Nova head clearance

[lundahl]

Hi.

I just tried to shim my 5 port Nova i bit tighter. But it ended up at 0.005. Do you guys think that is to tight?

Fredrik.L

 

[shoboat]

Fredrik,

We have run them as low as .003" but we like .006" usually on our 21's.

Test It And See,

Mark

 

[lundahl]

I will test it out.

Fredrik.L

 

[Jeff Schindler]

.05mm? If so that's too tight. I would target .20mm (0.008in) to .15mm (.006in) at the tightest.

 

[lundahl]

Hey Jeff.

No not 0.05 mm 0,005" so about 0,13mm.

Fredrik.L

 

[RodneyPierce]

.006

 

[steve wood]

.007 inches

 

[dave roach]

.0065 inches

 

[dwilfong]

.00666"

 

[anthony_marquart]

unless you have every single other variable nailed,.. I think .006 -,009 is just fine...

 

[RobertWurster]

I took out 1 of the thick stock shims and put in a 4 th shim and she runs good on 60 % .

 

[steve wood]

I run .005 but not with a stock head button

yeah you got run about .19 volume with the .007 clearance,and aluminum button,.17 to .18 volume with brass

 

[shoboat]

Greg,

You should try more intake timing.

Too much exhaust timing is not the way to go with a great pipe design.

More fuel More Power!

Thanks,

Mark

 

[TUNLBOTR]

It would be best to read all (intake/exh) for complete picture.

old KBs use to have 125 intake, where my RB C5 pulls 131 intake. Shoboat has the key to pulling props under his numbers.

Then you can calculate blowdown.

Total exhaust open duration

minus

Total transfer open duration

divided by 2

Above 30 degrees low end starts going away quickly.

.0065 works well on O/B's, but I am also switching to a Orlic Button, which carries slightly smaller bowl volume over stock button.You have to account for the total package to hunt the beast inside the motor.

186 exh

minus

131 intake

div.by 2

=27.5 blowdown.

It aint no slouch at any point in the range.

 

[shoboat]

Geg,

I am sure you have a good set of numbers.

I was just passing on that some blow up the exhaust timing numbers and then

They cannot shorten their pipe enough. I had a great 21 motor a few years ago that went

81 MPH in oval trim and it only had 178 degrees of exhaust timing. It could pull a pipe at

7" or less.

Thanks For Reading And Sharing,

Mark

 

[Jack ODonnell]

My pipe is 6 3/4 right now and I cant get it shorter.

I find 190 / 132 works for me with 221 total intake closing 68 atdc

Greg; The 190 exhaust timing you are talking about,is that the total exhaust port . J.

 

[RodneyPierce]

If Im a betting man, probably just the center port roof. Ive been taught to leave the satellite ports alone, and just raise the port roof on the center "big" port. And so far, it seems to be working for me just grand.

 

[dwilfong]

The side ex ports should open up first to get the flow going the right way.

Some eng like a small ex port it keeps the heat in the eng.

Big ex ports do not flow better thy just slow down the velocity of the charge exiting the cyl at one point.

Only need enough to get the spent gases out.

as far as clearance slam it down till it just hits then back it off.

When I stopped messing with the ex and spent the time on the transfer angles and port size the power came up and the pipe got shorter.

If you get more flow in the cyl you can pump up the CR and tighten the squash.

Open the intake at BDC and close at 65 ATDC let the eng breath with the bigger transfers.

If you have a good pipe it will take control of the eng the intake is just a hindrance at that point.

You just need the intake valve to run at off pipe conditions.

This is what I have found in my testing.

Every one has there point of view this is just my take on it.