e55amgrocket

Well as part of my build i have decided to raise my compression ratio slightly. We measured everything awhile back to get the combustion chamber volume and dish volume of the piston. A couple days ago i went by to talk to my engine guy and he said that 10.5:1 was to high in his opinion and my stock compression is 8.18:1. I said no its 9:1 from what i've read and heard. Today i went by and we CC'd the head and piston again. 60cc chamber on the head with spark plugs installed and -30cc dish on the piston. The only thing we guessed at was deck height and used the deck height at ZERO. At .015 which is what it most likely is brings it down to 7.97:1. We took some CC's away because of air bubbles just to be safe and the highest we could possibly see it was at 8.47:1. I also CC'd a E55 head and N/A head and they were the same. I have heard of car maker's not being 100% accurate but this far off is a lot. It's pretty impressive how strong these car's run with that compression also. Look's like i will be going to 9.5:1 now.

e55amgrocket

For those wanting to check the math:
Combustion chamber volume-60cc's
Piston-30cc dish
Stroke-92mm(3.62in)
Bore-97mm(3.82in)
Gasket bore-98mm(3.86in)
Gasket thickness-.024in

GT-ER

Those numbers gave me 8.5:1...interesting for sure.

e55amgrocket

What did you use for deck height? I'm pretty sure it's around .015 and not zero. Make sure when you out in deck height you don't put a - for negative. If you put the height as negative it actually mean the piston is up out of the hole

GT-ER

You're right, I put 0.

blackbenzz

Nice find Someone actually providing useful info

e55amgrocket

I don't understand how it is so far off of what they say it is.

55fanatic

Excuse my noob ness as I am very not engine tech savvy, but if the supercharger is in boost would that pressureize what's in the cylinder? In essence increasing its volume as it makes everything more dense?

GT-ER

Yes, but static compression ( which is what is being discussed here ) does not change since it's mecanical.

Firechaser

You will enjoy the 9.5:1 ...did it on the GN and it helps nicely on the bottom end.

e55amgrocket

Yea I was actually going to do 10.5:1 but seeing how low we start out at now 9.5 sounds better. Don't want to get the tuning window to narrow either. I'd like to be able to drive it long distance and not worry about when it will be on 93 pump. With 10.5 the tune would need to be spot on with pump. I did some searching and see they list the SLR engines at 8.8:1 I'm going to call MAHLE tomorrow and see what dome volume the SLR piston has. Looks like part of the power increase they get is from a little more compression also.

DVC

Wow, great find.

I guess AMG likes to round up...like the displacement.

Firechaser

9.5:1 seems to be the magic number for hyper boosted builds. Gives us more room in the boost department

e55amgrocket

I've seen and gone a lot higher in the past but as I know you also know that's asking for trouble if your tune up isn't perfect. My engine builder has a 4.6 mod motor making 1800hp with 11.1 CR but got last season and added .3 degrees of timing and melted 7 of 8 pistons the first pass. The "fuse" as they say gets really short quick. After doing a lot of talking with my engine guy and another well known engine builder I'm going to switch from Q16 to VP Import. The Q16 evaporates so fast and is full of lead and neither of them like it for boosted applications.

dtohio

Used your numbers with zero deck and got your 8.18 also. Smart to check before mods. I checked my 33 ford v8 years ago and also found a different number than advertised. This could be explained simply by the mfg. so often making changes and the imformation not being updated correctly, which is common.

e55amgrocket

My builder said more often than not the OEM figure isn't 100% accurate.

e55amgrocket

I'm also 99.9% sure deck height is around .015. With a .024 gasket you know it's inside the deck and I would be amazed if it was a zero deck block. That puts it at 7.97:1

adianaty

They probably spec it at max material condition given the conservatism of the German engineers versus at nominal. Still hard to believe it would be that far off.

WHTEVO

Wow, i wouldn't have thought it was so low in these cars. Most boosted cars i have messed with have always been in the mid to high 8:1 ratios from the factory anyways so i guess it could be right.

Aaron

e55amgrocket

I thought the same thing about it being so much lower than what we've all thought/believed it was but the numbers don't lie. My builder and I were sure we messed something up somewhere or a wrong number so we checked them again and using a plate. With compression that low it makes sense why these cars respond so well to mods and getting more air in and LOVE timing. Looks like 9.5:1 is where I'll be at and hopefully it doesn't throw Jerry any loops tuning it either.

PACougar

Steve, I've been pretty interested in trying the VP Import after reading up on it awhile ago on VP's website. My VP guy couldn't give me any useful reasoning for picking one above the other. Can't wait to hear if you like it more than the Q16.

e55amgrocket

I've used the VP Import in the past and loved it. I didn't love the price and I get dealer pricing and its still over $100 for 5 gallons. I've thought about running the Renegade PRO 120 also and have a rep from renegade close to me that is pushing it and said its comparative to VP Import. My price on the PRO 120 is $640 a drum delivered to me. Import is around $1100.

PACougar

I think the last 5 gallons I bought of the Q16 was like 98.00 after tax, so the Import isn't sounding to bad. I'll have to look into what Renegade has to offer and if they have any reps near by.

e55amgrocket

Wow!!! I pay around $67 I think. I know retail on VP Import is around $145 around here. VP factory in Delaware is about 2 hours from me so maybe that helps prices some.

400RWHP

I can absolutely believe the numbers are low. Look at this from a car manufacturer's standpoint rather than our tuner point of view:

1. They want margins of safety everywhere because some bonehead is going to run 86 octane fuel in his AMG because of God knows why.

2. They want margins of safety for warranty purposes in general, and want the car to be able to go 200k miles in stock form. Reliability.

3. They like to 'parts bin' as much as they can because it keeps manufacturing costs low because they don't have to reset tooling for different applications.

Also don't forget who built these cars. As Clarkson said when driving the R8 'For instance this stalk is supposed to activate the GPS but doesn't do anything. Oh, I shouldn't have said that on national television, the German who is responsible will most likely be shot in the morning for his mistake'.

I only make this statement as Germans are some of the most highly regarded, exacting engineers on the planet and I would be more apt to trust what they say about something they built than a random engine builder(no offense he may be the Stephen Hawking of engine builders).

Just food for thought. But as WHTEVO eluded to, as a tuner on DSMs and quite a few other cars I would rather see lower static CR than higher. It gives so much more room for boost, timing and leaner AFRs with a wide safety margin.

Regards