Teach me about an exhintake cam swap for a Miata

I'm not sure why this has been off my radar. Either it is pointless, rare, or I've been sleeping.

Looks like the exhaust and intake cams have identical lift, but the duration is longer on the exhaust. So, a 2nd exhaust cam swapped into the intake side (with the rear tang cut off) should let the engine breath better. Sounds like a cheap upgrade short of machine work (particularly for someone who has a free exhaust cam laying around).

Any thoughts? Experience? Is it internet lore, or is there some merit to it?

FWIW, this is for an E85 300 hp turbo 1.8L. Might get some porting soon, but no valve work or machining until maybe someday in the future.

Looks like it works:

http://forum.miata.net/vb/archive/index.php/t-18329.html

Interesting. I had never heard of it before, but it sounds cool. I did find a page where a guy just rotated his timing gear 19 teeth instead of redrilling it.

I've never seen what I consider to be a good A-B comparison myself. Seems to me most of the gains come from adjusting the cam gears, which isn't done with the stock intake cam.

Do whatever my friend Quinn did, he's making power.

http://clubroadster.net/vb_forum/89-roadster-projects/16110-quinns-nb-thread-itb-content.html

In reply to sesto elemento:

That very nice, and I'll read more later, but it's a completely different plan. He's milked about 185 hp out of a 1999 Naturally Aspirated NB with ITB's running pump gas. Impressive.

But mine is a 1994 NA with a turbo running E85. Very different animal.

His car has higher compression ratio, improved head, variable intake, better flow, solid lifter cams, more duration, and more lift. It started with 140 bhp, compared to my 128 bhp. Different exhaust, different intake, everything.

I am not sure he is doing anything at all I can use.

MrJoshua wrote: Interesting. I had never heard of it before, but it sounds cool. I did find a page where a guy just rotated his timing gear 19 teeth instead of redrilling it.

Yeah, I saw that.

GameboyRMH wrote: Looks like it works: http://forum.miata.net/vb/archive/index.php/t-18329.html

Hmm..interesting. By that thread, I would come to the EXACT OPPOSITE conclusion- that it doesn't work at all.

The OP appears to have done it, and is claiming gains, but he moved timing around, switched gears, etc., so he doesn't actually know what the gains were from the cam.

Later in the thread, they claim the specs are identical on the intake and exhaust cams.

Anyone know the facts?

It's a common mod on the KA24DE motor in the Nissan 240sx to swap from the stock 242/248 int/exh to 248/248. It increases top-end by increasing overlap, but you need to be really careful with overlap in a boosted engine, or you'll be pushing the intake charge out the exhaust. If you did swap, you'd likely need cam gears to adjust for that.

SVreX wrote: In reply to sesto elemento: That very nice, and I'll read more later, but it's a completely different plan. He's milked about 185 hp out of a 1999 Naturally Aspirated NB with ITB's running pump gas. Impressive. But mine is a 1994 NA with a turbo running E85. Very different animal. His car has higher compression ratio, improved head, variable intake, better flow, solid lifter cams, more duration, and more lift. It started with 140 bhp, compared to my 128 bhp. Different exhaust, different intake, everything. I am not sure he is doing anything at all I can use.

IIRC that's 185 whp too.

If you're bothering to do all the work, why not just regrind the cam to something that will make power anyway

In reply to Dietcoke:

Because cheap race car.

I HAVE an exhaust cam. I am NOT a machinist. Cam regrinds do not fit in a $20XX Challenge budget.

SVreX wrote:

GameboyRMH wrote: Looks like it works: http://forum.miata.net/vb/archive/index.php/t-18329.html

Hmm..interesting. By that thread, I would come to the EXACT OPPOSITE conclusion- that it doesn't work at all. The OP appears to have done it, and is claiming gains, but he moved timing around, switched gears, etc., so he doesn't actually know what the gains were from the cam. Later in the thread, they claim the specs are identical on the intake and exhaust cams. Anyone know the facts?

From what I'm reading someone is claiming that '94-'97 NA Miatas have the same specs on the intake and exhaust cams. It's true that it would be very difficult (for DIY'ers without fancy tools) to separate gains from the cam swap itself from gains due to any timing changes, and it doesn't seem that anyone's done that yet.

The point about overlap is a good one, I think that's the best argument against putting this cam setup on a boosted engine.

I was under the impression that the blowing charge out the exhaust concern was a bit overblown.

I don't think they are talking about blowing charge out the exhaust. They are concerned about duration, and overlapping exhaust and intake, and backblowing compressed cylinder air back through the intake instead of the exhaust.

In reply to SVreX:

I'm sure you are right. I know so little about it that I was trying to be vague and just get the feel across-and I still said it wrong.

The thing I think I am trying to say is: for years I have read the argument that you don't want to go too aggressive on cams for turbo cars used to dismiss the need for upgrading the cams at all. Deep in a few of the threads where people who know a lot about turbos and cams were talking they basically said it's not a worry for most of the discussions where people think it is.

The "feel" I got without full understanding of the facts is that its not usually a problem. Based on my "feel" (which is what most people saying don't upgrade turbo cars cams are using) you are making such a small change that it has very little possibility of mattering that you are turbocharged.

I get what you are saying (and don't know much about it).

But I think the concern being voiced here is a little different.

"Aggressive" usually refers to lift. More lift means more air (which is why we hear about "lumpy cams"). But it can also refer to duration- the length of time the valve is open. More duration would also mean more air (but also relate to the timing).

"Overlap" is a different issue. It's describing a condition when the duration of the intake cam overlaps the duration of the exhaust cam, and both sides are open at the same time (at least partially).

This would leave the exhaust stroke pushing air out through both the exhaust and intake valves, or it could leave no compression if the duration overlapped the power stroke.

I've got the head apart. The lifters are collapsed, and I need to deal with them. Dropping in a 2nd exhaust cam would be super easy at this point (I have a 2nd). But this particular idea has not convinced me quite yet.

Please don't rely too much on the particular terms I use to validate or disprove my statements-I don't know the right terms so I am likely using them wrong. I just know that this particular type of question has come up many times and when people who aren't just parroting common internet wisdom show up some of them say the common opinion is wrong. Of course I am just parroting less common internet wisdom, but I like the less common stuff so it would be enough for me to try to find that rabbit hole and dive into it to see if the uncommon wisdom people have a point. That clear anything up?

all I know about this.. 16v NA fiat engines based on the 124/132 series engines do this to make more power. It is a rare set up on this side of the pond, but Guy Croft, a noted Fiat/lancia tuner says in those engines it can make about 220 to 240hp at 8 grand

Strictly personal experience: Powerworks advertised their supercharger kit as producing 200 TQ and 200 HP at the wheels on a stock Focus Zetec engine. This was proven to be true on numerous installations.

I had the Ford Racing Stage 2 cams in my motor when I put the supercharger kit on my car. Those cams had a little more lift and because of the duration had more overlap. My engine with the kit produced 188 TQ and 223 HP when first installed. Using adjustable cam gears to dial out some of the overlap we could bring the TQ up to 200 but the HP dropped to 205 at those settings.

So IMO, overlap does affect output in boosted applications. You can play with it but you may not like the results. I ended up setting my cams back to 0 and staying with the 188/223.