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How Far to Overbore a B20?

I have a '69 P1800 with the stock B20B with a Weber 32/36 DGV. I am almost ready to paint the car and the engine will come out and be rebuilt. I have an ipd street performance cam for it already. I plan to bore it out. I already have a Clifford header and 2-1/4" exhaust. First question: how far can I bore the block and still have good reliability? A person who works at ipd suggested that I could bore to 2.4L from the stock 2.0L. Is that correct? If not, is 2.2L OK? If I went that far with the bore, and coupled with the exhaust and the head mods I am going to have done (a local guy ports, polishes, and installs larger Chevy valves in the heads along with slightly changing the combustion chambers), how much horsepower might I be able to expect? 150? 160? More?

Also, my car has the 4.56 rear end ratio. Will it help lower the RPMs much at 60 (currently about 4000) if I switch to a 4.30? Or should I switch to a 4.10? Thanks for your help!
Greg Simon

Cameron says: My guess is that the "person at " who mentioned a 2.4 liter B20 was referring to increasing displacement by both boring the cylinders and stroking the crankshaft.

I've never seen a B20 that was bored to 2400. I have heard about one that some folks in Redondo Beach, CA, built several years ago. Rumor was that it was very fast and very prone to overheating. I would not bore the cylinders to this degree.

By boring the B20 to accept B21 pistons, you can easily and safely reach 2130 cc. Stroking the crank to allow the pistons to travel farther away from the combustion chambers -- thus further increasing displacement -- is the better way to reach approximately 2400 cc.

Things to take into consideration would be the camshaft and carburetion -- I don't think that a 32/36 DGV or DGEV would supply adequate fuel for such an engine. ipd's Street Performance cam may or may not be a good match for 2400 ccs. You would be steep on the learning curve with such an engine.

TRS offers 2395 stroker kits. They aren't cheap. Topi would be able to offer better information than I on one of these kits.

A 2130 can be made to perform well with a 32/36 Weber, Fuel Injection, DCOEs (or variants) and with one of several cams. I have used the Street Performance cam in a 2130 with excellent results: 178 horsepower measured at the flywheel.

Porting the head is a good idea. I've heard from two schools of thought on polishing the ducts. Pro: smoother ducts create less turbulence and help facilitate flow. Con: semi-rough ducts create mild turbulence that helps to atomize the fuel mixture and results in a better burn.

About oversized valves: if they're too large, the valves themselves can serve to shroud the port and actually decrease flow in and out of the combustion chambers. An alternative would be to use high ratio rocker assemblies: these are available in 1.6, 1.65 and 1.7:1 ratios instead of the standard 1.5:1. These also are not cheap.

Potential benefit of high ratio rockers: more valve opening without higher cam lift or steeper cam lobes. Potential downside: greater likelihood of a valve and a piston making contact if the timing gear should fail -- this would be a bad thing.

I realize that I'm not really answering your original question. What I can tell you is this: a 2130 cc B20 with a Street Performance cam, a ported and cc'ed head, the exhaust you describe and suitable induction can make an easy-to-drive, strong and reliable engine.

One last note: an associate and I are currently collecting parts to assemble a 2395 stroker and we hope to have it together and running within the next year. I'll certainly be keeping notes as we progress.

Phil says: If 60 MPH with 4.56 gears gives you 4000 RPM, 4.30 gears will give you 3772 RPM and 4.10 gears 3596 RPM. I can testify that Cameron's 2130 motor coupled to a 4.10 rear end pushes one back into the seat quite convincingly...

Greg wants to know more: I have one more thought after reading your comments. You've convinced me to go with 2130 cc for the bore, but I know ipd does not sell pistons for the 2130 cc unless it is a '74 or '75 engine. Do you know of anyone who sells 2130 cc piston and ring sets for a '69 B20 and, if so, what the cost is and who the manufacturer is? I would ideally like to get the original Mahle pistons if possible. Also, would there be any other parts that should be changed with an overbore such as that?
Greg Simon

Cameron answers: The most likely reason for this, Greg, is that the '74 and '75 B20s use 24mm wristpins: the same size as the early generation B21. Earlier B20s use a 22mm wristpin. What this means is that later B20s will accept B21 pistons and rings with no modification required other than boring the cylinders. Also, the '74 and later B20 engines have some additional castings in the block between each pushrod that offer greater structural rigidity than the earlier blocks.

Check the flywheel: if there are 8 bolts securing it to the crankshaft, it is a '74 or '75. If there are 6 bolts, it's an earlier B20.

It is possible to use an earlier B20 for a 2130 project; this would require having a machine shop ream the wristpin end of the connecting rods to 24mm and giving up on the idea of using wristpin bushings. Depending on who you ask, this might not be a bad idea -- and it might not be a good idea.

To make this even more complicated: the internals of the early blocks are standard while the laters are metric. What this means is that the flywheel and rods are matched to the crank, the wristpins are matched to the rods, the pistons are matched to the wristpins. This requires greater modification overall to reach 2130 cc.

Alternative: get a later block from a salvage yard and have the cylinders bored about .125" over and assemble. Much easier.

There are a few sources for oversized pistons for the early blocks: I've never seen one, I've never spoken to anyone who uses them and I've never come across any hard data that supports them or leads me to think that they'd be better than -- let alone as good as -- Volvo or Mahle B21 pistons.

I'd be interested to know if there are any others out there with more information.

More information: I saw you were interested in 2.2 liter pistons for early B20 engines. Dave Rauch at Competition Motors in Burlingame, CA (650-342-3111) managed to obtain a set for me and is currently rebuilding my '73 1800ES FI engine with them. They're forged and look great, but I'm not sure if any more are available from his source.
Lee Powell

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