Ok, I am curious. Will it make much of a diffrence with the 1-5/8 or the 1-3/4 primaries? This is for the 91 in the sig. I plan on a Vortec crate, with other boltons and good prom tuning, the rating should be like 360-380 at the flywheel. Question is. Will the 1/8 diffrence hinder or help me. If 1-5/8 will sufice then a set of the dynomax coated headers are a lot cheaper. They can always be cut and mated to dual cats. the headers will be installed way b4 i get the motor, so im planning ahead, but the motor will go as follows SDPC Vortec Crate Motor, LPE 213/219 cam and 1.6 RR's,
SLP siamesed ported runners, ported plenum, 52 mm tb, AFPR, 24# injectors, custom prom. the exhaust will have the random tech cats, and spintec exhaust. The cats need replaced now, so thats 1st, then the headers, ill just be swapping the mufflers. So tell me what ya think?

 

This is really a question for the exhaust board but...

I had SLP coated 1 5/8 headers on my L98 and they are now on my ZZ4 even though SLP recommends 1 3/4 for the 350ci engine. This is an ongoing debate.

With the crate motor, I would most definetly go with 1 3/4. With the stocker, even with mild mods, I would go with 1 5/8. 1 5/8 flows plenty up to about 375-400hp and retains lots of low end power.

If you do go with 1 5/8 and then switch the motor like I did, rest easy knowing that if you spent an immense amount of money to switch to 1 3/4 headers, people that have done that only see about 6hp increase on the top end, at a sacrifice for a little low end.

 

I put a set of SLP 1 5/8 on my 305 with no head or cam work.. Just pulleys,airfoil,and a Dynomax Ultra Flow muffler and I gain 7 hp on a dyno.. Which I was impressed considering 221 RWHP before headers is pretty good.. So I thought everything was flowing great and I would see no increase but 7 hp after the install wasn't that bad.. I was thinking more about 10 RWHP gain but no luck... you gotta pay to play... I will be swapping a 350 sometime in the future so I know they will work great on that aswell...

Just thought I would let you Know..

 

Stiffler,

Is your car an auto or 5 speed?

 

5 speed

 

I would go w/ the 1.75" just because it's more likely to match up better w/ the D-port exhaust ports on the vortec heads.

Personally, I just checked out the plug access on the 1.75" dual cat SLPs on a friends 91 and I'm not impressed, but I don't know what ones are better.

Matthew

 

Plug access isnt a concern. Im use to spending rediculous amounts of time changing them. Thanks for the replies guy. Looks like it'll be the 1-3/4 ones

 

how about u sell that hunk of junk to me for $3.00 hehehe yea man.. go with the slp 1 3/4 are u gunna get the coated... i gotta see them in ur car next time we all meet up!

 

If you plan on adding bigger heads,intake,etc., then go with the 1 3/4 now. What little power you give up now, will pay dividends later.

Especially in the cost to install and weld up the dual cat system. Also, while you are at it- get new converters and have a custom after cat y-pipe made with the smoother flowing Flowmaster Y-Collector.


I am putting on my 1 3/4 SLP's this weekend. They have got to make more power than the crappy log type manifolds.

 

yeah i would follow what slp recommends and go with the 1 3/4..

 

I did that and plunked down my $ only to realize I went faster thru stock manifolds and when I contacted them about their performance guarantee they tap danced around the issue by having me install a AFPR which I did and still no good so they said for a basic bolt on L98 car 1 5/8s may have been the way to go and the person who sold them to me should have told me that but if I buy a cam from them and do other work the 1 3/4s will be better. They gave me a bunch of excuses as to why I didnt go any faster even tho I provided slips with identical 60fts in similar conditions so make sure you figure out what you PLAN on doing since they dont always give you the returns you think youre investing in. I guess when I add a cam and heads the 1 3/4s will shine funny how they never mentioned that when I still had my $ in my hand...

 

ill prolly go with the 1-3/4's. After the motor swap, it should be a good benifit. Ovet time ill be modding the motor with all the stuff, less the new intake and heads. Then ill just have it all moved over to the crate. The crate comes with the hot cam but ill be usin an LPE cam. How you think the hot cam will work in an LG4. Bit extreme, but an interesting thought.

 

If you still have the stock intake pieces, then the 1 3/4 headers will not do very much.

If the motor doesn't have enoughair volume going in, then if the headers and exhaust are too big-it's like hooking up a firehose to your garden faucet.