Guys, I was wondering if any of you think it would be worth it to put aftermarket runners on my 1990 IROC L98?

I heard GMHTP mag got 5-7 HP/TQ across the RPM band with them on a 305TPI with un unported plenum.

What could I expect on my 350TPI with a ported plenum?

Its basically stock with K&Ns, TB bypass, UD pulley, modified airbox, AFPR, Accel ignition, Dynomax cats, 3" catback...

 

What sort of aftermarket? You talking about some siamese runners? When you start upgrading intake parts and what not, you need to look at other aspects. For example a bigger throttle body is good, but better if you get headers. it doesn;t do any good to get more air in if you can;t get it out just as quickly.

 

Well in my opinion you can expect more of a gain for the simple fact the 305 and 350 intakes are the same. So by going to aftermarket runners (depending on the kind) it can greatly shorten the overall runner distance. Which means more air for the cyclinders to fill with. The ported plenum should help you out even more especially if you went with some SLPs that were siamized cut.

For the money though you might just think of another upgrade. Those runners can cost up to 400 bucks. You could get a nice set of headers for less and see a similiar gain.

 

You will see more gain than the 305 with stock plenum for sure.

How much is hard to say, but if you really dig into the SLPs and siamese as far in as you can you could see some great gains across the powerband. Lose an insignificant amount of torque, gain a significant amount of horsepower!

 

Anyone one think headers would be more ideal to do? Or would these mods be better much equal in benefit, time and labor?

By the way, do they make headers for dual cat cars? Ones that would bolt up to the stock Y?

 

go with headers instead. i felt an improvement right away, unlike with a set of SLP runners i used to have. even if you get more airflow in, it'll still be restricted by the exhaust manifolds.

 

Runners AFTER headers, definitely. I didn't think about that. The runners will show gains only when the exhaust has been taken care of, cause our exhaust... (sucks)

** and yes they make headers for our dual cats. However, if your cats are stock I doubt they're functioning anymore! You can gut them, to get rid of the (now useless) honeycomb material and get headers with the dual cat y-pipe; or you can junk them and replace them with a new single catco cat for around 50 bucks and get headers with a single cat y-pipe. Either way is good - the single catco cat flows well, and so do gutted dual cats

 

Most headers for Dual cats come with another 2 pipes that you then cut and weld onto your intermediate pipes (stock pipes) going into your cats. I would recommend a pair of shorties unless you want to custom fab up an exhaust system for long tubes. I would say go with 1 3/4" tubes too you could get 1 5/8" for a little more tq but you might end up wanting to upgrade them later for a different motor... I'm going through this problem now.

 

GMHTP is a great mag...but remember that SLP practically sponsors the mag, and they did that Dyno test AT SLP.

Food for thought. That's all. Take it for what you want.

 

What about porting the exhaust manifolds?? I heard about that before too.

 

There is not much to get out that. Some people do it because they absolutley can't afford headers, and they have the time and skill to port. Headers DESTROY ported stock manifolds.

 

I totally agree with the above statement. The only real reason to port out manifolds is if you're broke and can't afford headers or if you're in a class that doesn't allow for headers. Main difference is that the headers offer bigger ports and each port is kept by itself in the tube... there's not a big conflict of trying squeeze 4 ports into a little 2" or 3" tube then drump it out. Also theres more of an equal length of the tubes. If you get headers you won't be let down... I swear I gained a easy 20 tq+ from headers and catback on a stock 350.

 

This is probably a little late to answer but here it goes anyway.

I would suggest spending your money on the headers as well. There are stock eliminator guys running mid-11s with the factory runners. The headers should be worth almost twice the power output for just a bit more money. Port the plenum and port match the runners. Save the lower intake and runners until after you get a cam and/or heads.

And no SLP doesn't sponsor the whole magazine. We went to their shop, put on their parts and told the readers exactly what we found by installing them. Nothing but the truth.

 

So you work for them, and were on that article?? Sweet job man.

I would get headers first also. But I cant see doing aftermarket runners unless your doing it for emmisions reason. Go with the HSR if you can, more gains for about the same money. ALso you will be able ro realize more gains in future mods with the HSR