guttedmiata

Ok, installed new cams on a mild ported head with 1mm over valves, hoping to pick up some power.

Cam specs:
Intake: 104deg centerline, 230deg duration @ .050, .360" lift

Exhaust: 112deg centerline, 238deg duration @ .050, .377" lift

Overlap of 18degrees which is only 1degree more than stock overlap.

Lost 50whp over pretty much the whole rpm range. Boost is from a 1.2 liter twin screw supercharger and climbed to about 17psi @ 6500rpm with the stock head and cams. Now boost never goes above 15psi around 4500rpms and then actually falls.

I could try retarding the exhaust cam, but with only 1degree overlap more than stock, it doesn't really seem like I should have to, even with a little more lift and duration. Ran out of time on the dyno to try it and see what the results were.

Also went from 2.25" collector and exhaust with a quiet muffler to a 3" collector and exhaust with a couple straight through mufflers.

Don't know if I'll have time to call the cam guy tomorrow during business hours, so I thought I'd post it up here and see what kind of input I get.

Full_Tilt_Boogie

iTrader: (1)

Twin screw is a dynamic compressor and should not require shitty cam profiles to build boost, no?

My money is on the mechanical timing being off a tooth or more.

TorqueZombie

iTrader: (6)

The old garden hose vs coffee straw analogy comes to mind about the boost drop. If the charger is doing what it did before and just a better flowing head it could explain the drop in boost. Less restriction = less boost measured/needed? Not sure how it would make less power though.

guttedmiata

mechanical timing is not off. Cams were degreed in with adjustable cam gears.

Yeah, a boost drop wouldn't bother me so much due to less flow restriction, but I wasn't expecting a power loss.

Leafy

iTrader: (1)

hmmm whats the dynamic compression ratio change with those cams?

Also whats the actual valve area overlap change? IE you only increased overlap by 1° but if you added a whole lot more lift and flow in that time then that could be a problem. Pretty much the only way you can lower boost and reduce power on a blower setup is to be the air straight through the head and into the exhaust. Or if you made it flow better but also killed the dynamic compression ratio. Did they do a full re-tune on the dyno? There's probably a bunch more spark advance to be had.

guttedmiata

lift increase is minimal, but duration is a fair amount. No idea how to determine the new dynamic compression ratio. I agree, it seems like we're just blowing the pressure out the exhaust. Just emailed Mike at Jones Cam Design that did the cams to get his $.02 on retarding the exhaust.

Tuner was at the dyno trying everything and he is good. Travels across the country tuning cars. 20deg advance and leaned way out and still wouldn't change by more than a few hp.

Leafy

iTrader: (1)

Calculatin DCR Wallace Racing: Dynamic Compression Ratio Calculator I hope 18psi doesnt see this because he said he'd rape me if I ever linked to an online calculator ever again. But **** it, I confirmed this one to work by doing the actual math out that is more complicated to explain.

guttedmiata

According to that calculator the dynamic compression increased from 8.22 to 8.60

guttedmiata

hmmm.... been comparing the new cams to the figures here all along Solomiata : Engine : Camshaft specs, but when I run the opening and closing numbers listed for the stock grinds I get much more duration than is listed in the table. More than the new cams. So I'm wondering if the duration listed on the stockers is correct or the opening/closing values or neither one.

Hope Mike has measurements still off the stockers.

2manyhobyz

iTrader: (4)

Does the same concept apply for SC as well for turbo? From what I understand forced induction (turbo) the less overlap the better. I would think you would want the exhaust closed so the cylinder fills and less gets pushed out.

Leafy

iTrader: (1)

The turbo problem with overlap is normally to prevent exhaust back flow because most turbo setups run at higher exhaust manifold pressures than intake manifold pressures. So the exhaust gases back flow during overlap sometimes and can even get into the intake manifold.

Full_Tilt_Boogie

iTrader: (1)

Thats how it is with a positive displacement compressor, like a roots (JRSC, etc.), but I dont think its like that with a twin screw.


OP,
did the AFRs change any? do any retuning?

Leafy

iTrader: (1)

Twin screws live in a weird where they are both a pump and a compressor. Some of the compression happens inside the twin screw, like a compressor, but some of the compression happens in the intake manifold, like a positive displacement pump. Its very confusing.

miata2fast

iTrader: (2)

Lets start with some basics first. Am I to assume you have solid lifters? Have you double checked to see if your valve lash is correct, and you don't have any gaping holes in the valvetrain?

Motors have a way of becoming injured when major changes are done. You might want to do a compression and leak down test before going any further.

If all is well, then I would double check your math and how you centered the camshafts. You may have a gaping hole there.

guttedmiata

Already answered. Top notch tuner was involved.

guttedmiata

Yes, solid lifters and yes lash is set.

The compression test isn't a bad idea. Hadn't even considered that. It is a brand new rebuild. Doubt that's it as all the plugs look the same.

Gonna try moving the cams and see what happens. I'm just hesitant to think I'm gonna pick up 70-80hp doing it.

miata2fast

iTrader: (2)

Having a new set-up and a 50 hp loss makes me think you have bugs to work out. Don't assume everything is just right.

TNTUBA

iTrader: (1)

http://horsepowercalculators.net/sup...and-cam-timing

Cam Timing with Supercharger - S2KI Honda S2000 Forums

Two good articles on Supercharger Cam timing.