Not sure if this goes here but I feel like it might.

I'm trying to decide between a high revving destroker 339 LS3 and a stroker 416 LS3 with really long gears.

So I've got a 1990 miata with around 125,000 miles on it I got for $2000 about two weeks ago. On the test drive I discovered cut front springs, minor dings and evidence of a minor rear ending (apparently a ~10mph accident, the rear bumper cover and end panel are really the only damage) while picking through the car I knew this was the perfect project car right off the bat, there's only a wee bit of rust on the driver's side quarter panel and it's been taken care of), even with the cow-print white-and-peeled paint job at the price screamed told me it was a worthwhile investment, especially since my fiancée and I now both have miatas and work on our own cars. The motor in mine right now has recently developed a clanging noise in the front end. My trans is pretty much done but the #3 cylinder is starting to lose compression and the engine is starting to leak oil. Needless to say the car is in great shape overall, but the old boat anchor 1.6L short nose is on it's way out.

I've decided to go with an LS swap using a monster miata kit with a T56 and a 31 spline ford 8.8 rear end with a modernized LSD. I'm looking at a 15" 8.5 rear drag tire at the strip and something like a 15"x 7.5 on the street since after that we start running into issues finding wheels, tires, and staying street legal, so traction is probably my biggest issue.

On to the debate at hand. In an attempt to help with traction at launch I'm trying to lower the torque down low using a destroker build, the question I've got is which aluminum block and crank to go with- Ideally I'd like to keep the high revving nature of the miata so I'm looking at possible combinations, but I also think just running a really low gear would accomplish the same thing. I've seen Richard Holdener's videos on the 8000RPM 339 Destroker LS3 build and that's roughly where I want to start, but I'd like to move the power band further up in the rev range and try for more rpm since I really like the sound. The end goal is to have a reliable daily driver but I also put 40,000 miles a year on my car currently (that will drop to about 10,000 or less next year) so it may eventually see weekend warrior status until then. On the other hand I could go tried and true with a much higher power potential, but at that point I start running into drivability issues.


If I go with the destroker 339:

• Lower torque
• More forgiving
• Glorious high rev noises
• Upgrade path is fairly limited
• Initial investment is higher, but possibly a buy once, cry once deal.
• Somewhat more unique
• More realistic options for gearing
• More daily drivable

If I go with the stroker 416:

• Higher torque
• Harder on tires at launch
• Requires more finesse at launch
• Delicious and instantly recognizable LS sound
• Initial investment is lower as I'm using off the shelf parts
• Tried and true
• Upgrade path is fairly open ended.
• Option to use any forced induction, including a twin screw supercharger becomes viable vs basically being stuck with a turbo kit for more power.
• Requires ultra long gearing to be realistic as a daily driver.
• Demanding to drive, but that's what a serious sports car or super car should be

I think the answer is probably the latter and the former is just could-not-should mentality. But i'm interested to hear other people's input. The gearing is a big thing for me cus i'm not sure exactly how to figure that out, sure 200 mph sounds like a thrill ride fun but let's keep it at the track. I've seen how hard 600 is to put down in an NA at modern highway speeds with a T56/3.73 rear, soooo I'm trying to avoid that.

I have around $25,000 put aside for the mechanical parts and labor on this project and will be doing the majority of the swap in our garage with my fiancee (Who is actually a better mechanic than I am so I trust her judgement, and we both do all our own maintainence, it's actually how we met), but would like to stay under that.as much as possible. staying away from the LS7 because of the thinner cylinder walls and astronomic crate engine cost. Aerodynamics and suspension are going to be an issue, especially since the car already dances around in the rear end around 100, but I've got a plan for that. Ideally I'd like to eventually do a widebody kit (or flared fenders) to help if I need more tire but lets start with looking at a 205/X/15 (X is 45,50,or 55, lower means **** ride and higher starts rubbing) street tire maximum.

 

Based on what I read, there is nothing to gain by destroking. I'd think 3.42 gears would help with keeping from totally roasting the tires off. A Miata is significantly lighter than a TA. I'd guess you'd be in the 2400# area.

 

Yep. 2400lbs or so, maybe 2500 with me in it. Of course I can strip the car down to about 2200 at that point by removing weight, AC doesn't work, the sound system is currently trashed, the spare can come out at the track, etc. I think my original plan involved 3.28 or 3.42 gears but I'm sure with a good suspension setup and a good bit of tire and aero I should be able to keep the power on the ground.

So i'm pretty set on the 416. Would eventually adding more torque down low with a twin screw supercharger be a solid reason to look at 3.28s or 3.08s? I'm interested in keeping to my buy once cry once philosophy.

 

My Miata is 2400# with a 471 whp LS3 in it. I have 3.23 gears in mine, helps with traction (sort of) and mileage (31 mpg highway with my mild cam).

I've had the car running for 6 years, but did my first track day in it yesterday. Hot sticky rubber helps a lot with traction. On the street, 2nd gear never hooks, but once the tires were warmed up on track, it was not an issue. I have 205s on the car, on 8" rims ... road race rubber, not drag rubber.

I'd just stick a 5.7, 6.0 or 6.2 in there. I don't think destroking buys you anything, because the ultimate useful rev limit of the engine isn't the bottom end, it's the stock plastic intake, or any intake of that general style. Staying with a stock displacement engine is cheaper than either a destroked motor or a stroked one. I have a FAST 102 on mine, and yesterday I hit the rev limiter at 7000 rpm a few times (tach doesn't keep up with the crankshaft) ... it pulled hard all the way into the limiter.

You **could** use a FAST102 with some of the adjustable length runners in the intake (LS3 style heads only, I believe). You give up a lot of low end torque that way, and if it's a daily, the torque is nice.

I also would suggest the V8 Roadsters kit. I think it's the best one out there. If I was looking to swap in a Ford, I'd go with Martin at Monster Miata.

 

I did look at v8 roadsters as well as flying miata, and as i'm not intending to keep AC, power steering, etc it may be worthwhile to give those a second look. Was there anything that you think justified the price difference between the Monster miata and V8R/FM kits? $3000 of my budget is nothing to scoff at.

And is going with an TKX a valid option over a T56 for weight and fitment savings or should I just be like "yea buy once cry once."

 

What the most important goal for the car?

Sounds like the most reasonable engine solution is a stock bottom end LS3/6.2 with a ported MSD Atomic intake & matched low lash solid roller cam aka LLSR cam. You can turn ~7,500+ rpm etc. Far more workable with $25,000 budget for the whole project.

The destroked motor will be a big waste of money and accomplish little else. Spend more, make less power. The 416 is also a poor choice if the car is driven 40,000 miles a year. The typical 4 inch stroke motor probably isn't going to last you three years/120,000 miles with out a rering or two. The 416's lower fuel economy will also hurt.

T56's are OK but a properly built Tremec Magnum is probably what you want for this sort of build. Something like an RPM Transmission Level 6 Magnum etc.

Or if you want to get stupid with the RPM, contact Lingenfelter and ask abou their 8,000+ rpm flat plane crank LS build. Probably need way more than $25,000 for just the engine.

 

I’m pretty sure the MSD Atomic won’t clear the stock Miata hood. My FAST **barely** clears, and I think the MSD is taller, especially outboard.

It’s a much nicer looking car with an unmodified hood, IMHO.

 

Yeah, I left all the AC, PS, etc. off my car as well, to save weight on the nose and keep the F/R balance good.

I like the V8R kit because they designed and engineered it so well. The subframe mounts the engine as far back as possible (my passenger cylinder head is maybe 1/8" from the firewall) for better weight distribution. The pickup points on the front subframe retain all of the stock suspension and steering geometry. They offered a complete solution, including frame rail stiffeners, trans and diff mounts, Driveshaft and rear axles. Maybe for a drag car, you don't care about steering geometry being sports car perfect. To my mind, the Monster kit was developed because V8R was causing a bunch of LS engines to go into Miatas instead of Ford engines. I like Coke better than Pepsi, too. :-)

If you do as I and others have suggested, you'll more than save that $3K by not building a non-standard stroke engine.

Is a TKX cheaper? Lighter (probably, it's smaller)? Looks like it has enough torque capacity. They also claim it will shift at 8000 rpm. If you went that way, I'd get a TCET17805 ... I like the gear ratios better. Nice long overdrive for cruising, and closer intermediate gears. A V8 Miata certainly doesn't "need" 6 gears.

 

You are making more out of this than it needs to be, The difference in stroke between the two engines isn't going to make one rev higher than the other at least not enough to make a difference, You can build a 339 to rev to 8k all day long but you can also make a 416 rev to 8k and make more power, Both will need premium parts to live and neither will live a long life, That's just a fact of building high revving high performance engines. If you want to make power without revving it to the moon then cubic inches will win every time in torque and HP output. Increasing RPM's to make power comes at a cost in increased wear and increased chance of catastrophic failure.

 

I see. In all honesty, this is a glorified daily driver that will see occasional track use, so going with anything that is going to severely hurt the overall fuel economy weighs heavily. Longevity removed from modifying stroke/bore/internals amounts to unnecessary headaches and that's the thing I need to avoid. Thanks. I'm going to revisit looking at different camshaft profiles as they'll go further dollar for dollar towards putting the power band in the place where it needs to be than the wiggle room any stroker or destroker will.

The car seems light enough that it really won't need the PS and AC might be useful on long road trips but those are so few and far between it'll be better to just rent an econobox or fly anyway. The decision on a 5 speed TKX vs T56 is relevant because the TKX is lighter by 36 pounds dry vs dry, can be had for around $500 less, and will hold at the power levels I'll be starting with. You still have to buy a clutch and flywheel kit, tail shaft and a bellhousing on both, so those become a wash. I think i'm on the surface streets for maybe 5 of the 124 miles I drive (lucky me, I drive opposite traffic most of that). so I'll spend the vast majority of my time in top gear.

I prefer pepsi unless it's vanilla coke, since finding vanilla pepsi without cherry is basically impossible around here.

No replacement for displacement and a century of R&D.

I figure it's better for me to just use FI if I ever want to go to higher power levels instead of redesigning the wheel. I doubt the car will ever want to see 200mph so I'm leaning towards:

LS3 (Exact mods to be determined but nothing in the bottom end)
TKX TCE17805 or T56 Magnum F 2.97/.63
Ford 8.8 Rear End
3.08 or 3.28 posi
Still debating on the swap kit, have to think more on how important handling is to me.
Aero and Suspension work

Speaking of gearing:
IT appears the both the 3.08s on the TKX 2.87/.68 with 225/55/15s and the 3.28s on the T56 Magnum F with 2.97/.63 with 215/55/15s move the car at 70mph at 2000RPM in top gear which is realistically the bottom of where the engine will live the majority of it's life, should I concern myself over 500 RPM in either direction or when does RPM begin to significantly affect MPG?

 

My car with 3.23 gears and the .5 overdrive in 6th is usable all the way down to 1100 rpm in top gear. I don’t usually cruise it below 1250, which is ~52 mph, but if traffic slows, it can coast down to 1100 and pull back up.

With a non-crazy cam and a good tune, the engine will run fine a lot lower than 2000 rpm.

 

Regarding the rpm & mpg questions, my 1999 T/A has 168,000 miles and has been a daily driver. Bought new & car has had three key configurations.

I'd expect your Miata to able to get slightly better mpg due to less weight etc.

Lid & catback etc 99 T/A ~weight 3900 pounds w/full tank & driver (300 pound driver me) LS1, T56, 3.42 gears
323whp & 335 wtq
23.5 mpg normal half city & half interstate
32.5 mpg highway consistently

Same 99 T/A HCI LS1 w/LS6 intake, TEA S2 706/5.3 heads, 224/224 cam, 1 3/4 Hooker headers, T56, 4.10 gears
418 whp & 404 wtq
20 mpg normal half city & half interstate
27 mpg highway consistently

Same 99 T/A mild 416 LSA, RM ported LS3 heads, ported Fast LSXR intake, 237/245 cam, 1 7/8 Kook headers & true duals, T56 RPM Magnum L6, 8.8 with 4.10 gears
517 whp & 471 wtq
15 mpg normal half city & half interstate
21 mpg highway consistently

Get the A/C on in heavy city. traffic and the 416 will drop to ~10 mpg. The feeding a 416, 93 octane gas is like feeding a hungry fat man at a fancy a la carte restaurant twice a week aka expensive.

 

You have a few things working against you with that 416 option: narrow tires, light weight, and a short wheelbase.

There are a number of benefits to going with the shorter stroke if you look outside of dyno numbers:
- The shorter stroke will decrease windage.
-It will allow for a longer rod and better geometry. ie. Sideloading the piston is drastically reduced.
-The 4.8 crank and rods will allow you to run a factory piston.
-The shorter stroke will have less reciprocating mass.

You could also just go the route of getting a 4.8 or 5.3, but the destroked engine will give you the benefit of having the larger bore. Will it cost the same as a stroker? Not if you utilize factory components in the rotating assembly. Will you make less power? Probably. Will you make less torque? Definitely. But I think the smaller V8 will make the finished car a better overall package instead of trying to play around with gear ratios for the sole purpose of defanging the 416.

 

Footsteps- Listen to Grubinski. He has been there and done that when it comes to LS Miatas. Heed experience....

 

My fiancee said a lot of the same things but she's also the type of person who doesn't want any mods on her cars aside from handling stuff, which is her area of expertise.

@grubinski Any benefit to using a GM Rear end over a 8.8?

 

I used the Getrag because you can fit a slightly bigger exhaust pipe around it, and it’s not going to see standing start abuse. If you think you’ll be doing much drag racing, I’d go with the Ford. The Getrag is also pretty noisy, clunking on/off throttle.

 

Grub - any thoughts on if the shorter stroke sets you up for boost (dwell time at tdc for example)?

 

Smokey Yunick was a big fan of higher rod/stroke ratio to improve dwell at TDC. It raises cylinder pressure and torque a bunch. But I don’t know if it matters more or less on a boosted engine.

In a Miata, a 9.0:1 compression 4.8 with a stock cam would be pretty damned fast. 🙂

I think the OP should build something straightforward, and not get too worried about tricking out the engine to the Nth degree.

Also, I just remembered… I’m not even sure a 4” stroke crank would fit in the car with the V8R kit, and maybe others. The pan is *very* shallow in front, to give clearance for the steering rack. It would take extra work to solve that.

 

Just a thought, since the idea of a PD blower has been mentioned by the op multiple times. DON'T. You don't want to hang all that weight on the front end of a car that is only really liked because of how well balanced it is. By the time you do the blower with integral A/W intercooler, fluid pump, necessary heat exchanger for the intercooler, and whatever else, your well balanced car will be a nose heavy pig. Not to mention the AWD system you would need to keep the tires from going up in smoke, lol.

 

I am not trying to be contrary but Smokey was incorrect. Unless your rod/stroke ratio is awful, like under 1.5/1, and your wristpin is still safely up inside of your cylinder bore and holding your piston straight, your cylinder pressure and torque are not affected.

As an example, the LS7 had a rod ratio that was just barely over 1.5:1, and its factory rev limiter was set to 7100rpm. It was the highest revving engine sold by GM up to that time, as far as I know.

The rod ratio does have some potential to give a different signal in terms of cylinder filling to the cam, but it is minimal. In the real world just connect your pistons to your crank with the best rod you have and don't worry about it.