Hi Guys, I am building my first superchaged engine. Its a vintage style 327, 0.030, 461 fuelly heads with a mild port 1.94 and 1.60 valves, fairly fat solid cam 242@0.050 on 110 centres. Blower is a 6/71 running around 8 psi with a single 850dp. My issue is with compression. Here in Australia, piston availability is very limited for 327's and with the forged trw piston i am looking at buying (L2165 f30), the comp seems to work out just under 9:1, with the piston 0.010 down the hole and a standard felpro gasket. I realise less compression would be better,(easier to deal with), but i just wonder if I could get away with 9:1 on 98 octane? Thoughts Guru's?
Running 8 psi on pump gas will not work I would guess your compression would be at least 13 mabe 14 to 1. I have a 8 to 1 motor 7 psi puts me at 12 to 1 and running race fuel. 5 ponds under driven keeps me in pump gas. My 2 cents
Another issue with using a 6-71 on a 327 is the crank snout. They're pretty weak on a 327 and are/were a know issue back in the day. Even running a blower 10 to 20% under puts a real strain on that snout. If you want a blower motor, I'd suggest you build an 8:1 350. Here's some good info. http://www.blowerdriveservice.com/techcharts.php
OK, so if i dont turn the blower as hard and only run say 5psi, and i can lose half a point of comp somewhere, maybe with a thicker head gasket...? I was hoping that running the bigger cam, i could bleed off some low speed cylinder pressure, to help keep it out of detonation. My 327 has a modified for twin keyway factory steel crank, is there something that specifically makes the 327 crank snout weak? I can run a snout support if i absolutely have to, space is pretty tight in there. I dont plan on turning the engine past 6000.
The snout is weak because it is so damn short and small. If you don't flog it too hard and have everything balanced well, you should be okay. Just don't run a bunch of boost and the CLOSER your blower pulley to the block, the better! You might even consider a 2" drive setup instead of 3" - it will put less strain on the crank (at least in my opinion).
Do not let under driven fool you in to thinking less power. 5psi over is going to blow the guts out of your motor or melt the tops of your pistons on pump gas. I run my motor under driven on pump gas It is built for this I have over ten thousand in my motor you want to piece a 327 together and run 5 psi over please send pictures after it ****s the bed. My last 2 cents Also roots blowers make boost at low rpm by the time you get to 6000 rpm the part will be over.
i ran mine at 10% under on a stock 9.5 to 1 used 390 FE ford with 2" belt. drove it every day on pump gas, just did not tighten the belt real tight. drove it for two years like that with a motorcraft 4bbl. carb. no problems
Wow, 98 octane. Wish we had that here. A wider lobe separation like 112-114 would be ideal or your just wasting fuel having a lot of overlap w/a pressurized intake charge. I had a SBC w/9:1 comp. running 10 lbs. on half and half 93 pump gas w/low lead av gas which is about 100 octane and did fine. You may just get away with 8 lbs. on 98 if your carb and fuel system are set up right and doesn't go lean. Also the main issue with the crank would be the balancer or hub spinning on the snout. I've also had good luck using a aftermarket balancer or hub w/a tight press fit, new crank keys and a hardened bolt at or under 10lbs. A steel crank helps too.
me thinks i would have to look for a different motor...and those 461 heads???save em for a high revvin high compression motor...sbc looks like a sbc..people don't have the slightest clue as to what size it is without looking for some block numbers...the advice on the cam over lap is dead on..ya don't get up into the 112-114 spread, well you'll just be pushing raw fuel thru the exhaust...go with a 400..get the hi nickel block..509 if memory serves...drop some low popers in it and get some of the 76cc heads and hog them out...ya can get past 500 hp....really quick...and the torque will match hp..and it is mind blowing..and drive it anywhere.. was drivin every day, rain or shine..550 hp +...pump gas..and this is a baby blower to boot...400 40 over.....
just a thought....if they are flat-top pistons , couldn't you machine a little dish into them? i know chevy had used dished pistons in later 350's
Lose the heads and get some GM 883 76CC heads - that will put your compression in the right range and you can keep the pistons. I don't think machining the pistons thinner would be a sound idea in a supercharged engine Good luck man!! As far as a pulley on the snout - get your crank double keyed and use a hub. Don't use a cast balancer. I have a single keyed crank with a billet SFI damper running the blower on my 283. It works fine. If you hae the crank out get it double keyed. What ratio do you have to run to get your 8 psi??? have you read all the tech on www.blowerdriveservice.com ??? great stuff there. I ignored all the recomendations and my motor runs smashing. If you follow their advice you'll have no worries.
WoW!!!!! Heaps of p***ionate responses. Wasnt quite ready for that. Couple of small clarifications..... 5 - 8 psi (peak) is what i would like to run, i will change the drive ratio to achieve that boost level. My guess is with my cam cubes and carb, 10% underdrive should get me the 5psi max i need for the street. On Australian 98 octane fuel, with a double keyed steel crank, 8.5:1 comp, my solid cam and 30 degrees total timing pulling out 2 degrees per psi. It should still make over 400hp on 5psi (peak), sound bad***, and live. I will be inefficient, but definately enough to turn 6.50 15 whitewalls for as long as i can hold it straight. On race day. I can change the top pulley so it makes 8psi, throw in 116 octane, switch to 1 degree per psi, and it should go 10.80 at 125 on slicks. Maybe. How does this sound?
Sounds like fun. Make sure you at least use aftermarket head bolts and good head gaskets like Fel-Pro. By the way you could run more than 8 lbs. w/race fuel, maybe 10,12,14? lbs. Copper head gaskets and O rings would be a good idea too especially over 10 lbs.
You'll probably get away with a bit more timing than you think as well. Like 32 total, even without a btm.
Found myself some dish top forgies "AT LAST", so the 327 is go. Crank is being dual keywayed and tapped for a crank bolt to match the romac balancer. comp should be 8.6:1. Still a little high, but I am going to give it a go. Here's hoping i dont burn it.
You've gotten some good advice. I'd second the thought on reducing compression by getting heads with bigger cc chambers and not using a thicker head gasket. Most street blower cams do have 112-114 degree lobe centers. You will be better off not running the crank balancer - the belt and blower seem to dampen things enough and it reduces the crank snout strain. Have fun with it.
BDS's website says they make the most power with 110 degree lobe center cams with gas and recommend the 114 seperation for use with methanol or alchohol. I wonder what the difference is.
114 degrees seperation will give higher cylinder pressure (and boost pressure) then an equivalent 112 degree can, as there is less overlap between intake and exhaust. Higher cylinder pressurewill lead to higher HP.
O-ringing and using copper headgaskets may be 'traditional', but there are better solutions out there these days like Cometic headgaskets for instance. I will be putting a pair on my own blower motor. The ones I'll be using are 0.100" thick by the way to lower the static piston-compression down from 9.5 to 8.2:1. Also, I don't think proper piston-quench (.030-.040") can easily be achieved in a blower motor.
You can't calculate compression ratio from static CR by multiplying it by the boost as a proportion of atmospheric pressure, such as: 8.0:1 × 7.35 psi (= 1/2 of 1.7 psi) = 12.0:1. Yes, I know the tables that do this ar all over, bu they shouldn't be - the results are all wrong. They don't get complaints because the actual math is a bit annoying and their method gives a conservative answer - but ask yourself, how does a motor run with 8:1 × 14.7 psi = 16:1 CR on gas? The answer is: it doesn't. The actual compression ratio, of course, doesn't change at all. The change is how much more pressure is developed during the compression stroke from the existing CR when the charge is already pressurized? The calculation can be done with your Windows calculator, if you know your static CR, boost pressure, and atmospheric pressure. To approximate the effective compression ratio from the static compression ratio and boost, where: ECR: effective compression ratio. CR: static (nominal) compression ratio; e.g. 8:1. B: boost, in pounds per square inch (psi). A: atmospheric pressure, in pounds per square inch (psi); 14.696 is approximately correct for sea level and 70° F. ECR = CR × ((B + A) ÷ A)^.74074 Example 1, an engine with 8.0:1 static compression ratio and 10 psi of boost at sea level: CR = 8.0 B = 10 A = 14.696 ECR = 8 × ((14.696 + 10) ÷ 14.696)^.74074, or 8 × (24.696 ÷ 14.696)^.74074, or 8 × (1.6805)^.74074, or 8 × 1.469, or 11.75. The effective compression ratio is 11.75:1 (whereas the incorrect method predicts: 8 × (24.696 ÷ 14.696) = 13.44:1).
My only blower motor at the moment has such low compression (H-D flathead 82" motor with M45 Eaton) that the math doesn't really count. It's about 6.4:1 (may go down if I need more lift clearance for the intake valve) with perhaps 15 psi (may also go down if I can't keep head gaskets in it!). No intercooler, blow through using Mikuni CV.
My old 250" Chevy L6 using Corvair E/F turbo was about 8.5:1, only got 8 psi out of it but I suspect poor exhaust system + leaks to blame. H-D 84" shovelhead (1978?) with Corvair F turbo - complete failure (turbo too big, A/R too big) barely kicked the needle off the scale until over 4,500 RPM, maybe 3 psi max. Eaton M90 on Chevy 235: still in progress (and most of the work still to do), slow blower drive since trying to use the original pistons at about 8:1, so hoping for 6-8 psi.
Blower motors seem to be a lot more knock resistant than NA engines with similar ratios, and IDT it's due to better mixing etc. The cylinder pressure "spike" (highest point) is smaller than on an equivalent static high ratio motor, and also comes in later in the stroke where the rod angle is better.
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