What is the ideal CR for a street/strip roots blown Hemi.I thought I had 8.5 to 1.Now that the engine is out and we are measuring what was there it is coming up at 7.4 to 1.The piston is 100 thou in the hole.This is NOT was I expected to find.The camshaft was ADVANCED 6* also.And the rings were Moly. Not too happy with my findings BUT,,,,,I got 5 years and well over 10k miles before the ring failure.
The iron heads I had on there are 170 cc,,the aluminum ones I want to use are 183 cc and un shrouded. So the new pistons will make up for the shortcomings of the last build.Now,,,make it a true8.5:1 or higher???
Wonder what I was lacking with that cam 6* advanced,,,,,,,,,

Don't know if the cam at the 6 degrees was worth anything. The blower makes up for a lot of little things that you would otherwise notice. It's not like you are the first guy to stick a huffer on a Hemi. I'm sure that there is legions of info on it and buku people that have a ton of experience. It really doesn't get any better in our hobby than what you're doing. Who doesn't want to play with BLOWN HEMI'S??? Hopefully someone with some knowledge will chime in here with their experience. Good luck with that thing!

Thanks Dave.This vision has been in my head since attending the '71 Summer Nats at National Speedway here on Long Island.I was 13 YO
What better place to find pointers and info than HERE!! Thanks to Tom

I doubt seriously if it needs the cam advanced that far, if at all. Less compression allows more boost. All about what kind of fuel you want to run.
Doug

Nice picture!Your cranking psi was about 160 wasn't it?

What type of fuel and how much boost are you planning on running for your street driving?
The last blown pump gas Street hemi motor I built and dyno tested(4.250 bore with 4.375 stroke) had 9.2 to 1 mechanical compression with a Littlefield 10.71 12 % under driven for CA pump gas with a set of the Carb shop 1050 CFM, I think custom built blower Dominators carbs. That motor made 927 HP at 7300 RPM with 33 degrees total timing on 1999 91 Octane CA pump swill with 7 lbs of boost We had made a bunch of pulls testing the timing and jetting first to get those results
We switch the upper and lower pulleys to make the blower have 13% over drive which made 12.5 lbs at 6500 RPM , we did first switch the fuel to Union 114 octane race gas and took 8 degrees timing out of it so it had 25 degrees revved up, the motor started to go into detonation at 6600 RPM so we stop pulling it. That pull made 1024 HP and was still gaining HP at 6500 and started loosing it at 6600 RPM We made one more pull and moved the timing up to 27 degrees and the motor hated that
IHTHs

Started at 30* and was down to 24* when the rings failed.

Mike,I never did a compression test on it.BUT,,,,leak test before my last track day was already 18%.2 passes and it was done

I'm also on the fence about coated bearings & pistons

FWIW, the static compression ratio on my old 528 blown Hemi was 7.8:1. Personally, I wouldn't go any higher than 8.5:1. Here is a pic of one of the Ross blower pistons I used, with tapered tool steel pins. Notice how far down the ring grooves are.

Nice piston,not to thin by the valve pocket where itis had trouble.

Mike,that's where it needs to be.Found my pistons to be 100 thou in the hole.

My blown hemis have a 10.5-1 compression and I run alcohol. I use brad fat heads and make 60 lbs of boost. The pistons hold up fine. Yes I use tool steel rings and pins.

IMO 8-1 is too low. I would want to be around 9.5-1 if I were you. I think your piston design is fine.

With the piston .100 in the hole you are far away from the edge of the piston[valve relief] and could have a much thicker edge there.Will you do a clay check on the valve to piston?

I want to raise the top of the piston,not sure if I should make it zero or leave it down a little,,,,,,,
9.5 to one isn't too high for a mainly street driven engine.Guess I could underdrive to 3 or 4 lbs for pump gas steet driving.Would the higher compression give me more power with less boost?How does that change the intake manifold air temp???

The higher compression ratio will give you better throttle response at the lower RPM when driving under vacuum
Which will also allow the intake manifold to remain cooler as air does not heat up under vacuum, it is a bunch cooler than with boost
The amount of under drive will determine what RPM the blower starts to make boost and the amount of boost will determine how much heat will be made at what lbs. of boost. The bigger the blower, 6.71,8.71, 10.71 or 14.71 the less heat it will make at the same amount of boost due to it turning slower than the smaller blowers will have to make the same amount of boost on the same engine Bigger is better when it comes to blower size

So if I start with 9.5:,it will respond better to higher levels of boost with more octane?I would also think that at that static CR,The pump gas limit would be,,,,like 8 lbs
I'm also replace the STOCK '66 iron head with can old set of Direct Connection Dart raised port heads that were ported by Total Flow.

If your going to aluminum heads you can raise the mechanical compression ratio a little bit and still be safe on pump gas, I wouldn't though Keep it safe at 9.5 to 1 and use the O.D. ratio on the blower to make more power on better fuel when you need it
As far as what is safe at 8 lbs. of boost will be determine buy which spark plug heat range, how much timing, the jetting and the local weather conditions, if it starts to not feel right, lift up your right foot!

Knowing when to lift is what saved the bore! It just gets better,,

I believe the physics dictate that the lower CR theoretically will make more power at the same boost level. A larger volume to pack air and fuel into. More air and fuel equals more power, yes?

Because there is no free lunch it will be down some at no boost tho. I've forgotten how big your engine is but if it's a bunch over 426, would it be down enough to really matter?

Kevin

It's 4.385 bore X 4.5 stroke.Trying to pry the information from people in the know.I am working with 2 piston manufacturers and deciding which way to go.