493 Stroker Cam and Intake Selection

[383man]

COOL, NICE CAR . Had a 63 once.

Thanks for the kind words. Ron

 

[nstyle73]

A little cleaning and off for porting.

 

[wyrmrider]

Read through this thread again

some comments

Post one

OP might be happier with the DP since not track and 3:55 gears see post 3 and several others, 10

235 cam will not like 2000 rpm

post 2 285 mopar grind a bit too much in a street car- yup BVVC

#5 custom grind- most likely not enough info o comment on 238

#7 EDM liftrs YES cam is for 3.73 gear

#8 better details

#9 dbl check that compression 75 cc heads and a “six pack” deck clearance

#23 really ueful

#25 (me) still sounds right

Had not followed this thread till today

#26 Agree with 1.6 on the intake



Let me postulate about inverse flank (jones) and flat flannk (most everyone else)

the inverse flank allows about 8 degrees less duration for a similar profile around max lift

That's 4 degrees on the open side and 4 degrees on the closed side

SO

if you want the intake to close at a certain point BTDC the LCA would spread out with the Jones and overlap would be less-- what's not to like

DO NOT PICK A CAM WITH THE LCA as a parameter – IT IS A RESULT

some other examples

Asymmetrical lobes vs symmetrical lobes and various combinations of flat flank or inverse



SO how do you pick when the intake should close?

intake charge has velocity and mass (and nitro is different than gas or LPG)

after BDC the intake charge starts to slow down and if the valve is open too far it can stall, and if you shut it too early it can rebound or reverse, (and either way you loose cylinder fill)

so you squeeze the intake closed at a rate to keep the speed- velocity of the charge up

finally just before the charge has no more energy you shut the valve and compression can start

In other words you work from the air flow BACKWARDS through the valve train to the cam profile.

Same drill on the other three events- If not a Bonneville motor you have to factor in all the use info.

Now how do you pick a cam out of a cam list- well unless you are Joe Sherman picking a cam for a 383 chevy when he has built hundreds- don't do it.

This is why picking a cam from .050 is way too coarse

Pick from VALVE events you need and work backwards. Usually at .006



#27 later

 

[wyrmrider]

#27



On the right track on the overlap however at .006 the difference would be more pronounced



You want the overlap to maximize pull on the intake by the exhaust

But not blow your intake out the exhaust

Zoomies, headers, stock exhaust and boat wet exhaust or through the prop are all different

Here's the problem

Flat flank rollers have to start opening the intake “early” inorder to get it open when the piston is moving fast- this gives more overlap than really necessary

Overlap is also a place where long rod/ short rod makes a difference and why stroke and rod length is a necessary parameter

(also needed for that intake close calc discussed last post)



so the exhaust close- intake open points (and the rate if you have time/ duration not having to open As fast as possible) is a design parameter

You do not want the intake to open too early (flow stalls and is late getting going) or too late

unfortunately the trade off to too early opening to get the valve open mid stroke is usually made with the bad idle and fuel economy going out the window acceptable

VVT anyone

Inverse radius cam will increase torque in all ranges



28

Most box cams have narrower LSA and more overlap because they are trying to put 10 pounds of air in a 5 lb sack

VERY rarely does the exhaust need to be higher than the intake- look at your flow charts

Isky hyd grinds are reasonably gentle to be installer forgiving, rev well and low warranties. Want a cam that runs you have to talk to Ron





29 EXACTLY



31 let us know how it turns out and also post in the stroker thread

FWIW I've mostly used Isky lifters. Springs. etc because I had the long deal there



33 I like DP also most of the time, more experience with tunnel ram than single plane but then I have not run low geared no OD street cars in a long time



#36 include Jones on your street roller search- I can vouch that he has rollers that are easy on the valvetrain (and some where you have to change springs on a regular basis) so stay away from rebuild every week race cams on your street car. The Indy buicks had no problem with high rpm valve train stability



37 It can be done if you do not go for a “bottom of the page” grind

 

[383man]

#27



On the right track on the overlap however at .006 the difference would be more pronounced



You want the overlap to maximize pull on the intake by the exhaust

But not blow your intake out the exhaust

Zoomies, headers, stock exhaust and boat wet exhaust or through the prop are all different

Here's the problem

Flat flank rollers have to start opening the intake “early” inorder to get it open when the piston is moving fast- this gives more overlap than really necessary

Overlap is also a place where long rod/ short rod makes a difference and why stroke and rod length is a necessary parameter

(also needed for that intake close calc discussed last post)



so the exhaust close- intake open points (and the rate if you have time/ duration not having to open As fast as possible) is a design parameter

You do not want the intake to open too early (flow stalls and is late getting going) or too late

unfortunately the trade off to too early opening to get the valve open mid stroke is usually made with the bad idle and fuel economy going out the window acceptable

VVT anyone

Inverse radius cam will increase torque in all ranges



28

Most box cams have narrower LSA and more overlap because they are trying to put 10 pounds of air in a 5 lb sack

VERY rarely does the exhaust need to be higher than the intake- look at your flow charts

Isky hyd grinds are reasonably gentle to be installer forgiving, rev well and low warranties. Want a cam that runs you have to talk to Ron





29 EXACTLY



31 let us know how it turns out and also post in the stroker thread

FWIW I've mostly used Isky lifters. Springs. etc because I had the long deal there



33 I like DP also most of the time, more experience with tunnel ram than single plane but then I have not run low geared no OD street cars in a long time



#36 include Jones on your street roller search- I can vouch that he has rollers that are easy on the valvetrain (and some where you have to change springs on a regular basis) so stay away from rebuild every week race cams on your street car. The Indy buicks had no problem with high rpm valve train stability



37 It can be done if you do not go for a “bottom of the page” grind

Nice post and good reading. Thanks for posting it. I run pump gas in mine so in the mist of me having a custom grind I also considered when the intake closes to keep the cylinder pressure within the parameters to run pump and not have detonation. Course that's just one part of the cam selection as you pointed out since comp and many other items also effect cylinder pressure. Ron

 

[wyrmrider]

Thanks Ron
Let's look at overlap for a second
Lazy factory cams have way more overlap than they need
If you are running open headers you get much more exhaust pull on the intake
Through the mufflers even with headers not so much and with stock exhaust practically none
Since the piston at TDC is NOT MOVING it is not moving any air
So very much overlap in a street car is sorta counterproductve- and bassackwards in a stern drive
Exhaust has inertia even after TDC so you hold the ex open and close it using similar mass/ velocity logic as I posted on the Intake Close and as a bonus you get some pull on the intake
Lazy cams including "chevy grinds" HAVE to start opening the Intake early to get it open where it needs to be- All intake flow BTDC and near TDC is from exhaust pull.
Opening the intake too fast/ early kills opening flow (and you do not get it back) but it's done because you have to to get power.
Another reason to use a higher ratio rocker and cam profile optimised for the .904 lifter. It allows you to match the valve opening rate to the exhaust pull and then the piston demand.
And here again is where short rod/ long rod makes a big difference (and you can make either work but usually one is better than the other for each application, get it backward and not run as well.
The OHC motors can use much shorter cams and still make them work than can we OHV guys
So think about accelerating the intake mass and getting the velocity ramping up quickly vs bogging.