L98 TPI questions.
Race Director
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From: Kingman AZ
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St. Jude Donor '06-'07-'08-'09-'10-'11-'12-'13-'14-'15-'16
I had a nice long discussion with the guys from Corvette Plenum at Carlisle a couple of years ago. Their work has been featured in some of the hot rod and Corvette rags with great results. They do CNC porting of the L98 plenum, runners and intake manifold to open them up so they can breathe better.
I don't think they are in business any more with so many people moving on to the C5 and C6 though.
They explained to me that you cannot just open up the intake without doing other related modifications.
In order to get more power you will need to install a cam that takes advantage of the extra fuel. You need to either port the heads or purchase aftermarket heads. Headers and a less restrictive exhaust are required too.
Don't forget the roller rockers, aftermarket ignition and high stall torque converter either. Then you need to go to lower rear end gears and a custom tune to get it to run fast and well.
Gets expensive once you start adding things up!
I don't think they are in business any more with so many people moving on to the C5 and C6 though.
They explained to me that you cannot just open up the intake without doing other related modifications.
In order to get more power you will need to install a cam that takes advantage of the extra fuel. You need to either port the heads or purchase aftermarket heads. Headers and a less restrictive exhaust are required too.
Don't forget the roller rockers, aftermarket ignition and high stall torque converter either. Then you need to go to lower rear end gears and a custom tune to get it to run fast and well.
Gets expensive once you start adding things up!
Team Owner
Joined: Oct 2004
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From: altered state
St. Jude Donor '05
Do both the runners (replace with aftermarket) and the base. There is a ton of meat there to work with (base) and it will need it, pretty puny inside.
Other than matching the plenum to the runners there isnt much gain there kind of a while youre at it thing. Few ponies tops.
CP :isnt around for a reason
Other than matching the plenum to the runners there isnt much gain there kind of a while youre at it thing. Few ponies tops.
CP :isnt around for a reason
Last edited by cv67; May 28, 2013 at 12:25 AM.
Team Owner
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From: SE NY
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The "holdup" is in total runner length, ~21" I believe.
I was able to extend my HP range by doing the siamese base mod, using stock runners & plenum. I just cut 1" x 2" deep holes betweem each pair of tubes in the base.
That way the base could pull air through two runners instead of just one. The result was a substantial extension of the HP range up top. I had my '88 shifting at ~6k RPM instead of nosing over around 5.2k RPM.
Alternately you can purchase a BM base with AS&M runners to allow your L98 to breath better up top.
I used an AFPR to match fuel flow to air flow increase at WOT.
I was able to extend my HP range by doing the siamese base mod, using stock runners & plenum. I just cut 1" x 2" deep holes betweem each pair of tubes in the base.
That way the base could pull air through two runners instead of just one. The result was a substantial extension of the HP range up top. I had my '88 shifting at ~6k RPM instead of nosing over around 5.2k RPM.
Alternately you can purchase a BM base with AS&M runners to allow your L98 to breath better up top.
I used an AFPR to match fuel flow to air flow increase at WOT.
Team Owner
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From: Athens AL
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Runner length is the primary holdup to making more HP numbers, its "tuned" for low rpm performance where the high port velocity increases torque.
So to make it simple, you'd be better off replacing the plenum and runners with another system if its a choice between that and the manifold, but you'll want to port the manifold or replace it as well.
So to make it simple, you'd be better off replacing the plenum and runners with another system if its a choice between that and the manifold, but you'll want to port the manifold or replace it as well.
Race Director
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From: Overland Park Kansas
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In theory, siamesing the base (at the top) sounds good – and 65Z01 seems to like the result. As pointed out, air can be pulled from adjacent tubes. In practice, I THINK we’ve seen better results via complete siamese of SLP tubes. (You can’t really siamese [mandrel-bent] stock tubes.) Base siamesing is probably less affective [than tube siamesing] due to pressure wave disruption -- by having “holes” firing bursts into adjacent chambers. In short, call it cross-chamber turbulence. Also, it takes some large modifications to create significant change.
Given the choice, I would replace the runners WAY before the base though. (In fact, I’d never replace the base w/o a runner swap.) Larger tube runners can [somewhat] help flow but the OEM runners on a large-tube base isn’t going to do much at all. Also, “funneling” air to a tighter opening is more beneficial by increasing air speed of the intake stream. Unless you pay for porting – or figure out how to do it on your own, leave the darn thing alone.
Team Owner
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From: SE NY
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Any pictures of this mod?
Given the choice, I would replace the runners WAY before the base though.
While working in Mesa I visited AS&M and bought their LT runners. Once back home I decided to go with the siamese stock base instead and sold the runners to a friend who already had the BM base.He had seen NO gains from the BM base with OEM runners. After he installed my LT runners he saw a 10chp gain with 1mph increase in trap speed.
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Burning Brakes
Joined: Aug 2011
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Yep, I have pics of the base & lower end of the stock runners. I blended the openings to minimize turbulence. Send me an IM if interested and I'll post my email address.
While working in Mesa I visited AS&M and bought their LT runners. Once back home I decided to go with the siamese stock base instead and sold the runners to a friend who already had the BM base.
He had seen NO gains from the BM base with OEM runners. After he installed my LT runners he saw a 10chp gain with 1mph increase in trap speed.
While working in Mesa I visited AS&M and bought their LT runners. Once back home I decided to go with the siamese stock base instead and sold the runners to a friend who already had the BM base.He had seen NO gains from the BM base with OEM runners. After he installed my LT runners he saw a 10chp gain with 1mph increase in trap speed.
Race Director
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From: Overland Park Kansas
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Consider yourself enlightened. IMO, MR, SR, or FIRST are all easier options than a modified TPI setup. Even the XT Pro Flow and modified LT-intakes have been used to avoid the TPI drama.
Safety Car
Joined: Feb 2000
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From: Austin Texas
I did it because I wanted something fairly stealthy. I had everything black crinkle powder coated before assembling it...hard to tell it that way.
I'm putting out 425 FWHP and 520 FWTQ and it is very fun to spank a lot of the LPE Super Ram and others on track.
Next time...I'm just dropping an LS7 in and to heck with stealth.
Burning Brakes
Joined: Aug 2011
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Depends on what you want to accomplish...
I did it because I wanted something fairly stealthy. I had everything black crinkle powder coated before assembling it...hard to tell it that way.
I'm putting out 425 FWHP and 520 FWTQ and it is very fun to spank a lot of the LPE Super Ram and others on track.
Next time...I'm just dropping an LS7 in and to heck with stealth.
I did it because I wanted something fairly stealthy. I had everything black crinkle powder coated before assembling it...hard to tell it that way.
I'm putting out 425 FWHP and 520 FWTQ and it is very fun to spank a lot of the LPE Super Ram and others on track.
Next time...I'm just dropping an LS7 in and to heck with stealth.
Race Director
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From: Overland Park Kansas
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Thread Starter
Le Mans Master
Joined: May 2006
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From: indy indiana
I really don't want to lose my bottom end torque..thats where these l-98 shine and love the take off. not really concerned about 5K rpm run ups, for me that gets into " Danger, Danger, Will Robinson" territory.
Race Director
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From: Overland Park Kansas
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If you did a cam, did a bowl job/spring refresh to the heads, and swapped to a short-runner intake, you'd be happy.
Otherwise, I think you're making the right decision.
Trading to an LT1/4 [someday] is good option to consider.
Otherwise, I think you're making the right decision.
Trading to an LT1/4 [someday] is good option to consider.
Max G’s
Joined: Nov 2008
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From: Monroe OH
NCM Sinkhole Donor
Vader has a good write up on what mods are good for the TPI. From my experience, if you are going to stay mostly stock, go with the 3 piece undrive pulley set, large tube runners, remove the pre cats and install a new main cat. Just those mods will make the car run a lot better. 
Race Director
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From: Overland Park Kansas
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Vader has a good write up on what mods are good for the TPI. From my experience, if you are going to stay mostly stock, go with the 3 piece undrive pulley set, large tube runners, remove the pre cats and install a new main cat. Just those mods will make the car run a lot better.
Also, swap out mufflers for non-stock units -- like glass packs, chambered pipe, and/or resonated tips. Magnaflow makes a great resonated x-pipe muffler that would be good for the belly (in place of the center converter).
Safety Car
Joined: Feb 2000
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From: Austin Texas
New (universal) pre-cats and no main cat would be better. Better flow thru two cats versus cramming two pipes thru one. Less resonance too.
Also, swap out mufflers for non-stock units -- like glass packs, chambered pipe, and/or resonated tips. Magnaflow makes a great resonated x-pipe muffler that would be good for the belly (in place of the center converter).
Also, swap out mufflers for non-stock units -- like glass packs, chambered pipe, and/or resonated tips. Magnaflow makes a great resonated x-pipe muffler that would be good for the belly (in place of the center converter).
This after having a Flowmaster L98 system with large center pipe and 2-chamber mufflers.
My mid AND top end improved, AND I can now hear and the car is strong everywhere. Once again I could actually HEAR the stereo!
Thank god for Corsa!
Max G’s
Joined: Nov 2008
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From: Monroe OH
NCM Sinkhole Donor
New (universal) pre-cats and no main cat would be better. Better flow thru two cats versus cramming two pipes thru one. Less resonance too.
Also, swap out mufflers for non-stock units -- like glass packs, chambered pipe, and/or resonated tips. Magnaflow makes a great resonated x-pipe muffler that would be good for the belly (in place of the center converter).
Also, swap out mufflers for non-stock units -- like glass packs, chambered pipe, and/or resonated tips. Magnaflow makes a great resonated x-pipe muffler that would be good for the belly (in place of the center converter).
Race Director
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From: Overland Park Kansas
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Unless you're open to the tailpipe exit, you'll lose a lot of the scavenging effect either way. I still think the main converter would kill that too.
From a strick sound perspective, one center converter was the worst. Two front converters is the better.
From a strick sound perspective, one center converter was the worst. Two front converters is the better.