Increasing Velocity of IHP 35

[pratik_mahale]

While cocking my Pegasus the plastic spacer between the receiver & cocking arm broke gun was in working condition but cocking arm can make scratch marks on receiver so to replaced the Plastic spacer I visited Precihole Factory on last Saturday

Dr. Shirsat immediately replaced broken plastic part I was ready live but just then few people come to Dr. Shirsat with their defective Gemo CFX air rifle.
Doc opens the gun & the main spring was broken which doc replaced in few minutes.

While Doc was repairing the Gemo I observe that the diameter of receiver is just 25 mm & the weight of the piston also very less Gemo is using Top Hat to increase the piston weight but still its very less then IHP Piston I ask Doc why Gemo is giving more power then IHP Doc said Gemo has a longer Piston stroke so its generate more power.

I started thinking how we can increase the Piston stroke in IHP I got one idea
I ask Doc for some Moly grease & one SX100 Piston Seal for my IHP 35.

On Sunday I open my IHP 35 .177 clean internal parts
Now to increase Piston stroke I decided to cut the Piston by 2.5 mm which took me around 1 hour with hacksaw then I polish the piston top with metal file
Clean the piston & fix the Seal with Moly grease which Doc gave me
Reassembled the Gun with original IHP spring no Top Hat & fire more than 50 shots before I chrono my IHP with Master Shot flat head pellets .177

I was getting min 675 to max 728 fps ( Avg 703 fps) with normally pushing the pellets in the barrel & with pellet seating min 787 to max 807 fps( Avg 795fps ) velocity.

Reading before the modification
Avg 665 fps with normally pushing the pellets
Avg 713 fps with pellet seating

There are three major changes has effect the velocity
1) by cutting the piston by 2.5 mm I have increase the piston stroke
2) as I cut the piston the weight of the piston also reduced (50 to 75 grams)
3) SX100 Piston Seal & Moly grease which is much better then original IHP seal

IHP tunning.jpg

Accuracy I still not check but I feel will be the same as before

[airgun_novice]

Dear Pratik,

The difference in fps with just 2.5mm reduction is wonderful. What is the given thickness of the piston ? Can 10mm (1cm) be shaven off ? Can optimum thickness be kept and say a stainless steel or brass disc be attached/ spot-welded for smooth travel ? If so, then you can look at the neighborhood of 1000 fps for the IHP. BTW, howz the fitting of the Pegasus seal vis-a-vis IHP 35 ? Any gaps ? Any idea if receiver diameters of IHP 25 and IHP 35 are same ? (Thinking if I could experiment on my N25 )

Feel free to borrow my angle grinder if you think it will help more than the Axo. Good job experimenting - keep it up!

regs
AGN

[brihacharan]

Hi Pratik,
> Good show man
> BTW its no rocket science - Increase "Swept Volume" = Increase in Velocity & Power.
> By slicing off 2.5mm from the piston you've increasd the stroke length thus allowing more air to be compressed.
> Its fortunate that you did not exceed 2.5mm as otherwise you'd have had cocking problem.
> I am venturing to opine that this procedure could be considered as part of the "Tuning " process to increase power through enhancement of velocity.
> However accuracy should not be sacrificed - awaiting your tests for accuracy asap.
Briha

[pratik_mahale]

Thanks AGN & Briha Sir,

No idea about receiver diameters of IHP 25
May be we can slice about 5 -7 mm but then need to make threading in piston for the screw to fix the seal but Slicing more piston part need to be check first or else as Briha Sir told may be some cocking problem

[mercury]

Pratik......good experiment.

i have increased the swept volume on one of my rifles. it worked for me , but its kind of tricky. its not just the piston that needs to be worked on...there are a few other factors.
my observations....

besides increasing the swept volume...weight of the piston...spring power...addition of piston weight/ or not and weight of pellet ......its a combination of all these in the right proportions that can have give you a smooth / consistent shooting rifle with an fps gain.

in your case...please, i am not being negetive.....the better fitting seal and molly could have played a more significant role in fps increase...besides the advantage of a shortened and lightened piston.

another point you need to be aware of......with lengthening the stroke and lightining the piston it could lead to piston bounce.....add too much piston weight and you will have piston slam. you got to get the balance right.

rather than reduce the thicknrss of the piston ....holes csn be drilled in the piston to lighten it. optimum thickness can be mantained..and again rather than weld /attach metal...JM old school buttons can be glued to the piston.

the benifit of light and heavy pistons and the pros and cons on it effect on fps are subjects of endless debates and have ruined perfectly good rifles !!

increasing the swept volume/ lengthening the stroke , to be more effective, is more than than taking out 2.5 mm from the piston....a lot more needs to be taken off and thats the tricky part.

but go ahead and experiment Pratik.......thats the learning.

[pratik_mahale]

Thanks Mercury

Can you explain how you increase the swept volume in your rifle?

in your case...please, i am not being negetive.....the better fitting seal and molly could have played a more significant role in fps increase...besides the advantage of a shortened and lightened piston.

May be as I am not an expert in air rifles just doing some R&D don’t know which change has more effect need some more experiment

[brihacharan]

[quote="mercury"]

The benifit of light and heavy pistons and the pros and cons on it effect on fps are subjects of endless debates and have ruined perfectly good rifles !!

Increasing the swept volume/ lengthening the stroke , to be more effective, is more than than taking out 2.5 mm from the piston....a lot more needs to be taken off and that's the tricky part. That's the learning.

> Well said mercury
> Again as you rightly put it "The debates & experiments will continue unabashed"
Briha

[Basu]

A virtual increase of 1.54 cc in swept volume is very insignificant to understand wheather or not it is directly responsible to enhance FPS.reported figure shows that in case of normal pushing pellets ,there is an increase of 10 FPS.It is also seen that the extreme spread is very high with Precihole seal.I am of the opinion the power enhancement is caused by the new seal and moly most, less due to addition in swept volume.

Basu

[pratik_mahale]

Today got time to test the IHP 35 accuracy is same as before but power is equal to Pegasus.
I have only one scope which was fix on Pegasus so can’t compare the accuracy

[pratik_mahale]

Getting 20 FPS spread when seating the pellets I personally feel not bad but it’s not because of the Precihole Seal is mainly duo to some bend in the IHP Spring

[vsraja]

i also trimmed the front part of the piston.I feel the increase in power.I have trimmed nearly 4 to 5mm from the piston top.I feel the increase in power.Still there is a scope to improve.

[pratik_mahale]

Be ready for cocking problem if you overdo it

[Basu]

Dear vraja,
A mere increase of 2.5 CC , will not enhance much of its velocity .
Rather use of proper breech seal and piston seal will help.

Basu