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

RSTPerformance

RSTPerformance

New member
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).]
 
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).

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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608
 
[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.
 
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
 
Originally posted by joeg:
[I thought if you went oversize on the pistons, that also raised compression.]
B]
Click to expand...


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
smile.gif


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

Stephen
 
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-
 
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?
 
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
wink.gif
 
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
 
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.
Click to expand...

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
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!
 
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
 
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?
 
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.
 
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
 
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
 
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.


------------------
Peter Linssen
ITB Opel Manta
Pacific NW Region
 
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...
Click to expand...

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.
 
At least no one has asked about stroking the motor.
eek.gif


Scroll-up a couple posts, Greg.

Just talking hypothetical, now!
 
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