My shot at computing "interference" point for a more aggressive cam

[drj434343]

I have a rebuilt B21F, 0.024" off the head, 0.027" Cometic, piston TDC zeroed with the deck. My best guess is ~10.5:1 CR.

I'm currently running a B cam with stock K-jet. To try and better utilize my higher CR, I've obtained a 017 dizzy from a B23E and a K cam.

I wanted to see if the K cam would be interference in my engine. Here is what I think I know.

Stock 398 head combustion chamber height = 12mm

12mm - 0.6096mm (head shave) + 0.6858mm (Cometic) = 12.0762 mm working height

K cam lift = 11.95 mm

I've read to expect 0.001" rod stretch per 1K RPM, so 8K RPM = 0.2032mm max stretch

12.0762mm (working height) - 11.95mm (lift) - 0.2032mm (stretch) = -0.077mm INTERFERENCE

Working backwards a bit, the break even point for stretch is 0.0049", which means that the engine will become interference at 4900 RPM.

Can the stackup I did here be considered accurate? Are there other contributors I'm not including?

 

[John V outside agitator]

where to begin?
You're doing it wrong.
Here loo at this:
intake
Duration exhaust
Duration Overlap lift
intake lift
exhaust lobe centers
intk / exh degrees
ret/adv ................................................................................valve events intake / exhaust..btdc abdc bbdc atdc
256 256 38 .469 /11.9mm .469 /11.9mm see text zero .............................19 57 57 19


Intake opens 19 degrees Before TDC, closes 57 degrees after... so at TDC its only opened a liiiiiiiiittle tiny amount. Max valve lift will be somewhere about right in the middle or right around 104 degress AFTER TDC.

So remembering the crank only goes 180 degrees from TDC to BDC where's the piston when the crank has turned 104 degrees? way down the hole..

Many of the furriner Cam specialist list "valve lift at TDC"

here's a way way wicked cam for a volvo 8v SOHC from Piper in Pommieland:
http://www.pipercams.co.uk/pipercams/www/product_nav.php?type=C&cat=CM&man=29&engine=89

Lookie: fawking formatting!
Part No. VB18B320
Application RACE
Power Band 3700‑7500
Power Increase
Duration 314? 314?
Valve Lift 0.506″....12.85mm 0.504″/12.80mm

Timing In 53‑81---Ex 81‑53
Full Lift 104? 104?
Lift @ TDC with clearance In 0.180″/4.57mm Ex 0.178″/4.52mm

Valve Clearance In 0.016″ / 0.41mm EX 0.018″ / 0.46mm

Now that's a nice camshaft But look at the highlighted thing lift at tdc, and max lift at 104 degrees . No worries of interference even with this much nastier, rad-der, much funer-er cam (That's the one beauty of valves inline with bore)


And we just got some contemptuous condescending crap from the guy who wrote about what so many of you have taken as a trurth, the absurdity, the insane folly to sugggest "rods stretch 001' per 1000 rpm".
That misunderstnding or misreading was shown to NOT be a rule of thumb with the simple thought excercise: Is the rod stretching .001 at 1000 rpm? no is the rod stretch .001 at 2000 rpm? no--
See?
Alsow a moments thought and we see theorertical stretch is entirely dependant on weight of half the rod and 100% of pistons weight x speed

Ie a 880 gram piston and the reciprocating half of an 890gram rod at 7000 rpm is going to stretch a lot more than a 340gram piston and half of a rod 140g lighter... at 7000 rpm.

So when I say "You're doing it wrong" do you see the error?
All your assumptions and methods are all wrong.
Conclusion: mill the hell out of the head and get a gnarly cam, just get some valve spring..

(And note: when I say mill 050-060 off it is because even with that K cam the crank is still only going 180 from BDC to TDC BUT intake valve is open for gawddamn near enough to 1/3 of crank rotation from BDC to TDC and it probably isn't making much compression when there's a big 44 or 46mm hole open in the head until 57 degrees ABDC (60 woulb be 1/3 of 180)

So in order to have ANY compression worth a damn in the 2/3 of rotation left to do everything in, you gotta mill the hell out of the head.

also. don't guess. measure.
Don't believe, know.

No, go and sin no more.

 

[Canuckvolvo]

John is right, during normal operation, it won't interfere, but you're probably concerned about if/when you break a timing belt at 6000rpm. If you do proper maintenance, that should be an issue.

In case you are worried about that, keep in mind, the valve protrudes into the combustion chamber, even when it's closed. Probably something like 2mm, so you have to take that into account.

Also, the spark curve for the 017 dizzy is VERY different from that of the standard b-cam dizzy. It will probably suck with the B cam. Match the spark curve with the cam profile, not the static compression ratio (well in truth you want to match with the DYNAMIC compression ratio).

 

[drj434343]

OK, I thought it was a safe assumption I was talking about belt breakage here, so my fault. I see John is making the argument for why interference isn't possible during normal running, which I understand. TDC never happens during full valve lift is a much simpler way of saying that.

I'm thinking about a worst case scenario break, one valve stops at full lift and the bottom end is still rotating. So to that end, the only useful comment from John was that there can be no uniform rule regarding rod stretch. If we don't even have a ballpark for these engines, then there is no point in even considering interference computations; the stretch could be 0.001" or 0.1" or anything in between.

I'm also planning on changing to the 017 dizzy at the same time I go to the K cam for a better match.

 

[JohnMc]

Install cam.

Rotate to cam lobe 'tdc'.

Measure how much valve protrudes down past plane of cylinder head.

Subtract from that the thickness of the HG.

Add/subtract from that the piston deck height at TDC.

Voila. Add in some picometers for rod stretch if you feel like it.

I'ts not really something I'd worry that much about. If your'e that worried, just get a nice blue kevlar timing belt and replace it at half the suggested mileage. Don't ever ponder going 16V, that ish will self destruct if the timing belt jumps a couple of teeth.

 

[John V outside agitator]

OK, I thought it was a safe assumption I was talking about belt breakage here, so my fault. I see John is making the argument for why interference isn't possible during normal running, which I understand. TDC never happens during full valve lift is a much simpler way of saying that.

I'm thinking about a worst case scenario break, one valve stops at full lift and the bottom end is still rotating. So to that end, the only useful comment from John was that there can be no uniform rule regarding rod stretch. If we don't even have a ballpark for these engines, then there is no point in even considering interference computations; the stretch could be 0.001" or 0.1" or anything in between.

I'm also planning on changing to the 017 dizzy at the same time I go to the K cam for a better match.

Here's another useful comment:
Don't break belts, not when they only cost $16

And remember, dozens and dozens of people mostly LURK.
Many people have asked similar, and similarly vague questions about valve interference..
Most of the answers here blabbed on and on about everything except reality

Rubber band goes sproing-g-g! and depending on revs the belt has fallen off, rotating the cam and closing most of the valves somewhat. It is highly statistically and mechanically unlikely that the cam will stop precisely balanced on the top of some lobe(s) the valve leaving valve at MAX LIFT

But DON'T BREAK BELTS..
Don't worry. be happy.

 

[vvpete]

I have a rebuilt B21F, 0.024" off the head, 0.027" Cometic, piston TDC zeroed with the deck. My best guess is ~10.5:1 CR.

Agreeing with what John and others have said, except that your static compression ratio even with flat top pistons and zero deck height is any higher than 9.7:1 for a B21F given what you've done. You'll have to shave a lot more than .024 to go over 10:1

I did a B12F build and the 398 head shaved 0.020, .035 HG, flat top Mahle forged pistons, and ended up at 9:5:1, which included a fair amouint of chamber porting work, ~58cc.
Measure it. Don't guess.

 

[John V outside agitator]

Agreeing with what John and others have said, except that your static compression ratio even with flat top pistons and zero deck height is any higher than 9.7:1 for a B21F given what you've done. You'll have to shave a lot more than .024 to go over 10:1

I did a B12F build and the 398 head shaved 0.020, .035 HG, flat top Mahle forged pistons, and ended up at 9:5:1, which included a fair amouint of chamber porting work, ~58cc.
Measure it. Don't guess.

That's why I said lop off 050-060! Getting dynamic comp over 10:1 is FUN! Yep yep yep...
We did Tbricker "freya' 2.1 with a KG8 cam and we needed to get static up to around 11.7 to have 10.2 dynamic.. But it was a big eye opener for him how strong it pulled.

 

[badvlvo]

Here's another useful comment:
Don't break belts, not when they only cost $16

And remember, dozens and dozens of people mostly LURK.
Many people have asked similar, and similarly vague questions about valve interference..
Most of the answers here blabbed on and on about everything except reality

Rubber band goes sproing-g-g! and depending on revs the belt has fallen off, rotating the cam and closing most of the valves somewhat. It is highly statistically and mechanically unlikely that the cam will stop precisely balanced on the top of some lobe(s) the valve leaving valve at MAX LIFT

But DON'T BREAK BELTS..
Don't worry. be happy.

Installed and tensioned properly, without oil leaking on it, with a proper cover, properly functioning tensioner, it should not fail. They don't just break at random, if it breaks you are doing something wrong.

 

[smokeyfan1000]

Change the T- belt @ 35, 000 mile intervals instead of the Volvo recommended 50,000 miles if you're worried about belt breaking .....

 

[CAPT_BLOTTO]

john your posts are always so colorful

 

[John V outside agitator]

Installed and tensioned properly, without oil leaking on it, with a proper cover, properly functioning tensioner, it should not fail. They don't just break at random, if it breaks you are doing something wrong.

Yep. It's not really stressed. Even if guys end up with those dumb Bugpack VW aircooled springs with dumb 140lbs seat pressure its still not that bad...it is only valve springs that resist..
On both Volvos and Ford 2300 SOHC, those that I've seen break are either drenched in oil forever, or so ancient they're older than me, and I'm old as dirt.

Note that the Ford valves are ever so slightly canted over and never ever say damage in any of the cheap TB-like Xrattistas who ran these weather checked, fraying belt to the failure point..
As I often have said, there's something to be said about valves inline with the bore like Volvo, or say VW watercooled.
So, belt breaks and at worst the piston gently closes the valve.
The imprint in the carbon tells you what happened..

 

[John V outside agitator]

Agreeing with what John and others have said, except that your static compression ratio even with flat top pistons and zero deck height is any higher than 9.7:1 for a B21F given what you've done. You'll have to shave a lot more than .024 to go over 10:1

I did a B12F build and the 398 head shaved 0.020, .035 HG, flat top Mahle forged pistons, and ended up at 9:5:1, which included a fair amouint of chamber porting work, ~58cc.
Measure it. Don't guess.

Numbers are good.
Knowledge is good

 

[satanburns1]

my OEM belt has no cover soaked in oil with a newer tensioner and I've been abusing it for over 100k. I have a belt that I took off a customers car at 50k Interval sitting in my pile o parts. I don't worry but this motor is stock, beat and being replaced. and the fresh one will have kevlar...


Posted from Turbobricks.com App for Android

 

[vvpete]

Here's another useful comment:
Don't break belts, not when they only cost $9

Fixed

Last belt I bought from Tasca in July. A Volvo belt, less than a Conti, and won't strech under load like Conti belts by 40k. I can buy 8 volvo belts, change them at 40k and still have more $$ left in my pocket than if I bought a Kevlar belt.

If I really wanted to ensure belt integrity, I would convert to the round tooth belt. (if it can be done?). Never seriously looked into that, tho.

 

[towerymt]

If I really wanted to ensure belt integrity, I would convert to the round tooth belt. (if it can be done?). Never seriously looked into that, tho.

Can. I use round tooth on my 8v B23 with a crank gear from Knox. Others have sold B21/23 round tooth crank gears.

 

[smokeyfan1000]

Also remember to periodically re-adjusted the timing belt tensioner, about 500 miles after installation & then, ...say every 3500-5000 miles.

I do......

 

[satanburns1]

Also remember to periodically re-adjusted the timing belt tensioner, about 500 miles after installation & then, ...say every 3500-5000 miles.

I do......

every season change I let the tensioner spring out a little more. I know Im pushing my luck but who cares since stock won't hit. that's why I say currently @ stage -5. I just refill the oil weekly and beat it. the fresh setup will be fresh and very well cared for.


Posted from Turbobricks.com App for Android

 

[smokeyfan1000]

I have stock cam, and adjust my belt tensioner @ every oil change or so............