Is 9.0:1 compression ratio too high for 10lbs of boost?

[Larry Callahan]

Is 9.0:1 compression ratio too high for 10lbs of boost on pump gas on a fuel injected 383?

I am trying to pick out a cam and I am trying to figure out if I can get away with 9.0:1 compression ratio on pump gas with 10 pounds of boost. I know 8.0:1 or 8.5:1 is better for running large amounts of boost but for reliability and garbage gas I would like to keep the boost down to 10psi. I am afraid if I run 8 or 8.5 to 1 that it will be a lazy pig when not under boost.

Thoughts?

[XLexusTech]

Do you plan on running an intercooler?

I was just reading this on the prochrger site

The primary issues that determine the type of fuel needed are whether the engine is fuel-injected or carbureted, the compression ratio of the engine, and whether or not the supercharger system is intercooled.

For Intercooled ProCharger EFI/TPI applications with compression ratios less than 9.5:1, boost levels of 14-17 psi can be safely run with full timing on pump gas, and will produce horsepower gains of 75-100% (depending upon the boost level and the motor specifications). For 9.5:1 EFI/TPI applications running without an intercooler, boost levels above 5 psi will require the use of ignition/timing retard on pump gas, and will produce horsepower gains of 35-45%. Boost levels above 12 psi should generally be avoided even with racing fuel on a 9.5:1 motor. Of course, lower compression motors will be able to run more boost, and higher compression motors should run less boost, everything else being equal. All Intercooled ProCharger systems for street applications are designed to allow the use of pump gas with full timing and will not affect daily drivability.

For carbureted motors, the rules are slightly different. Carburetors deliver the vast majority of fuel in a liquid state, and as this raw fuel atomizes from liquid to gas, a chemical state change actually occurs. Due to this endothermic reaction, which draws heat and cools the incoming air, a carbureted motor can safely handle more boost than a comparable EFI/TPI motor. For carbureted engines with compression ratios of 9:1 or less and boost levels in the 8-14 psi range, pump gasoline works very well. Compression ratios of 10:1 and higher require lower boost levels, higher octane fuel, intercooling, or some combination of the above. Compression ratios in the 7or 8:1 range can usually handle 12-20 psi on pump gasoline.

[Blown353]

I used to run 15.4 psi non-intercooled (although blowthrough) with 9.25:1. California 91 pump gas.

With EFI I also had no problem at the same boost level non-intercooled on the same engine, although EFI doesn't suck as much heat out of the intake charge as a blowthrough carb and I did have to pull 2 degrees of timing out compared to the blowthrough carb setup.

I'm now running just over 14 psi with an intercooler, also at 9.25:1, still EFI, and I'm running 1.5 degrees more timing than I was with the non-intercooled blowthrough setup.

That being said, I don't see 10 psi on 9:1 being much of a problem especially if you coat the pistons & combustion chambers. The coatings really help with boost. As with anything, careful tuning is the key.

One thing to think about... at 8.5:1 a 383 won't be all that lazy off boost. An 8.5:1 1.6 liter in a Honda may not have any HP when off boost, but an 8.5:1 383 cubic inch small block will still go pretty good off-boost.

I'm going twin turbo later this year and sticking with my currently 9.25:1 for now and still expect to run 11-12 psi on pump gas... however when I build a new 400 cube shortblock I'll be dropping the CR down to around 8.5:1.

[Larry Callahan]

Thanks for the info guys. I am running a Ron Davis intercooler and it sounds like I will be fine with either 8.5:1 or 9.0:1 with my situation.

I am not looking to squeeze every last HP out of it. I want plenty of power on tap when I want it and nice and reliable.

[DeltaT]

I am running 9.7:1 right now on my 383 with a large FMIC, topping out at 12.4# of boost, on Cali 91 pump gas. I am adding alky injection for hot days and strip use, mostly for insurance. IMO if you run a centri blower, your target CR should be ~9.5:1 for snappy off-boost power and reliable on-boost performance. 10 pounds should fit into the 'safe zone' nicely. For extra insurance, I added the J&S Safeguard Knock Computer and dashboard monitor. I think it's paid for itself already by allowing me to tune and have a stronger safety net.

Jim

[Larry Callahan]

I called Comp cams and they recomended this cam for my motor. I guess I need to do more reading up. The last thing I want to do is pick the wrong cam and compression ratio.

http://www.compcams.com/Cam_Specs/Ca...?csid=150&sb=2

[kmcanally]

I am no expert but me thinks that cam is for NA motor...the LSA on cams for boosted applications needs to be more than 110 degrees...something in the order of 114-116 degrees.

[Blown353]

Larry, is this going to be turbo or supercharged? Rest of the engine specs?

The cam is very mild for sure and the LSA is a little narrow. I occasionally see 110LSA blower cams on marine applications where they're run wide open for long periods of time as the closer LSA helps keep the exhaust valves cooler; but that's for an entirely different application.

Just for reference, I'm still narrowing down cam selection for converting my Procharged motor over to twin T61's but the new turbo cam is probably going to be 236/230 @ .050, .540/.540 (will be .576/.576 once I get the 1.6 Jesel setup on the heads), 114 LSA, probably installed at a 110 ICL. My current Procharger cam is .542/.580, 242/254 @ .050, 114 LSA, installed at 108 ICL.

[ty1295]

I run 9.6:1 on iron heads so far up to 7psi no problem

You want a 114 or larger LSA I run a 224/224 114 custom

At 6psi based on my et's I am at 570hp or so on a 350.

http://videos.streetfire.net/video/T...oon_161854.htm

[DeltaT]

This is for twin turbos?

Jim

[shmoov69]

My last engine was 9.5 CR and it lasted about 17,000 miles and could squeez 17 PSI out of it on good gas. Then it split the cyl wall at the dragstrip. Because the cheap piston skirts were laying in the pan (for whoknows how long!!??!). I normaly ran it at 10-12 PSI. But remember this is with smaller turbos and intercooled. That cam seems extremely small. Surely I am reading that lift wrong? 334* straight? I believe mine (which is small too, cam I am talking about!) is around 480* lift and 220* duration flat tappet hyd. I will have to dig out the specs later though to be sure. I don't know if you talked to a friend, but if not, call back and go through it with another guy to get his take. Or call a buddy of mine at FAST named Jay Adams. He also used to be a tech at Comp, then a Cheez at Lunati, now back at FAST. He can point you in the right direction. Tell him you know me, oh and also Steve Strope, since he is buddies with him too. That way he knows you are not just some hoser from the street! LOL!

[kmcanally]

Surely I am reading that lift wrong? 334* straight? I believe mine (which is small too, cam I am talking about!) is around 480* lift and 220* duration flat tappet hyd. I will have to dig out the specs later though to be sure.

The .334" is lobe lift which must be multipled by the rocker ratio to get the actual valve lift. So with 1.6 rockers it is .534"

[shmoov69]

ahhh, I understand now! My cam cards have always said the "lift" to be the larger number. But, what do I know!! LOL

[RSX302]

I called Comp cams and they recomended this cam for my motor. I guess I need to do more reading up. The last thing I want to do is pick the wrong cam and compression ratio.

http://www.compcams.com/Cam_Specs/Ca...?csid=150&sb=2

Who am I to go against Comp Cams? I agree with the others, the cam they suggest seems on the mild side. I'm sure it will work fine but not the most IMHO.

I was looking at these?
12-415-8 [10] http://www.compcams.com/Cam_Specs/Ca...?csid=171&sb=1
12-467-8 [10] http://www.compcams.com/Cam_Specs/Ca...?csid=176&sb=1
and the radical cam:
12-419-8 [10] http://www.compcams.com/Cam_Specs/Ca...?csid=172&sb=1

I bumped my 302 from 8:1 (when you were in it) to 9.7:1. I still can run 91 oct at 12psi intercooled boost. Much funner!!!

My cam is a 114LC/110 Intake centerline with 284/290 adv dur (224/230 @ .05) -1 deg overlap. Motor idles at 15-16" vacuum and runs very smooth. I wasn't able to find a better cam for max power on my blown motor. Although Comp Cams says this cam was good to 6000rpm; I reached 7300 rpm peak horsepower with the turbos.

The higher Lobe center gives you better vacuum for computer controlled engines. Unless you want the lope?

Blown motors like higher exhaust duration for max horsepower as well. Just don't get into overlap.

I would probably say if your looking for more bottom end torque. Stay with Comps suggestion. But I think with the 383 and turbos "That won't be an issue!!"

[Larry Callahan]

I have been talking to some other people and they agree as well. One guy at Comp suggested a very mild cam. I will let you all know in a day or two what we come up with and let you guys see if you agree.

[eville]

I'd call UD Harold of VooDoo/Ultradyne fame.
He is an occasional poster on Chevelles.com in the performance section.
He'll have his number posted up there for you to call.

[Mike Norris]

Hi Larry,

Pretty much as most have said here i agree that the 9.0 to 1 is fine for 10 PSI with proper tuning on pump gas and should make good power.

For a cam a lot depends on the heads and I am always partial to the 224/230 cams as mentioned here on 350-ish ci engines. If it is a GM roller block I would go with a 08-503-8 (XR276HR) but have it ground on a 114 LSA and use 1.6 rockers.

Another good choice is the nitrous cam 08-301-8 (NX276HR) and again have it ground on a 114 LSA and use 1.6 rockers.

Hope this helps and I look forward to getting over and seeing the car soon.

Mike Norris

[Blown353]

Mike, what's your reason for more exhaust than intake duration on a turbo build? I'm still doing the research for my cam but it seems most guys I have spoken to with SBC turbo builds have anywhere between equal duration to somewhere around 4-6 degrees more duration on the intake than the exhaust-- I occasionally see people running turbo cams with more exhaust than intake duration as you suggested, but they seem to be in the minority.

Not questioning you at all... I'd really like to hear your input on this, I always like to learn. Does your preference for increased exhaust duration have anything to do with the typically lower I/E ratio of most out of the box SBC heads, or are you using the increased exhaust duration to bias the powerband more towards the top end?

One thing I should mention (and it's my standard disclaimer as it cost me an engine rebuild) is never buy a cast core roller cam, even a hydraulic roller. If ordering from comp specify you want the -9 (billet) core and get the optional pressed on iron distributor gear so you can run a long lasting GM Melonized gear on the distributor. The standard comp cast cores they use by default on their HR grinds can and will track lobes when you up the spring pressure and they often suffer from distributor gear wear issues.

[Mike Norris]

I guess I should stipulate that this is pretty much the norm for the LSx vehicles I build and and they work very well. I do not do any SBC cars and it is possible that they are quite a different beast, but my understanding was that the "traditional" split worked well for street cars with lower boost.

I think some of this comes down to the old "ask five people and get five different answers" kind of question. The only way to know for sure what your combo wants is to actually try 2-3 or even 4 different cams, peferable on an engine dyno and see what you get if you are that paticular about the results. Turbo gurus such as Nelson and Kenny D should ahve a great baseline also.

Hope this makes sense as I am better at building the cars the explaining theory

Mike Norris

[ty1295]

I personally don't feel the random tech guy on other end of phone specing your cam has anymore knowledge about a turbo motor as anybody else.

Although I have a comp cam, I trusted a friend that does a lot of turbo motors builds/tunes and has done many dyno runs. It was worth it for me to spend a few extra bucks and pay him for his time/knowledge I trusted to spec my cam.

I am very happy with what we selected. My Et is showing over 560hp and I still have the boost turned all the way down.