Hi all,
I am the original of a 1985 C4 with the Z51 handling package. Thanks to some of you on this forum who talked this newbie through on replacing a failed water pump. I will browsed this forum to get more performance out of the L98. So far,
1. I have done the throttle body bypass.
2. Installed a K&N air filter and I enlarged the air box to allow more air to the intake.

I noticed that I got better throttle response after doing items 1 & 2.

So, what is the next step for higher performance. I have read the discussion on the smog pump removal and I am not so sure that it help much relative to performance. Should I been taking a look at headers or should I continue to focus on getting more air for the intake?

Thanks and I look forward to your responses.

 

Removing the AIR is more of a cleanup thing that you'd do when you add headers, because most won't have the AIR fittings for it anyway.

Headers were the next step I went on my 86, which is basically the same as 85. It woke up the car a good bit.

Intake can be as simple as portwork for now, but going further requires some more thought as to your hoped final outcome for the car.

 

Headers are going to give you the best wake up at this point

 

A good friend had pulled his Intake took the replacement plenum and intake gaskets laid them in place drew with a sharpy then took a Dremel and opened up the intake and plenum ports. It seemed to wake up things

 

Thanks for all of your responses. Looks like headers and possibly port work are my next steps. I checked the internet and found names like TPIS, Hooker, Headman, American Racing, etc.

Regarding TPIS, I checked their web site but they almost never have the headers for the 85 C4 in stock. I don't mind paying for quality for I have read that many on this forum liked their products.

American racing have a nice set of 1.75" diameter stainless long tubes. I don't plan on racing so perhaps 1 5/8" diameter tubes are the way to go. I basically want to cruise around town with better performance than stock. What do you suggest?

Headman, Hooker, and the rest are less expensive. I don't know anything about their quality assurance. Not that I have many choices but do you suggest using any of these brands when TPIS and American Racing do not have the headers that I need? I am not real handy so I will have to pay some shop to do the install.

 

header and free flowing exhaust. stock exhaust manifolds are a restriction. I've never tested TPI. Sounds like the stock setup is flowing 90% of the head flow. I guess going for 92% could gain 8hp.don't know.

 

hedman fit good on my 84.

 

Thanks burning brakes. Not sure that I understand the 92% number. Does this imply that the header and free flowing exhaust will only yield 92% of the head flow or roughly 8hp?

 

Thanks mike1111. Sorry for these basic questions. What is the material of the Hedman headers? Is it 1 5/8" diameter tubing? Did you have to paint the headman unit with high flame retardant material? How long did your headers last?

 

Find a local dyno operator (preferably with a dynojet) and pay for a couple of dyno pulls to see where you're at now. ~220 wheel hp would be a reasonable expectation for a near stock L98.

The magazine test times from 1985 are all pretty slow (like 14.5 seconds) so my power estimate may be on the high side. Long tube headers would be the last thing that I would recommend.

 

Thanks tequilaboy for yours suggestion. When you say take it to the dyno for testing, do you mean get a plot of hp / torque vs. rpm. By looking at the graph, we can then decide on which direction to go for performance improvement? Or do you mean take there to have the engine tuned before doing any more modifications?

If long tube headers is the last thing that you recommend, what is the first bolt on to add? Best regards.

 

yeah I was unclear for sure. I was referring to head flow without intake manifold vs head plus intake flow.
flow testing an intake manifold and head together, they will flow a percentage of what the head flows. for example, a head that flows 200cfm, adding an intake they could flow 90%. that's 180cfm together. porting intake could increase flow to 92%, 184cfm. It also may not flow that good. not going to flow 100%. I've never tested TPI. just going off of other people's tests.
I've tested same heads that come on 85, with a manifold that flows 237cfm on its own. together they flow 92% of head flow. 196 x 92%= 180cfm.

 

my headers are new and ceramic coated. did get 14ga. steel. 1.625 tubing. don't know how long they will last.

 

220hp at wheels is impressive from a car that has 230hp at crank stock.

 

Thanks mike1111 for the information.

 

You guys are ignoring the fundamental problem with the TPI intake: the resonant tuning that helps add power at around 2500rpm creates a brick wall above about 4000rpm. It doesn't matter how much the runners and base flow with a steady flow of air: once the are trying to move resonant air (pulses created by the intake valves at a frequency half the crank RPM) the Helmholz characteristics take over. Porting this thing won't do much at all. To improve power substantially, you really need to consider a different intake with short runners.

 

Speaking from experience and many conversations here locally with AS&M... The TPI has a restriction/choke around 4500rpm, so regardless of pressure wave tuning, a big mouth intake and larger smooth runners and matched/ported plenum, will make a big difference for the ENTIRE powerband. It's not just runner size but the smoother runner creates more velocity and flow. I've done this on the 1986 many years ago and with a ZZ9 TPIS cam (and springs, rockers, headers) it easily revs past 5,500, and clearly above 300hp at the wheel on the mustang dyno. The Dyno was used for tuning up the holley spark table rather than repeated WOT pulls, but of course HP and TQ is easily available real time, I let John with AS&M do that work as he's done this same combination many times. The car ran good when he started, and mostly just confirmed the spark timing strategy but allowed for more spark advance above 4500-6000 to optimize.

Progression was first a better flowing intake/cam/headers as stated above, and then next step down the road was a Lingenfelter Super Ram upper with ported from 38mm to 40+mm runners, with the same big base (added power above 5K without any noticable loss down low)... next step after that was ported heads, next step was TPIS 1204 Mini Ram (short runner) which may have made more at 6K on the dyno, but lost a lot below. The first jump was the biggest by far if you look at overall power from 3K to 6K... even if 6K was not the strongest, but still quite usable (biggest improvement 5500 to 6200 was was the Super Ram though, and everyone said you can't go above 5,500 they were wrong) and the rods/springs were probably at the limit there.

The next step after that starting 2 years ago (we'll ignore this one here, but only from historical accuracy) was a biggest step with 4.125bore aftermarket block, 3.75 forged crank, more agressive custom billet cam... and going with AFR210 Comp heads meant new intake was required, for me a bigger Miniram was necessary to be able to bolt to the 210 heads (I think 1205 but not 100% in case there's a 1206, certainly not 1204 Miniram, and it needed a little work to line up). The larger displacement was key to properly utilize the short runner MiniRam. I did contemplate a custom welded intake but the 1205 sized MiniRam was the easier solution. Pipemax and Performance Trends software was pretty much right on the money with accuracy of the changes, if you want to model and design/optimize to your liking and have the tenacity to accurately input dimensions especially CSA's and other details.

 

Thanks MathewMiller. I take it that intake head flow improvements comes after headers if I could only do 1 improvement at a time. Regarding the resonant frequency, does it mean at that frequency, the engine will literally shake?

 

AZSP33D, thanks for sharing your experience and your very detailed explanation.

 

Big tube runners/ported intake and cat delete. Also bump your timing (93 octane) and increase your fuel pressure with adjustable regulator. This will get you into the 13’s in the quarter around 100 mph for least $$.