Compression or Flow? HE or Pre HE ( Flathead)

I’m building a low budget XJS for racing.

I have a couple of each Engine type and I can’t decide which way to go.
Stock they both make the same power and torque.
The Flathead ( pre HE ) has 7.8-1 compression with a. Stock total advance of 38 degrees
The HE has an 11.5-1 compression ratio with a total of 17 degrees total advance.

My total budget is $300 so the only modification I can afford is a local cam grinder can give me a little more lift and duration by regrinding my cams. Something around .406-.410 lift instead of the stock .375 maybe 4-6 degrees more duration ( should raise the power peak closer to 6600 instead of 5500 the stock cams yield.)
I do have a T5 manual transmission I can use later but for now they both will use a GM turbo 400 with a manual valve body.

Weight wise I expect to be around 3000 pounds
Which way would you go and why?

 

That's a roll of the dice, isn't it? On my USA models, the 83 HE I now have is rated at 266 hp, my 78 Pre-HE was rated at 244. Are you sure your pre-HE is only 7.8 compression? That seems very low, but I understand that some were that low. If I had to choose between the 2, I would go with the HE due to the higher compression. I would be very surprised if you could ever gain enough flow to make up for the low compression of the pre HE. Unless you supercharged it. I understand the HE heads do not flow great but the cam regrind will help that some.

 

Yes, Europe was 9:1, North America 7.8.

I'd go HE for more power at lower revs. Plus better fuel economy, which means fewer stops in the pits.

 

Is Both an option? Put pre-HE heads on the HE short block? That way you get the better flowing heads with the higher compression. It is definitely the most work. I'm not sure how your time figures into the $300 budget.

 

No, it doesn't work that way. I think Pre HE heads on an HE block give you something like 20:1 compression. Basically turns it into a diesel. Heads and pistons need to match, all HE or all flathead.

 

I think combining preHE heads with HE engine gives a 23:1 compression ratio, one of the forum Elders said so and thus it is Gospel.

23:1 is diesel territory, I'll donate money to the project if he goes that route and builds a V12 turbo diesel race car! And if anyone thinks diesel is slow just check this out:

 

We need clarification. The carbed engines in America produced 242 hp DIN net which is just Slightly more than SAE net.
European engines Had ( because they were 9.0-1 compression and didn’t have the timing changes were close to 284 hp ). ( with one partial year exception in early 1980 they had 10.-1 compression and 299 horsepower.)
The fuel injection engines both pre HE ( 7.8-1 ) and HE ( 11.5-1) had the same 262 horsepower in the US
European engines post HE were 12.5-1 compression

one other difference if you look at the Pre HE ( Flathead) the exhaust port is exposed to less aluminum that the HE exhaust port. That slight difference does make a difference when exposed to the 1300 degree exhaust gas.

Finally. The HE because of the poor head design tends to stratify the fuel charge.. cruising at lower RPM it’s not a problem but when pushed flat out on the race track causes hot spots and the propensity to blow head gaskets. The solution is to richen the fuel mixture Something Jaguar makes very hard to do. Usually requiring a trip to AJ6 engineering.

 

Racing is all high revs. 3500 rpm is rare. Usually only in the pits or on pace lap. Normally up near or past redline. Because of the mild lift the engine runs out of breath around 5500 rpm but the valve springs have been tested to 8300 RPM before any float occurs.

Something few people understand. Just how strong that engine is. A Chevy 350, 5.7 liter has smaller bearings both main and rod journals even though it takes much more load per cylinder. The Chevy Crank weighs about 25 pounds and is usually cast iron
The Jaguar crank is 78.8 pounds and is forged of EN 40 steel and then nitrided for hardness.
Plus the short stroke in combination with very long connection rods puts almost zero angularity pressure. What that means is 200,000 miles from new the rings will be still be good The cylinder walls won’t be worn. And the piston speed will be so slow pistons will last forever.
Pistons start and stop changing directions at the top and bottom of each stroke. In the middle they will be at peak speed. The longer the stroke the faster the piston speed ( and stress).
Take a Chevy V8 ( 350, 5.7 liter) it has short 5.7 inch connecting rods and travels 3.50 inches up and down.
The Jaguar has something like 6.25 inch long rods but only has to travel 2 .75 inches that reduced angularity minimizes wear and pressure plus lowers piston speed.
The factory tested the V12 to 8300 RPM no valve float. No problems.

 

high compression plus retarded ignition make 262 horsepower DIN net.

Low compression plus better ignition advance makes 262 horsepower DIN net.

the racers ( including me when I had the budget ) all used the better flowing heads ( flatheads ) because we made the pistons whatever compression we wanted and could still run the better timing because we had race gas.

Lister was making over 730 horsepower in their race car but pretty much the upper limit modification wise is 450 horsepower in the HE due to poor flow.

 

Why not breaking stereotypes? I know someone converting salvaged pre-eHE v12 to 2-stroke. He's not willing to share any details so he must be quite lucky with this concept. In theory it's not that complicated...

 

2 stroke V12 , NOW that would be interesting , something like the Famous GMC 2 strk. Diesel engines, those i am very familier with!
Damn woudnt that turn some heads, give it a shot NOS, and alcohol mix, or ACETONE to speed up combustion of the Diesel fuel!
ron

 

Wait: a Jag V-12 Diesel would be awesome in a boat. What else would I have to change?

 

First you push the red button then you push the pink button . Be sure to run the unicorn for good luck.

In other words forget it. It’s a pipe dream.

 

just like yours, pipe dream.

 

I'd thought as much. But I have heard of one, I think Kirby mentions it in the book. I know how to use gasoline on a boat, but diesel has its advantages... oh well.

 

Compression needs to be like 22.5:1, or some high ratio, glow plugs need to be in place of spark plugs, turbo with intercooler piped in, uprated headstuds. the rest can be figured out with a after market ECU...it COULD be done however a fair bit of effort involved.

 

If a high compression pre-HE was a choice, that would be best, better flow, and higher compression. Since that was not a choice, I would be very surprised if the low compression flathead could make more power than the HE.

 

Absolutely agree 299 horsepower with the rare 1980 10- 1 compression Flathead (Pre HE ) as compared to the 262 DIN net the 1975-1980) Flathead We have here in the states.
You guys even have 284 horsepower with 9-1 compression DIN net
But why do they call it HE when it has the same power as the pre HE?

Since it’s a race car I can cut open the little air intake funnel, draw cold air from in front of the radiator and pick up some power that way. I can also advance the timing a few degrees and still use pump gas
Between those two items I really should pick up 35- 50 extra horsepower. Then without mufflers Converters another 25-30 horsepower.
but it should be the same for both engines shouldn’t it?

There is one more “free” option. E85. That’s worth a solid 10% gain.Maybe little more with the 11.5-1 compression since Alcohol will tolerate more advance.

The problem is With the cam regrind. I’m now actually above the 450 horsepower ceiling AJ6 reports for the HE.

 

HE stands for High Efficiency. In typical road car normal driving, the HE uses half the fuel of a pre HE.

 

But the same power from the same sized engine? You’d think if they kicked the compression up so much the power would increase accordingly? No? No increase in power.
Ok so take credit for a mile or so extra fuel mileage at partial throttle. But maybe that’s too complex?
So how much credit do you give for dumping that ancient ( designed in the 1940’s ) Borg Warner and replacing it with the GM Turbo 400 which was already used by most big GM cars.
Any credit to the rear end change? From 3:31 or 3:07 to 2:88 ?
Then there is the changes in the EFI and size of injectors?
Oh and the body changes?

Does meeting the new California emission laws mean nothing? Just good luck or something?