How much compression is too much?

620ci RB, B1 MC heads, cam is .850" lift solid roller, duration 286/306 @ .050", carbureted on gas. Bore 4.600", stroke 4.665", piston 10.3cc dish volume, combustion chamber volume 52cc, head gasket compressed thickness .066", 4.600 bore size".
How much compression ratio would be considered too much? This is a bracket engine, to be raced on a weekly basis. The engine was brought to me with 16.4:1. I can change head gaskets to get it to 15:1 with out doing anything else.
The question is, how much is TOO much?

Thanks, Brian

What's the lobe separation on the cam?

I can change head gaskets to get it to 15:1 with out doing anything else.
Losing your quench isn't a good idea unless your VE is very low.

What does your chamber wall look like adjacent to the intake valve?

Sounds pretty healthy - for a bracket race engine the compression probably is a little much unless you plan on running C16 for fuel - Personally I would be a little nervous increasing the head gasket thickness given its already .o66 - maybe the pistons could be milled a little deeper if there is room - but with an engine that big a new set of pistons might be the best solution if you want to lower the compression

I'd run the thicker gaskets and not worry about the quench.

I gotta ask how do you have a 4.600 bore size? What block is it?

Leave it as is and run methanol.

I have heard that quench effectivness diminishes as rpm goes way up?

Buddy of mine had the same problem with C/R. He called Koffels and they said the B1 heads don't tolerate much over 15.3. I'd go with the thicker head gasket too. And I must ask what block is that to get 4.600 bore?

No reason for a basic bracket engine to be at that kind of compression ratio for sure. My little B1 was at 15.5 and worked pretty well for what it was. The one from the dragster is 15.3-1 and works pretty well too.

Id like to see the piston that gets you that kind of compression number for sure. I though I saw it was a world block somewhere but maybe not. 4.600 aint gonna leave much room between the cylinders on a 4.800" BS block for sure.

One of the guys on here did an experiment with a 440 and went from bad quench to tight quench and saw the biggest gains as the RPM went up, big time, like 50 HP. I think it was jesse lackman but don't quote me on that.

Show a picture of the piston, most aren't finished before installation IMO

If it's been working?? Just use it as is... Just dont lug it in some heavy azz turd car and changing the combo to much... Just my 2 cents...

Or drag the junk over to Greeneville TN and have the Johnsons dyno it...

If you do consider running methanol, a very sharp engine builder told me 15/1 is about the max that works well. Over that, more fuel is needed to run well, eliminating the advantages, and it is hard on parts.

Koleno

Crane cam, lobe separation 114.

I can post a picture later tonight, but it is finished. Diamond custom piston, comp height 1.530", intake pocket depth .340" (2.600" titanium valve), exh .390". Piston weighs 654 grams. Not much left to take off of it.

The only way to know if you have too much compression is to test it. Flame travel inside the combustion chamber is very complex. There probably isn't anyone who can read a post on the internet and then tell you if you have too little or too much compression. Even if you had a bunch of pictures of the piston and the combustion chamber I doubt anyone could give you a positive answer.

Personally I think that is way too much compression for a bracket engine. The tuning window will be narrow and you'll have to run expensive race fuel. If the car is being raced just for fun then what is the point of the sky high compression ratio?

Personally I think that is way too much compression for a bracket engine. The tuning window will be narrow and you'll have to run expensive race fuel. If the car is being raced just for fun then what is the point of the sky high compression ratio? [/quote]

This is my line of thinking also Andy. I'm thinking any advantage (IF there is any) to having this much compression is out wayed by the toll that it will take on the engine parts (rings, rods, bearings piston pins, head studs, ect.).

And just so everyone understands, this engine was running when it was brought to me. The last two passes in a 2200lb door car was 5.13 @ 139mph (1/8 mile), even with the problems that I have uncovered. In working through the issues on this engine, I'm hoping to head off as many future issues as I possibly can. I feel like this much compression is just not needed.

Thanks, Brian

I believe it was David Vizard that had a compression pyramid in one of his books. It seems the beginning of diminishing returns start at 13.5-14:1. After that, not much to gain and fuel gets real expensive.

No real advantage with that much compression, over say around 15.1 comp but if its together as stated you can run it the way it is, just use the correct fuel. Your wider LSA will help with the extra comp. I would just leave it as is and then on next tear down, address the comp issue.

Diesels run up to 25.1 ratios, fx high rpm 18000 rpm gas motors run 17.1 comp to assist at that rpm. So you are at Top level comp with your motor.

Also, for Just a Bracket motor, you really don't need a 620 ci as well. To me you have a Max built et motor. Send it to me and I will send you a reliable 448 Bracket motor. No Charge.

Run VP Q16 and go haul a$$ with it.

I think it's a 2.400 intake valve. I ran 16.3-1 with my B1/MC 572 with Sonoco Maximal and it made good power. The only question is the 4.600 bore and whether toy can get the head gasket to stay sealed.

Are you on 4.800" bore centers? That's a lot of compression to seal a head gasket with not a lot of room between bores.

The Koleno blocks are on a 4.8 centers, with a 4.60 bore that gives a .200 space center of sealing surface.

You are correct, it's real big!

Brian

Read an article years ago in National Dragster, written by Warren Johnson. He did very extensive testing on compression ratio's. He said anything over 16:1 did not make any more power.
Now my opinion: As gomangoCuda said, switch to methanol. It will love the 16:1

my sediments exactly.........just make sure carb is calibrated for it.

It’s one thing to run a heads up program with usually 4 rounds max compared to a bracket program with 7 or more rounds. How much does Q16 cost a gallon again. I think 55 gallons of alcohol was around 165.00
Last year.

Q16 is expensive and you have to use a lot of it. It is also really foul smelling and will eat away at the carb and the fittings over time. I did a Q16 engine for awhile and have no desire to go back to it.

But it all depends on what the owner wants to do. If it is a bucks up program with a crew chief and a tire wiping monkey in the pit crew then who cares. They can handle all of the maintenance and the driver gets to go down the track a little faster.

Before running alky, be sure and check with some people that have put a lot of runs on a B1 big motor with that compression. It would be rather expensive if my info proved correct

I've been running Q16 since 08/09 and never had any corrosion issues mind you I drain it usually after every race or after a week at max.also at that level of performance I don't think cost of fuel would be a major concern and for the performance gain definitely worth it.

Why not Renegade Pro 116 + ? Last price I had at the end of the year was just over $10 gallon when bought by the drum. Not oxygenated but plenty of detonation resistance.
Doug