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454 BBC: What compression ratio should I go with?

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454 BBC: What compression ratio should I go with?

Postby zwede » Wed Oct 25, 2006 1:23 pm

I'm finalizing my 454 rebuild and have to decide what CR to pick. The pistons (forged) have a solid dome that I will have flycut so I can get pretty much any CR between 9.3:1 to 11.1:1 depending on how much dome I remove.

I intend to run premium (93 octane). I have to be able to make it cruise on 91 in case I go on a road trip but I don't mind retarding the timing a little on 91.

Engine: 454 BBC. Edelbrock aluminum heads. Hydraulic roller cam, 224/230 115 LSA, +4 dgr. Port-EFI, fully programmable with wideband O2. I can set any air/fuel ratio I want as well as any timing advance map.

Car is fairly light at ~3200 lbs but has a tall over drive. 2000 rpm =75 mph. It's a 5-speed manual.

Usage is daily driver. It will be driven in high temps (100F). Car has good cooling system that keeps engine below 200F.

So I was figuring somewhere in the 10:1 to 10.5:1 range is reasonable? More? Less?
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Cam vs. compression ratio

Postby Hardcore » Wed Oct 25, 2006 3:18 pm

The cam you have will only work best in the 9.5:1/9.75:1 range. Any higher than this number will make detonation a problem to control. You will end with piston/ring problems. Forged or not. Depending on the head material (aluminum) go to the higher number, I would run it lower with cast iron. That 224 degree is more like an RV style/gas mileage cam in any BB as opposed to any type of performance deal. You need to be up at the 230+ duration minimum.

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Postby zwede » Wed Oct 25, 2006 3:43 pm

Everyone makes fun of my cam. :)

But still, the 115 LSA moves the intake closing later which will bleed off some cylinder pressure, right?

So no 10.5:1, but maybe 10:1?
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Postby Ed-vancedEngines » Wed Oct 25, 2006 4:12 pm

My suggestion if possible is to get a cam with more .050 duration numbers. I agree with Hardcore that the cam you have selected would work good in lower compression BB Chev. Increasing rocker ratio could increase the effective duration some, but probably not enough for that combination. I do love higher compression street engines when all is a matched combination though. So to me it is looking like choices of ruining your compression or picking a bigger cam. If it were me, I would build it at appx 10.50 -1 to 10.90-1 and get a cam with some higher duration events. I have no problem with the 115 Lobe Separatiopn in a street driver but would probably chose something like a 112 Lobe Separation. The wider lobe separation will give a higher cylinder pressure in most instances with a big broad torque curve, and the smaller lobe separation would give a more peaky torque curve with less cylinder pressure in most instances.

You do have a benefit of fairly light vehicle so that can allow you a little more compression too. How radical you can go is partly up to you and what end result you seek. The cam you selected should have a very smooth idle.

The reason I said in most instances is because there is really no absolutes. A cam with an Intake centerline of 115 deg and an Exhaust centerline of 105 deg would be considered a 110 LSA cam. A cam with an Intake Centerline of 112 deg and Exhaust Centerline of 108 deg would also be considered to be a 110 LSA cam. Niether cam would act or run the same but they would both be sharing the same numbers if all else were equal. There are many more differences that exist in cams with the same numbers too. But to try to help some, the cam companies do publish some basic info about what kind of use the car will see in the cam books.

Ed
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Postby MadBill » Wed Oct 25, 2006 4:47 pm

The aluminum heads and EFI will both help a ton re useable CR. There are several (albeit smaller bore) production engines with 11:1 CR or higher, designed to run with the usual large factory safety factors on 91 octane.
Rather than saying "Go to a bigger cam so you can use more compression." it would be more logical to stick with your first instinct: Select the correct cam based on desired performance characteristics, then, based on the intake valve closing point, match it with the appropriate CR. Google "dynamic compression ratio' and you will get millions of hits and will learn more than you ever wanted to on the subject! Here's one: http://cochise.uia.net/pkelley2/DynamicCR.html
"There's no product that can't be made cheaper and worse."
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Postby zwede » Wed Oct 25, 2006 5:21 pm

The cam is one I've had for years and I'm kind of fond of it. With 92% leakdown in multiple cylinders, 8.6:1 CR, full exhaust and street tires it still ran a high 12 and over 112 mph. Fixing the leakdown & bumping CR can only improve that.

So if I stick with this cam, I might get away with 10:1 but that's about it?

I guess if it pings I will have a good excuse to stick a bigger cam in it!
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Postby rmcomprandy » Wed Oct 25, 2006 7:24 pm

In my oppinion, 10.5/1 will be fine when you're simply driving it around; it's when you "lean on it" a bit you will experience some detonation.

224° intake installed at a 111° intake centerline, (as you have noted), might be OK if it were ground on a little tighter seperation in order to induce somewhat more low speed mixture dilution so, with you're combination 10/1 is definately on the high side.
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Postby Ed-vancedEngines » Wed Oct 25, 2006 7:54 pm

For what it is worth about his advising;
Hardcore aka Bill Mitchel was building and designing engines with Joel Rosen & Motion Performance, way back when I was only wishing I had one of them. His 454 engines were winning races before I ever built my first one. :)

So I do look at his advice with much admiration. I may not be in agreement with everything that he and Joe Sherman say, but you can bet your bottom booties that I have admired and respected them both for many many years and I do even now pay attention when the either of them speak.

Ed
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compression

Postby bigjoe1 » Thu Oct 26, 2006 12:40 am

Listen up Ed ( JUST KIDDING) I dont like your cam, but if you are going to stick with it, I feel you can run 10.5 no sweat. the Aluminum heads are going to save you here. The biggest thing that make an engine ping is under hood temp. If you run into problems down the road, look into a hood scoop, or something like loovers,or some sort of cold air setup.The next most important thing about street hi compression is the exhaust system. The more restrictive the system, the more likely it will have pinging problems. in order to run high compression on the street.you MUST have a zero back pressure setup. BEEN THERE< DONE THAT. JOE SHERMAN RACING
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Postby zwede » Thu Oct 26, 2006 8:31 am

Thanks Joe. I should have mentioned I do have a cold air setup in the form of a functional L88 hood.
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Postby Cobra » Thu Oct 26, 2006 11:23 am

Does this engine use a knock sensor? A knock sensor may be cheap insurance if you are forced to run sub-par gas. Have you looked for a local source of 100 octane pump gas? Don't you hate building engines to survive on junk gas? E-85 is something to be considered when building a street engine. As Joe related, anything done to reduce intake and underhood temperatures will help your cause. This includes reducing the running temperatures of parts, like your brakes. There is a good reason virtually all road racers run brake ducts.
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Postby zwede » Thu Oct 26, 2006 1:57 pm

Good point, Cobra. My EFI system is compatible with a knock sensor. I just never bothered to hook one up. With 8.6:1 it just was never an issue. :)

In other news, I got the Probe P2465F's in today. Man, they're pretty.
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Postby 63 Nitrous Ratt Vette » Thu Oct 26, 2006 2:42 pm

zwede wrote:Good point, Cobra. My EFI system is compatible with a knock sensor. I just never bothered to hook one up. With 8.6:1 it just was never an issue. :)

In other news, I got the Probe P2465F's in today. Man, they're pretty.


Wait till I tell Jim Moore you are over here still trying to justify that weenie cam, ha ha .
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