Yesterday when I took my block to the builder who is putting together my 572 Hemi he broached an interesting topic regarding compression ratio. He said that 700HP wouldn't be an issue on pump gas, but wondered outloud if I didn't plan on driving the car much why not go to 12.5 or 13.0 and (in his words).."make it run like a hemi should". What say the group on his thinking? Realistically the dart is going to be a toy and most likely will be driven less than 200 miles a year.

This will be a 4 speed, that currently has a 4.88 dana in the rear. Thoughts?? Thanks in advance.

Do it and cam accordingly.

Well if you only get 5 mpg you will need 40 gallons of gas to drive 200 miles. Pump premium at 3.50 a gallon is $70 a year, 110 leaded is about $10 gallon so it will cost $200 or $130 a year extra, if $130 a year is gonna break your buget than you probably should not be driving a gen II hemi. Also the leaded race gas tends to last longer in storage than pump gas.

LOL...take it from a past Hemi owner....money is no object when you mash that right pedal for the first time....

Really not a cost issue when it comes to fuel. I was wondering if cooling, starting or other issues would be present with increased compression. Thanks for the replies.

What comp were you thinking?

Initially I was thinking about a motor that was running 10.5 to 11.5 compression and running a mixture of 110 race gas and non ethanol 91 octane. Builder was talking in the 12.5 to 13.5 range with race gas only.

If you do that your life will be miserable. My buddy has a car like that and it's such a hassle/expense to drive it he doesn't drive it anymore, he can't enjoy it and now talks about selling it. Stick with pump gas. This isn't the old days. You can make all the power you want on pump gas with no downside.

That's why I asked. Car will never see the track and I would think 700hp in a A body should make one smile just fine...

I'm building a 572 pump gas hemi now for a local customer for his 1970 Challenger street and show car, it has a Holley EFI system with dual 1100 CFM 4150 throttle bodies,Stage V heads, rockers and EFI manifold. It should make well over 700 HP (probably nearer 800+ HP)and a little(40Ft.LBS+)more torque than HP on Oregon 91 octane pump swill
I do make more HP per C.I. on my wedge stroker motor builds than I do with a hemi motor with similar C.I. and compression
On your deal I would shoot for 10.5 to 11 to 1 with aluminum heads and right or under 10.5 to 1 with iron heads
That C.I. with a 4 speed and street tires in a A body should be fun, especially at WOT

I have personally run 11.6:1 in an iron block alum headed hemi on the street for years. My current all alum 605 is 11.2:1 and neither are engine had any issue with pump gas. I know of multiple folks running 12:1 in big Hemis on pump gas.

So if your starting from scratch, go big or go home. Use the thinnest .027 Cometic gaskets at 12:1, and later, if it does give you an issue, switch them out for thicker .041's and back it off..

Now with all this dizzying thoughts of massive Hp improvements, remember, you need good eye on your timing and your carb jetting.

If your going to put some 25 year old reproduction carbs on it and a stuffy dual plane intake....You may want to reconsider.

PS...My 800hp 605 Hemi is for sale

Motor will have an original magnesium cross ram with matching Hollies. Aluminum heads and made to look like the super stock motors in the 68 hemi cars. Got to sacrifice some efficiency for the look I guess. Thanks for the replies.

I spoke to Ray Barton about this subject when I was choosing parts for my 540. He suggested 10.5 with the 250* cam. I thought I wanted more comp than that, but he said the extra hassle with the fuel wasn't worth the few extra ponies.

I took his advice and my mild street 540 made 700hp on the engine dyno. I run 91 pump with no problems. I probably could have went with more comp and still used pump premium, but you never quite know where the line is between just enough and too much.

Anybody have a program that can figure the difference in HP between 10.5 and 12.5 comp ratios on the OP's combo?

Yes that would be interesting!

I don't think for you that going out of the range of pump gas will do anything for you.Itsa streeter going for the look.Would it matter to you if you had 650HP or 800? No Your not going to the track and will never have traction.Mild pump gas build will require less maintenance.Build it right and get YEARS of fun

You are probably correct, but will be running Caltracs and 13-14 inch slicks most likely...have to be prepared for those new Demons and ZR1 corvettes....lol. Thanks for the replies.

For what you will spend on the hemi, you might as well go buy the demon.

I'd keep it pump gas friendly. My 605 is 10.4-1 and it will run on anything you put in the tank. It also makes enough hours and torque to scare the Bejesus out of you.
I can't really see an upside to 13-1 for a street.

Sure, I will go right out and buy a demon at 25k over list. I might be dumb, but not stupid!

My street tires are 16.5X31,I race on 10.5w both M/T tires.Please don't think your getting ANY traction on the street from a dead stop weather your 10: or 15:1!