Don Terrill’s Speed-Talk

I am wondering how strongly we need to adhere to the camshaft manufacturer's suggested compression ratios.
I am swapping cylinder heads and if I keep the same pistons, I will lose 0.85 of a point -from 12:1 to 11.15:1 due to the larger chambers of the new heads.
The cam is a Crane 138801, and it states a minimum of 12:1 which will require a new set of pistons if I want to maintain that 12:1.
http://www.cranecams.com/product/cart.p ... il&p=24638

Will it be OK to drop nearly a full point with this cam?

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?

My understanding is that you need to compare DCR's of the different cams on the engine to see what the effect will be. A CR drop will mainly affect low speed torque. At higher rpm, your engine might see the actual static CR, which is determined by the octane rating of the fuel you use.

So in essence you use as high a static CR as doable with the fuel you run and the type of heads. Put in the cam numbers and see where you end up on DCR. If the DCR is around 8-8.5 you're golden (if this is street gas). If you have got everything else under controle you might go a tad higher (cooling, quench, flame travel)

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...

pdq67

Is it possible to mill the new heads to gain back compression, or are you running out of piston to valve clearance?

How big is the engine? That cam does not look particularly large for a 454. I can't imagine it being too big for a 11:1+ BBC.

Those catalog cam "compression requirements" are useless as teets on a boar, and based off of nothing but some 1960's rule of thumb.

CamKing wrote:Those catalog cam "compression requirements" are useless as teets on a boar, and based off of nothing but some 1960's rule of thumb.

The cr "requirement" is a guide line.
It will run fine at 11:1cr. based on just the cr change it will now make about 3% less power than before.

If the new heads flow better then the old heads you may end up with more net power than before.
Even with less compression ratio.

You can play with the installed position of the cam a bit to find the sweet spot for power and torque.
If it was a good cam before it will still be a good cam.

Cams and cr.... every cam in every engine will make a bit more power with 12:1 cr,, than 11:1
and even more power at 13:1cr
The difference is going to be approx 3 to 4 % per compression ratio change.

run it.

CamKing wrote:Those catalog cam "compression requirements" are useless as teets on a boar, and based off of nothing but some 1960's rule of thumb.

Awww crap, just when I thought I found the EASY button.

I have an old NOS Blazer cam, mechanical, 300/310 adv...238/248@.050, thinking of using in a 340 SBM with only 10 to 1 compression, maybe 9.75 to 1 ?

is there a rule of thumb of min cranking compression so I could check it before buttoning it up, I do have another hyd cam a bit smaller--the old "Hemi" 284 cam

Dodge Freak wrote:I have an old NOS Blazer cam, mechanical, 300/310 adv...238/248@.050, thinking of using in a 340 SBM with only 10 to 1 compression, maybe 9.75 to 1 ?

is there a rule of thumb of min cranking compression so I could check it before buttoning it up, I do have another hyd cam a bit smaller--the old "Hemi" 284 cam

At 238/248 I would guess its a hot street. 92 oct, aluminum head, good tune, 190-205 would be good under 180 will be a dog.

Dodge Freak wrote:I have an old NOS Blazer cam, mechanical, 300/310 adv...238/248@.050, thinking of using in a 340 SBM with only 10 to 1 compression, maybe 9.75 to 1 ?

is there a rule of thumb of min cranking compression so I could check it before buttoning it up, I do have another hyd cam a bit smaller--the old "Hemi" 284 cam

it will run very well This cam is still sold as the F-238/3200-2-14 cam. It is not "old" at all. Degree it in so the intake closes @46 deg ABDC @ .050" lifter rise.
needs a 3000-3500 stall or 4 speed and some 4.10's 4.30's in a Dart or Duster.

The valve lash is .022" hot.. try setting the valve lash cold at .018". A lot easier and pain free that way and you will be more likely to take care of periodic lashing with a pain free method.
The net lash opens up a bit when it gets warmed up.

real good "nitrous cam" for that lil 340. GOT NITROUS?!

on a low compression ratio engine (7.5:1 to 8.5:1cr) this cam makes a real good street strip supercharger cam in a 340 360. Or even a 318. A good match to a Weiand 142 blower on a low cr 340.

Why would you want to reduce the compression (torque output/broadness) of the engine by dropping it almost a point of compression? Is the purpose to rid the engine of cylinder pressure, or is it for the purpose to change fuels to save money? Just curious.

In many cases, the suggested items listed in a camshaft catalog are non-sense. In reality, many should read as so:

Poor low end power, poor mid range power, adequite top end power. I other words, this is the wrong cam for your engine so we want you to band aid the lack of cylinder pressure which yields poor velocity at low and mid rpms, with additional rear end gear ratio and additional converter stall. The lighter your car can be, the less disasterous this camshaft will appear.

The below means nothing and is absolutely useless without having an understanding of LSA and intake install. It also is useless information unless you understand the size of the pump and the piston speed, of which is determined by bore and stroke, and rod length and stroke.

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.

Strange Magic wrote:Why would you want to reduce the compression (torque output/broadness) of the engine by dropping it almost a point of compression? Is the purpose to rid the engine of cylinder pressure, or is it for the purpose to change fuels to save money?

It is real simple and just this simple... He is not looking to drop the compression ratio. But the new heads have a larger combustion chamber than the old ones .
This is not going to kill this motor. It will still have lots of cylinder pressure, torque and power.
If the heads heads are that much better he may well end up with more net power.

You can play with the cam installed position a bit +/- to good effect to find the new sweet spot.
The engine will show you what it likes best . Do not worry about the cranking pressure.
The only thing that matters is how it runs down the track.

If the cam was a good cam before, it will still be a good cam>>

It is real simple and just this simple... He is not looking to drop the compression ratio. But the new heads have a larger combustion chamber than the old ones .

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.