i have a few questions about compression pressures.
i read in hot rod magazine that cranking compression could be used as basis for cam selection.
My motor has 13-1 compression and a somewhat big cam(comp cam 294s 294 duration , .525 lift).my cranking compression is 190 psi.ive always used race gas with this motor.i work for lincoln/mercury and find that the lincoln LS motor have cranking compression of 210-220psi.

1 if the LS motor runs on pump gas could my engine run on pump gas with a 190 psi cranking compression ????

2 is my cranking compression about right for my compression ratio and cam profile????

3 do i have too much or too little cam?????

dave

 

Your confusing cranking pressure for BMEP
or brake mean effective pressure. This is the pressure in the cylinder at rpm.

If you could spin your motor fast enough and read the cylinder pressure, you'd see it is highest right around peak torque.

Cam size will have some effect on the octane requirement of a motor, but not that much.
With 13:1 compression ratio you'll need hi octane reguardless
of your cam selection.
A too large cam may lower cylinder pressure a little but it will also
probabily slow your car down more than you'd like.
You should select a cam based on the cylinder head and induction flow and tuning ( rpm range).
If you want my opinion on your present cam select, I'll need to know a lot more about your car and motor.
Like intake design, cylinder head flow#'s header dimentions
trans type and converter stall speed and diameter, rear gear ratio, tire size (dia) weight of the car. and present performance.
The 294s is a very virsatile bracket cam. Don't be suprised if it
turns out to be the best choice.
You have other options that can allow u to use pump gas to race with.
You could lower the compression ratio to 10:1.
You could switch to alcohol (methanol) ( cheaper cost and more horsepower)
You could add a water (water/methanol) injection system.
Water injection works the same on a high compression motor
as it works on a turbo motor. It lowers the inlet air temperture and cools the airfuel mix in the combustion chamber to within
the tolerance of the lower fuel octane.
When water evaporates into steam in the combustion chamber it absorbs a lot of heat.
You may loose a slight amount of power by displacement of the air by water(steam) ( volumetric efficiency) but not much. You'll still be ahead in power, than if you lowered the compression ratio. or retarded the timing. By mixing the water with a 50/50 mix of methanol the alcohol adds to the burn (power), while cooling too.
A 50/50mix of water and alcohol is safe too (nonflammable)
This has been found to give the best cooling and power return.
You may or may not need to increase the timing to adjust the spark because of the slowed burn rate. while injecting water.
There are many systems and design plans and information on the internet.
This is proven technology. It works.
You probabily won't need a computer controlled system.
A cheaper simpler fixed flow system will probabily do fine.
Because you only need it at full throttle and your operating range
(rpm range is limited)
Do a search on the net for water injection, detonation and turbo.
It's all there.

http://www.carrollsupercharging.com/

http://www.dawesdevices.com/water.html

http://www.aquamist.co.uk

http://www.turboneticsinc.com/spearco/h20inject.htm

http://www.cnw.com/~redline/waterinjectionkit.htm

 

Well...

I, like most people posting their theories on this board, don't build engines commercially but are ready to spout an opinion. In answer to your questions;

1)I kinda doubt it.

2)I think your cranking compression is fine so long as you want to use race gas

3)I'd say your cam is fine so long as you use it with race gas. A shorter cam would be even harder to find fuel for!

The cranking compression gives a good idea of low-rpm performance vs. cylinder filling etc. You could have crap cylinder heads and induction but very high cranking compression so the number is only one indicator.

Have a read of this site: http://victorylibrary.com/mopar/cam-tech-c.htm They have an informative article concerning cam timing and how it affects the cranking compression.

I hate to post a link just like the previous posting, but the article is worth reading.

 

So far we've missed a major component relative to cranking pressure ...cam overlap.


Oh..er... I mean Lobe Seperation Angle!

 

BTW daverr, is this an everyday motor or a racer (surely)? For most 13:1 motors, this is a small cam given appropriate induction.

 

Racereno, I agree with your last comment that 13:1 compression 'deserves' very good induction and therefore a bigger camshaft.

I disagree that valve overlap or the lobe separation angle come into play at cranking speeds. Once the intake valve has closed and the piston has reached TDC you have your cranking compression. What happens afterwards with the exhaust valve opening along with the beginnning of the next intake cycle won't affect cranking compression. The exhaust valve is closed before the intake valve is.

I agree that it would affect the cylinder filling at normal engine speeds though.

 

Cam properties have a HUGE effect on cylinder pressures during cranking; far more so than at speed. Agree or not, it's a fact, not possible to argue about it. You might as well disagree that the earth is round.

At peak torque RPM, whatever that is for a particular motor combo, the cylinder fill (and therefore pressure) reaches its max. If you stick a huge cam in a motor, that tends to bump the peak efficiency RPM up (duh) at the expense of low-RPM (such as cranking) efficiency (duh again). i.e., a big cam will cause a low reading at cranking RPM, all other things being equal.

The Lincoln motor probably has such a small cam, combined with relatively high static compression (10½:1 or so I would expect) that it gives high readings on the gauge.

You can't compare one engine's compression gauge readings to another like that. It's meaningless.

 

Perhaps you should read my post again carefully.

I didn't argue the fact that "cam properties have a huge effect on cylinder pressures during cranking".

I stated that valve overlap and the lobe separation angle do not affect cranking compression.

The sole valve event that determines cranking compression (between two camshafts) is the point at which the intake valve closes; that is the point at which compression starts to build in the cylinder. A hotter camshaft has a later closing point than a stocker.

One could not simply choose camshaft A with X degrees Lobe Separation Angle and say that it will generate more cranking compression than camshaft B with Y degrees LSA, unless cam A has an earlier intake valve closing point.

 

rustydawg, I understand what you're saying (and I should have said an appropriate induction/exhaust) but the proof to what I'm telling you is in what you say...

the trouble is that the exhaust valve is opened BEFORE the intake is closed. HOW long before matters a BUNCH.

RB83L69 is correct, there are sooo many variables you can't always compare.

 

Please explain to me what you mean by

"the trouble is that the exhaust valve is opened BEFORE the intake is closed. HOW long before matters a BUNCH. "

Then I will make an effort to explain my point.

 

Thanks for the reply everyone.ive read alot about this topic via your internet links.Heres another thing i found out about this subject.my trans which is a th350 ,torque converter reacts to engine torque therefore lowend should be made as strong as possible for a particular engine combo.an engine that has real low cranking compression will cause a torque converter to stall too low.

racereno my car at first was going to be a strip only car but i think ill have it back on the street to for special occassions (not daily driven though race gas is like 4 bucks here).my power brakes still work surprisingly.

right now the engine is out of the car ,on the engine stand.It was at the engine dyno like a month ago.i have my dyno numbers posted here.it made 450hp at 6600rpms.

my engine is a 350 bored .060 over,13-1 world product sportsman heads,team g intake,750 holley,comp cams 294s(12-224-4).

dave

 

daverr see the following..
Rustydawg I rescind my comments: you are completely correct.

Let this be a lesson to all, never let an old person near a computer messege board during a party (we become dyslexic).

My thoughts were reversed (I think)... the intake opens before the exhaust is closed (well duhh!) therefore the scavaging has an effect on CR only at speed. I think this agrees with what you are saying.

 

Racereno, you hit the nail on the head there, the cranking cylinder pressure wouldn't change with differing amounts of overlap.

Daverr, it sounds like you have a seriously powerful combination there with no costly exotic parts to speak of - just that expensive fuel! Perhaps water/alcohol injection would let you run it on pump gas.