Realistic SL mpg and K/N air filter

[JScottB]

We just got our 98 SL a few weeks ago and it has responded incredibly well to some TLC but it's gotten to the point that I'm not sure if I believe it. At first it got 25 mpg, I replaced a faulty ETCS and it jumped to 31 mpg which knocked me out. I then installed a K/N air filter, not a cold air intake, just a replacement for the factory air filter, and now the crazy thing is getting 38 mpg. I know K/N boasts a possible 10% gain in mpg but mine was more like a 20% gain going from 31 to 38 mpg. I opted for the K/N filter more so for the possible gain in mpg than for an increase in HP but I wasn't expect that kind of improvement and the old paper element filter was actually pretty clean. For those of you who use the K/N filter, have you seen an increase in mpg and if so, has it been a substantial gain? Keep in mind that our SL has a SOCH engine, 120k+ on the OD and the 38 mpg is an average of both highway and in town driving. I'm still not sure wheter to believe the 38 mpg just yet but if it can pull the same mpg again when I fill it up again next week, I'll let you know.

[RC1488]

I get about 38 MPG with hard highway driving (NYC) in my '02 Sl1, and 38 mpg with light highway driving in my 01 SC1. So yes...your car should be always above 35 even with city driving. If its less..somethings wrong.

[JScottB]

I tend to agree with you. The jump from 25 to 31 mpg was easily explained by the replacement of the faulty ETCS, I just had a difficult time believing the jump from 31 to 38 mpg was solely due to replacing the original filter with a K/N filter especially when the original paper filter was actually pretty clean. I drive the car soley to work and back but my son uses it when I get home and that can make things inconsistent because where he drives and how he drives it can throw things off. The engine could have also been continuing to burn off carbon deposits that had built up during the time it had a faulty ETCS and gradually running more efficiently as those carbon deposits cleared.

[RC1488]

You will also see a jump in MPG if you put on better tires. I have Michelin X Radials right now on the '02 and '97 and saw a 2mpg increase

[JScottB]

What's driving me nuts now is that my son is just itching to swap a dohc SL2 engine in it just to get 20/30 more HP. I told him if he wanted to go through all that mess, why not stuff a 2.2 ecotech from an L200 in it or maybe ditch the whole front wheel drive thing, go rear wheel and cram a Pontiac 455 of mine in it. Yeah I know I'm talking stupid but the point I'm trying to get into my kids head is that as unglorious as the SL1 engine may be, it does have potential to make substantially more HP with out having to swap engines.

[RC1488]

True true, however for the amount of work into the SOHC, its just smarter to put in a DOHC. However, make him do the research, and he'll see its not as easy as putting in a DOHC motor It may change his mind about it.
A DOHC engine would put the car around 124 hp, over the 100 hp of your SOHC.
Also, the ecotech will not be a plug and play operation. New mounts, axles, tranny, PCM, etc etc will be needed. This means some major crafting, time, and $$$$$

[JScottB]

Back when Saturns first came out, there was about a 30 HP difference between the SOHC and DOHC. The main difference between the 2 engines was of course the cyl head but also the fuel injection sytem and Saturn engineers found that when installing the FI from an DOHC onto a SOHC, the SOHC gained 20 HP wich meant the majority of the HP the DOHC had over the SOHC came from the FI and not from the cyl head and oddly enough our 98 SL1 has a FI that looks identical to the FI used on the early DOHC's. Basically what I'm asking is other that the cyl head, what is different between a later model (98) SOHC compared to a 98 DOHC?

[RC1488]

Basically nothing other than the head, compression ratios, PCM and all its tables. As you said when the s-series first came out, changing the fuel injection system would yield great gains. however the eary SOHC model came with TBI or Throttle Body Injection. This is very similiar to a carborated fuel injection system. The DOHC only came with multi port fuel injection. This is why puting on the DOHC FI system on the SOHC car would bring about such a great difference.

In the '98 year, both SOHC and DOHC use Multi port fuel injection. So if you did do a SOHC to DOHC swap, You wouid re-use your existing FI system including the fuel rail and injectors.

Hope that helps