TPITuned Port Injection discussion and questions. LB9 and L98 tech, porting, tuning, and bolt-on aftermarket products.
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Chevy 305 Heads and Engines De-mystified A little more truth a lot less fiction
There’s a lot of talk on various web forms about the Chevy 305 and its cylinder heads. Most of what I’ve read is negative; a lot sounds to me like second-hand regurgitated opinion and just a little first hand knowledge. I’ve owned a few 305s pre and post 1987 engines, carbed and tune port injected, stock as well as modified. I have also owned a flow bench for some 20 years or so, a number of computer engine dyno programs as well as an interest in developing engine technology. The 305 is interesting to me.
The pre 87 motors did what they were designed to do they made just enough power to get by and they got reasonable gas mileage The post 87 motors were a little better with roller cams, different heads and fuel injection.
So why not hop this thing up that’s what most people who own one want to do, it’s what’s in the car or truck right.... what can we do with it? are the heads any good? Good question, the first and most immediate response is “replace it with a 350” or “the heads are crap”.
So what about the heads ? The pre 87s are somewhere comparable to the old 283 power packs but with a better spark plug location, not really designed to flow a lot of air, just get the job done on a small motor. After 87 the motor came in TPI & TBI configurations which included swirl port heads. People say the swirl ports are no dame good the design is restrictive, But is it really ? I’ve read numerous articles on various heads being flowed but never any definitive testing on swirl ports themselves; I wonder how so many people can say these heads flow poorly when there’s no hard data to back up these statements.
I’ve flowed all kinds of Chevy heads and the day came when I flowed a set of swirl ports I wasn’t expecting much cause I like everyone else heard they weren’t great, the only thing I did to them was a 4 angle seat grind and back cut the valves, I guess I would have to say I was pretty dame impressed, Chevy swirlport heads flow !!! they will easily outflow the Vortec, in fact they kick the Vortecs butt particularly at high lift, the swirl port heads short coming is far from being its intake port, Its the exhaust port that is stunningly bad, the good news is this ports bad short side radius can be easily modified by anyone with a dye grinder and the improvement is huge.
The other significant short coming of the 305 as I see it, is the pistons, they all have some varying degree of dish with a tiny ineffectual quench ring, not conducive to better combustion and making power. D-dish flat tops are the best piston for this motor imo. Not everybody has access to a milling machine like I do so that they can make there own d-dishs from a flattop but I think this motor will run nicely with flattops if mid-grade gas is used.
Camshafts in these motors are pretty mild to say the least; I found the LT-1 cam worked really well in my TPI motor. I like how the roller motor cams can be re-ground, You can have your cam re-ground to a whole host of different profiles or even a close copy of a big name grind.
I’m always amused when I read people saying 305 swirlport heads don’t flow, How did the swirlports get loped in with the old heads of yesterday ? I think the person or persons who designed the swirlport intake port were darn smart, maybe a little smarter than there detractors ? People who poo poo these heads quit simply do not know what they are talking about. I wonder how many perfectly good sets of modifiable heads have been replaced (with older style poorer heads even, in some cases) or discarded because of the ill-informed advice given in various forms ?
What motor is going to make any kind of power if it can’t pump its exhaust, that’s the 305 swirlports crippling handi-cap. The 305 swirlport with a little exhaust port work and a hi-lift cam with some duration isn’t a half bad little motor and That’s the truth!
As a side note; 305 heads can be modified for 0.500 plus lift by using a set of Vortec take out valves purchased cheaply on e-bay, the intakes need to be cut down to somewhere around 1.84” to 1,88” or so and re-faced, what ever diameter you think you’d like to try. Vortec valves are just single grove (hence the ability to go high lift) so you have to use positive type valve guide seals. Also I’ve tried porting the intakes and found little gain, the port as it stands is that good.
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Hey petetinga,
Thanks for sharing your thoughts! In the spirit of "hot-rodding" and "turning a wrench" - I'll work on a set of heads that I have from a 1993 or 1994 5.7 - 4 bolt main engine. The last set of heads that I blueprinted were on a "30-over" 429 Cobra Jet engine... now that was FUN!!
