In general an engine will develope it's highest cylinder pressure at idle because as the engine speed increases the period of time in which the intake valve is opened and able to allow air into the cylinder becomes shorter. The cam timing stays the same it's just that things are happening faster.

The amount of pressure developed is of course affected by things like cam timing, compression ratio, etc. And of course I am talking about a naturally aspirated engine


OK, so I'm done stating the obvious. Here comes the question.

What sort of cylinder pressure, both in terms of PSI and total pressure (PSI x Piston Surface Area) is the LT1 able to tolerate and still maintain decent reliability?

For that matter, am I barking up the wrong tree trying to calculate things this way?
Should I just be looking to calculate my static and effective CRs?

 

I'm not sure that you stated the obvious.... it might be true at idle with WOT, but that isn't practical. As soon as you throttle intake flow (e.g. - at idle) to limit the amount of air flowing into the cylinder volume, decreasing its density, cylinder pressure is greatly reduced.

And, while the time available for the valves to be open decreases with increasing engine speed, the driving force in the form of vacuum created by the falling piston, is also increasing proportional to the engine speed. Valve open time decreases, but velocity increases. You'll get the highest volumetric efficiency at some intermediate point, determined by cam profile and head/intake flow characteristics.

Or am I missing your point?

 

We had a long discussion of this a number of months ago. Maybe a search will turn it up?

In any case, a generally accepted formula for estimating mean cylinder pressure (MEP) is: MEP = (HP X 792,000) / (displacement X rpm).

For most motors, the maximum BMEP occurs at peak torque. Using my motor as an example with a 15% correction fastor from rwhp to hp at the flywheel.

MEP = (775hp*792,000)/(385x5,000) = 318psi

I have no idea what hp it makes at idle and WOT, but using the equation it would need to make >155hp at 1000rpm to exceed the 318psi seen at peak torque, which seems pretty unlikely. The equation does illustrate why you shouldn't spray a big nitrous shot at low rpm though! Makes BMEP go through the roof.

The issue with maximum cylinder pressures as opposed to the mean presure is tied into fuel octane, timing, etc.. Detonation produces large pressure spikes and also tend to cause severe local heating. This is what breaks pistons. Mean pressures will never be high enough with an NA street motor to break even stock parts, provided there is no detonation.

Hope this makes sense, it's late and I'm tired. Am I misunderstanding your question? Are you talking about dynamic compression? If so, it won't break parts either, no matter how high, unless it causes detonation. We had a long discussion of this last month and I strongly urge you to read it. Very educational.

Rich Krause

 

Well the question really comes down to this...

I've been re-examining a lot of what I thought I knew and learning some new things on the way.

Basically, I've got a rebuild coming up and I'm trying to pin down a number of things, such as what sort of pressures I can ask a cylinder to hold before stuff breaks.

This involves compression rations, I know. Static and efective.

But it's not the whole story I think.

I'm delving far more into this sort of thing and stuff like cam timing, LSA, exhaust flow as a percentage of intake flow at .050 to .500 lift, etc.

So, I'm asking around, trying to find out what people have figured out over the years and see if I can put some of that together.

 

What you are talking about is called dynamic compression ratio. The DCR is a function of the static CR and the valve timing (as you suggest). Maximum DCR is largely determined by the fuel used (octane primarily) and the design and material used for the heads. Aluminum allows a higher DCR than iron and the reverse cooled LT1 heads are even more tolerant because they run quite cool. Here is a DCR calculator, which you can download and use http://cochise.uia.net/pkelley2/DynamicCR.html.

This is the topic that resulted in a very good discussion within the last couple of months. You really should do a search. Chuck Riddeck ("Mr. Horsepower") was kind enough to post to the thread. He related his own experiences developing an LT1 and gave some general guidelines. To sum them up, he stated that on pump unleaded ~9:1 was the maximum permissable DCR and he advised erring a little on the lower side as this assumes everything else is spot on.

But in any case, the issue with parts breaking is absolutely not a mechanical consequence of the DCR. It is as I previously stated: the maximum DCR refers to how high the DCR may be before detonation is inevitable. In this context, it's detonation that breaks parts. What you can mechanically, if you want to push the limits, is to use good quality forged pistons. They are tougher than hypereutectics and will be more tolerant of occasional detonation. A very useful trick is to have the piston crowns ceramic coated with a thermal barrier. This will keep them much cooler which supresses detonation and help preserve the mechanical integrity of the crowns as well as making hp by keeping heat in the combustion space. Coating the chambers will also help by preventing detonation and keeping the heat where it's supposed to be.

Hope this helps.

Rich Krause

Addendum: I just wanted to add that when running a SC combo, DCR should be much lower to avoid detonation under boost. My combo is optimized for 100 octane unleaded (I run it quite rich with retarded timing on pump 94 unleaded). My static CR is 8.5:1 and my dynamic CR is 6.75:1. However, I run 14lbs of boost and a 200hp nitrous shot at the track! I have coated piston crowns and am going to have the chambers (or at least the exhaust valve face) coated this winter.

Rich

 

Listen to Injuneer and Rich, and Mr. Horsepower if you find his posts.

These guys are good. Heed their advice.


The DCR stuff is right on the money.

 

I have been trying to search for this thread but I am getting too many hits.

Does someone who read that thread or posted on it remember the title?

 

It's great to learn and think about these things, It's a great hobby trust me, but you have an advantage here- the results from everyone else's installations here.

What exactly will the car be used for? (used to determine powerband)

What gearbox and rear drive ratio ? (also used to determine powerband.)

How built is the bottom end so far ?(redline question)

You can't have an engine with a powerband for drag racing, road racing, and daily traffic driving- there is no free lunch (no until we get VVT)

So what is your goal? And what is your budget?

Hope this helps,
-Christian

 

Hey Rich, how can you do such a low DCR with 8.5:1?

I am using 5.7" rods 9:1 CR and 42º ABDC for the valve closing... so.. it nets me a 8.27:1 if running 12psi it will go to 15:1

 

Like the Chinese guy said: Sum Ting Wong here. If your static CR is 9:1 I don't see how your DCR can be as high as 8.27:1 unless you are using a tiny cam, which you are not.

What is the "seat timing" (= "advertised duration). LSA and ICL on your cam and what are the particulars on your combo (bore, stroke, gasket size, piston dome/dish, cc volume). I will run those number and see what I get.

Rich Krause

 

4.03
3.75
.039gasket
5.7rod
31cc dish
58cc heads

 

You are right, the difference is the cams.

Run my specs and you will see a big difference.

214/224 (266/276)
114lsa 114icl
4.040"
3.75" stroke
74cc chambers
-21cc dish
5.7" rod

Rich Krause

 

I get a 7.84 for you...

 

I'll run it agian. My brain isn't working so good today (bad cold). Recehcking all entries gives 6.9. I am using the calculator from http://cochise.uia.net/pkelley2/DynamicCR.html, which is my favorite.

Try your numbers using that one!

Rich Krause