As I'm building my 331 I know I'm going to have a supercharger on there. Would I get more HP out of the engine if I lower the static compression (maybe closer to 7.5:1 or 8:1) so that I can have more boost? Or would I get the same amount of HP with less boost and higher engine compression?
well many supercharged engines have lower compression ratios because of the higher compression ratios created under boost, these can cause detonation.
Sure, more boost will increase the compression of the engine, but I'm wondering whether there will be any significant advantage to me building a lower compression engine for a 174 roots blower or if I should leave it at 9.5:1 and just run maybe around 6-7 lbs of boost.
You can run at 9:1 CR and be safe with the boost later on...You just wont be able to run alot of boost only plan on putting 4 to 7 lbs to it.If you want to run big boost...over 10 psi then yeah a 7 to 8:1 CR is going to be the place to be.The advantage is in how much boost you plan to run...Build accordingly.
i read in car craft that you get about 12hp per pound of boost, about 2 hp per degree of timing and about 6 hp per point of compression.so your better off to run lower timing and compression with more boost,you'll get more power to the wheels when your on boost.off boost it could get to the point of being a dog when off boost. i would go about 8.5-9 comp and a little lower timing and boost for street driving,might be able to run a higher comp ratio if your tuning is good.with that comp you can run a good timing curve and 5-8 psi of boost i would think.if you want more power for the track you could swap the pully on the blower for more boost and run better gas or knock back the timing.
It sorta depends on what supercharger you'll be running and and how much over-driven it is [do you already have the 174 roots blower?]. Look at the pressure vs rpm you will be running and then look at the adiabatic efficiency in that range (you'll need a compressor map for that specific supercharger). If you are not able to create a lot of boost, then higher compression is possible. If you are capable of producing 20+ pounds, then not so much. Also, if the supercharger has a very low adiabatic efficiency (like a roots blower), then it will limit how much compression you are able to run because you will have very high intake air temps. Will you be running a charge air-cooler (intercooler)? All of these have an effect on how much compression vs boost. If you can get away with stuffing 20+lbs of compressor boost at a decent temperature, then I vote for low compression. This is all assuming you will only be running low octane (pump gas) and not race gas. If you are using a stock block 302, then you will reach the limits of the block either way. If that's the case, which way is cheaper? I'm not so much a supercharger guy, but I love forced induction. I have a turbocharged 408 Clevor waiting to go into my Maverick when the time comes. It's low compression (around 8:1) but if I were to do it again, I'd run somewhere in the range of 10:1 compression with boost I would love to see a study of compression vs boost and the effect that it has on thermal efficiency and mechanical efficiency of an engine. I'm sure there are some SAE papers on it, just need to do some digging on their site.
If you do already have the blower, and know what pulley you will be using to create a certain amount of boost, then you should be able to come up with a theoretical maximum compression ratio. I would talk to a machinist that has experience with roots blowers and one that can spec you a custom cam. Heck, you may just want to talk to a dedicated cam grinder and he can tell you what compression would be ideal with a certain cam. I think that would maximize the potential of this setup presuming you already have the blower.
When planning an engine for use with forced induction you have choices to make. Running a lower compression makes more power because you can run more boost that allows more compressed air and fuel into the cylinders but it will use more fuel. Running higher compression means less boost pressure and a higher likelihood of detonation. It also limits the amount of fuel and air that will go into the cylinders. You will get slightly better mileage. These rules are especially true with the "ROOTS" type of blower - less so with centrifugal compressors - like a turbo or belt-driven "fan" type supercharger. The more volume in the cylinder at bottom dead center allows more air to be forced into the cylinder. This actually raises the combustion pressure at top dead center. If you start at 10:1 compression then you can't add much more pressure without getting detonation but with 7.5 or 8:1 compression you can fit a lot more mixture into the cylinder and get the power from it. In a typical 302 the maximum compression ratio that is safe is between 10:1 and 11:1 depending on the cam and air/fuel ratio that you use. Adding a supercharger to a 10:1 engine is asking for serious damage, even if you richen the mixture to help keep things cool. You can add water injection to help keep detonation under control but you need to keep the reservoir full or you destroy the engine. If you want to get the most from your supercharger then begine with the lowest compression and add boost to compensate for the low compression. A blower with 12 - 15 pounds of boost in a 7.5:1 engine will provide a lot more power than 6 pounds of boost in a 8.5:1 engine and both will be near the top pressure levels acceptable with gas from the local gas station pumps.
Take it from someone that has went both ways, If its a street strip motor, Go no lower than 9.0-1. If its strictly race then go as high as the gas you can afford. Most that have never ran a roots WET type blower don't understand the latent heat of vaporization rule. A WET type roots is way more efficient than either a centrifugal or a dry type roots. I have personally saw the intake temperatures of each. And a wet type blower is lower than a centrifugal with a air to air inter cooler. I made the mistake of building a low compression street blower motor, DONT DO IT. Between static and dynamic cylinder pressure,The Dynamic cylinder pressure is really the one that will get you into trouble first. But this can eaisly be controlled with the correct cam shaft. And N20 makes one of the best intercoolers there is. As always this is just my opnion BASED on personal experiance
A blower motor running 7.5-1 with 15 psi of boost will have a compression ratio of 14.6 -1. A blower motor running 8.5-1 with 6 psi of boost will have a compression ratio of 12.0-1 compression. But if you want to compare apples to apples, Run 10 psi of boost on the 8.5-1 motor, This will give you 14.3-1 compression. This in turn will not heat the air as bad and give you MORE power because you are not turning the blower as hard. (less parasitic drag on the engine). Or run a 9.0-1 motor with 8 psi of boost and have 13.9-1 compression and even more power and even less heat. And when your not under boost the motor wont feel like a dog trying to catch its breath. As always this is just my opinion and others will vary.
great! that's exactly what I wanted to know . . . so I'm gonna keep it at 9:1 and adjust the boost from there. Bryant and I were discussing our limitations and it seems that the stock 1995 block I have would probably be safe in the 500 hp range and with the 331 and blower I might be pushing it. thanks guys, I feel good about this decision.
If you are gonna run the blower and N20 i would look into getting a good stud girdle also. And yes you will be pushing the limit with both. Just better be sure of your tune.
