View Full Version : Pistons for 4.0 OHC
11-18-2007, 01:50 PM
Hello. Looking to up the stock compression ratio on a 2000 OHC engine without purchasing forged pistons. Will the SOHC pistons fit and yield a higher CR?
The project is swapping out the 2.8L in my TVR (British) with a 4.0L. Delta cam, up CR a bit, inject with Megasquirt, SM plenum. Not going for high revs and thought forged pistons would not be required. The SOCH CR of 9.7:1 would be nice, but don't have the head cc info to know what the ratio would be after the swap (assuming it's possible).
Anyone done this before?
11-18-2007, 02:10 PM
You need to read the 4.0 Rebuild article on the front page, Technical Section of this site.
The answers to all your questions are: It depends. . . .
If you aren't going for high revs leave the 4.0 alone - you are going to think you put a V8 in the thing anyhow.
The 4.0 OHV engine that you have currently is rated at 9:1 compression ratio with the factory pistons - you can install inexpensive hypereutetic pistons from a 90-94 4.0 and it'll bump the cr up to 10:1 - the flattop SOHC pistons would take the cr up to almost 11:1 (not a good idea).
11-19-2007, 10:12 AM
Thanks guys, I will read the tech article. Aren't the earlier pistons heavier than the late pistons (later being the 8 bolt crank with smaller, lighter, counterweights)? Apparently the difference is small enough it can be compensated for? 50 grams in the piston I believe I read - that's a bit to be taken off the pistons, isn't it... I have never personally ballanced the crank but wouldn't that mean adding weight - a bit harder than removing it?
11-19-2007, 10:37 AM
Ah yes - the Doug Anderson article. I talked with Doug several months ago when I was beginning the project and selecting a good 5 speed manual gearbox. He's a great guy. Unfortunately we didn't discuss pistons at the time.
It appears I would need pistons from a 90-94 and a crank from a 97-98 for the heavier bob weights. Thanks for the reminder to read this again! 10:1 should do nicely, though 9.5 was my goal. I will have to be careful not to skim too much to clean up the heads and deck; that's all the compression I want. Perhaps a bit of head chamber cleanup will keep it a bit lower.
Anyone use "SI" stainless valves? They are very reasonably priced.
FYI - the transmission combo I verified with Doug prior to purchasing was mating the OHC with the Tremec 5 spd from the 2006-7-8 Mustang. Everything bolts right up, but unfortunately there aren't after market flywheels and clutches at the moment - too new. The unique Mustang starter (manual different from auto) has me stumped at the moment (from the salvage yards). May have to purchase new from Ford...
11-19-2007, 11:19 AM
Ok, going from a 2.8 to a 4.0OHV would be similar to swapping in a V8 without the weight bulk and it's effects on working room and balance.
Do remember that a 4.0OHV makes more power than a pre EFI-HO 5.0.
I briefly had a TVR 280 into which I swapped another engine
the guy I sold it to shitcanned that engine for a 5.0 and promptly
totalled the car, killing himself in the process. (Darwin Award)
Even assuming you started off with a 2.9 EFI engine instead of the 2.8 (carb)
in the TVR an OHV 4.0 would feel like someone added a turbo to your engine.
The 4.0 makes more torque than a 2.9 at any point between 1100rpm and 4100rpm.
(and more torque than a carb 2.8 at any rpm between Idle and the rev limiter)
I'd be real concerned with breaking things (the gearbox and axle)
with a stock 4.0, let alone a Stock 207hp 4.0SOHC and I'd question
the sanity of messing with a 4.0SOHC considering the EEC-V management system
also I'm not sure you won't get into clearance issues with the bulky oil pan of the 4.0SOHC
and what exactly is wrong with the stock cams and valves in the SOHC heads?
You are talking a doubling of power in your car if you simply swapped in
a used 4.0 engine from a junkyard Explorer.
Are you talking Tasmin-280. TVR280i or an S-series?
I'm not saying this is a bad idea, I'd love to take the car for a drive,
I'm saying there are hidden stumbling blocks that may leave a set of
new permanant marks on your shins...
how much is enough?
I know the TVR I'd like to have ais a clean, unmucked with Tasmin 200into which I'd unhesistatingly install a 2.3Turbo:)
11-19-2007, 12:01 PM
It's a 280i. An ‘86, known as the series 2. The stock 2.8L has the Bosch Jetronic injection and a bit of a cam. The Euro version boasted 150HP, but US was down about 10.
The car is not light, though it is fiberglass with a tubular steel frame - around 2700 lb. It's not a car that has a lot of get up and go, but it handles great. The 2.8 has to go. The low output, 4 speed gear box (5 speed never imported) and the ghastly mechanical Bosch injection system all will be set aside for a larger lump and electronic injection.
I chose the 4.0 because it was still a Cologne engine - preserving the V6 lineage. I am not concerned about the drive train strength as most of these cars had Rover V8's in Europe and they have the Jag independent diff which is pretty strong. Coupling it with the upgraded T5 from the late Mustang will greatly improve the power and touring revs. I thought about a 5L V8, but am probably going down a path that's more expensive per horse power, just to preserve the heritage.
11-19-2007, 03:17 PM
OK - In my case, the engine I'm rebuilding has the 8 bolt crank - which according to the way I read the Doug Anderson article - has purposely been constructed with different weights. He mentioned something about causing a vibration at idle but he was not really detailed in his discussion.
I'm rebuilding - I know where there are .010 90-94 piston that I could bore my block for - but aren't those the very ones that is supposed to cause the vibration? I'm assuming they would hit 9.5 or 10.0 to 1.
11-19-2007, 06:35 PM
As I said, why not just work with a stock 4.0SOHC?
207hp and ~40% more torque will make it quite "snappy"
And if that isn't enough don't increase compression...
There are aftermarket means to install the Eaton Blower from
a Thunderbird Supercoupe on a 4.0SOHC.... :)
BTW, you have far greater faith in the Jag rear than I do, but with
a V6 or small block V8 I agree it probably isn't an issue.
Like I said, I'd like a clean Tasman 200, which originally came with a 2.3Liter
Pinto 4cyl with a carb.
a 2.3turbo with an intercooler makes 190hp and 265ft/lb stock
And with only minor adjustments to the wastegate actuation
system can EASILY make 250hp and upwards of 300ft/lb
But since your car started with a cologne V6 that seems to be the way to go.
Better a Cologne than an Essex engine like the earlier TVR's
FWIW I've never been a fan of the Rover V8, but it was never a "torque monster"
And it's torque that breaks things not horsepower.
Get 6 bolt 95-97 cranks and you won't have to worry about imbalance with the pre-95 pistons.
SI valves are great replacements that offer inproved flow over the stockers due to their undercut valve stems - I use them exclusively when I do up a set of heads.
11-19-2007, 09:24 PM
The way I read the article, and the note under the crank picture is:
There are two flavors of 8 bolt cranks. The 97-98 casting 975M-AC that has the heavier bob weights for the earlier heavier pistons (with smaller dish in piston). In 99-2000 casting XL2E-BA had the 8 bolt pattern and lighter bob weights for the lighter and lower compression pistons (when used with the later smaller heart shaped combustion chamber retained the 9:1 compression ratio).
So you must look at the crank you have. If it's the earlier 8 bolt casting you can use the 90-94 heavier pistons with smaller dished top (made for the larger head chamber of the earlier heads) and get 10:1 CR with the later heart shaped head casting.
I must source an earlier 8 bolt crank for my engines, which are 1999 and 2000 models, so I can use the earlier, heavier pistons and 10:1 resulting CR. I'll check the cranks first, just in case.
Can you please confirm this Bird? My combo with the late model Mustang trans works with the 8 bolt flywheel, so I would want the matching crank (97-98). I had a Ranger flywheel but the position of the starter ring gear didn't match the late model Mustang starter - when I finally got the Mustang flywheel I found that the starter they gave me was incorrect; for an auto. I wish I still had the Ranger flywheel to compair some dimensions. I may have been able to use it with the much cheaper pressure plate and clutches available.
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