[Imperial]

Hi, new to the forum. My dad and I bought a 1970 Lemans Sport with a 350 and 3-speed auto trans. Currently the only aftermarket thing we have done to it is cherry bomb mufflers. my dad and I want to beef it up this winter and Get it to 300-350 whp. We were already planning on putting some new headers and a qudrajet carb, what else would help it get it to those numbers? Thanks.

[Imperial]

Also we were thinking about 4-speed manual swapping it.

[steve25]

A 3 speed auto or a 4 speed does not matter anywhere near as much to your future plans as does the rear gears the car has.
If the car does not have AC and currently has a 2bbl carb then you likely have 3.08 rear gears and that will need to change FIRST so you can cam the motor to get to 300 hp no less top 300 Hp.

You 2bbl motor was rated at 255 Hp.

The highest hp 350 Pontiac ever made put out 325 Hp.

That was in 1969 with heads that not only breathed better than yours but gave the motor near 2 full points more of compression than you have with your 1970 heads.

Also the 4 bbl cam that 69 motor had was 3 steps bigger then your 2 bbl carb 254 cam.

If I am not mistaken the rear gears that cars with the 325 hp 350 came with where 3.55.

[unruhjonny]

Just incase it's worth mentioning, a transmission swap will not net a single extra horsepower - but it may make the car more fun to drive around.

To be brutally honest, you're going to need to do a LOT to get 300-350hp;
That engine as it sits is 2 barrel carbureted, it has 8.0:1 compression and a factory rating of 255(b)hp;
So you're looking for roughly 20% to 40% power increase.

Let's compare your car's engine against 70-72 GTO's - because they have the horsepower level you're hoping for);
1970 GTO "base" 400: rated at 350(b)hp; 400-4bbl; roughly 10:1 compression; uses large valve cylinder heads; has a larger cam (067); same exhaust manifolds
1971 GTO "base" 400: rated at 300(b)hp; 400-4bbl; roughly 8.5:1 compression; uses large valve cylinder heads; has a larger cam ('067; same as 1970), same exhaust manifolds
1972 GTO "base" 400: rated at 250(n)hp; 400-4bbl *other than casting numbers, this was in essence the same engine as used in 1971)*

A four barrel swap may yield a seat-of-the-pants power increase when going from the primaries to the secondaries, but I'd wager any power increase would be minimal.

Headers will net a gain, but will probably massively outflow your engine as it is set up (or as it will be post four-barrel swap).

Some people may knock the 350 (for 1970 those would be casting #11) heads, but I am guessing that you can leave them installed as the small valve configuration won't be the "power bottle-neck" that some might suggest.

I'd instead suggest you look at a camshaft upgrade to compliment a four barrel carb swap;
I believe that the two barrel carb you have is rated for 500cfm;
Most Quadrajet 4 barrel carbs have a 750cfm rating.

If you swap cams, and don't want to mess with cylinder heads, stick to a grind with factory lift ratings - for most Pontiac grinds the rating (with 1.5:1 ratio rockers) was around 0.406" lift.

If you went to a '067 type camshaft, and a GOOD dual plane intake, you probably wouldn't hit your aspired horsepower level, but odds are you would see a sufficient enough increase to be quite pleased with the net result.

I'd suggest a factory 1968-1972 4 barrel intake manifold, but if you are dead set on an aftermarket one, I believe the one which comes closest to flowing as well as a factory unit would be the Edelbrock Performer RPM;
Any other dual plane intake manifold will actually flow WORSE than a factory cast iron intake manifold.
If you chose a factory intake manifold used after 1972 you will need to change your valley pan (the part below your intake) to accommodate the EGR protrusion on the underside of 1973-1978 Pontiac intake manifolds.

Summary of my suggestion (with a bit more info):
- leave heads
- leave exhaust manifolds
- swap to a 067 Pontiac grind camshaft
- swap to a 1968-1972 Pontiac cast iron intake manifold
- swap to a 1968-1972 Pontiac 4bbl carb (with choke mechanism to match intake)

My opinion is that headers will only help the engine if you do a cam swap to compliment the 4bbl swap;
Headers alone won't get you near what you're looking for.

There are older articles comparing intake manifolds, and exhaust changes - those might be worth reading.

I hope this helps.

[b-man]

A 350 that can put 300 to 350 to the wheels is a pretty hot 350 that will require a lot more than the usual cam swap, intake, carb and headers.

Honestly I’d start looking for a 400 or better yet a 455 to swap in there since you’re going to spending quite a bit of money in your pursuit. A 350 is a nice engine but the wrong one to start with when you’re wanting 350 horsepower to the rear wheels.

A 350 horsepower 350 will put around 280 to the wheels, making one horsepower per cubic inch with a 350 won’t be cheap or easy and forget about 350 to the wheels.

Let cubic inches get you closer to your goal without the need to build a high-strung 350 which won’t be all that street friendly. A near stock 455 will make 400 horsepower without much fuss and be fairly docile around town, so there’s your 300 to 350 to the wheels. Bonus it will pull a tall rear end gear easily so no need to swap to numerically higher gears.

[JLMounce]

I echo what everyone else is saying here. 300-350hp at the rear wheels is going to be a tall order for your engine. is it possible to build a 350P to make that kind of power, yes. But it's not a bolt-on endeavor.

With the engine you have, you've got to contend with very low compression, cast pistons that won't hold up to much abuse, cast rods that won't want to spin as fast as you'd need this engine to and an oil pump that isn't really up to the task either.

In it's stock build configuration, doing a 4bl swap, with a small cam upgrade like a factory 067 cam or maybe something like a Comp 250H and headers with an upgraded exhaust is probably the limit. You're probably talking somewhere around 275 crank hp.

Now, going to a more performance oriented rear gear is also going to help you out a bit here, and the torque the engine makes will make it feel like it has a bit more pep.

If your goal is 350 to the ground, streetable and reliable, I like others agree that a swap to a 455 or stroked 400 is a good option. Depending on what your budget is, I'd probably go to somebody like Len Williams and have a basic 455 made. He markets a 425hp stroked 400 that is a nice piece and will get you to your 300-350 wheel hp target.

Or, if you want to get a little hairy and mess around with your reliability you always have this option. https://www.youtube.com/watch?v=izBrYc7LI7Y

I doubt an engine like the one referenced in the YouTube video above would hold up for very long. But it would be cool while it held together.

[formula kid]

If you want to make good hp swap to a 68-70 pontiac 4 bbl intake. All part will be a swap from the donor engine to your 350. Oh and bye the way your engine is a 354 cid Pontiac rounded down to make it simple for people to remember. Get a 4 bbl Quadra jet from the same period and fine tune it. It will work great when you adjust the secondary flap to open correctly. Next will be the headers try to find a set with I believe 1 5/8 primary tubes. If you are going to which from an auto to 4spd trans you can install the 068 cam if you stay with automatic then the 067 is you best bet. You could save money by changing the rocker arms from the 1.5 to the 1.65 on one side I don’t remember intake or exhaust side. If you can find an old H-O catalog check which to change on your year 350. Nunzi’ catalog will have the same recommendation. Recurve the distributor and you will be happy. Should you want more get big valve heads # 12,13, or # 48 from 1970 if you have the extra cash

[Imperial]

Thank you for the input, we'll probably start with the new carburetor, cams, and intake manifold. I figured the manual trans swap wouldn't add any power (just bored from dailying an auto) but when we do the trans swap, we'll change the rear gears. Again, thank you so much for the input.

[JLMounce]

Originally Posted by Imperial

Thank you for the input, we'll probably start with the new carburetor, cams, and intake manifold. I figured the manual trans swap wouldn't add any power (just bored from dailying an auto) but when we do the trans swap, we'll change the rear gears. Again, thank you so much for the input.

You might find that doing some of those small things and making sure the car is right and tight is all you need.

My wife had a 1970 Chevelle with a basic Chevy 350. It was one of those Gm Goodwrench crate engines. With the intake and headers, it maybe made 225hp.

That said, because the car was well sorted, had a nice sounding exhaust and a more performance oriented gear set. It was a blast to drive. It wasn't "fast" but it never left you wanting for more.

If you decide to try and run headers with your car, it is possible it has a set of heads that do not have the outer most bolt holes drilled. This makes sealing up a set of headers difficult it not impossible. Before buying headers, check to make sure those holes are drilled and tapped.

Good luck with the project!

[hurryinhoosier62]

Originally Posted by Imperial

Hi, new to the forum. My dad and I bought a 1970 Lemans Sport with a 350 and 3-speed auto trans. Currently the only aftermarket thing we have done to it is cherry bomb mufflers. my dad and I want to beef it up this winter and Get it to 300-350 whp. We were already planning on putting some new headers and a qudrajet carb, what else would help it get it to those numbers? Thanks.

A lot of guys are going to suggest you “find a 455”; depending upon where you live this could prove nearly impossible. I live in the Ohio Valley, just northwest of Louisville, KY. Pontiac 455s, as well as 428s, are nearly impossible to find within 4-5 hours of me. Bart’s suggestion of a 400 makes all kinds of sense; good Pontiac 400s are still relatively easy to find. To make 300-350hp at the rear wheels you are going to have to make 400-450hp at the flywheel. 450hp out of a 400 is a pretty “hot” street engine, but installing a stroker crank kit in a good 400 block will make it achievable. I would love to see you do it with a 350, but you are looking at major expenditures to achieve it with a 350 unless you install a stroker kit and modern aluminum heads.

[PAUL K]

I think 350's are good street engines and respond to modifications just like any motor. I think one needs to be realistic about power numbers and I prefer flywheel dyno numbers or quarter mile times instead of RWHP. Way too many variables with a chassis dyno. I've had 720 HP engines show 480 RWHP and 680 HP engine show 550 on the same dyno. I've also tested parts that pick up power 15-30 on a dyno and at the track but show no difference on a chassis dyno.


With that being said if, you'll see a nice performance gain with a good cam, headers and improving the intake tract and possibly changing heads to raise the compression.... and replacing the cherry bombs with a good flowing muffler. If the engine needs a complete rebuild and it's the original engine, it'd be a good idea to keep it. At that point you could install a 4.00" or 4.25" stroke crank, and spend some money to improve the heads. There's 350's out there making over 500 HP and 400 is achievable pretty easily with a sensible plan.

It sounds like your looking to pep up the current motor without a rebuild. A good cam, headers, Performer intake manifold with a Q-jet or vacuum operated secondary carburetor will make a big difference. Enough so that you'll be needing to address the differential with a posi (assuming it's open now) and possible up the gear a little to a 3:23-3:55 depending on your driving style.

[b-man]

A realistic goal for doing some bolt on modifications to a 1970 350 would be something in the neighborhood of 220-250 rear wheel horsepower.

Doing a head swap to aluminum aftermarket heads, a pretty hot cam in the region of a factory RAIV cam and headers along with a calibrated Quadrajet on a stock iron intake would get you close to your original goal of 300-350 RWHP and it won’t be inexpensive.

After you get your stick shift trans in there you’ll need a 3.90, 4.11 or 4.33 rear gear to go along with the big cam and heads.

A 350 with 350 to the wheels is certainly doable, but not with a few bolt-ons keeping the stock 1970 #11 heads.

Do a lot of research before throwing money at that 350. Either be more realistic about what it’s able to produce within a tight budget using a few of the usual hop up tricks or get serious and set yourself up with a 400 or 455 and go from there.

Those Cherry Bomb mufflers are a neat blast from the past but you’ve already chosen the wrong mufflers for an engine that’s going to meet your lofty expectations.

[Imperial]

Hi B-man, thank you for the input. Originally we were planning on getting a muffler delete and h pipe but the guy working on the car said it was too loud (buzzkill lmao) so he threw on some cherrybombs. the current whp goal is 220-250 with my new list upgrades.

[rolling money pits]

Can aluminum heads even work on the 350s? Isn’t there some kind of valve shrouding or some such performance deterring issue going on?

Unless you’re dead set on hopping up the 350, I’d follow the advice of searching for a 400 block, then going to town.

Can research info, gather parts as required, take time to assemble, etc. etc.

If you’re dead set on giving your butt dyno a workout, and plan to build a different engine, can always drop a couple hundred dollars and go to the David Freiburger school of quick and cheap power. Be warned though, that school is an enter at your own risk one, lol.

[Imperial]

Originally Posted by rolling money pits

Can aluminum heads even work on the 350s? Isn’t there some kind of valve shrouding or some such performance deterring issue going on?

Unless you’re dead set on hopping up the 350, I’d follow the advice of searching for a 400 block, then going to town.

Can research info, gather parts as required, take time to assemble, etc. etc.

If you’re dead set on giving your butt dyno a workout, and plan to build a different engine, can always drop a couple hundred dollars and go to the David Freiburger school of quick and cheap power. Be warned though, that school is an enter at your own risk one, lol.

After getting input from everyone else I threw out the 300-350 whp goal. It's a lot cheaper to just throw on some bolt-ons instead of swapping out the block.

[b-man]

KRE offers a 260 cfm head with smaller valves on the order of 2.06/1.66 or something close to that with a 65cc chamber that’s ideal for 350 builds so they’re readily available.

It’s not so much the small bore not able to make power it’s trying to get more than 1 horsepower per cubic inch without spending close to 3k on aluminum aftermarket Pontiac heads to achieve that no matter the displacement.

The LS1 has a bore (3.90”) just about the same size as a Pontiac 350 (3.875”) so with the right size valves and a high flowing head (260 cfm LS1, maybe 170 cfm #11 350 head) getting one or more horsepower per cube is not a problem with a small bore size.

[SD455DJ]

I might as well throw my hat into the ring…lol…with my thoughts on hopping up your ’70 350. I would follow the ’69 350HO recipe by adding ’70 #13 heads with their 75-78 cc chambers to keep your compression in the low 9’s (#12’s would get you 9.5-ish compression), add a cam like the Summit 2800 (204/214/112 @ .422”/.444” lift…kinda like a high-lift 067 cam), add a factory 4-bbl intake (’68-’71 to work with the exhaust cross-overs) and a suitable Q-jet. Replace the standard oil pump with a 60 lb. unit and the HD driveshaft. I would keep the log exhaust manifolds and add Pypes 2.5” mandrel headpipes followed with a complete 2.5” system with Magnaflow/DynoMax UltraFlo mufflers. 350HO’s came with 3.55 gears standard, so add that and a new limited slip diff and you’d have a very strong combination. I would keep the convertor near stock for better drivability and cruising performance if you keep the TH350. I always go for the hot stock factory looking sleeper.

Here is a ’69 350HO dyno sheet run with log manifolds with 2.5” mandrel bent headpipes, 067 style cam, and 10.6 to 1 compression. It was .030” over and a fresh rebuild, so maybe a bit more work/cost than you want to tackle. With a full point (+) less compression it would be in the 300 hp range. But all the pieces would work well together on a short block in good condition.

Dennis

[jhein]

If for whatever reason(s), you have the motor out of the car and torn down to the short block, you're not too far away from a Butler 4.25 stroker kit. That gives you 400+ cid with a 4.25 stroke (more torque than a stock 400). That, plus a set of aluminum heads would kick a$$.. Obviously that means a full rebuild, but it would get you pretty much everything you want in a fresh motor that you can have confidence in.

[fairwayhit]

I think I would just put a supercharger on it.

When my 557 block gives out I’m going with a blown 350


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[b-man]

Originally Posted by fairwayhit

I think I would just put a supercharger on it.

When my 557 block gives out I’m going with a blown 350


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So would you spend a pretty good amount of money to supercharge a 350 in unknown used condition or would you completely rebuild it first using a few upgrades like forged pistons and rods for starters?

It seems pretty clear after reading the responses from the original poster that he’s not interested in or able to afford a supercharger, aftermarket heads, stroker rotating assembly nor a top of the line fully blueprinted 350 HO Pure Srock engine.

All fine suggestions and examples but unfortunately none of that’s in the cards here.