Larger pistons??

RSTPerformance

RSTPerformance

New member
Can you get a lot of gain by running pistons that are 1mm larger?? It seems to me that the CB originally made it this amount because it wasn't enough to make a big difference but it was enough to repair an engine if needed. Has anyone seen/heard of any real gains from going the 40 over?

Stephen
 
You would need a Dyno, but everything else remaining the same, there should be a small power increase.

That's why you hear the expression: "There's no substitute for cubic inches" (even though it might only be a couple cubic inches in this case)
 
Don't forget that just by going to oversize pistons you'll also increase compression ratio.

I let you ponder that...

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MC
#14 gp VW Scirocco
 
Originally posted by racer14itc:
Don't forget that just by going to oversize pistons you'll also increase compression ratio.

I let you ponder that...

smile.gif


MC
#14 gp VW Scirocco
Click to expand...


I was under the impression that the compression would stay the same. It may raise a tiny bit since your recess depth doesn't get deeper in comparison to the larger dia. piston. However if you change nothing but diameter I wouldn't think it would raise it by that much. What is the equation to measure the compression? That would be the easiest way to figure it out.

Does anyone have any Dyno results??

Stephen
 
Originally posted by Scirocco#28:
It will raise the comp ratio, but the amount is negligable. My .040 motor still needed a healthy cut to the head and block (stock heights to start with) to make my allowable ratio.

Steve
Click to expand...

What is a "Healthy Cut"?

Also does anyone have the equation for compresion? How does SCCA measure it? I want to probably shave the head over the winter but I want to know how to calculate it!

Stephen
 
Yep, slight increase. Since the volume at TDC doesn't change (assuming piston is flush w/ the deck), but the total cylinder volume increase (because of the larger diameter piston), the ratio of total volume to volume at TDC (the CR) goes up slightly

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MARRS #25 ITB Rabbit GTI
SCCA 279608
 
The 'healthy cut' on my engine was in the .010 to .030 range total
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Not sure what it would be for you. Cutting from the head will raise comp ratio faster than taking it from the block. So we took the bare minimum for straightness off the head and then took the rest off the deck height. Ended up with a CR of 8.9:1 which is good enough and close enough for me. This is the 1st motor I've built to the limit of the rules, and I must say that it works like a champ. No hp #'s yet but I will in a few weeks hopefully. Probably in the 103-105 range. It feels a lot stronger than my now illegal HT engine from last season. Good thing I drive a VW on the street, at least the motor isn't collecting dust.
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Steve

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87 ITB Scirocco
www.geocities.com/highspeedconnectionracing

[This message has been edited by Scirocco#28 (edited September 02, 2002).]
 
From what I know about Sciroccos ,Steve,[which is not a lot!]good JH motors"legally"
put out mid 90's for hp on the dyno.
Dino
 
JH is a good 15hp shorter than a HT, both with legal cams. and compression. If ya got a Jh and 10-1 and a G cam than that is about the same as a stock /legal HT.
So; if the JH- GTI is running with a HT Golf , the GTI has too much scoot...
40 over is 3-5 hp. If the clearances are right and ring gap is proper. "Total Seal" makes the power for a long time. Read the instructions!!!, No oil on the rings for installing rings!!!. deck the block a tad to flatten the block and bump compression. .020 is fine.
.03MM
 
Originally posted by Vantage #51:
From what I know about Sciroccos ,Steve,[which is not a lot!]good JH motors"legally"
put out mid 90's for hp on the dyno.
Dino
Click to expand...

You're so modest saying you don't know a lot about Rocco's
smile.gif
Willy was telling me he put down 100hp prior to the July races. And he pulls me most everywhere.

I might have to get the butt dyno recalibrated. Maybe this motor just feels stronger and has better torque over the whole rpm range. I'm going to have some actual dyno time in the next week or so, that should give me a good idea as to what I'm running for power. The HT last season put out @ 103hp, with a stock bore. But it didn't last more than the last weekend at that setup. More likely was in the 98-100 range for the most part of the season. I won't be shy when I find out what my #'s are. And if they are higher than what everyone would expect, then I'll have to give my engine builder a pat on the back
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Steve

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87 ITB Scirocco
www.geocities.com/highspeedconnectionracing
 
You guys are both too modest. Had a heck of a time cutting the old cage out of the car Derek. I cant wait to hack out the perfectly good sunroof to get the hoop to fit....
 
Corey and I will be putting it in next week.
We were given'er so hard Corey had the case of the grinder melting and hissing at him : )
As well I was bleeding like crazy after I wacked my head on the corner of the camera mount. Its sore today. I have to get some metal tubing for the attachmentsand then some 2" x 1/8" Metal flatbar for corner braces.

I really dont want to hack the sunroof and weld her tight. Its a super clean little car. But hey I got it for 600$ CDN : )

Andrew
 
Steve,

That's a sweet engine build. Where on earth did you find brand new manifold bolts?! Seriously, where can I get some of those good-looking .040 over pistons?

The stock JH made 90 HP with the solid head and 92 HP with the '88-'89 JH hydraulic head. They took away our compression ratio but NOT our head (since Bentley and VW both confirm later JH engines came w/ the hydro head). The orig. stock exhaust manifold was terrible, so I can't imagine any well-built LEGAL JH with a good header or Techtonics downpipe making less than about 99 BHP, IMHO. Sure, the 10:1 C.R. was worth a few more (my guess is 3-5) HP, but NOT 15!!!

As some know, my pro-built 0.020 over JH only lasted one race plus half of another warm-up this year. Frankly, it did feel slow compared to last year's HT, but it was also a nightmare inside, as attested to by the sloppy aux shaft bearing which had the timing chattering, and which may have led to a lack of oil pressure to #1 rod. It also had some of the most mismatched ports I've seen in years. I still can't believe I paid money for that piece of junk.

If I ever get around to building a new LEGAL JH, I think it can still be competitive. In the meantime, I'm going to try the old 0.040 over 1.7 that's been gathering dust for a few years... At 160lbs lighter (must be the smaller valves), it just might be OK
smile.gif


Eric
 
BTW, the 90-92 Cabriolet with the 10:1 2H engine (basically the HT but w/ Digifant injection) still made only 94 BHP with the same or similarly restrictive exhaust manifold (and a bigger throttle body). Since the Digifant engines in the Golf made 105 BHP vs. only 102 BHP for the CIS, Digifant was not the cause of any BHP reduction. Therefore, the 10:1 C.R. vs. 8.5:1 C.R. seems to be worth only 2 BHP, FWIW...
 
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