Ex/Intake 274 degrees cam timing...Math's guru come here!

[mazdasly75]

Before you ask question about ex/intake cams swap please read http://www.wihandyman.com/forum/show...take-questions and http://www.wihandyman.com/forum/show...R-cams.-(BP26) for more informations.

I recently reground a set of 2 exhaust BP06 cams for my project http://www.wihandyman.com/forum/show...l=1#post719261

The goal now is to install the cams, by having stock BP26 turbo overlap of 10 degrees. From there, I'll tune it with the adjustable cam gears.

THIS POST HAS BEEN MODIFIED, AS I CHANGED THE MATH FOR A BETTER MATCH

What we have to know, from Mazda's technical data:

The stock BPT cams specs:

-intake open BTDC 2, close ABDC 51 = 233 degrees
-exhaust open BBDC 59, close ATDC 8= 247 degrees
-total overlap of 10 degrees

Stock BP 05 engine cams specs:

-intake open BTDC 5, close ABDC 48= 233 degrees
-exhaust open BBDC 56, close ATDC 14= 250 degrees
-total overlap of 19 degrees

The 274 cams from Colt Cams add 24 degrees from a stock BP05 cams, open 12 earlier and close 12 later. **Geoff from Colt cams told me that the cams centerline didn't move.**

I was about to pull out the engine, lol, to install the degree wheel, cause I couldn't figure out exactly the re-grind into the intake side, till I went deeper in this post http://www.wihandyman.com/forum/show...l=1#post678095 after I've been able to do the math and find a place to start with.

Remember:
turn the cam clockwise --> to advance
turn the cam counterclockwise <-- to retard

Basically, to install the exhaust 274 cam to this spec: open 86 BBDC - 8 ATDC, you need to retard <-- 1 tooth + retard <-- 2 degrees @ cam gear.

Intake spec: 2 BTDC - 92 ABDC, place the BP05 exhaust cam notch in the "I" mark, find the opposite tooth of the timing mark, advance --> 2 teeth + retard <-- 8 degrees @ cam gear and that's the mark you have to put to the top.

I'll take picture and post later...


**THOSE CALCULATION ARE BASED FROM A COLT CAMS RE-GRIND FROM 2 BP05 EXHAUST CAMS ONLY**

[mazdasly75]

**Post erased, all information are in the previous post

[irishkev90]

Wow, I could never have figured that out. Mind = blown.

Sent from my HTC Incredible using Tapatalk.

[bomdoutscort]

Nice post!!!

[gofanu]

"The goal now is to install the cams, by having stock BP26 turbo overlap of 10 degrees."

On what basis do you think this is a good idea? Seems crazy to me.

"Intake spec: 2 BTDC - 92 ABDC"
You are opening the inlet late, and then leaving them open way too late, with piston halfway up, giving low compression and reversing flow back through the induction side.

"exhaust 274 cam to this spec: open 86 BBDC - 8 ATDC"
Now you open the exhaust far too early at halfway down, and dump what cylinder pressure and power you still have, and cook the exhausts.

If it runs at all, it will be a pig, and roast pork soon after.

Overlap is not a starting point, but a mostly undesirable side effect of long cam timing. Seems to me you want to keep the in/ex centerlines more or less where they were, possibly retarding inlet a few degrees for the turbo, though I don't know much about turbo setup.

Since you spent the money and have the phone number, have you asked the cam guy about this?

FRM

[mazdasly75]

"Intake spec: 2 BTDC - 92 ABDC"
You are opening the inlet late, and then leaving them open way too late, with piston halfway up, giving low compression and reversing flow back through the induction side.

Well, your statement is both true and false at the time. If the engine would be N/A, that would be a pig for sure....now it will be a pig till you reach 3000rpm, after that it's going to be another story. Have you ever read about "miller cycle" or "Atkinson cycle"?, both of these therm are using 5 cycle of an engine. How? by letting the inlet valves open while the piston is in its compression cycle, thus hurting volumetric efficiency. The inlet valve remains open up to 30% in its compression cycle and suppress air back to the intake manifold, unless you ask for load and compressor charge air and will hold that pressure into the cylinder. Other way, hybrid engine use the same principle, "otto or atkinson cycle" but controlled by the variable cam. The benefits are a lower fuel consumption and reduces emissions as well. With a turbo charge, it's going to be the same, except that you loose huge power till the lag period is gone.From there, I can tell you, the high speed power benefit the long cam duration.

"exhaust 274 cam to this spec: open 86 BBDC - 8 ATDC"
Now you open the exhaust far too early at halfway down, and dump what cylinder pressure and power you still have, and cook the exhausts.

At that level, piston's almost down, might be missing 5% of it's stroke...half way of the crankshaft rotation, but piston's deep in its cylinder. Again, long duration will benefit turbo spool. If advance, will spool earlier, make power faster, but will suffer at top end.

Overlap is not a starting point, but a mostly undesirable side effect of long cam timing. Seems to me you want to keep the in/ex centerlines more or less where they were, possibly retarding inlet a few degrees for the turbo, though I don't know much about turbo setup.

Overlap is the worst enemy of turbo charged engine. The last thing you want is blowing air mixture in the exhaust. It has real negative effect on combustion temperature and could damage an exhaust valve or melt a piston.

Like I said, it's a base timing of the cams. The cam gears has 14 degrees of advance and 14 of retard to play with. Also, keep in mind those numbers are from advertised cams, it's 274 but, in reality, at 0.050" of lift, it's 210 degrees...

Thanks for sharing your opinion, a lot of peoples will find answers to their questions through this