Jetting a 468 with blower
 

A buddy of mine just got his motors back from the machine shop. Looking to richen things up a tad for safety. Here is his specs

468CI
Merlin oval port heads
B&M 174 Supercharger making 5PSI
10:1 CR
Fairly aggressive solid cam (blower grind)
1050 Dominator.

Right now he is running 98 jets, and 6.5 PV. He was going to switch to a 10.5 PV in order to richen it up, but I think that will just make the PV open sooner, and have no effect on WOT???

My theory is that with the little blower, and 10:1, his pistons are gonna be real hot! What would be the best way to get more fuel in? Going up several jet sizes??

That power valve should be used on a motor idling with 21lbs. of vacuum and with his cam and blower it'll just hang open all the time no help at all. Bigger jetts is the only way.

with that compression and a blower real hot is a understatement...grenade is closer...did he do the the math on static compression ratio @ 5 psi ??? and running in a boat ?? example a merc 525SC runs 7.5 comp....so tell you friend better look at race gas for it to live or the pin has been pulled...

Wow! I have a procharged 454, 8/1 compression,9PSI and I'm running 102's. I believe that is the largest jet available...might have to custom drill? Maybe you should just have a valve where you can just dump all the fuel the pump will push straight into the top of the carb when it gets in boost mode! ;)

I run a similar setup on 470cid engines. 9.5, 5psi, dominator, 6.5 pv, 731 cam. With 98s all around my a/f was round 11.6-11.7 wot. I moved them to 96s all around and the a/f numbers are 11.9-12 wot. I've run pretty hard with it this way so I am leaving it for now. My cruise is way fat though with around 11 a/f so I plan to go to 4.5 pvs. My vacuum at cruise is 5-5.5 so that seems right.

BTW my dynamic compression with 9.5/5psi is 12.4. With the 10:1 he would be around 12.75:1. He needs to stay rich and run 28-30* max timing and get 93+ octane fuel. I run 93 and about 30* total timing. I have aluminum heads so that helps me some and I run them cool at 140-150 water temp. If he has iron heads he might be precariously close to the edge. If the merlins are alum, he should be good.

Depending on the Dominator, the high speed air bleeds tend to be too big causing the fuel mix to go lean on top end no matter how big the jets are. Going to a smaller air bleed will have the same effect as going to a bigger jet. The air bleeds are on the top of the carb near the venturies. I believe the ones closer to the center of the carb are the high speed and the ones closer to the outside are the idle air bleeds. My carb had .037 air bleeds stock that I changed to .027. I was able to drop down 8 jet sizes as a result and the fuel curve seems better. That engine is going to need a lot of fuel to cool the piston tops. Do the air blled change and jet it untill you have a lot of color on the plugs. I would mix in race gas also to help it live.

Btw, size the power valve to open near full throttle but not during low speed operation like docking. Most boats like 3.5-6.5. Any larger numbers and it will be open during cruise killing your gas milage and sparkplugs and stalling around the docks.

I would also eliminate the thermostat and use a restrictor plate instead. Keeping the heads as cool as possible should be a goal

All good advice. The 6.5 PV size should be fine. I would just increase jet size. I too think that 10:1 with a blower is not very reliable for the long term. I would actually bring the boost down 1-2# with larger pulley. 10:1 by itself needs 93 octane. He's gonna needs some race fuel.

What does it take to convert a dominator with press in air bleed to removal air bleeds? Is there a kit with drills and taps and such? That would be fun to play with but mine are not changeable unless I am missing something. Sorry for the hj MT.

with 10-1 static I dont think you can do anything to keep it from detonating even at 5 lbs that little blower is spinning hard.

Got the cam specs from my buddy.

Solid Flat tappet
.581 in
.615 ex
244 intake, 256 exhaust @ .050

Total timing 34* he said.

A solid cam, 10:1 CR engine is really not a good platform for a small supercharger. The only reason to run a solid cam is to spin a lot of rpm. That supercharger is gonna make a lot of heat with more rpm and increase the risk of detonation.

I gained 9mph with my similar setup when adding the blowers. I run hypereutectic pistons too so there is clearly no detonation. My aluminum heads might be making the difference though and mine is half point less compression. I don't know that I would try this with iron heads and pump gas, but for me it has worked quite well. I am running 90mph now with a straight 1988 vee bottom. Blowers are spinning at 11850rpm at WOT and that seems to be ok.

Not sure, never ran into that. They are some small threads, I think I would have a carb shop do it unless you have your own machine shop.