Camshafts - Questions about Regrinds

[IronFreak]

I'm looking for some answers to questions that have spawned as I think about the theory of reground cams.

The concept of a reground cam in laymen terms is when the cylindrical shaft or base circle of the cam is ground down thus changing the profile and height of the lobe.....(Bump)....

All that I understand, if I'm missing something there feel free to add or correct me.

Onto the rocker. before the grind, when the rocker is riding on the base circle portion of the cam its a zero lift, so when you grind it, technically it becomes a "new zero" but in reality its below zero in relation to the old specs. I understand that this slack can be taken up in the eccentric, and they also make over-sized eccentrics.

This does make the the ramp or flank of the cam from zero lift to the lobe center line much more severe. Does this also add a significant amount of stress to the rockers or other areas of the valvetrain?

By removing (grinding) material are you weakening the overall integrity of the cam? What process is used to grind? Is heat being taken into account on how it effects the base materials?

I'm not for or against these, I've been entertaining the idea actually. I just want a better understanding of how changing the cam profile (specifically the base circle, you're making it smaller than then stock specs) effects everything else that comes into play.

Thanks in advance for the feedback.

[fawlty_towers]

Hello here is some cam info:
Before grinding, the centers are checked for accuracy & the ends and the middle of the camshaft are measured to ensure less than 1000th in. margin of error. Then they are ground against a grinding wheel using a grinding-profile or master replication that matches the core. When a lobe center is brought in tighter, the powerband or operating range is shortened, which is used by racers to keep the car at maximum horsepower at a specific rpm. Also, light weight pistons and connecting rods reduce inertia and stress. Computer controlled engines usually require a wider powerband. So when selecting a camshaft it is imortant to take in consideration of the variables: car weight, trans. type, converter stall speed, etc., and larger engines require more fuel to operate.

Hope this helps. There is more tech info & engine parts here:

http://www.bmpdesign.com/technical/

http://www.bmpdesign.com/category-exec/category_id/46/search_year/1988/search_model/36

http://www.bmpdesign.com/category-exec/category_id/96/search_year/1988/search_model/36

They have done a car like yours and can answer a number of questions. Have a good one.

[QUKBMER]

I run a 272 regrind cam from Ireland Engineering and works great !!!! Of course there is a BIG difference in price between regrind and new but it works fine for me.

[bmwstudent]

I have about 8k on my rebuilt engine with a bimmerheads head and 272 regrind. So far I've had the valve cover off twice and no problems so far. I have the I.E. rockers too.

[SkiFree]

This does make the the ramp or flank of the cam from zero lift to the lobe center line much more severe. Does this also add a significant amount of stress to the rockers or other areas of the valvetrain?

I'm entertained by an answer so far.

Your thinking is on the right path, just maybe too far....

Strictly speaking, yes you are right, the ramp angle increases and the pad contact changes (more to the nose). However, it is not as aggressively as you might think. Meaning it isn't too "severe" or add a significant amount of stress to the rocker. Take into account that the nose of the rocker isn't something we see break on the m20 because of added stress. The only nose failures on an M20 I've seen are pad dislocation and/or aggressive wear due to poor lubrication, NOT because of a cam [conversely on an M10 race engine running a 336° cam will usually will run a rocker with a longer pad or machine specifically to compensate].

As I alluded to this is the same sort of thing you'd see on a more aggressive race cam with a full base circle.

....... And reground cams are typically parkerized after being ground.

**EDIT: when you read about this in muscle car or hotrod circles they CAN and DO change the ramp angle pretty aggressively sometimes. So consider what they say but tone it down quite a bit for our M20's.

EDITED FOR POOR LATE_NIGHT GRAMMAR.

[slammin.e28]

I earned a lot today.

[IronFreak]

I'm entertained by an answer so far.

Your thinking is on the right path, just maybe too far....

Strictly speaking, yes you are right, the ramp angle increases and the pad contact changes (more to the nose). However not as aggressively as you might think. Meaning it isn't "severe" or add a significant amount of stress to the rocker (also take into account that the nose of the rocker isn't something we see break on the m20 because of added stress (the only nose failures I've seen are pad dislocation and aggressive wear due to poor lubrication, NOT because of a cam [on an M10 race engine running a 336degree cam usually will run a rocker with a longer pad or machine specifically to compensate]).

As I alluded to this is the same sort of thing you'd see on a more aggressive race cam with a full base circle.

....... And reground cams are typically parkerized after being ground.

**EDIT: when you read about this in muscle car or hotrod circles they CAN and DO change the ramp angle pretty aggressively sometimes. So consider what they say but tone it down quite a bit for our M20's.

Thanks! I don't really know the stock specs (I guess I could have looked around) but there was a few missing piece when I was thinking about all of this. I appreciate the clarification.

I earned a lot today.

LOL!

[slammin.e28]

LOL!

...a lot of knowledge.

It's still early.....only the 2nd cup o' coffee in.

[TobyB]

I'm with Andrew on this-
yes, it does add stress.
But not enough to matter.
Unless you're spinning well over 7k.

And in theory, the lobe ramps and flanks are adjusted to compensate for the
reduced base circle, so that the base circle doesn't affect the opening and closing rates.

Again in theory, the rocker pad radius would be adjusted, too, to match the cam.
In practice, the cam's ground with the stock pad in mind...

And yep, with an M10, much over 315 degrees was running out of stock rocker pad- and
even the 315 grinds were pretty effing close...

t

[kickinindian]

quick question on regrinds do you NEED larger eccentrics for a mild 27x size cam?

[SkiFree]

Depends on your head. Two main factors determine if you'll need larger eccentrics.

1) How much was removed from the base circle on the cam.

This naturally depends on how aggressive the cam profile is, but it also can vary from cam to cam depending how much material the grinder decided to remove from the cam to get a clean surface (ie trying to clean up some shallow scoring or pockmarks).


2) What your valve seat position is in the head.

A new valve seat and/or new valves that have not been back cut (new valves are intended to be back cut and are intentionally thicker). If you can imagine this can pull the valve down (meaning more space between the top of the valve and the eccentric).

-----------------------

In practice, while I have seen a 272 reground cam require oversized eccentrics they are few and far between (but not outside the realm of possibility).