90stang5speed Rear Mounted Turbo Project Thread

did he reduce the plug gap any?

I have been too short in some of the information I have provided. Once you are up and running, have a good tune, have maxed out your possibilities, have access to a g-tech or track time try it. I wouldn't even consider it until your consistent time wise(et) and provided you still have your old ones and can do it for free, or on the cheap. Same with a cam swap now. Info only.


The exhaust gasses want to go back into the motor ,it called reversion.
Too radical of a cam and you can blow boost out the exhaust(overlap),or let spent gases back in, causing detonation. This especially happens before full boost, once the wastegate opens you are ok.
This is the same reason the 114 cam lobe is better than you 112 lobe center line cam. Go to the Weiand website and look @ the blower cams, or nitrous cams,114-116 lobes to keep valve events tight and build cylinder pressure, making(especially a rear mount) spool faster.

Another reason to run tight lobe centers, like 116-118.Most cam companies either don't have access to that type of lobe, or don't get the reason to do it.
It IS the opposite of a N/A car for sure. You need more in and out on them, and both valves open help draw in the fresh charge. If used in the rear mount set up massive amounts of hot spent gas backs up to the motor, It would be easier to go back into the motor than spool the turbo.

Someday call Bullet Racing cams and tell them you have a rear mount set up and see what they said.
They were the only company that figured it out ever.
At 1st it was speculation on my part, then after 5-6 custom grinds I could see the benefit. Beware that a 114 and higher lobe will run out of steam and not tach to 8,000 , but you shouldn’t need to if you are @ full boost around 2500 to 3000. The less compression your combo has, the more overlap, the bigger the turbo, the longer it will take to boost.
You do have a dam big turbo!(for a rear mount). Depending on the trim, your 76 in your configuration is good for 850 to 1000 hp. On smaller engine combos, below 6.0 I usually would run smaller twins.
The a/r's are tinny on the rear mount turbo's compared to even a small 4cyl Honda front mount. You may decide to go smaller or larger on the A/R to help tune the boost in to a usable rpm for your combo.

I hope this helps.Yep, its controversial but hey, you got a rear mount set up that is controversial to begin with.
Hang in there and keep up the good work.

90stang5speed said:

I get the velocity idea but, which would be better, smaller volume at higher velocity or higher volume with a little less velocity? My buddy with the twin turbo VR4 has experienced spark plug blow out & compressor surge due to restrictive exhaust before he upgraded it.

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The more boost and compression, the more spark you need. Higher boost levels, even higher compression will "blow" the spark out.

The big boys always use a capacitive discharge system, like Msd-7al.
Nitro cars use a magneto for similar reasons.
Did the vr4 increase the boost levels?