Hey guys,
i have a '87 GTA with basically a cutout at the y-pipe and home made air intake, and an airfoil.
i have been consistantly running 9.009's at 76mph at the local 1/8 mile. i am going to be bracket racing this season, but want to run faster.
I just ported my plenum and put on some siamesed runners and port matched them as well.
What ET and Mph gains should i expect from this?

Also, i just put on a cowl hood, some 15x3.5 and 15x8 draglites with 26x10.5 Et streets, BMR boxed LCA's with relo brackets, and i am thinking about yanking the front crash bar and sway bar for weight.
Any ideas what you think i will run?
I am going to the track tonight to find out.
here is a picture of my car from 2 weekends ago.
i will post pictures after tonight as well.

 

Id say around .2 and 1 mph with the runners and by porting the plenum. Man those times are awesome for a nearly stock gta. Thats an L98 under the hood right?

 

Pretty much stock. cutout right where the y-pipe comes together, home made 4in intake with k@n. airfoil, pulley. no smog pump

well i actually ran an identical pass...i am rather disapointed.
9.009@ 75.67 1.99 60ft
with the draglites and the hood in weight reduction, and the runners in performance i thought i would dip into the 8's for sure.
my previous tires were with 245/70 16's this time i was on 26x10.5 15 Et streets. it really seemed like to much tire.
but its not any taller than the street tires.
I am stumped here. it was even cooler outside, and i iced down the intake.

 

If you are going to get runners get a better intake manifold as well. Thats where you will see the biggest gain. Check out my car domain site. I have almost full bolt ons on my L98. So if you have any questions about aftermarket parts I could let you know how they worked on my car.

 

None, you just flattened out your torque peak without doing anything for your peak airflow so your maximum hp should be about the same. Your biggest restrictions on the intake side are your stock intake/airbox assembly and then the heads. _Everything_ in the intake flows at least 25cfm more then the heads top out at, and you could probably use another .050” or more lift then the stock cam gives you to take advantage of even that.

You’ll be lucky if you run the same time, if you ran a great 60’ on that run you may run slower since you won’t have the same midrange.

Probably break even with the lower weight.

The stock L98 is limited to about a mid 1.8 60’ without steeper gears and converter. You messed with your midrange and probably aren’t capable of doing that anymore and would probably need about another 2-400rpm stall to come out even.

Based on your second post looks like you ran about as expected.

 

You said it was cooler out, but where was the actual DA compared to the last time your ran? How was the humidty compared to what you ran last time. I've seen countless times where ambient temps were 10-15* cooler but humity is higher thus cancelling out any benefits the cooler air might have offered.

Kind of the reason why fall is THE time to chase down best slips, cuz you have cool DRY air as opposed to just cool air.

Find a buddy with a weather station or when you budget allows get one for yourself, very good tuning aid for picking a number to stick on the window once you realize how your car reacts to the chaning weather. Not needed for once in awhle fun racing, but once you get serious about trying to win events, it's a must have item.

 

That's right.. to a point. It only made a few hp on my car, but moved the power range up about 200 rpm, depending on your gears and what rpm you trap at it could help you. But if you plan on running a daily driver/stip setup, tpi is the way to go and ported at that. It's pretty fun to leave the car in 1st (automatic) and hold it at 3400 rpm going about 25, and shift into 2nd and bark the tires. Flat out throttle, youll notice a difference.

 

this is one of the only instances where ive seen mentioned that the TPI intake isnt the restriction in the L98 motor, and that its actually the heads

ive seen flow data that states the completed intake assembly flows right around 200cfm, while at the L98 cams peak lift values, the heads only flow 175cfm. after reading that, i wonder what the results would be if the intake was left intact and the heads were changed on a TPI car. it seems as though everyone who modifies these engines does it the other way around though

 

glad this post is taking some interest.

Not sure what the DA was...but outside temp was about 61-63 the first time, then after the runners the temp was about 56-58.
Not a big difference in temps. But once again i dont recall the DA.

I am surprised to hear someone say the stock mess o snakes are not restrictive, and that the heads primarly are.. well to my knowledge they both are very restrictive.
Thus being the reason for used runners selling for $300 used on ebay, and corvette,vortec,and afr heads among others being a great replacement for stock heads to make power.

My take on the runners was that by putting them on and making a descent port match would increase the flow velocity in the still very restrictive intake and increase torque, not flattin it out. But either way an increase in flow with something restrictive should net me some sort of gain IMO.
But due to the results and what you are saying Crossfire TA , i assume your right.
I am about to take these runners off sell them,and put some stockers back on.
The car has 3.27 gears.

 

I have an Lb9 and did not note any real gains after siamesing the runners and mildly porting the base, removing the plenum wall until I also added headers. It pulls hard past 5200 now.

 

If you’re lucky.

The stock cam gives you something like .404” lift total, and basically you want to figure that the airflow that you can actually use is at least .050” under peak lift, and I’d be shocked if most L98 heads flow much over 160cfm at that point. The complete intake assembly usually flows about 195-205cfm/port.

Just for ****s and giggles, I played with some numbers, and with the track times I have from my mostly stock Formula 350 (even stock exhaust manifolds…) at it’s race weight of 3710 it should be making slightly more then 290hp at the crank. I took and entered that combination as close as possible in desktop dyno and got 292hp with the stock L98 heads. Then I increased the valve size and found a head flow file that should flow close to what the intake does (afr 190 heads, around 190cfm @ .300 and 230cfm @ .400, so for this comparison a little lower then what I’d want to see at .300 and a little higher at .400), and gained 47hp with no other changes. Coincidently, that’s about what one of the head manufacturer’s claimed HP gain from swapping a set of their heads onto an L98 head equipped stock engine.

Corvette L98 heads out of the box actually flow less then the iron L98 heads, their only advantage is that they’re aluminum and have tiny chambers to increase compression.

There’s a lot of utter bull**** out there about the TPI intake. The fact is that I hate the thing, but at the same time, I know that it’s not as bad as people make it out to be, and more accurately, it’s not what people make it out to be at all. Yes, it’s a tuned runner intake, but everyone’s “it’s has killer low end” is just not the case. The tuned length puts the first useful harmonic right in the low to mid 3K rpm range, not anywhere near the right off idle that people assume. If you want off idle torque, compare a properly running crossfire 305 to that L98 and you’ll be shocked how much torque that ancient tech 305 puts out, there is no comparison coming away from a stoplight.

My real point was that you have to match parts to each other if you actually want to go faster. In this case you actually took some reasonably well matched parts and made a few changes that mismatched them.

If I was starting with a stock L98 car and wanted to go faster with bolt ons, I’d start with the typical good cat back, some matching headers, and then a converter (for my use I’d probably start with something with a lockup, fairly good efficiency and a stall in the low 3xxxrpm range, unless I soon planned on intake changes to justify a little more stall, if I was recommending something to someone else that intended to put mostly street time I’d err on the conservative side with something that would stall in the 2800-3000rpm range).

If I was you now, I’d probability keep the parts and match stuff to them as you go, and probably get that converter next anyway, since that will cover up/fix the bottom end loss/slight mismatch that you caused.