Luckily all of them have the same bolt pattern.

I wasn't aware that Chevy had gone with two slip yokes. That sets my worries at ease a bit.
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