corrected vs uncorrected comp ratios???

bclapham

LIFETIME SPONSOR

Nov 5, 2001

4,340

0

• Apr 21, 2004

• #1

Hi Guys:

i am reading up on Bells book, and i get the impression that he has little time for the way corrected comp ratios are calculated.

however, from reading Erics book, i see that when he suggests exhaust port changes, he then states that the head should be milled down by the same amount as the port was raised so as to keep the comp ratio the same.

so, what is everyone doing, i assume since Erics book is more recent and he has more experience with jap MX engines that this is the way forward.

so this leads me to further questions.

1. what is the best way to measure exhaust port height?

2. what is the best way to measure deck height?

3. manufacturers often quote two comp ratios, eg PV open and PV closed- since the PV doesnt actually seal against the piston, i assume that the higher comp ratio for PV closed does not figure in to the grand whole scheme of things?

thanks

BC

 

J

JTT

~SPONSOR~

Aug 25, 2000

1,407

0

• Apr 23, 2004

• #2

Anybody?? I'd also be interested in understanding this better.

 

Rich Rohrich

Moderator / BioHazard

LIFETIME SPONSOR

Jul 27, 1999

22,839

16,904

Chicago

• Apr 23, 2004

• #3

bclapham said:

2. what is the best way to measure deck height?

Click to expand...

I'll answer #2, but I'll leave the other questions to Eric. He'll explain it better.

The best way to measure the deck is with a dial indicator and a simple stand (sometimes called a deck bridge)that sits on the deck. The dial indicator is zeroed out on a perfectly flat surface and then the stand is dropped on the deck in a few places. Plus or minus readings from zero shows the true deck height.

 

Attachments

• deck_height.jpg

  50.7 KB · Views: 112

M

MikeS

LIFETIME SPONSOR

Jun 12, 2000

716

1

• Apr 23, 2004

• #4

I think the old way to measure height is to use a flat machists ruler. Not to accurate but it gives a general idea. I am sure the pros have a real way...Eric , Rich.

A buddy of mine works for a large machine shop. He said they have a Computer aided device that can interpret a piece and then draw it out. (CAD). I asked him if he can do a cylinder for a bike. I gave him an old RM85 cylinder and he said he will give it a try. I guess I will owe him a paint job from my body shop if he gets it to work out. He said he has done alot of complex shapes.

 

bclapham

LIFETIME SPONSOR

Nov 5, 2001

4,340

0

• Apr 23, 2004

• Thread starter
• #5

Rich: thanks for answer #2- i am quickly realizing that there is probably lots of power to be had with a stock engine, just by tightening clearances up and getting things where they should be. i just dont have the $$$ to put down on tools, i was hoping you was going to say something daft like a tooth pick, straight edge and some calipers, LOL!!!:)

OK guys, i have more questions, the next one being secondary, or rather crank case compression ratio- how much effect does small changes make on how the engine should "theoretically" work?

for example, say you use an 8mm longer rod and spacer you loose crankcase compression- is this significant? in my case, i dont think it will be since ive got a bigger piston and that must have increased that ratio? (assuming a bigger piston increases that ratio)

 

Rich Rohrich

Moderator / BioHazard

LIFETIME SPONSOR

Jul 27, 1999

22,839

16,904

Chicago

• Apr 23, 2004

• #6

Bruce - The indicator stand costs $21 and you can pick up a dial guage for $9 from J&L or Enco. You spend more than $30 on graphics :)

 

bclapham

LIFETIME SPONSOR

Nov 5, 2001

4,340

0

• Apr 23, 2004

• Thread starter
• #7

Rich Rohrich said:

You spend more than $30 on graphics :)

Click to expand...

LOL- actually i dont, nearly everything i own comes of ****- my riding buddies are actually ashamed of me with my mismatched bike and kit!

i wonder if i can get one of them dials in mms?????? :eek:

when did e b a y become a swear word???

 

Rich Rohrich

Moderator / BioHazard

LIFETIME SPONSOR

Jul 27, 1999

22,839

16,904

Chicago

• Apr 23, 2004

• #8

bclapham said:

when did e b a y become a swear word???

Click to expand...

It always was in the past to discourage people from using the forums as a feeder to their online auctions. The software upgrades broke the filter but Okie fixed it recently.

Last time I checked they will still sell you one of those crazy mm dial guages but you have to order it in English instead of English. :laugh:

 

bclapham

LIFETIME SPONSOR

Nov 5, 2001

4,340

0

• Apr 23, 2004

• Thread starter
• #9

LOL- i am off to UK next week, i might pick one up then, and some proper sized drill bits as well!

 

B

billyghog

Member

Jan 31, 2004

12

0

• Apr 26, 2004

• #10

bclapham said:

I might pick one up some proper sized drill bits as well!

Click to expand...

Letter, Number, and Fractional?


:laugh:

 

bclapham

LIFETIME SPONSOR

Nov 5, 2001

4,340

0

• Apr 26, 2004

• Thread starter
• #11

milimeters lad!! have i taught you nothing?????