Cam Suggested Compression Ratios..

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F-BIRD'88
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Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 »

Here are two cams that are almost identical..
My old Crane flat tappet 134781 that requires 11-12:1 comp. http://www.cranecams.com/product/cart.p ... il&p=24459
..and a Comp Cams 11-219-4 http://www.compcams.com/Company/CC/cam- ... d=458&sb=2

Both cams have 248° Intake with the Crane 10° more Exh. -all at 0.050".
Both have a 110° LSA and about the same lift..
Why does the Comp only need 10:1 and the Crane 1-2 full points more for roughly the same camshaft?
Is it all just where the intake valve closes?


The short answer is these two cams are more alike than different.
They will both run very well and very similar in the engine. The actual difference will be small.
Each of these two cams will run good at 10:1 cr and each will run equally well at 12:1 cr and the power increase
that both will see
will be approx the same for both as you increase the compression ratio.
ALL high perf cams and race cams like more compression ratio. More makes more power.
less cr makes less power.

Do not get all wrapped up the cam catalog descriptions for each cam.

12:1 makes more power than 10:1. It is just that simple.
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Re: Cam Suggested Compression Ratios..

Post by wyrmrider »

It's that simple unless you have to retard the spark due to detonation or bad fuel (which always happens at the worst possible time)
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Re: Cam Suggested Compression Ratios..

Post by pdq67 »

SM,

I SAID THAT--------

I USED to use this with a grain of salt.

Advertised duration cam families vs static compression ratio's here.

248/250/252 = 8 to 1;

258/260/262 = 9 to 1; (The 266 can go either way).

268/270/272 = 10 to 1; (Same with the 274 can go either way).

278/280/282 = 10.5 to 1; (Same with the 284).

288/290/292 = 11 to 1; (Same with the 294/296).

and

300 = 12 to 1.

Seemed to work pretty good, but these are for the older cams.

There are a whole bunch of things that affect this such as; aluminum head vs iron, camber shape and type; quench distance and headgasket thickness; of course fuel octane; timing; etc., etc...

Look at the cam cat's and you will see the old cams I'm talking about so take a hike.

pdq67
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Re: Cam Suggested Compression Ratios..

Post by Dodge Freak »

It has iron heads MP 360 heads, based on the 308 castings. No nitrous and its got KB hyper pistons, had to go .040 over and at the time $250 seemed much better then custom forged. Has a 10 inch, 3,000 stall and 3.91 gears 26.5 inch tires in back. Motor has over 5,000 miles on two seasons and 6,300 it revs cleanly to. Right now it has a hyd 220/230@.050 cam on 110 lbc

Had my engine shop machined and assemble the short block, they must have done a good job with everything including the ring gaps...I really didn't want hypers but they kept saying it be OK and so far it is but was told never to use nitrous, lol
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Re: Cam Suggested Compression Ratios..

Post by Dodge Freak »

Don't forget pdq67, they used to sell "racing gas" at the local fuel stations. Oh have things changed :(
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Re: Cam Suggested Compression Ratios..

Post by BlackoutSteve »

Strange Magic wrote:F-Bird, I clearly understand what he wrote. I am asking him, the topic starter why he would do this or settle for this. I've read through his posts, not interested in anyone elses posts. I would like to hear this from him, rather than others taking pop shots at what might be going on in his own mind.
The situation is about that simple.. I have a 454 that has 12:1 with a pair of Edelbrock heads that I have somewhat "outgrown" or so it seams. I won't go into details but discussions with Larry Meaux raised a few clues from his formula and my real dyno sheets (BSFC in particular) that suggests that this was the case. Those Edelbrock heads have ~106cc chambers after 0.043" milling, and I feel thiat milling them was a mistake. I really don't want to mill the new heads.
The new heads, AFRs that I am looking at buying have 113cc chambers. If I simply bolt these new heads on, I will go from 12 to 11.15:1 and am concerned that power may be (overly) adversely affected. I am well aware of the advantages of a high comp ratio, which is why I want to maintain the 12:1 I built into this engine in the first place.
My question is how much the lower ratio will hurt that cam's ability to make it's intended power and will if hurt that cams ability and do more harm than simply expect 3% less from the missing point..
The car is a street car on pump gas (and water injection) that would probably be happier with the lower comp from a "streetable" point of view.. As mentioned, the superior flow of these new heads should (should!) make up that 3% and then some, or of course, I'd be wasting my time.
If I need to maintain the 12:1, that's fine, but it means a new set of custom pistons and down where I am, that's easily $1500 plus rings and balancing etc on top of the head swap.

I tried a DCR formula on Wallace Racing's website but there is no input for LSA. http://www.wallaceracing.com/dynamic-cr.php
Slightly more to the story is I have gone from a Crane 138101 with a 110° LSA to their 138801 that has a 106° LSA to help move this heavy car. (The previous 138101 was damaged when a roller lifter failed, and thought this was a good time to reconsider my cam choice.)
Comparing those two cams is almost no different in the Dynamic results as their LSAs are not considered.
http://www.compcams.com/Technical/FAQ/LSAproperties.asp
Is there a more detailed calculator out there?
Last edited by BlackoutSteve on Thu Feb 21, 2013 2:27 am, edited 1 time in total.
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Re: Cam Suggested Compression Ratios..

Post by bigcam406 »

i used the OP's cam in question a couple of years ago in a LS-6 i built.it was a pump gas deal,10.25:1 compression,worked very well.
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Re: Cam Suggested Compression Ratios..

Post by BlackoutSteve »

Do you have some basic details on that engine? Did you ever have it dynoed or ran at the track?
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Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 »

These calculators do not tell you anything about the real pressures in the RUNNING engine at speed.

IF you already own and have used the 106 LSA cam before and it worked, it will work again.
I would not shave the new AFR heads either. I would install and test. @ 11:1.

I don't think that cam was or is going to be ideal (the perfect cam for this motor) anyways

If you already own that cam:
Run it.... play with the lash and the installed C/L. The car will show you what it likes.
(you need to play with the collector extensions to make that cam work.)
If your car won't "move" put some gear in it. Most street cars are severely under geared
Typical all converter and no gear



All you will achieve with these DCR calcs is wasting time.

By the way.... water does not burn. Something to think about of you want to go faster.

I'd re verify the VTP clearance of the new heads before running it.
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Re: Cam Suggested Compression Ratios..

Post by BlackoutSteve »

Thanks.. No I haven't ran this cam before, so it will be interesting.
An engine builder I spoke to who suggested to tighten the LSA said the loss in comp won't be an issue. Like we all seems to be expecting, these better heads will easily make up the difference.

The water injection works very well. :)
No, water doesn't burn and neither does the 80% Nitrogen that is also consumed with the oxygen and fuel. The engine doesn't self destruct, and water is almost free. It's nothing but an intercooler. I don't use it for any additional power.
I like it because it has allowed me to stop using (read: trying to buy, afford, carry, blend in) VP or AvGas and keep the high comp and cam streetable.
It's far more practical than race fuel in my application.
Think of it this way. What would you choose? A ZZ572/620 on pump, or a ZZ572/720 on pump and H2O.. Cake and eat it.
The advantages and benefits or water injection are well documented, ..for some time now. :wink:
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Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 »

Think of it this way. The water displaces air and fuel that does burn to create power.
On a N/A motor it has the effect of makeing the engine SMALLER. Its like shrinking the engine CID.

Now your 454 is a 445 CID engine. and less powerfull less volumetricly efficient . IN a N/A motor you loose more than you gained by adding compression beyond what good pump gas will support. If you have to supress knock with water.

A supercharged engine is different. You can effectively double the CID of a engine with 15lbs boost. Now your 454 is effectly a
900+CID engine. So if by using water injection you give up a bit you are still way ahead of the game.

You will only need to belend in a very small amount of 110 octane unleaded to your 92+ octane pump gas to protect your new 11:1cr AFR head engine.

DO you already own this 106LSA cam now....or..... It would not be my choice for this motor. @11:1 or 12:1
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Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 »

The whole problem with water injection on a N/A motor is its very hard to inject jsut the right amount of water and no more,,, and at the right time.

On a turbo motor the inlet air ( charge air) is always too hot. water is always needed when ever there is boost.
And it will always tolerate a little extra water as the power net loss is small compared to the gain of boost.

The operating conditions of a street driven N/A motor are different; and they are not constant.

On a N/A motor the air inlet temp is not always hot. It varys a lot... especially on the street.
Sometimes its hot, sometimes not so hot. SO the correct amount of water needed is always different under different conditions that are always changing. You will be injecting the WRONG amount of water ( too much or too little) most of the time. You wil get it wrong more often than right at any one time. When you get the amount wrong you loose power.
( casue the excess water absorbs too much heat and slows the combustion too much and displaces space othewise beter used by air and fuel that creates heat and pushes on the piston.
You are either absorbing not enough heat or too much heat. Your water injection does not adjust for the temperature of the engine inlet air. Which is never constant.
plus the water displaces air and fuel.

Its great for towing to supress knock on a N/A motor on cheap fuel. But not very efficient for a 1/4 mile blast
on a N/A motor. You will be further a head with a cr that will run on pump gas or pump gas + a bit of race gas blended.

than 12:1 and water injection. you are adding 4% by loosing more than 4% power. the result is a net loss of power.

Make it run on pump gas. So you can run max power spark advance. Put the water back in the fish tank where it belongs.

Put some gear in that heavy slug of a car. It will move.

Water injection does very effectively supress excess heat and detonation that results but always at the cost of some power.
The difference in power and torque between 11:1 and 12:1 is 4%. But if you have to inject water to do it on pump gas
you loose that 4 percent and more....

I would try it at 11:1 on pump gas. If it knocks and you cannot get rid of that no how, remove the heads and take a bit out of the combustion chamber to lower the cr and test again. 10.9 10.8 etc. or get better gas. They are not all the same.
Do you have a effective Ram Air system on your car? Cool air is always better, especially if that cool air is at a higher pressure than what is under hood, which is not always equal to ambient pressure but is ALWAYS hotter.
Employ some of the tricks Greywolf does to run on pump gas. Cool air is better.
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Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 »

You cannot look at the camshaft in isolation. It is a part of a system. The whole car is the system.
The engine is a sub part of that system.

If you want to analyze the cam you have to look at the whole system.
It is very very hard to over cam a BBC. I bet even at 11:1 the cam is actually too SMALL.
Please list the whole combo,,, car and all.
and some of your test time slips.
The more info the better.

anything you do to enhance engine torque (especially low rpm torque below the torque peak) will make the engine more likely to knock. A 106LSA cam in this motor is a mistake that will get worse with the new better heads. and pump gas.
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Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 »

You cannot look at the camshaft in isolation. It is a part of a system. The whole car is the system.
The engine is a sub part of that system.

If you want to analyze the cam you have to look at the whole system.
It is very very hard to over cam a BBC. I bet even at 11:1 the cam is actually too SMALL.
Please list the whole combo,,, car and all.
and some of your test time slips.
The more info the better.

anything you do to enhance engine torque (especially low rpm torque below the torque peak) will make the engine more likely to knock. A 106LSA cam in this motor is a mistake that will get worse with the new better heads. and pump gas.

If you have not purchased this cam....don't.....
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Re: Cam Suggested Compression Ratios..

Post by BlackoutSteve »

I already have the camshaft and it was a recomendation from a reputable engine builder down here.
This engine is not a clean slate, so it's not THE choice of cam he'd make. We're simply choosing what was in it (the 138101), but with a tighter LSA. However, it's sounds like you think it's more than a little bit wrong and I'm wondering why.

Without getting off topic on the water injection, I used to blend high octane (30%) and boy, what a pain in the arse that was in a car that gets 7mpg on the highway. So the H2O is a very good high comp fix that did not involve changing or modifying parts.

Here is a basic list of what I have..

69 Camaro. 4000lbs. (I use the factory cowl induction, so plenty of cold air. :wink:)
5200 converter, PG 1.76 Low, 12Bolt 4.56:1, 28" tire.
Sorry, haven't been to the track yet as I have just finished the resto, then the lifter crapped itself.

MKIV 454 (4.28x4.0) 6.135" rods.
Existing Edelbrock 60459 heads, soon to be AFR 300cc Magnums.
Victor Jnr, HP1000 4159 Holley.

With the 138101 cam and Edelbrock heads.. I was a little disappointed here..
Image

This is a the car.. I post this to show that it's a street car and not a tube-chassis track only car. In other words, it's a compromise. :)
Image
Last edited by BlackoutSteve on Thu Feb 21, 2013 6:16 am, edited 1 time in total.
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