Wow, do the lower pulleys off the X308 XJR fit the STR engine? Wonder whether a pulley ratio of more than 2.87 is possible

 

6.93"/2.5" = 2.77 . Everything is possible with different pullies but that requires excellent intake to get full advantage. Spinning the SC takes always power - even the TS one. There are a lot of other things to consider at that kind of boost levels if you don't want to blow up your bottom end or lift the heads. My new setup should be enough strong for high boost excluding a few parts. I just don't want to check anymore where are the limits.

 

Yeah I get what you mean, it's nice just having everything working reliably and not breaking. The way I drive my car sometimes, I'd probably push my luck too far with more boost on this baby TS. I thought Avos was running completely stock internals on his at 700+HP or are you shooting for more?

 

. Avos made ~630hp with stock 4.2 bottom end, but the 700+ was with stronger oversized pistons etc.

 

I see, I was too optimistic about how strong my engine is then! Those 2.6 KBs you guys have are beasts for sure.

 

We don't know where are the hp or boost limits of stock 4.2. Someone should blow up the internals first I have seen limits of stock 4.0 composite head gaskets as well as limits of stock 4.0 cast pistons in case of detonation in one cylinder.

 

That's interesting! My XKR 4.0 has lower 153mm ~ 6.02". I thought xjr 4.0 and xkr 4.0 engines are the same.

 

Haha! Send me one of your big lower pulleys and we can find out I think my stock one is 6.2" and blower pulley now is 2.16" so maybe it wouldn't end too well. My mechanic is actually keeping me a spare STR engine for such eventualities.

 

As I know the stock 4.2 lower pulley is 160mm = 6.30".

 

There is no difference between XJR and XKR stock pulley sizes.

 

Interesting, so my pulley ratio is actually 2.91 and at 6100rpm the SC is spinning at 17,792rpm.

I wonder what the max recommended rotor speed of the twin screws is? 20,000rpm seems like a nice round number

 

KB says max 18K

 

Max recommended constant autorotor 1.7 speed is 13500rpm.

 

Here's what KB say's about it:
Actual Size and Ration of 13# Pulley

I prefer the 12500 rpm range sound wise (so for cruising speeds).

 

Thanks Avos, good read. I've driven my current set up very hard on occasion and so far nothing has blown up. I'm still wondering how they made 507rwhp with the 1.7 spinning it at only 16,000rpm on a stock dual cam 4.6 mustang. Our engines aren't that different are they?

 

What certainly matters is how well the air can flow into the SC, and then afterwards into the cylinders, so you can't just look at sc rpm alone. Next to that there are so many more factors playing a role into net rwhp, there is just to little info to make a comparison.

 

Very true, I guess my intake elbow is far from efficient and blocks off a lot of the inlet of the supercharger so it's unfair to compare it against the tests KB did using a full intake like you made on the 2.6 kits. I'm sure if the full inlet of my 1.7 was used, I'd see more comparable power. Are you able to elaborate on how you modified the stock intake on the 2.1 KB to support 455rwhp? I might try to be a copy cat, if you don't mind that is.

 

The XKR has more room at the back of the engine then your STR, you can't create the easier flow into the SC there with the stock elbow without making a fully new one. The basics are simple, just make sure there is a nice path for the air to flow, ideally in a straight line from the filter into the SC ;-)

 

I see, I thought you made 430rwhp on a STR with the 2.1 KB with all stock intake. I didn't realise a custom intake elbow was made there.

Do you think 500rwhp would've been possible on that car with more aggressive pulley ratio, bigger MAF and ported TB? I could do with a project after I fit my 8.8" LSD

 

You talked about the 455 rwhp, that was with my xkr with the 2.1 KB, hence my answer.

Now you refer to the 430 rwhp for the STR, and am sure there would have be some more room, but I don't know if you could have reached 500 rwhp there, it would have been a struggle at least using the stock intake elbow design. I ditched that setup for a new intake design with the 2.6H as a more balanced setup with much more room to grow hp wise.