Did I screw Up? With my cam.

[concobra]

Hello RodKnock

That is a reasonable assumption on you part and in general makes good sence. Let me try to explain the variables.

There are three issues that present themselves here. 1. Very low static compression of 9.5:1. 2. Very large camshaft duration. 3. Cylinder filling ability. These three specifications work in a dynamic fashion to each other.

1. The low static compression of 9.5:1 yields a volume of 119.7 ccs at TDC on Big Boss' engine. This is a rather large area and will take a lot of ignition timing to get the flame pattern to fill this area during initial combustion. In addition the spark plug location on the original FE heads in way to one side which also contributes to poor flame travel. The more timing to move the flame across this area the better. So to speak.

2. Cam timing as it relates to static compression determines actual or Dynamic compression. Dynamic Compression is defined as the numerical number associated with the event when the intake valve closes and the cc displacement at this point verses the cc displacement with the piston at TDC. With the cam at 6 degrees advanced this number on Big Boss' engine calculates to 7:1. This is still a very low compression number as it relates to both performance engine building and toleration to detonation with 91 octane pump gas. Therefore using a lot of timing will help overcome this by creating more force against the piston which is the primary job of high compression. In other words we are cheating the engine into thinking it has more compression.

3. Cylinder filling. With all due respect to original low riser heads these do not have a good design and do not move alot of air/fuel mxture well on their own. That is why this cam helps to some event by having a lot of lobe duration to give the head ports time enough to fill the cylinders. Even with the cam's help the cylinder filling is not good. If you can't get it in you can't make big power. ( The old " An engine is nothing more than an air pump" theory holds true here)

Given these conditions is why I recommended these specifications to Big Boss.

Let me share one point as proof. My Current Cobra runs a solid roller with 256 degrees of intake duration, a static compression of 12.1:1, and a cylinder filling capacity of 105% percent as registered on the dyno. I run this thing on 91 octane all the time without any signs of detonation heard or as inspected on the piston tops or rod bearings. One consideration though. I keep the RPM above 3000 before ever flooring the gas pedal. But this should be normal practice with any usage of a performance / upper RPM engine.

Regarding your comment about being an expert. I am sure you know quite a bit and I truly enjoy a dialog about engines. I am not an expert either. I just enjoy engines. The only thing I am an expert in is making my wife mad when I spend too much on my own engine. (She is mad quite often.)

Concobra

[RodKnock]

Concobra, thank you. I learned something. My engine is being built as we speak and I think I focus too much on the big picture such as the static CR and not the full complete picture. I'm not going to steal big-boss'es thread, but I was concerned about static CR and I went a notch down on the cam as well as the static CR using the Edelbrock heads and maybe now I shouldn't have. Hmmm.

Regarding wives, my wife as well as everyone else I seem to run into, thinks I'm nuts for buying a car without and engine and trans. Almost no one seems to understand how you can spend so much money (I bought a Kirkham) and NOT get an engine, transmission and a running automobile.

[RodKnock]

Concobra, one more comment. How are you always able to bring the RPM up to 3,000 before flooring it. Will your engine ping on 91 octane if you were to take it to the track whether it be a 1/4 mile or road race type track?

[concobra]

Hello Again Rodknock

Very good question as I launch around 2000 RPM. So how do I do it? I cheat when at the track. I put in racing fuel there. If your at the track your there to race and racing fuel makes more power against a high compression engine. Use what helps to win.

If I did stay with pump 91 I still believe I would be OK regarding detonation. Reason being: I launch around 2000 RPM then step into it slowly to avoid as much spin as possible. Once past first gear the rest of the shifts only drop the RPM to above 4000 so the problem is self eliminating.

So you have a Kirkham. I recently saw one of these at a locale show here in Central Oregon. Man what A nice car. Exceptional craftsmanship, engineering, and component quality. Good luck on your build and I hope you will post some pictures and let me know when it is done.

As far as wives go. I am sure we are all very lucky to have ladies that will put up with the ridiculous addiction we call a hobby. Maybe they are smarter than we think. At least they are rid of us in the shop all day!!!!

[RedCSX1]

Two tips from me.

If you still want to stick with that cam then purchase a set of edlebrock heads from Keith Craft. Pay now or pay later. trust me its easier now before you put it in the Cobra.

Do go to Patrick at Pro systems for a carb. And more than likely he will tell you not to run annular discharge like he told me. Annular discharge carbs are designed for high RPM /WOT. Think Nascar or Drag racing.

[big-boss]

concobra, thanks again for the info-damn good info!
Do you have a number for Pro-systems? Seems like there site is down.

[concobra]

Hello Big-Boss

Patrick's number @ Pro-System is 727-490-5717. You will probably have to leave a message but he is real good about calling back.

Good Luck with you build!!!

Concobra

[496fe]

I like big boss's plan. As everyone here knows, the biggest limiting factor are the heads. I also have a low compression, big displacement engine with a cam that is a little milder (KC's 252/262 duration). The only thing the engine doesn't do well is rev off idle. If that's real important, you can always run 20-degree initial timing and use a retard box.

Unfortunately, you need to port the head and the manifold. You need at least 300 intake cfm in a big motor. KC's Stage 2 heads are close I think.

I know that it's taboo here, but nitrous would realllllllly wake this motor up. My motor is street-friendly - I drive it at 1,500-2,000 RPM's all day, and it still has a little over 725 HP when spraying. You need to plan ahead a little before getting on it.

I am not sure if you can pull the cam with the engine in the car. If you can, I would run the cam as it is and go back later.

The information from concobra is right on and we are lucky to have him among us.

Good luck.

Brian

[ItBites]

Concobra is correct that even with a mis-matched cam/head/compression combination, it can be made liveable with special considerations in driving style, overlooking shortcomings and unusual tuning, but you still will have a mis-matched set-up. Not great at anything, but not bad either, if you're willing to live with the shortcomings.

This does not take away from the fact that this cam is not a match for the intended application. Selecting too large of a cam is the most common mistake newbies and "experienced" builders make and this cam is too large. This engine will not be a high rpm, high hp screamer, nor will it be a smooth driver at low rpm, nor will it have low end torque, nor will it be easy to drive in traffic. It will have the best of no world in this engine. Yes the engine will run.

I do not understand why anyone would advocate using a mismatched cam by the argument that with compromise tuning and driving style, it can be made to work at some compromise level. For all the money and effort we put in our cars, most folks will be way happier with a cam that is matched to the application. Why compromise on using a cam that can be made to work instead of getting a cam that works perfectly, given that cams are cheap in the grand scheme of your Cobra and engine build.

None of the engine builders (KC for example) would put this cam in this combination since they have to be concerned about the HAPPINESS of their customer. This engine will not bring happiness unless the owner is willing to live with all the mentioned compromises in what is otherwise a no-compromise car. Big-Boss, please call KC or one of the cam companies and please get a cam more suited to your happiness.

[patrickt]

There is an "emotional commitment" to this cam, and Big-Boss is obviously anxious to get the car up and running. That is understandable; however, it also acts to cloud one's judgment.

[496fe]

I designed my engine using desktop dyno software. If you take the time to get head flow numbers (they are available for every FE head imaginable) and plug in manifold, carburetor, valve size, etc. you can get very close to real life. My power curve from the desktop dyno printout almost perfectly overlaid the actual chassis dyno printout. The actual HP was 4 HP different than the software prdicted!

I found that there are too many variables in play to accurately predict power characteristics of any engine/cam configuration. You need help from a computer. I know I will get flamed by the old-school builders, but if you don't have the experience of a KC, a Southern Automotive, or a Gessford, you need help, especially with the large displacements available recently.

[big-boss]

I just got off the Phone w/ Patrick @ Pro Systems and my solution will be here next week.
Patrick says, I will be OK and should like the power and economy? Yes-He said his analysis indicates I should get good milege. It will have a 1000 body, small venturies but very big throttle plates, mech secondaries, double pumper. When it comes I will pass the word.

[concobra]

Hello Big-Boss

It reads as though your conversation with Patrick went well. I am sure you will be pleased with his product.

At the risk of sticking my nose in your business again may I ask if you have considered the issues associated with cam break problems caused by changes in engine oil over the last several years? I believe that this has been covered on this forum before but, given my own sad personal experience with this, and not wanting to see anyone go through this I hope you don't mind me asking.

I have a option on this that is a strong possibility to prevent this from occuring if you would wish to know.

Concobra

[big-boss]

Yes, I want to know- I will start another thread.

[CHANMADD]

Lot's of good advice here!!!