I,ve run blowers on my rods for years. Just got my first blown Hemi on the road. Got a 354 with a 6/71 and a manual 5 speed. I have underdriven the blower using some pulleys I had lying around. Wanted to iron out any teething problems with the car before turning the boost up. Just wondering what ratios others are running?
10%-15% under is a good place to start. Experiment from there. How wild is the cam and what are you running for compression?
I run a 6-71 at 6% underdriven on a early 392 hemi with aluminum heads....Works out to a 11 to 1 and works well with 93 octane fuel we have.. So far never had any detonation issues...
It all goes back to the boost,,,,,,every combination is a little different . But,,,,,Hemis can take more than other engines,,,,at least for a while . A good bottom end,,,and a decent setup,,,,always goes a long way . Tommy
Yeah, it depends on the condition of the blower. No crystal balls here... You're missing a LOT of details..! Static compression ratio, lobe overlap (dynamic compression ratio), lobe degrees open, etc, etc. How well...or not, is it set up ? loose, tight, teflon strips, race tight..? Is the block o-ringed, heads o-ringed Best get yourself a pressure gauge and monitor the pressure closely. Mike
The real killer that everyone is missing is detonation. If you cant control the heat in the combustion chamber bad things happen. Plug gaps close up; rings get pinched; piston domes collapse etc. Using old technology limits what you can do to control the heat. The timing is critical. It cant be emphasized enough. A early hemi likes between 28 and 32 degrees max. Under boost conditions that needs to come down. So your left with A. Backing off you timing running around retarded until you step on it. B. Setting the timing normal and under driving the blower so much the car makes very little power. C. The best option i have found is running a MSD-7 with boost retard. A map sensor is screwed into the manifold. As the boost comes up the timing is retarded. Usually about a degree per pound of boost. This keeps everything happy. The nitrous and turbo guys do the same thing. With the MSD box you can run the blower at any over drive you want safely. A nice street ratio is around 5-10 over depending on the blower. A boost gauge is highly recommend so you know whats really going on. Anyone that talks about pully ratios and not boost pressure is not with it. Manifold design; engine size; valve size; port size etc all effect the boost pressure. A single blower put on 100 different engines will give you 100 different pressure readings. So want to know what a pully change does to your engine. Thats were the boost gauge comes in to play. Alot of guys are stuck in the old days before boost retard boxes were invented. I run one on my hemi. I run it at 25 percent over on the track. On the street its at 9 over and still made a thousand hp at the tire. I first started to run the msd on my 354 after i broke alot of parts because of detonation. Look into this easy fool proof part.
Running 8:1 compression. Isky 270 mega_hyd cam , 112 deg lobe centres. MSD 6BTM. Running 10 degrees initial timing,28 total all in by 2500rpm. 5 speed Tremec .Trutrac 3.7 diff and 30.5 inch tall tyres. 6/71 with 2 Edelbrock 500 cfm carbs. Got a couple more pulley set ups here to try next week.Learnt about detonation the hard way 30 years ago.
What's your compression ratio? How many actual pounds of boost are you producing now? To establish a baseline. Racer X is experienced here. ☆edit I see 8:1 ratio
With aluminum heads you can subtract one full point of compression. They draw alot of heat from the chamber.
I am putting together a street-only iron-head 354, with a 6-71, with intention of driving it everywhere. It will be impressively quick, but not utterly irresponsible. Given the expense of building in the first place, and the expense of making a mistake, I might do what is unthinkable, and unspeakable here, and move towards something that can be tuned with a laptop, for at minimum the ignition timing/boost retard. I have altitude change considerations in my motoring area, too. A whole lot of tradition included ruining parts and disabling your car. I don't want to participate in that part. I want to have fun, not single-handedly fund an entire tow company.
That is what I am leaning towards. Anything more extensive than that would require a much larger cash outlay. I have everything but the MSD box in-house already.
Lock the distributor. Program the timing retard. Install a map sensor. Now if you change elevation or put good gas in it the tune can be changed. Hide the box and no one will know. I use a braided line to the map with a high quality clamp. I have blown plain rubber line off the map.
I am not worried about the MAP sensor being visible, or the box. I am going to get a full-ration for building a mid-engine Model A pickup, with a blown Hemi.
The Jalopy Journal
