Stock Runners? Modified 355
 

Im in the process of putting a tpi back on my 86 iroc. The original engine was removed(305 tpi), and I put 355 with high flowing heads and a moderate hyd roller cam in its place. I am trying to figure out if I need to go with better flowing runners.(slp?) I have upgraded the injectors (22lb) and am programming a chip to match. I have a holley 670 carb that seems to be well matched to the engine, but am excited to get the tpi back on.
Sorry if youve had to answer the same kind of question a bunch of times, but any advice would be much appreciated!

Re: Stock Runners? Modified 355
 

What specs on cam?
Stock TPI intake is known restriction to top end performance so anything you can do to improve flow will add power potential

Re: Stock Runners? Modified 355
 

Yeah, ive heard that the runners were the most restricted part of the top end. The cam Im running has 206-212 duration and 480-487 lift. With the carb Im getting power up past 6000rpm, Im hoping to get good flow from the tpi and not lose the top end power.

Re: Stock Runners? Modified 355
 

The stock top end can support 400hp/500ft lbs, You think you'll exceed that with the 355?

Re: Stock Runners? Modified 355
 

But flatlines at 4500
Aftermarket intakes will run past 6K

Re: Stock Runners? Modified 355
 

More like 4750/5000, but it comes down to the Owners desired usage of the vehicle. On the street, 4500 is plenty. For drag racing higher rpms might be. But a higher rpm range also costs you bottom end torque.

For a primarily street car, I'd gladly sacrifice the above 4500rpm horsepower to gain bottom end torque. For the drag strip I'd go for rpms too.

I constantly see people quick to dismiss TPIs rpm range when no consideration is given to the intended usage.

Re: Stock Runners? Modified 355
 

I think a FAST TPI intake would work well with your combo. It will allow you to keep that low end torque, but let you breath uptop and not waste your cam. Your set up with that intake and a spot on tune will make a really fun street car that should be easily into the 12's.

Re: Stock Runners? Modified 355
 

Thanks for the info guys. I forgot to mention that my heads have been ported and polished too. Ive been trying to find cfm specs on the stock runners compared to slps. Im wondering how much of a difference bigger runners can make? What is a Fast TPI intake?

Re: Stock Runners? Modified 355
 

are u keeping the 273's? then if you are you better go for a low end torque street car. How are u using the vehicle? track only? street?

Re: Stock Runners? Modified 355
 

I would love to change the rear end gears to 3.73s, or 3.42s. Its just a street car, but not a daily driver. I feel like the gears are way too high right now. Do you guys think that upgrading the runners sacrifices low end torque?? I am just kind of wondering the difference in performance between stock and aftermarket runners, and I guess trying to confirm that they are the most restrictive part of the intake? Maybe someone has done this upgrade and was happy or unhappy with the results?

Re: Stock Runners? Modified 355
 

Unported the SLPs will show very little gain. You have to port the !!!! out of them to be worth it. then you need a bigger or ported base.

Re: Stock Runners? Modified 355
 

The heads are your biggest bottle neck with the TPI motors. You have updated heads so I think you will see nice gains. I have the Edelbrock intake set up on mine. And I'm pretty happy.

Re: Stock Runners? Modified 355
 

you will lose a little low end,it will mainly shift your powerband up a little. the same torque you had at like 3k rpm will now be seen at 3500 but you may see like an extra 10 ft/lbs. That is basically how it works. Just so you know, if you are running a 700r4 transmission,then 342's would be the max gear i would use. I have 342 and almost have no 1st gear now and feels like i need to shift again after 65. So that gives u an idea. I say stick with the stock runners UNTIL your requires bigger runners.

Re: Stock Runners? Modified 355
 

i think your looking at like 10hp on a stock motor and they cost $300+.

Re: Stock Runners? Modified 355
 

Before you look into a gear in the rear end, think about the cost. You probably have a series 2 carrier. Series 2...you can only go to a 3.08 gear....its not worth the $$ in my opinion.
Anything more then that your going to have to jump to a series 3 carrier. And if your going to be spending the $$ I would go with 3.73....I have had them in my car for 6 years...wouldn't change a thing! People who think 3.42's are the best....haven't driven a car with 3.73's LOL!!

Re: Stock Runners? Modified 355
 

you can pick up a used rear for $150 from a junkyard. Sometimes with 342 or 323's in them. I remembering reading an article on a 91' TPI car that went from 342 to 373 and gained nothing. Maybe a better 60ft time.

Re: Stock Runners? Modified 355
 

Remember he only has 273's. I gained like 3 tenths from my 342 swap! Feels like 100 ft lbs of torque. Even my wife said the car felt like it had a supercharger or something on it

Re: Stock Runners? Modified 355
 

Yeah, I would like to use the 3.73s.. I think. What kind of rpm are you running at 60mph?

Re: Stock Runners? Modified 355
 

suit yourself

Re: Stock Runners? Modified 355
 

and imagine what she would have said if you installed a 3.73 :thumbsup:

It's not like hes going to the track racing every week. Hes an in town driver who like the streetlight to street light race. Or likes the feel of goosing the gas pedal off the start.

You want to get even better throw a little stall in it. Do some research, a stall out of a S10 truck / blazer bumps up the stall a little, that's cheap too. And that's just a stock GM converter. I think our cars stall around 1200 this bumps it up to 1700.

Re: Stock Runners? Modified 355
 

Remember the 3.07 1st gear of the auto gives plenty of torque multiplication
In 1st gear ,2.73 rear w/ T700 is equal to old skool 3spd auto with 3.4's
3.42 is equal to old skool auto w/ 5.10's

Probably running out of top end w/TPI

Re: Stock Runners? Modified 355
 

https://www.thirdgen.org/forums/tran...4-11-whos.html

here is a link to the mph / rpm's it shows 3.73's also