supercharger experts, i need your thoughts. i gathered my info from charts on the dyer's blowers website and books. my engine is a 354 chrysler hemi. it will have 8:1 forged pistons, polished and shotpeened factory rods with better bolts and stock forged steel crank, nothing exotic. i'll use head and main studs instead of bolts. the block will be decked and the heads surfaced to promote good sealing. this will receive a 6-71 with 6 97's on top. this is going in a model A coupe. tires are 7.00 firestones, i know, they'll go up in smoke pretty quick. but i also plan to run a set of cheaters. my original plan was to limit boost to 8 lbs for several reasons. first, both hot heads and my engine builder agree the above engine mods will survive 8 lbs of boost on a street engine. start inching towards 10, and most certainly by 12 lbs, more engine mods are necessary. second, doing some math i found in tex smith's book on hemis, a 354 making 8 lbs of boost at 6000 rpm requires 945 cfm of carburation. figuring good 97's might flow 155, six will get me 930, so i'm in the ballpark. now come the questions. the pulleys i currently have are 32 and 35 tooth gilmer style 1/2 inch pitch. running the smaller 32 tooth on the bottom achieves around a 9 percent underdriven ratio making a maximum of 10 lbs of boost at 6000 rpm. doing the math, that requires over 1000 cfm of carburation, something 6 strombergs aren't capable of. let's say i do run this set up. providing they are jetted correctly and i don't go lean, will the lack of flow starve the supercharger therefore limiting the boost levels? if the 10 lbs is maximum boost at maximum 6000 rpm, am i safe in thinking that one, i'll never see 6000 rpm, i'm thinking more like 5000 to be tops. and two, the strombergs will limit the boost because at that rpm level they start limiting flow. so will running those pulleys at a lower rpm yield less boost? i recalculated carburation flow demand, this time plugging in figures for a 354 making 10 lbs of boost at 5000 rpm and came up with 855 cfm, well within the limits of 6 97's. i guess the overall question i have is do you think i'm on the right track in my reasoning? the charts say maximum 10 lbs at 6000 rpm. well if i don't plan on spinning the engine that high plus the effect the carbs might play in limiting boost, am i safe to say my maximum boost will be more around 8 lbs and therefore, my engine mods are sufficient? dyer's website did say cam profile and heads do have an effect on boost also. so if i bring a milder cam into the picture, one that's not designed to maximize performance, i'm even closer to my goal of 8 lbs of boost. or do i swap out the 35 tooth top pulley for a 38 or 39 tooth and base all my calculations on maximum figures, something the engine will never attain?
I'm not an expert but have dinked around with a smaller blower. I'd say run it and see what boost it builds, stuff like changing the rear gear in my car alters how the boost comes on and how long it sees peak. Certainly sounds like you've done your homework, time to adjust for the reality factor of an unusual (and very cool) combination. Also a mild cam will raise boost levels by letting less air into the cylinder. If you wanna drop boost, cam it up. The blower will compensate for what the big cam loses in low speed driveability. Good luck
I agree with Shifty. I've only delt with Paxton SN60 blowers that run 6psi, but what I can tell you is unless you are planning to race it competitvely and have good funding, go on the lower psi range. Your safe at 8psi. It's easy to grenade a blower motor if things get out of hand. I would think at 8psi set up like that it should be a pretty screamin' mill, and very cool looking. I wish I could help more but I haven't delved into 6-71's yet -Dean
a cam with a big over lap bleeds off alot of cylinder pressure. the 944 gtr race cars i use to work on made 3 bar of boost but the cams kept what was in the cylinder down.
It all depends on your effective cylinder pressure and a lot of that depends on your cam. I'm running about 8.8 static compression in my 354 with about 7# boost. It likes mid range gas or better amd I am using a boost timing retard to help.
Something else to think about is that more boost does not equal more power. Aside from the fact that the air gets hotter as the overdrive goes up, boost is also a reference to the amount of air that is not going into your cylinders or how restrictive your engine is. Things such as the ports on your heads, the design of your pistons, the length of your rods, and the size of your valves, can all have a differing effect on not only the boost on your gauge, but also on the airflow into your engine. Airflow, and air temperature are what should be measured on a blower engine, however there is no easy way to measure these, so we use a boost gauge, which isnt the best information, but it is better than nothing. My $.02 is to run the pulleys that you have, and if it doesn't perform the way that you would like for it to, change the pulleys. You can get used pulleys for $40 - $50 each, so buy a few, and play around. Brian
You could also rent some time on a load-bearing dyno, and see what boost levels you hit. So you looking for 400-500hp? And as noted, the milder cam will result in the engine making less power, so the boost will go up. I think it's better to build more engine power and boost low. On the other hand my '98 Formula makes 867rwhp @ 25 psi.
Your logic is right on, and by limiting the rpm redline you're limiting what you're asking the engine to do. I'd also invest in a 'blower' cam with more exhaust duration to let that boost out... Also, please remember that the WORST thing you can do to a blown engine is to run it lean under boost. The heat generated by this is what melts stuff. So, run it a tad rich if you can to ensure all is good. A bit of a rich mixture will help keep everything cooler. Naturally, make sure your cooling system is up to snuff. I don't know what diameter your primary exhaust headers are, but they should be a bit bigger than non-aspirated engines typically need. Running 7 - 8 lbs of boost shouldn't be too hard in that 354. If you've upgraded to ARP hardware on the bottom end and you're using head studs like you say, you're fine. I'd surely dyno test it to guarantee a solid tuneup and even fuel distribution. All that being said- odds are good you still won't be able to get grip with old-school style cheater slicks. The modern tires are SO much better, you'll get a harder launch on a street-legal drag radial than you will on a hard cheater slick. But, it'll be fun! And there's no denying the cheaters have the right 'look'. I guess I'm just saying that even with a conservative tune and a 5,000 rpm redline, that huffed Hemi will easily overpower those tires. ~Scotch~
thanks for the replies. i know i can make more power with different setups. i'm willing to make sacrifices to power and to spend more time learning and tuning to run the 6 carbs. once i reach a certain level, it will be wasted with limits the tires and suspension place on the whole scheme.
Driving around all day with 10lb pulleys and 6 97s won't bother you at all, but once you are full throttle making 10lbs and spinning 6k rpm you better have enough fuel. 6 97s bother me a little...(but I run TWO 750s on my 671.) My advise is to run 6lbs with the 97s to be on the safe side. Then, you can get her to a dyno shop that can check out your air/fuel and see if you have room to step up the boost. Nice combination so far. Be mindfull of your cam profile and that little motor will scare the piss out of you even at 6 lbs. Good luck, -Abone.
but how much boost i'm i making at 2000 rpm? and with the agreed upon limitations in my choice of tires and chassis design, when, if ever, will i see full boost at 6000 rpm? i know fuel concern is important. my basic question was on how much boost, so i can design the engine modifications and fuel system to handle it properly.
Depends entirely on butterfly position at any given rpm. If you are just cruising along at 2k rpm you will show vacuum. If the butterflies are at WOT and you are passing 2k rpm as the engine accelerates you will show boost. Nobody is going to be able to tell you exactly what boost you will make at any given rpm because there are way to many variables.
thanks again for your replies. this thread has been very informative to me. i plan to keep everyone updated and maybe seek more advise in the future. there's only so much knowledge to be gathered from books. one needs to experience first hand to fully achieve all that is to be learned.
Given the style of blower you have, you will have power all over. Car I'm thinking will feel like a has a big motor under there. When my Formula was supercharged with a Vortech YSi it felt like a 600ci engine even though it was a centrifrugal blower that I spun to 7000. Boost is just a value, but one of my engine builders states it as follows... If your engine makes say 350hp at the crank, then 15psi would make double that power, or 700hp at the crank since it's double the atmosphere. You can work it from there. Now if your motor stock was say 10:1, every full point drop in compression reduces total power by about 3%. So an engine that makes xxx hp at 10:1 will make 6% less power at 8:1. I'm guessing that you probably also need to reduce the timing, and run colder spark plugs. I'd look into plugs that are 3 heat ranges colder than stock, and gap them at like .035 for starters. I think I would probably spend two hours on a load dyno just to see what it's doing.
i'm running the truck heads. i don't have the casting numbers here in front of me. since they were on a 58 dodge truck, they have the 57 and later 392 size 2.00 inch intake valves and 1 3/4 inch exhaust. they come with hardened exhaust seats. they're getting new bronze guides and stainless valves. headers will have 2 inch primary tubes.
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