Planning an engine for power at low revs (long post)

L

LincolnMarkVII

Active member
I'm not sure if this should go in here, or in hardcore inline, or in the turbo/supercharger section. Also, sorry for the long post ahead..


I could rant on for ages about my plans for the car.. but to simplify matters, I'll try to be brief. I have a 200/C5 in my '83 Fairmont now, and a line on a '76 250. This car will probably never see a track, since I'm not interested in racing.. this is strictly my nimble (but safe) around-town toy. Fuel economy is also a priority, as I want it to be efficient enough to use regularly without worry. It's 2900 lbs. as is. I'll probably keep it automatic.. AOD if I use the 250, modified 5R55E (inspired by "Project Frankentranny") if I use the 200. I'd like to convert to fuel injection. I'm thinking 3.55s or 3.73s out back, but in a Cobra IRS. I'll also upgrade braking and do the usual structural reinforcements to match. I may lower the car slightly, but we're talking an inch or two at most. I also want the car to be user-friendly enough that any member of my immediate family (mostly middle-aged women) could get in it and drive it safely without worry, if they needed to. To that end, I may consider one of those newer ignition boxes that includes a crude traction control function.


The part I need the most input on is the engine. I want to build something designed for power at low revs, with no real care if the engine ever sees much above 4000 RPM. I want instant power with no lag within that range. I am open to either a turbo or an Eaton M90, although most of the friends I've discussed this project with are biased towards turbos. (I'm biased towards superchargers.. I've always wanted one) I don't have a problem with things like having a custom header fabricated to try to enhance low-end grunt, if need be. I would be interested in the CI aluminum head, IF it would be suitable for this application. My guess is that I would need a truck/RV type cam, but this sort of thing is why I'm posting asking for advice. I've never bought an aftermarket cam before.

Unfortunately I have to cut the post short because a friend just contacted me needing help.. but I'll try to fill in more details later, as needed.

Thanks for taking the time to read this, and any thoughts on this would be appreciated.
 
well your basic goals then would be to:

1) improve the breathing efficiency of the engine
2) get the most out of the fuel before it is exhausted

so yes, the RV cam is good, and an appropriate exhaust is good. Do a search for the words 'torque' and 'exhaust' and you'll come up with some good info. Your stock valve train will handle your requirements nicely.

What are you doing for a carb? A good single barrel will be fine for what you are after. Stay away from any spark control valve carb.

Put on a good ignition such as a DSII with a hot spark, or a DUI unit. Make sure the advance curve is good. Open up your spark plugs to 0.055".

Other mods will get you some, but limited bang for your torque $$$.

Your chassis mods and gearing are for a higher rpm and higher hp engine. Not a problem, but not consistent with the torque goals.

Have fun planning and executing :D

John
 
The oil pan on the 250 won't fit a fox-body, it'll have to fabbed up.
I get great performance around town without too many mods, and it doesn't scare my wife to drive it. Stock cam works great up to about 3600rpm, but no top end. 4000rpm @ 85mph is WOT on my car, with 3.08 rear.
Go for the double-roller timing chain so you can advance the timing.
I run the stock exhaust without any smog stuff, no cat, but I do have egr hooked up and use the Holley 1946. I've adjusted the accelerator pump shot so that there is no bog, and plenty of get-up-and go. I've also swapped out the heavier of the distributor springs for one from the Mr. Gasket kit.
It's all I need at the moment, considering the speeding tickets I've accumulated this year, and I'm reluctant to go any further and swap in the modded head, cam, and two bbl carb :!: :x
 
To catch up on this..

I have the stock 1 bbl. now, but when the time comes I'll be converting it to EFI. I was thinking about the possibility of adapting the EFI setup from a Super Coupe to it.. but I'd probably just go Megasquirt.

I knew about the oil pan part.. I was thinking about trying to pick up a second Fox 200 oil pan to base a custom 250 pan off of. If I could pick up a couple extra quarts of capacity in the process, that would be even better.

My fiance is used to driving my '89 Mark VII with just the stock HO, AOD, and 3.08s out back. This car (at almost a thousand pounds lighter and similar eventual horsepower levels) will definitely be quicker.. but hopefully not so bad that she couldn't drive it safely.

I believe I already have a Duraspark II on it.

The chassis mods may not be strictly necessary for my build, but I want the car stiffened.. both to tighten it up, and to help it last longer. I want this car to last a very long time. It only has 101k miles on it. I'd like to see triple that if I can.

Exhaust behind the header, I have two lines of thinking with this.. I may just run a 2.5" single to a Cherry Bomb glasspack, or I may run something a little more complicated like single 3" up to a Warlock, and then 2.5" pipes over the rear axle with one of those electric cutouts on the third outlet. If I do that, I'd dump each pipe out right behind the rear wheel. I kinda like the thought of pushing a button to make it louder when I get a wild hair..

If I went with an Eaton M90, I'm thinking I could fab up a header with REALLY long primaries.. as much pipe as I could cram into that side of a Fox body. If I went with a turbo, I don't know what I'd do about the header. I was reading up a bit on header design and how to tune it for low-end torque or not, but I don't know how a turbo would impact that. I don't know if I should plan to run pipe around to the open side of the engine (like to the missing smog pump's location above the PS pump) or what.

BTW, my goal of "around 200 RWHP" is just an arbitrary figure.. if I got 250 or 300 while still being 100% streetable and reasonably fuel efficient, I would not be complaining one bit.
 
Just curious, is that an '83 Aussie Fairmont, or an American one similar to your little picture there?
 
8) as with any engine build you need to take a systems approach. since you have already decided the rpm range you want the engine to work best in, you have already made the hardest decision. the first thing is to build the basic short block for reliability and durability. you dont need forged pistons, unless you want to run more than 10psi boost pressure. you want the rotating assembly balanced, but dont spend the money on blueprinting as it is not worth the money for the results.

now for cam selection, pick one that works in the off idle to 4500 rpm range. classic inlines has one that is real close to what you want. more lift, less duration, wide lobe centerlines, and low overlap are what you are looking for.

next the head. the best head is obviously the new classic inlines aluminum head. mike has done a spectacular job on that head.

next would be headers, in this case you want a small tube header for best low speed torque. something in the 1.5" primary tube range.

set your compression ratio around 9.5:1 with a wide lobe center cam, otherwise you run into detonation problems. since you are using efi, you want to use a decent sized throttle body, but like a carb not too large. i would say a 60-65mm TB will work real nice.

set your timing around 10 degrees initial if you use the classic inlines aluminum head as it has a fast burn combusiton chamber.

now to the biggest choice, engine displacement. the long stroke of the 250 means a lot of low speed torque, but the taller block deck can be a problem. on the other hand the 200 fits real nice, and while it doesnt have the low speed torque of the 250, it also doesnt mind running at higher rpm. for a light car like the fairmont, the 200 will do real nice. by the way if you decide on using a supercharger, drop the compression ratio to between 8.5 and 9:1. again keep the boost pressure below 10psi.

i think the 200 with above modes, blower or not, and 3.73 gears will make an excellent street engine in the fairmont.
 
Thanks for all the feedback.

It's not entirely necessary, but I feel I should point out that this plan calls for 3.73s with an overdrive transmission. Currenly the car has a C5 and a 2.73 rear end. According to my spreadsheet, an AOD (which I'd probably use with the 250) has a .67 OD which knocks 3.73s down to 2.5 final drive.. numerically slightly lower than 2.73. I get to around 3k RPM at 70 now with 185/75R14s.

As an aside to that.. I realize the C5 is a pretty hefty lil' bugger. I think in lurking here I saw that someone said they thought the C5's torque converter was probably in the neighborhood of 80 lbs. I've also read here that people tend to think an AOD is a little too massive for a stock small-block 6. Thirdly, I've read it postulated elsewhere that a lot of rotating mass like that was bad for top end, but good for low-end torque. Given that I'd be building the engine well past stock, and given my goals, does an AOD seem a better fit for the built 250?


If my questions seem a little ludicrous at times, it's because I'm REALLY good at crunching raw numbers like gear ratios, but don't have much experience testing those numbers behind the wheel. I have a spreadsheet that has a column for transmission ratios, a column for a possible gear splitter, a column for rear end gearing, a column that calculates final drive ratio for those figures, and then a few more columns that take tire size and final drive ratio and output three different MPH figures at different RPMs.. (1000, 4000, and 6000) I can regurgitate a ton of numbers, but I have little idea how those numbers will actually feel. When I give a number like a 3.73 rear gear, it's because I've taken my current setup and applied a different transmission's set of gears to it and tried to make the last gear's final drive ratio close to what I have now. If it turns out that the built engine would better handle a lower highway RPM, I'd bump down to 3.55s or so without hesitation.

Wow, didn't mean to write a novel there. Sorry.


Edit: One thing I forgot to ask: How would the step up to 1.6 roller rockers affect the build? Let's say I went with that idle-4500 RPM cam that was mentioned above. Would I be throwing things off significantly? I'm a little fuzzy with cam specs since I've never messed with aftermarket cams or valvetrain improvements. (yes, I know, I'm biting off quite a bit here with this project)
 
8) an 80lb torque converter :shock: :shock: :shock: :shock: not likely since the entire trans (C5) is only 125lbs or so. the AOD is heavier, but not by much. and yes the rotating mass of the AOD is greater than the C4/C5. remember that the C5 is a version of the C4. rotating mass hurts performance through out the rpm range. the only time rotating mass is good is when you have a heavey duty engine that pulls heavy loads, but doesnt turn rpms above 3000 rpm, rather like a diesel engine, or heavy duty gas engines. for a street engine like you are building, eliminating as much rotating mass, with in reason, is the best way to go. the AOD will do a fine job for what you want.
 
Hooray for the modern marvel of overdrive, right?

OK, so it was an exaggeration about the C5 converter weight, then. Good.

I'm still leaning towards building the 250 if I get it, because of the bellhousing bolt pattern and the bit of extra torque.

I could offer up a desired boost figure, but my guess is that boost will be dictated by the size of the gap between my power goals (we'll say 200-250 RWHP) and what the rest of the engine would do on its own. I am definitely interested in the aluminum head. I have no problems with the cam that has been recommended. I'm not too squeamish about dealing with an EFI conversion, even if I've never done it before. I like to think I could do it with 7 PSI and no intercooler and cast pistons, but I also like to think I'm 21 again and thin as a rail and make James Bond look like a nerd.. but what I like to think doesn't necessarily have any bearing on reality.

I want to build this right the first time, not tinker around with it and see what I can get away with using whatever's on it at the time. Even if I wrecked this car between now and when I'm ready, I'd pick up another I6 Fairmont or Zephyr and use that for it. (maybe even a wagon)
 
Sounds like you're right up my alley with your Fairmont. Check some of my posts and see if there's anything you might concider doing differently based upon them. The only thing I see is that you shouldn't bother fabricating a special long tube header for a forced induction motor. The exhaust is pressurized and it won't matter how far it has to travel to get out. It just wants out. Equal length headers are applicable in turbo motors, i.e. ramhorn style headers, but that's to equalize flow to the turbine. Also might I suggest getting a copy of the Falcon Six Performance Handbook? This is a fantastic resource when it comes to dealing with our sixes.
 
:twisted: A word of warning. Unless you use a modern 280 degree camsahft, and Mikes head, and EFI or a good set of DCOE Webers or a triple 500 CFM Holley carbs, 200 rwhp is not achievable without a turbo or supercharger.


:wink: Your gearing and assumptions seem bang on.

I'm sure you understand that power is a compromise of fuel economy, engine charactersitic and rev range. Its like this...power is cubic inches times rpm, devided by an aspirations index. You cannot create 302 or 351 power without ending up with the engine being short sheeted of either low end torque or having to scream harder than a V8.


As an example, Chrysler Australia experiemented with a 185, 205, 216, 248, 280, and 306 gross hp variants of one 265 cubic inch engine in 1972. It started with a 2-bbl I6 grocery getter, then upgraded in stages from a 17.3 second quarter mile plodder to a 14.4 second road racer. First upgrade got dual out headers on the 205 version, then dual exhasts on the 216 version, then triple carbs on the 248 hp number, then a 280 degree cam at 280 hp, then a wild 302 degree cam for the 306 hp version.

Mopar found that you could exceed the performance of a 351 Cleveland with a very high tuned I6, but becasue it required a 302 degree camshaft, the engine was always very noisey and required a quite a lot or revs to do the same work a hot V8 could do easily. The stock 185 hp engine made power at 4500 rpm. The 306 hp at 5300 rpm. The peak rpms were 5000 for the 185 hp version and well over 6000 rpm for the hi po version. In real terms, the rear wheel hps were 115 for the 185 hp engine, and about 200rwhp for the so called 306 hp variant.

By about 1993 on wards, Ford Australia proved that 165 rwhp is achievable with a 264 degree cam. To get past that 165, you have to decide to loose low end torque, and gain extra revs at the top end to make 200 rwhp. Today, not even the latest non turbo 24 vlave twin cam 4.0 liter engine yields more than 185 rwhp, so you better realise that you will need to get a custom cam, carb, head and exhast to even get close.


What you are attempting to do is sort of an XR6 Falcon or Eta concept engine, aka 1985 BMW 525e, but on a bigger scale. That is , a reduced rev, high torque engine with just enough power to satisfy a performance oriented driver.

You are attempting to create what Ford Australia has done on all its I6 engines since 1976 untill 2007... optimise low end torque, and get as much power as you can for as little revs as possible without increasing engine capacity.

Your key requirement for just 165 rwhp must be

1. that the peak rpm is less than 5000 rpm, and
2. the maximum power is at 4500 rpm,
3. with peak torque at 2500 - 3000 rpm.

Eliminate a supercharger, as brake specific fuel economy nose dives when any engine is boosted. If, say , a 250 supercarged M90 engine verses a 302 both had , say, 200 rwhp, then the V8 would use less fuel. Unlike a turbo, a supercharger auguments an engines total displacement from indle to maximim revs, and that requires additional fuel. A turbo engine can operate at next to no boost at part throttle, while a supercharged engine is always behaving like an engine with 25 to 65% moe capacity, depending on the blower.

Any righteous attempt at an EFI four our I6's requires a whole new custom intake manifold. So think 2V Classic Inline head, or start making you own custom intake set-up now. You'll find the best power gain for every pound of gasoline used is not factory port EFI, or a SC 3800 style Supercharger, but port on port carburation like DCOE Webers, triple 2300 350 CFM Holleys or Mikes EFI system. The problem with any stock carb 250 is that at low rpm, there is very little inertial ram on a single carb engine, and you loose low end torque and power. Your best aims are to do what ex I6 and V8 Ford Australia engineering consultant said in 1990...the 250 six is best served by pulse tuning the intake by multiple carbs, and limiting the rev range to 4800 rpm. You can then fatten out the torque curve.

If you take a totally stock 250 I6, wack triple Weber throttle bodies on it, you gain well over 25% more power, even if you do nothing else. So a stock 1-bbl might do 95 hp net, and maybee 75 hp at the wheels. Add triple carbs, you get 100 rwhp. With a good dual exhast system, 25% more. With a combination of carb sizing or injector tuning, and a better camshaft, you won't get much over 165 rwhp (225 hp net) if you preserve a 4800 to 5000rpm rev limit. A great cylinder head (like Mikes), you will easily get over 200 rwhp, but you then have to trade off low end torque and look at a 280 degree or more cam, and extend the rev range to 5500 to 6000 rpm, peak torque at 3500 rpm, power at 5200 to 5500 rpm, and then there is 200 rwhp with ease.

Basically, you can get the performance of a 1996 302 Cobra SVT type engine with GT 40 heads and intake with any 250 block. Thats about 260 to 270 hp, or 190-200 rwhp. It's not hard to do, but you won't do it unless you have a large cheque book. :wink:

Here's to fame and fortune! :wink:
 
Thanks for the feedback. Let me try to expand a little on my mindset, so everyone can tell me I'm on crack.. I mean, err, where I can go from there. :D (yeah, maybe I am crazy)

I view this car as being a project to make a car *exactly* how I want it. I would, for example, have no problem with learning how to fabricate carbon fiber body panels for half the car.. (which is on my list of hope-to-dos) and then doing so.. but then trading the weight savings off elsewhere like adding Dynamat or more structural reinforcements or whatever. Yes, all for a daily driver. (and yes, I'd want to paint over the carbon fiber.. I don't care for the "carbon fiber look")

Why? Well, for one: I've had new cars before.. but they were never really what I wanted them to be. I wouldn't hesitate to spend ten grand (or more) fixing this up exactly how I want it to be. It already looks more or less how I want it to look.. (except the paint, and no tint on the windows, and I'd probably black out most of the chrome) it has exactly the options I want and no more, (A/C, PS, PB, rear defroster) and it's just the right size for me and no larger.

Honestly.. I try to think so long-term on a project like this, that I think about things like whether I could go .020, .040, .060 over instead of .030 then .060 over on the pistons so I could get one extra rebuild/refresh out of it. The car has 101k miles on it right now. Barring accidents or my untimely demise, I'd love to keep the car to 300k or more.



Another angle that's just starting to form in my head is that I'm trying to start a small powdercoating business, with the hopes of doing regular automotive painting as well after that takes off. I suppose if I needed to, I could view the car as a rolling business card for that.. but all that's just justification to give to people who aren't into cars. The above motivations come first.



Anyway, on to the engine. I would gladly use the new CI aluminum head in this. You've swayed me back in the direction of using a turbo instead of a supercharger. I really don't want to mess with carbs, though. Maybe it's because I'm part of "the video game generation", but I don't have a problem with converting to EFI and dealing with fiddly little computer nonsense. I like being able to get in and just turn the key and start it right up, not having to remember which vehicle of mine needs how many pumps of the gas pedal when it's cold to get it to start.

I've thought about things like whether I should use Megasquirt or whether I could try to adapt the EEC from a Super Coupe. Same number of cylinders, boost, and a similar displacement to the 250 I'm trying to trade for. Maybe, maybe not. Of course, now with me leaning back towards a turbo instead..


I don't drive with a lead foot. Much of my time is spent at no more than half throttle. For those times I do stick my foot in it, I would expect it to help itself to plenty of fuel. Think of my driving like a common house cat.. it can lay around all day and then suddenly get up and fly across the room for no apparent reason. I suppose that means I'm either looking for a turbo, or looking to quit caffeine. For those times when I'm "flying across the room", I want the car to be a sheer blast to drive, going straight and going through curves way too fast. For those times when I'm not, I want it to be nice and quiet and well-behaved and reasonably comfortable. Too much to ask? Hopefully not.


Didn't mean to write a novel there, but hopefully that clears up my mindset (crazy) a little. I don't have to have precisely 200 at the rear wheels.. this isn't the plot to some cheesy Discovery Channel show. "Can he make it to 200 rear-wheel horsepower, in the middle of the deadly Bering Sea..?" I do think it would be nice if the engine were more than a match for the stock 5.0 HO in my Mark VII, though. (even before the significant weight difference between the two cars) Yes, to most car guys, that makes no sense.. spending thousands of dollars to make it match or beat an engine I already own.. but I don't make sense. If I just wanted 200 rwhp for the sake of it, I already have a carbed 351W sitting on an engine stand just waiting for a few upgrades. Instead, that's what I'm trying to trade for the 250.
 
LincolnMarkVII, not to hijack, do you have a good source for a frankentranny or are you going to build it yourself?
 
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