High Altitude Engine Choice

[Echo]

Been contemplating my engine choice for my future scratch-build and need some advice. I've got a rather unique problem, however. I live and will be driving my scratch-built GT40 up at 7000-8000 feet.

My two engine options have been either a 302 based stroker (~347, 3.4" crankshaft) or going with a 351 based stroker (~408, 4" crankshaft). Here's my pros and cons list:

302:
- Pro: Lighter rotating assembly: faster acceleration at the same horsepower.
- Pro: Lighter overall weight, better weight distribution.
- Pro: Component cost is cheaper.
- Con: Less power and cubic inches than a 351W stroker.
- Con: Weaker block.

351:
- Pro: More power, higher mechanical advantage.
- Pro: More cubes.
- Pro: Stronger block.
- Con: Heavier block, higher rear-biased weight distribution.
- Con: Component cost is more expensive.
- Con: Heavier rotating assembly.

I'm going to be running one of these engines in front of an Audi transaxle. It'll be a road car that I want to have fun in. I'd like to feel fast as can be, make small children cry, that sort of thing. Since I'm losing a quarter of the performance simply by being at this altitude, I'm not too worried about the transaxle. I'm not going to be dumping the clutch, track racing, competing with it, or anything like that. Just a fun road car.

Which engine would you run (keeping costs in mind, cost vs performance, etc)?

 

[Julian]

Simple - turbo or supercharge whatever engine you use. Then you won't have much of a power loss at altitude...

 

[Echo]

Simple - turbo or supercharge whatever engine you use. Then you won't have much of a power loss at altitude...

True.. but I would prefer to keep it naturally aspirated.

 

[Jon Briggs]

I would go with a high revving 302 or 289. Revving it to 7000rpm will make grown men cry let alone small children!

If you want naturally aspirated at that altitude then your best choice would be fuel injection.

 

[Echo]

I would go with a high revving 302 or 289. Revving it to 7000rpm will make grown men cry let alone small children!

If you want naturally aspirated at that altitude then your best choice would be fuel injection.

From what I've been reading online (and we know how dangerous that can be ), the 408 stroker should be good up to 6500-7000 RPM redline, with the 347 good to about 7000-7500 rpm.

I'm leaning towards the 408, simply because it'll feel faster due to the higher launching torque from the mechanical advantage, but I'd still like to hear more opinions.

Plans are to megasquirt the motor. Full control from a laptop, and low installation cost.

 

[Randy V]

From what I've been reading online (and we know how dangerous that can be ), the 408 stroker should be good up to 6500-7000 RPM redline, with the 347 good to about 7000-7500 rpm.

I'm leaning towards the 408, simply because it'll feel faster due to the higher launching torque from the mechanical advantage, but I'd still like to hear more opinions.

Plans are to megasquirt the motor. Full control from a laptop, and low installation cost.

If you plan on running a stock block 347 to 7000-7500 RPM - I wouldn't plan on doing it very often or very long... Look into the new Boss block or Dart / World Products blocks.

The other option other than using a compressor of sorts would be to increase your static and dynamic compression ratio.. Probably something on the order of 11.5 - 12:1.. You'll want to review density altitude tables for your altitude along with sea-level to determine the optimum ratio.

Mind you - if you ever bring it down to sea level - you better have some really good high octane fuel for it..

 

[Echo]

If you plan on running a stock block 347 to 7000-7500 RPM - I wouldn't plan on doing it very often or very long... Look into the new Boss block or Dart / World Products blocks.

I was just speaking about general redlines (assuming properly built valvetrain, bottom end, etc), not necessarily what I'd do to it .

The other option other than using a compressor of sorts would be to increase your static and dynamic compression ratio.. Probably something on the order of 11.5 - 12:1.. You'll want to review density altitude tables for your altitude along with sea-level to determine the optimum ratio.

Mind you - if you ever bring it down to sea level - you better have some really good high octane fuel for it..

My primary reasons for not using a compressor are mostly based around cost. It's cheaper and easier to throw more cubic inches at it than a full forced induction system. I don't want to ruin the beautiful GT40 180* header exhaust note with twin turbos, and I've got serious moral issues with superchargers.

Increasing the CR is definitely a good option. We haven't quite reached the combustion section of my thermodynamics II course, so I'm still not quite sure how much I can raise the compression ratio with 91 octane gasoline and not have detonation (even pinging). I'd love to hit 12:1, but like you said, if I ever take it any lower in altitude, I'll need some racing fuel .

As I stated in the original post, I'm not going to be racing this at the track, competing professionally, etc. I'm interested in a very fast feeling car, not necessarily the fastest car (does that make sense?). I'm a very visceral person, and I love the feeling of being in a car, planted in my seat, with a glorious V8 roaring away.

There is a doubt in my mind that a 347 stroker wouldn't meet my final requirements, but that's just based on other experiences that I've had in cars. I haven't ridden in a 347 stroker GT40..

Based on what I've said, does anyone think a 347 would be the better option? Or should I stop worrying about it and punch out a 351W to 408 cubes?

 

[Randy V]

If it were me - I would totally ditch the notion of running an Audi transaxle with the torque of a 347 or greater engine. While there have been some inroads made on the strengthening of the Audi box - you can only wring out so much capacity out of that small gear-box and ring/pinion interface...

At the weight of these cars (~2400#) - you'll find that even a 300 HP engine will move you along at a rate that your senses will appreciate.

 

[Howard Jones]

408 @ 10 to 1 CR, good alum heads (2.20 intake/1.60 exaust for the big cubes) Hyd roller cam (for the bad oil we have avaiable today) 750 holley (it can be tuned just fine for a given altitude, what carbs don't like much is changes in altitude). You should be able to get 1hp per inch on pump gas with this spec. You will fine that hyd lifters/cams systems really won't work very well above 6500. That's ok and good for the wallet.

400HP is right in the sweet spot for a 2500-2600 pound GT40. That ought to net you about 350HP at the wheels. Just right for a road car in my opinion. It's also near to but within the limmits of a good Audi box (if you drive it like its your money).

 

[Garry]

I tend to agree with Howard, however,Randy brings a good point to the weaknesses of the Audi box. Let's see what the NZ'ers have to say. Jac Mac is a really good source of info and they use a lot of the Audi boxes down under.The Porche box would be better if you can't budget for a ZF or a ZFQ. There are some occasional deals on the Porche box from some of the vendors who post on this forum. I'd want to hear from them as well.
Garry

 

[Mike Pass]

My 2c.
With 20% altitude loss from a 500hp big motor you are left with 400hp. 20% loss from a 400hp motor leaves you with 320hp. Start with more and you end up with more. How much power do you want at altitude? If you want good performance from a natuarally aspirated motor at altitude use a cam with small overlap, high compression and a good flowing manifold/cylinder head combo. ie good pumping efficiency. My vote goes to an alloy block 428ci (351 size block) with high lift road cam ( or high ratio rockers) and some good heads/manifold. Less weight to shift makes the power and torque that you have more effective.
Cheers
Mike

 

[Rick S.]

I agree with Howard & Mike, where the recommendation is for larger displacement. I would keep compression ratio modest.

 

[Jac Mac]

I'm going to be running one of these engines in front of an Audi transaxle. It'll be a road car that I want to have fun in. I'd like to feel fast as can be, make small children cry, that sort of thing. Since I'm losing a quarter of the performance simply by being at this altitude, I'm not too worried about the transaxle. I'm not going to be dumping the clutch, track racing, competing with it, or anything like that. Just a fun road car.

Which engine would you run (keeping costs in mind, cost vs performance, etc)?

This paragraph plus another post where you mentioned you will likely take it on a trip to lower altitude spells it all out.
From this I take it that when you find a nice chunk of winding highway you will likely downshift to 2nd & pretend your a Le Mans driver from the past as you accelerate up thru the gears, then back off with a look of satisfaction & guilt smeared all over.
You have less chance of breaking a trans if you build a high RPM combo with a light flywheel / clutch package than if you use a heavy big torque cube monster...... BUT the Hi RPM deal will dictate a solid cam etc which apart from cost will require more frequent maintenance ( valve lash etc), are YOU up for that?... I dont mean paying someone else to do it...I mean you...... To me driving a car like this should be out to max RPM in each gear with RPM dropping to the 'sweet spot' on each change up, you dont get that with a 400 cube long stroke monster with a 6k limit as the steering corrections keep you too busy.

 

[Keith]

This paragraph plus another post where you mentioned you will likely take it on a trip to lower altitude spells it all out.
From this I take it that when you find a nice chunk of winding highway you will likely downshift to 2nd & pretend your a Le Mans driver from the past as you accelerate up thru the gears, then back off with a look of satisfaction & guilt smeared all over.
You have less chance of breaking a trans if you build a high RPM combo with a light flywheel / clutch package than if you use a heavy big torque cube monster...... BUT the Hi RPM deal will dictate a solid cam etc which apart from cost will require more frequent maintenance ( valve lash etc), are YOU up for that?... I dont mean paying someone else to do it...I mean you...... To me driving a car like this should be out to max RPM in each gear with RPM dropping to the 'sweet spot' on each change up, you dont get that with a 400 cube long stroke monster with a 6k limit as the steering corrections keep you too busy.

Whoaaa, I got all moist when I read that Jac, you are so damned right mate....

 

[Jac Mac]

Whoaaa, I got all moist when I read that Jac, you are so damned right mate....

I didnt add the bit that once he had achieved the ' satisfied/guilty' look to go back & try it again one gear up at all points where your on the limit of tire adhesion & you dont really have enough power/throttle left to create a bit more oversteer with, yet you realise that to lift off its going to send you down the road in a huge tank slapper, you know the feeling, where you can feel that vacant lot being created in your gut!

Never on a public road though!!!!

 

[Cliff Beer]

Me personally, I'd go for lightness and a higher/faster revving motor given your "elevated" position. The 347 doesn't want to rev past about 6,500 too much IMHO unfortunately. Ideally, I'd want an aluminum block SBF in a 331 with some great heads, larger valves, and solid cam/high lift, along with EFI/ motronic (or similar) for easier/better spark and fuel control to deal with the altitude changes.

As has been pointed out above, you don't need a crazy amount of hp/tq to make a 2,400lb GT40 move out and scare the snot out of you....

 

[Brian Hamilton]

From my experience, the 331 stroker built on the 302 engine is a much better street engine. Now, if you have the availability of E85 in your area, then stay with about 12.5:1 and just use that fuel all the time. It loves compression, both mechanical and artificial (boost). Now, the reason I say the 331 is a better street engine, is the 347 strokers usually put the wrist pin up in the ring lands. This is a known issue with the 347 stroker and one that most Mustang guys just shrug off. The 331 solves that problem and is just a dynamite engine setup. The 408 is a really great choice, as is the 427 stroker. You can order a tall deck 351-W and make yourself a 460+ cube motor, but honestly, I think the 408 or even the 418 cube motor with the right heads and valvetrain along with nice compression will work out great. Remember, if you have E85, go friggin nuts! In all reality, a boosted engine is really the best way to go for high altitude performance, but you said you want to keep it NA, so go for it man.

Laters,

Brian

 

[Echo]

The conclusion I've come to is to stick with a 302-based engine. After weighing the pros and cons and reading everyone's response over and over again, I simply like the lighter rotating mass, lighter overall mass, and smaller package of the 302. That, combined with the deep gears and limited strength of the audi transaxle will make for a better combination, I think.

My plan at the moment is to go with the 3.4" stroker crankshaft. From what I've been reading online, the oil-ring issue was solved years ago, and with any high quality pistons, shouldn't be a problem. The longer stroke will bring the torque curve down a bit, which is better for a street motor (not that the 3" stroke, or 3.25" stroke is bad..). The only thing I do worry slightly about (which I am told isn't something I should be worrying about) is the rod/stroke ratio of the 347 (1.588:1) versus the 331 (1.662:1). Although the more favorable rod/stroke ratio of the 331 will lead to a theoretical longer life and in theory, more torque (due to the higher piston dwell and therefore better chamber filling), I'm not sure if it can compete with the additional stroke of the 3.4. Then again, the rod/stroke ratio of the 408W stroker is only 1.563:1, which is worse than the 347..

If anyone wants to see my numbers, here they are:

That is assuming the 5N code Audi 016 transaxle. The "Static/Dynamic Compression Ratio" is assuming sea level conditions, and the "Effective Static/Dynamic Compression Ratio" is assuming the inputted elevation (adjusting for 7500 feet, in this case). The camshaft specifications are for the Ford Racing Z303, but that is just an interim value to run the equations, I'm not set on the valvetrain/cylinder heads yet. The jury is still out about the lifter/camshaft combination (solid vs hydraulic). The solid has a bunch of advantages, I just don't want headaches with it down the road.

My plan for the transaxle is to stick with the 5N gear ratios. I don't think the 4.11:1 gears will be a problem with the 325 nitto tires, but I could always swap to the 3.89:1 AAZ gears if it's a problem. Thoughts? I'm also planning to put in a Porsche LSD to help with traction.

Thanks everyone!

 

[Tom]

Echo,
I've been driving my Cobra in and around the Denver area (5k to 14k ft) for several years. What works out here first and foremost is EFI. That alone will give you instant response at most any elevation in Colorado. That equates to an 'explosive' feel when you drop the hammer, while the carburated car ahead or behind you is still trying to clear its throat and spewing exhaust smoke.
Since you're not considering a turbo or supercharge then I'd opt for a few more cubic inches vs going with a strictly high-revving motor. You'd be left behind at the stop light while you're trying to modulate the clutch, revs, and wheelspin, till you get your 'revs' up. This is where the higher torque motor rules in this thin air. Revving a motor under load to redline at 8000 ft feels like it takes forever vs trying to do the same in lower elevations. Besides who drives in traffic revving past 8k (besides a Rotary or crotch rocket). It's fun (and intimidating for the other driver) when you leave the high-revving/low torque cars behind at the stoplight without much effort or in another case, when you're coming out of every sharp corner in the mountains. It also attracts less attention from the cops.

BTW, a 331 would be a great compromise if you want to go 302 based. Check out the new Boss block and go large bore!

Just my experience from driving different types of cars at altitude....

 

[Will Parker]

Can someone explain what is meant by "favorable rod ratio" and what makes one combination good and another bad?

Thank you