Why are the 73 302HP so low?


New Member
Mar 26, 2003
Well I really know the answer, or at least one part. Emissions. They are rated at a depressing 150HP or so. I have a 1973 Mustang Mach 1 in yellow like in gone in sixty seconds 1 or the orginal. It has around 54,000 miles on it. My goal is to get it to 350-400 HP NA or Forced Induction. I just am new to the V8, I have a DSM that I'm doing a head swap on, because inter- ferring heads brake valves when timing belts brake at 100,000. what a long project I got into there lol. Anyway. I want to be able to run about 91-92 octane. I really would love to leave the bottom end alone to get to my goal. By the way it's a 302 2v and it's auto, not sure what tranny. I would love to get to 400 by changing the carb, heads, cam,intake, and maybe a super charger, I know there very involved, but I think I can do it. It sounds very nice with the thrush side pipes that were installed back in the mid 70's when my dad bought it. He's only the second owner, and bought it with about a 1000 miles on it back in late 73 early 74. I know to get to my goal I need to elimanate the emissions, but not sure how, and what I can safely remove. I'm 18 and have finally got the green light to mod my dads baby.

I would love to keep the compression down, and lumpy idle doesn't matter, because I like that anyway. I have a mustang I bought myself, but it's a 1995 black V6 vert, thats fun, but will never look or feel the same as the last of the classic mustangs . Lets face it 74 was horrible, and from then on got worse. Then they beacme better again in the 80's, but not as much as the classic's. I plan to become a tunner and master tech someday, and all this "schooling" will help out when I go to UTI or WYO-Tech.

Thanks for any help
  • Sponsors (?)

I had one and I think it was more like 120 HP but it has been a while. The big reason was low compression. If I recall, it is 8 or 8.5:1. That plus big chamber heads, cam, cam timing and ignition timing were the culprits. To get the compression up, you'll need flat top pistons and older heads. The cam can be mild, like a Summit 3600 (same profile as a Performer), which is very smooth but offers more power than stock. And you can curve the timing so it comes in faster. You can also have 351 valves put in the heads and gasket match them, to help with flow. Try to find unsmogged heads with just the humps in the exhaust ports. They can be ground out and you'll add some there if you don't want to gasket match. I think you'll not be able to meet your power goal without either adding a blower or turbo charger, unless you yank the engine. You'll need to use 89 octane if you bumping the compression with flat top pistons. I currently have a '74 block with '68 heads in my '65 convert. body, modified as described, and with good ignition, induction and exhaust, it dyno'd at exactly 300HP when the engine was new. Have 155,000 miles on it now and it runs almost as well. I also use Mobil 1 15/50 which I think accounts for the long life and maybe even a little of the power.
Well, few more questions, what are you going for? Strip or street car? Would be easier to make that power with a big cam and a single plane intake, but it's street manners would be less than likable. Are we talking engine HP or RWHP?

The problem with leaving your bottom end stock is most likely the bolts and the pistons. You'd probably want larger intake valves, thus the stock pistons would need be atleast flycut. I don't know how those pistons would hold up against boost, I doubt they were forged in 73...but I don't know...

You'll need everything you listed, along with headers and exhaust. Well no the S/C...but it would make the power come easier. If you're going to go with one, plan it out ahead of time, as you'll want a different cam and induction than if you went NA.

I don't know about the integrity of driveline in that time, but with 400RWHP, the tranny and rear will probably need to be souped up as well.

My .02
I have the same delima with my 302 only it's from 79. Stock it made 140 hp, I have a 4 barrel carb on it though and the heads crank and intake are from an 85 HO engine "big deal though" lol Anyway, I just did a cam swap, crane cam 272 adv duration .484 lift. It really woke up my engine, my old cam was .380 lift! My next plan is headers. If I were you, I'd start with the cam swap, and look into getting a 4 barrel carb on there as well.
One word for that 73 Stang------ Cleveland. No way you'r gonna get it up to 350-400 hp with that 73 short block. It's got weak rod bolts and Grand Canyon sized dishes in the pistons. That power can be made on 87 octane gas. Been there-done that but with a 5.0 roller motor. For a car as large as the 73's are, you need more torque to move it. That's why I recommend a larger motor such as a Cleveland. Bolt in swap on a 302 equipped 73.
I agree with Hearne 100%.

To get the HP you are looking for out of that 302, you would need compression, aftermarket heads and cam, full headers and a host of other mods.

That can be had far cheaper by swapping in a cleveland. Even swapping in a later model roller 302 with some minor mods would be a better bet than modifying what you have.
Despite what some people might think, I am not all about spending gobs of money.
The mid 70s 302s can be warmed up rather inexpensively. Get some 65 289 heads and bolt them on...if you want spend a little money on porting and polishing...this will but the compression up nicely, add to that a good cam/intake/carb combo and you will have a lively little street car.

We did this to my buddy's 66 that has a 78 302 in it...and he was happy with the results....of course you always want more and he is talking about a SC crate motor now....but you get the idea.
thanks to all the responses, but I don't agree it's a boat, I've seen way bigger cars than this. Anyway I want it to have it "behave" on the street and take it to the drag strip every so often. It appears I will have to tear the engine down, which is what I didn't want to do, because I've never done it. I forgot to mention I already knew it had low comp, thats why I thought of SC, or turbo. I have a turbo on my DSM,and just not sure if it would look right on a stang. If I went with a supercharger, would 9-9.5 compression pistons be ok or should I stay in the 8-8.5. If I do rebuild it, I may just find another engine and rebuild it, and posibly stroke it. I would like to avoid that for own reasons. Does anyone know what the pistons can handle? Surly they could handle 8-10 psi? Since it has 55,000 on it I would raher not rebuild it. Also I know in the DSM world if you use after market pistons they are not as reliable, and piston slap is very common. If I swap on 289 heads, would any year work, and what compression would I be at then? Do they work well with mild cam, and a super charger? I would want all the componts to be able to work with the supercharger. I would like to get the cam, heads. then intake, carb, and super charger. Roots style. Next not sure what supercharger will fit the 302 nicly, and what intake will work. How much will a good set of 289 heads run me, and roughly how much to prt and polish them. What about Boss 302 heads, would they work, I know the engine itself was a higher compression.

Thanks Derek
Um, you want lower compression if you want to high run boost, but they need to be strong, forged pistons, not stockers. The stock pistons wouldn't stand 10 psi. Parts you want with a S/C...same as NA, you just need to make sure you can get plenty of fuel, you don't want to be running lean. All you could really use is your block to make that kinda power, so the more knowledgable guys are right...a whole new engine would be a lot cheaper. :(
More cubes or a 85+ HO would be the best bet.
what do you mean I'm all over the map. I'm just looking at it from different angles. I don't know much about old V8's and need some info. I'm more from throwing a big fuel pump, injectors,bigger intercooler, and bam more boost for more power. I'm needing more help on what I need to do to get to my goal. Would 300 at the flywheel be more realistic goal. I just think the mach needs a litle more bite to go with it's looks. I'm not sure how involved a S/C would be, so I think I'll go all motor, and focus on 300-350 flywheel horse power. So would 289 heads, a good cam, intake, and carb, and vac advance kit get me there, or will I still have to put better pistons in it.

So I'm going to stay NA and try and get 300-350HP using heads,cam,intake,and carb.

I just need to find the right combo. I don't really care if it stubles or has a lumpy idle ether, as long as it's still somewhat streetable.

I also need to know what all I can remove in terms of emissions. I want to clean the engine bay up a bit. I also have a tick in one of the heads. If the lifters are non adjustable how do I fix this tick? Also when I pull the heads, whats a good Head gasket to buy.

I want to leave the bottom end alone. Is my new goal somewhat realistic?

Thanks Derek

PS I already have the exhust covered.
One more thing I would like to keep the same block, I just think putting a newer engine in it, takes away from it, but thats my own opion.

I also know all about running lean, which causes detonation, which causes pistons to melt, which isn't good. If I should swap in a 351 cleaveland, how much easier would my 300-350 HP goal be. Plus they are very hard to find. Would my 302 with 289 heads, cam, intake,4 barrel carb make 300-350 flywheel HP or will it take more.

Thanks Derek
Tell a Boss 351 it's a boat :rlaugh: and you might hear the driver laughing over the sound of the exhaust on his way to a 13 sec. 1/4 (in stock trim mind u)

BTW my '71 Coupe weighs 3100lbs - (currently running a modded 302), and has outrun modded GTs and Camaros. It's quicker than my '85 GT and that car is no slouch! This '71 will get a nitrous system bolted on here sooner or later in preparation for the 460 big block cause I do like overkill.



  • SIDE1.jpg
    52.1 KB · Views: 284
the problem is that a 302 just isn't enough motor for that car. It is a boat(for a 302 anyway), the only way to deal with that is with cubic dollars or cubic inches. Forget about aftermarket heads, forced induction, etc. For the money you'd spend on that you could drop in a mild 351W or Cleveland and have better results and streetability.

You need to set a specific goal for the car and be prepared for the conseqeunces. Building a 300 horse 302 is easy and cheap, but a 300 horse 351W is cheaper and will pull that boat around alot easier. Not to mention if you throw big heads and cam on that 302, you'll also need to change your torque converter and rear gears. Don't get hung up on keeping the stock block. A 351W and a 302 look identical, only difference is the 351 is about an inch wider and little taller. Valve covers look the same, intake looks the same, and heads look the same, the 351 is just scaled up a notch.

IMO, ditch the sidepipes. If you think they're cool you'll need to upgrade to shag carpet, pleated velvet seat covers, find the best AM 8 track radio available and trip out to 1976 all over again. But thats me, whatever floats your boat dude.
I just found it odd that you were staying away from the short block because you have never done one, but talking about all sorts of supercharging options, which is a lot harder. No dis intended.

If you find an original 4 barrel cleveland, they were rated at 300hp from the factory, although that was at the flywheel.

To get close to your 300hp goal, you will have to get rid of everything but the short block and replace it with better stuff. Better heads, better cam and valvetrain, better intake and carb, better exhaust, etc. Doing so will eliminate all your emissions as well.