Let's talk high compression and pump gas

Great article gentlemen.

If this doesn't make it into the tech archives, I'll be disappointed.


I'm wondering how affected a 9.0/1 CR 462" late Buick engine in a street roadster (2400#) is when you gear it up for a lower rpm level during fast highway cruising?
Which can be pretty fast considering the 75 mph speed limits here on the long and lonesome desert highways.
Just keeping up can be the hard part and 80 + is common.

The engine in this car has a very mild cam, hydraulic flat tappet and 260-266 degrees advertised duration.

Gearing will go from 3.70 to 3.00 with no other changes.
Right now, rpm's are running 3300 rpm at 80 per.


The 31 on 32 rails roadster I'm building now also has a 462" engine which should come in at 10.0/1 or 10.5/1 CR and if I can swing it, auminum heads.
Bigger hydraulic flat tappet cam with 288-294 advertised duration and 112* lobe centers.
I have an almost identical cam with 118* lobe centers, but prefer to use the 112* cam at first.

Gearing on this 2200# car will have the engine spinning about 2900-3000 rpm at 80 mph.

Total all-in timing on both engines will be 32* BTDC and an intitial of 8* on the 32 roadster and perhaps up to 12* initial on the 31.

With such light cars, lugging the engine doesn't seem to affect things.
I stay away from lugging them, but if I accelerate from say 20-25 mph in 3rd I don't floor the throttle until about 35-40 is reached.

Driving technique seems to help, but it may not be a worry with the 9.0/1 CR of the 32's engine.

 

Exactly.

Overlap happens at the end of the exhaust stroke, and the beginning of the intake stroke, so it would have no effect on the dynamic compression. Its the intake valve closing as you get to the compression stroke that changes your dymanic compression.


Most of the high performance engines I build are close to 11:1 compression and are all run on pump premium. Cam, compression, and a good quench are all it takes. I am sure you could run much more compression like others here have with some real research and tricks like reverse cooling, cermic heat barriers, and super tight quench in a efficient chamber.

Even the engine in my 57' with a TH700R4, and 3.36 rear gear, is 9.2:1 and only ever gets run on 87 octane.

 

Hello,

Here is an old (1997) Hot Rod Magazine article about building an 11 to 1 350 to run on 87 pump gas. It is a ten year old article and could probably use some updates, but it has some good ideas.http://purplesagetradingpost.com/sumner/techinfo/350%20chevy%20engine.html

Hope this helps somebody get started

Darrell
Atlanta Ga

 

C9, I'm not sure exacty the end result in your scenario, light weight, but high drag (my A 2D sedan is @ 80, I expect your roadster is in the same boat). I use the Dynamic Compression ratio calculator on the Keith Black site I read about it here, with carbs I try and keep the dymaic under 7:1, as I'm a wus when it comes to the possiblity of bad fuel. I have heard that some do well up to 9:1 (dynamic) on pump premium with aluminum heads and FI.

3.00 from 3.70 is a huge jump, do you have anything in between?

 

that is correct, bored and Stroked, one way to think about it is if you advance a given camshaft you generally see an increase in cylinder pressure but overlap hasn't changed

 

Thanks for the calculator address.

3.00 and 3.70 is all I have for the 32's 28 spline axles right now, but Pic A Part is only a couple miles away.

I have a 3.25 31 spline locker set up for the 31 on 32 rails roadster, but it's a little ways from getting on the road.
Should be about right with the 28" tall tires on it.

The 32's rear tires are 30", but I do have a pair of new 28" tires . . . just need to find the correct backspace wheels for using them.
The wheels they're on fit, but are so close to the body they don't look right.

I need some slot mags 8" x 15" with 3" backspace.
The wheels the abovementioned tires are on have 4" backspace.
Doesn't sound like much in the backspace dept, but it is.

Funny part is the 10" x 15" wheels on the 32 have 2 7/8" backspace as do a spare pair I have.

Seems like all the 7" slot mags I'm seeing lately have 4" backspace.


The 3.00 may turn out to be too high.
I started the car with a 2.75 diff in it and it was way too high.

What I ought to do is pull the top back off and run a couple of tanks through it and see how it does MPG-wise, but probably not too much difference.
I got 15 mpg running topless down to Seligman a couple weeks back, but half of it was running 75 or so along with a couple of full throttle blasts when passing.

Does seem like the top adds a lot of aero drag, whole lot of flapping going on....

 

I remember this article and a few others from awhile back -great stuff!
Another favorite article is this one "Cooler Heads For Hotter engines" I believe this is where the reverse flow cooling idea for the LT-1 came from in 1992 and possibly why it stopped in 1998 Jack Evans click!.

I used to race an LT1 and Ill never forget how much I learned from that design. I always wanted to build something similar with what I had laying around "retro" but with a few things adapted from the LT1 design.

This is what Im currently building.
3.48x 4.155 combo trw 2311's shaved to produce around 11.1 with a 70 chamber. To complicate things even more the heads are cast iron 041x heads with chambers re-contoured and polished. Lots of work & time going into the piston and chamber work area. I also went with a single top tunnel ram plumbed for new LT-1 style reverse flow cooling and Ill be using Evans cooling products.

 

I run a 426 wedge at 11 to 1 with 253@050 on the street often with 92 octane although it much prefers 94 from Sunoco which is available here.
The whole reason for raising compression above say 10 to 1 is that we need to recover the compression lost by the extended intake lobe cam duration. Nothing happens in the compression stroke until the ntake valve closes. ie there is no positive pressure OF SIGNIFICANCE created till the valve closes. So say if you have a small block Chevy and the intake doesnt close till the piston is .125 up the hole on the compression stroke then you dont have the same compression ratio as the static measurement will tell you you do because you only have 3.48 - .125 =3.47875 stroke to malke the squeeze with. The only way ro recover the lost squeeze is with high domed pistons . That is the single most important speed secret there is. The trick is finding the balance between getting more charge in the cylinder to squeeze and having enough stroke to squeeze it. Does a long cam allow more static compression ratio. Certainly. But like everything it has its limits. Best to error on the conservative side.
BTW a 383 should not be pinging. If it is it has too much total advance. In a mod form we never run the vacccum advance and limit the total to 32 to 36 degrees. Different combustion chamber designs require different timing. I wouldnt sell the open chamber mopar heads short. They are cabable of some very extreme perfomance. They are open to eliminate dead air space (where fuel doesnt burn which raised emmissions. (HC)) but we discovered that they also make wonderful power. Now though with the aluminum aftermarket heads avaialble it is simply often less expensive to go that route then do a set of iron ones and they already have better flow than the best ported iron. By the time you buy stainless valves ,retainers , springs and do the machine work you can easliy pay for a set of aftermarket heads that will give you the same or better results. Dont discount though the effect long cam tming has on what fuel you can run. Static compression ratio is almost meaningless . Dynamic is what tells the real story.
Don

 

Great thread,Dolmetsch you most certainly cut some cast in your day.
Sometimes getting a point across is difficult by way of writing with out illistrations but this post sums it up best.
Another thing to mention is ALL gasoline has the SAME btu rating.
All gasoline has the same ability to make the same power.
The difference is the ability to RESIST detonation,hence octane rating.
Just like static compression octane is just a number.Good building practices will allow the limits to be raised.
As mentioned motorcycle technology shows what good chamber design can do,ever wonder how those sport tuners get 500 horse out of a 4 cyl ?
Lots of testing here and lessons to be applied.
Good stuff.

 

I've built this engine twice the first using Ford rods but because of the extra rod weight it required $200 of heavy metal to balance. The second motor used off the shelf 400 6" rod pistons and after market 6.25 rods was actually cheaper to build. Both motors used Edlebrock heads and a hydraulic roller cam 215 @.050 intake and 220 exhaust on 112 centers The last one was pulled on a dyno with 87 octane and no detectable detonation Long rods and good quench go a long ways.

 

Variable everything helps. Knock sensors, ignition advance and retard cam advance and retard, constant fuel supply adjustment. Add to than aluminum heads and blocks to help control combustion chamber temps and tight quench, long rods to increase dwell time.

There are things that can be done to our old engines but we will never achieve the constant engine management that is achieved with a good computer.