Rebuilding a 1.8L for 250whp
 

This is one of my first posts here but I'm not new to this. Been on CR.net but thats turning into too much of a stance game over there on who can get lower. :blah:
Wanted to get your feedback and advice since you all are using your roadsters to maximum potential. Here's some background on my situation:


I'm just starting an awesome course at my Community College (needed one more class for my major, so I'm taking auto classes to fill up my time for the next two semesters). Automotive Engine Building in one of the nation's best machine shops. We have the most state of the art machining equipment and my teacher has ~40 years of experience in the industry.
He's assured us all that if we can dream it, it can be done with the equipment we have in the shop. I didn't realize how in depth this class was going to be. I was also not aware, it didn't say anything in the course description, that we would need an engine to personally rebuild. Looks like I'll be picking up an engine in the next 2 weeks to rebuild!
I figure while I have such an awesome opportunity I'm going to build a motor for boost as that's the plan for the car. So now I'm in a rush to find a motor to rebuild. I wanted to post my goals, plans, and build specs here and get some feedback thats miata specific instead of my muscle head instructor. This semester will be rebuilding the bottom end, next semester the head.


The car will be supercharged with a Rotrex aiming for a reliable 250whp without breaking the bank.

1.8L VVT BP Engine with low compression (50) in #3 from 2ndChanceRoadster
Flyin Miata Engine Rebuild Kit with all the seals and bearings etc.(Flyin' Maita)


Bottom End:
Manley Forged Connecting Rods (949racing)
Supertech Pistons 84.00 mm 9.5:1 (949racing)

Head:
Supertech Stainless intake Valves 34mm .6mm longer(949racing) should i get oversized?
Supertech Stainless Exhaust Valves 29mm .6mm longer (949racing) should i get oversized?
Supertech Valve spring kit (springs, retainers, seats) (949racing)
Supertech Valve Seals (949racing)



Does this all sound right? Are there any supporting parts I would need with this setup? Let me know what ya'll think of these build specs!

Anything outside of rods and pistons is not needed for a 250 whp build. Even at that you may not even need to build the engine 250 whp. Rotrex superchargers don't kick in like a rocket. AaronBC (not know if he's on miataturbo) made a 5 speed hold up for years (and still is) to 300 whp on a 1.6 rotrex, for track use. Which is unheard of. Show how smooth the power band is. It's not hard to think that a stock engine can easy hold 250 for a long time.

But if your going to rebuild it may as well aim above 250. ;)

Thanks for the input. The class requires an engine to build so I figured I'd make full use of the [practically] free experience and machine equipment! Obviously with the above mentioned build I could probably reach 300 but then the drivetrain would be strained if I'm correct.

I'm not concerned about the levels of this build however I know theyre attainable... what I would really like to know is should i do over-sized valves, 9.5:1 compression, any other support while I'm in there!

Well if you really want to and have a buget to why not just do it. But if 250 is the goal, any heavy head work like overside valves, cams, and porting, is unneeded. I got 248 out of a stock engine (rotrex of couse) with 100k miles on it. With a rebuild engine you should get even more. 9.5:1 compresion will be a good idea as 10 is high for boost.

Okay good to hear. I guess I'll shoot for 300 whp then! The low compression in one cylinder makes me nervous, 50 seems pretty low, I wont be able to diagnose it until it gets here in a couple weeks. With compression that low it's probably a valve am I right? couldnt just be a ring right? I'll plan on oversizing valves in case the seats are damaged. Do a 3 angle valve job while I'm in there.

Anyone know the limit of our valve train? I'd like to do valve springs while I'm at it if necessary for 300 whp, but if the situation calls for it can I run new valves on the stock valve springs with no issues?

low compresion could be may things. 50 is really low. can be a valve not seating, broken ring or ring landing, or the worst would be a crack in the head or block. the heads aluminum so good chance they have valve seat inserts. Never looked my self. even if the don't you can always just bore out the seat and ram in a insert. you'll do a lot of inserts any ways so may as well try it on your own car first befor trying a customer's car.

Yes, aim higher than 250. I ran 250hp on a stock motor for 3 years no problem.

If you are building an engine anyway, you may as well slap a medium sized turbo on there instead of the SC and get yourself ~350-400hp. Even FaeFlora ran 400hp on his GT3071 without much in the way of problems. Well, nothing a sawzall couldn't solve.

Aim higher. My car has been at about 250whp since 2006 on a stock motor. (Likely higher for at least 4 years of that.)

Just get a BORGWARNER S366 AND GO FOR 1000WHP BRO

I would go with 8.6:1 pistons as well so that you can run more boost. That is of course if you wake up and go turbo instead of SC. FYI if you put the right turbo on our motors you will get way more top end and more torque EVERYWHERE. Just look at Soviet's dyno charts for proof.

In the modern world, i wouldn't go near that low for compression when talking about a built motor.

When you speak of the modern world you are taking into account the archaic technology of our engine design correct? I am no expert so you could be absolutely right and I would like to hear why if you are correct. I have also heard on this forum many times that 8.6:1 is the way to go to open up more room for boost without det in our engines.

I'm not an expert either, but for the very modest goals of the OP, he could quite easily get away with a 10:1 motor, i'd expect.

For ALL the power, then i'd probably be in the 8.6-9.0:1 range myself. But if i was looking for ALL the power, i wouldn't be dicking with a BP, either.

He wants to go rotrex (not a great plan imo)
If he sticks to his plan, he doesn't want some super low comp bruiser that takes 30+psi, he wants something with relatively decent cr (9.5-10.0) and focus on balancing and lightening the living crap out of the internals, and opening up the valvetrain as much as possible for maximum revs and maximum flow.

I know if he stays rotrex he will want higher comp pistons that is why I said that is only if he wakes up and goes turbo. My point though is that if you are gonna go all out and build a motor with headwork and turbo it why would you stay at 250rwhp. You have already done all the hard work and created an engine that can take the power, the rest is relatively easy to extract another 70rwhp or so.

Its easy to say "shoot for the moon" but you guys need to realize how much more money and work a reliable 350-400 is before you start throwing around numbers....So how about lets be a bit reasonable and let Fae be Fae?

BTW he made less than 400 on a 3071 with 25psi and w/i. So nothing about that was "easy" or "reliable". Stop swinging e-dicks and be realistic. Let the man hit 250whp and then he can decide if he wants 300, 400, or 500.


PS: I'm all for big turbos and high hp despite how my post sounds, but telling a n00b building a BP for the 1st time to go for "ALLOFIT" usually results in Fael

The reason I stated an extra 70rwhp is because that would bring him to 320ish to the wheels and I have seen that done many times and when the engine is built I see very little in the way of problems unless they are harcore tracking it. From what it sounds like the OP will have this as either a DD type car or weekend toy but not a hard core track rat. 320rwhp can be reliable on the streets fairly easily with a fully built motor.

Yes, because 320 is reliable but 321 will blow sh!t up:facepalm:

Nevermind. This is pointless.

He just needs to go with a BORGWARNER S366 and shoot for 800

I did not say that 320 was a magic number. I am going with a median number from quite a few different dyno graphs from actual members on the site that have all run about the same rwhp numbers within +-10rwhp without issues on the street, all with built motors. It seems that these same members then have to make a whole different set of upgrades to move beyond this point and much closer to 400rwhp. Don't nitpick it so much.