I'm new to porting, so I've got a few questions. I have read through the archives quite a bit and I know a little more on the topic, but I'm still kinda confused.

I've got an IROC with an LB9 (305 TPI). What would be the best approach to freeing up the intake (porting/siamesing/both)? I want to keep some of my low-end torque, but I want to increase my max RPMs too. I'm going to be adding headers and an exhaust system so I don't want to have too much low end torque to put too much strain on my rear end.

Currently, I plan on removing the interference in the plenum. Would it be a good idea to siamese the plenum-to-runner openings (on the plenum), since I'm siamesing the plenum-to-runner openings on the runners (I'm guessing "yes" since it seems logical)?

How about on the runner-to-base; would siamesing help any? I think I read somewhere about this decreasing low-end torque by a lot.

And on port-matching... where is the best place to do this? On another post, I read that you should not port-match the runner-to-base area.

How critical is it to do a "quality" job? Obviously, I want to do the best job I can, but is it extremely important to get perfectly-angled porting and siamesing?

Last question: how to clean the intake afterwards... can I just spray some water through all the pieces and ensure that it's dried?

Thanks very much everyone; you're all so helpful on this board.

 

Here are some thoughts for you blue86-

First off, siamesing the intake is essentialy shortening the length of your intakes 'runners'. So, when doing this you would want to start at the begining of these runners, which is at the runner-plenum intersection, and work your way down them. Changing the effective length of your runners can play a big role in determening your engines rpm range. Shortening your tpi's runners will increase your engines power at high rpm's, however it will only do so at the cost of your low and mid rpm's power. From my understanding, even the slight siamesing associated with SLP's "Siamesed Runners" isn't recomended on a 305 unless it is heavly modified. You might want to reconsider doing this unless you have a cam and heads that can take advatage of the increase in breathing abillity.

On the other hand, cleaning up the plenum and gasket matching the rest of the intake would be benifitial for your 305. This will be equally effective at each of the intakes mateing surfaces. I have heard though that you shouldn't gasket match an 'upstream' part, unless the coresponding 'downstream' part is also matched (ie. don't do the intake base, unless you're doing the heads also).

Finally, after this porting, special care should be given to ensure none of these aluminum shavings make it into your engine for obvious reasons. A hose in the back yard with some suds would be a good start, followed maybe by some compressed air. However you do it, I wouldn't be in too much of a hurry.

This should give you some food for thought!

 

you arent going to have to worry about breaking your rear end with too much torque from a 305

 

You are aware that until you swap the cam out of that thing you're not really gonna go anywhere. Seriously, don't spend dollar one until you do a cam swap, peanut cam cars just don't respond to mods very well at all and you'll just be dissappointed.

Beyond that, maybe i'll touch this post later if i have time, gotta runt ot a meeting though....

 

 

You'd have to really rag the hell out of your car to break the stock rear with any NA 305. Even a forced induction 305 could keep a 10 bolt in it easily depending on actual power level. Plenty of folks running 11s and 12s on the stock rear.

 

You are very right, But age does take its toll. If you dont take care of it, it wont take care of you. I've seen V6 cars kill a rearend.

 

Yeah, I was planning on definitely putting in a new cam... I've heard good stuff about the ZZ4 cam in the 305/350. How much would that cam alter my emissions? If money allows, I may go for some new cylinder heads (but I'm not counting on it).

I'm also assuming that the stock heads are not already port-matched to the intake?

I have lost the positraction function of my rear end due to its usage... I'm going to have to get that fixed (along with some new gears in the process). I'm just afraid of overpowering my 16-year-old rear end that's got nearly 70,000 miles on it.

 

I have a 15 y/o rear with 150k +, and I'm going to spray the hell out of it this year. I think I should be ok.

 

If you're swapping in a new carrier and gears, exactly what 15 yo part are you worried about breaking?

 

splitting the housing straight in half.

btw, i had over 220k on a stock rear end when i pulled it, and the posi still worked every now and then, and i beat on it all the time. so i wouldnt be too worried about the rear, especially if your rebuilding it anyways.

 

I was talking to a guy I know who's real knowledgeable about GM cars... he said that a stock 10-bolt rear on my '86 IROC will probably handle about 250hp and still be driveable; 300hp absolute maximum. Now, this is assuming that I'd be hitting it hard from stoplight-to-stoplight... I won't be doing that all the time, but it would be nice to know that I can do it often without breaking stuff . I'm not sure exactly what would break, but I trust what this guy's saying. I didn't mention to him that I was going to fix the positraction and add new gears, so he may be talking about the limitations of my 16-year-old stock rear. I was reading the following link about improving my 10-bolt, so that may be an option.

http://www.ws6transam.org/10bolt.html

 

A properly set-up 10 bolt w/ a nice aftermarket carrier should be good well into the 12s behind a foot braked automatic as long as you're otherwise 'nice' to it, plenty of folks are doing it. Not that you can't have fun with it, you just can't be off doing donuts and crazy harsh driving all the time, or sure stuff will break. But cruise to work, the occasional on-ramp or stop light stuff, and track time should be fine.

 

Sounds good. I'll still have to see how much power I'll be producing once I'm done with all my mods... gotta get it dynoed.

 

You Can Do all the Donut's and Burnout's you Want...


It's When the Tires Hook That you Have problems

 

I've broke both a 10 bolt(9 yrs old in a TBI 5 speed) and a 9-bolt (11yrs old in a TPI 5 speed), both pretty much stock at the time on the street in regular street tires( I do love to bang gears though). Currently have a '96 4th gen 10 bolt rear with a stud girdle rear cover, T5 tranny is dying slowly now.

Siamesing a base, I started out by using a 1/2" drill and drilling down the center section about 5", then finished with a die grinder w/ cutting bits and sanding rolls(~6+ hrs of work and still wasn't professional looking at all) Cleaned with lots of high pressure water or hose water, which took at least 2 or 3 rinsings to get all the metal powder out.

I don't know why everyone wants torque so bad, w/ 5 or more liters of displacement lack of torque on the street should never be an issue.

 

Beast5spdGTA, you siamesed your base? I'm just curious... what engine do you have in that car?

 

It's a 305 TPI, I also put in a 214/220 cam, 1.6 RR, AFPR, pullies, along with the base, on top of headers, SLP chip, homemade cold air, cat, and muffler that I already had. Unfortuately I only picked up .2 sec and ~1 mph in the 1/4, but I think a custom chip will help out some since I picked up 3 mph and ~.3 sec in the 1/8 mile with a missed shift/rev limiter (1-2), which probably hurt a little.

On the street the car feels alot stronger and seems to pull to 5500(rev limiter) with ease. The only loss of torque that I noticed is below 1500 rpm(like 5th gear at 40 mph).