Dynamic compression differences between cams...

I am putting my 460 together now. The guy I got the motor from used a Lunati 41607 solid flat tappet. Specs are; RPM range 4000-7400, duration @ .050, 265 int, 272 exh., Lift .603 int., .609 exh., Lobe sep = 110. Intake closes at 58.5 degrees ABDC.

http://www.lunatipower.com/Product.aspx?id=2288&gid=247

I think this cam will be too much for what I want to do with the car. Weekend cruising to car shows/cruises, and probably to the track three or four times a summer.

The motor has TRW 2443 pistons with the domes cut down to .100", compression should be around 10.5:1, Eagle "I" beam rods, stock crank, ported D3s with 2.25" int and 1.725" valves. I don't have any flow numbers on the heads, but he said it was dynoed at 400HP to the tire through a c-4, and ran consistent 10.80s(with uncut domes and 50/50 mix pump and race gas, victor intake and 850 Demon carb). I am going to use a Weiand tunnel ram with two 750s, a C-4 with a 9" converter(stall yet to be decided) 29" drag radials, 3.50 or 4.10 gears, which ever is more appropriate. I want to run the car on straight 93. No real set in stone performance goal, just as fast as I can with the parts I have.

This brings me to the dynamic compression question, with the Lunati 41607 I will be at 8.71:1 according to a calculator I found online. From what I read the 8.71 would be OK.

I was looking at smaller Lunati solid flat tappet cams, particularly 61641. RPM range 2600-6800, duration @ .050" 241 int, 249 exh, lift .599" int, .623" exh, Lobe sep 110, Intake closes at 46.5 degrees ABDC.

http://www.lunatipower.com/CamSpecCa...rtNumber=61641

They both use the same recommended springs which I already have, but after running the dynamic compression calculator it comes in at 9.35:1. The smaller cam has a much more realistic RPM range for what I have, but the dynamic compression would seem to be too high.

Should I keep looking for something different?

I am not dead set on a solid cam, but from the reading I have done here, the solid flat tappet cam seems to be the way to go for me. Thanks.

Jess

I must say, and commend your ability to understand the "useful" operation of the internal combustion engine within a given application. Myself, I think your move to the "smaller Cam" was in the right direction, for drive-ability and the engine's intended usage. With cast iron cylinder heads, street driving and extended idling along with low octane gasoline, the DYNAMIC compression is pretty damn good! I do understand the use of the tunnel ram, plenty of "eye appeal", smaller carburetors (600 CFM or 450 CFM) may enhance drive-ability and still offer good street performance. Wish your project goes well.

Happy motoring,
Dave.

so can you change your dynamic compression by advancing or retarding a cam ?

bruno wrote:so can you change your dynamic compression by advancing or retarding a cam ?

I'm not sure. The cams are both ground with 4 degrees of advance built in. Seems like all of the cam timing events would change when the cam is advanced or retarded, maybe someone who knows more about it will comment...

Jess

bruno wrote:so can you change your dynamic compression by advancing or retarding a cam ?

Yes.

These dynamic compression calculators look at the intake closing point. Problem with them is to know accurately the point at which the valve closes or more correctly stops flowing. This is hard to determine and using advertised cam numbers can put you off a fair amount.

I'd also say unless everything was measured, you don't really know what the compression ratio is. I would start by measuring the head and piston cc's and deck to piston clearance, then properly calculate the CR. Once that is done can get a good idea where you stand as far as pump gas use.

If you want something that will work with what you have and sound good and run good. The old 250 -262 @ .050

on a 108 will work and work good, is it perfect .... well no. but it works in anything. lol, Or something close to that.

The tunnel will work very well on the street and add torque down low with the 750's.

You will need a 3000-3500 converter to make it run as fast as it can , or if it is street only a 2500 will work.

May take a little tweaking to get the dual 750's right but should be sweet.

IDT-572 wrote:If you want something that will work with what you have and sound good and run good. The old 250 -262 @ .050

on a 108 will work and work good, is it perfect .... well no. but it works in anything. lol, Or something close to that.

The tunnel will work very well on the street and add torque down low with the 750's.

You will need a 3000-3500 converter to make it run as fast as it can , or if it is street only a 2500 will work.

May take a little tweaking to get the dual 750's right but should be sweet.

Is this the one you are referring?

Comp duration 256 int., 266 ext., lift .589" int., .615 exh., lobe sep 108*. Dynamic compression works out to be 7.60:1.

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=1021&sb=2

Jess

I have ran that cam and the Lunati 252,260 roller in 10 to 1 and 11 to 1 compression 429 and 460,s with a performer rpm and stealth intakes and ran very very well on pump gas.

Can't speculate on the dynamic compression but cranking compression on my engines were around 180 lb.

Dynamic compression ratio means LITTLE unless you're talking about a DENSE and completely CLEAN Air/Fuel charge ... which isn't the case in a high performance engine.

To much and uncontrolled cylinder pressure is what causes detonation, (presuming the ignition timing is correct), and there are a few different ways to lessen cylinder pressure and still run higher dynamic compression ratio.
HINT: One is controlling the amount of internal EGR at lower engine speeds which has a whole lot to do with what can and cannot be used.

Although my reply has little to do with the original topic, it is an interesting story of dynamic compression. The Elliott brothers found a way to measure dynamic compression in the mid 80's. Using this tool they would build the engine and tune each cylinder to produce the maximum consistent power from each cylinder. From the article I read, cylinder timing, cylinder cam timing, cylinder compression ratio and cylinder cooling along with several other areas were explored. All of this is way over my head, but the Elliott Thunderbirds were fast during the 80's!

Dave.

LivermoreDave wrote:Although my reply has little to do with the original topic, it is an interesting story of dynamic compression. The Elliott brothers found a way to measure dynamic compression in the mid 80's. Using this tool they would build the engine and tune each cylinder to produce the maximum consistent power from each cylinder. From the article I read, cylinder timing, cylinder cam timing, cylinder compression ratio and cylinder cooling along with several other areas were explored. All of this is way over my head, but the Elliott Thunderbirds were fast during the 80's!

Dave.

What they were doing was/is called "compression balancing".
By doing this, every cylinder had a different compression pressure, (the most that particular cylinder could possibly use), based upon that particular cylinder's detonation threshold.
It was because of them that NASCAR put a 14/1 limit on compression ratio at that time and only will allow a prescribed amount of track testing for EVERY car.

I found a different calculator online that adds 15 degrees to the intake closing ABDC number. When I run the numbers of the Lunati cams though this calculator, I get dynamic compression numbers similar to what I got before for the Comp cams stuff. I'm not sure why it is this way.

Anyway, I spent a couple evenings looking at a bunch of cams and reading about each, and I have narrowed it down just two. Hydraulics. Either a Lunati 61605, or a Comp XE284H. I am leaning towards the Lunati because of the couple extra degrees of duration seperation. Any thoughts on which would be a better cam for my application? Thanks.

Jess

Mustang-junky wrote:I found a different calculator online that adds 15 degrees to the intake closing ABDC number. When I run the numbers of the Lunati cams though this calculator, I get dynamic compression numbers similar to what I got before for the Comp cams stuff. I'm not sure why it is this way.

Anyway, I spent a couple evenings looking at a bunch of cams and reading about each, and I have narrowed it down just two. Hydraulics. Either a Lunati 61605, or a Comp XE284H. I am leaning towards the Lunati because of the couple extra degrees of duration separation. Any thoughts on which would be a better cam for my application? Thanks.

Jess

They both should make adequate power and have about a 3 "fun to drive" factor; (on a scale of 1 to 10). .

rmcomprandy wrote:

Mustang-junky wrote:I found a different calculator online that adds 15 degrees to the intake closing ABDC number. When I run the numbers of the Lunati cams though this calculator, I get dynamic compression numbers similar to what I got before for the Comp cams stuff. I'm not sure why it is this way.

Anyway, I spent a couple evenings looking at a bunch of cams and reading about each, and I have narrowed it down just two. Hydraulics. Either a Lunati 61605, or a Comp XE284H. I am leaning towards the Lunati because of the couple extra degrees of duration separation. Any thoughts on which would be a better cam for my application? Thanks.

Jess

They both should make adequate power and have about a 3 "fun to drive" factor; (on a scale of 1 to 10). .

So, based on the details I posted in my first post, and our conversation, how far up your "fun to drive" scale could you get me with a custom cam?

Jess

Mustang-junky wrote:I found a different calculator online that adds 15 degrees to the intake closing ABDC number. When I run the numbers of the Lunati cams though this calculator, I get dynamic compression numbers similar to what I got before for the Comp cams stuff. I'm not sure why it is this way.

Jess

It is that way because as I explained before you don't have proper information to determine dyamic compression. Even then it does not tell you all you need to know.

Mustang-junky wrote:

rmcomprandy wrote:

Mustang-junky wrote:I found a different calculator online that adds 15 degrees to the intake closing ABDC number. When I run the numbers of the Lunati cams though this calculator, I get dynamic compression numbers similar to what I got before for the Comp cams stuff. I'm not sure why it is this way.

Anyway, I spent a couple evenings looking at a bunch of cams and reading about each, and I have narrowed it down just two. Hydraulics. Either a Lunati 61605, or a Comp XE284H. I am leaning towards the Lunati because of the couple extra degrees of duration separation. Any thoughts on which would be a better cam for my application? Thanks.

Jess

They both should make adequate power and have about a 3 "fun to drive" factor; (on a scale of 1 to 10). .

So, based on the details I posted in my first post, and our conversation, how far up your "fun to drive" scale could you get me with a custom cam?

Jess

A 10 ... however, as I tried to explain, you have to give up something to get another something. In this case a 10 wouldn't make as much power so, it is only you who can decide what you would be best satisfied with having.
Put "fun to drive" at one end of the scale and "maximum street horsepower" at the other end then choose exactly where you want the end result to be ... you simply can't have it all.