plug heat range advice

[jetski9010]

I am helping my friend put back together his superjet. The engine was already built when I bought it and it had B7HS plugs in it. The compression is around 190-200 psi (with a harbor frieght gauage) and the squish is around .050. The guy my friend bought this from didnt say anything about running pump gas or not so he was running it with 91 octane last summer but only rode it a few times. I went over everything with him and we are putting it back together now. First question do you think he would be safe with just pump gas, Pump gas mixed with octane booster, or should he mix 50/50 with 100 octane race fuel. Also would the B7HS plugs be right for this setup or should it be bumped to an 8 plug?

 

[spence]

Either bring the compression down to the 175-180 range or mix in some race gas

 

[Crab]

go to the 8's.

 

[tipafew]

My gut reaction is 50/50 mix and 8's would be wise.

 

[Waternut]

What kind of ignition are you running and what kind of riding do you do? The safest is certainly running 50/50 race gas but being in CA kinda sucks for you since 100/91 is still only 95.5 which is barely higher than the 93 most states get. However, with the right combination of ignition timing and riding style, you may or may not be ok to run with 91.

 

[brianelliott0414]

I am running a 5 mil stroker with total loss. It has 200lbs compression with a Snap-On gauge. I dont know how the squish band is set up, but I do know that I ran it last summer (75ish tanks of gas) with 92 octane at 40:1 with br8's. I think it mostly comes down to the rider. I dont ever do any full throttle pulls more than 75-100 yards MAX without burpin' the throttle a bit, and I am always throttleing it up and down when just cruising making sure I get plenty of fuel in there to keep it cool. I also keep my eyes on the plugs to make sure it is never leaning out because that would obviously be something very seriousley against my favor. SO, for me, I run higher than reccomended compression on pump gas in my ski, However that is the only thing. My cr250 is at the 180 mark on 92 with a br8 and I am still weary, and my snowmobile is also at the 180 mark and I mix 50/50 with 92 and 114 Trick. Basicaly the only reason I mix in the sled is because in one ride I can change elevation by several thousand feet which is going to cause it to run lean.

 

[Matt_E]

Why not a 100LL (AvGas) mix? :dunno:

I run 200psi on a big bore with a 3:2 100LL/92 pump gas mix.