Part I

Hours of research on miata.net reveals that the you can install an exhaust cam in place of the intake to increase lift and duration, increasing HP. This can be done on 94-97's and on 99-00's. To time the cam properly, it's suggested to get an adjustable cam gear, or, drill a new locating hole in the stock cam gear 197.5* clockwise from the "I" mark on the cam gear facing as you would read the mark.

The following is specific to a 99-00 head, or a 94-97 head w/ 99-00 cams swapped in it.

Stock intake valve timing :

intake valve opens at 8* BTDC
closes at 49* ATDC.

Now remove the intake cam and install an exhaust cam in its place. Assuming you drill a new locating hole at 197.5* clockwise from the "I" mark, and align the "I" mark as normal, the new numbers would be

Exintake w/ relocated hole 197.5* clockwise from "I":

intake valve opens at 10.5* BTDC
closes at 51.5* ATDC

Little more overlap and a slightly delayed intake valve closing, both of which are generally good for HP on a N/A motor. Combine this with the added lift, it's no surprise 8-10 whp gains are typical on otherwise stock miatas.

In general, retarding intake valve closing (they close latter IE a higher number of degrees after top dead center) increases peak HP but hurts low end HP. There are reasons for this. I can elaborate on them later if needed. But in general this is true up to a point.

Well, it turns out our cam gears have more than 1 tooth on them. I counted 46. That's 7.82609 cam degrees per tooth. Or 15.652174 crankshaft degrees. So installing a cam a tooth off shifts the valve opening and closing timing w/ respect to crank position ~15.6 degrees. That's a lot.

The "I" mark on the cam gear is centered on the top of a tooth. 197.5* counter clockwise from here doesn't land directly on the top of a tooth. Darn. So, how close are we off?


EDIT: I deleted half of this cause the numbers are backwards yet again. Don't play with angles at 2 am. Will switch the stuff around and repost when it is damn sure correct.

 

pat, if the engine turns clockwise, advance is going to be counterclockwise. ie earlier valve event.

 

Uh, no. Pretty sure it's turn clockwise to advance, turn counter clockwise to retard. Say the lobes are pointing straight up on cylinder 1. Cam spins clockwise. You want to advance cam timing and have the valves open and close sooner. If you turn the cam clockwise, the lobe gets closer to the bucket. So the that valve will now open sooner.

 

my bad, you're right. the cam must turn clockwise. I think I was thinking sprocket relative to cam. carry on.

 

http://www.diymiata.com/exhintake.html

 

managed to score an exhaust cam from a '94 1.8 for $75 shipped. Can't wait to install and get some results

 

So I know you said the OP said no questions but is it decided that this is only good for an N/A application? Will it show benefit on a turbo setup as well?

 

Haha, that's an old saying, the "no questions, only answers...". I need to redo my drawings and post up. This was intended to be a writeup though, not a discussion. But I posted a bunch of drawings I made that were assbackwards. I gotta redo them and put them back up. Maybe tomorrow I'll do it.

But yes this will help any 1.8. FWIW I put a mark on the cam gear I shipped you that would be the ideal mark to use without redrilling the gear. You'll see when you get it. Just pretend the new mark is the "I" and line it up as usual.

 

Oh Ok, well i just re-drilled the one i have here using the template on DIY miata. Is your mark and my redrill going to put the cam in the same place or did you do something different?

 

pics of said markings?

 

if pat didn't take any I'll snap a few when it arrives at my house...

 

excellent, thanks

 

Like I said I'm gonna redo the drawings and I'll post them up, maybe tonight.

 

Credit goes to Chris Charlton for this pdf.

The pioneer of the exhintake cam install is "Protege GT aka Josh Cosford". He is running it on a naturally aspirated Protege 1.8. He set the cam timing for more overlap. Good for NA. Turbo is a different story.

I think I posted many times at m.net in my RX-7 AFM rants that part of the reason for the 8-10whp gain from 'only' the Exhintake cam is that it helped to correct the pig rich condition that happens at high RPM. Might be 50% cam, 50% AFR's getting closer to correct. My first hand knowledge of this is that I ran the stock 1.8 with standalone ECU/wideband and tuned. Installed the Exhintake, retuned and no way got 8-10whp out of it. Maybe half that if I used my imagination.

HRK is making a ton of power for the boost level he is running. So, maybe the 'stock' 197.5° overlap setting is fine at lowish boost.

If you install this cam and are serious about getting the most out of it, get adjustable gears. I don't have my EGT installed yet so I have an excuse for not really setting the cams correctly.

We discussed cam timing a bit and tried to get some people with real experience to chime in.
https://www.miataturbo.net/forum/t14902/

 

Got some Results...

I dyno'ed my car yesterday with the exintake cam indexed as PatsMX5 instructed. I put down 102HP/105TQ. Now I'm not sure how usefull that info is seeing as I dont have a before dyno.
Mods:
Exintake
DIY MS NO AFM
Toyota Cops
Stock Header Gutted Cat FM Turbo Catback
170k Mile 1.8L that burns oil...

I think i would have made more power if i didn't tune my car for 12.5 afr at wot and went to the more ideal 13.0-13.5 AFR.

The only strange thing is why is my torque higher than my HP? Didnt think that really happened on a 4 cylinder...

Im still waiting on the dyno operator to give me the dyno graph files...

 

Retarding the intake cam should help peak HP. You got a dyno plot? 12.5:1 is about right for max power N/A. I wouldn't go any leaner, that's for sure. I run 12:1 at WOT myself. What timing map you running?

 

It did seem to loose a bit of power when i tried leaning it out. The dyno graphs got fucked up and where waiting on the dyno guy to resend them. Also i stopped making power at about 5800 which seems like an intake manifold issue judging from a manifold swap we did on one of the turbo cars that also stopped making power at 5800 with the stock manifold. I did sound good on the dyno. The exintake changes the intake note for sure...

 

 

So if im standing in front of the car which way should i rotate the cam? Should i do one tooth or take the cam gear off and redrill it?

 

look at that pretty blue cloud