rebuild advice on 3ltr sc motor
 

I need to rebuild the topend of my 3ltr and was considering a change to 3.2ltr pistons and cylinders. Is this is pretty straightforward mod or are more complicated internal mods necessary? I plan to run the current cis until I can swap to carbs.
I think my question might make it obvious that i'm still green behind the ears!!!
Thanks in advance for your replies!!

This conversion is known as a "3.2 short stroke" or "3.2SS". If you do a search on "3.2SS" you should find lots of info.

Here's the main points:
- It's pretty straightforward though not inexpensive.
- The conversion increases the bore from 95mm to 98mm.
- Factory 3.2L motors used the same 95mm bore but a longer stroke.
- You can get Mahle P&C for something like $4K.
- You can get later 3.2L cylinders, have them bored out and use them with 98mm JE pistons. (You can't use your 3.0 cylinders as cores). That will run you around $2.3K

Note: I have been unable to find 98mm 9.5:1 compression JE's. I've purchased 2 sets of "9.5:1" JE's that measured out to 10.5:1. Hopefully the situation has been corrected by now. Always CC your motor, never assume.

-Chris

Thanks Chris!

I talked to my mechanic and he basicly talked me out of of doing the mod.

What were the reasons?

Wayne

I currently run a 3.2 SS based on an SC.
Admittedly there are a lot of other mods done to the motor, but I am possibly one of the happiest Porsche owners arround.
I started with the Max Moritz 98mm conversion that i purchessed second hand. The power increase was about 8%, as expected with the capacity increase. Definately worth it.
One point to remember is that if you intend to go to carbs, is that the SC type piston desigen is not the best for carbs. It was designed for CIS.
If you wanted to use carbs in the future I would use a set of 98mm JE's. The JE piston desigen will work well with carbs and CIS with no ill effects.
Just my 0.02 worth.

The initial reason I managed to get talked out of it was because I think I really am not sure of what i want. I dont plan to do any racing, possibly some a/x. I have been doing some reading of other posts and I am leaning toward a milder mod on the motor.

As far as why my mechanic convinced me not to do the 3.2 mod......it was due to total expence for power and how much of that power I would actually use.

could someone give me some feedback on a 964 cam regrind and larger exhaust porting?

Thanks

The 964 cam will help a bit as will back dating the exhaust to the earlier SSI setup.
The exhaust porting is as large as you would like to go. The intake ports are actually too small. If you wanted to open up any ports you should open up the intake ports.
The only problem with this is a loss in bottom end performance due to the reduction in the intake air velocity with the larger ports. There would be an improvement in the top end performance, especially with the 964 cams.

Mahle Motorsport offers the 98mm P/C conversion kits for 3.0L applications in 2 versions: one for CIS cars like you stated and also a carb/mfi/efi version where there are larger valve relief's in the piston dome to allow use of more appropriate camshaft profiles with the "hotter" injection. These sets also typically have a higher advertised compression ratio than the CIS sets.

The price is more or less the same between the CIS & Carb/MFI/EFI sets, which is to say expensive. If you are doing this on a budget, the JE's will be a cheaper alternative.

Ralph

Correct. In the original post, it was mentioned to run 3.2 P&Cs until a carb conversion was done. Well, you can't simply slap carbs on a motor and get power - you need to swap out the pistons and cams as well. Read Chapter 4 of the Engine Rebuild Book - it clearly explains this...

-Wayne

Thanks Wayne,

I have been going through your book and I have found it to be very helpful. I read it during my down time at work and eves at home. I would like to have your opinion on whether going to the 964 cam is a good option considering I am, like many others, on a somewhat limited budget. I still would like to ad carbs later. but for now limit the work being done to higher rate springs and the cam, mentioned previously. If it helps, all I primarily am looking for quicker acceleration for the twisty roads and ax.

Thanks a million for your input!!

Rookie 911 owner
Alonso S

Alonso

Putting a slightly "hotter" cam like the 964 in a 3.0 CIS motor should give you some more horsepower. Reading posts, it seems the concensus is that this works and doesn't have nasty drawbacks like terrible idle.

But why do you want stiffer valve springs? You only need stiffer springs if you are using a really radical cam which slams the valves closed or has other tricky characteristics. The 964 is not such a cam. Or if you are going to rev the engine way up. Opinions on what way up is varies. Some get worried above 7,000. I think I got along just fine in a race motor with stock springs at 8,000 rpm.

Now reving the motor up another thousand RPM is a classic method of gaining horsepower. That's why Formula 1 engines can make so much horsepower from such tiny engines. What do they get - something like 800 hp from a 2.0L engine at 19,000 rpm? Something like that - I don't follow those things exactly because I am never going to run an F1 motor. So if you rev your motor quite high for what it was originally designed to do, you certainly need stronger springs to control those valves (I'm simplifying this a bit here).

But if you take a stock 3.0 and rev it up beyond maybe 5,500 rpm you start losing horsepower (on mine the optimum shift points are about 6,200 depending on which shift is involved). Why? Because it is starting to have real trouble breathing. It breaths best down around or a bit below 4,000 rpm, where the torque peak is (the torque peak is always at the point of peak volumetric efficiency because that is where the most air gets pushed - or sucked, if you like - into the engine). Above that the beneficial effects on horsepower of more RPM keep the horsepower climbing even as the breathing and thus the torque falls off, but that doesn't last forever, and at the HP peak the falling breathing takes over and HP goes down above that point (for best acceleration you shift above the HP peak, but that's another topic).

So if you put a 964 cam in your 3.0 engine, we can assume you will raise your torque curve because you will get better breathing. It should move up and to the right on a standard graph. Or perhaps just move to the right. But will it raise the curve such that optimum shift points are now 7,200 rather than 6,200 or so? I'm dubious (did Porsche use stiffer springs on the 964s?), but that is the kind of fact you really want to know before spending money on stiffer springs.

Stiffer springs cost a little bit of power (it takes extra power to compress them compared with stock) and increase valve train wear some. More to the point, they cost money. There might be something else you could do with that money to cut time off your autocross runs, or give you a bit more acceleration for spirited highway driving.

Frankly, if you drive your SC to its limits often on public roads you ought to be put in the looney bin because those limits are so high already they will exceed your ability to figure out when some guy turning left on a country road isn't going to expect that dot in the distance to hit him before he completes his turn, or will think he can see far enough around a curve that he can make his turn or pull out onto the road. Suspension, for instance, will probably help you more in autocross than power. Practice will do more than either suspension or power until you get pretty good at it. But we all do like more power. Power is good (at least as long as you can afford the fuel).

Once you know the crude version of the theory (pretty much all I know - kind of a qualitative understanding), you can do a smarter job of putting the pieces together by taking actual data from others and seeing what their results were. Maybe somebody who has put a 964 cam in a 3.0 engine (and kept the CIS and didn't port the heads or bump the compression and so on) would give you his dyno sheet. That way you could calculate what the shift point RPM is (another subject). If it is above 7,000, maybe the slightly stiffer springs would be a benefit worth paying for.

Walt Fricke

When I do a 3.2 or 3.0 motor with factory pistons and 964 cams I follow Bruce Anderson's advice and use a stiffer valve spring as insurance. Not because of throttle induced high revs but because these motors are invariably connected to 915 transmissions. :D

It's easy enough to kiss the pistons with the stock cams when you miss a shift. When you switch to 964 cams (w/their longer duration and slightly higher lift) the valves are lingering longer in the way of the approaching piston. In short, the stiffer springs are insurace against missed shifts.

-Chris

I get a lot of calls from people that just bent a valve running an auto-x or D.E. Most are 3.0 and 3.2 engines that have missed a shift.
If you move the shift point up to 6500, the performance springs are good insurance for the reasons Chris pointed out.
The 964 cams are a good upgrade, but I doubt porting the cylinder heads will be worth the $$.

For a CIS and/or Carb discussion, check out this thread. Look in particular for Tyson Schmidt's comment on the conversion. I'm offering this only as a point/counterpoint argument.

Secondly, did you say what year your 3.0 is? There were, for the five-year-run of 3.0s, two U.S. variants of the engine, provided you have a U.S. 3.0. Both do vary in their personalities.

Surely stronger valve springs are insurance, but if you're not racing the car in a way where you can miss a shift, I'm not certain the value is worth the cost. If you are, or find yourself driving aggressively, then I'd change the valve springs out, particularly if you're putting in a more radical cam - not necessarily a 964 cam, but for a cam like a 20/21, which is slightly more radical than a 964 cam.

As is, the stock 3.0 valvetrain is supposed to be good up to 7,300 RPM.

Nah, I would save your pennies and do the full conversion to higher compression pistons and 'S' cams. I think that putting the 964 cams in is a good option if you still need to smog the car, but it's really only a mild improvement over your other options. Not worth the $$$ if you're going to remove them later on...

-Wayne

Whoops, you forgot to post the link?

-Wayne

Sorry...
http://forums.pelicanparts.com/showthread.php?s=&threadid=236055&highlight=CIS+vs +carbs

Ok. This is getting really interesting. The motor is from a 78sc made for the US. What kind of changes would I need to perform, other than pistons and new rings, if I do decide to go with your recommendation, as I'm sure will be the case. Is buying a set of used pistons a resonable way to go? Could the SC cam be regrounded to "S" profile?

I plan to run CIS until My wife hopefully gives carbs for my next B-DAY. By the way I am also doing a backdate to the very simple look of a 1972 911s with sc flares in the rear as well removing weight. front and rear f/g bumpers from renspeed will be used to go with the ducktail already installed. The car will be painted gulf blue and have an absolutely spartan interior with modified 914 seats by added bolstering and perlon carpet kit as well as aluminum floor boards/footrests. I look forward to posting pics of my car in its current state and updates as they happen.

Thanks a million to everyone for all your feed back and advice.

Alonso:)

Alonso,
The SC cams cannot be used to make S cams. Its better to start off with a billet. The SC cams can be ground to 964 specs easily
until you get the carbs on your birthday..

John,

How will high comp pistons and the "S" cams run on CIS? I see the CIS as a temporary situation. I just wonder if I might have to deal with detonation or other dangerous hazzards?

By the way.......I assume you make these cams. right?
If so how much????

Alonso S

Like a dog.

If this motor is to be done in two stages then I think you'd be best off doing a stock rebuild at this point. Hopefully your pistons and cylinders are usable and don't need replacing. If you can swing it back date the exhaust system to pre-75' if it isn't already.

In stage two you can change cams, P+C"s, and induction. In stage two since you'll be changing P+C's go to 98mm to make a 3.2SS accompanied with the cams and induction and the motor will be a screamer. Scary fast.

Your ports are plenty big at this point so I would'nt touch them. They actually will be adequate for a 3.2 as well but if you want a little more high end then you can port down the road. Really though for a street motor I'd only port the exhaust to 37 or 38mm.

We all go kind of nuts when we get to thinking about hot rodding our 911s - so many options, power up to 500 hp or more if you want to spend the money. But the money is the sticking point that keeps most slightly sane here.

Stock Euro 3.0s from '80 or '81 on made a listed 204 hp (something like that). That's as much as the US 3.2 Carreras of the time. All you need for a nice upgrade to your 3.0 with its highly desirable big intake port heads and runners is a set of 9.8/1 Euro cis pistons and the early exhaust ('74 or earlier) or SSIs or the like (or race headers if you live where you will never need heat). If you have your heart set on a cam, then the 964 regrind of what you have. You can use your CIS - it will run fine and with your light car you will have great performance on a budget. There is some suspicion that an engine like this makes about 230 flywheel hp.

And you are on a budget, you say. If you weren't, you would probably be thinking of something really different, like a 3.6 transplant. Or at least thinking of EFI instead of carbs. You can't run high compression pistons made for carbs with the CIS with a whole lot of benefit (in fact, they should not work as well as what you have - the funny geometry of the CIS piston tops is there for a reason), and you sure can't run the hot cam that would work well (or well enough) with carbs in any interim using CIS. So why not decide to

a) hop up the engine you have to Euro spec and maybe a bit beyond with the CIS you have and get driving your personal rocket ship sooner and with more money left over, or

b) save up your money for your own idea of a killer motor and use what you have as you lighten the car.

Then, again, I suppose if you like building engines, the parts you buy now for an interim engine which won't work on the ultimate engine could be resold at half or so of what you paid for them so you wouldn't take such a hit at the next upgrade.

As I said, we all tend to go a bit crazy. But as you catch on to which parts will work with which other parts a pathway may unfold for you.

Walt Fricke

(happy his street 911 also races in a stock class, so his money for that car gets frittered away on tires and suspension instead of engine parts)

I went to a 3.2 SS with 964's, SSI's, Dansk 2 in/1out and CIS because I was concerned with smog testing. I'm really pleased with the results. I think for a street car it was a nice upgrade. I was still able to keep the car sorta stock and have the option of bolting the old exhaust system and emission crap onto the engine and make it possible to pass an emission test if need be. Of course, after a few weeks of reading about 3.6 transplants I too get the bug for more power. All in all it was a nice way to go at the time. My situation required a major rebuild..new pistons and cylinders so it wasn't that outrageous to go to 3.2ss. YMMV

Alonso,
Bobby is right, S cams and cis would be a dog. LOL
I like Walt's ideas. Especially running what you have now,
and doing the 3.6 conversion. I think you will find most of the guys
who did a 3.2 or 3.4 upgrade are happy with the reuslts, but like Ron, they wonder if a 3.6 conversion would have been better.

Taking a completely different approach to any engine modifications, depending on if your car is a '75 or newer model with a 915 gearbox, you can always switch out the ring and pinion to a 7.31. That'll get you some fairly serious power via the gears.

Hello, dd.

This is quite a common little bit of confusion.

Power is the rate of doing work..and we can conveniently measure it by a functional product of torque and revs per minute.

The power at the flywheel at any particular RPM is of course identical whatever gearing we use..

The power at the wheels is always a little less because of losses in tyres and trans...but can never exceed flywheel HP, and varies with it...

Changing the gear ratios cannot increase HP at the wheels at any particular flywheel speed or HP..it can only shift it to a higher or lower road speed..
This is of course just a matter of being prepared to shift into the optimum gear for power when you need it..

And some very very hot motors have such a narrow power (rev) band spread that close ratio intermediate gears are needed to optimise acceleration through the gears..


In a Porsche road car, there is little to be gained from gearing..four speeds are enough, the fifth being an economy gear for cruising..

But the real importance of gearing relates to cornering in race conditions..the ability to have a suitable gear for every corner on a particular track is very helpful..

Kind regards
David

David - I have not at all found that to be the case. In fact, with every car I've modified, gearing changes have had the best results over many engine modifications. I have a stock 3.0 with a 7.31 ring and pinion. My car has also been lightened quite a bit. It's no myth to me that my car can keep up with and pass 964s and also catch turbos. That isn't perception on my part.

Secondly - if there is any gear that is not useable with a 7.31 r/p, it would be the first gear, which I'm usually in for maybe two seconds before shifting to second. But overall, a 915 with a 7.31 r/p is very useable all the way to fifth gear.

Yes, I realize where power is developed, and gearing doesn't literally increase power. But that's the beauty of gearing changes. It doesn't put additional strain on the engine, and yet makes driving a much faster and immediate experience. Plus, one doesn't have to resort to bragging about power, but can rather expound that "less is more."

Hmmm..

How do you explain your observation?

If the power at the flywheel is the same...the revs are the same..the speed the same..what does it matter whether you are in ( say) 3rd rather than 2nd?

Kind regards
David

Power: I think we are dealing with the difference between engineering definitions, and a slightly more colloquial approach.

A few observations:

When I got my first 911, a '77 Euro with '74 headers but otherwise stock, I thought I had died and gone to heaven in terms of performance (coming off VW beetles and busses, to be sure). It had, and has, all the highway performance anyone could really put to sensible use. I could go up mountain passes so fast I had to slow for curves I hadn't noticed before, and I could rack up speeding tickets at a rapid rate.

That being said, we gearheads have a thing for performance, whether we need it for any practical purpose or not. And when we get hooked on track driving (which is very good for the driver's license points and insurance costs by satisfying most of the need for speed), we do have a use for more performance.

But I think that it is really uncontrovertible that you can improve performance, up to a point, with gearing. You can get better 0-60 times and 0-100 times, and even 0-120 times in the pre-964 cars I know best in this way by tightening up the ratios. That's why Porsche used to offer "airport" gears and "hillclimb" gears. Yes, you are not increasing power (horsepower), but you are able to increase acceleration up to a point. The one point you probably can't do much about with gears is top speed - you need X horsepower to push the car through the air, it increases exponentially as speed increases, and once you set your top gearing at what - the horsepower peak of the engine = the hp requirements for a speed in top gear - , that is probably it. Typically, with short gears you give up top speed in order to get better acceleration. And assuredly you typically give up fuel mileage: higher rpms = more air in = more fuel in.

I put a 7/31 into an 8/31 tranny. Wonderful unless you were doing long highway trips, where gas mileage dropped due to running at a higher RPM for hours at a time. And some folks don't like droning along at 4,000 rpm (or higher) for hours at a time. I built a short track tranny geared so that 5th at 8,000 rpm was 120 mph. Worked great, got a jump on guys with more horsepower (and with cars of comparable weight to mine) because 120 was just a dream on those short straights with fairly tight corners leading onto them.

The ultimate in transmissions is the variable ratio tranny, where the engine RPM is constant (I can never remember if you use the torque or the HP peak for this, but I think it is HP) and acceleration occurs because the gear ratio increases. Infinite ratios without the losses that happen during shifting.

And yes, with fairly broad power bands beyond a point tightening up the ratios may not help. I put a 60 mph first gear into the race tranny I now use and thought I'd start beating guys out of slow corners. But that wasn't happening. Why? I think because I had to shift when they didn't, and lost in shifting time what I made up in better initial acceleration.

But the 7/31 with 8/31 gears is a nice combination if you don't mind the higher RPMs for long, high speed cruises.

Practical - well, what is. But it will pep up the performance up to any realistic street speed.

One thing which was very helpful to me in sorting through this kind of thing was to chart (and later to use Excel) delivered torque at the rear wheels with speed and gears and rpm: chart torque at 500 rpm increments along the left side, multiply that with the gear ratio times the rear end ratio and tire radius at those same increments in the next columns, and toss in columns which show what speed relates to a particular rpm in a given gear. Then graph all this for each gear. Delivered torque increases up to the torque peak, then falls off as speed/rpm increases. As you upshift, the rear wheel torque inverted U shaped curve falls lower and to the right on a graph where speed increases to the right.

Here is a chart Craig Seko did - sorry, not a 911 but it was the first one I had handy. Thrust is torque times gearing here. You can see how changing the rear end ratio by changing tire diameter changed things. And that if you stuck a lot more gears in there you could increase the area under the curve, but not beyond a basic limit: peak torque. These charts are usually done to find the optimum shift points, which are where the lines cross.

http://forums.pelicanparts.com/uploa...1127932410.gif


I think dd and ppp both know all this stuff, probably better than I do - they are just crossing terminologies and maybe philosophies a bit.

Sorry about getting further off the topic of how best (based on one's personal criteria) to make a '78 SC engine more powerful.

Walt Fricke

Revs are quicker, gears are better matched to the engine's capabilities. And most importantly, I have power through 6,500 in fifth gear. I'm using the entire engine until it becomes unsafe to rev it higher.

Standard 8.31-geared SC cars run out of breath at, I believe, at 5,500 RPM.

Hello again.

The engine is unchanged?

If so, the torque and HP curve is also unchanged.

The engine will run out of breath at the same revs.

Nothing you can do to the gearing will affect the engine.

Kind regards
David

You guys need to set up a timed event, one guy starts out in 3rd gear and then shifts to 4th. The second guy starts out in 2nd and shifts to 4th.
Make it a flying mile somewhere out on a closed course, of course.

My money is on the guy starting out in 2nd . But it might be a lot closer than I expect.

The gear ratio change does a lot for the "butt dyno" numbers.
I have heard about small changes on the Dynojet chassis dynos by running the car in 3rd gear vs. 4th. I hear it is more to do with software than actual increases in whp.

Hello, John.

Yes there will be samll differences I'd say..

The efficiency of gears is lower ( losses higher) with more change in shaft speed from input to output.
So, second should be slightly less efficient than third....

But, of course, to maintain the same road speed, the final drive will be different..

and the lower FD will be slighly less efficient than the higher one..

Overall, these things are very very small, and likely to cancel out.

On the rolling road of course, tyre losses dominate...

Kind regards
David

Timed event? I'm game. One problem though is wheel size, tire diameter and weight all play part in what I've accomplished with my car. It isn't like the 7.31 r/p is the only modification taken into account. The main thing I want to get across is it isn't exactly only about the engine. In many cases that I've seen on the Pelican board, the engine is the very last item in a 911 that needs to be considered for modification. Your driver ability is first, your weight second, your suspension third, your wallet fourth, fifth and sixth, then your engine. :D

And David, unless my tach is a liar, I'm pulling power at 6,500 RPM. Not great power as I might between 4,000 to 6,000, but at 6,500 RPM, the car isn't slowing down any, nor is it riding off velocity, either.

Sure, you have power at 6,500.

Nothing you can do to gearing will affect the engine torque curve..

Kind regards
David

...you should try the modification. Unless you've run out of money from doing engine work. ;)

Hello again.

I have most variations of trans ratios round here, so I do know..

kind regards
David

Just out of curiosity, what would the ratios turn out to be?

Here is a Dynojet rear wheel torque and HP curve chart for an SC with a stock '82 engine and '74 headers and straight pipes. Other SCs in similar configuration are about the same, depending partly on cam timing - mine was set toward the torque end and my peak torque was higher than that of various buddies. And their peak HP was above mine. I took the car to the dyno about five years ago. Good thing, too, as my glasspack muffler was nicely loud but cost me buckets of power - I thought I had messed up the engine rebuild until I put something better on.

http://forums.pelicanparts.com/uploa...1127953397.gif

Engines "start to run out of breath" at their torque peak. But the beneficial effects of RPM increases on horsepower outweigh that until the horsepower peak. And you keep going beyond that so that your rear wheel thrust in the higher gear will equal that in the old lower gear right at the shift. Doing it any other way will cost you, as you can see from the thrust/speed/gear graphs.

In lower gears the car will still accelerate well into the 7,000s because torque and power have hardly fallen to zero up there. I used to run my 2.7 up to 7,200 at one point on one track because I could save a shift - slow corner to slow corner, with not enough space in between to make shifting up worth it. That car's rev limiter was easy to tweak so this could be done and I didn't have to sit on the rev limiter or back out. But that would be silly on a long straight.

I make peak torque on this engine to be somewhere around 3,800 rpm, but the curve is nicely sort of flat around there. Peak horsepower is around 5,500. With stock gears and 245/45-16s, the second to third shift is best at 6,250 rpm according to the formula, and the rest of the shifts are about 6,200 (the stock first is so low you just run up to redline and shift - that's the best you can do). This keeps you on the fattest part of the HP curve and gives the best accelleration.

Were I to plug in 7/31 with 8/31 gears "short" box numbers, I'd find that I'd be shifting at a lower RPM, and the drop between gears would be less (I'd start at a higher RPM in the new, higher, gear). Thus at both the start and end of 3d and 4th, and the end of 2d and the start of 5th, I'd be at a higher horsepower (and rear wheel thrust). So I'd accelerate faster. Unless we are going for top speed records as in Bonneville (slightly different considerations there), I'll do better with the short box than the stock box.

So gears matter for performance, if acceleration is the measure of it. Of course they don't make the engine more powerful, but they get the most that it has to give. Isn't that what dd is saying?

As to camgrinder's experiment, the way to compare apples to apples is to do this with the same car. The practicality of this depends on what gears you have and what speed you start at. Skipping third is going to have you revving way beyond where the power is in second, and starting out way below where it is in 4th. Which means you'll be putting down a whole lot less thrust for a long time as you slowly wind out. Starting in 3d (say the start speed is 30 mph?) will be kind of slow for a while, but until you get up to 6200 rpm (or so) you will be way ahead of the game, I posit, for that speed range where you are lugging in 4th. Since acceleration falls off so much with speed, you should be better off sacrificing at the low end. Put another way, you'll be screwed up in both gears part of the time one way, but only in one gear the other, and not where it counts as much. That's why the books recommend setting your gearing so you use it in equal speed blocks - 60-80, 80-100, 100-120, that kind of thing.

Now when that '78 gets its 9.7/1 CR CIS pistons, good exhaust, and 964 cams, his torque curve is not only going to be a bit higher (due to the CR boost), it is going to shift to the right with the better breathing, which will make a large difference in horsepower and performance, I predict. (My 2.7 race motor has a torque peak at 6,000, and a HP peak at 7,100, and has 50 more RWHP than the 3.0 even though it has 5 ft/lbs less peak torque). Add the planned weight loss and he'll run rings around us stockers even without shorter gears. Now a tricky question would be to compare the effect of gears only versus engine only in terms of straight acceleration, which I think is what he has in mind.

Walt Fricke (it is us track guys who rate seat time ahead of everything else)

All other things being equal, if a gear ratio cuts the rotational speed
of the wheels in half, the torque (to the wheels) is doubled.
The HP required to sustain a set velocity is dependent on load factors such as wind resistance, tire resistance etc.
I had a new '89 Gt Mustang that would go 145 or so in 4th gear. If I remember correctly the car would stop pulling about 5200 rpms in 4th. Shifting to 5th gear (.68 overdrive) would limit the top end to about 125 mph. The engine was now out of the powerband and not able to push the boxey Stang thru the wind.