# 15:1 compression!



## cabby85 (Feb 28, 2009)

I currently have a 1.8 JH motor CIS injection :thumbdown:, I found a really good deal on a set of "4 VW Wiseco Pistons. 1.8L Board .080 over. Actual bore is 3.268in. Has one race on them (maybe 15 miles?) Found illegal in class, and had to be removed. Like New Condition! These are flat top pistons so if you're looking to up your compression ratio, these will do the trick! With .040 off the head, these pistons should give to a compresstion ratio of 15:1" for only $275 I'm thinking these a TT 288 cam and dual carbs would be a mean setup. Questions are what other issuses would I end up with cooling, conneting rods, valves, etc
Allen


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## wantacad (Apr 4, 2003)

are you building a race motor yourself?


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## antichristonwheels (Jun 14, 2001)

you are running alcohol right?


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## Prof315 (Jan 5, 2009)

cabby85 said:


> I currently have a 1.8 JH motor CIS injection :thumbdown:, I found a really good deal on a set of "4 VW Wiseco Pistons. 1.8L Board .080 over. Actual bore is 3.268in. Has one race on them (maybe 15 miles?) Found illegal in class, and had to be removed. Like New Condition! These are flat top pistons so if you're looking to up your compression ratio, these will do the trick! With .040 off the head, these pistons should give to a compresstion ratio of 15:1" for only $275 I'm thinking these a TT 288 cam and dual carbs would be a mean setup. Questions are what other issuses would I end up with cooling, conneting rods, valves, etc
> Allen


Rods are a must and as soon as you go above 11 to 1 with a big cam it becomes an interference motor. (valves can hit the pistons) maintenance becomes VERY important.


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## MkIIRoc (Feb 20, 2005)

Why are rods a must?


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## cabby85 (Feb 28, 2009)

i am wanting to build a motor, to put in a 79' rabbit. I was gonna do a ABA but then i found these at a good price, its not really gonna be a race car but more like a sleeper. It seems this would cause to much problems and ABA would be best
Allen


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## Prof315 (Jan 5, 2009)

MkIIRoc said:


> Why are rods a must?


The cylinder pressures involved with a 15 to 1 motor will beat the rod bearings to snot without a good set of cap screwed rods. 

Example: Our race car is a 14 to 1 1.8L with wiseco pistons and scat rods. 3 seasons of racing and things are still ok.

We have a customer with a 14 to 1 1.8L same pistons, factory rods with arp rod bolts resized by the best machinist in central FL. he's on his 3rd motor in 3 years all due to rod/rod bearing failure. Now he replaces the rod bearings every 3rd race.


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## MkIIRoc (Feb 20, 2005)

Thanks I was just curious on your opinion. I used stock rods on my motors and on my friends mini stock motor. He's running about 14.5:1 compression and everything is going good.


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## chois (May 12, 2000)

Obviously would need appropriate fuel. 112 minimum IMO.


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## ABA Scirocco (May 30, 2001)

Prof315 said:


> The cylinder pressures involved with a 15 to 1 motor will beat the rod bearings to snot without a good set of cap screwed rods.


Rods are really nice to have but they're not a MUST. We've built quite a few race engines with compression ratios of 13:1 or little higher, many with stock rods and what works better than anything else to extend bearing life is an extra 0.001" clearance on the rod and main bearings.


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## Three Wood (Nov 29, 2008)

I am not expert in piston engines by any means but I must admit that I don't understand how the type of connecting rod can lead to bearing failure, assuming that the rod itself is not failing.


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## vwturbofox (Jul 31, 2010)

rods are not a must. i have been driving my boosted 8v 15 psi 10:1 for 6 months daily and i drive it hard stock bottom end .


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## Eganx (Apr 30, 2004)

cabby85 said:


> its not really gonna be a race car but more like a sleeper



15:1 is pretty high for a street driven car.....race gas is not cheap either. I would seriously think about that before buying. sunoco HCR plus(leaded 114 octane) is rated at 14:1 cr and up.......over $10 a gallon...ouch


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## vwturbofox (Jul 31, 2010)

yeah if this will be street driven you will waste you money 15:1 comp and it wont be a sleeper at all. spend the money for turbo on an 8v for daily best bang for you buck. 15:1 and 288 and dual carbs on street driven :thumbdown:


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## Eganx (Apr 30, 2004)

vwturbofox said:


> yeah if this will be street driven you will waste you money 15:1 comp and it wont be a sleeper at all



ha....I didn't even think about that....with 15:1 the exhaust note will be a dead give away


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## Three Wood (Nov 29, 2008)

Three Wood said:


> I am not expert in piston engines by any means but I must admit that I don't understand how the type of connecting rod can lead to bearing failure, assuming that the rod itself is not failing.


I inadvertently may have found my answer while reading a different forum about another subject entirely.

--
Don't take me wrong here, you must take into account the bearings surface speed and area but they are not the major factor in determining clearance. Factors yes, primary factor, no. You need to consider Internal component deflection and distortion when determining bearing clearance as well. *We still routinely hone rods oval. Wider at the parting line than top to bottom. Why, because the rods distort under dynamic conditions and the parting line pulls in and wipes the bearing. When the bearing parting line touches the journal, BOOM out the side of the block it goes.* We have had to oval Titanium rods .0075 at the parting line in order to make them live! Titanium has a very high modulus of elasticity in comparison to steal so the deflection and contortions of the rods big end had to be compensated for. 
--


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## Seax_Smith (Jun 1, 2007)

if you are building a drunk car... try to bump the compression more toward 16:1.

Where are you planing on getting gas if you are going the gas route? 15:1 and pump gas = fail.


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## Glegor (Mar 31, 2008)

Seax_Smith said:


> if you are building a drunk car... try to bump the compression more toward 16:1.
> 
> Where are you planing on getting gas if you are going the gas route? 15:1 and pump gas = fail.


i had 11:1 compression in my gti, and it would hardly run on anything besides premium. it really liked non-ethanol premium..


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## 88Jetta350 (May 4, 2007)

Glegor said:


> i had 11:1 compression in my gti, and it would hardly run on anything besides premium. it really liked non-ethanol premium..


My Syncro runs 10:1, on a properly set up 32/34 Weber DMTL carb, and will ping in the summer on anything but premium....


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## Three Wood (Nov 29, 2008)

That's kind of surprising. I've seen 12:1 engines with much larger bores (more detonation prone) and iron heads (more detonation prone) run just fine with 38 degrees total timing, on pump gas.

Were you guys running decent camshafts with the compression? I'm thinking that you would want at least 250-260 degrees at .050 for streetability with high compression...

After all, you wouldn't throw a huge cam in an engine without ditching the stock intake and exhaust manifolds, right? Everything has to be approached as a package.


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## Seax_Smith (Jun 1, 2007)

Form what I understand, larger engines tend to be less finicky. When you get down into the 2.0's and lower, little differences become bigger and bigger. Like an actually 270* cam will make a 1.8 digi idle a little rougher than a 2.0 digi/ABA hybrid. The only real difference in those 2 engines are teh 0.2L and the slightly longer stock of the ABA.

I beleive the same goes for compression, that smaller displacment engines become more finicky when you start squeezing the compression, etc. Larger dispalcements tent to be more forgiving and the greater rotating mass doesn't hurt either.


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## Glegor (Mar 31, 2008)

Seax_Smith said:


> Form what I understand, larger engines tend to be less finicky. When you get down into the 2.0's and lower, little differences become bigger and bigger. Like an actually 270* cam will make a 1.8 digi idle a little rougher than a 2.0 digi/ABA hybrid. The only real difference in those 2 engines are teh 0.2L and the slightly longer stock of the ABA.
> 
> I beleive the same goes for compression, that smaller displacment engines become more finicky when you start squeezing the compression, etc. Larger dispalcements tent to be more forgiving and the greater rotating mass doesn't hurt either.



this may be true on some of your engines, but ive never seen this phenomenon.. 

maybe your 2.0 hybrid idled nicer because it wasnt a crappy old 1.8 with dried up, hard seals everywhere? good injector seals have alot to do with how well your engine idles..

my gti was a different animal when i fixed all the intake leaks. thought my cam was super lumpy, but cams actually make vacuum leaks more appearent.


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## WaterWheels (Aug 14, 2005)

Seax_Smith said:


> Form what I understand, larger engines tend to be less finicky. When you get down into the 2.0's and lower, little differences become bigger and bigger. Like an actually 270* cam will make a 1.8 digi idle a little rougher than a 2.0 digi/ABA hybrid. The only real difference in those 2 engines are teh 0.2L and the slightly longer stock of the ABA.
> 
> I beleive the same goes for compression, that smaller displacment engines become more finicky when you start squeezing the compression, etc. Larger dispalcements tent to be more forgiving and the greater rotating mass doesn't hurt either.


More or less correct.


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## Seax_Smith (Jun 1, 2007)

Glegor said:


> this may be true on some of your engines, but ive never seen this phenomenon..
> 
> maybe your 2.0 hybrid idled nicer because it wasnt a crappy old 1.8 with dried up, hard seals everywhere? good injector seals have alot to do with how well your engine idles..
> 
> my gti was a different animal when i fixed all the intake leaks. thought my cam was super lumpy, but cams actually make vacuum leaks more appearent.



wasn't a leaky, beat up 1.8.... was all buttoned up tight and runing well. 

given that everything else is basically the same in the two engines, the (eproxomate) 10% increase in displacement and the greater rotating mass will give a slightly smother idle in the 2.0.

Start changing things around like better exhaust or exhaust diameters, intake flow etc.... the change becose much less appearent to non existant, but if the two engines are runningnthe samew basic air intake, exhaust, etc, the 2.0 will idle slightly smother.


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## Three Wood (Nov 29, 2008)

Seax_Smith said:


> Form what I understand, larger engines tend to be less finicky. When you get down into the 2.0's and lower, little differences become bigger and bigger. Like an actually 270* cam will make a 1.8 digi idle a little rougher than a 2.0 digi/ABA hybrid. The only real difference in those 2 engines are teh 0.2L and the slightly longer stock of the ABA.
> 
> I beleive the same goes for compression, that smaller displacment engines become more finicky when you start squeezing the compression, etc. Larger dispalcements tent to be more forgiving and the greater rotating mass doesn't hurt either.


You're looking at two different factors, here.

Cam specs do need to be detailed to the engine's displacement, because what the cam does is expose a certain amount of airflow potential to the cylinder. A smaller engine needs less airflow for the same VE so its ideal cam profile for a given situation/cylinder head is necessarily going to be different.

Generally speaking, larger engines are more detonation prone than smaller engines. Smaller bores are less likely to detonate because everything is closer to the spark plug. Detonation is when the end-gases get heated up/compressed enough by the expanding flame front that they auto-ignite all at once, in an explosion. It's a lot easier for this to happen when the bore is larger... I've seen one engine builder say that, for his application, there was no point to bores larger than 84mm, because anything gained in displacement was lost in reduced detonation resistance.

Notice sportbikes have compression ratios in the 11:1-13:1 range and run just fine on pump gas, without needing the direct injection tricks that passcar engines need. Smallish bores, and relatively large cams for the engine size. It's a balance.

Rotating mass has absolutely nothing to do with detonation resistance... not sure what it has to do with anything. Idle quality has nothing to do with it either, you can have a long duration cam that will bleed off cylinder pressure at lower RPM that idles just fine and dandy, but it will kill the midrange torque. Compromises. That's kind of why the OEMs haven't been doing much with really high compression...


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