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Howcan I increase compression on newly rebuilt engine?

Iím wondering what the best course of action would be to increase my compression ratio. I think I had the wrong people rebuild my 2.0L 914-4 engine. I've had it now for almost 20k miles, and it still only reads about 80-90 PSI/cylinder (I measure it every couple of months and have seen no drop in numbers). I run Mahle pistons with the dish indentation. Also, my damn engine builder put VW bus heads on the thing (I have a sneaking suspicion they couldíve taken my Porsche 2.0L heads but didnít check ahead of time which heads the engine had). I have removed the head gaskets. I donít know if the combustion chamber is different between the 2.0L Porsche heads and the 2.0L VW Bus heads. I am looking for a solution now to increase my compression ratio, but am afraid it will entail possibly Euro pistons, real Porsche 2.0L heads, and thatís assuming my stroke is correct. Any other ideas?

Ben
Old 03-22-2002, 09:48 AM
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Compression ratio and compression while related, can differ and still be okay in both cases. example, my race engine has 11:1 compression, but because of a lot of valve overlap that compression is down, plus the rings are not as tight as in a street engine. Make sure you are doing the compression test correctly (no offense here) as follows: have valves adjusted exactly to spec, fully charge the batt, warm engine up well(hot), remove all plugs, hold/block throttle wide open, pull coil wire, insert gauge, crank for a set time/# revs and then record the measurement. As for the heads, I don't think there is much difference although DD probably knows the exact answer? Sometimes engines also take a very long time to seal up, especially if synthetic oil was used right away after a rebuild? Others may have some additional answers. Good luck.
Old 03-22-2002, 10:52 AM
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Ben, I would do the compression test as John has outlined. 80-90 sounds pretty low to me.
The 2.0l bus heads stock have small valves compared to the 2.0l 914 heads. The spark plug is in another location too. The combustion camber is also different. I looked at your web page and it said you rebuilt a 1.7 into a 2.0l. 1.7 heads came with pretty small valves too. I would look at the build sheet from the mechanic to see what parts he used. Did he change the valve size on the 2.0l bus heads ? 44 webers are pretty big carbs for that motor unless you are running a different cam too.
Geoff
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Old 03-22-2002, 11:12 AM
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I meant I took a car that was originally 1.7L and put a GA cased 2.0L engine into it. I have done the compression test as prescribed above. The valves are small, and the carbs are big. Got the carbs b/c they were $100 for the setup. I've dropped 32mm vents into them and rejetted. The main symptom I wish to eliminate is low power at low RPMs. I experience bogging under 2000 RPM, and believe this to be caused by my low compression. Like I said-I had the wrong people built my engine. I wouldíve done it myself but thought, "Why don't I let the professionals do it?" Worst mistake.
Old 03-22-2002, 11:23 AM
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Is there any way you can tell if the sheet metal is originally what you gave the machine shop? Also do you have the D-Jet intake pipes that came with the motor(if it came with them)?

If the shop put bus heads on a motor that had 914 2.0L heads they would also have to put on 1.7-1.8L top tinware as the spark plug holes in the sheet metal won't line up, and you wouldn't be able to get plug wires on the plugs.

If the intake pipes were on the motor see if they have 3 or 4 bolt holes, 4 means 1.7-1.8L heads 3 means 2.0L 914 heads.

Really the best thing to do is remove the heads and have them fly-cut and 42 intake 36mm exhaust or bigger valves put in, if your cam can support it.
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Old 03-22-2002, 11:50 AM
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That's a good point. It was primarily VW shop, and is well renouned, but Im not sure how much access they had to 914 tins. The tins line up, and if I were to guess I would say they didn't swich tins. Ahhh, now I think I can tell myself that they didn't rip me off head wise. The engine came from a parts car that had extensive PO mods, no FI, so who knows the history of the engine. The heads do have 4 bolts, but I have checked the number and it matches a Bus 2.0L head with oval exhaust ports. The problem is that when I had the engine rebuilt I was just getting into 914s and didn't know to check if I had true 914 2.0L heads.
Old 03-22-2002, 12:01 PM
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Those PO's can be buchers, I had a 1.8L that had one 914 1.8L heads 41mm int 34mm esh, and one bus head, 39.3mm int and 32mm exh. No wonder I couldn't the the single carb to run right. On top of that the PO let some exh leaks at the head go for so long that the one 1,8L head I had was ruined, the exh ports flange were melted.


The bus heads aren't bad, they crack less then the 2.0L 914 heads, but they have smaller valves then a 1.7L because the busses needed to build torque and had L-jet with emission controls. If you put big valves in them they flow OK.

Also try spraying in some sort of compression booster, I use PB Blaster "tune up" catalyst. do it just before the comp test to see if it boosts numbers, if it does you problem is the rings not the heads.
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Old 03-22-2002, 12:15 PM
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Now you are getting me worried. I've got an exhaust leak because of a stripped exhaust stud that I can't get out. I suppose I could snap it off and then HAVE to get it out.

BTW, my compression on a 1.8 is running ~ 130 psi. I put rings in it a while ago and left the sealing rings out.

80 - 90 does sound low, and the carbs sound big for low RPM driving.

Karl P.
Old 03-22-2002, 02:18 PM
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Ben, look for the build sheet or parts list for what they put into the motor.
Maybe they used bus p&c's which are cheaper and low compression (7.3 to 1) to go with the bus heads.
Also maybe they set the deck height too big so that could throw the compression of too.
Try squirting some PB Blaster into the cylinder when doing a comp test to see if it builds higher compression. Maybe the ring gap is set too big.
Make sure the valves are adjusted right and not set too tight.
Geoff
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Old 03-22-2002, 04:15 PM
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Leak down

After you fiddle around compresion testing till you are fed up. Try doing a leak down.(technically not leak down but) Quit guessing at what might be wrong and find a definative answer. You don't need alot of special equipment just an air compressor and a way to connect it to the spark plug hole. (be creative) Air sound comming from oil filler is rings. Air sound in perspective intake or exhaust is valve (try adjusting them again).

At best your motor is over carbed hence the bog at low RPM. Period... If the cylinders are all close in compression (run it) and deal with it. As it was said before if the car has a "CARB cam" how big and how long is the duration? I don't know how to measure it without a dial indicator and degree wheel (see pain in back side while motor is in).
Compression ratio as said before does not have anything to do with valve size, carb size. But rather bore, stroke, Deck hieght, Combustion chamber cc., that is it.
Don't confuse cylinder CFM flow rate or ability to move air through the cylinder with compression or compression ratio.

P.S. don't lose a washer down the intake (still trying to recover from that one).
Old 03-22-2002, 05:04 PM
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Joe, Don't you just hate carbs....and those little washers, BTDT,
Just another reason to like FI.
Geoff
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Old 03-22-2002, 07:00 PM
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For KARLP, bite the bullet and purchase "easy outs" (screw extractors. Drill exhaust stud as to size required (centered) and run the screw extractor in - the stud will come out. Takes a little time and effort ~ may save a head.

And on the 44s, you can possibly change the venturi size down, which effectively increases velocity thru the carb (read better fuel atomization and less "bog" at lower rpms). Commensurately, you may have to rejet to avoid a to rich mixture. Any loss will occur in the upper rpm range.

Just an idea!
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Old 03-22-2002, 08:39 PM
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Screens

A guy that works for me came up with what started as a joke but actually has some merit. Put a screen between two gaskets under the manifold. This is the second time something has weaseled it's way into the intake. the 1st time I seen it and was able to go in after it.
Q. why do parts only fall in a port with an open valve?
A. It's the only reason for fallng in the first place.


Let me tell you fishing out something from a combustion chamber is tough. But I now have cool little hooks made from bike spokes.

Sorry for jump'n on the thread dude.
Old 03-23-2002, 03:54 AM
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