Good compression & leakdown results - SNC question

[wineguy]

Hey all...

(hope this is in the right forum section - apologys in advance if it needs to be moved)

I've been a mostly reader/lurker/searcher for quite while -- only posted here a couple of times and have been following the 1.6 MP62 development closely. After lots of reading on m-net & here, I've determined that when the time comes to go with Forced Induction, it will (hopefully) be with an FFS system. Hopefully, because in doing the homework, there's one snag that might not work out well with a supercharger.

First, the good news -- last April, I ran a compression check:

#1-188
#2-193
#3-193
#4-194

So far, so good. Secondly, I finally got around to getting a leak-down test done. As a side note, it was with a Harbor-Freight unit that I replaced the two guages with a pair from McMaster-Carr, so I'm reasonably confident it's somewhat-reliable. Test pressure on all cylinders was 80 psi:

#1 at 4%
#2 at 1%
#3 at 6%
#4 at 3%

Not bad, if I say so myself! :taz:

This is all on a 1990 (red - is there any other color?), 44K miles, dino to synthetic about 4 months ago, with various suspension and exhaust mods. So the big hangup is... SNC. Yep, that's right, the dreaded early Miata bolt/keyway potential problem. No replacement of the timing belt/water pump yet, but did go in and loctite and re-torque the bolt properly -- it wasn't as tight as it should be, but there was no keyway damage, only a bolt that needed to be tightened. I do remember seeing a post in a thread somewhere that Tom indicated he did not recommend the use of the early SNC with a SC. Oh yeah, BTW, done the seafoam a couple of times, and have used the vacuum/water injection to work on de-carbonizing the piston tops -- still have a ways to go with that.

So, the question and request for advice revolve around the strength of the crankshaft I've got -- I was hoping not to have to rebuild and/or replace the motor to install a SC, but in lieu of this I may have to postpone the SC plans. Or, I could be the first 1.6 SNC with an MP62 to find out how big of a pulley it takes to snap it off completely! Probably won't happen -- I'm not in the self-destruct or "drive-hard-until-it-breaks" mode...

As far as background information goes, the car is mostly for enjoyment on the street, driven to work occassionaly and on weekends when tthe opportunity arises, and with the local Miata club when I can make the runs. No autocross yet, but I would certainly look forward to trying that to learn more about the car and being able to drive better.

Lots of good information here -- keep up the good work!

[magnamx-5]

The SC puts aditional laods on the crank but so long as the key stays torqued i don't think you are going to flex the crank nose at all. As you have read and probably surmised the real isue here is the torque of the bolt. I think Tom has the ability to get kits out at the moment if you decide to pull the trigger abit sooner than later. :D GL man

[wineguy]

The SC puts aditional laods on the crank but so long as the key stays torqued i don't think you are going to flex the crank nose at all. As you have read and probably surmised the real isue here is the torque of the bolt.

Thanks for the re-assurance… I’m assuming the that the lower/crank SC pulley bolts to the front of the crank using the existing four bolts on the regular pulley, and not the crank itself – is that correct? If I've read correctly, the early SNC failure has to do with the bolt coming loose and allowing the key and keway to get chewed up. With this in mind then, it's not so much the horsepower load on the front of the crank to turn a SC as much as it is keeping the bolt tight, so that the key & keyway can do their job properly under additional loading.

So as long as I bolt it up properly, get it tuned, and check/maintain the crank bolt at proper torque, I *shouldn’t* have problems with it. :cheers2:

Any other words of advice?

[magnamx-5]

Yep as far as i can tell you just swap the balancer and it's 4 bolts i would suggest getting some grade 10+ bolts for these 4 bolts as i have had them fail on me and the hamonic balancer fell off not a pretty picture doing 50 mph down the highway. ;) Luckily i was about 1/2 ablock away from work and coasted in to park and retrieve parts :D

[wineguy]

Yep as far as i can tell you just swap the balancer and it's 4 bolts i would suggest getting some grade 10+ bolts for these 4 bolts as i have had them fail on me and the hamonic balancer fell off not a pretty picture doing 50 mph down the highway. ;) Luckily i was about 1/2 ablock away from work and coasted in to park and retrieve parts :D


I hope that the damage was minimal - I can't imagine the racket that thing made under the hood when it came loose... :eek:

I looked at the CAD drawings closely for the coldside - I'm assuming the hotside crank pulley mount is the same - that would have pretty much answered my questions.

Thanks for the info. Now to keep scraping up the bucks to place the order. I already have a flywheel & clutch ready to go in -- just need to get a Roundtuit!


(miata) + (clutch kit):hammer:

[Tom @ Fast Forward]

Actually, all you replace is the four bolts. You pull the old ones and install longer ones that come with the kit. The new ones are grade 12.7 but the grade is not all that critical. People who have sheared those bolts on other companies kits and the one that happened on one of my kits were not due to the hardnes rating of the bolts. It was due to the new pulley not seating properly down inside against the stock crank pulley. It the one case on my kit, they left out the spacer. That left a gap inside and that sheared the bolts. Which were grade 12.7.

Correct:

http://www.FastForwardSuperchargers.com/crank-pulley-alignment 1.jpg

Incorrect:

http://www.FastForwardSuperchargers.com/crank-pulley-alignment 2.jpg

[wineguy]

Great ! ! !

Thanks for the clarification & pics...

[magnamx-5]

Nah luckily a new pulley and backing plate + bolts put me on my way. Jerry rigging it in the parking lot after work to get home was not fun though. Yes as Tom pointed out if the bolts are good and the pulley is seated correctly you will have no troubles. Tom do your pulleys have a reinforced area or spacer around the bolts like the stock puley?

[Tom @ Fast Forward]

I thought you have a turbo?

The bolt holes are very close tolerance. There is a centering ring on the back of the pulley so that it is as centered as possible on the crank pulley. Not sure what you are asking? If I didn't answer your question well, re-phrase and ask again.

[magnamx-5]

no you answered it just fine i just figured if my stuff was moving around with out the laod of a SC then some provision would need to be there to be reliable ya know ;)