1991 RS 305

85 305 TPI. I plan to remove plenum dividers at runners, and siamese stock runners as much as safely possible, total this would shorten runners around 1-in. EGR walls arlready removed. With my set-up (mods in sig) would I get a worthwhile gain with a Edelbrock or Accel base, or would porting out/cleaning up the stock base be sufficient? I'm keeping in mind the fact that the TPI set-up was origionally intended for the 305. Thanks in advance.

mrr23

her's the difference between stock runners, siamesed stock base, LT1 vs siamesed SLP runners, Edelbrock base, ZZ4 cam

1991 RS 305

Has anybody tried a before and after comparison of porting the stock base only? What were your impressions? I hear the stock base will flow well when ported, though not quite as well as say an Edelbrock , etc. Comments, or pictures anyone?

mrr23

well the #14 dyno is with a siamesed stock base and stock runners. still didn't peak past the 4800 mark. there was another guy that siamesed his also. but used different runners.

Captain C

You will hurt your performance on your 305 if you do the runners or the intake!!!!! If you had a modified 350 it will work fine. The mods you are doing are for top end and you don't have enough cam for top end, besides the car should do almost 140 on top as is. All you will do is lose your low end (quickness) if you do those mods.... Whatever you do, do not use the Edelbrock TPI runners, they are a royal pain in the rear to get sealed!!!!!!!!! Use SLP

Guest

I would port the stock base on a 305, you will gain plenty of airflow doing just that.

Captain C

With a stock engine he will be pulling to 5000 RPM. He doesn't have a cam in it so there is no reason to port his intake for more power. If he had a stock 350, don't port the base, add runners. Even on a modified 350 you will gain more from a runner change than a base manifold change. That's not to say you won't gain with the manifold, just that you gain more from the larger runners than the larger manifold. On a 400, you will see a large gain from the manifold.

On my Camaro, I haven't run the stock intake on the 350, only on the 305. On the 350 with the Edelbrock intake and stock runners it pulls to 5000 hard and flattens out, after 5200 it's dead. With the runners installed it pulls to 5500 then flattens out, after 5700 it's dead.

All I'm really trying to say is that the stock runners are more restrictive than the stock intake. You also have to remember the stock 305 heads don't flow worth didly. They have 1.84 intake and 1.50 exhaust valves. Change to some World Products S/R torquers for 305's and get some 1.94's with 170 CC intake ports. Then you will notice a power increase with the runners or intake....Maybe! If you change them with a "stock" 305 engine you will lose more low end than you gain back on top end. Bottom line is, he has a "stock" engine.....

Guest

I totally disagree with everything you said. I know of nobody who has not benefitted from porting the base on a 305, including myself. Besides, its a cheap mod.

Captain C

Is your 305 stock other than your intake mods??????

Mooose

What book did YOU read? This is absolutely wrong. The runners are the least of the restriction problems on these. The base is the biggest bottleneck in the 3rd gen TPI system.

Captain C

I got the information from an old Fuel Injection Specialties brochure I ordered several years ago from them. I've also seen the difference made from runner changes on a stock base manifold on my buddy's stock 350 and runner changes on an Edelbrock base manifold on my non-stock 350. Those are what I base my opinions on. My buddy's car ran like a stripped ape after the runner change. My car ran virtually identical ET's. What I gained in top end just made up for what I lost in low end. I have just reinstalled the larger runners because I like the top end pull and the cool factor of the big port runners alone is worth it. As to around town drivability or stop light to stoplight quickness, the stock runners are better. (Short stoplights obviously)...

The ports on a 305 stock head are smaller than the ports in the intake manifold or runners. Again, this is for a stock 305 that I say they would be better off. If it's a modified 305 or even a stock 350, by all means go for it, you will definitely improve!!!!!!

All you have to do to win me over to your argument; is show me hard data to back up your claims for quarter mile ET improvements on a stock 305. That's all you have to do and no one has offered any so far……

Captain C

Just an FYI for what it's worth.

When I pocket ported my heads (World Products S/R) and upgraded the valves to 2.02/1.60 with back-cut stems it made a change in how well my chips worked.

I have an 85 Vette chip for use in my 85 computer and a TPIS custom Stage V chip for use in my 89 computer (attached via an adapter harness).

With the 85 chip and untouched heads it would pull hard off line to about 3500 then start laying over. With the TPIS chip in the 89 computer, it didn't start pulling until 3500. Then it threw you back in the seat like a 4 barrel kicking in. Made peak power at 5150 rpm.

After the head modifications:

The 85 chip pulled from off line to 4500 before laying over. The TPIS chip started pulling from 2500 rpm. Peak power was now made at 5500 rpm which is really the limit of the cam....

Moral of the story, you make any changes to airflow, you need to recalibrate your chip for best performance.....

nasty355

i would think that some minor port work would help out just from smoothing airflow.i ported my lower and upper on my setup i couldnt believe how much you can realy take out of them.if i was running a stock 35 i would do a mild port on the lower match everythign and smooth good but i wouldnt go crazy grinding out massive amounts of materiel.as for the rest on the upper i matched everything real good then smoothed eveything i could.oh yeah and grind out the wall behind the t-body.again dont go nuts just remove the wall(quite a bit of materiel)and match and smooth everything else.with a "stock"305 thisshould work well without shifting the rpm range upward.maybe someone else will chime in on this.eric

1991 RS 305

Thanks all. I probably will port-clean up the stock base for more flow. Still haven't decided if I will remove the plenum runner dividers, and siamese stock runners. This in effect, would shorten runners around 1-in, but I haven't decided if this would help/hinder torque, and overall power. I want to do NOTHING to cause torque loss, just "enhance" the current system much as possible, while keeping "stock" appearence. Comments?

Guest

https://www.thirdgen.org/techbb2/sho...hlight=runners

1991 RS 305

Thanks madmax. Porting the base and siamesing plenum & upper runners just might be my ( I hate to say it) "winter project".