I have a 305 and am in need of headers. I know the 1 5/8 headers are recommended for the 305 but I plan on a 350 or 383 swap and I dont want to buy headers twice. What kinds of negative effects if any will I see by going to the larger 1 3/4 size? I am thinking Hooker 2055's coated with no air tubes. I am a novice when it comes to headers and I have done as much reading as possible but I need real world answers. The going rate seems to be $500 (Hookers) to $650+ (SLP's) for a good coated header. I dont want to **** away that much money on something that will get ruined somehow or cause problems with my crappy *** 305! Help me make an informed decision.

 

I don't think it has all that much to do with Cubic Inches, more rpm than anything and hp. Either will be fine, but you won't see any real negative effects from 1 3/4 on a 305.

 

He is correct on this one

 

As you said 1 5/8" would probably yield better results but since you are going with a bigger motor in the future there is no point in getting 2 sets. Also, anything is better than the stock manifolds and you will still see power gains.

 

Ofcourse I am

 

Just get the 1 3/4" and be done w/ it. The only downside you might see is a little bit more loss in torque over the 1 5/8" due to lack of velocity of the exhaust gasses. It won't hurt anything though.

 

correct me if i am wrong but dont they only come in a 1 5/8 version?

when i bought mine like 2 months ago i never saw a 1 3/4 version of them in any of my research

so if you want 1 3/4 shorties, SLP is your only choice

 

Correct and correct.

 

Sal, you will probably notice a slight torque loss. I noticed it on my L98, but you've got the 3:73 too. That'll help even it out.

I'd go with the 1.75 SLP's!!!

 

I agree. Those will be a good header for both.

 

Ok I must have read for too long when I saw the 1 3/4 on the 2055's. Well I really want the SLP's but I dont like the $600+ price for the coated headers. Oh well I guess I gotta pay to play.

 

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1 5/8 is recommended for the 305 by a few companies. The 3.73's will help with the slight loss of low end torque (basically just get the damn thing going) from the use of 1 3/4 headers, according to the way I read it. The ceramic coating will add dollars to the final cost but it will stop corrosion, increase exhaust flow, and keep the temperature of the headers lower (longer header life). The pretty finish is just an added benefit. it all comes down to what you consider too much money to pay for headers!

 

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im running slp 1 3/4s on my 305 with 3.73. i have a lt1 intake so that killed some torque as well. but it still has enough torque down low to burn off 1st and most of 2nd if i can overcome the awesomeness of spohn suspension, but thats on crappy tires, on a crappy road, and really beating hard

 

Of course it has something to do with cubic inches. How could it not?

Primary tube diameter is all about exhaust velocity. The larger the tube, the less the velocity at a given RPM. With a small displacement engine, at low RPMs, the exhaust will get "lost" in large diameter primary tubing, and velocity will "die" - killing the scavaging effect. RPMs have to be higher in order for the velocity to stay high enough to produce the scavaging.

With a larger cubic inch engine, the exhaust mass, and therefore volume, will be higher for each exhaust pulse. Therefore, the velocity will be higher at the same RPM as the smaller engine. Therefore, you can use larger primary tubing and still maintain velocity. Or, put another way, the velocity will be higher at a lower RPM in a larger displacement engine than a smaller displacement engine.

The smaller the engine, the higher the RPM for proper exhaust velocity for good scavaging. The larger the engine, the lower the RPM for proper exhaust velocity for good scavaging. The larger the engine, the larger the primary tube for good scavaging at higher RPMs.

Cubic inch displacement is directly tied to primary tube diameter, and primary tube diameter is directly tied to power band. You can't separate the three.

 

I think I'm starting to understand engines finally... because I have a question for five7kid!

Isn't your explanation based on the assumption that the heads are the same? Or rather, the exhaust valve lift and diameter are the same? I'm just talking theory (because I don't know the actual flow numbers on different heads), but I'm guessing that the heads on a 350 have larger exhaust ports (or farther lift) than a 305, meaning the same header can have less of a scavenging effect on a 350 than a 305.

I know I'm wrong somewhere... where is it?

 

wouldnt the amount of exhaust be determined by how much air/fuel enters, and how much of it is burned? if there was 100% volumetric efficiency would exhaust still be necessary?

 

Exhaust is still necessary. You may burn everything up in the cylinder but it doesn't just disappear. "Matter is neither created nor destroyed, it simply changes form."(Chemistry class) So no matter what: as long as you're sucking stuff into the engine you're still gonna have to spit it back out.

 

Yes. There are engines out there that run at (or very near to) 100%... like competition blower motors running HUGE amounts of boost. They still need exhaust systems.

 

I understand where you're coming from, but there's something a bit wrong here.

The 305TPI (LB9) heads are VERY similar to the iron 350TPI (L98) heads. They have the same 1.84/1.5 valves and flow very similarly (meaning they will be a restriction on a 350 motor before they are a restriction on a 305). From what I've seen, they only flow 2-3cfm difference. The L69 HO carbed motors also had very similar heads to the LB9.

However, this is not the case for the crappy stock heads on L03s and LG4s. They are much poorer in design.

In any case, if the exhaust flow is LIMITED enough by the heads, then bigger headers would make scavenging even worse because there would be even less air coming into them at even slower speeds... if that makes sense. The idea is you want the air to get out of the cylinder as fast as possible after combustion. If you slow the air down at all, you reduce the scavenging effect.

 

A couple of minor details: L98 heads have 1.94/1.50" valves and 64cc chambers vs. 1.84/1.50" valves and 58cc chambers for LB9/L69/LG4 heads. However, the port volumes are very similar.

FWIW, I put 1-1/2" primary (long tube) headers on my '71 GMC 350 pickup a few years back, attaching to the same dual exhaust that I had installed on the factory manifolds. The difference was amazing, even more so than going from single exhaust to dual. The small primaries were ideal for the small valves/low RPMs of that 350 utilized to keep gas velocity up and promote good torque/pulling. 1-5/8" primaries would have moved the exhaust power band up, creating a mismatch with the rest of the combination.

My observations above assumed that all else remained equal, with only displacement changing. 2" primaries are often used on race 350 motors that rev to 7000, but that doesn't mean that all 350's should use 2" primary headers.

 

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Get 1-5/8"... 1-1/2" just sucks on any SBC if you wanna go fast.

 

Dont get the Coated SLP's. Get normal SLP's and have them coated. Their coating sucks. I have seen the pics. Pay ACS to do it. Also the 1 3/4's are best if you plan on putting some bigger cubes in.

EDITED: I have seen the pics of their coating after 6 months, and yes I own a pair myself.

 

Would the 1 3/4" headers be sufficient for an engine with around 400cid?