Go bigger or shave it, or a little of both?

[ITA_CRX]

What is the difference in cost between .020 over pistons and .040 over pistons? This should probably be your driving factor.

With the Honda's there is no difference in HP gained between .020 over and .040 over, but the .040 over pistons cost $400 more. Not exactly worth it.

Motor performance is not directly proportional to displacement. There are other things to consider, like the weight difference between the two piston sizes. It'll probably be insignificant, but it is something. Depending on how high your motor revs it may be worth more to have the lighter pistons.

Your biggest gains are going to be from the compression ratio. Seeing as either way you'll end up with 9:1 it might not matter which piston you go with.

Since you said money doesn't matter build two motors and dyno them. Then you will know for sure.

Basically no one can answer your question without knowing a ton about the motor you are building. Each motor is going to be different and all you can do is speculate. In all honesty, cost of the parts and ability to rebuild the motor should be your driving factors.

Jamie

[RSTPerformance]

Originally posted by ITA_CRX:
In all honesty, cost of the parts and ability to rebuild the motor should be your driving factors.

Jamie

In all Honestly your right money matters a lot for most of us IT racers. I wish I was speed source or any on eof those big shots but I'm not. The other thing that works against me is that noone has developed the engine that I am using... 2.2 Audi 5cyl. 10 valve. I did say money didn't matter because I am using this forum to learn. I'll point out a few key things that I have learned.... they don't make .040 pistons so I would have to have someone like JE make them for 100+ each. so I figure to do the overbore over the simple shaving will be at least 8oo bucks more. ya that is probably more than I would spend on any ITB engine. So with that said I would not be doing the overbore.

However this topic still interests me and if anyone has done or knows of some research on other engines that would be great. I really enjoy learning about this not just taking the easy way or cheap way out and that's why I asked the question the way I did (Money doesn't matter)

doesn't the GCR say the pistons have to be the same weight? I don't have it in front of me but if it does and you did get larger pistons couldn't you make them weigh the same amount?

I'm suprised some of the every few hour posters haven't chimed in! Kirk, George, Darin and the rest of you guys/girls have a ton of knowledge... any input?

Stephen

[This message has been edited by RSTPerformance (edited October 07, 2003).]

[ITSRX7]

Originally posted by RSTPerformance:

I'm suprised some of the every few hour posters haven't chimed in! Kirk, George, Darin and the rest of you guys/girls have a ton of knowledge... any input?

Stephen

Stephen,

I think the reason you haven't seen many posts from the 'regulars' is simple...they don't know the answer - and it's a good thing they didn't post yet. You have some good baseline advise above but the bottom line is that nobody has the experience with your motor to give you a precise answer. Now, would you have asked about a Honda 4cyl or a Mazda rotary...

AB

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Andy Bettencourt
06 ITS RX-7
FlatOut Motorsports
New England Region
www.flatout-motorsports.com

[apr67]

Ok, so with an audi 5 cylinder, lets think.

Most cars only have 4 cylinders to go .040 over. You have 5!

Pistons shouldn't cost much more than $125 each. That is custom (i.e. stock, but .040 over) made by ross or one of the other piston mfg's.

You probably want to be careful with decking the block and milling the head. Why? It can create a detonation area. This is one of those things that you are better off talking to an Audi guy about. Mike Flynn in Orlando raced an audi, may still race it on ocassion. Also, Jim Litehauser did worked with someone on one of these cars (I am assuming the ITB Audi, but they probably can point you in the right direction regardless).

Most motor builders understand chevys and fords. Some might be good with other motors, but very very few know much about the obscure motors used in most FWD cars that are raced in IT. Motors in IT last a long time, so it is better to spend your money ONCE and get it right, or atleast close.

[therooster]

RST, they do make .040 piston. I have some in my new motor. Cost around $450 for all five. Can not remember exactly where I found them, but it was some place out of Canada that imported them from Germany. I will look for the invoice and email you.

[RSTPerformance]

Originally posted by therooster:
RST, they do make .040 piston. I have some in my new motor. Cost around $450 for all five. Can not remember exactly where I found them, but it was some place out of Canada that imported them from Germany. I will look for the invoice and email you.

Please do! I searched for well over a year for them but everything was .060. over. (2.3 engine) Do they come with rings and everything? Please E-mail me with any info ASAP! thanks and make sure you use the below e-mail so raymond doesn't get it!

[email protected]

[lateapex911]

Originally posted by RSTPerformance:
.........thanks and make sure you use the below e-mail so raymond doesn't get it!

[email protected]

Ha ! Your secret is safe with us and the rest of the IT.com readers, and the internet community at large! Probably only a 0ne in one chance he'll find out!

And thank's for not including me in the "every hour" poster category!



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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[rcc85]

The ITCS says "Where a factory specification for original cylinder head thickness can be proven, a tolerance of .025 inch less than the service limit will be permitted. Under no circumstances may the compression ration be increased by more than one-half (.5) point".

Depending on combustion chamber design and overbore, shaving the head to the maximum might give you more than a one-half point compression increase.

That would, of course, be illegal.

[rcc85]

I also don't see where decking the block below the manufacturer's spec is allowed in the ITCS. Decking the block to manufacturer specs would be allowed under "blueprinting".

Bob Clifton
85 Dodge Daytona ITB

[lateapex911]

Originally posted by rcc85:
The ITCS says "...... Under no circumstances may the compression ration be increased by more than one-half (.5) point".

Depending on combustion chamber design and overbore, shaving the head to the maximum might give you more than a one-half point compression increase.

That would, of course, be illegal.

That figure refers to the manufactures specs right? .... and those usually have tolerances, don't they? So if the Audi spec reads 9.0 plus or minus .5, he could actually wind up at 9.99 and still be legal, right??

Just a rotary guy thinking out loud.....



------------------
Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[RSTPerformance]

Originally posted by lateapex911:
That figure refers to the manufactures specs right? .... and those usually have tolerances, don't they? So if the Audi spec reads 9.0 plus or minus .5, he could actually wind up at 9.99 and still be legal, right??

Just a rotary guy thinking out loud.....

I like Jake!!!

I'll look into what all the specs are when I get more time.... to late tonight and tomorrow packing and leaving for the glen right after work! I probably wont be able to get back on this until Monday.

[therooster]

" thanks and make sure you use the below e-mail so raymond doesn't get it!"

What is in it for me? I need to know some of your secrets so I we can attempt to take 1-2-3 at NHIS!

[Ralf]

Try contacting TSR Performance over here in the UK. Their catalog shows AUDI 5 Cyl motors. They might be able to get you the proper sized pistons and also answer your questions since they also do head work.
www.tsr-performance.com


------------------
Ralf
ITB Golf GT
Parked till '06

[MMiskoe]

I would bet on the head/block shaving for the following reasons - lower weight pistons, smaller surface area in the combustion area, block would be rebuildable if it scored a cylinder, pistons are cheaper as you alreay have 5 of them, you won't have to mess with oversized head gaskets. The change in cam timing is pretty damn small, I've seen the math on it, could get it if you want, send an email if you want it.

Find a machine shop that does nothing but build & dyno race motors. R&R down in Londonderry, impressive shop. When I was there they had a big block Mopar thing on the stand loafing at 350hp, the owner commented that most of their stuff starts at 6or700hp.

Matt