Here are typical flowbench figures for triumph heads & the power you can expect to get:-

Does this sound like a black art?

Why do engines very rarely develop their potential power?

Quite simply because the majority of tuning companies (& most of the OE manufacturers) specialise in DETUNING the engines.

How is this explained?

The reality is based on various components and their design.
Mostly, design remains far from optimal quite apart from production spread.
So, as you can see it's far more accurate to represent the original output power based on head flow then SUBTRACT using a 25% degrade in head flow performance to show what you can expect to see.

This is for a number of reasons.

OEM camshafts are very mild, with lift that doesn't often achieve optimal flow.

(On the OEM Triumph head this is about 0.360").

Modified camshafts are usually very poor, with profiles that weren't actually developed for that engine at all.

(As you can see the TR5/TR6 CP Pi camshaft, always comes very close to optimal!)

Exhaust silencers & inlet systems can be/are constrictive.

Most tuners selling after market exhausts, have no idea what they are doing & are not interested in giving value for money.

Some OEM cars were VERY far from their real potential. (Eg. GT6 engine)

Above is a graph showing the various influences on head flow as a function of modification.

This is not silly conjecture, it's the result of very careful testing of each individual component.

As you can see, the largest improvement is made by changes to the seat and valve shape, then as lift passes 0.250" it is overtaken by the short side radius of the inlet port.

A law against obscenity?

What is quite amusing is to see the kind of weird theories expounded.
Several people speculated on the much increased perfomance to be had, down-drafting the Triumph head.
(Even D Vizard stated this!)
The held belief is that gas has difficulty going around a corner!
This theory is hilarious, had it not been tested.

The fact is;- you can modify the Triumph head in this kind of way UNTIL you can literally SEE into the chambers...and yet it STILL flows LESS!
It's even funnier when you see the kind of techniques being used to modify Dolomite 4V heads.
JOIN THE RANKS OF THE DETUNERS, you simply will enjoy, NOT BEING ALONE at least, & doing more "arm-chair detuning" over the internet in your spare time, with others..
Contrary to belief, almost NO HEAD combo will ever achieve better than -20% degrade (call it a detune) unless the exhaust ports & manifold are really good.
In this case it may be possible to see it close to -10% or better in truly exceptional cases. (Better than -20% is usually only possible on 4V engines).

It MUST be said, the treatment of 4V engines SEEMS TO BE, even further from optimal if you take into account the figures shown here.
(The Dolomite engine is 65bhp short of its potential target FROM THE FACTORY!)

The excellent Spen King designed Dolomite and Jaguar 4V engines fall SO FAR short of optimal, that they literally become laughable!

(The Jaguar 24V engine is MUCH worse, running about 120bhp short of its proper target!)
This really SHOULD NOT BE.
4V engines are inherently more stable & lower loss than their 2V ancestors

How on earth could sane people & so called "highly qualified engineers" do things this badly?

How to use this anti-bullshit guide:-

For some idea how to calculate the potential power of your favourite engine, enter the head flow figure referred to in the 1st column cross referenced of the 6th column "marked -25%"

Stab the calculator by multiplying that figure by the number of cylinders.
Next if you are SO SURE your tuning skills are exceptional, stab the figures from the "5th column" (no pun intended) into your calculator.

You are probably already DREAMING!

An example:- If you think you are a TOTALLY AMAZING head porter:-
You just got your TR6 head to flow an exceptional 140cfm (good luck!)
Multiply 30.8 x 6
Answer 185bhp.
YES this is ALL you will get with Webers.
(In other words only about 10bhp more than a bog standard really well put together TR6 pi with the right exhaust system on!)
Gutted? You bet it's a lot of money for spending an extra €3500 on Spanish webers that don't work right, a hot cam & a "flowed head"!

If you can con your rolling road to exaggerate the figures & your exhaust and Pi system is perfectly up to scratch
Multiply 32.8 x 6.
Sorry this is IT. 197bhp, not 220, not 240 or 250. You can't cheat proper maths.

197bhp is ALL you will get, on a good day with engine temp at 60C.

If you eventually manage to get the right exhaust valves & achieve this big improvement in exhaust ports done here, multiply
37 x 6.
This gives 222bhp. again NOT 251, not anything like it!

SORRY to break your dreams!

Rough n ready flow calculator:-
CFM Flow figure 25"	10" equiv	bhp per cyl (cfm/2.2)	-10%	-20%	-25%	eg.
80	52	23.6	21.2	18.9	17.7	STANDARD 1500 spitfire
90	58	26.4	23.7	21.1	19.8
100	64.5	29	26.1	23.2	21.75
105	67.7	30.8	27.7	24.6	23.1
110	71	32.3	29	25.8	24.2	STANDARD TR5 Pi
115	74	33.6	30.2	26.9	25.2	STANDARD GT6 11
120	77	35	31.5	28	26.3
122	78.7	35.8	32.2	28.6	26.9
125	80.5	36.6	33	29.2	27.5
128	82.5	37.5	33.8	30	28.1
130	83.9	38.1	34.3	30.5	28.6	GOOD modified Spitfire
133	85.8	39	35.1	31.2	29.2
135	87	39.5	35.6	31.6	29.6
138	89	40.5	36.4	32.4	30.4
140	90.3	41	37	32.8	30.8	Claimed to be best head ever done for a TR6 race car (N COLL)
142	91.6	41.6	37.5	33.2	31.2
145	93.5	42.5	38.3	34	31.9	Really GOOD modified GT6 2L
147	94.8	43	38.8	34.4	32.3
150	96.8	44	39.6	35.2	33
152	98	44.5	40	35.6	33.4
155	100	45.5	41	36.4	34.1
160	103.2	47	42.3	37.6	35.3
165	106	48	42.2	38.4	36
220	142	64.5	58	51.6	48.4	STANDARD Dolomite 16V
230	148	67	58	53.6	50.2
240	155	70.5	63.5	56.4	52.9
250	161	73	65.7	58.4	54.75
260	168	76	68.6	60.8	57
Heads from Standard Triumph & GTT Conversions
Head	Head Number	Max figure inlet@25"	Figure converted to 10"@0.450"	MAX potential Power per cyl worked back from CFM figure (/2.2)	degrading factor -10%	degrading factor -20%	degrading factor -25%	Original power	OEM Shortfall	Action required to get -20%	Action required to get -10%
OEM. GT-6 Mk2	517528	113	73	33	29.9	26.4	24.8	98 (130 on Pi)	50 (18)	Change camshaft to TR5 with Pi	not poss
OEM. Spitfire 1500	TKC1155	80	52	23.6	21.3	18.9	17.7	70	1	n/r	not poss
OEM. 1300/1500 TOLEDO	218139/40	90	58	26.4	23.7	21.1	19.8	71	8	Change cam to Mk3 type, maybe exhaust manifold etc	Not poss
OEM. TR5/6 Pi	516816	109	70	31.8	28.6	25.4	23.9	143	0	POSSIBLE.Blueprint/balance then change inlet filter barrel.change exhaust silencers	Change to good 6-3-1 manifold. Mod metering unit
OEM. Dolomite 16V	UKC1418	220	142	64.5	58.0	51.6	48.4	127	67!	NOT POSS*.But Notice how this head flows DOUBLE the TR6 head! Blueprint/balance then change inlet filter design, change exhaust silencers,longer cam duration (doesn't need more lift).	Not poss* because lack of space
Modified Heads:-
Head	Head Number	Max figure inlet@25"	Figure converted to 10"@0.450"	MAX potential Power per cyl worked back from CFM figure (/2.2)	degrading factor -10%	degrading factor -20%	degrading factor -25%	Original power	OEM Shortfall	Action required to get -20%	Action required to get -10%
MOD. GT-6 Mk2	517528	145	93.5	42.5	38.3	34	31.9	155	30	Change camshaft to ideal profile CR increase etc.	POSSIBLE to -10% = 230bhp.
MOD. Spitfire 1500	TKC1155	80	52	23.6	21.3	18.9	17.7	70	1	IMPOSSIBLE Needs larger valves	not poss
MOD. 1300/1500 TOLEDO	218139/40	130	84	38	34.2	30.4	28.5	114-145	0 posssible with mild cam.	This head needs to be used as a basis.Can get better than -10%!	Only doable on Mk3 engine
MOD. TR5/6 Pi	516816 219016 etc (makes no diff)	140	90	41	36.9	32.8	30.75	180	5	this equates to a maximum poss of 197Bhp.	-10% = 221bhp. POSSIBLE. Some claims to reach 250bhp. Theoretic max is 246bhp IMPOSSIBLE
MOD. TR3/4	late head	155	100	45.5	41	36.4	34.1	165 MAX	0	this equates to a maximum poss of 165Bhp. NB:- the TR4 engine especially in ENLARGED form has EXACTLY the same bore/stroke as the Jaguar AJ6 3.6L engine (which is a modern 4V engine) This means the ideal peak power is no higher than 6800rpm.	POSSIBLE. Some have claimed to reach 225bhp. The 100% Theoretic max is only 180bhp so more is IMPOSSIBLE
MOD. Dolomite 16V	UKC1418	260	168	76.2	68.6	61	57.1	228	18!	It's perfectly possible to get 245bhp from this engine without excessive revs or over hot cams. (doesn't need & must not use much more lift).	space & cooling considerations, but the engine is capable of a genuine 275bhp+.