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To Bore or Not To Bore LZ20E


sssr20det510

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OK so i picked up a z20E that has less than 49k miles ( the car that it was pulled from said 49k 81 200sx) but the engine has been rebuilt at some point And has factory flat top pistons (maybe 2cc of valve relief) Im building this engine for a dirt track 510 so im contemplating just slapping an closed chamber head and calling it good with big cam and diff springs. OR bore it out and put milled ka24e pistons in it and balance the rotating assy. How much more power do you think can be made from the bigger pistons?

 

 

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Probably minimal gain for the expense involved. Any gain you see will likely be from the resulting increase in compression rather than the actual bore increase. You can get the same effect by shaving the head. If it were mine, I'd slap a closed chamber head on it and see how it runs. If you find a problem with the bottom end (rings, piston slap, etc) you can always go with plan B and it won't cost you any more than the price of gaskets. What's the calculated compression difference either way given the same head?

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Probably minimal gain for the expense involved. Any gain you see will likely be from the resulting increase in compression rather than the actual bore increase. You can get the same effect by shaving the head. If it were mine, I'd slap a closed chamber head on it and see how it runs. If you find a problem with the bottom end (rings, piston slap, etc) you can always go with plan B and it won't cost you any more than the price of gaskets. What's the calculated compression difference either way given the same head?

 

the reason I went with the z20e was for the longer rods and rod/stoke ratio of 1.773

 

Assuming a 2cc valve relief on the pistons I get 11.08 compression using a 41cc closed chamber head. The stock L16 head was semi open and 38.5cc. They had small valves and ports so I assumed you would want something like an A-87 closed head for the larger Z20 size motor.

 

KA24E pistons cannot be used on a Z20E motor. Sorry but the tops of the piston would stick up above the block deck by over 2mm. IF you were to mill the KA24E pistons by about 1.75mm (basically making them flattops) you would have the required .3mm above the block maximum amount allowable. With a crushed gasket thickness of 1.2mm this gives the minimum clearance for rod stretch at high RPM so they don't hit the head. So I worked this out...

 

Z20E block bored 4mm to accept KA pistons milled 1.75mm sitting .3mm above the deck making a 2138cc motor with...

 

A 41cc closed chamber head.......................... 12.45

A 45.2cc open chamber head.......................... 11.51

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I personally would just run a ball hone through it, and some new rings, as long as the bore is good. And dont forget to put some arp rod bolts in. I'm sure you will be spinning the piss out of it...gotta love those long rods, makes a nice little screamer.

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Mike i have a few a87 closed chamber heads and access to a w53 closed chamber and a u67 open chamber. Ive been keeping my eye out for a 219 or v912 head. If i do pull it apart i will bore it but just seeing as its low milage rebuilt if its worth doing for the extra displacement. Or not

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Assuming a 2cc valve relief on the pistons I get 11.08 compression using a 41cc closed chamber head. The stock L16 head was semi open and 38.5cc. They had small valves and ports so I assumed you would want something like an A-87 closed head for the larger Z20 size motor.

 

KA24E pistons cannot be used on a Z20E motor. Sorry but the tops of the piston would stick up above the block deck by over 2mm. IF you were to mill the KA24E pistons by about 1.75mm (basically making them flattops) you would have the required .3mm above the block maximum amount allowable. With a crushed gasket thickness of 1.2mm this gives the minimum clearance for rod stretch at high RPM so they don't hit the head. So I worked this out...

 

Z20E block bored 4mm to accept KA pistons milled 1.75mm sitting .3mm above the deck making a 2138cc motor with...

 

A 41cc closed chamber head.......................... 12.45

A 45.2cc open chamber head.......................... 11.51

 

I have a question Mike, How much is safe to mill from the top of the piston? Can you run a closed chambered head with this combo safely? What about using KA24DE pistons?

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The KA24E piston has a raised edge around the top. You could safely mill it off.

 

Z20E block bored 4mm to accept KA pistons milled 1.75mm sitting .3mm above the deck making a 2138cc motor with...

 

A 41cc closed chamber head.......................... 12.45

A 45.2cc open chamber head.......................... 11.51

 

12.45 is very high octane gas, probably110 aviation fuel.

 

KA24DE motors have smaller combustion heads so the pistons are dished even more than the KA24Es. The DE pistons may not even fit the L or Z rods.

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Probably minimal gain for the expense involved. Any gain you see will likely be from the resulting increase in compression rather than the actual bore increase.

Maybe I oversimplified my statement. What I meant was that the increase in compression will allow you to take advantage of the larger cam and high octane fuel more so than simply increasing the bore alone.

Example:

Chevy 350 + .60 over and 8.5cr with large cam, stiff springs, 850cfm, headers = gutless pig.

But,

Chevy 350 std bore and 11.5cr with large cam, stiff springs, 850cfm, headers = strong runner.

Make more sense?

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haha, that's an old wives tale that was definitively put to rest in the early 1990s. A low comp V8 engine, otherwise built like a high comp one, will give lots of HP. You can even put a big cam in it. It won't be a pig if built right. Any yes, it will run even better with high compression.

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You gain (very roughly) about 3.5 % per point of compression. That's about 7 hp on a stock L20B at 10.5 compression at it's max hp RPM. Less and lower RPMs and you need higher octane gas.

 

Always maximize engine size if at all possible. A 2.2 motor will make about 10% more power than an L20B, all else the same. Cost about the same to build, run on reg gas and not have to 'work as hard'.

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You gain (very roughly) about 3.5 % per point of compression. That's about 7 hp on a stock L20B at 10.5 compression at it's max hp RPM. Less and lower RPMs and you need higher octane gas.

 

Always maximize engine size if at all possible. A 2.2 motor will make about 10% more power than an L20B, all else the same. Cost about the same to build, run on reg gas and not have to 'work as hard'.

 

Yeah...I'll stick with my larger bore LZ23 with 8.5 compression and run on reg gas.....no pinging/detonation...run on...etc.

Most likely still have more torque than the smaller bore/higher compression engines that have to run on the more expensive/high octane fuel

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I guess I'm not following you. If I understand correctly every race team ever has done it wrong. NASCAR is 12:1, superbikes in the 12-13:1 range, Infinity Indy engine at 14.5:1.

 

RACE

 

Racing is a whole new world where winning is the goal. Spare no expense, risk anything, try anything that will shave tenths off your lap times. The law of diminishing returns does not apply here. So what if the motor only lasts for 5 hrs. of racing and needs re-building? Much in the racing world does not translate well for street use. Not saying a compression increase doesn't work... it does by making the engine more efficient, but there are drawbacks and expense involved that isn't considered in racing but the street driver must. The closer you get to a race engine for use on the street, the closer it comes to being undrivable.

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Sounds like he's building more of a race engine than a street cruiser.

Im building this engine for a dirt track 510...closed chamber head...big cam and diff springs

The initial compression increase is a side effect of the head swap. Depending on the bore size (displacement) either quite a bit or alot.

11.65 for 85mm bore and 13.06 for 89mm bore thats figuring on a 38cc combustion chamber head

I understood the original question to be whether it was worth it (expense, time, trouble) to increase bore. This is where I totally agree about diminishing returns. If it's truly a low mile rebuild, I would leave it alone. Just my frugal opinion. An unlimited budget scratch build would be a different animal all together.

...just seeing...if its worth doing for the extra displacement. Or not

Again, race motor.

We Run Race gas

Jeremy didn't go into great detail but I would assume that he is using a complimentary fuel, exhaust and ignition system to make the longblock shine.

I don't feel that increasing bore size is pointless at all. If this was a scratch build I would definitely answer "all of the above" when going over the checklist if things to improve performance. There are soooo many variables that come into play here. By increasing displacement you would essentially make the rest of the components act "smaller" in comparison. On a street oriented, long life, DD type build it makes total sense to increase displacement and stay relatively mild with a tame cam and lower pump-gas friendly CR. Jeremy is building a race engine. With his combination of parts the CR increase is happening on it's own. That will allow him to take advantage of race oriented parts/tricks.

My point is that taking a bone stock L20b in a '79 620 and simply increasing bore size will yield almost no measurable performance gain. Likely the same would hold true by taking the same stock L20b and increasing the CR from 8.5 to 10.5 with no other changes. BUT, by starting with a race oriented package like Jeremy's, there would be a more noticeable gain from the resulting proper high compression rather than only increasing bore size. So; Jeremy's current combination but with 8.5 CR = lame. Jeremy's current combination with 11.65 CR = cool. Jeremy's combination with 89mm bore AND the accompanying 13.06 CR = obviously even cooler. All other things being equal, I would choose compression over displacement if I had to choose for a race engine. It goes without saying that I would take both options given the opportunity.

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