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View Full Version : Go bigger or shave it, or a little of both?



RSTPerformance
10-06-2003, 12:28 AM
This is kind of a complicated question that may need more explanation for a good answer but here goes....

The rules for IT.... we are allowed to go up .5 in compression and we are allowed to go up 1mm in piston size (.040) Everything else in the engine stays the same.

I can either shave my block .030 and get 9.02:1 compression ratio
or
I can shave my block .020 and go up from 81mm to 82mm in piston size and I will get 9.02:1 compresion ratio

Both of these numbers use slightly rounded figures and yes 9.02 is .02 more than the .5 allowed but the math is way easier to explain with these numbers!

My questions are....
Which situation yeilds more HP.
Which situation yeilds more Tourque?
Which engine would make a better IT car?

Stephen

[This message has been edited by RSTPerformance (edited October 06, 2003).]

Bill Miller
10-06-2003, 06:17 AM
Stephen,

All else being equal, I would take the one that has the higher displacement. Don't get caught up in exactly how much the head/block is milled. Something else to take into account is that the more you mill the head/block, the more you will tweak the valve timing (unless you go w/ an offset key to return it to stock).

------------------
MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

joeg
10-06-2003, 07:03 AM
[I thought if you went oversize on the pistons, that also raised compression.]

Not a big fan of head shaving because of the cam timing issues.

RSTPerformance
10-06-2003, 09:12 AM
Cam timing issues??? please endulge a little into this an edumacate us less fortunate who may not understand!!!

Raymond "we will be faster next year with some motor work!!!" Blethen

RST Performance Racing

RSTPerformance
10-06-2003, 09:48 AM
Originally posted by joeg:
[B][I thought if you went oversize on the pistons, that also raised compression.]
B]

That is the reason that if you go over .040 and only shave .020 you get the same result as shaving .030

Thanks so far for the replies. I'll keep spitting questions or info at you if you need it. I really appreciate the help http://Forums.ImprovedTouring.com/it/smile.gif

And Raymond go away don't read this thread you build your engine your way and I'll build mine http://Forums.ImprovedTouring.com/it/smile.gif This was my "secret" question so I could beat you!

Stephen

Prophet
10-06-2003, 10:37 AM
The way I understand it, is this. Valve timing for a SOHC or DOHC motor is run off the crank pulley. There is a certain distance from the crank to the head. When you mill the head even the slightest you reduce the distance from the head to the crank. In essance the cams will run slightly faster (maybe even slower) and also move closer to running out of time with the crank.

I have also been told that the tensioner plays into the whole timing issue. I have someone helping me prep my motor and he says we should mill the head, but I have reservations on this issue.

-Jeremy-

apr67
10-06-2003, 11:18 AM
Jeremy.

Milling the head on an OHC motor does not make the cams run faster or slower. The ratio between crank and cam rotation is set by the pullys. In any motor that I have ever seen the ratio is, crank rotates twice while the cams rotate once. That is why the crank pullys are half the diameter of the cam pullies.

Milling the head affects cam timing. Depending on the motor it can advance the timeing or retard it, just a tiny bit. You can return it to factory spec per the rules or leave it alone.

One thing that no one is taking about is the affect of milling the head on the combustion process. And who is to say you want to take the same amount off on both sides? Hmm?

Quickshoe
10-06-2003, 11:41 AM
The way I understand it:

Milling the head/block moves the cam closer to the crank. You have to offset the cam sprocket on the cam to put the valve timing back to stock.

I don't think that there is any real advantage (HP) to any of the options you have. Either way you are getting approximately .5CR gain. Going max overbore means you'll need a new block or sleeve when it needs to be rebuilt in the future. However this will give you the max legal displacement and the higher CR. Don't know how much dirtier your combustion chamber will be...?

Milling the head will put the valve closer to the piston. How much clearance do you have when the valve is at maximum lift?

Go Bigger or Shave? I've always heard that shaving it will make it look bigger http://Forums.ImprovedTouring.com/it/wink.gif

Joe Craven
10-06-2003, 12:09 PM
The rules state that you can bore the cylinders .040 over standard size. It's listed in the ITCS under 17.1.4.D.1.j

"Engines may be bored to a maximum of .040 inch over standard bore size. Factory oversize replacement pistons or their exact equivalent shall be used.

If factory standard gives a tolerance of +.002, then one can only overbore to .040 + .002 max. If one bores a bit more to get additional piston to cylinder clearance, then I'd say that they could lose in a protest.

37 ITB

RSTPerformance
10-06-2003, 12:34 PM
Originally posted by Quickshoe:

I don't think that there is any real advantage (HP) to any of the options you have. Either way you are getting approximately .5CR gain. However (going max bore) this will give you the max legal displacement and the higher CR.



Ok so we are getting closer to my answer.

Is it better to have that extra displacement and the same CR if you can or is it better to just take the .030 off? Which produces more HP and which produces more torque? Or am I just digging at something useless? Remember Money doesn't matter and I don't care about rebuilds this is a one time deal (Pretend we are a pro team with unlimited budgets http://Forums.ImprovedTouring.com/it/smile.gif but we want the best motor!)

I could shave the head to about .039 before you see little dings. No I've never tried but I know tons of people that rally them!

PS: I know I am being annoying but you guys are my best resource for learning this stuff. I stubled across this idea by accident. I asked some others and they told me just to stick a turbo in it and forget it!


Stephen

Quickshoe.... If ony I joined this forum when I was still in college!

67ITB
10-06-2003, 03:16 PM
Stephen,
Why not do both ???
1. As I understand it the overbore pistons alone wont raise the CR at all because the piston shape MUST stay the same… dome/dish/valve, ring thickness pin height relationship and all that good stuff .bla bla bla (ITCS page 10 of IT section) not trying to quote rules, trying to learn more myself.
2. So as I read it you can go .040 over, AND deck the head. But the increase in displacement “may” eliminate any increase in compression.
3. As for decking the block, is that a factory thing ? I couldn’t find it in the rule book,
but it sounds like the way to go, as you can keep the stock “swirl” of the combustion chamber. I know the VW’s have a lip that would be eliminated if shaved to much
Something that decking the head would almost certainly mess with

Again this is as I understand it and I have never built an IT motor, but I have talked to a guy that has done a “few” hahaha

Lastly, you boys don’t need to be going any faster. Just pull a plug wire off and run on 4 cylinders =)
Good luck with what ever you do

Matt

MarkL
10-06-2003, 03:49 PM
67ITB says:

1. As I understand it the overbore pistons alone wont raise the CR at all because the piston shape MUST stay the same… dome/dish/valve, ring thickness pin height relationship and all that good stuff .bla bla bla (ITCS page 10 of IT section) not trying to quote rules, trying to learn more myself.
2. So as I read it you can go .040 over, AND deck the head. But the increase in displacement “may” eliminate any increase in compression.
=============================================

Actually, if you overbore the cylinder any amount, it increases the displacement of the cylinder, and if you left the combustion chamber volume the same, you are now trying to squeeze more uncompressed air/fuel charge into a smaller volume: compression ratio (volume at BDC including gasket thickness and combustion chamber volume) divided by volume at TDC. The compressed ratio...get it?

joeg
10-06-2003, 04:20 PM
Decking a block is effectively the same thing as decking/milling the head.

However, it is easier to shave the head rather than the block and you can take a deeper shave at the head.

Decking the block may bring the deck surface too close to the piston's ring lands; you should only deck a block no more than a few thous and it is done more to ensure flatness and promote gasket sealing rather than bumping compression.

It also gives you the same cam timing issues as shaving an OHC head.

Here is more food for thought--"stroking" the stock crank to increase displacement and CR. This is done with offset grinding the rod journals. You need a standard crank and a set of maximum sized bearings for the deepest undersize grind on such crank. You obviously also need a skilled crank machinist and a rebalance. This process moves (lowers) the center centerline of the rod journals in relationship to the block's deck. We are talking small changes here.

That would be borderline illegal because it changes the bore/stroke relationship ever so slightly, but it will bump compression.

Cheers.

badal
10-06-2003, 05:18 PM
Borderline illegal?
The stroke is specified, no allowance for altering it.
The .040 overbore is allowed.

Stephen, I would do the bore first. It will get you more displacement, more HP, more torque, and will help unshroud the valves.

Do the math and figure out how much more displacement you will get. Then figure your hp per cubic inch. If you get 3 more cubic inches, and you were making one HP per cubic inch, you have gained 3hp from the overbore, plus a little from the CR increase.

This assumes:
Pistons are available
It is safe to overbore your type of engine that much.

------------------
"Bad" Al Bell
ITC #3 Datsun 510
DC Region MARRS Series

67ITB
10-06-2003, 05:23 PM
MarkL,

I see exactly what you are saying about the combustion chamber.
Thanks for the example.

joeg,
Getting back to the block, is decking it legal in IT ? or is this a manufacture spec ?

Thanks for the info

Matt

gsbaker
10-06-2003, 06:00 PM
Originally posted by Quickshoe:
Go Bigger or Shave? I've always heard that shaving it will make it look bigger http://Forums.ImprovedTouring.com/it/wink.gif


Quickshoe,

At least no one has asked about stroking the motor. http://Forums.ImprovedTouring.com/it/eek.gif

------------------
Gregg Baker
Isaac, LLC
[email protected]

gsbaker
10-06-2003, 06:04 PM
Seriously, I don't know the answer offhand, but if you guys want to get into the heavy stuff, you might want to slip into: http://eng-tips.com/



------------------
Gregg Baker
Isaac, LLC
[email protected]

itmanta
10-06-2003, 07:50 PM
Stephan, your question is hard to answer due to the fact that the cumbustion process happens differently in every motor. Your answer is best answered by someone who has done development work on the engine in question.

In general you will definately want to go with the largest overbore.
As far as raising compression. As mentioned before milling the head or decking the block will effect cam timing. This will effectively change the torque curve of your motor in relation to RPM. This could work to your advantage or against it. Either way the change will be small, maybe a few hundred RPM. The one advantage to decking the block would be to tighten the squish area helping VE and reducing the chance for detonation.


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Peter Linssen
ITB Opel Manta
Pacific NW Region

Quickshoe
10-06-2003, 11:52 PM
Originally posted by itmanta:
...The one advantage to decking the block would be to tighten the squish area helping VE and reducing the chance for detonation...

Speaking of squish area...If you mill the head/increase the bore without being able to reshape the head, are you creating a nice place for detonation to occur? Also, If the VE/BMEP falls off more than the Volume is increased won't the net power be less?

Interesting topic...I'll let the guys with the money and time figure it out.

joeg
10-07-2003, 07:05 AM
At least no one has asked about stroking the motor. http://Forums.ImprovedTouring.com/it/eek.gif

Scroll-up a couple posts, Greg.

Just talking hypothetical, now!

ITA_CRX
10-07-2003, 08:29 AM
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
10-07-2003, 09:35 AM
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
10-07-2003, 10:20 AM
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

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

apr67
10-07-2003, 10:32 AM
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
10-07-2003, 04:38 PM
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
10-07-2003, 05:48 PM
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
10-07-2003, 06:22 PM
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! http://Forums.ImprovedTouring.com/it/wink.gif



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

rcc85
10-07-2003, 08:31 PM
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
10-07-2003, 08:36 PM
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
10-07-2003, 08:42 PM
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
10-07-2003, 10:33 PM
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.....



http://Forums.ImprovedTouring.com/it/eek.gif 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
10-07-2003, 11:01 PM
" 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
10-09-2003, 11:57 AM
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 (http://www.tsr-performance.com)


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

MMiskoe
10-12-2003, 07:53 PM
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