too much compression vs. cam timing, what happens?

[dbusch]

ran a SBF engine recently with 15.5 compression (accidently, the pistons were ordered to yield a 14.0 compression ratio). the cam was designed for a 12-13.0 compression engine. thinking it was 14.0, i ran Sunoco Maximal in it and let it rip.

The engine would not make good power after back to back pulls and actually misfired a bit thru the rpm range. it was a horror to get it started, and would keep running a bit after shutdown. But, when we got it good and cool, it was a beast. It made good power and the misfire was gone. after i got it home, i checked cranking compression, and it was 270!!!

Question is, was it detonating or was some other deal going on here? I figured someone on here as done this (advertently or by accident) and has some feedback on this.


BTW, I do plan on going with a different cam more suited to the compression ratio before delivering the engine to the customer.

[larrycavan]

ran a SBF engine recently with 15.5 compression (accidently, the pistons were ordered to yield a 14.0 compression ratio). the cam was designed for a 12-13.0 compression engine. thinking it was 14.0, i ran Sunoco Maximal in it and let it rip.......

That's an interesting way to put that...What makes the cam designed for 12-13? Overlap & duration values?

[panic]

IVC.

[larrycavan]

IVC.

Seems like there's got to be more to it than just IVC.....

[MadBill]

Well, of course it's normally the other way around: You pick a cam that optimizes performance with the other chosen components in the required RPM range, then, based on fuel usage, altitude, etc. (and flame travel across the dome willing), use the SCR that gives the required DCR with the cam's particular IVC..

[KennyM]

IVC ! This reminds me of the time I filed the intake bleedoff closing ramp off a Briggs single cylinder cam and set the valve lash wide to boot. Nearly broke my wrist when I tried to start it from the combination of compression, firing and not setting the flywheel at the right point before pulling the rope.

In your situation, the combustion chamber plays a role in how much you can get away with for fuel octane or dieseling occurrs at shut down. If the cam duration is quite short, this traps a lot of air at compression and raises the octane requirements.
I would think that normally this is not a usefull combination for a performance engine that makes power in the upper rpm ranges. To get there, the cam duration and overlap needs to greatly increase to favor that. In doing so, it lowers the cranking compression into an acceptable range, as a normal result. (overlap and intake closing point)
What are your cam specs for opening and closing points?

[dbusch]

IVC point was at 58*, with an intake centerline of 110*.
Great for 12.0-12.5 compression, not too good for 15+...LOL!

[MRE]

Are you suggesting Sunoco Maximal isn't optimal for 15-16:1 compression? Just going off memory here, but I thought Maximal was a fairly high octane fuel. About 116 or 118 octane? Is there something about it aside from the octane which would make it detonation prone?

[dbusch]

Sunoco is great for stuff in the 15-16.0 compression range. the problem is when you mismatch the cam timing for the compression ratio. there becomes a point when the engine will tolerate only so much dynamic compression, and at this point Maximal is not enough. I doubt C16 would be enough either. Once cranking compression exceeds about 240 lbs, things start to get ugly. Perhaps serious attention to engine cooling etc might get around it. I know this engine ran really well with 120* water temps, but over 150*, forget it.

[Strange Magic]

What is the current stroke, bore and rod length?
What are the specs on that camshaft you have in this engine currently?

270 pounds of cranking compression is quite excessive and will seriously do damage to the wrist pins, rods and main bearings. Depending on how long you ran this engine for will determin to the extent of how much you should pull this engine down to check for signs of component failure.

Red maximal has a heavy spec gravity and is used for nitrous and very large displacement engines. Octane rating is only a small portion of the fuels specs and an engines fuel requirements should never be solely based upon octane rating. Other key factors such as lead content and specific gravity should be considered prior to the choice of octane rating. Some of the most powerful engines run on octane ratings that are not necessarily considered high, but considered to be moderate in the ratings.

P.S. If the compression calculation was grossly missed the first time, how do you know if you have a clear understanding on exactly what it is this time?

[dbusch]

i honestly dont know where i came up with 14.0 compression. must have slipped up on the calculator...LOL!

722 cc for 4.065 bore, 3.4 stroke

50cc for clearance volume (8.5cc gasket, 1cc to top land, -9.5cc for piston, 50cc chamber)

this comes out to 15.4 compression.

Yes, 270 is high, and i wouldnt have run it if i knew it was that high. Maybe i should tear it down to have a look at everything. I made probably about 7 dyno pulls on it...

[Ed-vancedEngines]

A tighter valve lash could help some but that would only be some.

If it would work with your combination and application, you could increase the exhaust duration some which should give it more overlap. I said exhaust becuase if you increase intake duration it can pull away from your midrange response and make it worse coming off turns or if drag racing at shift recovery. Increasing the exhaust duration over what is optimal might take a very few top rpm horses but the car would still have good response. Along these same lines of thinking you can increase rocker ratio on the exhaust if you have valve to piston clearance for it. That will increase the effective duration from valve lift to almost the nose but will not change seat to seat duration, which will also end with more overlap.

Only other thing to do is go back to the drawing board and figure what went wrong. Maybe a different cam design completely is in order.

You might also consider to degree your cam a little different too. If youhave piston to valve clearance to do it.

Probably the best bet is to change the cam though.

Man, that dynamic compression must be a ton.

Ed

[airflowdevelop]

I'm sure your top rod bearing shells are real happy now....

I have seen motors with too much compression, and improper cylinder head / valve timing develop a "hysterious miss" as I would call it, this is pretty common, almost sounds like a slightly lean miss....these motors like to be overly rich or dead cold to make any power. When everything is correct, it will make more power than currently...that is the bright side!


Hey, if we didn't make mistakes, we would have holes in our hands....
Dennis

[John Haskell]

A problem at times un-noticed, is refered to as 'rough combustion'. It means what it says. ------ When you get into differing air quality based on humidity/temperature, it gets worse.

We all know how the chamber is everything, so your typical tub style of chamber of the Ford/23*Chev, makes high-high compression less desireable as reacting smoothly.

One customer claimed a 23* GM alcohol package cranked 320 on the dyno. Pretty much could not drive it this way. (Sprint). But he did this to teach himself what did what, -------- and the cam/piston was quickly changed to be further tolerable in every way, but it was alcohol.

I can say in my past experience, that when you drop compression & you gain power on gasoline fuel, it's rough combustion. ------ Look into this concerning how touchy your Ford is & you should learn what's happening as I read into your question. -------

Nitromethane taught me this, way to long ago.

[Ed-vancedEngines]

Hi John,
That is why I say that I don't like mountains. I am a compression and quench freak about getting things the way I want them. But as much as I do like as high compression as is tunable I prefer to get a better burn with evidences of a more complete flame pattern. So I am notorius at chopping tops off of Mount Everest dome pistons, and also cutting back the side to at least a 45 deg angle followed by slightly rounding it all breaking edges. I will lose a little compression in the process but I gain power and tuneability.

Ed