Technical MOTOR, Compression Timing

Here's one I wrote quite a while back.
Old hat to some, perhaps new to others.
Anyway, here tis....

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Compression Timing

Compression timing is my own name for a little trick that I’m sure most of you have heard about.

Something I read about many years ago, thought it was a good idea, but never got around to trying it.

It goes like this:
Get the basic short block built up.
No cam as yet.
Install the starter and flywheel/flex plate.
Locate true TDC.
Bolt on your degree wheel.
Install one or both fully assembled heads. although #1 cylinder will have a pair of the “valve clearance checking” springs. (Simply a very light duty spring, easily compressed by hand and basically used to prevent valve-piston conflicts.)
#1 cylinder is also the only cylinder with lifters, etc. installed.
Install the cam, straight up.
Check valve-piston clearances as usual.
If ok, change the cam timing to about 8 degrees advanced.
Check clearances again.
If ok, change the cam timing to about 8 degrees retarded.
Check clearances again.
A bit of extra work, but much cheaper than a broken valve or damaged piston.
Since most cams are run either straight up, or up to about 4 degrees advanced the 8 degree figure will give you a good safety factor.
Remove the “valve clearance checking” springs and install the outer springs that you plan to use.
Again, on #1 cylinder only.
Gotta start somewhere, so set the cam at 4 degrees retarded.
Hook up a screw-in compression tester to #1 spark plug hole.
Spin the engine over with the starter and make note of the compression pressure.
Do this a couple of times and average it out.
Repeat this test with the cam installed straight up and also with it advanced 4 degrees.
And you may even want to do it at 2 degrees advanced or retarded depending on which way the compression figures are going.
For me, 4 degrees one way or the other is about as far as I wish to go, although I have heard of a couple more degrees being plugged in.
Key thing is, piston-valve clearance.
Or put another way, bring more $$$.
Once you find the highest compression pressure set the cam there and finish bolting the engine together.
An easy and very good method to have the cam timing set at the max HP levels right at the start.
A couple of other things enter into it, but for most of us, it’s not a big deal.
This will do the trick.




The other bit alluded to in the timing the cam for maximum cylinder pressure also involves some re-thinking about ignition timing.
Since I’ve had this little technique in the back of my mind for a few years and figured I’d give it a try on the next engine we build up, it may give us a running start there.

I was very fortunate to get the information about timing from a very reliable source and he points out that less ignition total advance is required.
Something along the lines of 28-30 degrees BTDC, all in.
Since he was probably talking about the SBC and maybe the BBC which run all in timing figures of about 34-36 degrees BTDC, it looks like our 462" Buick will be retarded quite a bit.
That’s ok, it’s not the numbers that make HP, it’s having it right.
Buicks, at least the 455's, recommend a total advance figure of 32 degrees BTDC all in.
That’s for both the stock and hot rod engines.
Due in part I think to the 4 5/16 bore and 3.9 stroke.
And of course the combustion chamber enters into it.

Since we’d decided to start with 9/1 forged pistons and as little brother points out, the high compression forged pistons are not too much more expensive we really should start with a set of 11/1's at least.
He likes compression, and lots of it, with 15/1 being his personal best. (On an alcohol 498" Olds engine)

One thing desired with our 462" Buick powered dry lakes 31 A roadster project is to stay away from racing gasoline and additives.
No special reason, just because, and it’s just easier to find and run 92 octane pump gas.
Depending on the heads we use, the 11/1's should end up at 10.2/1 or 11/1.
And maybe just a touch lower if we enlarge the combustion chamber slightly by unshrouding the valves.
Running around 4-6 degrees less of ignition timing ought to do the trick as far as using pump gas goes and BMEP/Torque figures should be better at the retarded position regardless of octane used.
92 octane pump gas vs. the racing stuff anyway.

Big thing to keep in mind here, is that compression ratio and compression pressures are two different things.
Compression ratio being in effect a theoretical figure.
Compression pressure being where it’s at, as far as the combustion chamber is concerned.

And compression pressure can also be called the “effective compression ratio”.
A figure easily changed by several small things, but the easiest by simply changing cam timing.

One small thing to keep in mind if you’re running say a 10/1 CR quench type engine using a .040 compressed thickness head gasket.
If detonation raises it’s ugly head, simply adding another .040 compressed head gasket won’t do the trick.
It will cut the CR down to say 9.5/1, but since the quench and swirl characteristics of the combustion chamber are altered, most times the detonation problem becomes worse.
To combat detonation you may have to:
Run a higher octane fuel - not always easy or desired.
Retard and/or change the curve on the ignition timing - easy, but not always what we want, HP goes out the door.
Richening the mixture helps too - although if you go too far with the excess fuel bit to get rid of the detonation, you may end up killing some HP.
Unshroud the valves, which brings the CR down since the combustion chamber is larger.
Make a camshaft change, paying particular attention to the intake valve closing point.
Probably the best - since you’re involved with a performance engine and if you don’t go overboard with the cam timing figures it will still run ok on the street.
Provided the gearing is reasonable.
It helps too if you’re running a big block and the bigger the better.
They can handle more cam timing than the smaller small blocks and still be very tractable on the street.


Circle track guys running solid lifter cams, specially at dirt tracks will many times change the valve clearances depending on whether the track is loose or has good bite.
Good bite means the clearances get loosened up a touch effectively de-tuning the cam a bit and the torque peak comes on a little lower.
Same thing with a loose track, clearances get tightened up a touch raising the torque peak and spreading it out a bit.
If you think about it, you can see the looser settings effectively make the cam a milder one and the opposite is true with the tighter settings.
Lift starts earlier with the tight settings and the cam is in effect more radical.
All small changes for sure, but one the circle track tuners have done for many years.
Just part of the Black Magic they seem to drag out now and then.
For some, tuning is an art as well as a science.


An interesting short story about brother in-laws 1961, 413 Dodge with cross-ram etc.

It was stock and a strong runner.
Darned thing would pull the left front tire off the ground on the launch and break the front seat brackets now and then after he put cheater slicks on.
I could still beat him every time with my Olds engined 50 Ford coupe.
Rolling starts with street tires for both of us and usually done at 3000 rpm in low when my coupe was on the cam and pulling hard.
He got wise though, and started forcing the start at a much lower rpm.
Bog city for the coupe or slip the clutch and spin the tires.
After that, it was all over except for handing out the money.

An interesting lesson for both of us as far as using the strong points of a particular engine combo.
Especially in the street racing game.

So here’s the interesting part.
A lot of the early muscle cars had 11/1 or so compression ratios.
Granted, gas was pretty high octane in the “good old days”, about 102 on the left coast and the Blue Sunoco 105 good stuff on the right coast.
Although octanes were figured a bit differently then and the same gas today might come in 4 points or so lower.

The big Dodge ran good and ran hard and like any “normal” hot rod guy, brother in-law decided it was time for a big cam.

As you know, one of the very best ways to get the “right” cam for what you want to do is follow the mfg. recommendations.
Something that took me a while to learn.
Although I have made a few good choices.

Heck, we were hot rod guys, just give us the Ricky Racer cam and we’ll go out and kick some serious heinie.
Not a problem for the cam mfg., they’ll sell you what you want, but you better know what it is that you really want.
So with a smile, the cam guy hands over the Ricky Racer cam and we sped home and popped it in the big Dodge with big expectations.
One of those afternoon to after midnight thrashes.

Fired it up, broke it in, idled great.
Sounded wicked and mean.
Just the thing for us.

Course now the car detonated at mid throttle settings and mid rpm levels.
We did a few things, mainly to the distributor timing curve, but to no avail.
Brother in-law gave it up and just drove it.
The rattling drove me nuts, he got used to it, and it never did create any problems.
The car may have been faster, but not by much.
And I used to wonder why a simple cam change would create detonation in an otherwise good running engine.

Well, a few months ago I was doing a little reading and the answer was right there in black and white.
The car mfg. that built some of these big high compression engines set up the cam timing events with a very late close on the intake so as to bleed some compression pressure off.
Making an 11/1 CR engine equivalent to an 8/1 CR engine.
Since our Ricky Racer cam was a symmetrical grind and probably closed the intake valve too soon trapping too much cylinder pressure within for our street engine that’s where the detonation came from.
An interesting answer to a past problem, but one I had wondered about for a long time.

Too dumb at the time to think about:
When did the problem start?
What did you change?
The answer was there and very clear.

Course what young hot rodder wants to pull a for real Ricky Racer cam?

Heck, we be stylin now.......

 

AWESOME post, C9!!

That's something alot of guys don't often think about, but yes....using cam timing to bleed off the cylinder pressures at lower rpm levels helps to make an engine much more flexible over a broader rpm range. This 'trick' has been coming to the surface more lately as engine guys try to find bigger power numbers with today's pump gas offerings.

Tech-O-Matic material for sure!!!

 

I know what yer sayin about the big cam/ cylinder pressure deal! My motor's 11.6:1 and I run pump gas in it most of the time. I'll usually dump in some octane boost just for peace of mind, but it isn't necessary. I do run the good stuff when I can afford to smell good, and it does seem to run a little better, but I haven't seen or heard even one little hint of Detonation in 3 yrs of driving it!
It's an aluminum headed sbc, and the cam's a solid roller (268/270 @.050, .632/.632)

 

Good info C9 I also would like to try that.Not that I knew about it.Ive printed the post to save thanks.I love trying to choose cams I have every major cam catalog.Seems like the more comparison between them the more confusing they get.None of them make the same exact cam. The lift-duration-lobe centers will be real close BUT the usage and power band will differ.So what youre saying makes total sence to Me.So if you experimented with compression timing for power and detonation the information you need is on the cam card. That tells you opening and closing events,and ofcoarse you only get that when you buy a new cam.That also explains the difference of what I mentioned above.Am I right here? GOOD READ----FEDER

 

Think this belongs in TECH O-matic.

 

Hot Rod to Hell's aluminum heads allow about a half point increase in the CR before detonation rears it's ugly head.
From what I've read anyway.

There's a lot of info out there for most if not all of the cam mfg's.
I like Crower Cams and their catalog as well as the Crower info in the PAW catalog shows most of what you'll want.

Far as changes to lift, duration and lobe centers go, potential usage - depending on what you want to accomplish - will change.
And changes in any of them affect the power band as well, but it's not always real noticeable in seat-of-the-pants testing when the cams are fairly similar.

I have a KB cam and Crower cam with virtually identical lift and duration figures. Advertised duration runs in excess of 290 degrees for both
The difference between the two comes with the lobe centers.
The KB is 118 degrees and the Crower is 112 degrees.
The KB runs 118 degrees as that calms the racy cam lope down quite a bit and the cam is advertised as KB's biggest cheater cam.
It does have a racy lope when the engine is cold - even with choke/chokes off, but when fully warm it sounds a bit like an old tractor idling.
Just the thing for a sleeper motor.

The Crowers lope stays pretty much the same throughout the engines thermal range.
Which - with my absolutely unscientific testing methods (stab it and hang on) - makes the Crower the preferred cam when I run the dual quads.

Biggest real piece of advice I can give you is decide what cam you want to run - and try to be realistic about how you're going to use the car - then choose the next cam down.
Down meaning toward the mild side.

The present cam in the 32 is 260-266 advertised int and exh duration along with a good ignition and a 750CFM Carter.
It makes for a good crisp running street engine that responds to throttle like right now.

Lumpty lump sounds cruising through the drive in are cool, but having a quick responding engine is far better in my book.
Other advantages are, the redline recommendations are usually lower and that makes life easy for engine components.
Not to mention valve spring pressures are entirely within reason which leads to a long life for your cam.

I am - as you've probably noticed - a big fan of big blocks.
May as well start with some serious torque figures and go from there.
Along with that, I've semi-sorta decided that street big blocks need small cams and street small blocks need big cams.
All with the proper diff gears of course.

If you can't make torque, you may as well spin it and make some HP....

 

those were some great posts C9. what exactly does quench type mean? i have heard it used before but haven't really understood the term. is there another type as well? anyways great information. very much enjoyed reading all the posts.

 

My Son and I both use alum heads.From talking to the manufacturers they claim up to 1.5 points more compression before detonation over a iron head. And because of heat dissapation the iron head will make more power at the same compression 10 to one and below. Now granted thats with a worked over iron head that would equal the port-valve and runner size.The 428 in FEDSLEDS Merc has Edelbrock Heads and runs 10 1/2 to one compression.He does run premium fuel always and has noticed no pinging.Another thing you see now from the piston mfgs like Ross-JE-Probe and others is the lowering of rings.The FE ford is notorious for detonation and broken top rings.So now with alum heads and new lightweight pisons with lower rings some guys are claiming 11 to 1 comp with 92 octane. The aftermarket is really letting us tried and true non Bigblock chev owners make some real power these days,ITS GREAT!!---FEDER