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Join Date: May 1999
Location: Burlington, NC
Posts: 273
Big Bore Kit

I have a '74 914 2.0L w/ a stock engine. I am considering a Big Bore Kit w/ 96mm pistons. This kit claims to be compatible with stock FI. Also claims to increase HP by 25 to 30 HP.

I realize that there will be more heat in the engine and this is more stressful on the engine. My question is: how reliable are these kits and could I possibly cook my engine over time?

If you have experience with the Big Bore Kits or have an opinion let me know what you think.
Old 09-28-1999, 04:30 AM
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I put a big bore kit on my '75 1.8ltr. So far, I only have about 2000 miles on it so I don't know how reliable it's going to be. As far as the extra power goes, it is noticable, but 25 - 30hp extra? I haven't dyno tested it, but it doesn't feel like it. Also, I don't think the math supports it either since you would only be adding about 85cc. Maybe 4 - 5 extra hp?

BTW, absolutely no problems with the stock fuel injection beyond minor tuning.

[This message has been edited by randyle (edited 09-28-1999).]
Old 09-28-1999, 08:42 AM
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If the big bore kits are only supposed to give you 4-5hp, why not just put in the Euro pistons? I understand that they are a "bolt in" upgrade.
Old 09-28-1999, 10:15 AM
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I've got, what I think are, big bore euro pistons. I run a higher compression as a result. I cant comment on the performance increase because (as I've mentioned so many times) that I had my whole engine rebuilt/modified. Your increasing the amount of vacum the engine is capable of creating with bigger piston/cylinder. So an increase is to be had.
As far as cooking your engine, that depends on what the health of your engine is currently. Also you should have a professional do your engine work. That way he can check the crankshaft mating surfaces and check to see if the bore is aligned. Also you should make sure your oil cooler and overall cooling passages are unobstructed and in good condition. I dont think its a good idea to just pop on a big bore kit without atleast knowing what kind of condition your engine is in currently.

Generally my engine runs pretty cool, I've driven the car in stop and go and 4000 rpm in 5th on the highways in the middle of the night for 40mins. The engine doesnt seem to generate anymore heat than any of the other times I've driven it. Keep in mind I've had the inside of the engine case polished, reciprocating mass balanced, blah blah blah. (I'm sure someone out there is sick of hearing me yack about my engine). This contributes to a less stressful cooler running engine.

The claim of an extra 25-30hp probably would require some head work and an aggresive camshaft.
Old 09-28-1999, 11:29 AM
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Any of the 96mm bore P/C's have less sealing surface than the stock bore. Really isn't a problem except for the 1.7L, but there may be problems over time. One thing that helps (VW service bullitin from the 80's) is to assemble the engine without head gaskets (add a base gasket if you need to compansate for the higher compression).

The best thing to do (either with the big bore or the euro P/C's) is to get hold of an air/fuel meter and adjust the fuel pressure and the MPS sensor (look at the D-Jet article on the 914 fan web site). While you're at it go through and check that the entire system is up to par.

You *might* get 25+ hp from a big bore kit if you also put in a different cam that would need/use dual carbs, along with a race exhaust. That 25 hp may also be at a useless 6000 RPM, and at the cost of low end torque. My best guess is if you keep the D-Jet, use the big bore kit, and boost compression a little, you'll get a noticable 5 maybe 10 hp(w.a.g.), in a 914 this is big gain.

As for the heat, clean everything really well, clean the cylinder heads, remove the oil cooler and soak the fins (don't "dunk" it, you don't want solvent inside the cooler) in a gallon of carb cleaner, paint the outside of the cylinders flat black, clean the case and trans, make sure ALL the tinware gromets are in place along with the engine compartment seal (any missing piece of rubber is, in effect, a hole in your radiator), make sure the thermostat is working (please, don't take it out if you want your engine to last) and get a accurate oil and head temp gauge (the stock oil temp isn't very reliable). Watch the gauge, and look at past postings to see if your motor is staying cool. If not, look at putting in an extra oil cooler.

In a perfect world you would only put a big bore kit on a new motor, but if you have good oil pressure, the engine has not been beat on or it has low miles, then you should be O.K. Oil pressure is a good indicadtor of how tight the bearings are. Another good one is to measure the crankshaft end play at the flywheel, the motor dosen't need to be taken apart. Just use a dial indicator on the flywheel and push/pull the flywheel in and out. Also look in the vavle spring area, if it has dark black, burnt, oil chunks in it then the motor has probably been overheated to the point that the bearings are shot.

After putting in the new P/C's break them in properly. Do NOT use full throttle for the first 600-1000miles. DO NOT stay at a constant RPM (stay off the highway) for the same amount of time. DO change the oil at 500 miles then 1500 then the normal 3000 (I know, most of the metal in a new engine is from the bearings, but the rings lose some too). Read the Haynes book for all the stuff I forgot (or stuff I don't know )
Old 09-28-1999, 11:53 AM
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I have a 1.7L, actually two of them, and am looking for a supplier of a big bore kit for it. Seems like all the kits I have seen are for 1.8's, and 2.0's.
Old 09-28-1999, 05:11 PM
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Re: Big Bore (96 MM Pistons)
Yep! They are lots of fun whe added to a 1.7 Liter but I had to keep a close eye on the Head temp gauge I could push the engine way too hard in the hills and get it to start breaking up and miss. (No that was not normal driving at all, usually should have gotten a ticket if one of PA's finest was around) I had the car on Pocono race track to damn near 100 in fourth on driver's ed days this spring and it did not overheat to any problems, and that was with standard cooling. The thing I did have to do was convince a local machine shop to "Fly-Cut" the Heads to accept the bigger piston diameter and that was all. Or find a set of 1.8L heads, they will accept the bigger pistons already. Here in the Northeast most of these cars are long gone to the crusher.
The short stroke of the 1.7 crank and the bigger pistons combined with a set of weber carbs converted to 32 mm venturies was FUN I ran er way past the redline a lot. Unfortunately I had a substandard rear main seal from NAPA that failed as I was running a really great road with a lot of 90 degree corners (the old time farmers never ruined a good field with a road through it, and I for one thank them) and hills at the time. Damn I wish that the Germans used a red light for oil pressure, I saw the green light from the corner of my eye but didn't stop until the end of 4 linked corners. Too Late!! that 1.7L did not appreciate the abuse... By the way wouldn't a big bore for a 2.0L be more of a 103MM, the 96 is just 2MM bigger than stock. enough rambling on Bill K.
Old 09-28-1999, 06:05 PM
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The reason is that the pistons on the 1.8 and 2.0 are bigger than those on the 1.7, so it is easier to fit even bigger ones in. To fit the 96mm pistons and cylinders on a 1.7, you'll need to either get the heads machined to accept the bigger jugs, or get 1.8 heads outright. At which point, your 1.7 is a 1.8 except for the P&Cs.

Oh, wait--the 1.8 heads have bigger valves than the 1.7 ones, plus the early (W code) 1.7 cases didn't have the "windage tray" in them.

The late 1.7 with bigger valves and the cylinder sealing surfaces enlarged will be a 1.8 except for the P&Cs, distributor and FI.

Old 09-28-1999, 06:10 PM
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Dave thinking of putting 96 mms big bore kit on my 1973 914 with 1.7 do you have the kits and is there any problems besides machining the heads
Old 03-24-2017, 11:28 PM
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